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
- Stepper Motors - a well-documented solution capable of precise control but the affordable common 5 volt motor would give us a very slow-moving robot.
- Motors with Hall Sensors to enable
precise speed control eg
http://www.dx.com/p/12v-220rpm-dc-gear-motor-with-hall-encoder-silver-multi-colored-436609#.WeyCYGiCzIU
I am still searching for details on how to work with this.
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.
