GB2158617A - DC Brushless motor drive control - Google Patents

Abstract

Servo system e.g. for the head 12 of a disc drive employs a brushless DC motor 16 and is driven by a single power supply amplifier 18 which can provide a reversible drive current to the phases of the motor. Commutation of the motor is accomplished by electronic commutation circuitry 20. An encoder 24 provides position feedback signals to a microprocessor 28. During a velocity mode the microprocessor 28 generates a velocity profile and compares the actual velocity 26 of the motor with a desired velocity and generates a drive signal to the amplifier 18. The amplifier provides a drive current of direction dependent upon the polarity of the drive signal. During a subsequent position mode of operation, the commutation of the motor is frozen and the error signal is generated as a function of the difference in the actual and desired position of the motor. By freezing the commutation, commutation transients and positioning instability are eliminated. <IMAGE>

Description

SPECIFICATION DC brushless motor drive system BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to DC brush less motor drive systems and more particularly to incremental motion servo systems employing a full rotation brush less DC motor. Still more particularly, the present invention is directed to an incremental motion system for positioning the heads of a disk drive system with respect to concentric data tracks on disks of the system.

2. Description of the Prior Art Most prior art incremental motion servo systems employ brush-type DC motors. In such motors, coils of the different phases of the motor are coupled to the rotor through a commutator and current is delivered to the phases via brushes which surround the rotor. In a closed loop incremental motion system, the position and velocity of the motor is monitored and used to control the drive current applied to the motor. The direction of torque applied to the motor is controlled by reversing the directior of the drive current Brush-type DC motors have several characteristics which inhibit their performance in high accuracy incremental motion systems. The friction created by the brushes rubbing against the commutator results in a lack of accuracy when stopping the motor.In addition, such motors typically have either a low inertia but poor heat dissipation capabilities (e.g., pancake motors) or have low thermal resistance from the rotor to the case (i.e., good heat dissipation) but have a high rotor inertia. This results in severe limitations in sustained acceleration and deceleration activity or in a lengthy incremental motion time.

Stepper motors have also been employed in incremental motion servo systems. Although such motors provide accurate positioning at their null positions, they present problems in incremental motion systems since they are not really designed for high-speed slewing operations.

With respect to incremental motion systems for use in disk drive head positioners, systems which employ a linear voice coil instead of a rotational motor are widely employed. Although such systems provide high-speed positioning, i.e., low access times in moving the heads from an initial data track to a desired data track, they are prone to instabilities, expensive, inefficient and limited in heat dissipation capabilities.

Some prior art disk drive systems employ limited rotation DC torquer motors to position the heads.

In such motors, a rotor carrying a magnet is positioned within a ring-shaped frame having a coil wrapped around it, as illustrated in Figure 1. For a narrow range of rotation, e.g., 120O, the torque of such a motor is proportional to the level of current applied to it. Such motors are brushless motors and have the advantages of no friction and high heat dissipation capability. However, such motors are not very efficient, since a large portion of the copper of the coils of the motor does not contribute to torque generation. In addition, the coil has a high flux leakage and requires substantial shielding to prevent megnetic interference with nearby components.Because of the limited rotation nature of such motors, the encoder which is used to provide position information in a closed loop incremental motion system must have high resolution since every track position of the disk should correspond to a zero crossing of the encoder. Furthermore, the limited angle of rotation dictates the gear ratio of the drive mechanism which is used to couple the motor and the head. Because of this limitation, it is very difficult to achieve an optimum gear ratio for use between the motor and the load (head carriage mechanism and heads)ln a disk drive system, the inability to select an appropriate gear ratio may prevent the disk drive from being able to operate properly in various orientations, e.g., both horizontal and vertical, due to the varying effects of the weight of the load on the drive motor in different orientations.

Full rotation DC brush less motors have several characteristics which make them attractive for use in incremental motion systems. Such motors can be developed with extremely high torque-to-inertia ratios, thus resulting in excellent acceleration characteristics. In such motors, coils of different phases of the motor$ are wrapped around teeth which extend inwardly toward a magnetized rotor. A threephase DC brushless motor is illustrated in Figure 2.

Because of the configuration in which the coils are wrapped around teeth, such motors are quite efficient since all of the copper of the coils are exposed to the magnetic flux of the rotor magnets.

