WO1997026703A1

WO1997026703A1 - High speed bidirectional brushless motor controller using back emf sensing

Info

Publication number: WO1997026703A1
Application number: PCT/US1997/000636
Authority: WO
Inventors: David Jones; Stephen R. Topaz
Original assignee: University Of Utah
Priority date: 1996-01-16
Filing date: 1997-01-16
Publication date: 1997-07-24

Abstract

A high speed bidirectional brushless DC motor (14) using modified back EMF sensing. Two motor control chips (10, 12) supply control signals to a power booster circuit (24) which insulates the motor control chips (10, 12) from the motor (14), allowing higher motor drive voltages to be supplied to the brushless DC motor (14) from the power supply voltage limit of the motor control chips (10, 12). Back EMF sensing signals are provided to the motor control chip (10, 12) by the power booster circuit (24) via a center tap signal (30).

Description

HIGH SPEED BIDIRECTIONAL BRUSHLESS MOTOR CONTROLLER

USING BACK EMF SENSING

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to bidirectional brushless direct current (DC) motors. Specifically, the present invention relates to a device and method for driving a brushless DC motor at high speeds using a motor control chip which can not by itself supply sufficient voltage to the motor. Accordingly, the invention pertains to exploiting the advantages of the limited voltage controller chip while supplying higher motor drive voltages to achieve higher motor speeds.

2. State of the Art

DC motors in the state of the art develop rotational force through interaction of two magnetic fields. All motors have a non-rotating housing called the stator, and a rotating part, the rotor. The stator has means to generate a magnetic field, as does the rotor, and it is these two fields that interact to cause the rotor to turn.

Magnetic forces obey the familiar rule of "opposite poles attract and like poles repel". If the polarities of the two magnetic fields were fixed and unchanging, the rotor would simply align itself with the stator magnets according to the magnetic force rule and then halt. To continue rotation, one of the two magnetic fields must be constantly changed to stay "one step ahead" of rotation. The changing orientation of the magnetic field is achieved in a process called commutation. It is the means of commutation wherein the primary difference between motor types exists.

In a brushed DC motor, permanent magnets are mounted on the stator to produce the stator magnetic field. Electric current is passed through windings on the rotor to produce the rotor field. The brushes in a brushed motor are two switched contacts attached to the stator housing. The brushes make mechanical and electrical contact with a segmented set of contacts affixed to the rotor. As the rotor turns, different pairs of rotating contacts come under the brushes to cause electric current to flow in the appropriate set of windings that produce the rotor's magnetic field. The switch contacts and winding are arranged such that the magnetic fields of the stator and rotor produce maximum rotational force on the rotor. Electrical connection to the motor is made directly to the two brushes. Current enters the rotor through one brush, and leaves through the other. All that is required to operate a brushed motor is power to the motor. Motor control signals do not appear outside of the motor as control is achieved by the internal mechanical switches. Direction of rotation is determined by the polarity of the power supply voltage. Reversing the connection of the two motor leads causes motor rotation in the opposite direction. The motor reversal methods described in the five patents are electronic and electromechanical means to reverse the polarity of the voltage supplied to a brushed motor.

In contrast, a brushless DC motor consists of a stator and rotor, but their magnetic roles are reversed from those of the stator and rotor in the brushed motor. The brushed motor has permanent magnets in the stator, whereas the brushless motor has permanent magnets in the rotor. Thus no electrical connection to the rotor is necessary to produce the rotor's magnetic field, allowing deletion of the brushes and the segmented rotor contacts. A proper sequence of current through three (or more) windings on the stator provides the variable magnetic field to cause motor rotation. Electrical connection to the windings is direct, without need for brush contacts, and this results in significantly more reliable motor operation.

Since the brushless motor has no internal mechanism to switch current from winding to winding, an external controller must provide power to the motor. To determine when to switch power from one winding to another, brushless DC motor controllers must have some means to determine the angular position of the rotor. One way to achieve this is to add magnetic sensing hardware, or optical sensors may be employed. These sensors provide motor control signals, in the form of rotor position information, to the motor controller. The controller interprets these signals to determine the proper times to switch the motor power from winding to winding.

