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PID controller

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A proportional–integral–derivative controller (PID controller or three term controller) is a control loop feedback mechanism widely used in industrial control systems and a variety of other applications requiring continuously modulated control. A PID controller continuously calculates an error value as the difference between a desired setpoint and a measured process variable and applies a correction based on proportional, integral, and derivative terms (sometimes denoted P, I, and D respectively) which give their name to the controller type.

In practical terms it automatically applies accurate and responsive correction to a control function. An everyday example is the cruise control on a road vehicle; where external influences such as gradients cause speed changes, and the driver also alters the desired set speed. The PID algorithm restores the actual speed to the desired speed in the optimum way, without delay or overshoot, by altering the power output of the vehicle's engine.

The first theoretical analysis and practical application was in the field of automatic steering systems for ships, developed from the early 1920s onwards. It was then utilised for automatic process control in manufacturing industry, where it was widely implemented in pneumatic, and then electronic, controllers. Today there is universal use of the PID concept in any application requiring accurate and optimised automatic control.

Contents

Fundamental operationEdit

 
A block diagram of a PID controller in a feedback loop. r(t) is the desired process value or "set point" (SP), and y(t) is the measured process value (PV).

The distinguishing feature of the PID controller is the ability to use the three "control terms" of proportional, integral and differential influence on the controller output to apply accurate and optimal control. The block diagram on the right shows the principles of how these terms are generated and applied.

A PID controller continuously calculates an error value   as the difference between a desired setpoint (SP) and a measured process variable (PV), and applies a correction based on proportional, integral, and derivative terms.

The controller attempts to minimize the error over time by adjustment of a control variable  , such as the opening of a control valve, to a new value determined by a weighted sum of the control terms.

In this model:

  • P accounts for present values of the SP-PV error. For example, if the error is large and positive, the control output will be proportionately large and positive. Proportional control alone will always have an error between the set point and the actual process value, because it requires the error to generate the proportional response; so if there is no error, there is no response. This was the problem with the historic rotating-ball speed governor - it always had an error.
  • I accounts for past values of the SP-PV error and integrates them over time to produce the I term. For example, if there is an SP-PV error, the integral of the error will accumulate over time. The integral term seeks to eliminate the error by contributing a control effect due to the historic and present cumulative value of the error. When the error is eliminated the integral term cease to grow. This will of course result in the proportional effect diminishing as the error decreases, but this is compensated for by the growing integral effect.
  • D accounts for possible future trends of the SP-PV error, based on its current rate of change. It is sometimes called "anticipatory control" as it is effectively seeking to reduce the effect of the SP-PV error by exerting a control influence generated by the rate of error change. The more rapid the change, the greater the controlling or dampening effect. [1].

The balance of these effects is achieved by "loop tuning" (see later) to produce the optimal control function. The tuning constants are shown below as "K" and must be derived for each control application, as they depend on the response characteristics of the complete loop external to the controller. These are dependent on the behaviour of the measuring sensor, the final control element (such as a control valve), any control signal delays and the process itself. Approximate values of constants can usually be initially entered knowing the type of application, but they are normally refined, or tuned, by "bumping" the process in practice by such as introducing a set point change and observing the system response.


The overall control function can be expressed mathematically as:

 

where  ,  , and  , all non-negative, denote the coefficients for the proportional, integral, and derivative terms, respectively (sometimes denoted P, I, and D).


In the standard form of the equation (see later in article),  , and   are respectively replaced by  , and  ; the advantage of this being that  , and   have some understandable physical meaning, as they represent the integration time and the derivative time respectively.

Selective use of control termsEdit

Although a PID controller has three control terms,some applications use only one or two terms to provide the appropriate control. This is achieved by setting the unused parameters to zero, and is called a PI, PD, P or I controller in the absence of the other 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.