Recently, DC brush less motors have been developed which exhibit an extremely low cogging effect, making them even more desirable for use in incremental motion systems. In addition, the phases of a DC brushless motor are commutated by means of electronic commutation rather than brushes, thus elminating friction and facilitating precise positioning.

In an incremental motion servo system, the velocity of the motor is controlled by applying positive (accelerating) or negative (decelerating) torque to the motor. In typical DC brush motor systems, a change in the direction of torque is accomplished by changing the direction of current through the motor. In DC brushless motor systems, torque reversal is typically accomplished by altering the commutation sequence of the electronic commutator of the motor. This presents problems in high accuracy incremental motion systems, since switching the commutation state may result in commutation transients which cause positioning errors. In addition, if the desired position of the motor corresponds to a point at which commutation should occur, instability problems can result.

Thus, despite several performance advantages which could be obtained by employing a DC brushless motor in an incremental motion servo system, such motors have not been widely used because of several substantial disadvantages when attempting to incorporate them into a closed loop positioning system.

Summary of the invention The present invention is directed to a system for driving a DC brushless motor or step motor wherein the system includes a unique amplifier in which the reversal of torque applied to the motor is accomplished by changing the direction of current through the motor phases rather than by altering the commutation sequence. The amplifier may be used in various closed loop systems such as incremental motion systems and constant velocity systems.

In the preferred embodiment, the amplifer is employed in an incremental motion servo system which employs a DC brush less motor having electronic communtation wherein the amplifier provides a bidirectional drive current to the motor.

The servo system includes velocity and position feedback means providing signals indicative of the velocity and position of the motor. During a velocity mode of operation, the actual velocity of the motor is compared with a desired velocity by means of a microprocessor controlled digital-to-analog converter circuit and an error signal is generated to the amplifer to provide drive current to the motor to apply either accelerating or decelerating torque to the motor. During the velocity mode, the phases of the motor are automatically commutated at optimum points as determined by sensing means such as Hall effect sensors.

At some point prior to reaching the desired destination, the system is switched from the velocity mode of operation to a position mode of operation in which the motor is driven to its desired destination. During this mode, the motor is not commutated. Rather, the commutation state is held in the state it had at the end of the velocity mode. This avoids any instability which might otherwise result from commutation occurring at or near the desired destination position of the motor. During the position mode, error signals are generated to the amplifer as a function of the difference between the actual and desired position of the motor. In both the velocity and position modes, torque reversal is accomplished by reversing the direction of current supplied to the phases of the motor rather than by altering the commutation sequence.

Thus, the present invention facilitates the insertion of a DC brush less motor into a servo loop just as if it were a DC brush motor, i.e., a bipolar error signal drives the amplifier which in turn provides current of the desired magnitude and direction to the phases of the motor. This operation is facilitated by a unique amplifer in which the direction of current through the phases of the motor may be aitered simply by reversing the polarity of the input to the amplifer.

Brief description of the drawings The invention will be described with reference to the accompanying drawings, where: Figure 1 is a diagramatic illustration of a prior art partial rotation DC motor; Figure 2 is a diagramatic illustration of a full rotation DC brush less motor utilized in the present invention; Figure 3 is a block diagram of the incremental motion control system of the present invention; Figure 4A-E are waveforms associated with the incremental motion servo system of the present invention; Figure 5 is a block diagram of the electronic commutator and amplifier of the present invention; Figure 6 is a circuit diagram of the amplifier of the present invention; and Figure 7 is a diagramatic illustration of a delta connection of the motor phases.

Description of the preferred embodiment The following description is of the best presently contemplated mode of carrying out the invention.

This description is made for the purpose of illustrating the general principles of the invention and is not to be taken in a limiting sense. The scope of the invention is best determined by the appended claims.

Referring to Figure 3, a disk drive mechanism includes a disk 10 having a plurality of concentric data tracks on its surface. A magnetic transducer or head 12 is radially movable with respect to the tracks by means of a head carriage mechanism 14 to position the head 12 with respect to a desired data track. The head carriage mechanism per se does not form a part of the present invention and will not be described in detail. The head carriage mechanism is driven by a motor 16 which is a DC brushless motor having a plurality of phases. Although a two-phase motor could be employed, in the present embodiment of the invention, a threephase motor is utilized because of its reduced torque ripple. Other motors could also be employed, including DC step motors.

