Motor Torque Control Project

[beh188]

I am a mechanical engineer who needs some help with the electronics of controlling a motor. All design aspects of the project are open at this point, but the solution needs to be relatively simple and cost effective.

My application is that I need to turn a crank with a motor until the motor reaches a certain torque, and then the motor needs to turn off and trip a relay. The limit on torque is ~300 in.lbs. and it needs to be done with a gearmotor turning about 20-40 in.lbs. The motor will need to be capable of turning the crank in either the forward or reverse direction to the required torque. The power I will have available will be standard 110AC wall outlet voltage with about 10A AC to draw. Limiting the size and weight of the motor is important

I have been successful in finding many DC motors with the capabilities needed (Globe Motor, Bison Motor, Bodine-Electric). I have been looking for 24V DC motors because my thought is that staying at 24V would produce the smallest and lightest motor at the required torque. Also, I have been looking for a DC motor because I felt torque is most easily regulated. These 24V DC motors draw about 9A of current at the limit I would like to set.

My questions are related to the control aspect of the motor, though if anyone has a suggestion on motor selection let me know. My research for motor control seems to be pointing me in the direction of finding a DC drive with a current limiting functionality. (speed regulation is not important for the application). In addition to current limiting I also need to have some functionality to trip a relay and stop the motor when the current & torque reaches the limit.

Thanks for you help.

 

[dknguyen]

In theory you can figure out the torque of a well-characterized (well as in individually characterized DC motor) from it's voltage, current, and speed. In reality, it's best to just get a torque sensor which tend to be expensive and can be delicate. The easiest thing to do for a mech E might just be to use a clutch or the same mechanism that torque control screw drivers use, rather than measuring anything, especially if your torque threshold never needs to change.

DC brushed motors are by far the easiest to control.

 

[Ubergeek63]

Motor torque is directly proportional to the product of two factors: gap flux density and current.

Typically the higher the voltage, the lower the current, the less copper, the smaller the motor: but it does not generally work that way since the magnetic structure is really the limiting factor for a given HP.

Since HP is proportional to speed if you want something smaller and can live with it slower, you get a gear motor for something smaller and lighter.

And in the future when **broken link removed** gets a DC version out it will work much better for you.

Reversible is problematic as it requires a bridge of power components. Contrary to popular belief, you can not RELIABLY reverse a motor with an inexpensive relay. The DC current in the contacts causes the plating to migrate causing eventual failure.

That combined with arcing is why you see relays rated for 250VAC/28VDC. And the DC rating might be even less than the AC rating since they are assuming you are making and breaking the circuit with the contacts.