FOR YEARS, A DESIGNER TRIED TO THINK OF A WAY TO MAKE ENCODERS CHEAPER AND MORE ROBUST. NOW HE THINKS HE HAS ONE: EMBED IT IN THE VERY STRUCTURE OF A HIGH POLE COUNT AC MOTOR. THE RESULT IS A RUGGED MOTOR THAT PROVIDES 32,000 POSITION COUNTS PER REVOLUTION.
The ability to fine tune a motor's position comes with a high price tag, especially in smaller motors.
It takes an encoder, which typically costs as much as twice the price of the motor itself, and it is also the most likely part of the power system to fail, according to Don Labriola, president of Quicksilver Controls of San Dimas, Calif.
Labriola spent years looking for an alternative.
"I knew that when we energized a motor, it knows where to move," he said. "So something in its magnetic structure has that information."
The solution came to him while visualizing the inside of a motor during a long, boring drive. He could visualize the windings he needed. When he got back to his lab, he cut apart a few motors with a Dremel tool, stripped some wire, wound it into shape, and removed an insulating coating with fingernail polish remover.
Those first sensors showed that his idea might work. He then began designing sensor coils that he could integrate into the stator's windings.
Labriola started with his company's stepper motors. These motors divide each rotation into 100 or so steps, and they can move to these steps without a feedback sensor. Labriola, however, drives the motors with a four-quadrant vector controller (similar to a variable-frequency drive) that makes his motors act like servomotors. The drive takes advantage of the changing magnetic flux as the rotor teeth pass the stator teeth to provide very fine control over position. This is called microstepping, and it achieves up to 8,000 position counts per revolution.
Once Labriola embedded the encoder sensors into the motor's stator, taking measurements was a matter of timing. He likens it to running between a moving elephant's feet.
Here's how it works. The same processor that controls the servo drive voltage also controls the timing of the analog-digital converter that reads the voltage from the sense coils. So Labriola takes his measurements in between servo pulses, after the rotor has time to settle into position and 30 to 40 nanoseconds before a new pulse moves it to the next position.
"As long as you know and control the timing of the servo signals, and synchronize the readings, then a solid signal is available," he said. "We do not even need to shield the sensor wires from the motor wires for good signals."
The resulting NEMA 23 hybrid servo achieves 32,000 position counts per revolution at a lower price than conventional servo-encoder combinations. Since the encoder is part of the motor body, it is not vulnerable to misalignment or dust. It also retains the high sustainable torque at low speeds for which stepper motors are known.
Potential applications range from machine screws and belt drives to medical devices and solar panels. Labriola is working to expand the range of embedded encoder motors and improve their performance with high inertial loads.