Drive signals to the motor 16 are provided by an am pIlfer 18 under the control of an electronic commutator 20. In the three-phase motor system of the present invention, two of the phases of the motor are energized at a time, with the energization being determined by the commutator 20. The commutator 20 is an electronic commutator which automatically commutates the motor based upon the position of the rotor as indicated by Hall effect signals 22 provided by Hall effect sensors located in the motor.

A position encoder 24 is coupled to the motor 16 and provides a sinusoidal position signal with every null (zero-crossing) of the signal corresponding to a data track of the disk 10. As is known in the art, the encoder output may be inverted every half cycle by an inverter 25 to provide a signal in which the direction of the slope does not change, as shown in Figure 4B. A velocity signal is derived from the encoder output by means of a velocity extractor 26.

The position signal is applied to a microprocessor 28. When a seek command is applied to the microprocessor 28 commanding the system to move the head 12 to a particular data grack, the microprocessor compares the present location of the head 12 with the desired location and determines a velocity profile corresponding to the distance to be moved. The microprocessor generates an instantaneous velocity signal in accordance with the velocity profile as a function of a distance of the head from the destination track. This signal is converted to an analog value by a digitalto-analog converter 27 and applied to a summing junction 29.

The operation of the system is divided into two modes. During a velocity mode, the actual velocity of the motor as determined by the velocity transducer 26 is compared to the desired velocity as indicated by the velocity profile determined by the microprocessor 28. If there is an error between the actual and desired velocity, an error signal appears at the output of the summing junction 29 and is applied to the power amplifier 18 via a scaling amplifier 30 to change the torque applied to the motor 16 to cause it to either accelerate or decelerate.

The drive signal to the amplifer 18 is a bipolar signal, i.e., positive to signify acceleration and negative to signify deceleration. During the velocity phase, the commutator 20 automatically commutates the phases of the motor, i.e., causes the amplifier to energize selected phases of the motor, in accordance with the position of the motor as determined by the Hall effect sensors. The commutation is performed so as to ensure maximum torque availability from the motor 16.

During a seek operation, the position of the head 12 is monitored by the microprocessor 28 by counting nulls from the encoder 24 applied via a squarer 23. When the head 12 reaches the area of the null corresponding to the desired head position, the microprocessor switches to a position mode of operation. In this mode, a switch 31 is closed and the position signal from the encoder is applied to the summing junction 29 and the output of the summing junction indicates the positioning error of the motor with respect to the next null. If the junction output is positive, it is an indication that the motor has not yet reached the null and accelerating torque should be applied. Conversely, if the junction output is negative, it is an indication that the motor has passed the null and reversing torque should be applied.In the position mode, the output of the converter 27 is zero and the velocity signal from the extractor 26 is maintained connected to the junction 29 to provide damping.

During the position mode, a signal POS is applied to the commutator 20 by the microprocessor 28 to freeze the commutation state of the commutator in the state it was in at the end of the velocity mode. This signal is maintained throughout the position mode of operation, i.e., no commutation occurs during the position mode even if the Hall effect signals 22 indicate that the motor has arrived at a commutation point. If the motor overshoots its desired end position and a reversing torque is required, it is supplied by reversing the direction of current supplied by the amplifier 18 to the phases of the motor 16.

Thus, in both the velocity and position modes, the direction of torque applied to the motor 16 is controlled by reversing the direction of drive currents in the energized phases rather than by altering the commutation sequence of the motor.

During the velocity mode the direction and magnitude of the torque applied to the motor is controlled by the amplifer to achieve the desired velocity, whereas during the position mode, the direction and magnitude of the torque applied to the motor by the amplifer 18 is controlled to cause the motor to move to the desired destination position.

The automatic commutation provided by the commutator 20 is frozen during the position mode so that no commutation transients are generated as the motor nears its destination position and so that no instability problems occur.

The control system just described is similar to a conventional incremental motion control system for a DC brush motor. The unique power amplifier 18 of the present invention enables a DC brushless motor to be employed in this type of servo system.

