I laid out the bottom bat to hold the motors and some sensors. The two holes needed to be large enough for the whole wheel to pass through, in addition to providing some clearance if the tires squish or flex. Cutting the plastic bat was done using a Dremel tool, but a saber saw with a fine toothed blade would have worked better. The plastic cuts very easily, even with coarse blades. Cutting at a slow speed to prevent the plastic from melting and gumming up the blade will actually be quicker.
I aligned the top and bottom parts for drilling the holes for the stand-offs. I clamped both bats together and drilled them at the same time to insure that the holes line up. One accessory I wanted early on was a compass, so I used all aluminum hardware to minimize interference. I was able to buy 8-32 aluminum countersunk screws at the hobby shop; they are used for radio controlled cars.
The bottom didn't have much clearance, so using countersunk screws became a requirement. It is really easy to countersink the plastic using a countersink bit — even a larger (i.e., 3/8") bit would work. Using countersunk screws does give the robot a more professional look, too.
To mount the original gear motors, I put the flat, bottom surface against the bottom bat. I cut out two 1/2" x 4" x 0.032" pieces of aluminum — one for each motor. I drilled a 1/8" hole in each end and wrapped it around the gearbox. Then, I drilled in the bat on each end for mounting screws. For mounting the precision motors, I layed out a pattern on some 0.063 aluminum and cut two identical pieces.
System block diagram.
System block diagram.
Bending aluminum takes care. Hardened aluminum — like 2024-T3 or 6061-T6 — requires a setback when bending. A sharp bend will cause the aluminum to crack. Aluminum also has a grain that can be seen in the light. Tighter bends can be made perpendicular to the grain.
To attach the wheels to the motors, I needed a way to adapt the shaft diameter to the wheel hole size. Brass tubing (available at most hobby stores) is ideal for making adapters like this. The metric shaft of the precision motor fit loosely in some 5/32" outside diameter tubing, which fit snugly in the center of the wheels. I left the tubing about 1/2" too long on the outside and used a 5/32" du-bro collect near the motor.
When the collect was tightened, it squashed the brass tube to the shape of the shaft. I was careful to align the set screw of the collect with the flat on the motor shaft. The excess shaft sticking out of the wheel was flattened with pliers and bent over. I drilled a 3/32" hole in the tube and the hub of the wheel. Through this hole I put a 2-56 x 1" threaded rod and tightened some nuts around it. This provides positive wheel movement.
When I bought the Handy Board kit, it came with some optical sensors. I wanted to use these optical sensors for wheel encoders. I added a piece of plastic on the wheel side of the motor mount to allow attaching these sensors. For the actual encoder disk, I drew four intersecting lines in Xfig (www.xfig.org) and printed them out on a laser printer. These pieces of paper were glued to a plastic disk, slightly smaller than the wheel. I drilled a hole 5/32" diameter in the center of the disk and sandwiched it between the collect and the wheel with a little silicon glue.
I used 4-40 screws to attach all the smaller items to the bottom. The precision motors required 2.5 mm mounting screws to attach the motor to the bracket. All screws that penetrate the bottom are countersunk. The bats are about 1/4" thick, so countersinking too far is not a problem.
The chassis will vibrate constantly when the robot is in motion, so all screws are required to be locked in place.
Introducing the Pololu USB 16-Servo Controller
The features you need in a size that fits your robot:
- Control 16 R/C hobby servos - Dual USB and UART interface - Independent speed and range for each servo - 0.5-microsecond resolution - 50 Hz update rate - Upgradable firmware -Small size of 1.9" x 1.0"
Our new servo controller plugs into a USB port and looks like a serial device to your software; when you are ready to disconnect your robot from the computer, the servo controller can also communicate serially with your robot controller.
See www.pololu.com for a complete line of servo controllers starting at only $27.
Circle #100 on the Reader Service Card.
Robotics & Electronics
Was this article helpful?