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PID controller From Wikipedia, the free encyclopedia  HYPERLINK "http://en.wikipedia.org/wiki/File:Pid-feedback-nct-int-correct.png"  INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/thumb/4/40/Pid-feedback-nct-int-correct.png/300px-Pid-feedback-nct-int-correct.png" \* MERGEFORMATINET   HYPERLINK "http://en.wikipedia.org/wiki/File:Pid-feedback-nct-int-correct.png" \o "Enlarge"  INCLUDEPICTURE "http://bits.wikimedia.org/skins-1.17/common/images/magnify-clip.png" \* MERGEFORMATINET  A  HYPERLINK "http://en.wikipedia.org/wiki/Block_diagram" \o "Block diagram" block diagram of a PID controller A proportionalintegralderivative controller (PID controller) is a generic  HYPERLINK "http://en.wikipedia.org/wiki/Control_loop" \o "Control loop" control loop  HYPERLINK "http://en.wikipedia.org/wiki/Feedback_mechanism" \o "Feedback mechanism" feedback mechanism ( HYPERLINK "http://en.wikipedia.org/wiki/Controller_%28control_theory%29" \o "Controller (control theory)" controller) widely used in industrial  HYPERLINK "http://en.wikipedia.org/wiki/Control_system" \o "Control system" control systems a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured  HYPERLINK "http://en.wikipedia.org/wiki/Process_variable" \o "Process variable" process variable and a desired  HYPERLINK "http://en.wikipedia.org/wiki/Setpoint_%28control_system%29" \o "Setpoint (control system)" setpoint. The controller attempts to minimize the error by adjusting the process control inputs. The PID controller calculation ( HYPERLINK "http://en.wikipedia.org/wiki/Algorithm" \o "Algorithm" algorithm) involves three separate constant parameters, and is accordingly sometimes called three-term control: the  HYPERLINK "http://en.wikipedia.org/wiki/Proportionality_%28mathematics%29" \o "Proportionality (mathematics)" proportional, the  HYPERLINK "http://en.wikipedia.org/wiki/Integral" \o "Integral" integral and  HYPERLINK "http://en.wikipedia.org/wiki/Derivative" \o "Derivative" derivative values, denoted P, I, and D.  HYPERLINK "http://en.wikipedia.org/wiki/Heuristic" \o "Heuristic" Heuristically, these values can be interpreted in terms of time: P depends on the present error, I on the accumulation of past errors, and D is a prediction of future errors, based on current rate of change. HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-0" [1] The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve or the power supply of a heating element. In the absence of knowledge of the underlying process, a PID controller is the best controller. HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-ben93p48-1" [2] By tuning the three parameters in the PID controller algorithm, the controller can provide control action designed for specific process requirements. The response of the controller can be described in terms of the responsiveness of the controller to an error, the degree to which the controller  HYPERLINK "http://en.wikipedia.org/wiki/Overshoot_%28signal%29" \o "Overshoot (signal)" overshoots the setpoint and the degree of system oscillation. Note that the use of the PID algorithm for control does not guarantee  HYPERLINK "http://en.wikipedia.org/wiki/Optimal_control" \o "Optimal control" optimal control of the system or system stability. Some applications may require using only one or two actions to provide the appropriate system control. This is achieved by setting the other parameters to zero. A PID controller will be called a PI, PD, P or I controller in the absence of the respective control actions. PI controllers are fairly common, since derivative action is sensitive to measurement noise, whereas the absence of an integral term may prevent the system from reaching its target value due to the control action. Contents [ HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" hide]  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Control_loop_basics" 1 Control loop basics  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "PID_controller_theory" 2 PID controller theory  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Proportional_term" 2.1 Proportional term  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Droop" 2.1.1 Droop  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Integral_term" 2.2 Integral term  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Derivative_term" 2.3 Derivative term  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Loop_tuning" 3 Loop tuning  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Stability" 3.1 Stability  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Optimum_behavior" 3.2 Optimum behavior  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Overview_of_methods" 3.3 Overview of methods  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Manual_tuning" 3.4 Manual tuning  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Ziegler.E2.80.93Nichols_method" 3.5 ZieglerNichols method  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "PID_tuning_software" 3.6 PID tuning software  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Modifications_to_the_PID_algorithm" 4 Modifications to the PID algorithm  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "History" 5 History  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Limitations_of_PID_control" 6 Limitations of PID control  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Linearity" 6.1 Linearity  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Noise_in_derivative" 6.2 Noise in derivative  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Improvements" 7 Improvements  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Feed-forward" 7.1 Feed-forward  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Other_improvements" 7.2 Other improvements  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Cascade_control" 8 Cascade control  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Physical_implementation_of_PID_control" 9 Physical implementation of PID control  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Alternative_nomenclature_and_PID_forms" 10 Alternative nomenclature and PID forms  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Ideal_versus_standard_PID_form" 10.1 Ideal versus standard PID form  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Basing_derivative_action_on_PV" 10.2 Basing derivative action on PV  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Basing_proportional_action_on_PV" 10.3 Basing proportional action on PV  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Laplace_form_of_the_PID_controller" 10.4 Laplace form of the PID controller  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "PID_Pole_Zero_Cancellation" 10.5 PID Pole Zero Cancellation  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Series.2Finteracting_form" 10.6 Series/interacting form  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Discrete_implementation" 10.7 Discrete implementation  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Pseudocode" 10.8 Pseudocode  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "PI_controller" 11 PI controller  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "See_also" 12 See also  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "References" 13 References  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "External_links" 14 External links  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "PID_tutorials" 14.1 PID tutorials  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Special_topics_and_PID_control_applications" 14.2 Special topics and PID control applications[ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=1" \o "Edit section: Control loop basics" edit] Control loop basics Further information:  HYPERLINK "http://en.wikipedia.org/wiki/Control_system" \o "Control system" Control system A familiar example of a control loop is the action taken when adjusting hot and cold faucets (valves) to maintain the water at a desired temperature. This typically involves the mixing of two process streams, the hot and cold water. The person touches the water to sense or measure its temperature. Based on this