Durallymax (Customer) asked a question.
BRX PWM
I have a project that needs proportional control of valves. I am debating between using the built in PWM on a BRX(if they make it back in stock in time)or simply interfacing with a valve PWM driver that just needs an analog signal send to it.
Any suggestions to use built in vs a driver? From what I see driver will be less programming but more money. If I did use the BRX to do PWM, what type of interposing devices are typically used between the BRX output and valve coil (5W in this case)? SSR?
Thanks
What type of valve? Is it a stepper?
Proportional hydraulic solenoid valve for a mobile application. Not a stepper, just a plain coil, no feedback.
I'm curious as to others' opinions. Reading through some purpose built solenoid driver literature, it appears as though there's compensation circuitry built in to the driver to account for variable current draw as the coil heats up. You might be able to compensate for this within the PID loop (maybe not?). At initial glance, you might be better served to opt for a dedicated driver initially and experiment with the PWM card if this is a more than one-off.
That was my thoughts as well, the drivers seem to have a bit more going on then just taking a 4-20 or 0-10 or 0-5 and outputting PWM.
Plus I believe I can use them to set ramp rates and such allowing a simple discrete signal to be used for actuators that don't need PWM but would benefit from "soft" movements.
I think one generally wants to use a constant current diver for a proportional solenoid. The force a solenoid produces is directly proportional to the current, not voltage, while IF the resistance remains constant then the current is proportional to voltage. But if heating changes the resistance then the output will change if only the voltage is being controlled, but a constant current source will change the voltage to keep the current constant.
You said the coil was 5W, but didn't mention the nominal voltage. The nominal maximum output rating of a BRX output is 0.5A at 24V that is 12W, or 6W at 12V, that could nominally drive your coil if it is one of those voltages, but personally I'd want something a bit more rugged between the PLC and the coil, if one is buying something anyway I'd consider a driver made for the valve. One valve driver specs. I found says it uses a 1.2kHz PWM. The MOSFET SSRs ADC sell have a maximum turn off time off 1mS and turn on around half that, then considering one want's to modulate the duty cycle, I don't know that one could do more than 100Hz (if that) which might be less than optimal.