[rboyd]

I was hoping to use a robotic arm for a project I'm working on, and wondering if you guys could answer a question about motors. In my very limited research it looked like one of the factors that make the industrial (kuka, etc) robots so expensive was that they use backlash-free motors. What does that even mean?

I also saw a couple startups aimed at sub-$5k robots (like carbon.ai). Are they solving this problem in some novel way?

[monknomo]

Backlash free motors are motors where the output shaft begins moving as soon as the motor starts moving. In particular, when the motor reverses direction there is no "slack" to pick up before the output shaft starts to move. The slack is called backlash when talking about gears and motors and what have you.

It's important for robots to not have backlash, because as movements are repeated, each bit of backlash adds up into a potentially big cumulative error. It could end up with the robot operating outside of the intended design envelope, which might be a safety problem.

I don't know what the startups are doing.

[tgflynn]

Wouldn't that only be a problem with open loop control though ? If you have an encoder and use feedback would backlash still be an issue ?

[monknomo]

We're getting outside of my home hobbyist experience here, but I think you could guarantee the robot would be in a particular position, but the backlash might make it hard to say when the robot will get to the particular position. Using encoders on all the motors would require having inputs for each encoder, which can get complex.

My guess is you can go pretty far with janky parts if you don't run for long periods of time and also measure where they are.

[jonnycowboy]

I think you mean backlash-free gearboxes (ie: cable driven gearboxes, harmonic drives or spring-loaded gearboxes that always apply a minimal but constant tension).