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I don't know how to call the device I'm looking for (or need to create), maybe you can help me with terminology too so I can at least search..

I want something that either rotates or pulls with a certain (controllable) force, i.e. not like a servo/stepper motor which control position, but on the contrary, it should be able to "catch up" to the moving load fast or stay still if the load is too heavy, but provide a steady pull of a certain force. Do these things exist and if so, what are they called? Do you know of any such thing that does it out of the box?

Btw I'm looking for something that pulls in the range of 100-200 lb, so the constant tension things from sewing machines won't work :)

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  • $\begingroup$ Look up torque control and force control. $\endgroup$
    – SteveO
    Sep 27, 2019 at 11:33
  • $\begingroup$ If you wanted to implement this in code, impedance control is what you need. $\endgroup$
    – MorganStark47
    Oct 7, 2019 at 5:27

3 Answers 3

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If I understand your question correctly, it seems to me that you are looking for a DC torque motor (brushed or brushless) with a suitable sensor attached to the load or to the output shaft. The sensor measures whatever it is what you want to control, you need a processor to implement the control system algorithms, and a power amplifier to deliver electrical power to the motor.

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I would use a stepper motor with a spring and dash pot connection to the output shaft and a variable resistor or optical feedback measuring the difference between the motor and the output shaft.

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Modern motor control offerings are able to control position, velocity, and torque (force). Stepper motors give you good position control, moderate velocity control, and really poor torque control since they pass between pole location and have highly variable torque output.

A brushed DC, brushless DC, AC induction, or best AC permanent magnet servos can offer extremely precise control in all of these areas.

Without knowing you application it is hard to recommend the "best" motor, but in general, DC brushed and brushless servo motors can be purchased relatively inexpensively and are easy to find in the 10W to 1kW range. AC induction motors (with a position control VFD) are very common and while they offer poor position control compared to other options, they are easily in power ranges from 100W to 1000+kW. AC servos are king when it comes to dynamic control, power density, and availability. They can be easily found in power ranges from 50W to 30kW.

Looking at torque, Stepper motors have very high stall torque at low speeds. DC motors offer little torque at very low speeds and are best mated with a gearbox or made significantly oversized for the application. AC servos offer nearly flat torque response for their entire speed range.

In terms of cost, expect a closed loop system to run 2-3x (or more) beyond the cost of open-loop motor control. Closed loop is really necessary if you want to accurately control position or velocity with anything other than a stepper motor.

Feel free to expand on your requirements so we can help you pick the best solution for your application.

Source: I am an industrial controls engineer with a focus in motion control. Hobby robotics guy.

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