ApplicabilityEdit

The use of the PID algorithm (see below: Limitations of PID control) does not guarantee optimal control of the system or its control stability. Situations may occur where there are excessive delays: the measurement of the process value is delayed, or the control action does not apply quickly enough. In these cases lead-lag compensation is required to be effective. The response of the controller can be described in terms of its responsiveness to an error, the degree to which the system overshoots a setpoint, and the degree of any system oscillation. But the PID controller is broadly applicable since it relies only on the response of the measured process variable, not on knowledge or a model of the underlying process.

HistoryEdit

 
Early PID theory was developed by observing the actions of helmsmen in keeping a vessel on course in the face of varying influences such as wind and sea state.
 
Pneumatic PID (Three term) controller. The magnitude of the "three terms" P, I and D are adjusted by the dials at the top.

OriginsEdit

Continuous control before PID controllers were fully understood and implemented has one of its origins in the Centrifugal governor. This had been invented by Christian Huygens in the 17th century to regulate the gap between millstones in windmills depending on the speed of rotation, and thereby compensate for the speed of grain feed. [2][3]

With the invention of the high pressure stationary steam engine there was a need for automatic speed control and James Watt’s self-designed "conical pendulum" governor, a set of revolving steel balls attached to a vertical spindle by link arms, came to be an industry standard. This was based on the mill stone gap control concept.[4]

However, rotating governor speed control was still variable under conditions of varying load, where the shortcoming of what is now known as proportional control alone was evident. The error between the desired speed and the actual speed would increase with load. In the 19th century the theoretical basis for the operation of governors was first described by James Clerk Maxwell in 1868 in his now-famous paper On Governors. He explored the mathematical basis for control stability, and progressed a good way towards a solution and made an appeal for mathematicians to examine the problem. [5][4] The problem was examined further by Edward Routh in 1874, Charles Sturm and in 1895, Adolf Hurwitz, who all contributed to the establishment of control stability criteria. [4] In practice, speed governors were further refined, notably by American scientist Willard Gibbs who in 1872 theoretically analysed Watt’s conical pendulum governor.

About this time the invention of the Whitehead torpedo posed a control problem which required accurate control of the running depth. Use of a depth pressure sensor alone proved inadequate, and a pendulum which measured the fore and aft pitch of the torpedo was combined with depth measurement to become the pendulum-and-hydrostat control. Pressure control only provided a proportional control, which if the control gain was too high, would become unstable and go into resonance, with considerable instability of depth-holding. The pendulum added what is now known as derivative control, which damped the oscillations by detecting the torpedo dive/climb angle and thereby the rate of change of depth.[6] This development (named by Whitehead as "The Secret" to give no clue to its action) was around 1868.[7]

Another early example of a PID-type ship-steering controller was also developed by Elmer Sperry in 1911, though his work was intuitive rather than mathematically-based.[8]

However, it was not until 1922 that a formal control law for what we now call PID or thee-term control was first developed using theoretical analysis, by Russian American engineer Nicolas Minorsky, [9] Minorsky was researching and designing automatic ship steering for the US Navy and based his analysis on observations of a helmsman. He noted the helmsman steered the ship based not only on the current course error, but also on past error, as well as the current rate of change;[10] this was then given a mathematical treatment by Minorsky.[4] His goal was stability, not general control, which simplified the problem significantly. While proportional control provides stability against small disturbances, it was insufficient for dealing with a steady disturbance, notably a stiff gale (due to steady-state error), which required adding the integral term. Finally, the derivative term was added to improve stability and control.

Trials were carried out on the USS New Mexico, with the controller controlling the angular velocity (not angle) of the rudder. PI control yielded sustained yaw (angular error) of ±2°. Adding the D element yielded a yaw error of ±1/6°, better than most helmsmen could achieve.[11]

The Navy ultimately did not adopt the system, due to resistance by personnel. Similar work was carried out and published by several others in the 1930s.