The back EMF (and torque characteristics) of the three phases of the motor is illustrated in Figure 4A. One full phase of each signal represents a complete revolution of the motor. During such a revolution, many data tracks may be traversed, as indicated by the output of the position encoder shown at Figure 4B. The relationship between the Hall effect commutation signals and back EMF signals of the various phases is fixed as shown in Figure 4. The magnet flux is sensed by the Hall effect sensors which provide digital output signals in accordance with the polarity of the sensed flux as indicated in Figures 4C, 4D and 4E. These signals are logically combined in the commutator 20 to cause commutation to occur at each zero-crossing of a torque curve, as indicated by dashed lines in Figure 4. This commutation operation results in maximum torque capability from the motor.If the phases of the motor are interconnected in a delta configuration (Figure 7) the energization sequence to achieve maximum torque is indicated at Figure 4F. The "not" sign over a phase indicates that current through that phase should be in the negative direction to apply positive torque to the motor. If the motor phases are interconnected in a star configuration (Figure 5) current flows through only two phases at a time, and the energization states are indicated in Figure 4G.

Referring to Figure 5, the commutator 20 generates six control signals 35a-f to the amplifier 18 to control the energization of the phases of the motor. In Figure 5, the phases are shown interconnected in a star configuration. The commutation sequence will be the same for both star and delta configurations. The commutator 20 receives the Hall effect signals HA, H, and Hc and logically combines them to provide the six output control signals to the amplifier 18. In the present embodiment, the Hall effect signals are applied to a code converter 32 to obtain the desired commutation sequence, although it should be noted that discrete logic gates or other methods of logically combining the Hall effect signals to provide the commutation signals could also be employed.

The output of the code converter 32 is applied to a latch 34, the output of which is connected to the amplifer 18. During velocity mode operation, the output of the code converter is passed through the latch and applied to the amplifer 18. At the end of the velocity mode, the signal POS is applied to the latch, which holds the output of the code converter. As long as the POS signal remains high, the commutation state of the commutator 20 will be frozen and no commutation will occur during the position mode.

The amplifer 18 is comprised of three blocks 18A, 18B and 18C, each of which has an output connected to a respective phase of the motor. Each block receives the drive input from the scaling amplifier 30. Each block of the amplifer is operated in one of three different modes under control of the control signals from the commutator 20, two of which are applied to each block. In a first mode, a block can operate as either a current sink or source. In a second mode, a block operates as a current controller to control the level of the current as a function of the drive signal on line 30. In a third mode, a block is switched to a high impedance state so that current does not flow through it.

For example, for one particular commutation sequence, the block 18A may operate as a current controller, the block 18B as a current source or sink and the block 18C placed in the high impedance state. Current will therefore flow between the blocks 18A and 18B through the phases A and B of the motor. The direction of the current will depend upon the polarity of the drive signal from scaling amp 30 applied to the block 18A. In a different commutation state, the block 18A will be in the high impedance state, the block 18B in the current controller state and the block 18C in the current source/sink state. Current will therefore flow between the blocks 18B and 18C and through phases B and C of the motor, with the direction and magnitude of the current being determined by the drive signal from scaling amp 30 applied to the block 18B.

The operation of the amplifer 18 will be described with reference to Figure 6. Each block of the amplifer is comprised of an operational amplifier coupled to four output transistors and a pair of FET switches for controlling the state of operation of the block. The block 18A includes an operational amplifer Al, output transistors T1-T4 and FET switches S1 and S2. The block 18B includes an operational amplifier A3, output transistors T9-T12 and FET switches S5 and S6.

For purposes of illustration, it will be assumed that the block 18A is in the current controller state, the block 18B in the current source/sink state and the block 18C in the high impedance state. The states of the blocks are controlled by FET switches S1-S6 which are driven by the commutation signals from the commutator 20 on lines 35a-f. When the block 18A is in the current controller state, FET switch S1 will be turned on and FET switch S2 turned off. This connects the output of the operational amplifer Al to the bases of the transistors T1 and T2. With the block 18B in the current sourcel sink state, FET switch S3 will be opened to disconnect the operational amplifer A2 from the transistors T5 and T6 and FET switch S4 will be closed to provide a low impedance path for a six-volt reference voltage to be applied to the bases of transistors T5 and T6.In the block 18C which is operated in the high impedance state, FET switch S5 is opened to disconnect the output of operational amplifier A3 from transistors T9 and T10 and FET switch S6 is opened to remove the low impedance path from the reference voltage to the transistors T9 and T10, so that the reference voltage is coupled to the transistor bases only via a high value resistor 58.

