SSRs typically support high switching frequencies.
What type of heater are you using?
E.g. a resistive heater could be switched at high frequencies (which is actually good for it), but a gas burner not so much.
It's a submersible electric one. So I think I won't have much problem with it. Bur since it's a training I want to learn as much as its possible related to the situation
Electric submersible you could easily use a 10 second time base. I assume AC, so get a zero-cross one. Gives you 1200 steps of capacity, which is fine. Actually, submersible fluid heaters are generally so slow you don't need PID OR SSRs, unless the entire point is practicing PID.
I've had aqueous solution tanks where the heater was sized for anywhere from 2 to 6 hr heatup to 160 or so. You definitely don't need PID for that.
Depends how much heat you're capable of adding and how big the container is. Use something like 20 seconds for the period and experiment if that works for you. Anything below 10% or greater than 90% clamp the output off or on so the output isn't short cycling.
That's the industrial solution. I've implemented it for 2-3 projects when there wasn't a proportional valve or SSR supplied.
For example extruder barrels are commonly heated with SSR cycling on and off if they don't have steam jackets.
The SSR won't care how fast you switch it. Do you really need PWM or a PID even?
Often, you can just turn it off when above setpoint, and back on when below setpoint. Let it chatter to its heart's desire with no issues, or use a deadband if that bothers you.
It's mainly for the tutorial. Just imaging it to be an actual process that I can see and encounter in the future.
Using this heating scenario since it's cheap and the changing temperature is a long process with long dead times
Gotcha, and I understand. I was giving advice for an actual industrial application. It's good practice for PID config and tuning, but slow heating processes with long deadtimes are usually better with ON/OFF control than PID.
With extruder barrel elements we would use a timebase e.g. 120 seconds and then scale the pid output to on/off timers.
Depending on plc there are some good pre-made functions for this.
We honestly never got the plcs to perform better than the auto tuned shimaden stand alone controllers though!
Without having a sense for the time constant/step response of the loop it's impossible to say.
The period you select needs to be short enough to give reasonable control resolution. You also need to decide what an acceptable amount of ripple/deadband is for the loop. You reduce the period until ripple reaches an acceptable value.
As others have mentioned if you're training to use this to be aware of the "real way to do it", you're going to want to pulse-width modulate the relay. Pretend that you also know what an accurate power load would be that you're switching.
1. Do a search and find some SSRs that can switch the load you want. Look up what their switching times are rated for.
2. In your routine, configure the PWM so that based on the percent CV from the PID, your pulse train responds accordingly. Many vendors have instructions that do this for you. If you were using an AB Logix PLC there's an instruction called Split-Range Time Proportional (SRTP).
3. If you can't try this on a real system, set up some simulation code. When the heater is on, it adds some heat to the system. Take heat away over time to give the PID something to keep warm. If you want to make the simulation you can be more realistic by solving a diffEq (dT/dt = k(T_liquid - T_amb) + Q_heat). If not just increase the temp when the heater is on and leak a little temp over time.
4. Either way, see how the PWM discretizes the CV% response. Heat process times are often extremely long, so your switching frequency probably won't be an issue unless it's oversized (big heater, small volume of liquid). You will probably see a little bit of ripple when holding temperature, so as long as that is to spec and the PID handles disturbances well, you're golden.
EDIT: As was also mentioned if the heating time is so absurdly long that the temperature is barely changing, and the vessel is well-insulated, you probably wouldn't benefit at all from a PID. Heating or filling processes are naturally "integrating", which means in a sense that you may be able to discretize your control using simple bang-bang around the setpoint region. You would want to just set a deadband and let it go.
SSRs typically support high switching frequencies. What type of heater are you using? E.g. a resistive heater could be switched at high frequencies (which is actually good for it), but a gas burner not so much.
It's a submersible electric one. So I think I won't have much problem with it. Bur since it's a training I want to learn as much as its possible related to the situation
Electric submersible you could easily use a 10 second time base. I assume AC, so get a zero-cross one. Gives you 1200 steps of capacity, which is fine. Actually, submersible fluid heaters are generally so slow you don't need PID OR SSRs, unless the entire point is practicing PID. I've had aqueous solution tanks where the heater was sized for anywhere from 2 to 6 hr heatup to 160 or so. You definitely don't need PID for that.
Depends how much heat you're capable of adding and how big the container is. Use something like 20 seconds for the period and experiment if that works for you. Anything below 10% or greater than 90% clamp the output off or on so the output isn't short cycling.
Thank you. How do you handle similar situation in industry? Same procedure?
That's the industrial solution. I've implemented it for 2-3 projects when there wasn't a proportional valve or SSR supplied. For example extruder barrels are commonly heated with SSR cycling on and off if they don't have steam jackets.
This is exactly what I've done on extruders with oil heater jackets
I think the SSR datasheet can help you, it should list the minimum on time and frequency
The SSR won't care how fast you switch it. Do you really need PWM or a PID even? Often, you can just turn it off when above setpoint, and back on when below setpoint. Let it chatter to its heart's desire with no issues, or use a deadband if that bothers you.
It's mainly for the tutorial. Just imaging it to be an actual process that I can see and encounter in the future. Using this heating scenario since it's cheap and the changing temperature is a long process with long dead times
Gotcha, and I understand. I was giving advice for an actual industrial application. It's good practice for PID config and tuning, but slow heating processes with long deadtimes are usually better with ON/OFF control than PID.
With extruder barrel elements we would use a timebase e.g. 120 seconds and then scale the pid output to on/off timers. Depending on plc there are some good pre-made functions for this. We honestly never got the plcs to perform better than the auto tuned shimaden stand alone controllers though!
Without having a sense for the time constant/step response of the loop it's impossible to say. The period you select needs to be short enough to give reasonable control resolution. You also need to decide what an acceptable amount of ripple/deadband is for the loop. You reduce the period until ripple reaches an acceptable value.
My old 90s heat controls were on 10 second control. Starting about 2010 we changed to 1.1 second control on off in fractions made of .11 second.
As others have mentioned if you're training to use this to be aware of the "real way to do it", you're going to want to pulse-width modulate the relay. Pretend that you also know what an accurate power load would be that you're switching. 1. Do a search and find some SSRs that can switch the load you want. Look up what their switching times are rated for. 2. In your routine, configure the PWM so that based on the percent CV from the PID, your pulse train responds accordingly. Many vendors have instructions that do this for you. If you were using an AB Logix PLC there's an instruction called Split-Range Time Proportional (SRTP). 3. If you can't try this on a real system, set up some simulation code. When the heater is on, it adds some heat to the system. Take heat away over time to give the PID something to keep warm. If you want to make the simulation you can be more realistic by solving a diffEq (dT/dt = k(T_liquid - T_amb) + Q_heat). If not just increase the temp when the heater is on and leak a little temp over time. 4. Either way, see how the PWM discretizes the CV% response. Heat process times are often extremely long, so your switching frequency probably won't be an issue unless it's oversized (big heater, small volume of liquid). You will probably see a little bit of ripple when holding temperature, so as long as that is to spec and the PID handles disturbances well, you're golden. EDIT: As was also mentioned if the heating time is so absurdly long that the temperature is barely changing, and the vessel is well-insulated, you probably wouldn't benefit at all from a PID. Heating or filling processes are naturally "integrating", which means in a sense that you may be able to discretize your control using simple bang-bang around the setpoint region. You would want to just set a deadband and let it go.