Industrial controlEdit

 
Proportional control using nozzle and flapper high gain amplifier and negative feedback

The wide utilisation of feedback controllers did not become feasible until the development of wide band high-gain amplifiers to utilise the concept of negative feedback. This had been developed in telephone engineering electronics by Harold Black in the late 1920s, but not published until 1934. [4] Independently, Clesson E Mason of the Foxboro Company in 1930 invented a wide-band pneumatic controller by combining the nozzle and flapper high-gain amplifier, which had been invented in 1914, with negative pneumatic feedback from the controller output. This dramatically increased the linear range of operation of the nozzle and flapper amplifier, and integral control could also be added by the used of a precision bleed valve and a bellows generating the Integral term. The result was the "stabilog" controller which gave both proportional and integral functions using feedback bellows. [4] Later the derivative term was added by a further bellows and adjustable orifice.

From about 1932 onwards, the use of wideband pneumatic controllers increased rapidly in a variety of control applications. Compressed air was used both for generating the controller output, and for powering the process modulating device; such as a diaphragm operated control valve. They were simple low maintenance devices which operate well in a harsh industrial environment, and did not present an explosion risk in hazardous locations. They were the industry standard for many decades until the advent of discrete electronic controllers and distributed control systems.

With these controllers, a pneumatic industry signalling standard of 3-15 psi was established, which had an elevated zero to ensure devices were working within their linear characteristic and represented the control range of 0-100%. The metric equivalent for this is 0.2 - 1.0 bar.

In the 1950s, when high gain electronic amplifiers became cheap and reliable, electronic controllers became popular, and 4-20 mA current loop signals were used which emulated the pneumatic standard. However field actuators still widely use the pneumatic standard because of the advantages of pneumatic motive power for control valves in process plant environments.

 
Showing the evolution of analogue control loop signalling from the pneumatic to the electronic eras.
 
Current loops used for sensing and control signals. A modern electronic "smart" valve positioner is shown, which will incorporate its own PID controller.

Most modern PID controllers in industry are implemented in distributed control systems (DCS), 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. PID temperature controllers are applied in industrial ovens, plastics injection machinery, hot-stamping machines and packing industry. PID controllers are at the core of multirotor drones as well, in the form of self-stabilising flight controllers.

Electronic analogue controllersEdit

Electronic analog PID control loops were often found within more complex electronic systems, for example, the head positioning of a disk drive, the power conditioning of a power supply, or even the movement-detection circuit of a modern seismometer. Discrete electronic analogue controllers have been largely replaced by digital controllers using microcontrollers or FPGAs, to implement PID algorithms. However, discrete analog PID controllers are still used in niche applications requiring high-bandwidth and low-noise performance, such as laser-diode controllers.[12]

Control loop exampleEdit

Let's take the example of a robotic arm[13], that can be moved and positioned by a control loop. An electric motor may lift or lower the arm, depending on forward or reverse power applied, but power cannot be a simple function of position because of the inertial mass of the arm, forces due to gravity, external forces on the arm such as a load to lift or work to be done on an external object.

  • The sensed position is the process variable (PV).
  • The desired position is called the setpoint (SP).
  • The difference between the PV and SP is the error (e), which quantifies whether the arm is too low or too high and by how much.
  • The input to the process (the electric current in the motor) is the output from the PID controller. It is called either the manipulated variable (MV) or the control variable (CV).

By measuring the position (PV), and subtracting it from the setpoint (SP), the error (e) is found, and from it the controller calculates how much electric current to supply to the motor (MV).

ProportionalEdit

The obvious method is proportional control: the motor current is set in proportion to the existing error. However, this method fails if, for instance, the arm has to lift different weights: a greater weight needs a greater force applied for a same error on the down side, but a smaller force if the error is on the upside. That's where the integral and derivative terms play their part.

IntegralEdit

An integral term increases action in relation not only to the error but also the time for which it has persisted. So, if applied force is not enough to bring the error to zero, this force will be increased as time passes. A pure "I" controller could bring the error to zero, however, it would be both slow reacting at the start (because action would be small at the beginning, needing time to get significant), brutal (the action increases as long as the error is positive, even if the error has started to approach zero), and slow to end (when the error switches sides, this for some time will only reduce the strength of the action from "I", not make it switch sides as well), prompting overshoot and oscillations (see below). Moreover, it could even move the system out of zero error: remembering that the system had been in error, it could prompt an action when not needed. An alternative formulation of integral action is to change the electric current in small persistent steps that are proportional to the current error. Over time the steps accumulate and add up dependent on past errors; this is the discrete-time equivalent to integration.