The amplifier 18 is designed to work with a single power supply, which in the present embodiment of the invention is a 12-volt supply. Since disk power drives typically operate on a single supply voltage, it is important that the amplifer be able to operate utilizing only a single power supply. The amplifier of the present invention employs a single power supply yet provides a bidirectional current in response to a bipolar input signal.

The bipolar drive signal on line 30 will be applied to each of the amplifiers Al, A2 and A3 via resistors 43. However, only one of the switches S1, S3 and S5 will be closed (S1 in the case in which the block 18A is acting as a controller) and the output signal of only one of the operational amplifiers will be applied to the bases of its associated output transistors. In the block 18B, the switch S3 is opened and the switch S4 is closed and the six-volt reference voltage (obtained from the 12-volt supply voltage by a simple divider network) is applied to the bases of the transistors T5 and T6. In block 18C, both the switches S5 and S6 are opened to place the block in its high impedance state.

The interconnection of the transistors T5 and T6 is such that the application of the constant six-volt drive to their bases will cause a point 40 connected to the emitters of the transistors to be held at a six-volt level. If the point 40 falls below six volts, the transistor T5 will turn on until the six-volt level is again achieved, whereas if the point 40 rises about six volts, the transistor T6 will turn on. The point 40 is interconnected with a point 38 in the block 18A via resistors 44 and 46. The transistors T5 and T6 interact with the transistors T1 and T2 to control the current flow between the blocks.

The operational amplifiers Al, A2 and A3 are of a type which operate from a single supply and include zero as a common mode input signal. An example of such a device is a Motorola Model LM 358 operational amplifier. The op-amps operate off of the 12-volt power supply and for a zero input differential, their output is one half of the supply voltage, i.e., six volts. The output swing ranges from zero to +12 volts. A small bias (on the order of 20mV) is applied to the positive inputs of the op-amps via a resistor 75. Thus, in the static condition, a slightly positive voltage will be applied to the negative input and the op-amp output will be approximateiy six volts.With the block 18A operating as a current controller, the switch S1 will be closed and, for a zero differential input signal to Al, a six-volt output will be applied to the bases of transistors T1 and T2. Feedback via a resistor 72 and capacitor 74 will cause the point 38 between the emitters of transistors T1 and T2 to be maintained at six volts.

If a positive drive signal from the scaling amplifier 30 is applied to the operation amplifier Al, its output will fall below six volts (since the signal is applied to the inverting input). This will result in a negative base-to-emitter voltage at the transistor T2, and cause the transistor T2 to turn on. Since the transistor T1 is off, the transistor T2 will draw current from the transistor T5 of block 18B through the resistors 44 and 46. Thus, transistors T2 and T5 will turn on in response to the application of a positive input signal to the op-amp Al. The turning on of the transistors T2 and T5 in turn causes the transistors T4 and T7 to be rendered conductive.

Current will therefore flow from the positive power supply connection in the block 18B, through the transistor T7, through phase B of the motor, through phase A of the motor, through transistor T4 and through a sensing resistor 50 to ground.

The magnitude of the current will be deteremined by the manitude of the current will be determined by the magnitude of the drive signal applied to the op-amp Al through the action of current feedback from the resistor 50 applied to the positive input of Al via resistor 76.

If the input signal from the scaling amp 30 to the op-amp Al is negative, its output will rise above six volts. This will cause the transistor T1 to be rendered conductive, which in turn causes the transistor T6 to be turned on. Current will therefore flow through the transistor T1, the resistors 44 and 46 and the transistor T6. As a result, the transistors T3 and T8 will be rendered conductive, with a current flow path being crerated from the 12-volt supply in the block 18A through the transistor T3, phase A of the motor, phase B of the motor, through the transistor T8 and through a sense resistor 52 to ground. The magnitude of the current will again be determined by the magnitude of the drive signal through the action of current feedback, in this case from resistor 52 applied to the negative input of Al via resistor 68.Thus, the current flow through the motor phases will be opposite to that in the situation in which the input signal is positive. The amplifier therefore operates to receive a bipolar input signal and provide a bidirectional current through the energized phases of the motor as a function of the input signal.

