Hey guys,

I have a question regarding the outputs seen on most PID temperature controllers on the market. I guess where I'm really confused is what exactly the output looks like in response to tuning parameters of the PID algorithm. For the most part I've used these controllers for mostly just the on/off functionality as temperature swing hasn't been too critical. However, the app notes supplied by the manufacturer of the thermoelectric cooler I'm controlling says that a PWM output is necessary to maintain a tighter control on temperature when using the full PID function.
My question is, are the output waveforms the same across different forms of controller output types, specifically SSR, mechanical relay and DC pulse (0-10V)? I would think that there would be a frequency related to PWM control but nowhere in the spec sheets did I see anything mentioning it. And if this frequency is high enough, a mechanical relay probably wouldn't be the best choice due to the amount of switching that could possibly shorten the lifetime of the relay.

Thanks

 

PID usually implies that either the magnitute of the output voltage or current is varied according to the control law (not relay, not PWM). Some PID controllers include the analog power amp, so do not.

 

Ok, now I'm confused even more. So looking around comparing controllers I came across this one made by Omega. This is from user manual:
CONTROL OUTPUT 1 & 2
Relay
250 Vac or 30 Vdc @ 3 A
(Resistive Load); configurable for
on/off, PID and Ramp and Soak

Output 1: SPDT type, can be
configured as Alarm 1 output

Output 2: SPDT type, can be
configured as Alarm 2 output

SSR
20-265 Vac @ 0.05-0.5 A
(Resistive Load); continuous

DC Pulse
Non-Isolated; 10 Vdc @ 20 mA

Analog Output (Output 1 only)
Non-Isolated, Proportional 0 to 10 Vdc
or 0 to 20 mA; 500 Ω max

I can see your point when it comes to the Analog Output at the end, but for the first relay it states that it is configurable for PID and Ramp and Soak.

And regarding your statement about the control law not pertaining to PWM or relay, from the app note of the thermoelectric manufacturer:

Proportional controllers use proportional regulation to maintain a constant temperature with no swing in the control temperature. This is often accomplished by using a proportional integral derivative (PID) algorithm to determine the output value and a pulse width modulation (PWM) output to handle the physical control. When using a controller with a PWM output, a capacitor can be placed across the output to filter the voltage to the TEM.

Thanks,

Dave

 

For PWM the frequency is the repetitive frequency of the pulse itself and is fixed.
PWM output for PID control usually assumes a PWM compatible control device, a relay would not normally fall into this category.
And generally control a motorized valve or similar component.
Max.