FB_TemperaturePID

Description

This function block provides PID "Proportional-Integral-Derivative" A PID controller is a generic control-loop feedback mechanism widely used in industrial control systems. An "error" occurs when an event or a disturbance triggers off a change in the process variable. A PID controller attempts to correct the error between a measured process variable and a desired setpoint by calculating and then outputting a corrective action that can adjust the process accordingly temperature control with auto tuning.

Figure 7-235: The TemperaturePID user-defined function block

Arguments

Inputs

iEN	BOOL	Enable function
iREFTEMP	LREAL	Reference temperature [°C]
iTEMP	LREAL	Actual temperature [°C]
iSTART	BOOL	Start PID or auto tuning
iMODE	BOOL	FALSE-automatic, TRUE-tuning
iKP	LREAL	PID Proportional Gain
iKI	LREAL	PID Integral Gain
iKD	LREAL	PID Derivative Gain
iTC1S	BOOL	Sampling Time is 1s
iTC5S	BOOL	Sampling Time is 5s
iPROGCYCLE	LREAL	Execution time of the function [ms]

Outputs

oOK	BOOL	Function A function calculates a result according to the current value of its inputs. A function has no internal data and is not linked to declared instances. enabled
oCHECKSTABLE	BOOL	TRUE when checking if ambient temperature is stable
oTUNESTART	BOOL	Tuning is started
oTUNEOK	BOOL	Tuning is completed
oTAU	LREAL	System Time Constant[s]
oPWM	BOOL	PWM command for heater
oERR	BOOL	Function error
oERRID	INT	Function ID error (in case of oERR=TRUE)
oPOWER	LREAL	% of power requested from heater (100%=full power)
oTICK	BOOL	Pulse When the step gets activated, the action is activated for a single execution, and possibly once again when the step is deactivated every sampling time

Usage

Tuning Process

Tuning consists of three steps.

1. Check if the ambient temperature is stable: the measured delta_temp=Tmax-Tmin must be lower than 0.1*Tmax.
   This step takes 10 cycles (10*iTC5s or 10*iTC1s).
   The tuning fails (oERR=TRUE, oERRID=1) if the ambient temperature is greater than 0.1*Tmax, otherwise Tamb=(Tmax+Tmin)/2.
2. Start tuning Phase1: output oPWM is kept TRUE until the final measured temperature iTEMP gets over iREFTEMP/2. After that oPWM is kept LOW.
3. Start tuning Phase2: with oPWM kept LOW the temperature gets down until the final value is lower than [(iREFTEMP/2-Tamb)*0.368+Tamb].

After, PID gains are calculated as:

Kp=10
Ki=0.14
delta_time = time to complete Phase2

Kd=SQRT(delta_time)*7

The tuning is completed.

• 
  

• oTAU may be useful for setting the proper sampling time (1s or 5s).

Start PID Controller

Upon starting the PID controller, the output oPWM is modulated 5 times within the sampling time (blue line is oTICK, green line is oPWM) and each pulse length depends on output oPOWER (100%=full length).

Example

ST

//Run PID function with determined proportional, integral, and derivative gains

//send PWM output to command heater
udfbThPID( TRUE, 90, Temp_mes, TRUE, FALSE, KP, KI, KD, FALSE, TRUE, 1);
Tau_Calculated := udfbThPID.oTAU;
PWM_CMD := udfbThPID.oPWM;
Power := udfbThPID.oPOWER;
Tick := udfbThPID.oTICK;

FBD

FFLD

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