DerivativeEdit

A derivative term does not consider the error (meaning it cannot bring it to zero: a pure D controller cannot bring the system to its set point), but the rate of change of error, trying to bring this rate to zero. It aims at flattening the error trajectory into a horizontal line, dampening the force applied, and so reduces overshoot (error on the other side because too great applied force). Applying too much impetus when the error is small and is reducing will lead to overshoot. After overshooting, if the controller were to apply a large correction in the opposite direction and repeatedly overshoot the desired position, the output would oscillate around the setpoint in either a constant, growing, or decaying sinusoid. If the amplitude of the oscillations increase with time, the system is unstable. If they decrease, the system is stable. If the oscillations remain at a constant magnitude, the system is marginally stable.

Control dampingEdit

In the interest of achieving a controlled arrival at the desired position (SP) in a timely and accurate way, the controlled system needs to be critically damped. A well-tuned position control system will also apply the necessary currents to the controlled motor so that the arm pushes and pulls as necessary to resist external forces trying to move it away from the required position. The setpoint itself may be generated by an external system, such as a PLC or other computer system, so that it continuously varies depending on the work that the robotic arm is expected to do. A well-tuned PID control system will enable the arm to meet these changing requirements to the best of its capabilities.

Response to disturbancesEdit

If a controller starts from a stable state with 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 affect the process, and hence the PV. Variables that affect the process other than the MV are known as disturbances. Generally controllers are used to reject disturbances and to implement setpoint changes. A change in load on the arm constitutes a disturbance to the robot arm control process.

ApplicationsEdit

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 temperature, pressure, force, feed rate,[14] flow rate, chemical composition (component concentrations), weight, position, speed, and practically every other variable for which a measurement exists.

PID controller theoryEdit

This section describes the parallel or non-interacting form of the PID controller. For other forms please see the section 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   as the controller output, the final form of the PID algorithm is

 

where

  is the proportional gain, a tuning parameter,
  is the integral gain, a tuning parameter,
  is the derivative gain, a tuning parameter,
  is the error (SP is the setpoint, and PV(t) is the process variable),
  is the time or instantaneous time (the present),
  is the variable of integration (takes on values from time 0 to the present  ).

Equivalently, the transfer function in the Laplace domain of the PID controller is

 

where   is the complex frequency.

Proportional termEdit

 
Response of PV to step change of SP vs time, for three values of Kp (Ki and Kd held constant)

The proportional term produces an output value 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 constant.

The proportional term is given by

 

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 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.[citation needed]

Steady-state errorEdit

Because a non-zero error is required to drive it, a proportional controller generally operates with a so-called steady-state error.[a] Steady-state error (SSE) is proportional to the process gain and inversely proportional to proportional gain. SSE may be mitigated by adding a compensating bias term to the setpoint or output, or corrected dynamically by adding an integral term.

Integral termEdit

 
Response of PV to step change of SP 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 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

 .

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 overshoot the setpoint value (see the section on loop tuning).

Derivative termEdit

 
Response of PV to step change of SP vs time, for three values of Kd (Kp and Ki held constant)

The 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

 

Derivative action predicts system behavior and thus improves settling time and stability of the system.[15][16] An ideal derivative is not causal, so that implementations of PID controllers include an additional low-pass filtering for the derivative term to limit the high-frequency gain and noise. Derivative action is seldom used in practice though – by one estimate in only 25% of deployed controllers – because of its variable impact on system stability in real-world applications.

Loop tuningEdit

Tuning a control loop is the adjustment of its control parameters (proportional band/gain, integral gain/reset, derivative gain/rate) to the optimum values for the desired control response. Stability (no unbounded 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 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 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. 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. Usually, initial designs need to be adjusted repeatedly through computer simulations until the closed-loop system performs or compromises as desired.

