PID Controller

From SysCAD Documentation

Navigation: Main Page -> Models -> Process Control Models

General Description

The PID unit is used to simulate simple PID plant control functions. If the user requires more complicated controls that involve loops, then the Controller - General should be used.

A single PID controller unit can be used to simulate any number of independent PID controllers on the plant.
The control logic used to manipulate the variables is implemented once at the end of every SysCAD iteration.
The PID controller may be used in ProBal or in Dynamic mode. In Dynamic mode it closely simulates an actual plant PID loop, while in ProBal mode the time dependent factors have no relevance.
The Controlled Tags, the Output from the PID, and the Measured Tags, the Input to the PID, will have coloured triangles shown in the access window of those units. When the user right clicks on one of these referenced tags it will indicate which PID is referencing the tag. See Controlled and Referenced Variables.

Diagram

The diagram shows the default drawing of a PID controller.

This screen dump shows variables used and calculated in the PID Controller.

Inputs and Outputs

There are no connections to this unit.

Model Theory

The classical PID algorithm as shown below is used in the model:

$\mathbf {\mathrm{u(t) = K[e(t)+\frac{1}{T_i}\int e(t)dt + T_d\frac{d_e}{d_t}(t)]}}$

Where:

u(t) = the control action or output variable at time t;

e(t) = the error (or deviation) w(t) - y(t): the difference between the set point w(t) and the measured variable y(t);

K = the Gain;

T_i = the Integral time;

T_d = the Derivative time.

For a full description of the theory, please see the reference below.

References

Clarke D.W, PID Algorithms and their computer implementation, Dept of Engineering Science, University of Oxford Trans Inst M C Vol 6 No 6 Oct-Dec 1984

Data Sections

The default PID Controller access window consists of 2 sections. The number of sections in the access window will depend on the number of independent PID controllers required by the user.

The first tab contains general information relating to the unit.
The Info section contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.

Class: PID_Cont-1 - The first tab page in the access window will have this name.

First Section - Overall Data Fields

Tag / Symbol	Input / Calc	Description
Common First Data Section

On	Tick Box	The Overall PID unit will be enabled or disabled using this box. This means that all of the independent PID controllers in the unit will be disabled.
ShowCnv	Tick Box	With this option selected, SysCAD will display engineering units for tags such as Spt, Meas, Output, InMin/Max and OutMin/Max. Note that these will only be displayed after SysCAD has completed at least one iteration.
No_of_PIDs	Input	The number of independent PID controllers required. This may be any number from 0 upwards. The user may also change this number at any time. However, the unit will always add new PID controllers after existing ones, and it will delete them starting from the last controller. e.g. if you have 5 PIDs set up and you change this to 4, PID number 5 will be deleted. So if you do not want PID number 1, the best is to turn this off individually or use the priority field to change its display order.

Check Tags	Button	SysCAD will perform a check on the validity of the tags and functions used in the PID controllers.

State	Display	The state of the PID controller. If everything is correct and operating normally, the user will see 'OK' in this box. Otherwise the unit will flag an error message, which helps the user to locate the problem with the controller.
Error:
MSG_1 to MSG_2	Display	Any errors will be reported here. These messages are also displayed in the message window.

Individual PID Data Fields

Each independent PID has the following layout. The first PID will be displayed on the first page. Each page thereafter displays two PIDs.

