PID Controller with External Feedback

Owned by Gary Cane
May 09, 2022


Operator Menu Location: Controller Blocks


Operator Bitmap: 

 
Functional Description:
The PID Controller with External Feedback block implements the position form of a Proportional-Integral-Derivative control algorithm . Like the PID Controller block, it computes a control signal M, as the sum of proportional, integral (reset), and derivative (rate) control modes:

Unlike the PID Controller block, it computes the reset mode from an external feedback signal FB

where is the integral or reset time, expressed in minutes, and where delta t is the loop sampling time. If the signal that is fed back is simply the output signal M, i.e.,
FB(t) = M(t-(delta)t),
then, the PID Controller operator and the PID with External Feedback operator will function identically. The external feedback version is useful if you intend to do additional processing between the output signal M and the end actuator (as shown in the schema below). The PID Controller block should be used when output M drives an actuator directly.
The proportional and rate modes are based on measurements (samples) of an error signal E. For direct-acting control, this error signal is defined as
E = PV - SP,
where PV is the process variable and SP is the set-point. For reverse-acting control, the error is defined as
E = SP - PV.
The direct/reverse acting attribute of a PID controller refers to the relative direction of movement of the process variable and controller output. Under a direct acting configuration, an increase in the process variable results in an increase in the controller output. Under a reverse acting configuration, an increase in the process variable results in a decrease in controller output. The proper setting depends upon the process itself. In most processes, the controller should be set to reverse acting. (Hence, the operator defaults to reverse acting.)
The proportional mode term, , is calculated as the product of the proportional gain, , and the error term E(t),

The derivative mode term, , is obtained from

where is the derivative time, expressed in minutes. With this definition, a change in the process set-point can cause undesirable pulses in the derivative mode term. These undesirable pulses, known as derivative kick, can also result from discretization effects and from the noise-amplification effect of differentiation. Derivative kick can be avoided by having the derivative mode act on the process variable, rather than on the error. To avoid derivative kick is calculated from

where (delta)PV is the process variable change from one sample to the next. For direct acting processes, we have
(delta)PV(t) = PV(t) - PV(t-(delta)t)
and for reverse acting processes, we have
(delta)PV(t) = PV(t) + PV(t(delta)t)
The parameter befor B sub D (herein, z) in the equations above acts as a noise filter. Its reciprocal, 1/z, is called the "dynamic gain limit" and corresponds to the maximum amplification of the error signal. By default, the operator is configured to avoid derivative kick. And, the default value of the filter parameter is set to z=1.
The PID with External Feedback block provides a boolean input labeled A/M that indicates the operating mode: automatic or manual. If input A/M=LOW, the operator is in automatic mode and the output is computed as explained above. If input A/M=HIGH, the operator is in manual mode; the output is tied directly to the feedback signal, i.e.,
M(t)=FB(t) if A/M=HIGH.
And, the integral term is matched to the difference between the feedback signal and the automatic output:

With this arrangement, the transition between automatic and manual modes will be bumpless.
A sample schema demonstrating the use of this PID with External Feedback block is shown in the figure below.
In the figure, the PID with External Feedback block is used in conjunction with an external A/M Button block. Note that the Bumpless Transfer property of the A/M block should not be selected, since bumpless transfer between automatic and manual modes is built-into the controller block.


User-Defined Properties:
Object Name. A string label that identifies the operator
Display Minimum Pin. This property determines if input pin is visible in the schema diagram. Design Pad issues a warning upon processing the schema when the pin is visible but not connected. The warning message is not issued when the pin is not visible (when this property is not checked).


Minimum Output. The minimum output value
Display Maximum Pin. This property determines if output pin is visible in the schema diagram. Design Pad issues a warning upon processing the schema when the pin is visible but not connected. The warning message is not issued when the pin is not visible (when this property is not checked).

Maximum Output. The maximum output value


Output Range Auto-Center. When activated the initial output value of the PID will be set to the mid position of the minimum and maximum output. Otherwise, the initial output value is set to zero.


Display Direct Acting Pin. This property determines if the Display Active input pin is visible in the schema diagram. Design Pad issues a warning upon processing the schema when the pin is visible but not connected. The warning message is not issued when the pin is not visible (when this property is not checked).

Direct Acting. A boolean property that governs the relative direction of movement of the process variable and controller output. When the property is checked, an increase in the process variable results in an increase in the controller output. When it is not checked, an increase in the process variable results in a decrease in controller output. By default, this property is left unchecked (in most applications, a PID controller should be reverse acting.)


Derivative on Measure. A boolean property that governs the computation of the derivative-mode term. When this property is checked, the derivative mode term is based on the process variable; when it is not checked, it is based on the error. The latter case may result in derivative kick when the set-point is modified, or when the operator switches between automatic and manual modes. By default, this property is checked to eliminate derivative kick and to limit the noise amplification effect of differentiation.


Filter Parameter alpha. A scalar parameter used when the 'Derivative on Measure' property is checked. It serves to limit the effects of noise and discretization on the derivative-mode term.


Comments:
•The reset time ( ) and rate time ( ) inputs are measured in minutes
•The minimum reset time ( ) input is 0.001 minutes (if a smaller value is applied, 0.001 will be used as the reset time in the computation of the integral term)
•The minimum proportional gain ( ) input is 0.001 (if a smaller value is applied, 0.001 will be used as the gain in the computation of the output signal)


See Also:
PID Controller, A/M Button, Lead-Lag Controller