feedback they perform a control action to adjust the hot and cold water valves until the process temperature stabilizes at the desired value. The sensed water temperature is the process value or process variable (PV). The desired temperature is called the setpoint (SP). The input to the process (the water valve position) is called the manipulated variable (MV). The difference between the temperature measurement and the setpoint is the error (e) and quantifies whether the water is too hot or too cold and by how much. After measuring the temperature (PV), and then calculating the error, the controller decides when to change the tap position (MV) and by how much. When the controller first turns the valve on, it may turn the hot valve only slightly if warm water is desired, or it may open the valve all the way if very hot water is desired. This is an example of a simple proportional control. In the event that hot water does not arrive quickly, the controller may try to speed-up the process by opening up the hot water valve more-and-more as time goes by. This is an example of an integral control. Making a change that is too large when the error is small is equivalent to a high gain controller and will lead to overshoot. If the controller were to repeatedly make changes that were too large and repeatedly overshoot the target, the output would  HYPERLINK "http://en.wikipedia.org/wiki/Oscillate" \o "Oscillate" oscillate around the setpoint in either a constant, growing, or decaying  HYPERLINK "http://en.wikipedia.org/wiki/Sinusoid" \o "Sinusoid" sinusoid. If the oscillations increase with time then the system is unstable, whereas if they decrease the system is stable. If the oscillations remain at a constant magnitude the system is  HYPERLINK "http://en.wikipedia.org/wiki/Marginal_stability" \o "Marginal stability" marginally stable. In the interest of achieving a gradual convergence at the desired temperature (SP), the controller may wish to  HYPERLINK "http://en.wikipedia.org/wiki/Damping" \o "Damping" damp the anticipated future oscillations. So in order to compensate for this effect, the controller may elect to temper their adjustments. This can be thought of as a derivative control method. If a controller starts from a stable state at zero error (PV = SP), then further changes by the controller will be in response to changes in other measured or unmeasured inputs to the process that impact on the process, and hence on the PV. Variables that impact on the process other than the MV are known as disturbances. Generally controllers are used to reject disturbances and/or implement setpoint changes. Changes in feedwater temperature constitute a disturbance to the faucet temperature control process. In theory, a controller can be used to control any process which has a measurable output (PV), a known ideal value for that output (SP) and an input to the process (MV) that will affect the relevant PV. Controllers are used in industry to regulate  HYPERLINK "http://en.wikipedia.org/wiki/Temperature" \o "Temperature" temperature,  HYPERLINK "http://en.wikipedia.org/wiki/Pressure" \o "Pressure" pressure,  HYPERLINK "http://en.wikipedia.org/wiki/Flow_rate" \o "Flow rate" flow rate,  HYPERLINK "http://en.wikipedia.org/wiki/Chemical" \o "Chemical" chemical composition,  HYPERLINK "http://en.wikipedia.org/wiki/Speed" \o "Speed" speed and practically every other variable for which a measurement exists. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=2" \o "Edit section: PID controller theory" edit] PID controller theory This section describes the parallel or non-interacting form of the PID controller. For other forms please see the section  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Alternative_nomenclature_and_PID_forms" Alternative nomenclature and PID forms. The PID control scheme is named after its three correcting terms, whose sum constitutes the manipulated variable (MV). The proportional, integral, and derivative terms are summed to calculate the output of the PID controller. Defining u(t) as the controller output, the final form of the PID algorithm is:  INCLUDEPICTURE "http://upload.wikimedia.org/math/e/3/3/e3386d1b5511c8ce5b70a4ba8bcfc3e3.png" \* MERGEFORMATINET  where Kp: Proportional gain, a tuning parameter Ki: Integral gain, a tuning parameter Kd: Derivative gain, a tuning parameter e: Error = SP " PV t: Time or instantaneous time (the present) [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit&section=3" \o "Edit section: Proportional term" edit] Proportional term  HYPERLINK "http://en.wikipedia.org/wiki/File:Change_with_Kp.png"  INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Change_with_Kp.png/320px-Change_with_Kp.png" \* MERGEFORMATINET   HYPERLINK "http://en.wikipedia.org/wiki/File:Change_with_Kp.png" \o "Enlarge"  INCLUDEPICTURE "http://bits.wikimedia.org/skins-1.17/common/images/magnify-clip.png" \* MERGEFORMATINET  Plot of PV vs time, for three values of Kp (Ki and Kd held constant) The proportional term makes a change to the output that is proportional to the current error value. The proportional response can be adjusted by multiplying the error by a constant Kp, called the proportional gain. The proportional term is given by:  INCLUDEPICTURE "http://upload.wikimedia.org/math/3/f/8/3f80d26e2e621e0f123bb26e80e609e5.png" \* MERGEFORMATINET  A high proportional gain results in a large change in the output for a given change in the error. If the proportional gain is too high, the system can become unstable (see  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Loop_tuning" the section on loop tuning). In contrast, a small gain results in a small output response to a large input error, and a less responsive or less sensitive controller. If the proportional gain is too low, the control action may be too small when responding to system disturbances. Tuning theory and industrial practice indicate that the proportional term should contribute the bulk of the output change.[ HYPERLINK "http://en.wikipedia.org/wiki/Wikipedia:Citation_needed" \o "Wikipedia:Citation needed" citation needed] [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=4" \o "Edit section: Droop" edit] Droop A pure proportional controller will not always settle at its target value, but may retain a steady-state error. Specifically, drift in the absence of control, such as cooling of a furnace towards room temperature, biases a pure proportional controller. If the drift is downwards, as in cooling, then the bias will be below the set point, hence the term "droop". Droop is proportional to process gain and inversely proportional to proportional gain. Specifically the steady-state error is given by: e = G / Kp Droop is an inherent defect of purely proportional control. Droop may be mitigated by adding a compensating bias term (setting the setpoint above the true desired value), or corrected by adding an integral term. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=5" \o "Edit section: Integral term" edit] Integral term  HYPERLINK "http://en.wikipedia.org/wiki/File:Change_with_Ki.png"  INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Change_with_Ki.png/320px-Change_with_Ki.png" \* MERGEFORMATINET   HYPERLINK "http://en.wikipedia.org/wiki/File:Change_with_Ki.png" \o "Enlarge"  INCLUDEPICTURE "http://bits.wikimedia.org/skins-1.17/common/images/magnify-clip.png" \* MERGEFORMATINET  Plot of PV vs time, for three values of Ki (Kp and Kd held constant) The contribution from the integral term is proportional to both the magnitude of the error and the duration of the error. The  HYPERLINK "http://en.wikipedia.org/wiki/Integral" \o "Integral" integral in a PID controller is the sum of the instantaneous error over time and gives the accumulated offset that should have been corrected previously. The accumulated error is then multiplied by the integral gain (Ki) and added to the controller output. The integral term is given by:  INCLUDEPICTURE "http://upload.wikimedia.org/math/d/5/9/d593c27abdc1aecffb56d06d2a9ba8e3.png" \* MERGEFORMATINET  The integral term accelerates the movement of the process towards setpoint and eliminates the residual steady-state error that occurs