<<BACK TO PREVIOUS

Control for brushless motors needs no sensors

Electrical drives are widely used in cars. Up to 100 motors can be spread in a vehicle in body and powertrain applications. As the brushes in standard type DC motors are prone to attrition, brushless motors (also referred to as permanent magnet synchron motors or PMSM) would offer an additional advantage on top of being more compact and more efficient.

Yet brushless DC motors need complex sensor system technology and the processing of feedback information on the current position of the rotor for commutation and speed control. This has held back their widespread use.

A new way of sensing the rotor position and commutating brushless motors could change that. Elmos Semiconductor has developed a demo-board that can activate a motor without the need for sensor technology. From this new demo-board, called VirtuHall, the integrated circuits specialist intends to develop a family of motor driver ASICs and standard driver ICs. The development of a first chip has already started.

“The new board is being further developed. Our new proprietary principle has been in use for three or four months now,” said Erhard Müsch, chartered engineer and vice president of applications and systems for the company.

The board is suitable for brushless DC motors of all types and can control any motor size down to a 2.5W pump drive. It continuously detects the speed and position of the rotor and can either control the rotor commutation directly or provide the data via an interface to a microprocessor.

To detect the rotor position VirtuHall uses a fairly simple procedure, based on the fact that phase inductance depends on rotor position. The board detects phase induction relations by measuring neutral point voltages during the application of micro second test pulses to the windings. By sustaining the test pulses, the rotor position is detected during operation. A single algorithm serves to interpret the change of induction from rotor standstill to maximum rotor speed.

In contrast to existing procedures for rotor position detection VirtuHall has no critical transition area in which, for example, forced commutation in the low rev field is followed by a different measurement method in the higher rev area. Typically any such transition can result in unwanted disturbances.

The new detection principle results in a 30° resolution of electrical angle by simple voltage comparison. “By interpretation of analog values this resolution can be greatly increased,” said Müsch. “For our way of detection it does not make a difference whether the motor runs forward or backwards.” The measuring principle is also insensitive to voltage influences during generator operation.

The demo-board kit consists of driver software that enables a Windows computer to activate the system as well as the hardware for testing the new principle. The board provides connections for CAN, LIN, Ethernet and USB.