DUAL MODE CONTROLLER FOR A BRUSHLESS DC MOTOR

Abstract: A dual mode controller for a brushless DC motor provides driving signals to an electronic commutator in two distinctly different modes defined by a predetermined transition speed. An encoder supplies motor position signals to the controller for use in generating the driving signals. At speeds below the transition speed the driving signals are generated in phase with the position signals. At speeds above the transition speed the phase of the driving signals is substantially advanced. This reduces the back EMF in the stator windings and substantially increases the maximum no-load speed of the motor.

Description: DUAL MODE CON'ΓROIJLER FOR A BRUSHLESS DC MOTOR Background Of the Invention This invention relates to the field of controllers for DC motors. It has particular application to a brushless DC motor having a permanent magnet rotor and a three phase stator. Such motors generally comprise a transistor arrangement for converting a DC power supply into three phase alternating current for use by the stator of the DC motor. The design of an electric motor customarily involves a trade off between speed and torque requirements. Generally speaking motor output speed decreases linearly with torque, while the motor current at the same time increases with output torque. Especially in a motor with permanent magnets the maximum available speed for a given current and the maximum available torque for that same current are in a fixed relationship which is determined by the laws of physics, irrespective of motor size or design. If a permanent magnet motor is designed to deliver a certain output torque for a certain current, then the maximum output speed, for a given voltage, is automatically determined and cannot arbitrarily be increased. Therefore the design of a permanent magnet motor, for example, a brushless DC motor, involves a compromise between conflicting torque and speed requirements. It is known that the impedance of a brushless DC motor is primarily resistive at low rotational speeds and becomes increasingly reactive with increasing speed. This causes a phase shift of the magnetic fields which are induced in the stator, which in turn reduces the torque produced by the motor. The prior art has recognized that particular problem and has dealt with it by advancing the phase of the current supplied to the stator windings. The control systems for providing the phase-shifted current are generally quite complex and attempt continuously varying adjustment as a function of speed. Examples of such motor control systems are disclosed in Dishner et al U.S. Patent 4,835,448 and in Meshkat-Razavi U.S. Patent 4,651,068. Those systems do not address the problem of providing high no-load speed while retaining high torque capability during startup or under stalling conditions. Summary of the Invention This invention provides a control system and method for maximizing the no-load speed of a brushless D motor without loss of starting torque. The invention uses a dual mode controller which causes the motor to operate in one or the other of two different modes, depending upon motor speed. At low speeds the motor operates in a Base Speed mode where it has a maximum torque constant and develops a base no-load speed. When the motor reaches a predetermined transition speed, the controller switches the motor to a High Speed mode characterized by a continually increasing speed/torque ratio. By switching to the High Speed mode the controller causes the maximum speed of the motor to increase by a factor of more than 2.4 over that which could be obtained in the base mode. A position encoder is coupled to the motor and provides motor position signals to the dual mode controller. The dual mode controller uses the position signals to develop driving signals which it passes on to a conventional electronic commutator. During operation in the Base Speed mode the dual mode controller generates the driving signals in phase with the position signals. The commutator uses the driving signals to generate gate control signals for an inverter. The inverter responds to the gate control signals and to a DC voltage source by generating polyphase AC driving currents for the stator windings of the brushless DC motor. In the High Speed mode the dual mode controller advances the phase of driving signals a predetermined amount, dependent upon the number of current phases being supplied to the motor. In the case of a three-phase motor the phase advance is approximately 60 degrees or about 2.8 milliseconds at 3600 RPM. For rotation in the reverse direction, the controller adjusts the driving signals to maintain the desired advance. The advance is frozen for a short period of time following mode transition in order to avoid hunting between modes. Brief Description of the Drawing Fig. 1 is a schematic block diagram of a dual mode control system for a brushless DC motor. Fig. 2 is an illustration of signal waveforms associated with operation of the apparatus of Fig. 1. Fig. 3 is a plot of performance information for the apparatus. Fig. 4 is a detailed schematic diagram of a dual mode controller. Description of the Preferred Embodiment Fig. 1 illustrates a three-phase brushless DC motor 20 being controlled in accordance with the present invention. Motor 20 has three Y-connected stator windings PA, PB and PC driving a permanent magnet rotor 22. Windings PA, PB, PC carry AC currents IA, IB, IC respectively. These currents are supplied by an inverte 24 and are mutually phase-separated