Unfortunately, motor drive voltage is limited to the voltage supplied by the motor controller. Limited drive voltage ultimately means that the speed of the motor is also limited. Therefore, what is needed is a device and method for supplying higher motor drive voltages to the motor while still taking advantage of the motor control chips which make it possible to have a brushless DC motor.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and apparatus for providing a brushless DC motor using motor controllers which do not limit the motor drive voltage.

It is another object to provide a high speed brushless DC motor.

It is another object to provide a high speed brushless DC motor which takes advantage of the motor controllers to provide bidirectionality. It is another object to provide a high speed brushless DC motor which disposes a power booster between the motor controller and the windings of the brushless motor.

It is another object to provide a high speed brushless DC motor which uses modified back EMF sensing to determine motor winding voltages.

In accordance with these and other objects of the present invention, the advantages of the invention will become more fully apparent from the description and claims which follow, or may be learned by the practice of the invention.

The present invention provides in a preferred embodiment a high speed brushless DC motor using modified back EMF sensing. A motor control chip supplies control signals to a power booster circuit which insulates the motor control chip from the motor, allowing higher motor drive voltages to be supplied to the brushless DC motor without violating the maximum supply voltage limit of the motor control chip. Back EMF sensing signals are provided to the motor control chip by the power booster circuit via a center tap signal .

Another aspect of the invention is the use of two motor control chips to provide bidirectional operation of the brushless DC motor. In this embodiment, the center tap information must be supplied to both motor control chips to accomplish modified back EMF sensing. These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled m the art from a consideration of the following detailed description, taken in combination with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram of the prior art showing two motor control chips coupled directly to one brushless DC motor to supply motor drive voltage. Back EMF sensing is accomplished by measuring the motor winding voltages directly.

Figure 2 is a block diagram showing two motor control chips coupled to one bidirectional and brushless DC motor with an intervening power booster to supply higher motor drive voltage.

Figure 3 is a block diagram schematic of the detail of one of three motor winding circuits.

Figure 4 is a block diagram showing one motor control chip coupled to one unidirectional brushless DC motor with an intervening power booster to supply higher motor drive voltage.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the drawings in which the various elements of one preferred embodiment of the present invention will be given numerical designations and in which the preferred embodiment of the invention will be discussed so as to enable one skilled in the art to make and use the invention.

The present invention is useful in many applications which require a very high degree of reliability. For example, the present invention is intended for use m an electrohydraulic total artificial heart. The artificial heart is powered by a brushless DC motor mounted between artificial ventricles. The motor rotor turns the blades of an axial turbine that pumps fluid from one ventricle to the other, driving blood out of one while the other ventricle fills. Then rotation is reversed and the previously filled ventricle is emptied. This is repeated sixty times a minute to drive the heart.

In addition to motor reliability, experience with physiologically related systems has demonstrated the need to simplify the circuitry. This is accomplished by eliminating motor position sensors from the system. Back EMF sensing allows operation of the motor by deriving the angular position from signals present on the motor windings, without additional motor position sensing hardware. Circuitry examines signals of the motor windings to determine communication times. This "back EMF" sensing runs the motor without additional rotor position sensors, such as Hall effect sensors. The immediate benefits include reduced parts count and simpler wiring, as will be explained in detail later Figure 1 shows how in the prior art, two motor control chips 10, 12 attached to one motor 14 through winding connections labeled "A" 16, "B" 18, and "C" 20. Two winding connections 18 and 20 are swapped, as shown, to achieve different rotational directions, depending upon which motor control chip 10, 12 is activated by the software running on the personal computer 22. Those skilled in the art will recognize that windings A 16, B 18 and C 20 can be arranged differently when coupled to the motor control chips 10 and 12 to achieve the same results as understood by those skilled in the art.

Before continuing, it should be remembered that the motor control chips 10 and 12 used in this application were designed for use in computer disk drives to operate a brushless motor that spins a disk.

