hp172
Thanks for the schematic. So the circuit would need to operate for both foward and reverse of the motor, i.e. for opening and closing it. From what what I can see the schematic is for one dierection only and would seem tricky to get it to rotate the motor in both directions. I cant see any way at the moment of adapting the schematic to make the circuit operate for both directions.

What Mr Software said in his post about the auto-windows in a car is exactly what I am looking to acieve except with a butterfly valve instead.

The window in my car uses what is basically an auto-resetting circuit breaker, I forget the real name for it. Under normal operating current load the device is fine. When the motor stalls and the current goes very high, the breaker trips turning off the circuit and unlatching the relay. After cooling for a few seconds the breaker automatically resets. Are you looking for basically the same thing for your valve?

 

Has anyone suggested using a mosfet driven by a specialized, current-limiting ic, such as this one, and this one?

 

hp172
Thanks for the schematic. So the circuit would need to operate for both foward and reverse of the motor, i.e. for opening and closing it. From what what I can see the schematic is for one dierection only and would seem tricky to get it to rotate the motor in both directions. I cant see any way at the moment of adapting the schematic to make the circuit operate for both directions.

What Mr Software said in his post about the auto-windows in a car is exactly what I am looking to acieve except with a butterfly valve instead.

No problem. Just add a DPDT switch across the motor. But the car window suggestionnsounds best.

 

Why not use an H Bridge and an ADC to monitor motor current? Then you can adjust the PWM to ensure the current is limited. I actually have a patent on a very similar mechanism which is used in large scale photocopiers. The motor moves in one direction and it is tracking the winding current. As soon as the current increases to anything larger than a preset value, the motor is disabled.

You can also easily regulate current with some H Bridge drivers and in this case you would be operating in what is called Torque Mode. For many years, I was the Apps Engineer for the DRV88xx family of devices and this is something we did all the time.

Because there is a SENSE resistor and a current regulation engine, you can set which current you want to see in your winding by changing the VREF. What will happen then is that if your current is less than the ITRIP value, then you current will be whatever it is. But if the motor load becomes higher such that the current is above the ITRIP value, then the H Bridge will (automatically!) regulate to the programmed value.

I think Pololu and Sparkfun sells a gazillion different DRV88xx mini boards which you could try. You can also check my blog ebldc where I treat topics similar to this and explain how it all works, in the event you want to understand the inner workings. Good luck on your project!