GB2167253A - Method and apparatus for noise-quieting in brushless DC motors - Google Patents

Abstract

A circuit for driving a brushless DC motor which reduces the interaction of axial forces between the motor windings (W1, W2) and the permanent magnet rotor. The circuit provides feedback of the back EMF developed by the motor winding (W2) from which power is being removed to the motor winding (W1) to which power is being applied. <IMAGE>

Description

SPECIFICATION Method and apparatus for noise-quieting in brushless DC motors The present invention relates to electronic circuitry for driving brushless DC motors. In particular, this invention provides a method and circuitry for quieting audio frequency noise produced by such motors when driven by conventional circuit configurations.

At least one source of audio frequency noise produced by brushless DC motors is caused by the interaction of forces set up between the motor windings and the permanent magnet rotor when driven by conventional circuitry. Typically, convention circuitry comprises power transistors which alternately draw current through the motor windings from a power supply on demand derived from a signal produced by a Hall effect device as the rotor rotates. This scheme simply draws required current through the windings to control motor speed.

According to one aspect of the present invention, there is provided apparatus for driving a brushless DC motor having a plurality of windings, said apparatus comprising driver means for sequentially applying drive current to the windings of the motor; and feedback means, coupled to the driver means, for applying the back EMF developed by the motor winding from which drive current is being removed to the motor winding to which drive current is being applied.

According to another aspect of the present invention, there is provided a method for driving a brushless DC motor having a plurality of windings, said method comprising the steps of alternately applying drive current to the windings of the motor; and applying the back EMF developed by the motor winding from which drive current is being removed to the motor winding to which drive current is being applied.

In the accompanying drawings: Figure 1 is a schematic diagram of the motor driver constructed according to the principles of the present invention; and Figure 2 is a timing diagram of control and drive signals for the motor driver circuit of Fig. 1.

The spindle motor driver circuit of Fig. 1 energizes motor windings W and W2 in response to signals produced by a Hall effect device and a microprocessor. More particularly, a first cyclic control waveform 23 determines which winding W, or W2 is energized on the basis of the rotational position of the rotor as sensed by the Hall effect device (not shown); a second cyclic control waveform 24, of lower frequency than the control waveform 23, is used to slowly increase and decrease the average level of energization of the windings W and W2 in order to assist stability of the motor speed. Both control waveforms 23,24 are generated by a controlling microprocessor (not shown).

A current source 10 comprises Q,, Q2 and R1. The base of Q2 is coupled to reference voltage V4 and the base of Q1 is connected to control signal 24. The emitters of transistors Qt and Q2 are commonly coupled to reference voltage V3 through resistor R,.

Referring to Fig. 2, when control signal 24 is high Q, is off (i.e. cut off) and Q2 is on (i.e.

active). The voltage at the emitters of Q, and Q3 is approximately 4 volts. In the present example, approximately 4.2 milliamps of current is available from the collector of Q2.

When control signal 24 is low Q1 is turned on as current flows from its base. As current flows through R1, Q2 becomes back-biased and is turned off.

The motor winding to which power is supplied is selected by comparators A1 and A2.

When control signal 23 is low, motor winding W1 is selected by comparator A,. Conversely, motor winding W2 is selected by comparator A2 when control signal 23 is high.

Transistor Q3 functions as a switch when the output of comparator A, is low. Base current drawn through R2 causes Q3 to saturate thus providing short circuit from its emitter to collector. Transistor Q4 functions in the same manner in response to low voltage at the output of comparator A2 Transistors Q5 and Q6 operate as Darlington pair 12 to provide power to motor winding W,. Thus substantial drive current can be provided in response to minimal control current applied to the base of Q5. Capacitors C1 and C3 and resistor R5 are used to control the rate at which power is applied to the motor windings and to provide feedback of back EMF produced by de-energized motor winding W2 for reducing audio frequency noise.An identical circuit comprising Darlington pair 14 (i.e.

transistors Q7 and Q8), capacitors C2, C4 and resistor R6 is provided to drive motor winding W2.

