The Canadian motion control specialist Agile Technologies has received two US patents which, it says, will help to turn low-cost two-phase stepper motors into sophisticated servo motors without needing external sensors, as well as improving the robustness of sensorless motor drives.

The first patent covers a technique that allows open-loop microstepper control to change to closed-loop control once it has gathered enough data from the motor. "The transition from open-loop to closed-loop was the hardest part of the design, therefore we had to make sure the transition was seamless and robust," says Dan Rose, Agile’s application engineering manager.

An algorithm calculates the position of the motor’s rotor using sensing circuitry in the amplifier. When the motor reaches 30–60 rpm (depending on its design), it switches to servo control. Applying servo techniques to a stepper in this way is said to result in faster acceleration and deceleration, improved efficiency, better stall detection and prevention, and potentially higher speeds.

Agile has already applied the newly patented technique in its Silentstep controller launched last year and available in the UK from LG Motion.

According to Tom Geiger, Agile’s vice-president for sales and marketing, the new technology "will allow customers to specify motors at their full torque limits. This means that electrical engineers no longer have to over-size the motor for any given application – smaller-sized stepper motors can do the job of larger ones".

Agile’s second new patent is the first of five pending applications covering various aspects of motion control. It describes the removal of motor-induced noise to produce a clean back-EMF signal that can be used to optimise the commutation of brushless DC motors. The results are claimed to include 100% start-up reliability and better low-speed control.

"This breakthrough technology will allow for more robust running of motors, increasing their speeds and efficiency," says Chris Butlin, Agile’s vice-president of engineering. "Permanent magnet motors that induce a lot of noise from saliency can benefit greatly from this technology by being able to acquire a clear back-EMF signal where, formerly, this saliency would contaminate the back-EMF signal."