Servo - July 2009

favorite is the two-tiered Kik-step rolling step stool (Cramer Industries, www.cramerinc.com), about $50 from Amazon and other online dealers. I picked up a stainless model, but they’re available in 10 colors. Regardless of the finish, the round step stool is stable and virtually indestructible. The three supporting braces between the two steps provide ample protection to any electronics mounted to the top of the first step, and the spacious, hollow underside is perfect for a pair of drive motors and wheels and battery pack.

I mounted a CrustCrawler Smart Arm ( www.crustcrawler. com) on the top of the upper step, with a 7.2V NiMH battery mounted to the underside of the top step. I mounted ultrasound sensors from Parallax and an Atmel processor card on the larger first step, sharing the battery with the Smart Arm. The spacious underside encloses a pair of six inch wheels driven by Parallax 12V servo motors ( www.parallax.com) and two, 7.2V NiMH battery packs.

The Kik-step isn’t the perfect mid-sized robot platform. For one, steel is heavier and a little harder to work with than aluminum. The second is that, at an elevation of 14 inches, my CrustCrawler Smart Arm can’t interact with objects at floor level. These modest limitations aside, the price is right — especially if your goal is creating a swarm of mid-sized robots capable of autonomously playing bumper-cars.

The Kik-step is just one example of how you can save significant money by repurposing non-robotic hardware for your next robot platform. If you’ve developed a robot using an inexpensive, readily available platform, please send me a note so that I can share your experience with other readers. SV

Dear SERVO:

First let me say "great job" to Dennis Clark in Ask Mr. Roboto! I am a professional engineer and I play with robots on the side. I really enjoy his column and I learn something useful every month.

I am writing about the question that Sam Browman submitted and which was answered in the May ‘09 issue, about a mystery motor/ encoder. I have some professional experience with this sort of device and I have some information that you may wish to forward to him.

What Sam has, is, as Dennis noted, not a servo but only a servo motor. Surplus units usually consist of a brushed DC motor and a quadrature encoder (most new servomotors are electronically commutated, a.k.a., "brushless"). Sometimes the encoder also has a home (0 degree position) sensor — sometimes not — for use in absolute positioning applications. These motors are designed for use with a dedicated servo controller that contains an H-bridge to drive the motor, and logic (microcontroller or FPGA/ASIC) to receive position and speed commands, and carry them out by reading the motor's encoder and applying a PWM signal to the H-bridge to drive the motor in the correct direction at the correct rate. If Sam Googles "Servomotor Controller," "Quadrature Encoder," "H-bridge," and "PID loop," he will come up with a lot of information on this type of device. The Microchip.com site also has a lot of very useful info about building a servo controller from scratch, since some of the 24 and 33 series micros are designed specifically for motor control.

A five wire device is going to be a digital quadrature encoder rather than a tachometer. The encoders are usually wired as follows:

sometimes up to + 24. Most will run at 5V.

Black: Logic ground.

Green: Motor frame ground (usually).

Yellow/White: Digital outputs for Phase A and Phase B (no way to tell which is which without testing). Phase A and B are identical except they are 90 degrees out of phase with each other.

Quadrature encoders usually include all necessary debouncing and provide a clean square wave logic level output. When Phase A leads Phase B, the motor is (usually) turning clockwise as viewed from the front. When Phase A lags (follows) Phase B, the motor is turning the opposite direction. So, the phase is used to determine motor direction. Motor speed is determined by integrating one phase output (counting pulses over time). Relative (change in) position is determined by counting phase pulses; adding when A leads B and subtracting when B leads A. In some cases, absolute motor position can be determined if there is a "home," or 0 degree sensor. When the home sensor is activated, the position is reset to zero and you count from there. More often, the home sensor is not on the motor but on the driven wheel or pulley.

Sam will have to determine the resolution of the encoder, that is, pulses per revolution. This can vary from very low, say 16 pulses per revolution, to very high, say 2,048 or more. There is no standard, though the number (if any) marked on the encoder may give it away. He can disassemble the encoder and count the slits (optical) or protrusions (magnetic) on the encoder disc if it is low resolution. High resolution slit discs cannot be counted by eye. In that case, he is probably better off