Referring again to Fig. 2, waveforms 20 and 21 respectively represent the voltage drive waveforms for the windings W1 and W2 (these voltages being those present at the node between winding W1 and capacitor C3 and at the node between winding W2 and capacitor C4 respectively).

When control signal 24 is low, current source 10 is off. Assuming motor winding W2 was energized just prior to control signal 24 changing from high to low state, minimum operating charge still exists in capacitor C4. If control signal 23 is high so that comparator A2 has caused Q4 to turn on, capacitor C4 then charges through resistor R4 to the base of transistor Q8. As capacitor C4 charges toward the voltage level V5, Darlington pair 14 is turned on and current flows in resistor R5.

Thus, the voltage at circuit node 15 is fixed at approximately 1 volt. Feedback from C4 assures that the voltage remains fixed as long as Darlington pair 14 is not saturated.

The rate at which C4 charges, and consequently the rate at which the energization of winding W2 is reduced over several switching cycles, is substantially determined by the current flowing through R6. The current into the base of Q8 and into C2 is negligible because of the high gain of Darlington pair 14.

If C4 charged faster, the current flowing to ground through resistor R6 would increase the base voltage of Os thus turning it on more. If O, is turned on harder, more power is applied to motor winding W2 which increases the voltage drop across W2 and forces the voltage at circuit node 15 to decrease. If the voltage at that node decreases, current through R5 decreases, which in turn reduces the base voltage of Q8.

With continuing reference to Figs. 1 and 2, when control signal 24 is high, current source 10 is turned on. If control signal 23 is also high, more power is applied to the motor winding at a rate primarily determined by the rate determined by C4 discharging through R4.

Thus, the current from current source 10 is divided through resistor R6, on the one hand, and R4 on the other. The amount of current flowing through R6 is determined by Vb, of Q6 divided by R5. The balance of the current available from current source 10 charges capacitor C4 through resistor R4. At this time, the voltage at the C4 R4 node 15 is fixed at approximately 0.3 volts. By making the voltage at circuit node 15 different when power is applied to winding W2 than when power is removed from winding W2, the stability of motor speed is enhanced.

Capacitors C2 and C4 allow coupling from the winding which the circuit is not driving to the winding which the circuit is driving by fixing the voltage at circuit node 15. The back EMF generated in the winding not being driven is inverted and applied to the winding which is being driven during the middle of each phase of control signal 23. See for example, motor drive voltage 21 driving window W2 during positive phase of control signal 23 shown in Fig. 2. Approximately 6 volts of back EMF is being added to motor winding W2 from motor winding W1 during the first full, positive phase of control signal 23.

Capacitors C1 and C2 control the rate at which power is switched between motor windings W1 and W2 (C1, C2 are much smaller in value than C3, C4). For example, when transistor Q4 turns off and transistor Q3 turns on in response to control signal 23 changing state, the Darlington pair 14 turns off at a rate determined by the discharge of capacitor C2 through R6. Thus, as voltage 21 rises, motor drive voltage 20 decreases at the same rate because capacitor C4 provides coupling to circuit node 15. Thus, the voltage being removed from motor winding W2 is transferred to motor winding W, in a relatively short period of time.Capacitors C, and C2 also protect transistors 0, and Q7 from voltage breakdown owing to high transient voltages produced by motor windings W1 and W2 if drive current 22 were reduced too rapidly when power is switched from one winding to the other.

Referring again to Fig. 2, drive current 22 is applied to motor winding in phase with drive voltages 20 and 21. Thus, current is switched from one motor winding to the other approximately coincident with a change of state of control signal 23.

As stated elsewhere in this specification, when control signal 24 is high, current source 10 provides current to transistors 03 and Q4.

Control signal 23 determines which path the current shall take. When control signal 23 is high, current flows through Q4; when control signal 23 is low, current flows through 03.

The source of control signal 23 is a Hall effect device which monitors the magnetic field of the rotor of the motor being driven to determine the appropriate winding to which power should be applied.

