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Function BlockFB_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 11-201: The TemperaturePID user-defined function block

Arguments

Inputs

iEN BOOL EnableEnable signal for the drive, Hardware-Enable with 24V signal to X8, Software-Enable command by setup Software, fieldbus or permanently set. Both are required for enabling the drive. 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 FunctionA 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 PulseWhen 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:

KpProportional gain of a control loop=10
Ki=0.14
delta_time = time to complete Phase2

Kd=SQRT(delta_time)*7

The tuning is completed.

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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