As you know, I’m working on the long term project, using Mecanum wheels for a robot with omnidirectional driving capabilities.
Today, I received the gear motors for my Mecanum wheel chassis prototype:
These are 12V gear motors with a 1:50 gear reduction. The spindles are running at approx. 104 RPM.
To drive these motors from an Arduino Duemilanove, I use the motor shield from ladyada.net (at least for the prototypes).
Here’s a short video of testing the controller/motor setup:
Already having printed all 4 Mecanum wheels, I thought I would be nice to assemble the whole shebang and give it a test drive.
All combat takes place at night, in the rain, and at the junction of four map segments.
Robert De Niro in “Wag the Dog”
Well, there were some bumps on the road.
When designing the Mecanum wheel, I planned to use them with stepper motors. Although it’s generally nice to have exact control of the (stepper) motors’ RPM, I’m sure that the wheels’ slipping on the ground is quickly killing this advantage. Because of the much simpler controller electronics, I switched over to the above mentioned gear motors.
The main problem here: The stepper motors have a 5mm spindle, the gear motors have 6mm spindles. Thus, I ended up with 5mm bores in the wheels for the 6mm gear motor spindles.
I re-drilled the center bores in the Mecanum wheels with a 6mm drill. After that, the wheels fit on the motor spindles but -of course- now the captive M3 nuts for the spindle set screws didn’t fit anymore.
To solve this problem, I needed to file down 4 M3 nuts to about half their height:
Although this solved the problems with the changed spindle diameter, the whole process most likely didn’t enhance concentricity of the wheels. Printing a new wheel (without the rollers), takes more than an hour. Cleaning up the printed object, glueing in the ball bearings and assembling the rollers takes at least another hour. Since it’s a prototype anyway, I chose to go with the fast, easy and maybe less precise solution.
I mounted the four motors with some quick and dirty printed clamp assemblies to a plywood base plate.
The clamps were printed relatively quick and they give me some freedom in (re-) adjusting the motor/Mecanum wheel positions. However, this sort of mount mechanism might not be ideal for long time use.
And here’s what the first prototype looks like:
The black Mecanum wheel in the front right is the first I printed. I had no liquid rubber then and I didn’t find the time and mood to completely disassemble the wheel in order to apply the liquid rubber to the rollers, yet.
The Arduino controller runs a pretty simple test sketch in the following movie, to test the four general drive modes (forwards, backwards, left, right):
Now I need to beef up the robots sensors and firmware. I really like the idea to add a gyroscope to recognize wanted and unwanted direction changes. And, of course, the thing definitely needs some kind of collision sensors…
