GB2102222A - Motor control circuit for a brushless DC electric motor - Google Patents

Abstract

The motor has plural sets of windings 45,46 and plural drive sections 27,28 each operative to commutate one of the sets of windings. The drive sections are coupled to a power source 32 by a switching circuit 30,35,36 which, in response to the speed of the motor, places the drive sections and thus the winding sets either in a series or in a parallel configuration across the terminals of the power source. As a result, the winding impedance and the back electromotive force reflected back to the power source is varied in accordance with the motor speed, thus enabling high torque at low speed and low torque at high speed. <IMAGE>

Description

SPECIFICATION Series/parallel motor drive This invention relates to electrical circuitry for the control of DC brush less motors and, more particularly, to a system which provides two values of torque constant that are dynamically selectible during motor operation. Brushless motors are similar to conventional DC motors, in that there is a fixed proportionality, called the voltage constant, between the motor speed and the electromotive force produced by the windings.

This electromotive force is opposite in polarity to the applied voltage required to operate the motor and deliver power to a mechanical load. When a brush less motor is operated from a fixed source of applied voltage there is a certain speed, called the no-load speed, which the motor cannot exceed because its electromotive force at that speed is equal and opposite to the applied voltage. At this speed, no significant current flows in the windings and, therefore, little or no torque is produced.

Thus, the speed range of a brushless motor is bounded by the applied voltage.

Brushless motors have another similarity to conventional DC motors, in that there is a fixed proportionality, called the torque constant, between the motor torque and the winding current. When a brush less motor is operated from a power source where current is limited, the torque of the motor is proportionally limited.

In both conventional DC motors and brushless motors, there is a fixed relationship between the voltage constant and the torque constant for a given winding configuration. This relationship may be expressed as an equation V Torque K RPM where K is a characteristic constant for the motor.

During the design of a conventional DC motor or a brushless motor the limitations of current and voltage from the power source have the effect of forcing a compromise in the selection of a winding configuration. For efficient generation of torque at low speed the best design would have a large value of torque constant. This, however, would limit the no-load speed to a low value. To achieve a high speed, the voltage constant must be low, with a correspondingly low torque constant which limits the available torque.

In many applications for motors it is highly desirable to obtain a high value of torque at low speeds, and a high no-load speed with low torque. For example, this characteristic is notably useful in traction motors, which must achieve high acceleration at low speeds, and also sustain high speeds with moderate torque output.

Conventional DC motors are sometimes designed with two or more field windings having different voltage and torque constants, and are used with switchgear to selectively connect these field windings to the power source for optimum use of the motor's multiple characteristics. A brushless motor, however, normally has a permanent magnet rotating field and, therefore, the multiple field winding techniques cannot be used to provide multiple motor characteristics.

Nevertheless, with DC brush less motors there are cases where more than one torque constant is desired for operating flexibility and energy conservation. Furthermore, in applications where fast response is desired, such as in servo drives, the ability to rapidly change the torque constant is highly desirable.

The aforementioned features are provided in a DC brush less motor drive system which incorporates plural sets of stator windings and corresponding plural switching control circuits.

These switching circuits are so arranged that the commutated winding sets may be effectively operated either in series or in parallel, to provide two different motor constants. The selection of series or parallel is performed electronically in a manner which facilitates rapid selection of the winding connection regardless of the speed or torque being developed by the motor.

The present invention is directed to an electrical drive system comprising an electric motor comprising first and second members movable relative to each other and winding means having first and second winding sections for imparting a relative motion between said first and said second members, and drive means for applying power from a source of electricity to said winding means, characterized in that said drive means include first and second drive sections connected respectively to said first and second winding sections; and said drive means further comprising coupling means for alternately coupling said first and said second drive sections in series and in parallel arrangements thus selectively providing different motor characteristics.

When the motor windings are connected in series across the terminals of the power source, the impedance seen by the source is double that which would be presented by a single set of windings. When the windings are connected in parallel across the terminals of the source, the impedance seen by the source is half that which would be presented by a single set of windings.

The switching elements may be activated by a sensor of motor speed, such as a tachometer, to provide the series arrangement at low motor speeds and the parallel arrangement at high motor speeds. Thereby, the torque constant is increased at low speed to prevent excessive current drain from the source, and the back EMF is decreased at high speed to permit the drawing of additional current from the source.

In a preferred embodiment of the invention a brushless DC motor is utilized having two sets of stator windings wherein each set has three windings connected in a wye configuration. The commutation of each winding set is accomplished by a separate switching bridge.

Each switching bridge is formed of six transistor switches arranged in a bridge circuit for a pulse-width modulation control of the winding current, the duty cycle of the modulation being selected to provide a desired average value for the winding current. A logic array such as a read-only memory selects the specific ones of the bridge switches that are to be closed and opened for electronic commutation of the winding current.