Some processes have a degree of nonlinearity 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 gain scheduling (using different parameters in different operating regions).

StabilityEdit

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 diverges, with or without oscillation, and is limited only by saturation or mechanical breakage. Instability is caused by excess gain, particularly in the presence of significant lag.

Generally, stabilization of response is required and the process must not oscillate for any combination of process conditions and setpoints, though sometimes marginal stability (bounded oscillation) is acceptable or desired.[citation needed]

Mathematically, the origins of instability can be seen in the Laplace domain.[17]

The total loop transfer function is:

 

where

 : PID transfer function
 : Plant transfer function

The system is called unstable where the closed loop transfer function diverges for some  .[17] This happens for situations where  . Typically, this happens when   with a 180 degree phase shift. Stability is guaranteed when   for frequencies that suffer high phase shifts. A more general formalism of this effect is known as the Nyquist stability criterion.

Optimum behaviorEdit

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 rise time and 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.

Overview of methodsEdit

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 time consuming, particularly for systems with long loop times.

The choice of method will depend largely on whether or not the loop can be taken "offline" for tuning, and on 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.[citation needed]

Choosing a tuning method
Method Advantages Disadvantages
Manual tuning No math required; online. Requires experienced personnel.[citation needed]
Ziegler–Nichols[b] Proven method; online. Process upset, some trial-and-error, very aggressive tuning.[citation needed]
Tyreus Luyben Proven method; online. Process upset, some trial-and-error, very aggressive tuning.[citation needed]
Software tools Consistent tuning; online or offline - can employ computer-automated control system design (CAutoD) techniques; may include valve and sensor analysis; allows simulation before downloading; can support non-steady-state (NSS) tuning. Some cost or training involved.[19]
Cohen–Coon Good process models. Some math; offline; only good for first-order processes.[citation needed]
Åström-Hägglund Can be used for auto tuning; amplitude is minimum so this method has lowest process upset The process itself is inherently oscillatory.[citation needed]

Manual tuningEdit

If the system must remain online, one tuning method is to first set   and   values to zero. Increase the   until the output of the loop oscillates, then the   should be set to approximately half of that value for a "quarter amplitude decay" type response. Then increase   until any offset is corrected in sufficient time for the process. However, too much   will cause instability. Finally, increase  , if required, until the loop is acceptably quick to reach its reference after a load disturbance. However, too much   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 overdamped closed-loop system is required, which will require a   setting significantly less than half that of the   setting that was causing oscillation.[citation needed]

 
Effects of varying PID parameters (Kp,Ki,Kd) on the step response of a system.
Effects of increasing a parameter independently[20] [21]
Parameter Rise time Overshoot Settling time Steady-state error Stability
  Decrease Increase Small change Decrease Degrade
  Decrease Increase Increase Eliminate Degrade
  Minor change Decrease Decrease No effect in theory Improve if   small

Ziegler–Nichols methodEdit

Another heuristic tuning method is formally known as the Ziegler–Nichols method, introduced by John G. Ziegler and Nathaniel B. Nichols in the 1940s. As in the method above, the   and   gains are first set to zero. The proportional gain is increased until it reaches the ultimate gain,  , at which the output of the loop starts to oscillate.   and the oscillation period   are used to set the gains as shown:

Ziegler–Nichols method
Control Type      
P   - -
PI     -
PID      

These gains apply to the ideal, parallel form of the PID controller. When applied to the standard PID form, the integral and derivative time parameters   and   are only dependent on the oscillation period  . Please see the section "Alternative nomenclature and PID forms".

PID tuning softwareEdit

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 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.[22]

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.

Limitations of PID controlEdit

While PID controllers are applicable to many control problems, and often perform satisfactorily without any improvements or only coarse tuning, they can perform poorly in some applications, and do not in general provide optimal control. The fundamental difficulty with PID control is that it is a feedback control 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 in an observer without a model of the process, better performance can be obtained by overtly modeling the actor of the process without resorting to an observer.

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 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.

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 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.