by 120 degrees. As hereinafter described in detail, motor 20 has two distinctively different modes of operation; a Base Speed mode and a High Speed mode. In the Base Speed mode a Hall Effect encoder 2 monitors the rotation of rotor 22 and generates three position signals S1 , S2, S3. These position signals are applied to three separate lead lines, which for ease of understanding, are indicated by reference characters S1 , S2, S3, respectively. The Hall Effect link between encoder 26 and rotor 22 is indicated by the reference numeral 28. Position signals S1 , S2, S3 are sent to a dual mode controller 30 which transforms them into driving signals S1 * , S2 * , S3 * , respectively. The relationship between position signals S1 , S2, S3 and driving signals S1 * , S2 * , S3 * depends upon the operating mode, as shown i Table I and discussed in detail below. During operation in the Base Speed mode the two signal sets are the same. TABLE I si * S2* S3* n < n tr S1 S2 S3 n > = n tr & S R = 1 . S2 "1 S3 "1 sr 1 n > = n tr & S R = 0 S3 "1 sr 1 S2- 1 where : n tr = transition speed Sk _1 = invert of Sk; k = 1,2,3 Driving signals S1 , S«3- are applied to an electronic commutator 32 of conventional design. Suitable commutators for this purpose are commercially available and may be purchased easily. A general teaching of the configuration and operation of commutators for brushless DC motors may be found in Peters et al U.S. Patent 5,202,616 and references cited therein. Commutator 32 uses driving signals S1 * , S2 * , S3 * for generation of six gate control signals GA, GA' , GB, GB' , GC and GC . These gate control signals selectively control power switches SA, SA' , SB, SB' , SC, SC respectively. The six power switches are arranged in parallel with diodes 91-95. In the Base Speed mode the gate signals control the power switches such that current flow into motor 20 is maintained through two of the three motor windings PA, PB, PC at any one time. The current is supplied to different winding pairs in sequence and at the correct magnitude and phase to produce an electromagnetic torque of constant magnitude and direction. The waveform of the current has a frequency proportional to the frequency of the gate signals and a peak magnitude which varies with changes in the voltage of the DC power supply. The motor speed is controlled by adjusting the DC voltage. The operation of commutator 32 is such that periods of current flow through each individual winding correspond to periods of constant induced voltage in that winding and also such that the direction of current flow through each winding corresponds to the polarity of the induced voltage in that winding. The maximum rotational speed of rotor 22 occurs when the induced voltage in two of the three windings due to the relative motion of the magnetic field of the rotor balances the supply voltages being provided by inverter 24. During operation in the High Speed mode the driving signals S1 * , S2 * , S3 * are time-advanced by 60 electrical degrees by controller 30. This causes the currents IA, IB, IC to advance (but only by 30 degrees in phase), which reduces the effective induced voltage in the windings. Consequently the rotor speeds up until the effective induced voltage balances the supply voltage. In a typical application such a mode change will increase the maximum rotor speed by a factor of about 2.4. Fig. 2 illustrates the waveforms of the three voltages VA, VB and VC which are impressed across the three windings PA, PB and PC respectively. These voltages correspond to the position angles of the rotor relative to the stator windings PA, PB, PC and are sensed by encoder 26 for use in generating the three position signals SI, S2, S3. As illustrated in Fig. 2 the position signals switch from LO to HI when the winding voltage begins rising and from HI to LO when the winding voltage begins falling. Also shown on Fig. 2 are two sets of waveforms for the driving signals S1 * , S2 * , S3 * . Both sets of driving signal waveforms are for operation in the High Speed mode; the upper set being for rotor rotation in the forward direction, and the lower set being for rotor rotation in the reverse direction. In each case the driving signals are advanced, in the appropriate direction, 60 degrees relative to the corresponding rotor position angles. Controller 30 accomplishes the appropriate driving signal advancement through the logical transformations indicated above in Table I. In order to perform these transformations controller needs to know the rotor speed, rotor direction and transition speed. These parameters are determined by controller 30 as described below. Another control parameter is the transition time. When the controller 30 switches modes it freezes the values of S1 * , S2 * , S3 * for a sufficient period of time to prevent mode hunting. For a three- phase motor having 4 poles, a preferred freezing period is about 1/12 of a revolution. For other motor configurations the preferred freezing period would be somewhat different. Controller 30 may be readily implemented in either hardware or software. In one preferred embodiment controller 30 may comprise an EP600 PLD chip 40 and a comparator 42, as illustrated in Fig. 4. All of the digital logic for signal transformation is programmed into chip 40, while comparator 42 signals chip 40 when the transition speed has been reached. Chip 40 is programmed to change the mode at that time and to maintain the driving signals in a fixed form until a predetermined number of position pulses (3 pulses in the