The memory is addressed by signals from a sensor of the relative position between the rotor and the stator to update the selection of the bridge switches as the position shifts between the poles of the rotor and the windings of the stator.

The logic array is further addressed by signals of a pulse-width modulator to initiate and to terminate pulses of winding current for providing the desired average value of the winding current.

The series/parallel connection is accomplished by arranging the two sections in series with the positive terminal of the second section being connected by a diode to the negative terminal of the first section. The series arrangement is connected across the positive and negative terminals of the power source. Thereby, current can flow from the positive terminals of the source through the first drive section including the winding set coupled thereto, through the diode into the second drive section including the winding set connected thereto, and into the negative terminal of the power source. The series connection is transformed to a parallel arrangement by means of two switches, one between the two positive terminals of the drive sections, and the other switch between the two negative terminals of the drive sections.Upon closing the series/parallel switches, the diode becomes back biased and the two drive sections are then in parallel. Upon opening the two series/parallel switches, the two drive sections are in series. Thereby, the drive sections can be electronically switched between parallel and series configurations for selectively applying the desired impedance characteristic and EMF characteristic to the source of power in accordance with the speed and current drain of the motor. In the alternative embodiments of the invention, the foregoing speed may be sensed by a tachometer or by measurements of winding current.

The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings.

Fig. 1 is a schematic diagram, partially in block diagrammatic form, of the motor control circuit of the invention coupled to a brushless DC motor; Fig. 2 is a schematic drawing of the circuitry of switches utilized in the drive section of Fig. 1.

Fig. 1 illustrates a control circuit 22 constructed in accordance with the invention coupled to a motor 24. The control circuit comprises two drive sections 27 and 28 which are connected in series by a diode 30. The series combination of drive sections 27 and 28 is connected across the terminals of a power supply 32 in conjunction with switches 35 and 36. The circuit 22 further includes a comparator 38 and a speed threshold 40 for operating the switches 35 and 36.

Motor 24 is a four-pole brushless DC motor including a rotor 42 and two sets of windings 45 and 46. Rotor 42 includes two north poles, N, positioned alternately about the rotor with two south poles, S. Each set of windings 45 and 46 is connected in a three-phase wye configuration.

The rotor shaft is mechanically coupled, as indicated by a dashed line 48, to a position sensor 50, a tachometer 52 and a load 54. The tachometer 52 provides a voltage proportional to the speed of rotation of rotor 42, the voltage being applied to the comparator 38 to signal the speed of the motor. Position sensor 50 may be a photoelectric or magnetic sensor of shaft angle, an exemplary magnetic sensor being shown. The position sensor 50 comprises two sets of Hall devices 56, there being a total of six Hall devices which are positioned with equal spacing about the periphery of a magnetized disc 58. The disc 58 rotates with the rotor 42, due to the mechanical coupling therewith, and has a set of magnetic poles corresponding to the configuration of the poles of the rotor 42.The system further includes a logic arrays 61 and 62, a pulse-width modulator 64, a clock 66 and a torque command 68. Electronic commutation of the winding current is provided by logic arrays 61 and 62 in cooperation with drive sections 27 and 28 and position sensor 50. Drive section 27 has six switches 71, with individual ones of the switches being further identified by the legends A-F, for applying current to the windings 45.

Drive section 28 has six switches 72, with individual ones of the switches being further identified by the legends A-F, for applying current to the windings 46. The blocks representing the Hall devices 56 are understood to include circuitry for converting each Hall signal to a one-bit binary signal. Thereby, logic array 61 is addressed by a three-bit signal on line 75 designating the position of the rotor 42 relative to the set of stator windings 45. The logic array 62 is similarly addressed by a three-bit binary signal on line 76 from the remaining three Hall devices 56 designating the relative position between the rotor 42 and the set of stator windings 46. The logic array 61 has output lines connected to control terminals 79 of individual ones of the switches 71 A-F. Similarly, the logic array 62 has output lines connected to control terminals 80 of individual ones of the switches 72A-F. The terminals 79 and 80 may also be further identified by the legends A-F corresponding to the identification of the switches 71 and 72. Thereby, in response to the position address signal on the line 75-76, the logic arrays 61 and 62 apply commands via their respective output lines to the terminals 79 and 80 which activate respectively the switches 71 and 72 to impart current to the sets of windings 45 and 46 for rotation of the rotor 42 in a desired direction.The commands by the logic arrays 61 and 62 are continually updated in accordance with the relative position between the rotor and the stator of the motor 24 for electronic commutation of winding current.

