Servo - May 2008

engine. Figure 3 is a motor that I saw advertised on eBay. It is a Curtis & Crocker bipolar motor from the late 1880s that I believe is a fan motor. It was being sold as a collectable. Many electric motors of that era had open structures that exposed the internal wiring to errant fingers. Today’s liability lawyers would salivate over seeing such a motor on today’s market.

Motors for Robots

Steam engines, hydraulics, pneumatics, spring-wound motors, shape-memory actuators (Muscle wires), solenoids, and even twisted rubber bands have powered experimental, industrial, service, and toy robots for years, but we really rely on the good old electric motor for robot power. And that ‘good old motor’ was usually a DC permanent magnet (PM) motor. I won’t go into detail how each type of electric motor works, as I am sure the vast majority of SERVO’s readers are quite familiar with the repulsion and attraction of the motor’s armature poles and the field poles. We all know how the commutator at the end of the armature of a DC motor allows the brushes to continually change the magnetic fields on the armature to keep it spinning. There are numerous links on the Internet that will go into depth on all types of DC and AC motors.

Back in the ‘70s, robot builders relied on surplus businesses, junk yards, car parts, and toys for their motors. Motors have always fascinated me and I had amassed over 500 electric motors and gearmotors in my collection several years ago; most were from surplus places. I got to thinking, “why does anybody need so many motors? Am I ever going to build a hundred different types of robots?” Since I couldn’t come up with any good answers and with my wife’s urging, I managed to cull my collection down to about 50 of the best ones and then gave a bunch away at Seattle Robotics Society meetings, sold a bunch, and donated some to schools.

from a favorite surplus place of mine in Pasadena, CA that was about a 40 mile drive north from my home at the time in Long Beach, CA. C&H Sales had thousands of military surplus motors and the staff knew me well enough to let me wander in the back of the store, plowing through bins of complex military mechanisms to pull out the best motors. I could test them on-site for speed, torque, operating voltage range, current at various loads, and mounting methods. I almost always chose gearhead motors as these didn’t require a separate attached gearbox or belt reduction.

Some of the best gearmotors were planetary drives made by TRW and other suppliers to the government. Something that I found out rather quickly is a person cannot go by the listed current draw or voltage to determine the actual torque and output speed of a particular motor. As an example, a particular gearmotor may draw one amp at 28 VDC at full load and 20 RPM with 100 oz-in of torque, and another only 500 mA at 28 VDC for the same output. Look at a surplus motor catalog and you’ll see what I mean. Choose wisely.

Quite a few of these motors — especially the military variety — were 24 or 28 VDC, but would run nicely at a reduced voltage such as 12 VDC. Pittman made (and makes) a great series of gearmotors that operate nicely at 12 VDC, and I used many of these for robot appendages, grippers, and head motion applications. C&H has shrunk quite a bit in the past few years and now sells most of their stock on eBay. In the ‘80s, they had a great monthly catalog that went to quarterly several years later and was a great reference for their motors. Considering that these motors were surplus, they probably had been sitting on shelves or dark military warehouses for years and were not constructed using the latest technology.

AlNiCo and ferrite magnets seemed to be the technology for PM motors in those days. Universal motors

FIGURE 3. Curtis & Crocker bipolar motor.

were one of the most popular military surplus styles, though these motors were not particularly good for robot applications. They used an electromagnet winding for the field poles instead of permanent magnets and could operate on AC, as well as DC current. Vacuum cleaners still use this type of motor. Don’t use these for robots.

Another popular type of DC motor was the pancake motor or printed circuit armature motor. These motors have the advantage of compactness that is especially nice when coupled to a geartrain. They have low weight, are fairly energy efficient, and start and stop fast due to low inertia of the armature. I was given a box of PMI pancake motors that I used for all sorts of projects. I did find that they could be damaged when they were overloaded as the armature did not have an iron core to soak up and dissipate excess heat. I found this out the hard way when a pancake motor I was using had the copper-wound armature so cooked that it was warped.