Described below are a method and a device for driving a brushless direct-current motor during the displacement of an actuating element.
Brushless direct-current motors are already well-known. In the motors, the otherwise customary mechanical commutator, which has brushes for reversing the current direction, is replaced by an electronic circuit. Furthermore, brushless direct-current motors have a rotor equipped with permanent magnets and a stator having windings. The windings of the stator are energized with a time offset by the electronic circuit in order to generate a rotating field which produces a torque at the permanently excited rotor. The commutation pattern by which the stator windings are energized is determined on the basis of an evaluation of rotor position signals which are provided by Hall-effect sensors and supplied to the electronic circuit. The latter energizes the stator windings by way of drivers or amplifiers which have, for example, power transistors or power MOSFETs as electronic switches.
DE 10 2006 003 151 A1 discloses a control method for a brushless direct-current motor in which a desired number of revolutions of the brushless direct-current motor is predefined, the number of revolutions is converted into a required number of revolutions, a first supply current activating the brushless direct-current motor is supplied thereto, the revolution pulses generated during a rotation of the brushless direct-current motor are counted, the number of counted revolution pulses is compared with the number of required revolution pulses, and a second supply current dependent on the comparison result is supplied to the brushless direct-current motor. In order to bring the brushless direct-current motor to a stop, the rotation of the motor is halted either by setting the supply current to zero or by supplying equal multiphase currents to the stator windings.
Furthermore, it is already known to employ brushless direct-current motors as valve drives or throttle actuators. During their lifetime these are supplied with current for the majority of the time. In the absence of suitable measures, the torque required in the holding mode of the drives or actuators in order to keep them stationary is generated by the amplifier. This circumstance generates losses both in the motor and in the amplifier.
If the brushless direct-current motor displaces the actuating element from a position A to a target position B, then a position control function is active. In this case the motor is driven by a driving commutation pattern derived from rotor position signals. The rotor position signals are in this case derived from the output signals of, for example, three Hall-effect sensors. In principle it is possible to hold the actuating element at its target position by a control function of the type. However, the motor always moves a little due to the comparatively coarse position resolution, which is attributable to the fact that only the Hall-effect signals of the Hall-effect sensors are evaluated. This has a negative impact on the life expectancy of the gearing mechanism and also on noise generation. Consequently, after the target position is reached the brushless direct-current motor transitions into a holding mode in which it is driven by a commutation pattern providing a required holding torque.
It is disadvantageous in this case that a continuous flow of current must be provided to produce the holding effect. The strength of the current, i.e. the holding current value, is based in this case on the maximum holding torque which the brushless direct-current motor is required to make available according to its datasheet. Furthermore, a reserve must additionally be factored in to enable tolerances in series production to be compensated for.
Depending on the installation position and weight of the displacement element, the required holding torque, which holds the displacement element in its target position, may be very small. If the mechanical friction present is sufficient in itself to hold the displacement element in its target position, then the holding torque can have the value zero. With known drives, in a situation of this kind, the motor is nonetheless supplied with current at the full holding torque. This is uneconomic from the energy perspective.