PID and output hardware On/Off for temperature control

Hi al,
I have read about temperature controller that uses the PID algorithm with hardware output: mechanical relay.
How does it work? Normally the PID output is continuous. It will be clear if PID output control directly one power transistor. But with relay, I do not understand.

Regards.
Muoinho

Take a look at this current discussion:


Maybe it is useful for you?

best regards

Both mechanical relay and power transistor act as an on/off switch in the temperature controller. The different is power transistor can be switched faster compared to that of mechanical relay. If the temperature is higher than the desired temperature, the controller will issue an signal to turn off the relay. In the other hand, if the temperature is lower, the controller will turn on the relay. These are repeated to maintain the temperature as the desired value.

Hi
It is not on/off control, but PID control. If PID output is used to control one BJT in linear mode. It is ok. So if it is used to control one switch.
When does it control ON switch and When does it control OFF siwtch ?

Regards
Muoinho

muoinhohn said:

Hi
It is not on/off control, but PID control. If PID output is used to control one BJT in linear mode. It is ok. So if it is used to control one switch.
When does it control ON switch and When does it control OFF siwtch ?

Regards
Muoinho

Click to expand...

I think you are misunderstanding something: PID means proportional / integral / differntial and must not be related to analogue regulation. PID acts on the speed and value change in the input signal and you can also control a heater which can only be switched on or off.

In this case you would switch on when the PID output is at 50% or more otherwise your output would be switched off ...

hope this helps and best regards

Hi
I begin to understand . Thank you very much.
So i have one questtion. If i have the transfer function of the object, is there the method to decide the percentage where i can switch ON/OFF relay.
Perhap i make one stupid question. So with me, the control theory is difficult
Regards
Muoinho

muoinhohn said:

It is not on/off control, but PID control. If PID output is used to control one BJT in linear mode. It is ok. So if it is used to control one switch.
When does it control ON switch and When does it control OFF siwtch ?

Click to expand...

It depends on how you interprete the output of PID controller.

In your case, as the BJT works in linear mode (I assume linear mode (amplifier mode) is NOT switching mode, pls correct me if I'm wrong), the output of PID controller is a continuous analog voltage that will change the VBE and base current, and in turn the collector current, which flows through heating element.

Mosfet is a voltage controlled switch. Say we control the mosfet as a switch. The output of the PID controller (plus PWM generator) will be a PWM pulses with variable duty-cycle. If the duty cycle is higher, the mosfet will turn on longer in one cycle or period. If the mosfet is turn on longer, more power will be transferred to the heating element.

muoinhohn,

I am wondering why you use a PID regulator for a temperature regulator?

Normally I do it like this:
I have a reference temperature (the temperature I want to have) let us say 22 degrees and a hysteresis (the amount the temperature is allowed to differ) let us say 1 degree.

The algo works like this:
switch relais on if it is off and temperature is less than reference temperature - hysteresis (22 - 1).
Switch relais off if it is on and the temperature is more than reference + hysteresis (22 +1).

This works for all standard heating systems.

best regards

If you have a controll value (manipulated value or output commanded from PID) which is 50% and you chose to regulate with a PWM then your duty for transistor is 50% .The same situation is with relay with the exception that you can not switch so fast with the relay as you can with the transistor. The PWM frequency with transistor or MOS-FET can be as high as few MHz a ( usualy few tens of KHz) and with the relay it can be up to few Hz or lower. As you change your duty with the transistor you can change the duty with the relay.If you chose the PWM freq for relay 0.1Hz and you want Duty 30% you will switch relay on for 3 sec and off for 7 sec for 40% duty you will have 4sec on and 6 sec off etc. I hope that cleared the subject a little bit .
Greetings
Dragan

C-MAN,

Sometimes u need to control the Temperature CHANGE RATE, not the temperature itself. I think the Hysterises methodology will not be effective in this situation because you will be turning the relay ON and this is the only thing u can do to control the temperature. All other factors are out of your control, in this situation, a P, PI or PID algorithm is the best to use.

Hopy this is useful.

Yours,

Besides PI or PID, fuzzy logic technology has also been used to control temperature, e.g. the water heater in bathroom.

Hi al,
Thank for all your reponses. I have understanded the problem .
I need to control PID because the temparature is a line, but not a constant value.
Regards.
Muoinho