Consequently, everything required to operate the motor, including power handling electronics, is included in the motor control chip. The design of the motor control chip was influenced by the requirements of manufacturers of laptop computers. Anything that extends the life of an internal battery by conserving power is advantageous in a laptop. Thus, the motor control chip has a power conserving mode in which it can be "put to sleep", under software control, when access to the hard disk is not required. This sleep mode will be discussed later. Full speed bidirectional rotation is provided by the pair of unidirectional motor driver chips 10, 12 connected in parallel and activated sequentially. Each motor driver chip 10 and 12 takes care of one direction of rotation of the motor 14. In the prior art, the maximum supply voltage which the motor driver chips 10 and 12 can supply to the brushless DC motor 14 is restricted to the maximum voltage supplied to the motor driver chips 10 and 12. Because the motor control chips 10 and 12 are operating in parallel, it is necessary that they do not interfere with each other. The motor control chip manufacturer created the "sleep mode" mentioned previously, where the chip 10, 12 is inert, as its connections to the motor windings 16, 18 and 20 are put m a high impedance, or off, state. This allows multiple motor control chips 10 and 12 to be attached to the windings of one motor 14 , as long as only one motor control chip 10, 12 acts to drive the motor 14 at any one time. A system based on the architecture of FIG. 1 was built and tested, and bidirectional operation was achieved. However, the resultant motor speed was insufficient for the specific purpose of use m an artificial heart, a key limiting factor being the supply voltage that could be placed upon the motor control chips 10 and 12. Increasing the supply voltage beyond a specified level caused motor control chip 10, 12 failure. Techniques used to fabricate the motor control chips 10 and 12 necessarily restrict the voltage and current available to drive the motor 14. The present invention shown in FIG. 2 overcomes this restriction through addition of power handling transistors outside of the integrated circuit. The external transistors can operate at voltage and current levels much greater than those tolerated by the motor control chips 10 and 12. Our interfacing technique "boosts" the permissible range of motor applications that can be controlled through use of the integrated circuit motor controller.

FIG. 2 shows that to overcome the motor control power supply voltage limitation, a "power booster" circuit 30 was added to a drive system. The power booster circuit 24 "insulates" the motor control chip

10 and 12 from the motor 14, creating a region of limited voltage 26, and a region of higher voltage 28. Insulating the motor control chips 10 and 12 from the motor 14 advantageously enables higher motor drive voltages to be supplied to the motor 14 without violating the maximum supply voltage limit of the motor control chips 10 and 12.

It should be evident from FIG. 1 that when motor control chips 10 and 12 are connected directly to the motor 14, each of the winding circuits 16, 18 and 20 from motor control chips 10 and 12 to the motor 14 acts as a "two way street". In other words, electric current driven through the motor 14 by power transistor devices internal to the motor control chips 10 and 12 provides power to cause motor 14 rotation. At other times, when a winding circuit 16, 18 and 20 is switched off and provides no drive current, that winding circuit acts to provide back EMF information from the motor 14 to the motor control chips 10 or 12. This back EMF voltage waveform is then analyzed by the motor control chip 10 or 12 to determine correct timing for the next power switching sequence. Thus, drive current and motor timing information share a common pathway.

In contrast in the present invention, FIG. 2 shows that the power booster circuit 24 is designed to electrically separate the motor 14 and motor control chips 10 and 12 while maintaining the "two way street" nature of motor drive and timing information flow. In essence, the motor control chips 10 and 12 are fooled into thinking that they are still connected to the motor 14. The indirect nature of the connection requires addition of a pathway to provide center tap information 30 to the motor control chip 10 and 12, as shown in Figure 2. The center tap information 30 is not required in the directly connected configuration of FIG. 1 because the motor control chips 10 and 12 can derive center tap information 30 by averaging all the winding voltage waveforms from the motor 14. The separate system power supplies are shown m

FIG. 2. The power booster circuit receives power from a power supply 56 which provides higher voltages than what are used to supply the motor control chips 10 and 12. Typically, this lower voltage power supply 58 provides a voltage of only 12 or 14 volts, whereas the power booster power supply 56 is designed to provide much greater voltages to provide a high speed motor 14.

