Capacitive Technology Finds Use in BLDC Motor-Commutation Controls

Appears in Print As: Capacitive Commutation Catches On

January 13, 2011

Capacitive-commutation techniques now replace optical and Hall-effect circuitry in brushless-dc motors.

Authored by:
James Seiler
Motion Control Product Manager
CUI Inc.
Tualatin, Oreg.

Edited by Robert Repas
robert.repas@penton.com

Key points:
• Capacitive sensing offers low operating currents, broad programmability, excellent temperature range tolerance, resistance to airborne contaminants, low cost, and extreme durability.
• Commutation points of conventional Hall-effect and optical encoders must be mechanically aligned with the rotor position while capacitive switching points are programmable, speeding setup and alignment.
• Nominal accuracy of Hall devices degrade with a greater number of poles, reducing motor efficiency.

Resources:
CUI Inc., www.cui.com
“Here Come Low-Power Encoders,” Machine Design, May 19, 2009, tinyurl.com/2aknjx3

Capacitive-sensing technology is certainly nowhere new to the world of electronics. Its first wide use came in the 1960s with capacitive touch switches. But capacitive sensing is new to the field of rotary encoders that measure position, distance, direction, and speed in motor-control applications. Once exclusively served by Hall-effect sensors and optical technologies, commutation encoders now benefit from the use of capacitive technology.

The advantages of capacitive sensing include low operating currents in the microamp range, broad programmability, excellent temperature range tolerance, resistance to airborne contaminants, low cost, and extreme durability. However, capacitive sensing can be susceptible to EMI/RFI and water. Both problems are correctable with mechanical supplements such as water seals and EMI/RFI shielding.

My first experience with a capacitive-sensing-control device was in 1965. The new bank in town used a marble facade next to its elevators. Embedded in the marble were two concave depressions, one marked “Up,” the other “Down.” There were no buttons — just the two small depressions in the solid marble. Despite this, the up-arrow lit when my finger touched the “Up” depression. Buried beneath the marble plate was a copper disc that created one plate of a capacitor. My finger became a second plate of the capacitor when placed on the marble that acted as the dielectric.