with a pure proportional controller. However, since the integral term responds to accumulated errors from the past, it can cause the present value to  HYPERLINK "http://en.wikipedia.org/wiki/Overshoot_%28signal%29" \o "Overshoot (signal)" overshoot the setpoint value (see  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Loop_tuning" the section on loop tuning). [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=6" \o "Edit section: Derivative term" edit] Derivative term  HYPERLINK "http://en.wikipedia.org/wiki/File:Change_with_Kd.png"  INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Change_with_Kd.png/320px-Change_with_Kd.png" \* MERGEFORMATINET   HYPERLINK "http://en.wikipedia.org/wiki/File:Change_with_Kd.png" \o "Enlarge"  INCLUDEPICTURE "http://bits.wikimedia.org/skins-1.17/common/images/magnify-clip.png" \* MERGEFORMATINET  Plot of PV vs time, for three values of Kd (Kp and Ki held constant) The  HYPERLINK "http://en.wikipedia.org/wiki/Derivative" \o "Derivative" derivative of the process error is calculated by determining the slope of the error over time and multiplying this rate of change by the derivative gain Kd. The magnitude of the contribution of the derivative term to the overall control action is termed the derivative gain, Kd. The derivative term is given by:  INCLUDEPICTURE "http://upload.wikimedia.org/math/0/6/9/0691207bf621049b3e8bb98627a50f27.png" \* MERGEFORMATINET  The derivative term slows the rate of change of the controller output. Derivative control is used to reduce the magnitude of the overshoot produced by the integral component and improve the combined controller-process stability. However, the derivative term slows the  HYPERLINK "http://en.wikipedia.org/wiki/Transient_response" \o "Transient response" transient response of the controller. Also, differentiation of a signal amplifies noise and thus this term in the controller is highly sensitive to noise in the error term, and can cause a process to become unstable if the noise and the derivative gain are sufficiently large. Hence an approximation to a differentiator with a limited bandwidth is more commonly used. Such a circuit is known as a  HYPERLINK "http://en.wikipedia.org/wiki/Lead%E2%80%93lag_compensator" \o "Leadlag compensator" phase-lead compensator. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=7" \o "Edit section: Loop tuning" edit] Loop tuning Tuning a control loop is the adjustment of its control parameters (gain/proportional band, integral gain/reset, derivative gain/rate) to the optimum values for the desired control response. Stability (bounded oscillation) is a basic requirement, but beyond that, different systems have different behavior, different applications have different requirements, and requirements may conflict with one another. PID tuning is a difficult problem, even though there are only three parameters and in principle is simple to describe, because it must satisfy complex criteria within the  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Limitations_of_PID_control" limitations of PID control. There are accordingly various methods for loop tuning, and more sophisticated techniques are the subject of patents; this section describes some traditional manual methods for loop tuning. Designing and tuning a proportional-integral-derivative (PID) controller appears to be conceptually intuitive, but can be hard in practice, if multiple (and often conflicting) objectives such as short transient and high stability are to be achieved. Usually, initial designs obtained by all means need to be adjusted repeatedly through computer simulations until the closed-loop system performs or compromises as desired. Some processes have a degree of  HYPERLINK "http://en.wikipedia.org/wiki/Nonlinear_system" \o "Nonlinear system" non-linearity and so parameters that work well at full-load conditions don't work when the process is starting up from no-load; this can be corrected by  HYPERLINK "http://en.wikipedia.org/wiki/Gain_scheduling" \o "Gain scheduling" gain scheduling (using different parameters in different operating regions). PID controllers often provide acceptable control using default tunings, but performance can generally be improved by careful tuning, and performance may be unacceptable with poor tuning. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=8" \o "Edit section: Stability" edit] Stability If the PID controller parameters (the gains of the proportional, integral and derivative terms) are chosen incorrectly, the controlled process input can be unstable, i.e. its output  HYPERLINK "http://en.wikipedia.org/wiki/Divergence_%28computer_science%29" \o "Divergence (computer science)" diverges, with or without  HYPERLINK "http://en.wikipedia.org/wiki/Oscillation" \o "Oscillation" oscillation, and is limited only by saturation or mechanical breakage. Instability is caused by excess gain, particularly in the presence of significant  HYPERLINK "http://en.wikipedia.org/wiki/Lag" \o "Lag" lag. Generally, stability of response is required and the process must not oscillate for any combination of process conditions and setpoints, though sometimes  HYPERLINK "http://en.wikipedia.org/wiki/Marginal_stability" \o "Marginal stability" marginal stability (bounded oscillation) is acceptable or desired.[ HYPERLINK "http://en.wikipedia.org/wiki/Wikipedia:Citation_needed" \o "Wikipedia:Citation needed" citation needed] [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=9" \o "Edit section: Optimum behavior" edit] Optimum behavior The optimum behavior on a process change or setpoint change varies depending on the application. Two basic requirements are regulation (disturbance rejection staying at a given setpoint) and command tracking (implementing setpoint changes) these refer to how well the controlled variable tracks the desired value. Specific criteria for command tracking include  HYPERLINK "http://en.wikipedia.org/wiki/Rise_time" \o "Rise time" rise time and  HYPERLINK "http://en.wikipedia.org/wiki/Settling_time" \o "Settling time" settling time. Some processes must not allow an overshoot of the process variable beyond the setpoint if, for example, this would be unsafe. Other processes must minimize the energy expended in reaching a new setpoint. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=10" \o "Edit section: Overview of methods" edit] Overview of methods There are several methods for tuning a PID loop. The most effective methods generally involve the development of some form of process model, then choosing P, I, and D based on the dynamic model parameters. Manual tuning methods can be relatively inefficient, particularly if the loops have response times on the order of minutes or longer. The choice of method will depend largely on whether or not the loop can be taken "offline" for tuning, and the response time of the system. If the system can be taken offline, the best tuning method often involves subjecting the system to a step change in input, measuring the output as a function of time, and using this response to determine the control parameters. Choosing a Tuning MethodMethodAdvantagesDisadvantagesManual TuningNo math required. Online method.Requires experienced personnel.ZieglerNicholsProven Method. Online method.Process upset, some trial-and-error, very aggressive tuning.Software ToolsConsistent tuning. Online or offline method. May include valve and sensor analysis. Allow simulation before downloading. Can support Non-Steady State (NSS) Tuning.Some cost and training involved.Cohen-CoonGood process models.Some math. Offline method. Only good for first-order processes.[ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=11" \o "Edit section: Manual tuning" edit] Manual tuning If the system must remain online, one tuning method is to first set Ki and Kd values to zero. Increase the Kp until the output of the loop oscillates, then the Kp should be set to approximately half of that value for a "quarter amplitude decay" type response. Then increase Ki until any offset is correct in sufficient time for the process. However, too much Ki will cause instability. Finally, increase Kd, if required, until the loop is acceptably quick to reach its reference after a load disturbance. However, too much Kd will cause excessive response and overshoot. A fast PID loop tuning usually overshoots slightly to reach the setpoint more quickly; however, some systems cannot accept overshoot, in which case an  HYPERLINK "http://en.wikipedia.org/wiki/Overdamping" \o "Overdamping" over-damped closed-loop system is required, which will require a Kp setting significantly less than half that of the Kp setting causing oscillation. Effects of increasing a parameter independentlyParameterRise timeOvershootSettling timeSteady-state errorStability HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-2" [3]KpDecreaseIncreaseSmall changeDecreaseDegradeKiDecrease HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-3" [4]IncreaseIncreaseDecrease significantlyDegradeKdMinor decreaseMinor decreaseMinor decreaseNo effect in theoryImprove if Kd small[ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=12" \o "Edit section: ZieglerNichols method" edit] ZieglerNichols method For more details on this topic, see  HYPERLINK "http://en.wikipedia.org/wiki/Ziegler%E2%80%93Nichols_method" \o "ZieglerNichols method" ZieglerNichols method. Another heuristic tuning method is formally known as the  HYPERLINK "http://en.wikipedia.org/wiki/Ziegler%E2%80%93Nichols_method" \o "ZieglerNichols method" ZieglerNichols method, introduced by  HYPERLINK "http://en.wikipedia.org/w/index.php?title=John_G._Ziegler&action=edit&redlink=1" \o "John G. Ziegler (page does not exist)" John G. Ziegler and  HYPERLINK "http://en.wikipedia.org/wiki/Nathaniel_B._Nichols" \o "Nathaniel B. Nichols" Nathaniel B. Nichols in the 1940s. As in the method above, the Ki and Kd gains are first set to zero. The P gain is increased until it reaches the ultimate gain, Ku, at which the output of the loop starts to oscillate. Ku and the oscillation period Pu are used to set the gains as shown: ZieglerNichols methodControl TypeKpKiKdP0.50Ku--PI0.45Ku1.2Kp / Pu-PID0.60Ku2Kp / PuKpPu / 8These gains apply to the ideal, parallel form of the PID controller. When applied to the standard PID form, the integral and derivative time parameters Ti and Td are only dependent on the oscillation period Pu. Please see the section " HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Alternative_nomenclature_and_PID_forms" Alternative nomenclature and PID forms". [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=13" \o "Edit section: PID tuning software" edit] PID tuning software Most modern industrial facilities no longer tune loops using the manual calculation methods shown above. Instead, PID tuning and loop optimization software are used to ensure consistent results. These software packages will gather the data, develop process models, and suggest optimal tuning. Some software packages can even develop tuning by gathering data from reference changes. Mathematical PID loop tuning induces an impulse in the system, and then uses the controlled system's frequency response to design the PID loop values. In loops with response times of several minutes, mathematical loop tuning is recommended, because trial and error can literally take days just to find a stable set of loop values. Optimal values are harder to find. Some digital loop controllers offer a self-tuning feature in which very small setpoint changes are sent to the process, allowing the controller itself to calculate optimal tuning values. Other formulas are available to tune the loop according to different performance criteria. Many patented formulas are now embedded within PID tuning software and hardware modules. Advances in automated PID Loop Tuning software also deliver algorithms for tuning PID Loops in a dynamic or Non-Steady State (NSS) scenario. The software will model the dynamics of a process, through a disturbance, and calculate PID control parameters in response. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=14" \o "Edit section: Modifications to the PID algorithm" edit] Modifications to the PID algorithm The basic PID algorithm presents some challenges in control applications that have been addressed by minor modifications to the PID form. Integral windup For more details on this topic, see  HYPERLINK "http://en.wikipedia.org/wiki/Integral_windup" \o "Integral windup" Integral windup. One common problem resulting from the ideal PID implementations is  HYPERLINK "http://en.wikipedia.org/wiki/Integral_windup" \o "Integral windup" integral windup, where a large change in setpoint occurs (say a positive change) and the integral term accumulates an error larger than the maximal value for the regulation variable (windup), thus the system overshoots and continues to increase as this accumulated error is unwound. This problem can be addressed by: Initializing the controller integral to a desired value Increasing the setpoint in a suitable ramp Disabling the integral function until the PV has entered the controllable region Limiting the time period over which the integral error is calculated Preventing the integral term from accumulating above or below pre-determined bounds Overshooting from known disturbances For example, a PID loop is used to control the temperature of an electric resistance furnace, the system has stabilized. Now the door is opened and something cold is put into the furnace the temperature drops below the setpoint. The integral function of the controller tends to compensate this error by introducing another error in the positive direction. This overshoot can be avoided by freezing of the integral function after the opening of the door for the time the control loop typically needs to reheat the furnace. Replacing the integral function by a model based part Often the time-response of the system is approximately known. Then it is an advantage to simulate this time-response with a model and to calculate some unknown parameter from the actual response of the system. If for instance the system is an electrical furnace the response of the difference between furnace temperature and ambient temperature to changes of the electrical power will be similar to that of a simple RC low-pass filter multiplied by an unknown proportional coefficient. The actual electrical power supplied to the furnace is delayed by a low-pass filter to simulate the response of the temperature of the furnace and then the actual temperature minus the ambient temperature is divided by this low-pass filtered electrical power. Then, the result is stabilized by another low-pass filter leading to an estimation of the proportional coefficient. With this estimation, it is possible to calculate the required electrical power by dividing the set-point of the temperature minus the ambient temperature by this coefficient. The result can then be used instead of the integral function. This also achieves a control error of zero in the steady-state, but avoids integral windup and can give a significantly improved control action compared to an optimized PID controller. This type of controller does work properly in an open loop situation which causes integral windup with an integral function. This is an advantage if, for example, the heating of a furnace has to be reduced for some time because of the failure of a heating element, or if the controller is used as an advisory system to a human operator who may not switch it to closed-loop operation. It may also be useful if the controller is inside a branch of a complex control system that may be temporarily inactive. Many PID loops control a mechanical device (for example, a valve). Mechanical maintenance can be a major cost and wear leads to control degradation in the form of either  HYPERLINK "http://en.wikipedia.org/wiki/Stiction" \o "Stiction" stiction or a  HYPERLINK "http://en.wikipedia.org/wiki/Deadband" \o "Deadband" deadband in the mechanical response to an input signal. The rate of mechanical wear is mainly a function of how often a device is activated to make a change. Where wear is a significant concern, the PID loop may have an output  HYPERLINK "http://en.wikipedia.org/wiki/Deadband" \o "Deadband" deadband to reduce the frequency of activation of the output (valve). This is