Figure 3 is provided to illustrate one section of the power booster circuit 24 (see FIG. 2) . Three sets of the circuits shown in FIG. 3 are required, one for each motor windings 16, 18 and 20 (see FIG. 2) . From FIG. 3, it can be seen that two power transistors 32 and 34 connect the associated motor winding 16, 18 or 20 either to the motor drive supply voltage 36 (Ql 32 on, Q2 34 off) or to ground (Ql 32 off, Q2 34 on) . Back EMF information appears on the winding 16, 18 or

20 when the motor 14 is operating and both power transistors 32 and 34 are off. Two comparators 38 and 40 in each section control the power transistors' 32 and 34 gates and therefore the on-off state of the power transistors 32 and 34. The inverting (-) inputs 42 and 44 of the comparators

38 and 40 are driven by either the power state of the active motor control chip 10 or 12 (which now serves to provide a logic level instead of a drive current) or by the back EMF amplifier 46, through a limiter resistor 48, when the motor control chip's 10 or 12 power state is turned entirely off. The limiter resistor 48 prevents damage to the amplifier 46 output that would occur if the full current capability of the motor control chips 10 and 12 was imposed on the amplifier 46.

Power transistor Ql 32 is turned on only when the comparator 38 input exceeds the "HIGH TRIGGER VOLTAGE" level 50. Similarly power transistor Q2 40 is turned on only when the comparator 40 input falls below the "LOW TRIGGER VOLTAGE" level 52. As a result, the external power transistors 32 and 34 mimic the action of the motor control chip's 10 and 12 output, but at a higher voltage.

When the motor control chip 10 or 12 turns off its internal power stage, it expects to receive a back

EMF voltage wave form from the open circuited motor winding 16, 18 or 20. If the actual voltage waveform on the motor winding 16, 18 or 20 were presented to the motor control chip 10 or 12, it would damage the motor control chip's 10 or 12 circuits. To prevent this, the actual winding voltage is passed through an attenuator 48 to decrease its amplitude to safe levels. Additionally, a fixed, positive bias voltage is added to the attenuated waveform by a divider and bias network 54. The combination of this attenuation and biasing results m a modified back EMF waveform presented through the amplifier 46 (a buffering amplifier 46 with unity gain) to the high impedance motor control chip 10 or 12 input 16, 18 or 20.

An practical matter is to realize that the two trigger voltages 42 and 44, the attenuation factor caused by the resistor 48, and the bias voltage supplied by the divider and bias network 54 are chosen to have the following qualities. The voltage range of the modified back EMF waveform cannot exceed the HIGH TRIGGER VOLTAGE level 50, thus the back EMF amplifier cannot cause power transistor Ql 32 to turn on. Only the motor control chip 10 or 12 can turn on power transistor Ql 32 by presenting a voltage high enough to trip the comparator 38. The HIGH TRIGGER VOLTAGE level 50, is of course, less than the voltage of the power supply used by the motor control chips 10 and 12.

In a similar manner, the voltage range of the modified back EMF waveform cannot fall below the LOW TRIGGER VOLTAGE level 52, thus the back EMF amplifier cannot cause power transistor Q2 34 to turn on. Only the motor control chip 10 or 12 can turn on power transistor Q2 34 by presenting a voltage low enough to trip the other comparator 40.

Each of the back EMF amplifiers 46 also provide a modified back EMF waveform to the input 30 of an averaging network to produce a modified center tap voltage waveform. This modified waveform is presented to the motor control chip's center tap input 30 as shown in Figure 2. Since the individual motor winding back EMF waveforms 16, 18 and 20 are each passed through identical attenuation/bias processes, the derived center tap waveform, an average of the modified winding waveforms, is an image of the actual motor 14 center tap voltage. This modified image is as if the original center tap waveform had been passed through an attenuation/bias process identical to the individual winding processes. This yields a modified center tap waveform with the following important property. Specifically, the modified back EMF waveforms and the modified center tap waveform presented to the motor have been altered in amplitude and both are biased away from ground, but the timing relationship between them, specifically the time when the modified EMF waveform and the modified center tap waveform coincide, is unchanged from the timing relationship of the unmodified motor EMF and center tap waveforms. It is this event, detection of the waveforms crossing each other, that allows the motor control chips 10 and 12 to derive timing information sufficient to operate the motor 14.

