Servo - July 2009

point A the second phase is logic high. We’ll call this reverse. My decision about which quadrature sense line is the “first” one, and which logic level is which direction, are both arbitrary and can be set up in any way that is most convenient to programming.

The Test: Can We Actually Measure the Distance?

Okay, we’ve got the code working, now does the odometry actually work? Let’s do some basic math first. I measured my wheels and they are 5 cm in diameter. This means that the circumference of the wheel is πD, which is 5π = 15. 71 cm. In Listing 3, we see that I tell the robot to go forward 1,000 tics and then back in reverse 1,000 tics. If we have 360 tics per revolution, this means that 1000/360 = 2. 78 wheel revolutions; 2. 78 x 15. 71 cm is 43. 6 cm. On my wood floor, I marked out 43 cm with a couple of loose pieces of scrap debris and hit the reset button. My little robot faithfully rolled forward and stopped right at my mark and rolled back, but overshot its starting position because the flywheel in the LEGO NXT servo motor just kept on turning after I shut off the power.

The way the 754410 works is that it has no brake so we just drifted past. I decided to add a little braking power by running the motors forward again for 100 ms to act as a brake before I turned off the power. This worked

reasonably well and my robot stopped on the dime right where it started.

After running the motors to show that I did indeed achieve a well functioning system, I put a loop at the bottom of the main() function so that I could see the tic value changing and the direction bit changing when I spun the wheels. This was, after all, just a proof of concept experiment. For those faithful readers of Mr. Roboto, you may remember the November ‘08 column where I described a simple PID algorithm that could be used to control a motor’s speed. In that article, I described a sensorless system that measured a motor’s back EMF to gauge speed and direction. You can simply use this motor’s feedback sensor data directly in those calculations to give you another way to do a PID based motor control system. I’ll leave that as an exercise for the student, unless I get the urge to do such a thing and write about it …

It is so easy with these motors, it almost seems like cheating. The LEGO NXT servo motor is about $19 and you get a motor, gearbox, mounting system, simple axle, and quadrature feedback all in one package (you purchase the wheel connection separately). The motor has decent speed and torque for small robots and it isn’t a power hog either. Keep those questions coming to roboto@servo magazine.com and I’ll do my best to answer them. Have fun and keep working on those robots! SV