TIA Portal: hysteresis temperature

what do you need it for?

control of a heater / cooler?

SP = setpoint

T = measured value

H = hysteresis

// Heater

if(T<SP-H)  Heater:=1;

if(T>SP+H) Heater:=0;

this will cause temperature to fluctuate around setpoint

// Cooler

if(T<SP-H)  Cooler:=0;

if(T>SP+H) Cooler:=1;

this will cause temperature to fluctuate around setpoint

// Both

if(T<SP-H)  Cooler:=0; Heater:=1;

if(T>SP+H) Cooler:=1; Heater:=0;

if(T>SP) Heater:=0;

if(T<SP) Cooler:=0;

this will cause temperature to reach setpoint. Upon reaching setpoint both heater and cooler switches off

Should deviation exceed hysteresis action is taken (heating if too cold or cooling if too hot)

hysteresis must be high enough to prevent constant activation cooler->heater->cooler etc.

Hysteresis should also (in the previously posted situations) take into consideration heat "inertia" and "lag".

"Inertia": from time heat is turned off before existing heated / cooled elements stop transferring heat out/in

"Lag": how much time passes between heater / cooler elements and temperature sensors

For example, horrible lag is when temp sensors are placed in front of heating/cooling elements.  The transfer medium (typically fluid) needs to go all the way around the loop before any changes are sensed.

Also, the device(s) controlling flow on/off/rate through the heating/cooling element(s) have a part to play in the hysteresis.

The S7-1x00 PID_TEMP takes these into consideration automatically during commissioning.

In S7-300/400, the PID control also _can_ take these into consideration, but it must be properly configured and commissioned.

The said inertia will depend on site and also on condition which can change

for example

jacket cooling inertia will depend on coolant temperature in cooling loop

and also on level of cooled media in reactor/fermentation vessel/etc. and exactly

ratio of coolant mass in jacket vs cooled media in the tank.

moreover, if tank is not insulated ambient temperature will affect it as well.