Our interfacing circuitry shown in FIG. 3 supports communication of motor control signals from the motor controller chips 10 and 12 to the power transistors (Ql 32 and Q2 34) while maintaining the flow of back EMF information from the motor windings 16, 18 and 20 to the motor control chips 10 and 12. These two-way communications occur over common physical paths, namely the motor windings 16, 18 and 20.

In summary, the present invention, unlike the prior art, is not seeking to boost the motor drive voltage above the voltage supplied by the system power supply 58. Instead, a completely separate power supply 56 provides power to the motor 14. The present invention therefore isolates the motor controllers 10 and 12 with their smaller available drive voltages while advantageously maintaining the two-way communication path providing motor position information between the motor windings 16, 18 and 20, and the motor control chips 10 and 12. Typically, this pathway is through power supply lines 16, 18 and 20 to the motor windings as shown in FIG. 1. In the present invention, the power booster circuit 24 isolates the motor control chips 10 and 12 from the higher voltage power supply 58 of the power booster circuit 24 which is being used to drive the motor 14. While the preferred embodiment of the present invention shown in FIGs. 2 and 3 is designed to provide bidirectional operation of a high speed brushless DC motor 14, it should be apparent that the present invention is adaptable in an alternative embodiment to a system having only motor control chip such as the system shown in FIG. 4. This figure shows the same elements of FIG. 2, but with the elimination of motor control chip 12 because the motor 14 only has to travel in a single direction. Nevertheless, the motor 14 is still supplied with a higher motor drive voltage enabling high speed operation.

It is to be understood that the above-described embodiments are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements.

Claims

CLAIMSWhat is claimed is:

1. A high speed, brushless, direct current (DC) motor, said motor comprised of: a brushless DC motor having motor windings; a power booster circuit coupled at an output interface to the motor windings and supplying a final drive motor voltage thereto, having an input interface for receiving a preliminary, electrically isolated and lower voltage drive motor voltage which is used to generate the final drive motor voltage, providing a center tap output signal to provide a back electromotive force (EMF) information signal, and being powered by a first power source which also supplies the final drive motor voltage to the motor windings of the brushless DC motor; and a first limited voltage motor control circuit electrically coupled to the power booster to thereby supply the preliminary, electrically isolated and lower voltage drive motor voltage, having a center tap input which is electrically coupled to the center tap output signal to thereby receive the back EMF information signal to thereby generate the preliminary, electrically isolated and lower voltage drive motor voltages, and being powered by a second power source which is lower in voltage than the final drive motor voltage.

2. The high speed, brushless, DC motor as defined m claim 1 wherein the high speed, brushless, DC motor is capable of bidirectional operation.

3. The high speed, brushless, DC motor as defined m claim 2 wherein the high speed, brushless, DC motor is further comprised of a second limited voltage motor control circuit which operates in parallel with the first limited voltage motor control circuit, to thereby enable the high speed, brushless, DC motor to operate bidirectionally, wherein the first limited voltage motor control circuit controls operation of the high speed, brushless, DC motor m a first rotational direction, and the second limited voltage motor control circuit controls operation of the high speed, brushless, DC motor in an opposite rotational direction.

4. The high speed, brushless, DC motor as defined m claim 3 wherein the first limited voltage motor control circuit and the second limited voltage motor control circuit operate sequentially.

5. The high speed, brushless, DC motor as defined in claim 1 wherein the back EMF information signal is used to determine the angular position of a rotor in the high speed, brushless, DC motor.

6. The high speed, brushless, DC motor as defined in claim 4 wherein the first limited voltage motor control circuit is m a high impedance state when the second limited voltage motor control circuit is providing the preliminary, electrically isolated and lower voltage drive motor voltages, and wherein the second limited voltage motor control circuit is m a high impedance state when the first limited voltage motor control circuit is providing the preliminary, electrically isolated and lower voltage drive motor voltages, thereby avoiding interference.