To control torque of the motor 24 for rotation of the load 54 in a desired direction, the torque command 68 has two output lines, one line providing a torque magnitude signal to the modulator 64, while the second line provides an address signal to the logic arrays 61-62 to select the direction of torque. The current and rotation signals of the torque command 68 may be established by manually setting the torque amplitude and direction into the torque command 68. Alternatively, such signals can be provided by a computer (not shown) when the system is used as part of a computer control system (not shown).

The modulator 64 is driven by clock pulses of the clock 66 to provide a pulse signal which is modulated in accordance with the signals of the current command 68. As is well known, the inductance and resistance of the stator windings serve as a filter for smoothing the pulsations of the pulse-width modulated signals to provide a resultant winding current having an average value proportional to the duty cycle of the pulse-width modulated signal. The output pulses of the modulator 54 constitute a one-bit binary signal with an amplitude that varies with a logic 0 and a logic 1. Thus, the output signal of the modulator 64 on line 82, as well as the rotation command signal on line 84 shown fanning into line 82, provide a two-bit address signal on line 86 for addressing the logic array 62 in conjunction with the address on line 76.Also, the address signal on line 86 is applied in conjunction with the address on line 75 for addressing the memory 61.

Thereby, the logic arrays 61 and 62 are enabled to select the appropriate ones of the switches 71 and 72 for applying currents in the requisite direction through the stator windings, while electronically commutating the currents, to provide the desired direction of rotation of the load 54. In response to the portion of the address on line 82, the current paths provided by the switches 71 and 72 are interrupted repetitively in accordance with the duty cycle of the pulse-width modulation on line 82 to provide the aforementioned proportioning of the winding currents.

In accordance with the invention, the switches 35 and 36 provide for the connection of the drive sections 27 and 28 in either a series or in a parallel configuration. Switches 35 and 36 are of the same construction and are controlled by a common signal from comparator 38 to be simultaneously either in a state of conduction or non-conduction. When switches 35 and 36 are conductive a current path is provided across power supply 32 via switching bridge 27, line 90 and switch 36 and a parallel path is provided via switch 35, line 88 and switching bridge 28. Diode 30 is back biased and non-conducting under these circumstances. When switches 35 and 36 are non-conductive, a single series path exists across the power supply via switching bridge 27, diode 30 and diode 30 then switching bridge 28.

While the voltage drops across the drive sections 27 and 28 in the series configuration differ from the voltage drops obtained with the parallel configuration, the operation of the drive sections 27 and 28 is the same for both the series and the parallel configuration. Accordingly, the drive section 27 applies currents to the set of windings 45, and the drive section 28 applies currents to the set of windings 46 for commutating the current in both the series and the parallel configuration. The actual magnitude of the currents flowing in individual ones of the stator windings depends on the magnitude of the signal from the torque command 68, and also depends on the difference between the voltage drop across a drive section 27 or 28 and the back EMF induced in the corresponding set of stator windings.

With increased speeds of rotation of the rotor 42, the back EMF increases. As a result, the net voltage appearing across the stator windings, namely the difference between the applied voltage and the back EMF, decreases with a resulting decrease in the winding current.

Neglecting the effect of voltage drops through the diode 30 and through the switches 71 and 72 which may be regarded as negligible, the applied voltage is substantially equal to either the voltage of the power supply 32 or one-half the voltage of the power supply 32, depending on whether the parallel or the series configuration has been implemented.

Accordingly, in accordance with a feature of the invention, the foregoing series configuration is implemented at the lower speeds of rotor rotation, and the foregoing parallel configuration is implemented at the higher speeds of rotor rotation. Thus, during the lower speeds of rotation, the lower value of applied voltage and the reduced value of back EMF are both present for establishing the value of the winding current.

At the higher speeds of rotation, the larger value of applied voltage and the increased value of back EMF are both present for establishing the value of the winding current. Thereby, substantial winding current can be obtained at both the low speed range and the high speed range of the motor 24.

As a result, the motor 24 can be operated with sufficient torque at the low speed range while having sufficient current delivery to permit operation at higher speeds than would be heretofore possible.

The desired speed of rotation at which the transition between the series and parallel configuration is to occur is manually set at the speed threshold 40. Thereupon, the comparator 38 compares the actual motor speed, as provided by the tachometer 52, and the transition speed, as provided by the speed threshold 40, to provide an output logic signal signifying the relationship between the actual speed and the transition speed. The output logic signal of the comparator 38 is either a logic 0 or a logic 1 depending on whether the motor speed is below the transition speed or has attained and exceeded the transition speed. The switches 35 and 36 are operated to close or open in response to the value of the output logic signal of the comparator 38 to produce the requisite interconnection of the drive sections 27 and 28 in accordance with the speed range of the motor 24.

