Sunday, October 22, 2017

Robot Problem Solving - Gearbox Slipped a Cog

The "Rat" has become difficult to control in the last 3 weeks. It has the common mini wheeled robot setup of 2 x yellow plastic motor-gearbox units - one on each side with direction controlled by running them at different speeds. In the last 3 weeks it has become impossible to run forward in a straight line but it is OK in reverse. We have put in a lot of time on detective work and researching more complex designs.

I tried replacing our simple H-Bridge analogue power circuit with a more complex digital solution involving an Arduino Nano microcontroller giving PCM outputs from our screen light patch LDR circuits as analogue inputs. That gave even more random out of control crazy movement. And the power amplifier board gave an unpleasant noise from the PCM frequency even with motors disconnected. [EDIT - LATER 28 Oct 2017 - discovered the need to use "float" when doing some of the calculations on analogue inputs - this Arduino Nano solution is now looking better. The unpleasant noise issue remains.]

I also looked at

Suspicion then fell on the motor-gearbox units. The left one did feel harsh and rough turning the wheel to spin up the motor compared with the smooth feeling of the right one. But why would it be OK in reverse?

I bought 2 new units and swapped them in. The "Rat" was controllable again. Repaired! Hooray!

I then investigated the old units. I replaced the motors thinking maybe we had given them too much power and done some damage. The motor swap is relatively easy because the motor is only held in by an elastic plastic strap held by 2 hooks. That made no difference, the left hand unit was still harsh. I inspected more closely and it felt difficult to push the left motor into place while it was smooth to push in the right motor. So I pushed the left motor cog about 1mm along its shaft closer to the motor body. Smooth action!

"Slip a cog" is an old school metaphor for technology going wrong. But in the case of our robot that literally is what happened! I hope it is useful to share that this very common unit can develop a harsh drive problem and adjusting the cog position on the motor shaft can fix it.


  1. This is a excellent site, would you be interested in doing an interview regarding how you created it? If so e-mail me! FURUKAWA parts

  2. Wow! This can be one particular of the most useful blogs We have ever arrive across on this subject. Basically Great. I am also a specialist in this topic therefore I can understand your hard work. zf parts

  3. Heya i am for the first time here. I found this board and I find It truly useful & it helped me out much. I’m hoping to provide one thing back and aid others like you helped me. acim

  4. The next time I just read a blog, I hope which it doesnt disappoint me just as much as brussels. After all, It was my replacement for read, but I actually thought youd have something interesting to talk about. All I hear can be a number of whining about something that you could fix in the event you werent too busy trying to find attention. OMC electric shift outdrive repair 

  5. Hi there! I know this is kinda off topic however , I’d figured I’d ask. Would you be interested in trading links or maybe guest authoring a blog article or vice-versa? My site discusses a lot of the same subjects as yours and I believe we could greatly benefit from each other. If you’re interested feel free to shoot me an email. I look forward to hearing from you! Awesome blog by the way! Auto Glass Repair

  6. My brother suggested I might like this web site. He was entirely right. This post truly made my day. You cann’t imagine simply how much time I had spent for this information! Thanks! Used Mazda Parts