When control signal 23 is high and control signal 24 is low, current source 10 is turned off. When control signal 23 is high, Q4 effectively connects capacitor C4 to the base of transistor Q8 via resistor R4. Since no current is supplied by current source 10, Darlington pair 14 is turned off at a rate determined by the charging of capacitor C4 through resistor R6. The base current required by transistor QIl and the charging current-of capacitor C2 has negligible effect on the turn off rate of Darlington pair 14.

Capacitors C3 and C4 integrate current from current source 10 between the rapid phase transitions of control signal 24 to a slowly varying drive level 25 at the motor winding being driven. Thus, when control signal 24 is low, voltage drive level 25 linearly decreases; when control signal 24 is high, voltage drive level 25 linearly decreases and increases in phase with voltage drive level 25. It should be noted that voltage drive level 25 decreases as the negative magnitude of voltage 20 and 21 decreases.

The rate of integration by capacitors C3 and C4 is controlled by the current flowing through resistor R4 which current is the difference between the current from current source 10 and the current flowing through resistor R5 or R6.

Current flows from current source 10 when control signal 24 is high. Thus, the voltage on capacitors C3 or C4 charges at a rate determined by the current through resistor R4.

Since the voltage at circuit node 15 is fixed by feedback from Darlington pair 12 or 14, voltage drive level 25 varies linearly with integration of the current flowing through resistor R4. When control signal 24 is low, no current flows from current source 10 and the current through resistor R4 is equal to the current in resistor R5 or R5.

Resistor R4 helps stabilize the speed control loop by providing an immediate increase or decrease of the voltage at circuit node 15 as necessary to maintain constant level. The amount of such increase or decrease is determined by the difference between the current flowing through resistor R4 from current source 10 in response to control signal 24 when it is high, and the current flowing through resistor R4 to ground via resistor R5 or resistor R6 when control signal 24 is low.

Under ordinary load conditions, the drive current 22 of Fig. 2, effectively turns off at or near transitions of control signal 23. Since interaction of forces between the motor windings and the permanent magnet rotor are greatest during those transitions while is flowing in the motor windings, decreasing drive current 22 near such transitions substantially reduces those interacting forces and the resultant audio frequency noise.

When current is flowing in one motor winding at a transition of control signal 23, capacitor C, or C2 controls the rate at which drive voltage is transferred to the other winding. In addition, by controlling the rate of turn off of the drive voltage, capacitor C1 or C2 prevents voltage breakdown of its respective Darling pair caused by the inductance of the motor winding. Thus, when transistor Q4 turns off and transistor Q3 turns on, Darlington pair 14 turns off at a rate determined by the discharge of capacitor C3 through R6.

In addition to ensuring stability of the speed regulation loop, R4 regulates the flow of current from capacitors C3 and C4 which have been excessively charged during start up. During start up, power transistors Q6 and Q7 saturate, which drives circuit node 15 positive.

Resistor R4 maintains saturation of the power transistor which is applying power to a motor winding when the induced voltage, developed by the winding from which power is being removed, begins to decrease. Thus, resistor R4 limits the discharge of capacitors C3 and C4 to asure effective start up of the motor.

Claims (4)

1. Apparatus for driving a brushless DC motor having a plurality of windings, said apparatus comprising: driver means for sequentially applying drive current to the windings of the motor; and feedback means, coupled to the driver means, for applying the back EMF developed by the motor winding from which drive current is being removed to the motor winding to which drive current is being applied.

2. A method for driving a brushless DC motor having a plurality of windings, said method comprising the steps of: alternately applying drive current to the windings of the motor; and applying the back EMF developed by the motor winding from which drive current is be ing removed to the motor winding to which drive current is being applied.

3. Apparatus for driving a brushless DC motor, said apparatus being substantially as hereinbefore described with reference to the accompanying drawings.

4. A method of driving a brushless DC motor, said method being substantially as hereinbefore described with reference to the accompanying drawing.