Tag / Symbol	Input / Calc	Description
Tag.Cfg.[PID number]
On	Tick Box	This can be used to enable or disable the independent PID.
Name	Input	The user may give the PID a name that describes the control, e.g. Autoclave steam control.
Type	Original	Method which is based on the PID controller block in the Foxboro IA system (this method is not available in SysCAD 9.2 and later)
	Clark	This method is recommended for most applications.
	Clark FF	This method refers to controller Direct/Reverse acting in the form of Process Gain, as well as having extra fields such as Bias for Proportional control and Feed Forward cross reference.
UseSetPointTag	Tickbox	If this is enabled, then the user can use a SysCAD tag/function as the setpoint. If this is not ticked, then a constant value is expected for the setpoint. (Only applicable for SysCAD 9.2)
Current values
SptUsed	Output	The Set Point value used. The value is either a fixed user defined Set point value or it changes according to the user defined Set Point Tag.
Meas / Measured_Value	Output	The measured variable, or more commonly called the Process Variable or PV. The PV is measured and acts as an input to the Controller which takes action based on the value of the PV. Alternatively the PV can be measured and displayed so that the operator can perform manual control.
Out / Output_Value	Output	The current value of the Controller Output Signal (OP).
SetPoint
Spt / SetPoint	Input	The Target Value of the Process Variable (Meas), it should be between the minimum and maximum input values. Notes: In SysCAD 9.2, this field is only visible if the UseSetPointTag option is Disabled. In previous versions of SysCAD this field will be greyed out if the SetPoint_Tag field is used.
SetPoint_Tag	Input	This can either be a SysCAD tag or a function: Any SysCAD tag may be copied to this field as the set point tag. The user may type a function into this field. The first character must be an '=' followed by a valid expression. (See the PGM documentation for the required syntax.) The value of the calculated set point will be displayed in the SetPoint box. Notes: In SysCAD 9.2, this field is only visible if the UseSetPointTag option is Enabled.
Measured tag (input) and expected range
Meas_Tag	Input	The Measured variable (sometimes called the Process Variable or PV). The Measured variable acts as an input to the Controller which takes action based on the difference between the Measured variable and the Set Point. The Measured variable can either be a valid SysCAD tag or a function. If it is a function, the first character must be an '=' followed by a valid expression. See the PGM documentation for the required syntax.
InMin / Minimum_Input	Input	The expected minimum value of the Measured variable. If the relative difference between this Minimum value and the Set Point is too small, SysCAD will flag a warning and adjust the range that is used to carry out the control.
InMax / Maximum_Output	Input	The expected maximum value of the Measured variable. If the relative difference between this Maximum value and the Set Point is too small, SysCAD will flag a warning and adjust the range that is used to carry out the control. Note: If the actual value of the Measured variable moves outside of these expected values, the controller will not operate efficiently.
Controlled tag (output) and range limits
Output_Tag	Input	The tag that the PID Controller will set, or the Output Signal (OP). This must be a valid SysCAD tag that can be adjusted by the user, i.e. it MUST be a white field.
OutMin / Minimum_Output	Input	The minimum value that the PID controller may send to the Output tag, or Output Controller Signal (OP).
OutMax / Maximum_Output	Input	The maximum value that the PID controller may send to the Output tag, or Output Controller Signal (OP).
Settings
Auto	Tick Box	This emulates the auto/manual function of a controller. If this option is switched off, then the controller is in manual and will no longer control automatically. Instead, the user may set the output of the controller to a fixed value.
SptTrk	Tick Box	This field only appears in a dynamic project. If the controller is in manual (Auto check box is unchecked), then if this option is switched on, the setpoint of the controller will set to the measured value.
TrackMinMaxMeas	Tick Box	This field only appears in a dynamic project. If this option is disabled then the user will stop receiving warnings when the measured value is outside the specified range of InMin to InMax.
TrackMinMaxOut	Tick Box	This field only appears in a dynamic project. If this option is disabled then the user will stop receiving warnings when the output value is at either of the specified limits (OutMin or OutMax).
Reverse	Tick Box	The direction in which the PID controller will operate. For Methods 2 - Original and 3 - Clark: This refers to the process response, which is opposite of what the control engineers' definition of process gain. For example, for this model to work properly, the line of thinking should be as such: 1) Steam is added ↑ to raise ↑ the temperature in the tank. So the process response in the same direction, thus this should be left un-ticked. 2) Cooling water is added ↑ to reduce ↓ the temperature in the tank. So the process response is in the opposite direction, thus this should be ticked. For Method 4 - Clark FF: Direct/Reverse Acting is implemented in the traditional way, thus opposite of what the Original and Clark method does. 3) Reverse Acting (SP - PV) Used when the process gain is +ve. A process measurement increase causes a control output decrease. For example Inlet Flow to Tank Level - if the Tank Level is above Setpoint then we need to reduce the inlet flow. 4) Direct Acting (PV - SP) Used when the process gain is -ve. A process measurement increase causes a control output increase. For example Outlet Flow to Tank Level, if the Tank Level is above setpoint we need to increase the outlet flow.
Gain	Input	This uses the Proportional Control which is the main and principal method of control. It calculates a control action proportional to the error. Where Error is the difference between PV and SP. The Gain is applied to scale the size of the control action based on error. Using Proportional Control alone cannot eliminate the error completely. Gain represents the power of the control action. The larger the Gain, the stronger the control (or more difficult to control). Therefore, user must not use a Gain larger than really necessary as it affects the stability of the process.
PropBand	Input	The proportional band (PB) of the Proportional control. It is calculated as PB(%) = 100 / Gain. User only need to enter values for either Gain or PB, the other is calculated using the above formula.
Integral	Input	The integral time constant of the Integral control. Integral control is used to control towards no error, thus to assist the Proportional control. When a value is input in this field, we are using the PI Control. For Methods Original and Clark: The integral term can not be turned off. Its value is set to max(Ic,m_Period/2) where Ic is the selected integral reset time and m_Period is the period of execution of the PID controller. Integral cannot be turned off in the PID3 implementation as there is not a Bias. For Method Clark FF: Integral action is turned off when Ic=0.0 If Ic is non zero the Integral reset time used by the PID is set to max(Ic,m_Period/2) where Ic is the selected integral reset time and m_Period is the period of execution of the PID controller. When you do not use integral action you usually need to specify a Bias signal for the P Only or PD controller output to operate around.
Derivative	Input	The derivative time constant of the Derivative control. Derivative control is used to add stability. Note: This is generally only used in Dynamic mode. This would normally be 0 in ProBal mode.
Bias	Input	Only appears for Method Clark FF: For proportional controllers only. The controller output will equal bias when the error is zero, thus bias is adjusted so that the controller output are at their nominal steady state value. It has the same units as the control output.
Options
TrackSptRange	Tick Box	If this option is disabled then the user will stop receiving warnings when the setpoint value is close to the limits of the specified range of InMin to InMax.
Tune Rule	Input	The user may select a tuning rule to use when automatic tuning is used. The user may select between Proportional only (P), Proportional and Integral (PI) or Proportional, Integral and Derivative (PID).
Priority	Input	If more than one controller is used, the user may set the priority of the controllers. This changes the order the controllers are displayed. If the user wants to delete an unwanted controller, assign it with the lowest priority (thus the largest number) to move it to the last position.
FeedForward _Tag	Input	Only appears for Method Clark FF: Cross reference tag that specifies the amount of feedforward in units of the control output. Typically this value would be calculated in another controller.
FeedFwd	Output	Only appears for Method Clark FF: The value corresponding to that specified in the FeedForward_Tag.
Control Tolerances
AbsTol / Tolerance_Abs	Input	Absolute Tolerance, allows user to loosen or tighten the control. Use '*' to use project tolerance (see Project Commands for more information). Refer to Tolerance Testing for an explanation of how this is used.
RelTol / Tolerance_Rel	Input	Relative Tolerance, allows user to loosen or tighten the control. Use '*' to use project tolerance (see Project Commands for more information). Refer to Tolerance Testing for an explanation of how this is used.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