LinearityEdit

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 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).

Noise in derivativeEdit

A problem with the derivative term is that it amplifies higher frequency measurement or process noise that can cause large amounts of change in the output. It is often helpful to filter the measurements with a low-pass filter in order to remove higher-frequency noise components. As low-pass filtering and derivative control can cancel each other out, the amount of filtering is limited. So low noise instrumentation can be important. A nonlinear median filter may be used, which improves the filtering efficiency and practical performance.[23] In some cases, the differential band can be turned off with little loss of control. This is equivalent to using the PID controller as a PI controller.

Modifications to the PID algorithmEdit

The basic PID algorithm presents some challenges in control applications that have been addressed by minor modifications to the PID form.

Integral windupEdit

One common problem resulting from the ideal PID implementations is integral windup. Following a large change in setpoint the integral term can accumulate an error larger than the maximal value for the regulation variable (windup), thus the system overshoots and continues to increase until this accumulated error is unwound. This problem can be addressed by:

  • Disabling the integration until the PV has entered the controllable region
  • Preventing the integral term from accumulating above or below pre-determined bounds
  • Back-calculating the integral term to constrain the regulator output within feasible bounds.[24]

Overshooting from known disturbancesEdit

For example, a PID loop is used to control the temperature of an electric resistance furnace where the system has stabilized. Now when 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 for 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.

PI controllerEdit

 
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

 

where   is the error or deviation of actual measured value (PV) from the setpoint (SP).

 .

A PI controller can be modelled easily in software such as Simulink or Xcos using a "flow chart" box involving Laplace operators:

 

where

  = proportional gain
  = integral gain

Setting a value for   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.

DeadbandEdit

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 stiction or a 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 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.

Setpoint step changeEdit

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 some of the following modifications:

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 discontinuity present in a simple step change.
Derivative of the process variable
In this case the PID controller measures the derivative of the measured process variable (PV), rather than the derivative of the error. This quantity is always continuous (i.e., never has a step change as a result of changed setpoint). This modification is a simple case of setpoint weighting.
Setpoint weighting
Setpoint weighting adds adjustable factors (usually between 0 and 1) to the setpoint in the error in the proportional and derivative element of the controller. The error in the integral term must be the true control error to avoid steady-state control errors. These two extra parameters do not affect the response to load disturbances and measurement noise and can be tuned to improve the controller's setpoint response.

Feed-forwardEdit

The control system performance can be improved by combining the feedback (or closed-loop) control of a PID controller with 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 primarily has to compensate whatever difference or error remains between the setpoint (SP) and the system response to the open loop control. 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 without affecting stability. Feed forward can be based on the setpoint and on extra measured disturbances. Setpoint weighting is a simple form of feed forward.

For example, in most motion control systems, in order to accelerate a mechanical load under control, more force is required from the actuator. If a velocity loop PID controller is being used to control the speed of the load and command the force being applied by the actuator, then it is beneficial to take the desired instantaneous acceleration, 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 actuator 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 control system.

Bumpless operationEdit

PID controllers are often implemented with a "bumpless" initialization feature that recalculates the integral accumulator term to maintain a consistent process output through parameter changes.[25] A partial implementation is to store the integral of the integral gain times the error rather than storing the integral of the error and postmultiplying by the integral gain, which prevents discontinuous output when the I gain is changed, but not the P or D gains.

Other improvementsEdit

In addition to feed-forward, PID controllers are often enhanced through methods such as PID gain scheduling (changing parameters in different operating conditions), fuzzy logic, or computational verb logic.[26][27] 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. Another new method for improvement of PID controller is to increase the degree of freedom by using fractional order. The order of the integrator and differentiator add increased flexibility to the controller.[28]

Cascade controlEdit

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 setpoint 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 setpoint, usually controlling a more rapid changing parameter, flowrate or acceleration. It can be mathematically proven[citation needed] that the working frequency of the controller is increased and the time constant of the object is reduced by using cascaded PID controllers.[vague].