accomplished by modifying the controller to hold its output steady if the change would be small (within the defined deadband range). The calculated output must leave the deadband before the actual output will change. The proportional and derivative terms can produce excessive movement in the output when a system is subjected to an instantaneous step increase in the error, such as a large setpoint change. In the case of the derivative term, this is due to taking the derivative of the error, which is very large in the case of an instantaneous step change. As a result, some PID algorithms incorporate the following modifications: Derivative of output In this case the PID controller measures the derivative of the output quantity, rather than the derivative of the error. The output is always continuous (i.e., never has a step change). For this to be effective, the derivative of the output must have the same sign as the derivative of the error. Setpoint ramping In this modification, the setpoint is gradually moved from its old value to a newly specified value using a linear or first order differential ramp function. This avoids the  HYPERLINK "http://en.wikipedia.org/wiki/Discontinuity_%28mathematics%29" \o "Discontinuity (mathematics)" discontinuity present in a simple step change. Setpoint weighting Setpoint weighting uses different multipliers for the error depending on which element of the controller it is used in. The error in the integral term must be the true control error to avoid steady-state control errors. This affects the controller's setpoint response. These parameters do not affect the response to load disturbances and measurement noise. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=15" \o "Edit section: History" edit] History  HYPERLINK "http://en.wikipedia.org/wiki/File:Wiki_letter_w_cropped.svg"  INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Wiki_letter_w_cropped.svg/20px-Wiki_letter_w_cropped.svg.png" \* MERGEFORMATINET This section requires  HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit" expansion. HYPERLINK "http://en.wikipedia.org/wiki/File:Scross_helmsman.jpg"  INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/thumb/e/e5/Scross_helmsman.jpg/220px-Scross_helmsman.jpg" \* MERGEFORMATINET   HYPERLINK "http://en.wikipedia.org/wiki/File:Scross_helmsman.jpg" \o "Enlarge"  INCLUDEPICTURE "http://bits.wikimedia.org/skins-1.17/common/images/magnify-clip.png" \* MERGEFORMATINET  PID theory developed by observing the action of  HYPERLINK "http://en.wikipedia.org/wiki/Helmsmen" \o "Helmsmen" helmsmen. PID controllers date to 1890s governor design. HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-ben93p48-1" [2] HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-bennett84-4" [5] PID controllers were subsequently developed in automatic ship steering. One of the earliest examples of a PID-type controller was developed by Elmer Sperry in 1911, HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-5" [6] while the first published theoretical analysis of a PID controller was by  HYPERLINK "http://en.wikipedia.org/wiki/Russian_American" \o "Russian American" Russian American engineer  HYPERLINK "http://en.wikipedia.org/w/index.php?title=Nicolas_Minorsky&action=edit&redlink=1" \o "Nicolas Minorsky (page does not exist)" Nicolas Minorsky, in ( HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "CITEREFMinorsky1922" Minorsky 1922). Minorsky was designing automatic steering systems for the US Navy, and based his analysis on observations of a  HYPERLINK "http://en.wikipedia.org/wiki/Helmsman" \o "Helmsman" helmsman, observing that the helmsman controlled the ship not only based on the current error, but also on past error and current rate of change; HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-6" [7] this was then made mathematical by Minorsky. His goal was stability, not general control, which significantly simplified the problem. While proportional control provides stability against small disturbances, it was insufficient for dealing with a steady disturbance, notably a stiff gale (due to  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "Droop" droop), which required adding the integral term. Finally, the derivative term was added to improve control. Trials were carried out on the  HYPERLINK "http://en.wikipedia.org/wiki/USS_New_Mexico_%28BB-40%29" \o "USS New Mexico (BB-40)" USS New Mexico, with the controller controlling the  HYPERLINK "http://en.wikipedia.org/wiki/Angular_velocity" \o "Angular velocity" angular velocity (not angle) of the rudder. PI control yielded sustained yaw (angular error) of 2, while adding D yielded yaw of 1/6, better than most helmsmen could achieve. HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-7" [8] The Navy ultimately did not adopt the system, due to resistance by personnel[ HYPERLINK "http://en.wikipedia.org/wiki/Wikipedia:Please_clarify" \o "Wikipedia:Please clarify" why?]. Similar work was carried out and published by several others in the 1930s. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=16" \o "Edit section: Limitations of PID control" edit] Limitations of PID control While PID controllers are applicable to many control problems, and often perform satisfactorily without any improvements or even tuning, they can perform poorly in some applications, and do not in general provide  HYPERLINK "http://en.wikipedia.org/wiki/Optimal_control" \o "Optimal control" optimal control. The fundamental difficulty with PID control is that it is a feedback system, with constant parameters, and no direct knowledge of the process, and thus overall performance is reactive and a compromise while PID control is the best controller with no model of the process, HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-ben93p48-1" [2] better performance can be obtained by incorporating a model of the process. The most significant improvement is to incorporate feed-forward control with knowledge about the system, and using the PID only to control error. Alternatively, PIDs can be modified in more minor ways, such as by changing the parameters (either  HYPERLINK "http://en.wikipedia.org/wiki/Gain_scheduling" \o "Gain scheduling" gain scheduling in different use cases or adaptively modifying them based on performance), improving measurement (higher sampling rate, precision, and accuracy, and low-pass filtering if necessary), or cascading multiple PID controllers. PID controllers, when used alone, can give poor performance when the PID loop gains must be reduced so that the control system does not overshoot, oscillate or  HYPERLINK "http://en.wikipedia.org/wiki/Self-exciting_oscillation" \o "Self-exciting oscillation" hunt about the control setpoint value. They also have difficulties in the presence of non-linearities, may trade off regulation versus response time, do not react to changing process behavior (say, the process changes after it has warmed up), and have lag in responding to large disturbances. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=17" \o "Edit section: Linearity" edit] Linearity Another problem faced with PID controllers is that they are linear, and in particular symmetric. Thus, performance of PID controllers in non-linear systems (such as  HYPERLINK "http://en.wikipedia.org/wiki/HVAC_control_system" \o "HVAC control system" HVAC systems) is variable. For example, in temperature control, a common use case is active heating (via a heating element) but passive cooling (heating off, but no cooling), so overshoot can only be corrected slowly it cannot be forced downward. In this case the PID should be tuned to be overdamped, to prevent or reduce overshoot, though this reduces performance (it increases settling time). [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=18" \o "Edit section: Noise in derivative" edit] Noise in derivative A problem with the derivative term is that small amounts of measurement or process  HYPERLINK "http://en.wikipedia.org/wiki/Noise" \o "Noise" noise can cause large amounts of change in the output. It is often helpful to filter the measurements with a  HYPERLINK "http://en.wikipedia.org/wiki/Low-pass_filter" \o "Low-pass filter" low-pass filter in order to remove higher-frequency noise components. However, low-pass filtering and derivative control can cancel each other out, so reducing noise by instrumentation means is a much better choice. Alternatively, a nonlinear  HYPERLINK "http://en.wikipedia.org/wiki/Median_filter" \o "Median filter" median filter may be used, which improves the filtering efficiency and practical performance. HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-8" [9] In some case, the differential band can be turned off in many systems with little loss of control. This is equivalent to using the PID controller as a PI controller. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=19" \o "Edit section: Improvements" edit] Improvements [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=20" \o "Edit section: Feed-forward" edit] Feed-forward The control system performance can be improved by combining the  HYPERLINK "http://en.wikipedia.org/wiki/Feedback" \o "Feedback" feedback (or closed-loop) control of a PID controller with  HYPERLINK "http://en.wikipedia.org/wiki/Feed-forward" \o "Feed-forward" feed-forward (or open-loop) control. Knowledge about the system (such as the desired acceleration and inertia) can be fed forward and combined with the PID output to improve the overall system performance. The feed-forward value alone can often provide the major portion of the controller output. The PID controller can be used primarily to respond to whatever difference or error remains between the setpoint (SP) and the actual value of the process variable (PV). Since the feed-forward output is not affected by the process feedback, it can never cause the control system to oscillate, thus improving the system response and stability. For example, in most motion control systems, in order to accelerate a mechanical load under control, more force or torque is required from the prime mover, motor, or actuator. If a velocity loop PID controller is being used to control the speed of the load and command the force or torque being applied by the prime mover, then it is beneficial to take the instantaneous acceleration desired for the load, scale that value appropriately and add it to the output of the PID velocity loop controller. This means that whenever the load is being accelerated or decelerated, a proportional amount of force is commanded from the prime mover regardless of the feedback value. The PID loop in this situation uses the feedback information to change the combined output to reduce the remaining difference between the process setpoint and the feedback value. Working together, the combined open-loop feed-forward controller and closed-loop PID controller can provide a more responsive, stable and reliable control system. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=21" \o "Edit section: Other improvements" edit] Other improvements In addition to feed-forward, PID controllers are often enhanced through methods such as PID  HYPERLINK "http://en.wikipedia.org/wiki/Gain_scheduling" \o "Gain scheduling" gain scheduling (changing parameters in different operating conditions),  HYPERLINK "http://en.wikipedia.org/wiki/Fuzzy_logic" \o "Fuzzy logic" fuzzy logic or  HYPERLINK "http://en.wikipedia.org/w/index.php?title=Computational_verb_logic&action=edit&redlink=1" \o "Computational verb logic (page does not exist)" computational verb logic.  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-9" [10]  HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-10" [11] Further practical application issues can arise from instrumentation connected to the controller. A high enough sampling rate, measurement precision, and measurement accuracy are required to achieve adequate control performance. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=22" \o "Edit section: Cascade control" edit] Cascade control One distinctive advantage of PID controllers is that two PID controllers can be used together to yield better dynamic performance. This is called cascaded PID control. In cascade control there are two PIDs arranged with one PID controlling the set point of another. A PID controller acts as outer loop controller, which controls the primary physical parameter, such as fluid level or velocity. The other controller acts as inner loop controller, which reads the output of outer loop controller as set point, usually controlling a more rapid changing parameter, flowrate or acceleration. It can be mathematically proven[ HYPERLINK "http://en.wikipedia.org/wiki/Wikipedia:Citation_needed" \o "Wikipedia:Citation needed" citation needed] that the working frequency of the controller is increased and the time constant of the object is reduced by using cascaded PID controller.[ HYPERLINK "http://en.wikipedia.org/wiki/Wikipedia:Vagueness" \o "Wikipedia:Vagueness" vague]. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=23" \o "Edit section: Physical implementation of PID control" edit] Physical implementation of PID control In the early history of automatic process control the PID controller was implemented as a mechanical device. These mechanical controllers used a  HYPERLINK "http://en.wikipedia.org/wiki/Lever" \o "Lever" lever,  HYPERLINK "http://en.wikipedia.org/wiki/Spring_%28device%29" \o "Spring (device)" spring and a  HYPERLINK "http://en.wikipedia.org/wiki/Mass" \o "Mass" mass and were often energized by compressed air. These  HYPERLINK "http://en.wikipedia.org/wiki/Pneumatic" \o "Pneumatic" pneumatic controllers were once the industry standard. Electronic  HYPERLINK "http://en.wikipedia.org/wiki/Analog_circuit" \o "Analog circuit" analog controllers can be made from a  HYPERLINK "http://en.wikipedia.org/wiki/Transistor" \o "Transistor" solid-state or  HYPERLINK "http://en.wikipedia.org/wiki/Vacuum_tube" \o "Vacuum tube" tube  HYPERLINK "http://en.wikipedia.org/wiki/Operational_amplifier" \o "Operational amplifier" amplifier, a  HYPERLINK "http://en.wikipedia.org/wiki/Capacitor" \o "Capacitor" capacitor and a  HYPERLINK "http://en.wikipedia.org/wiki/Electrical_resistance" \o "Electrical resistance" resistance. Electronic analog PID control loops were often found within more complex electronic systems, for example, the head positioning of a  HYPERLINK "http://en.wikipedia.org/wiki/Disk_drive" \o "Disk drive" disk drive, the power conditioning of a  HYPERLINK "http://en.wikipedia.org/wiki/Power_supply" \o "Power supply" power supply, or even the movement-detection circuit of a modern  HYPERLINK "http://en.wikipedia.org/wiki/Seismometer" \o "Seismometer" seismometer. Nowadays, electronic controllers have largely been replaced by digital controllers implemented with  HYPERLINK "http://en.wikipedia.org/wiki/Microcontrollers" \o "Microcontrollers" microcontrollers or  HYPERLINK "http://en.wikipedia.org/wiki/FPGA" \o "FPGA" FPGAs. Most modern PID controllers in industry are implemented in  HYPERLINK "http://en.wikipedia.org/wiki/Programmable_logic_controller" \o "Programmable logic controller" programmable logic controllers (PLCs) or as a panel-mounted digital controller. Software implementations have the advantages that they are relatively cheap and are flexible with respect to the implementation of the PID algorithm. Variable voltages may be applied by the  HYPERLINK "http://en.wikipedia.org/wiki/Time_proportioning" \o "Time proportioning" time proportioning form of  HYPERLINK "http://en.wikipedia.org/wiki/Pulse-width_modulation" \o "Pulse-width modulation" Pulse-width modulation (PWM) a  HYPERLINK "http://en.wikipedia.org/w/index.php?title=Cycle_time&action=edit&redlink=1" \o "Cycle time (page does not exist)" cycle time is fixed, and variation is achieved by varying the proportion of the time during this cycle that the controller outputs +1 (or "1) instead of 0. On a digital system the possible proportions are discrete  e.g., increments of .1 second within a 2 second cycle time yields 20 possible steps: percentage increments of 5% so there is a  HYPERLINK "http://en.wikipedia.org/wiki/Discretization_error" \o "Discretization error" discretization error, but for high enough time resolution this yields satisfactory performance. