16 Nov 08 The Robot: Base, Wheels, Motors, and Sabertooth Motor Controllers

After attaching the 4 motors and brackets to the acrylic square, I found that it started to dip slightly due to the weight, and as I’d planned to put a 1.5kg lead acid battery in the center and I realised that this needed to be addressed. Rather than another visit to Homebase for some steel reinforcement, I just stuck (melted) two pieces firmly together with polycarbonate acid glue and then trimmed the edges with an electric saw.

Here is the base, the insulation tape all over the place is to hold down the connectors that I won’t be needing. The motors all contain encoders which I didn’t just want to rip out, so I’ve preserved the connectors for future usage, and just cut the – and + cables in a way that they can easily be reconnected to the connector if I ever want to. They were expensive motors so I didn’t want to ruin them!

If anyone is wondering why I didn’t attach standoff cylinders to the controller’s super large heat sink rather than attaching it directly to the acrylic base [which would normally be a bad idea], it’s because I didn’t have any standoff’s left, and the controllers are capable of 25A per channel. I will never drive them at higher than 4A, and the motors running on 4A for 30m or 2A for 2 hours solidly as a test didn’t generate any noticeable heat on the heat sink at all. At first I had also predicted the use of a fan to suck air in from the base, but I’m not sure it will be necessary, as nothing seems to get remotely hot so far..

I’ve also slightly indented the 4 points where the acrylic cylinders will be glued, just for extra stability. The motors are all wired to the two motor controllers, which has a junction box waiting for 12v now. The picoPSU should arrive some time this week, so hopefully I can get on with it.

The wheels are omnidirectional as they contain rollers. It’s a clever design and it seems to work well. Infact, I’m pleased with the way the motors and wheels ended up. Instead of having to work with two wheels and spending time on calculating angles for servo motors and turn radius, I can just attach 4 motors instead in the configuration that I have and using omnidirectional wheels. The motors will pull a lot of weight and I only have to concern myself with backward and forward for each motor, which in any combination will allow it to move in any direction. Hey, I’m not saying that I ‘invented’ this ingenious combination, just taking the credit for a smart move in implementing it! I have connected a power source directly across each of the motors to test. They are straight, and when I turn them all in the same direction, the board rotates around a ‘very almost perfect’ fixed axis which is great. I had in mind when I was positioning these, that I didn’t want to spend a ton of time in the software compensating for wheels that aren’t straight.

The motor controllers are Sabertooth 2×25A controllers. I only picked them as they support RC input, Simple Serial, Packetized Serial and analog voltage which means they have a lot of future potentially. I’m going to be controlling them via one of the serial methods, and I’ll decide which when the USB to TTL chips arrive. I’ve got one onboard serial bus and port only and I intend to use it as a console throughout so I won’t be able to use it for anything else, USB to TTL should be fine, I’ll only need to connect the ground and tx pins to the green terminal block you can see in the picture on the left. Unfortunately there’s nothing you can read from the controllers which would have been a really nice feature.

Now the most important things I am waiting for to arrive are the picoPSU, USB to TTL adapters and a 12V/10Ah lead acid battery and charger. Other than that I’ve only got misc bits on order such as LEDs, small mics for the ‘ears’, some relays, etc. Here are some more pictures:

Wheel Base

Base

Tags: 2x25A, analog voltage, battery, ground, lead acid, Linux, Linux robot, motor, motor controller, omnidirectional wheels, packetized serial, picopsu, rc input, sabertooth, simple serial, the robot, tx pins, usb to ttl

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