Brushless DC motors are now commonplace and are typically driven by signals supplied to the stator windings of the motor in accordance with the outputs of one or more Hall-effect sensors which detect the alignment therewith of the rotor poles of the motor.
Thus the requirement for brushes to effect commutation of the driving signals is obviated.
However, at low rotation speeds, smooth and stable control of the motor becomes difficult due the substantial time delays between successive changes of state of the Hall-effect sensor outputs.
We have now devised an arrangement which overcomes the limitations of existing control systems for driving DC brushless motors.
In accordance with the present invention, there is provided a control system for driving a brushless direct current (DC) motor, the control system comprising means for providing a measurement of the rotational speed of the motor by combining a first signal representing the rotational speed of the motor derived from a measurement of the back-EMF developed in the stator windings of the motor and a second signal representing the rotational speed of the motor derived from the output of means which sense the position of the rotor of the motor, such that as the rotational speed of the motor is increased from a first rotational speed to a second rotational speed, the combined speed measurement is determined to a greater extent by said second speed signal and to a lesser extent by said first speed signal, and means for supplying driving signals to the stator windings in accordance with the combined speed measurement and the position of the rotor of the motor.
Preferably at said first rotational speed the combined speed measurement is determined from said first speed signal only and preferably at said second rotational speed the combined speed measurement is determined from said second speed signal only.
Whilst the respective proportional contributions made to the combined speed measurement by each of said first and second speed signals may vary in one or more discrete steps between said first and second speeds, the respective proportional contributions preferably vary continuously with increasing or decreasing motor speed, thereby avoiding any abrupt transitions in the combined speed measurement.
Most preferably, within a region between said first and second speeds, the respective proportional contributions made to the combined speed measurement by each of said first and second speed signals vary linearly with respect to one another.
Preferably the position sensing means comprise one or more position sensing transducers. Most preferably the or each position sensing transducer comprises a Hall-effect sensor for detecting the alignment therewith of each of the rotor poles.
Also in accordance with the present invention, there is provided a method of driving a brushless direct current (DC) motor, wherein driving signals are supplied to the stator windings of the motor in accordance with a measurement of the rotational speed of the motor and the position of the rotor of the motor, the combined speed measurement being determined by combining a first signal representing the rotational speed of the motor derived from a measure of the back-EMF developed in the stator windings and a second signal representing the rotational speed of the motor derived from the output of means which sense the position of the rotor of the motor, such that as the rotational speed of the motor is increased from a first rotational speed to a second rotational speed, the combined speed measurement is determined to a greater extent by said second speed signal and to a lesser extent by said first speed signal.
An embodiment of the present invention will now be described by way of an example only and with reference to the accompanying drawings.