How to vary the force output of a motor while stalled?

Question

I want to provide a gradual increase in force provided to a piston (perhaps 0-100N over the space of a couple of seconds maximum), up till the point it begins moving, and then drive it to a varying speed after that.

I was thinking of using a linear actuator and applying a variable voltage to control the speed and force, but apparently using such motors in stall like this is generally a bad idea due to burning out the coils. However stepper motors and solenoid actuators are both designed for holding loads in place while applying a certain torque - would a stepper motor be able to do this without damage? Or could a solenoid actuator provide the gradual force increase in stall, and then the rest of the movement be accomplished using a seperate motor?

Many thanks in advance

Answer 1

There is nothing inherently wrong with running a motor in stall. The difficulty is that the power dissipation may be higher than the motor can handle long term.

Every motor can dissipate some amount of power safely indefinitely. In stall, the power being dissipated as heat is the full voltage times current. As long as that doesn't exceed what the motor can handle, you can run the motor as long as you want in stall.

Torque (or force for a linear motor) is proportional to current, and the power dissipation proportional to the square of the current. Both these are independent of the motor speed.

One way to run a motor safely at any speed is to regulate the current thru it. As long as that does not exceed the maximum current the motor can handle continuously, you can apply it continuously.

In your case, you want to control torque. That works out well with the above. Find the current to torque constant, and regulate the current to get the desired torque. As long as you limit this current to what the motor is rated for continuously, there is no special issue about the motor being stalled.

One possible problem for you is the "lumpy" nature of the torque over a magnetic cycle. This is also called "cogging". Stepper motors usually have high cogging, so may not be appropriate for what you want. A brushless DC motor being properly controlled can have quite low torque ripple. That is probably a better choice for you, since you are trying to control torque.