YABB 0.1

First bot should be a "quick and dirty" balancer that tries to either maintain the same position, or tries to maintain zero velocity, whichever is easier.

Body built from cardboard. Solarbotics GM9 motors and L298D motor controller. Standard 9.6V toy RC type battery for juice. Arduino controller. Wrote code to read the H48C. Wrote code for very simple PID controller based on a single prior value. Reused code to drive the motors using 3 wires per motor: enable, forward, reverse.

Motors are typically controlled by 3 wires: right high, left high, and enable. Enable makes the motor responsive to the right high and left high control wires. If enable is low, nothing happens no matter what the right and left control wires say. Right high directs power from the h-bridges high side on the right to the low side on the left. Left high directs power from the left high side to the right low side. If right is high and left is low, the motor spins in one direction. Visa versa and the motor spins in the other. If both are the same the two sides of the motor are connected to each other, and the motor brakes. That is, turning the motor generates an electric current that resists the turning. The resistance acts as a brake and regeneratively recharges the battery. Speed is controlled by PWM'ing the enable line. That is, turning it on and off rapidly. Longer the enable line is turned on relative to the time it's turned off, the faster it goes.

Bought a Parallax H48C 3 axis accelerometer from Hobby Engineering. At the same time purchased a Parallax HM55B compass module.

Problem: Debug output is scrambled. A lot of electrical noise from the motors. Trashes the serial connection between the bot and computer. I now suspect that electrical noise from the motors destroyed a bluetooth module I tried on a previous bot.

Purchased some opto isolators from Jameco. Also some OR gates so I can move from 3 wires per motor to two wires per motor: forward, reverse. Need a second battery for logic, just using a 9volt. Now I get clear debug output. While I was at it I also added "or" gates so that I could control the motors with two wires each: E = R or L. Only isue was that I now have to PWM both R and L rather than just E.

That fixed the noise issue, but later on I found out there are other was to do this that might have worked just as well. Primary being a .1uF capacitor across the motor leads. And a better power supply circuit. I may try those later.

Problem: Doesn't balance. Tends to over shoot. Tried tuning PID kp, ki, and kd parameters to no effect.

Perhaps the bot body is too short, making it very hard to drive the motors under the bot. As a friend said, it's easier to balance a pool cue on your hand than a pencil. Rebuild bot using 1/4" x 3" x 3' piece of birch, with the weight at the top. Batteries, breadboard, motor controller.

Problem: Still doesn't balance. In fact, no apparent change.

Perhaps motors aren't powerful enough. Replace GM9 motor with RM2 motor so that speed and torque aren't a problem. 3 times the torque and twice the speed. And a huge current draw at stall ... L298 tends to overheat. And have killed a few RM2's by stalling. At some point I need to put in a resettable fuse so I stop killing these things.

Problem: Still doesn't balance. In fact, no apparent change.

Careful observation shows that H48C readings do not reflect actual tilt when bot is falling. DUH! while falling there is no substantial change in acceleration. Only when I catch the bot do the X and Z readings change a bunch! Accelerometers alone are too slow. Gyros are supposed to be fast and are "supposed" to be used to balance bots. So, I purchased an IDG300 3 axis accelerometer and 2 axis gyro from SparkFun. I also wonder if it could be done with IR sensors. So I bought some of those from hobby engineering. Haven't done much with them yet, but I will. The IDG300 is $110 a pop. The IR sensors, $21 for a pair. The difference will be important if I have a dozen + bots flocking around.

And that catches us up to Jan 1 ...