For example, a temperature-controlled circulating bath has two PID controllers in cascade, each with its own thermocouple temperature sensor. The outer controller controls the temperature of the water using a thermocouple located far from the heater where it accurately reads the temperature of the bulk of the water. The error term of this PID controller is the difference between the desired bath temperature and measured temperature. Instead of controlling the heater directly, the outer PID controller sets a heater temperature goal for the inner PID controller. The inner PID controller controls the temperature of the heater using a thermocouple attached to the heater. The inner controller's error term is the difference between this heater temperature setpoint and the measured temperature of the heater. Its output controls the actual heater to stay near this setpoint.

The proportional, integral, and differential terms of the two controllers will be very different. The outer PID controller has a long time constant – all the water in the tank needs to heat up or cool down. The inner loop responds much more quickly. Each controller can be tuned to match the physics of the system it controls – heat transfer and thermal mass of the whole tank or of just the heater – giving better total response.

Alternative nomenclature and PID formsEdit

Ideal versus standard PID formEdit

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   gain is applied to the  , and   terms, yielding:

 

where

  is the integral time
  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   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   seconds (or samples). The resulting compensated single error value is scaled by the single gain  .

In the ideal form, shown in the controller theory section

 

the gain parameters are related to the parameters of the standard form through   and  . 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.

Reciprocal gainEdit

In many cases, the manipulated variable output by the PID controller is a dimensionless fraction between 0 and 100% of some maximum possible value, and the translation into real units (such as pumping rate or watts of heater power) is outside the PID controller. The process variable, however, is in dimensioned units such as temperature. It is common in this case to express the gain   not as "output per degree", but rather in the form of a temperature   which is "degrees per full output". This is the range over which the output changes from 0 to 1 (0% to 100%).

Basing derivative action on PVEdit

In most commercial control systems, derivative action is based on PV rather than error. This is because the digitized 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.

Basing proportional action on PVEdit

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. The modification to the algorithm does not affect the way the controller responds to process disturbances. The change to proportional action on PV eliminates the instant and possibly very large change in output on a fast change in SP. Depending on the process and tuning this may be beneficial to the response to a SP step.

 

King[29] describes an effective chart-based method.

Laplace form of the PID controllerEdit

Sometimes it is useful to write the PID regulator in Laplace transform form:

 

Having the PID controller written in Laplace form and having the transfer function of the controlled system makes it easy to determine the closed-loop transfer function of the system.

PID pole zero cancellationEdit

The PID equation can be written in this form:

 

When this form is used it is easy to determine the closed loop transfer function.

 
 

Then

 

While this appears to be very useful to remove unstable poles, it is in reality not the case. The closed loop transfer function from disturbance to output still contains the unstable poles.

Series/interacting formEdit

Another representation of the PID controller is the series, or interacting form

 

where the parameters are related to the parameters of the standard form through

 ,  , and
 

with

 .

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.

Discrete implementationEdit

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 discretized.[30] Approximations for first-order derivatives are made by backward finite differences. The integral term is discretised, with a sampling time  , as follows,

 

The derivative term is approximated as,

 

Thus, a velocity algorithm for implementation of the discretized PID controller in a MCU is obtained by differentiating  , using the numerical definitions of the first and second derivative and solving for   and finally obtaining:

 

s.t.  

PseudocodeEdit

Here is a simple software loop that implements a PID algorithm:[31]

previous_error = 0
integral = 0
loop:
  error = setpoint - measured_value
  integral = integral + error*dt
  derivative = (error - previous_error)/dt
  output = Kp*error + Ki*integral + Kd*derivative
  previous_error = error
  wait(dt)
  goto loop

In this example, two variables that will be maintained within the loop are initialized to zero, then the loop begins. The current error is calculated by subtracting the measured_value (the process variable or PV) from the current setpoint (SP). Then, integral and derivative values are calculated and these and the error are combined with three preset gain terms – the proportional gain, the integral gain and the derivative gain – to derive an output value. In the real world, this is D-to-A converted and passed into the process under control as the manipulated variable (or MV). The current error is stored elsewhere for re-use in the next differentiation, the program then waits until dt seconds have passed since start, and the loop begins again, reading in new values for the PV and the setpoint and calculating a new value for the error.[31]