Thus, despite employing a single power supply and providing a bidirectional drive current, the amplifer in Figure 6 is configured to provide negative feedback. The voltage across each of the sensing resistors 50, 52 and 54 (representative of the current through the motor phases) is fed back to the positive input of the operational amplifier on its own block and the negative input of the operational amplifier of the other two blocks. When a positive input signal is applied to the operational amplifier Al, and current therefore flows through the output transistors T4 and T7, the voltage across the resistor 50 is applied to the noninverting input of the op-amp Al resulting in negative feedback to the op-amp. If the current flow through the motor phases is reversed, the transistors T4 and T7 will be nonconductive and the transistors T3 and T8 will be conductive.In this instance, a positive voltage will be developed across the resistor 52.

However, the current direction through the motor phases can be considered to be negative and the positive voltage across the resistor 52 is applied to the non-inverting input of the amplifier Al of the block 18A in order to provide the desired negative feedback.

The commutation circuit 20 will cause the switches S1-S6 to be manipulated so as to cause one of the blocks 18A,18B and 18C to be a current controller, one to be a current sourcel sink and one to be in its high impedance state. The operation for any commutation state will be identical to the operation described above, with current flowing through the phases between different blocks as a function of the position of the switches S1-S6.

In each of the blocks of the amplifier, when a high impedance state is required, a large value resistor 58 is connected between the six-volt reference voltage and the respective pair of transistors in its block. When the switches S2, 54 and S6 are open, the bases of the transistors will be starved and the block will be in a high impedance state and no current will flow into or out of the block.

Each block of the amplifer also includes supply resistors 60 and 62 connected to the 12-volt supply and ground, respectively. Diodes 64 are coupled to the output transistors to minimize the effects of stored inductive energy. Resistor sets 66 and resistor 75 provide the bias to the positive input of the op-amp, and the feedback voltages for the blocks are applied to the operational amplifier inputs via resistors 68, 70 and 76. Voltage feedback from the emitter transistor pairs T1 and T2, T5 and T6, and T9 and T10 is provided by means of a resistor and capacitor 72 and 74 to remove a dead zone in response caused by the VBE of the transistors.

Although a system incorporating a three-phase motor has been described, the invention is not so limited. A two-phase motor could be employed, although the torque ripple characteristics of a threephase motor are preferable. The general operation of the system is such that an amplifier block is provided for each phase to control the current flowing between the phase connected to that block and another phase.

In summary, the present invention provides an incremental motion control system which employs a full rotation brushless DC motor which is automatically electronically commutated and in which current direction and magnitude through the phases of the motor are controlled as a function of a bipolar input signal. The system employs a unique single power supply power amplifier which includes a block corresponding to each phase of the motor, with each block being able to operate as a current controller or a current source/sink.

During the velocity mode of operation, error signals corresponding to the difference in actual and desired velocity are applied to the amplifier to control the magnitude and direction of current flowing through the energized phases of the amplier. Commutation is automatically accomplished to provide the maximum possible torque. During the position mode of operation, the commutation state is frozen in order to eliminate any possible instability and to prevent any possible instability and to prevent any commutation transients from affecting the positioning operation. It should be noted that since commutation is inhibited in the position mode the motor may move into a region of lower torque than would be obtained if commutation had been allowed to take place However, the loss of torque for the relatively small angle of action of the position mode is not significant.The slight loss of torque capability is greatly out- weighed by the inhibition of commutation transients near a desired position and by the ability to employ a DC brushless motor in a servo loop in which a bipolar drive signal is applied to the amplifier.

Although the invention has been described in terms of a system in which commutation is frozen during the position mode, in its broader aspects the present invention would not require such operation. This is so especially in situations where desired positions do not fall close to commutation points. More generally, the invention is directed to the use of a DC brushless motor (or step motor) in a closed loop system. Such operation is facilitated by the unique amplifier of the present invention in which drive current in different directions is provieded in response to a bipolar input signal. The system is not limited to use in incremental motion systems but could also be employed in other closed loop systems, such as constant velocity spindle drives for disk drive systems.

Claims (18)

1. A servo system comprising: a brushless motor having a rotor and a plurality of phases; electronic commutation means for commutating the phases of the motor at predetermined rotor positions; feedback means for providing feedback signals representing the velocity of the rotor; control means for providing a bipolar drive control signal representing the difference between the desired and actual velocity of the rotor; and amplifier means for receiving the control signal and providing drive current to the motor phases in response thereto in accordance with the commutation state of the commutations means, wherein the amplifer means operates to change the direction of torque applied to the motor by reversing the direction of current through the phases of the motor independently of the commutation of the motor.