described embodiment) have been received from encoder 26. Chip 40 generates a square wave of 50% duty cycle on an output line denoted by the reference characters SPD. The SPD signal is clocked by the position signals S1 , S2, S3 and therefore has a frequency proportional to the speed of rotor 22. In the illustrated embodiment the SPD signal comprises 6 pulses per rotor rotation, so for a typical rotor speed of 4,00 RPM, SPD will have a frequency of 400 Hz. This signal i applied to an inverting input terminal 48 of amplifier 42. A capacitor 52 is also connected to terminal 48. This capacitor is charged by the SPD pulses and is discharged between pulses. A pair of resistors 44, 46 are connected to a non-inverting input terminal 50 of amplifier 42. These resistors provide a reference voltage for amplifier 42. At high speeds the voltage across capacitor 52 is reset before it has time to reach the reference voltage on terminal 50 of amplifier 42. At low speeds the peak voltage across capacitor 52 exceeds the reference voltage. Amplifier 42 produces an output signal LO/HI which is fed back to chip 40 and sampled at precisely timed intervals. This signal remains HI for speeds above the transition speed. For speeds below the transition speed the LO/HI signal goes LO for brief time periods spanning the sampling interval of chip 40. The sampled state of the LO/HI signal is used by chip 40 in order to implement the logic of Table I. Chip 40 may establish the direction of rotation by checking the phases of position signals S1 , S2, S3. For forward rotation S1 will lead S2, while for reverse rotation the opposite will be true. Alternatively, a motor direction signal FWD/REV may be supplied to chip 40. In such a case chip 40 may be programmed to control the motor direction, as well as the phase advance. It will be understood that resistors 44, 46 and capacitor 52 are selected so as to provide an analog timing circuit which will time out during the inter-pulse period at a predetermined transition speed. This function could be provided by digital circuitry, either inside chip 40 or in another device. Chip 40 functions as a three flip-flop state machine to establish the operating state of motor 20 and thereby implement the logic of Table I. The techniques involved in preparing a state diagram and programming chip 40 are routine and need not be described in detail. It would also be a matter of routine programming to implement the logic of Table I in any one of numerous, commercially available microprocessors. Fig. 3 illustrates test data for a typical three phase brushless DC motor operated in accordance with this invention. Attention is drawn to the arrow 101 which indicates a mode transition between low speed operation and high speed operation. The plot of Fig. 3 illustrates three curves, each of which has two pieces, one for the Base Speed mode and one for the High Speed mode. Curve pieces 102a, 102b are a plot of speed versus torque. As shown by the curve piece 102b, the motor speed increased linearly with decreasing torque up to point 108, where the mode change occurred. The phantom line 103 indicates an extension of line 102b to show what the motor speed variation would have been without the mode change. It is seen that the mode change occurred at a speed of about 3800 rpm, where the motor was delivering a torque of about 31 ounce inches. Following the mode change, the speed increased rapidly with decreasing torque as illustrated by the curve 102a. Under no load conditions the motor reached a speed in excess of 12,000 rpm, whereas less than half of that speed would have been reached without the mode change. Curve portions 104a, 104b indicate the variation of DC current with torque for operation in the High Speed mode and Base Speed mode respectively. Curve portions 106a, 106b respectively show the corresponding torque per amp. It will be seen that during low speed, high torque operation the motor delivers about 2.5 ounce inches per amp, whereas for high speed operation it delivers a torque per amp ratio which decreases from about 1.6 down to about 1.1. Thus the invention effectively converts torque generating capability into increased speed when the firing angle is advanced approximately 60° relative to no load voltage. In a typical operation in accordance with the practice of this invention a brushless DC motor may be started under a near maximum load torque by supplying a driving current of appropriate magnitude. The voltage of the DC power supply is adjusted to maintain the necessary current flow. As the motor begins moving and gains speed, the torque may be reduced. This causes a further speed increase, and the process is repeated until the transition speed is reached. Meanwhile the motor operates in the Base Speed mode, as described above. Upon reaching the transition speed, the controller automatically switches the motor to the High Speed mode. As further torque reductions are made, the above- described phase advance will enable the motor to continue gaining speed. The motor operates in the High Speed mode for the prescribed minimum period of time and thereafter until torque increases or supply voltage decreases cause a slowdown to the transition speed, whereupon Base Speed operation resumes. While the method and the form of apparatus herein described constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to this precise method or form of apparatus, and that changes may be made in either without departing from the scope of the invention which is defined in the appended claims.

