I plan to build a robot that is likely to bump into a lot of things quite often. I know I can detect if a motor has stalled by measuring the amps it is pulling and would like to use that as a simple way to detect any collisions, but I'm worried that this might damage the motors if used a lot. Can this be applied safely without harming the motors over time?

  • 2
    \$\begingroup\$ I doubt that would work because bumping into things typically does not stall the tires. \$\endgroup\$
    – Janka
    Commented Mar 12, 2017 at 19:07
  • \$\begingroup\$ braking current may be comparable to collision current so it depends on Vinput as well. \$\endgroup\$ Commented Mar 12, 2017 at 19:14
  • \$\begingroup\$ is this a hobby or for an actual product? I ask because there is an annoying number of "stall detection" patents out there for what is essentially an obvious thing to do \$\endgroup\$
    – user16222
    Commented Mar 12, 2017 at 19:18
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    \$\begingroup\$ Speaking of patents, there is a very nice stall detection patent (expired) at google.com/patents/US5367236 . Have at it! \$\endgroup\$ Commented Mar 12, 2017 at 23:16

2 Answers 2


Motors work the same way as other electronics. (You need the smoke to come out before it stops working.)

Seriously, if you do not allow the overcurrent to heat up the motors, they will be fine. \$I^2 t\$ is what kills most things. Do not exceed that as specified by the datasheet.

For a small motor the thermal constant might be say 10 minutes or 600 s. Derate the full load to 99 % and assume the bump current is 500 %. This means that you have $${600 - 99\% \cdot 600\over 25} = 240~ms.$$

This means that you can bump into things a total of 240 ms of time in 10 minutes. This can be achieved by switching off the motor very quickly or derating the full load a bit more.


More likely depends on transmission used. If you use gearhead, planetary reductor, some other gearing combination, then this transmission will break before the motor itself will get damaged. A motor current limit will protect the motor, while it can protect the transmission by means of a inertial kinetic energy dump. The energy has to go somewhere.


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