2. A servo system comprising: a brushless motor having a rotor and a plurality of phases; electronic commutator means for commutating the motor phases at predetermined rotor positions; feedback means for providing feedback signals representing the velocity and position of the rotor; control means for generating a first control signal representative of the difference between the actual and desired rotor velocity during a velocity mode of operation and thereafter generating a second control signal representative of the difference between the actual and desired rotor position during a position mode of operation, said control means also for preventing commutation during the position mode of operation; and amplifier means for providing drive current to the motor phases in response to the first and second control signals, wherein the current to the phase is reversible to control the direction of torque of the motor independently of the commutation of the motor.

3. A servo system as in claim 2 wherein the control signals are bipolar and wherein the amplifer means provides drive current through the energized phases in a first direction for a control signal of first polarity and in the opposite direction for a control signal of second polarity.

4. A servo system as in claim 2 or 3 wherein the amplifier means is comprised of a plurality of blocks, one connected to each phase, wherein each block is controllable by the commutation means to operate as (a) a current source or sink, or (b) a current controller, wherein current flows between a block acting as a current source or sink and a block acting as a controller and through the associated phases.

5. A servo system comprising: a brushless motor having a rotor and a plurality of phases; electronic commutator means for automatically selectively energizing the phases of the motor at preselected positions of the rotor; feedback means for providing feedback signals indicative of the velocity and position of the rotor; and control means for (a) providing reversible current to the motor phases during a velocity mode of operation as a function of the difference between the actual and desired velocity of the rotor, and (b) maintaining the commutation state of the commutator means at the end of the velocity mode of operation and providing reversible drive current to the motor phases during a position mode of operation as a function of the difference between the actual and desired position of the rotor, wherein the direction of torque of the motor during both the velocity and position modes is reversed by reversing the direction of current through the phases independently of the commutation of the motor.

6. An incremental motion servo system comprising: a brushless motor having a rotor and a plurality of phases; electronic commutation means for commutating the phases of the motor at predetermined rotor positions; feedback means for providing feedback signals representing the velocity and position of the rotor; control means for providing a bipolar drive control signal during a velocity mode of operation representing the difference between the desired and actual velocity of the rotor and during a subsequent position mode of operation representing the difference between the desired and actual position of the rotor, said control means also providing a commutation control signal for preventing commutation of the motor during the position mode of operation; and amplifier means for receiving the control signal and providing drive current to the motor phases in response thereto in accordance with the commutation state of the commutations means, wherein the amplifier means operates to change the direction of torque applied to the motor by reversing the direction of current through the phases of the motor independently of the commutation of the motor.

7. A system as in claim 6 wherein the commutation means commutates the phases of the motor at rotor positions which result in maximum torque capability of the motor.

8. A system as in claim 7 wherein the commutation means includes sensing means to sense the position of the rotor with respect to each motor phase and logic means for receiving the output of the sensing means and generating commutation signals to the amplifier means to control the energization of the motor phases.

9. A system as in claim 8 wherein the commutation means includes latch means for latching the output of the logic means in response to the receipt of the commutation control signal from the control means, wherein the output of the latch means is provided to the amplifier means to control the commutation of the motor.

10. A system as claimed in claim 8 or 9 wherein the sensing means comprises Hall effect sensing elements,

11. A system as in any of claims 6 to 10 wherein the phases of the motor are interconnected and whdrein the amplifier means comprises: a plurality of blocks, one corresponding to each phase of the motor, wherein each block has an output connected to its associated phase, and includes switching means for selectively causing it to operate as a current source and sink, wherein the switching means are operated so that when one block is a current controller another block is a current source and sink, wherein drive current of a magnitude and direction determined by the block operating as a current controller will flow from one block, through its associated phase, through the phase associated with the other block and into the other block.

12. A system as in claim 11 wherein the motor has at least three phases, wherein each block is also switchable by the switching means to operate in a high impedance state to prevent current from flowing into the block, wherein the switching means are operated so that when one block is a current controller and another block is a current source and sink, one or more additional blocks are in the high impedance state.

13. A system as in claim 11 wherein each block comprises: input means for receiving the drive control signal from the control means and providing a current control signal in response thereto; reference voltage means for providing a reference voltage of predetermined value; output means for providing currents for the motor phases, wherein the input means and reference voltage means are selectively connectable to the output means via the switching means to cause the block to operate as a current controller or current source and sink, respectively.