7. A high speed, brushless, DC motor as defined in claim 2 wherein there are three separate motor windings in the high speed, brushless, DC motor, each of which are supplied with the final drive motor voltage as determined by the limited voltage motor control circuit, and wherein a sequence of supplying the final drive motor voltage to the three windings determines a direction of rotation of the high speed, brushless, DC motor.

8. The high speed, brushless, DC motor as defined in claim 7 wherein the first limited voltage motor control circuit is electrically coupled to the power booster circuit such that two of the three separate motor windings are switched in relation to how the second limited voltage motor control circuit is electrically coupled to the power booster circuit, to thereby drive the high speed, brushless, DC motor m opposite directions depending upon whether the first or the second limited voltage motor control circuit is supplying the preliminary, electrically isolated and lower voltage drive motor voltages.

9. The high speed, brushless, DC motor as defined in claim 3 wherein the second power source provides a voltage which is too high for safe operation of the first or the second limited voltage motor control circuit .

10. The high speed, brushless, DC motor as defined in claim 3 wherein the preliminary, electrically isolated and lower voltage drive motor voltages and the back

EMF information signal share a common path between the power booster, the first limited voltage motor control circuit, and the second limited voltage motor control circuit .

11. The high speed, brushless, DC motor as defined in claim 7 wherein the power booster circuit is comprised of three separate motor winding circuits, wherein each of the three motor winding circuits receives the preliminary, electrically isolated and lower voltage drive motor voltage, and determines whether the final drive motor voltage should be supplied to the motor winding of the high speed, brushless, DC motor which it controls.

12. The high speed, brushless, DC motor as defined in claim 11 wherein each of the three motor winding circuits is comprised of: a first circuit coupled to a winding for supplying the final drive voltage to the winding of the high speed, brushless, DC motor if the first circuit s activated; a second circuit coupled to the winding for grounding the winding of the high speed, brushless, DC motor if the second circuit is activated; and a third circuit coupled to the winding for providing a back EMF information signal to the first or the second limited voltage motor control circuit if the first circuit and the second circuit are in a high impedance state.

13. The high speed, brushless, DC motor as defined m claim 12 wherein the first circuit is comprised of: a first comparator having as an input at a negative terminal the preliminary, electrically isolated and lower voltage drive motor voltage and an attenuated motor winding signal, and having as an input at a positive terminal a high trigger voltage reference signal; a first power transistor for supplying the final drive motor voltage to the winding; and a level shift coupled between the first comparator and the first power transistor to thereby activate the first power transistor if the input signal at the negative terminal of the first comparator is greater than the high trigger voltage reference signal .

14. The high speed, brushless, DC motor as defined in claim 12 wherein the second circuit is comprised of: a second comparator having as an input at a negative terminal the preliminary, electrically isolated and lower voltage drive motor voltage and an attenuated motor winding signal, and having as an input at a positive terminal a low trigger voltage reference signal; a second power transistor for grounding the winding; and a gate driver coupled between the second comparator and the second power transistor to thereby activate the second power transistor if the input signal at the negative terminal of the second comparator is lower than the low trigger voltage reference signal.

15. The high speed, brushless, DC motor as defined in claim 12 wherein the third circuit is comprised of: a divider and biasing network to provide a DC bias to the motor winding signal; a unity gain amplifier electrically coupled to an output of the divider and biasing network for boosting a voltage of the motor winding signal; a resistor electrically coupled to an output of the unity gam amplifier for attenuating the motor winding signal to thereby provide the motor winding signal to the first circuit, the second circuit, and the first and the second limited voltage motor control circuit; and the center tap output electrically coupled to the output of the unity gain amplifier for transmitting the back EMF information signal to the first and the second limited voltage motor control circuit.

16. A method for providing a high speed, brushless, direct current (DC) motor while using a limited voltage motor control circuit, said method comprising the steps of : a) providing the high speed, brushless, DC motor; b) providing the limited voltage motor control circuit; and c) disposing a power boosting circuit between the high speed, brushless, DC motor and the limited voltage motor control circuit, wherein a motor drive voltage is supplied by the power boosting circuit to the high speed, brushless, DC motor, instead of using a lower voltage drive motor signal from the limited voltage motor control circuit.