Claims: CLAIMS : 1. Control apparatus for a brushless, polyphase DC motor comprising: (a) a position encoder for generating position signals indicating the rotational position of said motor; (b) a dual mode controller for generating driving signals which are in phase with said position signals when said motor is operating at a speed below a predetermined transition speed and which are advanced relative to said position signals when said motor is operating at a speed above said predetermined transition speed; and (c) power supply means for connection to a DC power source and generation of polyphase AC currents in timed relation with said driving signals for transmission to stationary windings of said motor. 2. Control apparatus according to Claim 1 wherein said power conversion means comprises an electronic commutator for generating gating signals in synchronism with said driving signals and an inverter for converting DC current from said DC power source into said polyphase AC currents in synchronism with said gating signals. 3. Control apparatus according to Claim 2 wherein said position encoder comprises Hall effect sensing means. 4. Control apparatus for a brushless, three-phase DC motor comprising: (a) a position encoder for generating position signals indicating the rotational position of said motor; (b) a dual mode controller for generating driving signals which are in phase with said position signals when said motor is operating at a speed below a predetermined transition —speed and which are advanced in phase approximately 60 degrees relative to said position signals when said motor is operating at a speed above said predetermined transition speed, said dual mode controller freezing the phase of said driving signals for a predetermined period of time following a phase advancement thereof; and (c) power supply means for connection to a DC power source and generation of three phase separated AC currents in timed relation with said driving signals for transmission to three stationary windings of said motor. 5. Control apparatus according to Claim 4 wherein said dual mode controller comprises means for freezing said phase of said driving signals for a period of time approximately equal to the time required by said motor to perform 1/12 revolution at said transition speed. 6. Control apparatus for a brushless, polyphase D motor comprising: (a) a position encoder for generating positio signals indicating the rotational position of said motor; (b) a dual mode controller for generating driving signals which are in phase with said position signals when said motor is operating at a speed below a predetermined transition speed and which are phase-advanced relative to said position signals when said motor is operating at a speed above said transition speed and for a predetermined settling period after passing upwardly through said predetermined transition speed; and (c) power supply means for connection to a DC power source and generation of polyphase AC currents in timed relation with said driving signals for transmission to stationary winding of said motor. 7. Method of driving a brushless, polyphase DC motor comprising the steps of: (1) supplying a DC voltage at a starting level; (2) generating a sequence of gating pulses at a starting frequency; (3) using said gating pulses to generate polyphase AC currents having maximum values corresponding to the level of said DC voltage and at a frequency proportional to the frequency of said sequence of gating pulses; (4) causing rotation of said motor by applying said polyphase AC currents to different stator windings thereof; (5) generating a plurality of sequences of position pulses, each of said sequences of position pulses indicating the rotational position of a different one of said stator windings; (6) applying a motion-resisting load torque to said motor; (7) sensing the rotational speed of said motor; (8) while said rotational speed is below a predetermined transition value, generating a sequence of driving pulses corresponding to, and in phase with, each of said sequences of position pulses; (9) adjusting the frequency and phase of said gating signals for synchronism with said sequences of position pulses; (10) increasing said rotational speed; (11) repeating aforesaid steps 3 - 10 until said rotational speed reaches said transition value; (12) after said rotational speed reaches said transition value, advancing the phase of said sequences of driving pulses a predetermined amount relative to said sequences of position pulses; (13) thereafter adjusting said rotational spee while maintaining the phase of said sequences of driving pulses in advance of said sequences of position pulses by said predetermined amount, said phase being so maintained for a predetermined period of time and thereafter until said rotational speed returns to said transition value. 8. A method according to Claim 6 wherein said motor is a three-phase brushless DC motor and said predetermined amount of phase advancement is a phase angle of approximately 60 degrees. 9. Method according to Claim 7 wherein said predetermined period of time is approximately equal to the time required for said motor to perform 1/12 of a revolution at said transition speed.

Inventors: DISSER ROBERT J, KLODE HAROLD, DISSER, ROBERT, J., KLODE, HAROLD
Applicants: ITT AUTOMOTIVE ELECTRICAL SYST, ITT AUTOMOTIVE ELECTRICAL SYSTEMS, INC.
Application year: 1995
Main classification: H02P6
IPC codes: H02P6