Equipments (Devices) > SCA06 > Ladder > Control

Block that performs the function of a discrete PID controller. From the input variables, it calculates the corresponding controller output.

Ladder Representation

PID_block

Block Structure

Variable Type	Name	Data Type	Description
VAR_INPUT	EN	BOOL	Block enabling
	SetPoint	REAL	Automatic reference (pre-control)
	ManualSetPoint	REAL	Forced reference (post control)
	SelectSetPoint	BOOL	Selects which reference to use
	Feedback	REAL	Feedback loop variable
	MinimumOutput	REAL	Minimum value of the controller output
	MaximumOutput	REAL	Maximum value of the controller output
	Kp	REAL	Proportional gain
	Ki	REAL	Integral gain
	Kd	REAL	Derivative gain
	TauSetPoint#	REAL	Time constant of the automatic reference in put filter
	Type#	BYTE	Controller type
	Action#	BYTE	Control action
	Ts#	UINT	Sampling time [ms]
VAR_OUTPUT	ENO	BOOL	Output enabling
VAR_OUTPUT	Output	REAL	Controller output
VAR	PID_INST_0	PID	Instance of access to block structure

Operation

On the positive transition edge in EN, Output receives zero value, and the block executes its functionality as EN is at TRUE level.

When enabled, this block performs a routine PID control with the Kp, Ki and Kd parameters chosen. The PID topology used may be the Academic or Parallel, depending on what is chosen in Type#.

Academic Form:

PID_topology_academic

Parallel Form:

PID_topology_paralell

The levels of the output signal of the controller are saturated at value MinimumOutput and MaximumOutput. The SelectSetPoint input level FALSE causes the SetPoint reference be adopted, allowing the controller maintains control over the process. When SelectSetPoint goes to TRUE level, the controller has no more domain, and ManualSetPoint becomes to be considered the output signal of the controller.

Action# will define the feedback operation. If Action# is DIRECT, the operation will be SetPoint – Feedback. If Action# is REVERSE, the operation will be Feedback – SetPoint.

Feedback receives the process variable considered as the plant output. Ts# receives the sampling time for the controller and # TauSetPoint receives the time constant for the input filter of the automatic reference.

When EN has FALSE value, Output remains unchanged.

The ENO value forwards to the next Ladder block the EN value after the operation is completed.

NOTE!

Effects of the alteration of gains on the process

•	If Kp decreases, the process becomes slower; generally more stable or less oscillating; it has less overshoot.

•	If Kp increases, the process responds faster; it may become more unstable or more oscillating; it has more overshoot.

•	If Ki decreases, the process becomes slower, lagging to reach the "SetPoint"; it becomes more stable or less oscillating; it has less overshoot.

•	If Ki increases, the process becomes faster, quickly reaching the "SetPoint"; it becomes more unstable or more oscillating; it has more overshoot.

•	If Kd decreases, the process becomes slower; it has less overshoot.

•	If Kd increases, it has more overshoot.

NOTE!

How to improve the performance of the process through the adjustment of gains (valid for the Academic PID)

•	If the performance of the process is almost good, but the overshoot is a bit high, try to: (1) decrease Kp 20%, (2) decrease Ki 20% and/or (3) decrease Kd 50%.

•	If the performance of the process is almost good, but it does not have overshoot and lags to reach the "SetPoint", try to: (1) increase Kp 20%, (2) increase Ki 20% and/or (3) increase Kd 50%.

•	If the performance of the process is good, but the process output is varying too much, try to: (1) increase Kd 50%, (2) decrease Kp 20%.

•	If the performance of the process is bad, i.e. after start up, the transitory lasts several periods of oscillation that reduce very slowly or never reduce at all, try to: (1) decrease Kp 50%.

•	If the performance of the process is bad, i.e. after start up it slowly moves towards the "SetPoint" without overshoot, but is still very far and the process output is less than the rated value, try to: (1) increase Kp 50%, (2) increase Ki 50%, (3) increase Kd 70%.

Block Flowchart

PID_flowchart

Example in Ladder

PID_example01

The above example creates a loop of a digital PID form with sampling time 50 ms, using the constants KP, KI and KD for control. Automatic reference SETPOINT, filtered by a first order filter with time constant of 0:01 is used. The error signal is calculated as the difference between the filtered reference and variable SAIDA_PLANTA. The controller output is saturated between the values 0.1 and 2.5 and sent to the variable ENTRADA_PLANTA.

Example in ST

The example below displays the instructions for applying the example above in the ST language.

VAR

SETPOINT, SAIDA_PLANTA, ENTRADA_PLANTA : REAL;

KP : REAL := 6.2;

KI : REAL := 1.3;

KD : REAL := 1.8;

PID_INST_2 : FB_PID;

END_VAR

PID_INST_2.EN := DI1;

PID_INST_2(

SetPoint:=SETPOINT,

ManualSetPoint:=1,

SelectSetPoint:=FALSE,

Feedback:=SAIDA_PLANTA,

MinimumOutput:=0.1,

MaximumOutput:=2.5,

Kp:=KP,

Ki:=KI,

Kd:=KD,

TauSetPoint:=0.01,

Type:=0,

Action:=0,

Ts:=50);

ENTRADA_PLANTA := PID_INST_2.Output;

DO1 := PID_INST_2.ENO;