	DLL:	Control1.dll	→	Units/Links	→	Control: PID
or	Group:	General	→	Units/Links	→	Control: PID

See Project Configuration for more information on adding models to the configuration file.

Insert into Project

Insert Unit

→

Control

→

PID

See Insert Unit for general information on inserting units.

Hints and Comments

If one of the PID controllers contains a tag that is invalid, SysCAD will not begin execution, but will flag that user that there is an error in the PID controller. Check the Messages window for a list of the invalid tags.
If a PID controller ceases controlling, it may be that the measured variable is outside of the user defined minimum and maximum values. If this is the case, adjust the minimum or maximum so that the measured variable falls within the limits.
If the response of a independent PID is too fast, it will bounce around and never converge. In ProBal mode, increase the proportional band until the response settles down. In dynamic mode, the integration factor should be increased, either alone, or in conjunction with an increase in the proportional band.
Users can take advantage of the controller auto-tuning functionality to help tune controllers in the project. This often result in projects converging much faster with well tuned controls and is especially useful if many controllers have been used. To find out how, please refer to Trend Window (with PID Controllers).
All control engineers using SysCAD will complain that the PID controller Direct / Reverse action is opposite to what they are used to, this is due to the fact that SysCAD uses the process response to determine the direction, not the process gain. For example, for this model to work properly, the line of thinking should be as such:
a) Steam is added ↑ to raise ↑ the temperature in the tank. So the process response in the same direction, thus this should be left un-ticked.
b) Cooling water is added ↑ to reduce ↓ the temperature in the tank. So the process response is in the opposite direction, thus this should be ticked.
The problem above can be rectified by selecting the newly implemented method 4-Clark FF under Type. Reverse Acting (SP - PV) Used when the process gain is positive. Direct Acting (PV - SP) Used when the process gain is negative.
Each PID implementation scales the measured variable and control output between 0-100%. The PID algorithm thus works in this scaled range. This scaling needs to be taken into account when computing the effective open loop gain of the process as seen by the PID controller.
- Process Measurement Scaling : Kin = 100/(Inmax-Inmin)
- Control Output Scaling : Kout = (Outmax-Outmin)/100
- Process Gain with Eng Units : Ku
- Unitless Process Gain as Seen by PID : K = Kout*Ku*Kin = (Outmax-Outmin)/(InMax-InMin)*Ku

PID Controller

From SysCAD Documentation

Contents

General Description

Diagram

Inputs and Outputs

Model Theory

References

Data Sections

First Section - Overall Data Fields

Individual PID Data Fields

Adding this Model to a Project

Hints and Comments

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