14. A system as in claim 13 wherein the output means comprises: a control stage comprising first and second transistor means of opposite conductivity having outputs connected to one another and control inputs connected to one another and selectively connected to the output of the input means or the reference voltage via the switching means, said control stage including impedance means coupled to the output of the transistor means and to corresponding impedance means of other blocks; an output stage comprised of third and fourth transistor means of opposite conductivity driven by the first and second transistor means, respectively, wherein the third transistor means is configured to pass current from a supply voltage to the phase associated with the block and the fourth transistor means is configured to pass current from the phase associated with that block to ground;; wherein in operation the current control signal is applied to the control inputs of the first and second transistor means of another block, wherein the reference voltage will cause the interconnected output of the first and second transistor means to be maintained at the reference voltage and the current control signal applied to said one block will cause one of the first and second transistor means of that block to conduct and thereby turn on either the third or fourth transistor means of that block, wherein the turning on of the first or second transistor means will cause the second or first transistor means, respectively, of said another block to be turned on to thereby turn on the fourth or third transistor means, respectively, whereby a current path is created from the supply voltage through the third transistor means of one block, through the phases associated with the one and another block and through the fourth transistor means of said another block to ground or through the phases transistor means of said another block to ground or through the phases associated with said another block and said one block, through the fourth transistor means of said one block and to ground.

15. An incremental motion control system comprising: a brushless motor having a rotor and a plurality of phases; feedback means for providing velocity and position feedback signals from the motor; and control means for receiving the feedback signals and operating the motor (a) in a velocity mode in which the motor is automatically commutated at predetermined rotor positions and driven with a reversible drive current to control the magnitude and direction of torque applied to the motor so that the actual velocity as indicated by the velocity feedback signal tracks a desired velocity, and (b) during a subsequent position mode in which the commutation state is maintained in the state in which it was in at the end of the velocity mode and the motor is driven with a reversible drive current to control the magnitude and direction of torque applied to the motor so that it moves to a desired position as indicated by the position feedback signal.

16. A single power supply amplifier for providing a reversible drive current to a DC brushless motor having plural interconnected phases, comprising: a plurality of blocks of current path means for controlling the flow of drive current through the phases of the motor, each current path means having a supply voltage terminal, a ground terminal and an output terminal, said output terminal connected to a single phase of the motor, each current path means including commutation switch means for causing the current path means to operate (a) as a current sink or source to provide current to its associated phase or accept current from its associated phase, or (b) as a current controller to provide current to its associated phase or accept current from its associated phase, wherein drive current flows through the motor phases between a block operating as a current controller and a block operating as a current source or sink, said drive current having a magnitude and direction determined by the magnitude and polarity of a bipolar input signal applied to the block acting as a current controller.

17. An amplifier according to claim 16 wherein the motor has at least three phases and wherein the commutation switch means is operable to cause the current path means to operate (c) in a high impedance state to prevent current from flowing into or out of the block.

18. A single power supply amplifier for providing a reversible drive current to a DC brushless motor, having plural interconnected phases, comprising a plurality of blocks, one for connection to each phase of the motor, wherein each block includes: a power supply terminal for connection to a single supply voltage; a ground terminal; an output terminal; first transistor means providing a first current path from the supply voltage terminal to the output terminal; a second transistor means for providing a second current path from the output terminal to the ground terminal; drive means for selectively rendering the first transistor means or second transistor means conductive, said drive means being interconnected with the drive of means of every other block input means for controlling the drive means, said input means receiving a bipolar drive signal; and commutation switch means for selectively connecting the output of the input means to the drive means, said switch means being operated so that at any time the output of the input means of only one block is coupled to its respective drive means, wherein when the input signal is of a first polarity the drive means of that block will render the first transistor means conductive and, via the interconnection between blocks, the drive means of a second block will render the second transistor means of the second block conductive to create a current path from the supply voltage terminal of the first block through the phase connected to the first block, through the phase connected to the second block to the ground terminal of the second block, and wherein when the input signal is of a second polarity the drive means of that block will render the second transistor means of the second block conductive to create a current path from the supply voltage terminal of the second block through the phase connected to the second block, through the phase connected to the first block and to the ground terminal of the first block.