Referring now to Fig. 2, a switch 71 comprises a transistor 100, a base driver 102 including a photodiode 104, a diode 106 connected with reverse polarity between the emitter and collector terminals of the transistor 100, a resistor 108 and a capacitor 110. The resistor 108 and capacitor 110 are connected in series between the collector and emitter terminals of the transistor 100. The base driver 102 includes a well known circuit (not shown) for applying base current through the base and emitter terminals of the transistor 100.

The base driver 102 is connected to the control terminal 79 by the optical coupling of the photodiode 104, the optical coupling permitting the voltage level at the terminal 79 to be set independently of the voltage appearing across the transistor 100. Upon the application of the base current to the transistor 100, the transistor 100 and the switch 71 are in a state of conduction terminating upon termination of the base current.

With reference also to Fig.1, current in a winding of the set 45, passes through the transistor 100 when the switch 71 is in a state of conduction.

The foregoing description with reference to a switch 71 applies to each of the six switches 71 A to 71 F of drive section 27, and also to each of the six swtiches 72 (72A-72F) of drive section 28.

The two switches 35 and 36 are also advantageously constructed in the same form as switch 71, it being understood that the transistors utilized herein have a sufficiently high current carrying capacity for supplying the current to all three windings of the winding sets 45 and 46. It is to be understood that the above-described embodiments of the invention are illustrative only and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiments disclosed herein.

Claims (12)

Claims

1. An electrical drive system comprising an electric motor comprising first and second members movable relative to each other and winding means having first and second winding sections for imparting a relative motion between said first and said second members, and drive means for applying power from a source of electricity to said winding means, characterized in that said drive means include first and second drive sections connected respectively to said first and second winding sections; and said drive means further comprising coupling means for alternately coupling said first and said second drive sections in series and in parallel arrangements thus selectively providing different motor characteristics.

2. The drive system of claim 1, characterized in that the drive means are coupled to said first and second drive sections in series or in parallel in accordance with a signal applied to said coupling means and indicative for the desired motor characteristics.

3. The drive system of claim 2, characterized in that the first and second drive sections are arranged in series or in parallel in accordance with the magnitude of said relative motion so that the impedance of said motor windings reflected back to said source is altered in accordance with the speed of said motor.

4. The drive system of claims 1 to 3, characterized in that it comprises diode means coupled between terminals of opposite polarity of said first and said second drive sections.

5. The drive system of claim 4, characterized in that said coupling means comprises a set of switches for alternately coupling and decoupling terminals of like polarity of said drive sections with a resulting alternation in the state of conduction of said diodes.

6. The drive system of at least one of claims 1 to 5, characterized in that it further comprises speed sensing means coupled between said motor and said drive means.

7. The drive system of claim 6, characterized in that each of said drive sections comprises a set of switches arranged in a bridge circuit configuration.

8. The drive system of claim 7, characterized in that it further comprises a position sensor of the relative position between said first and said second members of said motor, a memory device addressed by said position sensor for activating said switches of said drive sections, and a pulse width modulator addressing said memory device for modulating signals on control lines of said drive sections switches for carrying the amplitude of currents applied to said windings means.

9. The drive system of claims 1 to 8, characterized in that the electric motor is a brushless DC motor comprising a plurality of sets of windings and a position sensor for indicating the relative orientation between stator and rotor of said motor; and a separate drive switching circuit connected to each set of windings, each such drive switching circuit being operative to commutate a respective set of windings connected thereto; and series/parallel switching means operativeiy connected to connect said sets of windings in parallel or in series to thereby selectively providing different motor characteristics.

1 0. The drive system of claim 9, characterized in that said motor includes two sets of windings and wherein two drive switching circuits are provided, each connected to a different one of said sets of windings.

11. The drive system of claim 9, characterized in that it further includes speed sensing means coupled to sense the speed of said motor and connected to control said series/parallel switching means.

12. The drive system of claim 11, characterized in that said speed sensing means are operative via said drive switching means to connect said windings in parallel when said motor is operating below a predetermined speed and in series when said motor is operating above a predetermined speed.

1 3. The drive system of claims 9 to 12, characterized in that said drive switching circuit comprises a set of transistor switches in a bridge circuit configuration.

1 4. The drive system of claim 9, characterized in that it comprises logic control circuit means connected to each of said drive switching circuits and that the said position sensor controls the switching state of the drive switching circuits in accordance with the orientation between the stator and the rotor of said motor.

1 5. The drive system of claim 14, characterized in that a pulse-width modulator is connected to said logic control circuit means to control the current supplied to said windings by said drive switching circuits.