See alsoEdit

NotesEdit

  1. ^ The only exception is where the target value is the same as the value obtained when the proportional gain is equal to zero.
  2. ^ A common assumption often made for Proportional-Integral-Derivative (PID) control design, as done by Ziegler and Nichols, is to take the integral time constant to be four times the derivative time constant. Although this choice is reasonable, selecting the integral time constant to have this value may have had something to do with the fact that, for the ideal case with a derivative term with no filter, the PID transfer function consists of two real and equal zeros in the numerator.[18]

ReferencesEdit

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  7. ^ Sleeman, C. W. (1880), Torpedoes and Torpedo Warfare, Portsmouth: Griffin & Co., pp. 137–138, which constitutes what is termed as the secret of the fish torpedo. 
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  11. ^ Bennett, Stuart (June 1986). A history of control engineering, 1800-1930. IET. pp. 142–148. ISBN 978-0-86341-047-5. 
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  16. ^ Tim Wescott (October 2000). "PID without a PhD" (PDF). EE Times-India. 
  17. ^ a b Bechhoefer, John. "Feedback for Physicists: A Tutorial Essay On Control". Reviews of Modern Physics. APS Physics. 77 (3): 783–835. doi:10.1103/revmodphys.77.783. 
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  19. ^ Li, Y., et al. (2004) CAutoCSD - Evolutionary search and optimisation enabled computer automated control system design, Int J Automation and Computing, vol. 1, No. 1, pp. 76-88. ISSN 1751-8520. http://userweb.eng.gla.ac.uk/yun.li/ga_demo/
  20. ^ Ang, K.H., Chong, G.C.Y., and Li, Y. (2005). "PID control system analysis, design, and technology" (PDF). IEEE Trans Control Systems Tech, 13(4), pp.559-576. 
  21. ^ Jinghua Zhong (Spring 2006). "PID Controller Tuning: A Short Tutorial" (PDF). Retrieved 2011-04-04. 
  22. ^ Y Li, KH Ang, GCY Chong, Patents, software, and hardware for PID control: An overview and analysis of the current art, Control Systems, IEEE, 26 (1), 42-54. http://eprints.gla.ac.uk/3816/1/IEEE2pdf.pdf
  23. ^ Li, Y. and Ang, K.H. and Chong, G.C.Y. (2006) PID control system analysis and design - Problems, remedies, and future directions. IEEE Control Systems Magazine, 26 (1). pp. 32-41. ISSN 0272-1708
  24. ^ Cooper, Douglas. "Integral (Reset) Windup, Jacketing Logic and the Velocity PI Form". Retrieved 2014-02-18. 
  25. ^ Cooper, Douglas. "PI Control of the Heat Exchanger". Practical Process Control by Control Guru. Retrieved 2014-02-27. 
  26. ^ Yang, T. (June 2005). "Architectures of Computational Verb Controllers: Towards a New Paradigm of Intelligent Control". International Journal of Computational Cognition. Yang's Scientific Press. 3 (2): 74–101. 
  27. ^ Liang, Y.-L.(); et al. (2009). "Controlling fuel annealer using computational verb PID controllers". Proceedings of the 3rd international conference on Anti-Counterfeiting, security, and identification in communication. IEEE: 417–420. 
  28. ^ Tenreiro Machado JA, et al. (2009). "Some Applications of Fractional Calculus in Engineering". Mathematical Problems in Engineering. 2010. doi:10.1155/2010/639801. 
  29. ^ King, Myke. Process Control: A Practical Approach. Wiley, 2010, p. 52-78
  30. ^ "Discrete PI and PID Controller Design and Analysis for Digital Implementation". Scribd.com. Retrieved 2011-04-04. 
  31. ^ a b "PID process control, a "Cruise Control" example". CodeProject. 2009. Retrieved 4 November 2012. 

Further readingEdit

External linksEdit