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=24" \o "Edit section: Alternative nomenclature and PID forms" edit] Alternative nomenclature and PID forms [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=25" \o "Edit section: Ideal versus standard PID form" edit] Ideal versus standard PID form The form of the PID controller most often encountered in industry, and the one most relevant to tuning algorithms is the standard form. In this form the Kp gain is applied to the Iout, and Dout terms, yielding:  INCLUDEPICTURE "http://upload.wikimedia.org/math/6/8/d/68dcc9ea850cb4411290fa7bb6e47219.png" \* MERGEFORMATINET  where Ti is the integral time Td is the derivative time In this standard form, the parameters have a clear physical meaning. In particular, the inner summation produces a new single error value which is compensated for future and past errors. The addition of the proportional and derivative components effectively predicts the error value at Td seconds (or samples) in the future, assuming that the loop control remains unchanged. The integral component adjusts the error value to compensate for the sum of all past errors, with the intention of completely eliminating them in Ti seconds (or samples). The resulting compensated single error value is scaled by the single gain Kp. In the ideal parallel form, shown in the controller theory section  INCLUDEPICTURE "http://upload.wikimedia.org/math/3/e/e/3ee4a60db59c35d5e7c5b615796fe156.png" \* MERGEFORMATINET  the gain parameters are related to the parameters of the standard form through  INCLUDEPICTURE "http://upload.wikimedia.org/math/1/4/7/1472201539e8f1dc20a457cd8f01f5fb.png" \* MERGEFORMATINET and  INCLUDEPICTURE "http://upload.wikimedia.org/math/1/6/6/1665c88bcbccf4259ba592c39506c66c.png" \* MERGEFORMATINET . This parallel form, where the parameters are treated as simple gains, is the most general and flexible form. However, it is also the form where the parameters have the least physical interpretation and is generally reserved for theoretical treatment of the PID controller. The standard form, despite being slightly more complex mathematically, is more common in industry. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=26" \o "Edit section: Basing derivative action on PV" edit] Basing derivative action on PV In most commercial control systems, derivative action is based on PV rather than error. This is because the digitised version of the algorithm produces a large unwanted spike when the SP is changed. If the SP is constant then changes in PV will be the same as changes in error. Therefore this modification makes no difference to the way the controller responds to process disturbances.  INCLUDEPICTURE "http://upload.wikimedia.org/math/6/f/6/6f6c7fd7b8faaf75b3e1357575b1943b.png" \* MERGEFORMATINET  [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=27" \o "Edit section: Basing proportional action on PV" edit] Basing proportional action on PV Most commercial control systems offer the option of also basing the proportional action on PV. This means that only the integral action responds to changes in SP. While at first this might seem to adversely affect the time that the process will take to respond to the change, the controller may be retuned to give almost the same response - largely by increasing Kp. The modification to the algorithm does not affect the way the controller responds to process disturbances, but the change in tuning has a beneficial effect. Often the magnitude and duration of the disturbance will be more than halved. Since most controllers have to deal frequently with process disturbances and relatively rarely with SP changes, properly tuned the modified algorithm can dramatically improve process performance.  INCLUDEPICTURE "http://upload.wikimedia.org/math/4/1/4/414b2a2ee71117830f7032d7868bcbdd.png" \* MERGEFORMATINET  Tuning methods such as Ziegler-Nichols and Cohen-Coon will not be reliable when used with this algorithm. King HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-11" [12] describes an effective chart-based method. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=28" \o "Edit section: Laplace form of the PID controller" edit] Laplace form of the PID controller Sometimes it is useful to write the PID regulator in  HYPERLINK "http://en.wikipedia.org/wiki/Laplace_transform" \o "Laplace transform" Laplace transform form:  INCLUDEPICTURE "http://upload.wikimedia.org/math/6/d/b/6dbbb7aafc289a41c2345438e0814ab2.png" \* MERGEFORMATINET  Having the PID controller written in Laplace form and having the  HYPERLINK "http://en.wikipedia.org/wiki/Transfer_function" \o "Transfer function" transfer function of the controlled system makes it easy to determine the closed-loop transfer function of the system. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=29" \o "Edit section: PID Pole Zero Cancellation" edit] PID Pole Zero Cancellation The PID equation can be written in this form:  INCLUDEPICTURE "http://upload.wikimedia.org/math/f/0/4/f0426a8ffeb53a2778091b08fd4dfbc5.png" \* MERGEFORMATINET  When this form is used it is easy to determine the closed loop transfer function.  INCLUDEPICTURE "http://upload.wikimedia.org/math/2/7/d/27dbd17f1fc0f8487a823b831e1c60ac.png" \* MERGEFORMATINET  If  INCLUDEPICTURE "http://upload.wikimedia.org/math/2/8/f/28fa4750aaa1e6c9ee9da2b9798c8423.png" \* MERGEFORMATINET   INCLUDEPICTURE "http://upload.wikimedia.org/math/b/d/8/bd893bede981f7826a0dc81b86c8d281.png" \* MERGEFORMATINET  Then  INCLUDEPICTURE "http://upload.wikimedia.org/math/f/4/4/f44c74d2a8f08a5e1c470296223def9d.png" \* MERGEFORMATINET  This can be very useful to remove unstable poles [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=30" \o "Edit section: Series/interacting form" edit] Series/interacting form Another representation of the PID controller is the series, or interacting form  INCLUDEPICTURE "http://upload.wikimedia.org/math/c/b/7/cb7a7ece420cde9750cf8db2a0ea42ac.png" \* MERGEFORMATINET  where the parameters are related to the parameters of the standard form through  INCLUDEPICTURE "http://upload.wikimedia.org/math/f/a/5/fa5610da726ff3f635ff6d4c0e31f4b5.png" \* MERGEFORMATINET ,  INCLUDEPICTURE "http://upload.wikimedia.org/math/8/5/3/853b69dd0c9cabe19c0ceb642dcf0571.png" \* MERGEFORMATINET , and  INCLUDEPICTURE "http://upload.wikimedia.org/math/8/5/c/85c8f6c34da39678780658001e7ddae4.png" \* MERGEFORMATINET  with  INCLUDEPICTURE "http://upload.wikimedia.org/math/2/9/9/29939286f44565f70e4ff898c2ab7140.png" \* MERGEFORMATINET . This form essentially consists of a PD and PI controller in series, and it made early (analog) controllers easier to build. When the controllers later became digital, many kept using the interacting form. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=31" \o "Edit section: Discrete implementation" edit] Discrete implementation The analysis for designing a digital implementation of a PID controller in a Microcontroller (MCU) or FPGA device requires the standard form of the PID controller to be discretised. HYPERLINK "http://en.wikipedia.org/wiki/PID_controller" \l "cite_note-12" [13] Approximations for first-order derivatives are made by backward  HYPERLINK "http://en.wikipedia.org/wiki/Finite_difference" \o "Finite difference" finite differences. The integral term is discretised, with a sampling time t,as follows,  INCLUDEPICTURE "http://upload.wikimedia.org/math/2/1/3/213d3ca0edd0dc7905ebda889caaefcb.png" \* MERGEFORMATINET  The derivative term is approximated as,  INCLUDEPICTURE "http://upload.wikimedia.org/math/6/d/2/6d2d3de8ce2688bae28e47df223965f6.png" \* MERGEFORMATINET  Thus, a velocity algorithm for implementation of the discretised PID controller in a MCU is obtained by differentiating u(t), using the numerical definitions of the first and second derivative and solving for u(tk) and finally obtaining:  INCLUDEPICTURE "http://upload.wikimedia.org/math/a/9/b/a9beb5ee392aa86e76d734e9e761b148.png" \* MERGEFORMATINET  [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=32" \o "Edit section: Pseudocode" edit] Pseudocode Here is a simple software loop that implements the PID algorithm in its 'ideal, parallel' form: previous_error = setpoint - actual_position integral = 0 start: error = setpoint - actual_position integral = integral + (error*dt) derivative = (error - previous_error)/dt output = (Kp*error) + (Ki*integral) + (Kd*derivative) previous_error = error wait(dt) goto start [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=PID_controller&action=edit§ion=33" \o "Edit section: PI controller" edit] PI controller  HYPERLINK "http://en.wikipedia.org/wiki/File:PI_controller.png"  INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/en/thumb/2/26/PI_controller.png/300px-PI_controller.png" \* MERGEFORMATINET   HYPERLINK "http://en.wikipedia.org/wiki/File:PI_controller.png" \o "Enlarge"  INCLUDEPICTURE "http://bits.wikimedia.org/skins-1.17/common/images/magnify-clip.png" \* MERGEFORMATINET  Basic block of a PI controller. A PI Controller (proportional-integral controller) is a special case of the PID controller in which the derivative (D) of the error is not used. The controller output is given by  INCLUDEPICTURE "http://upload.wikimedia.org/math/1/c/f/1cf470dcf2bee139ccffa1fa5c395d9d.png" \* MERGEFORMATINET  where  is the error or deviation of actual measured value (PV) from the set-point (SP).  = SP " PV. 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gd3gd3^gd3FGXY`akl}~qrvwxy4567uvwxyzƾƶƮƦjl"h3Uj:!h3Ujh3Ujh3Ujh3U h30J h30Jjh30JU h30Jh3jh3H*U h30JH*>fghi%&*+,-D [\DEFGMN:;<= j)h3Uj(h3Uj'h3Uj&h3Ujp%h3U h36] h30J h30Jjh30JU h30Jjv$h3Uh3jh3Ujl#h3U<_jklRSef Z\ !"+= ׺ײ¨נѐjh30JU h30Jj8-h3Uh30J6H*]j+h3Uj*h3Uh30J6] h30J h30Jjh3U h30JH* h3H*jh3H*U h36]h38 Z#w*b{)egd3gd3gd3gd3^gd3gd3 ()*klmDEFlp˶˩ؚh30J6]h356\] h30Jj0h3U h35\j"0h3B*Uphjj/h3B*Uphh3B*phjh3B*Uphjh3U h30Jjh30JU h30J h30Jh32tlll m4noJpKqLqNqqqqqt tetftht^gd3gd368l l l_l`lglhltlullllllllll mnnnn\n^nLqMqwqxq{q|qqqqqqtttttItJtMtNtTtUtftgthtjph3UjPh3U h3H*j2h3Uh30J6H*] h30Jj1h3UU h30Jjh3Uh3h30J6]: involving  HYPERLINK "http://en.wikipedia.org/wiki/Laplace_transform" \o "Laplace transform" Laplace operators:  INCLUDEPICTURE "http://upload.wikimedia.org/math/8/6/9/869b109fde4154b61b23c48ae57e7a13.png" \* MERGEFORMATINET  where G = KP = proportional gain G /  = KI = integral gain Setting a value for G is often a trade off between decreasing overshoot and increasing settling time. The lack of derivative action may make the system more steady in the steady state in the case of noisy data. This is because derivative action is more sensitive to higher-frequency terms in the inputs. Without derivative action, a PI-controlled system is less responsive to real (non-noise) and relatively fast alterations in state and so the system will be slower to reach setpoint and slower to respond to perturbations than a well-tuned PID system may be.  Plot of PV vs time, for three values of Kp (Ki and Kd held constant)  INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/c/c0/Change_with_Ki.png" \* MERGEFORMATINET  Plot of PV vs time, for three values of Kp (Ki and Kd held constant)  ,1h/ =!"#$% Dd   S fF300px-Pid-feedback-nct-int-correctDd 4&  S :magnify-clip""Enlarge"Y$$If!vh5#v:V 6,534Dd ooH  C $Be3386d1b5511c8ce5b70a4ba8bcfc3e3\mathrm{u(t)}=\mathrm{MV(t)}=K_p{e(t)} + K_{i}\int_{0}^{t}{e(\tau)}\,{d\tau} + K_{d}\frac{d}{dt}e(t)Dd Whn  S J*320px-Change_with_KpDd 4&  S :magnify-clip""Enlarge"Dd oo  C B:3f80d26e2e621e0f123bb26e80e609e5P_{\mathrm{out}}=K_p\,{e(t)}Dd Wnn  S J*320px-Change_with_KiDd 4&  S :magnify-clip""Enlarge"4Dd oo   C Bjd593c27abdc1aecffb56d06d2a9ba8e3I_{\mathrm{out}}=K_{i}\int_{0}^{t}{e(\tau)}\,{d\tau}Dd Wnn   S J*320px-Change_with_Kd Dd 4&   S :magnify-clip""Enlarge" Dd oo   C BJ0691207bf621049b3e8bb98627a50f27D_{\mathrm{out}}=K_d\frac{d}{dt}e(t) g$$If!vh5"#v":V 6,5/ 34u$$If!vh55#5q #v#v##vq :V 6,534u$$If!vh55#5q #v#v##vq :V 6,534u$$If!vh55#5q #v#v##vq :V 6,534u$$If!vh55#5q #v#v##vq :V 6,534u$$If!vh55#5q #v#v##vq :V 6,534g$$If!vh5"#v":V 6,5/ 34$$If!vh55j5j5525u#v#vj#v#v2#vu:V 6,534$$If!vh55j5j5525u#v#vj#v#v2#vu:V 6,534$$If!vh55j5j5525u#v#vj#v#v2#vu:V 6,534$$If!vh55j5j5525u#v#vj#v#v2#vu:V 6,534g$$If!vh5#v:V 6,5/ 34$$If!vh5555Q#v#v#v#vQ:V 6,534$$If!vh5555Q#v#v#v#vQ:V 6,534$$If!vh5555Q#v#v#v#vQ:V 6,534$$If!vh5555Q#v#v#v#vQ:V 6,534Dd E1   S 6420px-Wiki_letter_w_croppedWiki letter w cropped.svg g$$If!vh5v5` #vv#v` :V 6,534Dd !p  S L,220px-Scross_helmsman Dd 4&  S :magnify-clip""Enlarge"Dd ooV  C 2B68dcc9ea850cb4411290fa7bb6e47219\mathrm{MV(t)}=K_p\left(\,{e(t)} + \frac{1}{T_i}\int_{0}^{t}{e(\tau)}\,{d\tau} + T_d\frac{d}{dt}e(t)\right)pDd oo$  C Bæ3ee4a60db59c35d5e7c5b615796fe156\mathrm{MV(t)}=K_p{e(t)} + K_i\int_{0}^{t}{e(\tau)}\,{d\tau} + K_d\frac{d}{dt}e(t)Dd oo  C B,1472201539e8f1dc20a457cd8f01f5fbK_i = \frac{K_p}{T_i}Dd oo  C ~B$1665c88bcbccf4259ba592c39506c66c K_d = K_p T_d \,Dd ooX  C 4B6f6c7fd7b8faaf75b3e1357575b1943b\mathrm{MV(t)}=K_p\left(\,{e(t)} + \frac{1}{T_i}\int_{0}^{t}{e(\tau)}\,{d\tau} + T_d\frac{d}{dt}PV(t)\right)Dd ooZ  C 6B414b2a2ee71117830f7032d7868bcbdd\mathrm{MV(t)}=K_p\left(\,{PV(t)} + \frac{1}{T_i}\int_{0}^{t}{e(\tau)}\,{d\tau} + T_d\frac{d}{dt}PV(t)\right)RDd oo  C BÈ6dbbb7aafc289a41c2345438e0814ab2G(s)=K_p + \frac{K_i}{s} + K_d{s}=\frac{K_d{s^2} + K_p{s} + K_i}{s}BDd oo  C Bxf0426a8ffeb53a2778091b08fd4dfbc5G(s)=K_d \frac{s^2 + \frac{K_p}{K_d}s + \frac{K_i}{K_d}}{s}2Dd oo  C Bh27dbd17f1fc0f8487a823b831e1c60acH(s)=\frac{1}{s^2 + 2\zeta \omega_0 s + \omega_0^2}Dd oo  C B628fa4750aaa1e6c9ee9da2b9798c8423\frac{K_i}{K_d}=\omega_0^2 Dd oo  C B@bd893bede981f7826a0dc81b86c8d281\frac{K_p}{K_d}=2\zeta \omega_0Dd oo  C B0f44c74d2a8f08a5e1c470296223def9dG(s) H(s)=\frac{K_d}{s}<Dd oo  C Brcb7a7ece420cde9750cf8db2a0ea42acG(s) = K_c \frac{(\tau_i{s}+1)}{\tau_i{s}} (\tau_d{s}+1)Dd oo  C B.fa5610da726ff3f635ff6d4c0e31f4b5K_p = K_c \cdot \alphaDd oo  C B4853b69dd0c9cabe19c0ceb642dcf0571T_i = \tau_i \cdot \alphaDd oo  C B885c8f6c34da39678780658001e7ddae4T_d = \frac{\tau_d}{\alpha}Dd oo   C BF29939286f44565f70e4ff898c2ab7140\alpha = 1 + \frac{\tau_d}{\tau_i}JDd oo ! 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X!kK 5 PM@$ǡݴm!9[mBM@$Cj7 0Tv!k<T@vU# } O!]a@`>S@@dWE0؇OPAU!k<T@vU# }8C'smXn4`TȎٝ ,@ d F& %JI1@nL@@J@)o`@ <}On.BJ& 40x%z,~AJ JYW[jm;,YB0`97FKrRipܓ MJq=Yw՗CXk@H~1 X LN26n2,{[1Y w_ \_sxtN.Pmm.3,z/,NR}Tk̜T {{M"xm^8uqbr ]_n`CxWt%ircպ䤤rq"lOy }W4964 ݃ ]:JsSNlghnyˮQCI99Is!k7(MvC-/:r "a>FACqiSZk09'UvApO|}ppF&1.|6SIENDB`Dd? ) S AÌChange_with_Kihttp://upload.wikimedia.org/wikipedia/commons/c/c0/Change_with_Ki.png(bFh<Yl/QndFh<YPNG  IHDRoafgAMA a IDATx^흉(_>>mըyogl$H @s__3^Gxޚ !#C: |${vҤ; <_7f\ _@:b)-!i >qHER¹ 2r!04..CoO)8\?`4ů?']pON}#;ܩw$su;pԻs>ߑΝzqG;XScnV`o=λbjP|3Qm&ډls5i2fw 3 Ai6&&q֐K<;=="OmJ~7'" Ź}Ӝ,2umuk h7v^o.\I5iD9pnK(Dڳ}-AC앢(:K? A% mյdU*%syܪA"\cj7έ uJ z! mt.:@`l8n,ں]5!sڹF-[Ήv#׭%[GΌv#׭%[GΌv#׭%[AZW, [ډW, [ډW, [ډW, +#DYk MJgihR>6EfAp &v+eB+ GXzzs.i"KxAzp! A(:Ky:Ap! 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ImprovementsK2r,http://en.wikipedia.org/wiki/PID_controllerNoise_in_derivativewo,http://en.wikipedia.org/wiki/PID_controller Linearityt%l,http://en.wikipedia.org/wiki/PID_controllerLimitations_of_PID_controloi,http://en.wikipedia.org/wiki/PID_controllerHistoryW,f,http://en.wikipedia.org/wiki/PID_controller#Modifications_to_the_PID_algorithm^ c,http://en.wikipedia.org/wiki/PID_controllerPID_tuning_software?t`,http://en.wikipedia.org/wiki/PID_controllerZiegler.E2.80.93Nichols_method:q],http://en.wikipedia.org/wiki/PID_controllerManual_tuningM>Z,http://en.wikipedia.org/wiki/PID_controllerOverview_of_methodsQW,http://en.wikipedia.org/wiki/PID_controllerOptimum_behavior sT,http://en.wikipedia.org/wiki/PID_controller Stability[Q,http://en.wikipedia.org/wiki/PID_controller Loop_tuningCN,http://en.wikipedia.org/wiki/PID_controllerDerivative_term:nK,http://en.wikipedia.org/wiki/PID_controllerIntegral_term`H,http://en.wikipedia.org/wiki/PID_controllerDroop+`E,http://en.wikipedia.org/wiki/PID_controllerProportional_term!BB,http://en.wikipedia.org/wiki/PID_controllerPID_controller_theory\;?,http://en.wikipedia.org/wiki/PID_controllerControl_loop_basics}<,http://en.wikipedia.org/wiki/PID_controllerf9-http://en.wikipedia.org/wiki/Optimal_control 964http://en.wikipedia.org/wiki/Overshoot_%28signal%29c3,http://en.wikipedia.org/wiki/PID_controllercite_note-ben93p48-1YF0,http://en.wikipedia.org/wiki/PID_controller 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