Control 2 outputs with one PID and one INPUT?

[ThePol]

I am sure the BCS can do this... but I am in the middle of my new brewery build and this control capability is essential to the design.

I want to heat my strike water with my 9000W RIMS heater, using (2) elements and ONE temp. input to the BCS. Can I control the (2) outputs (yes two SSRs) with ONE PID and with ONE temp. input?

Later in the day, during mashing... Id like to use (1) of those elements on a separate PID control and the same temp. input for the mash recirculation.

Basically, during different processes can I control an output by different means? Different PIDs, % output, ON/OFF

I am sure I can... but havent seen anything to let me know for sure.

Thanks.

[ECC]

Can I control the (2) outputs (yes two SSRs) with ONE PID and with ONE temp. input?

The BCS has 6 independent PIDs, and normally each PID is linked to its output. You can think of them as the Output0 PID, Output1 PID, etc.. Currently each output is hardwired to its associated PID, and you can't have one PID controlling two outputs.

Typically, when someone wants to use two elements with one PID, we suggest that they use two SSRs, and use one BCS Output to control them. This is the simplest way, and doesn't require any additional programming. In other words, one BCS Output is wired to the control side of both SSRs. That way one PID loop controls both elements, where they are just there for extra wattage. But your case is different, and it sounds like this isn't exactly what you are looking for.

The other way to go is to have 2 PIDs control both Outputs. The Alternate PID feature lets you grab the settings (coefficients/temp probe/etc) of a different PID, and swap them for a state. But you still have 2 PIDs controlling 2 outputs, so they may conflict... They should operate pretty much in lock-step, since they have the exact same inputs. But if they ever got out of step it would get weird..

Basically, during different processes can I control an output by different means? Different PIDs, % output, ON/OFF

Yes. The type of control for an output can be set in each state uniquely by selecting Direct, Duty Cycle (%), Differential (Ranco style), and PID. If PID is selected, the output uses it's default PID.

Welcome Pol!

[ThePol]

Okay... this will be an issue in my setup possibly.

I want (2) elements controlled by ONE PID for one process.

In another process I want to be able to control (1) of those elements only... which I cannot if both SSRs are controlled by the same output on the BCS.

In all honesty... during the process where I am running (2) elements I do not NEED PID precision, I do need differential temp. control though... I suppose I could control them separately with such control and things may get a little less weird when tied to the same output.

I appreciate your responses today and your interest in what we as HBers need as we get more and more technical!

[ECC]

Being able to select which PID controls which output sounds like a very good feature.. In fact, its really not a big change from how the current Alternate PID works, and I think its better / more intuitive. Instead of swapping the inputs of the PIDs, you would be able swap the Outputs.. People who are currently using Alternate PID wouldn't have to change their programming either.. It would also be useful for people who are driving large numbers of outputs. I'll look into adding this for the next firmware release.

In the meantime, there are plenty of options to get the desired behavior.

1. Use two PIDs, like I mentioned above. Adds a little weirdness with tuning, but would work quite well. Associtate both PIDs with the same temp probe, and enter the same setpoint in both PIDs.
2. Use two states with temperature exit condition. State0:RampUp would have both outs Direct ON. When Temp>=setpoint, exit to State1 where you have one Output hold temp via PID. I'm guessing you only want the extra element for quicker ramps, so one element should be able to maintain the temp.
3. Have one output controlled by PID, and the second output controlled differentially (or pid, doesn't matter).. Have the second output's temp setpoint a few degrees less than the first one. This gives the same behavior as above, but only uses one state. Basically gives you 2 temperature zones. Both elements are on for the majority of the ramp, but one drops off as it approaches the setpoint.

[ThePol]

Those are all viable solutions! The more I speak the BCS speak, the more it makes sense. Yes, running both elements on different PID temp settings would probably be the easiest and most intuitive for me. Set OUPTUT 1 to PID control and 165F while OUTPUT 2 is PID and set to say 160F.

The 9000W capabilty is only planned to be used for strike water (MLT) heating as I can flow very fast and utilize those BTUs at 100% output. When recirculating, my flow will be slower and 4500W will still provide enough power to exceed my flow rate.

This is why I say, I dont need PRECISE temp control for this function for mash in, as any discrepancies will be corrected in the RIMS during the PID controlled single element recirculation.

Looks though, like I should be able to use BOTH elements (outputs) on PID control and then have one drop off a few degrees short of the other. I cant wait to have some homebrew and tweak these details in my garage...

Id love to see the ability to use the same PID control to control multiple outputs... that would be sweet.

Thanks!

[jmdenmark]

This is a totally uniformed reply as I haven't even hooked my BCS up yet (waiting for some fittings) but if I understand correctly, couldn't you also create the configuration you are looking for by having two individual PID/Outputs control wiring to the same SSR and then used in different processes?

i.e. PID/Output0 control wire is spliced to control both the SSR0 and SSR1 serially.
and PID/Ouptut1 control wire is controlling SSR1.

By sharing the temp input in the process and ensuring the two processes can't run simultaneously wouldn't this accomplish the same thing?

If this is a complete misunderstanding, please let me know as I'm still trying to get my head fully around this too.

[ECC]

Wiring 2 outputs to the same SSR would create a short. If one output was driving high, and the other driving low, bad things would happen.

[ThePol]

I am comfortable with the previous work arounds, but I will be looking for... being the needy brewer that I am... the ability to use ONE PID to control MORE than one output in the next Firmware update. That would just be sweet (element and pump, two different pumps, two elements, two elements and a pump.... )

All of my SSRs and heatsinks are on thier way... Blichmans are on a truck somewhere, SS plumbing will be here in a few days... I will be out of town for a couple weeks