Have you considered taking a basic electronics class? It would help with a lot of the misunderstandings in your post. It sounds like you bought hardware that is easily capable of accurate temperature control, then settled for poor control rather than take the time to learn what you have and how to make it work properly.
@dannyjhu wrote:
The initial plan was to follow a post I found on this board where a poster was using an NI-9263 module and Crydom MCPC1225A solid state relay. This SSD allows for an input of 0-5 or 0-10VDC and then has a 0-120V output. The problem I ran into was that the NI-9263 is limited to 1mA output per channel, where the SSD I believe needs a minimum of 4 mA to operate.
The Crydom unit is an SSR (Solid State Relay), not an SSD (whatever that might be). It is not a simple on-off relay; instead, it turns on and off rapidly, synchronized with the AC period, with a duty cycle (amount of time it's turned on) determined by the analog input. This lets you get much finer control of the output, for example you can run your heater at 10% of full power rather than either 0% or 100%.
The Crydom unit you linked has multiple options for the input signal, including 0-10V, 0-5V, and 4-20mA (a common industrial standard). The NI-9263 is a +/-10V analog output, so you could use it with the 0-10V SSR. For a voltage control signal, the current needed is negligble. The only reason you'd need 4mA is if you decided to use the current-control (rather than voltage-control) model, in which case you'd need a current analog output such as the NI 9265. Your post completely confuses the two.
dannyjhu wrote:
While doing this I realized that the benchtop setup was having the Platinum series temp controller telling the SSD when to turn off and on. I wondered why can't I simple do this with my cRIO 9485 SSd module and the same Omega SSD.
Because the Omega controller is pulsing your relay and heater rapidly - using a PWM (pulse-width modulation) signal, similar to the Crydom relay mentioned earlier - to get fine control over the amount of power into the heater. Your code simply turns the heater on and off.
@dannyjhu wrote:
By changing the proportional gain (Kc) to 500 and leaving the rest as default my results improved greatly. With the Kc=500, and a set point of 50 degrees C the heater kicked on at 49.9 and kicked off at 50.1. The heater temperature continued to rise to 55 degrees C, mostly do to the watts of the cartridge heater and the fact that the heater is on one side of the aluminum block and the thermocouple is on the opposite side. When I increased the set point to 110 degrees C, the heater kicked on at 109.8 and kicked off at 110.2 and the heater tempeature rise maxed out at 112.5 degrees C. This is all in an acceptable range for my needs.
Please don't believe that this is PID control, because it's not. It's an overly-complicated on-off (or "bang-bang" controller). The Kc is meaningless if all you're doing is checking that the output is greater than 0, and if you add in any integral gain it will make things worse rather than better.
If you want to run your control this way, take out the PID and simplify your code. Turn on the heater when the temperature is below the setpoint and turn it on when the temperature is above the setpoint. You might get acceptable control this way, but you won't get a stable temperature - you'll always get some overshoot, then some undershoot as the system cools to below the set temperature, then overshoot again as it overheats, etc.
Not surprisingly, the faster you cycle the heater on and off, the better your control will be. If you couple this with a properly-tuned PID, you'll be able to achieve a stable temperature because the integral component will keep the heater on at a constant low power to offset cooling, and you'll have minimal overshoot when you increase the setpoint because the power will back off as you approach the setpoint.
@dannyjhu wrote:
I will go back at some point and take your suggestions of adding shift registers and setting up the loop to allow for an auto tune. I want to see if an auto tune imporves things compared to my trial and error settings. Thank you for your help.
Don't bother autotuning with the setup as you've currently described it. It will probably make things worse. PID only works when you have a variable (rather than on-off) output. First get your code to generate a proper PWM signal so you can vary the power to the heater, then attempt autotuning. Or use the Omega controller and let it handle everything.
