Tamiya Twin Motor Gearbox Rotary Encoder Project

My first small robotics project used modified RC Servos to drive the wheels. Although this was a simple approach, is wasn't very satisfying when it came to speed or position control. I really wanted to use a drive system that would provide me some feedback. I searched around and purchased some inexpensive gear motors with the idea of adding shaft encoders. There were some commercial encoder add-on out there, but they were pretty expensive and not very elegant solutions.

One particular drive system that caught my interest was the [Tamyia] Twin Motor Gearbox (TMG). This is essentially a plastic kit that incorporates two small electric motors and dual sets of plastic gears that could be configured for 58:1 or 204:1 reduction. I got a great deal on this item at [HobbyLinc.com], along with wheels and other accessories. Lots of robot sites sell this gearbox, and it can be used to run several plastic models (including a treaded Tank), and a cool round Lexan chassis from [Pololu.com]. I use the Pololu chassis for my Pucky robot.

I started researching Slotted Optical Switches. My biggest problem was that I needed the two fingers of the encoder to fit between adjacent gears, but I also needed them long enough to reach past the teeth and into the body of the gear. I had a very tight size requirement. After pouring through component catalogs (Digi-Key, Mouser, Newark etc) and searching the web, I finally located an optical switch from OPTEK that was small enough, and long enough to do the job. I ordered a set of the optical switches from Digi-Key and then set to work converting some gears into optical interrupters.

I made a little jig that I could use to hold a single plastic gear, and then I used a Spiral Cutting bit from my Roto-Zip tool in the drill press. I could plunge the drill down, and then rotate the gear with my fingers to cut a nice smooth slot.

When the Optical Interrupters turned up I was very pleased. The width and length were perfect. The only thing that was a little funky was the lead spacing. When you stacked two units next to each other, they didn't enable a 0.1 Inch spacing, the housings were a tiny bit too wide. Not to worry, a custom PCB would deal with that.

So next I made a prototype circuit on some perf-board. The interrupter is a basic LED photo-transistor pair, so I started out driving the LED with 10mA, but I found that the IR light got clean through the gear plastic. So I dropped the drive current down to to about 3mA and with a 4.7K resistor on the Photo-transistor I was able to produce a decent logic level change as the gear rotated. I experimented with mounting, and found that I could mount the Encoder board 1/8" off the gear chassis and the gears would spin freely and produce a good encoder output.

To try out the design on a real robot, I purchased a 1/8" Acrylic Chassis from Pololu.com (which was designed for the Tamiya TMG) and cut a hole in the center to expose the gear workings.

The proto-board sits flush on the chassis. In the photo on the right, I've outlined the encoders with a green line to make them more visible. I outfitted the chassis with an OOBOT-40 board from [Oricom Technologies] and Pucky was born. I needed to use the OOBOT40 board because it had a Quad H Bridge driver on it that I could use to run the two motors. See my [Pucky1] page for more details.

Once I had proven the basic concept, I decided to make a custom PCB to clean up the circuit. On small projects like this I love to use the PCB service offered by [ExpressPCB]. For just $59 I can design a 2.5" x 3.8" double sided board and get 3 boards back in the mail in 3 days. Too Cool. The 2.5" x 3.8" board size is their "mini-board" format, and since my encoder boards were really much smaller, I just duplicated the design to fit 3 encoders on one board. Since I had about 1/2" of unused space at one end of the board I decided to add a SN754410 Quad H-Bridge driver to the board and run the control signals to a common 20 Pin header. I arranged the signals on the header to make it easy to connect it to a MarkIII OOPic controller board (my favorite) with a straight through ribbon

At the left is a picture of the final boards after they had been separated. Notice the two narrow "Encoder only" boards, and then the larger "Encoder + Driver" board.

On the right is the populated Driver + Encoder board. I bolted this board directly to a Twin Motor Gearbox, using the 1/8" spaces that happened to come with the Gearbox kit. To convert it into a stand-alone robot, I added a plastic chassis (cut from a "For Sale" sign), a MarkIII OOPic controller board, and built my first Line-Follower. Naturally I had to go out and write an OOPic UserClass for reading the encoders and controlling the Twin Motors. You can read more about these projects once I create pages for them.