Anyone know of a good (and economical) way to make a motor controlled arm become a passive link on trigger? That is, the joint can be active (motor controlled) or passive (rotate freely). The tip of the arm weights 3kg and the length is 15cm. There's a 5kg load as well. I suppose the motor and gearbox will need to provide around at least 25Nm torque. A gearbox with that much torque would be very difficult to back drive.

I guess one way could be to use an electric clutch, but electric clutches are expensive and I don't think I have enough battery capacity to power the electromagnets. Another way I've thought of is to use planetary gears by braking the ring gear with a servo-controlled ratchet to make the link active. But planetary gears are hard and expensive to make as well. Anyone know of any alternative?

  • $\begingroup$ Did you consider equipping the system with force/torque sensing capabilities and thus use inverse dynamics to implement active compliance and/or zero-g control? $\endgroup$ Commented Mar 24, 2019 at 16:40
  • $\begingroup$ @Ugo I know only little about inverse dynamics, but AFAIK what you suggested wouldn't actually make the joint passive, right? $\endgroup$
    – John M.
    Commented Mar 24, 2019 at 17:22
  • $\begingroup$ Backdrivability can be implemented not only natively by means of passive compliance but also via active compliance where the motors are commanded using force/torque feedback to make the joint behave as it would be "passive". $\endgroup$ Commented Mar 24, 2019 at 19:00
  • $\begingroup$ @Ugo Oh, I'm actually trying to switch off the motor during passive mode. I'm not trying to make the motor behave like if it was passive, but I'm trying to make it actually passive without any work by the motor. $\endgroup$
    – John M.
    Commented Mar 24, 2019 at 19:10
  • $\begingroup$ @Ugo what are some tools and resources for designing active compliance? $\endgroup$
    – drerD
    Commented Mar 29, 2019 at 19:10

2 Answers 2


Depending on the motor type, you may be able to freewheel the motor. With a brushed DC motor this could be achieved with most commercial H-bridges, controlled with fast current decay.

Flyback diodes allow the inductive current in the motor to be dumped back through it and dissipated, or you can control the MOSFETs in the H-bridge to dump current to ground.

From your description it will be a significant amount of current, but it should be possible.


Here is my pitch: Collar!

Yes, you've read that right. You could use a same mechanism that is use in manual transmission. A collar that could slide and engage the arm with the gearbox.

The sliding action of the collar could be done by a small servo motor. A position sensor could be use on the arm and gearbox, so when you need to engage, both shaft will be in proper alignment to assure engagement/sliding of the collar.

I don't know if the price is good. It will also require sufficient room to operate.


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