How to detect DC motors stall current?

[Michele Panegrossi]

Hello!

I have a manual pneumatic regulator which I am planning to control by connecting a DC motor to it.

I can remove the cap and use a coupler to connect straight to the shaft. This is the easy part.

The regulators knobs allow for about 15 full rotation form the minimum to the maximum pressure settings and the torque increases considerably between the regulators being fully closed to fully open.

What I am struggling with is the initial setting of the regulator when I switch the system on. Encoders will not solve the problem unless they know where their zero point is.

First question is: do you know of DC motor with encoder that reports its zero point?

I though about rotating the motor anticlockwise until the regulator is at its minimum pressure. At that point the motor will stall. Is there a way to be able to detect that event with precision using some kind of sensor and an Arduino?

Or is there a better way of being able to detect when the regulator is at its minimum pressure using a motor?

I thought about  using a pressure sensor like this or like this and leave the whole stalling business alone.

Can anybody recommend a cheaper sensor / way of doing it or a better way?

I am working with pressures between 0.5 and 10 BAR max.

Many Thanks

Michele

[Adrian Godwin]

I would use a physical endstop switch or a separate pressure sensor. Given the high torque, you'll probably need a geared motor and detecting stall current can be imprecise.

A gray-scale encoder will give you absolute position. So will an Austria Microsystems rotary hall sensor. But both only for a single turn, which is useless for a regualtor.

If possible, I'd much rather use a pressure control valve with an internal pressure control loop, or a proportional flow valve with an external loop. A regulator is made for manual adjustment - there are more appropriate methods for mechanised adjustment.

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[Michele Panegrossi]

Hi Adrian,

thanks a lot for you advice! I agree with you, and I think the separate pressure sensor is the way to go.

I used these SMC electro pneumatic regulators before which do exactly what I need but unfortunately they are too expensive (£546 fro RS). There cheaper ones but they are too slow (6L/m) and they still cost around £250 each. One of the wires reports the actual output pressure (in volts) so you have your feedback loop there.

Are these the kind of valves you were talking about? DO you know where I can find examples of proportional flow valves with an external loop?

Basically I am trying to find a way to minimise costs of parts.

Many Thanks

Michele

[Adrian Godwin]

Yes, they're ideal if they're in budget. Or if you can find one secondhand.

Alternatively you can build your own pressure control loop with a valve, a pressure sensor and a PID controller. Ideally though, you want a proportional flow valve. Common air valves are on/off which is fine if you have a large reservoir or no need for smooth control. If you need something more like those expensive ones, you can use an SMC PVQ31 - still expensive (£70) but not £500.

Another way to do it would be to use your regulator with a slipping clutch between motor and regulator. Then you can drive it backwards for more turns than the regulator takes and rely on the clutch to slip. You still have to choose a torque that allows it to hit the endstop without damage whilst giving a reliable drive when off the stop, but that's easier than trying to measure the  motor stall because you're measuring the final torque, not the motor torque which is partially isolated from the load by the gearbox.

While we're on the subject .. I've got a need to move some pneumatic cylinders silently. This is something they're good at, but I have the problem that at the end of the stroke, I release the driving pressure and get a loud hiss. I could probably fix that with those proportional valves but they're too expensive - I want to use a 16-channel block of on/off valves. Does anyone know if there's a way to do this ? I haven't so far managed it with exhaust silencers but maybe there are better ways.