Brushless motor control system

Abstract: An all-digital control system for a brushless, three-phase, DC motor includes a main signal processor in direct digital communication with a drive controller. The controller generates a pulse width modulated signal having a duty cycle established by the command signal. A three-phase bridge generates a two-state control signal in response to the pulse width modulated signal. The control signal has an on-state which lasts for the duty cycle. A digital tachometer signal indicative of motor speed or position is thereupon processed and reported back to the main processor.

Description: This invention generally relates to an all-digital motor control system and, more particularly, to a system for controlling the speed or armature position of a brushless, three-phase, DC motor. Speed control systems for controlling the speed of a brushless DC motor are generally known. However, such systems rely, at least in part, on analog signals and analog-to-digital converters to convert the analog signals to digital signals for subsequent processing by digital signal processors. This adds hardware complexity and rigidity to the overall system, and the reliance on analog signals, at least in part, introduces an element of inaccuracy in motor speed control. It is a general object of this invention to advance the state of the art of speed control systems for brushless, three-phase, DC motors. It is another object of this invention to reduce the hardware requirement and system rigidity in such speed control systems. Another object of this invention is to provide all-digital accuracy in such motor speed control systems. In keeping with these objects, and others which will become apparent hereinafter, the invention resides in an all-digital control system according to claims 1 and 6. The system comprises a main digital signal processor for supplying a digital command signal indicative of a desired motor operation. A drive controller in direct digital communication with the main processor generates, for each phase, and in response to the command signal, a digital commutation signal to rotate the armature, as well as a digital pulse width modulated signal having a duty cycle established by the input command signal. The system further comprises switching means, eg. a three-phase bridge, in digital communication with the controller. The bridge is operative for generating, for each phase, and in response to each commutation signal and each pulse width modulated signal, a digital two-state control signal having an on-state which lasts for the duty cycle. The system still further comprises means in digital communication with the controller, for generating, for each phase, a digital tachometer signal indicative of armature position. The controller is further operative for processing the tachometer signal to generate a digital output signal indicative of the actual armature speed or position. The controller directly digitally communicates the output signal to the main processor. The main processor and the drive controller are interconnected by, and digitally communicate through, a plurality of optical fibers. No analog signals and, of course, no analog-to-digital converters, are used anywhere in the speed control system, thereby simplifying the hardware requirement for such system, and also eliminating any inaccuracies due to the presence of analog signals. A preferred feature resides in shutting down the system upon the elapse of a predetermined time during which no input signal is received by the controller. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. Fig.1 is a general block diagram of the overall all-digital speed control system according to this invention; Fig.2 is a detailed electrical schematic of the system of Fig.1; Fig.3 is a flow chart depicting part of the operation of the controller; Fig.4 is a flow chart depicting another aspect of the operation of the controller; and Fig.5 is a flow chart depicting still another aspect of the operation of the controller. The present invention is illustrated in terms of a control system for controlling the speed of a brushless three-phase, DC motor. Referring now to the drawings, Fig.1 is a general block diagram, and Fig.2 is a more detailed electrical schematic, of the overall all-digital motor speed control system of this invention. Reference numeral 10 identifies a brushless, three-phase, DC motor having an armature 12. Preferably, the motor is obtained from BEI Kimco Magnetics Division of San Marcos, California, as its part No.DIH 23-20-BBNB. This motor has a plurality of conventional Hall-effect sensors 14 mounted about the armature to sense armature position. The system includes a main digital signal processor (CPU) 16, preferably constituted as integrated circuit chip No.87C51-PLCC. The main processor 16 is in direct digital communication with a drive controller 18, preferably also constituted as integrated circuit chip 87C51-PLCC. Processor 16 supplies a digital input speed signal RX indicative of a desired armature speed to the controller 18 over line 20. The controller 18, as will be described in detail below, supplies a digital output speed signal TX indicative of the actual armature speed to the processor 16 over line 22. Controller 18 also communicates with the processor 16 over a RESET line 24. Lines 20, 22, 24 are high speed serial buses capable of transmitting data at 3.75 kbaud. Preferably, communication lines 20, 22 and 24 are optical fibers. However, the main processor and the controller may communicate by any parallel or serial communication means such as a parallel communication bus, a high speed serial hardwired interface or the like. Upon receipt of the input speed signal RX, controller 18 executes a software program. Controller 18 generates a set of six commutation signals, two for each phase of the motor, together operative for rotating the armature. More specifically, the controller includes an interior look-up table having a listing of six commutation bit patterns, each pattern representing a discrete command for the armature at an angular position spaced 60 electrical degrees from the previous armature position. The commutation signals are fed through, and processed in, a three-phase bridge circuit 26 and, optionally, through a bridge driver circuit (see Fig.2), wherein three position control signals, one for each phase, are output to the motor 10. The Hall-effect sensors 14 sense rotation of the armature and generate two-state Hall-effect signals which advise the controller 18 when to generate the commutation signals. This latter aspect of the operation of the controller 18 is displayed in the flow chart of Fig.3. The generation of the commutation signals is indicated by block 28. The reading of the Hall-effect sensors is denoted by block 30. If the controller 18 recognises that the state of the Hall-effect signals has changed (block 32), then the new state is saved (block 34) and the next commutation bit pattern is output to the motor (block 36). Thereafter, an internal counter operative for generating a tachometer (TAC) signal is incremented (block 38) prior to the next reading of the Hall-effect sensors. The tachometer signal is eventually processed to generate the aforementioned output speed signal TX. If the state of the Hall-effect sensors did not change in block 32, this indicates that the armature has not moved 60 electrical degrees and, hence, the controller attempts to read the Hall-effect sensors again in block 30. Controller 18 also generates in response to command data from the processor 16, a digital pulse width modulated (PWM) signal having a duty cycle established by said command data. The PWM signal is carried on a carrier signal having a frequency which, in the preferred case, is 3.90625 kHz. Controller 18 has an internal software PWM timer which, in the preferred case, establishes a PWM cycle of 256 microseconds. The PWM cycle has a high and a low state. The PWM output is allowed to continue running during the high state, but is reset to OFF in the low state. The command data controls how long the PWM timer runs; in the preferred case, from 14 - 242 µs. In this way, the duty cycle of the PWM signal is controlled from 5.47% - 94.53%. This aspect of the controller operation is depicted in Fig.4. Block 40 represents the generation of the PWM signal. The controller toggles and generates a two-state PWM bit (block 42) and tests the state of the PWM bit in block 44. If the PWM bit has a low state, then, as depicted in block 46, the PWM timer is reloaded from a command byte supplied by the processor 16. If the PWM bit has a high state, then the PWM timer is reloaded with the 2's complement of its existing value (block 48). As best shown in Fig.1, the PWM signal is fed to a drive logic unit 50 which, as shown in Fig.2, comprises three AND gates to which three of the commutation signals are conveyed. Unit 50 generates switching signal for the bridge 26. In turn, the bridge 26 generates, for each phase, the aforementioned modulated control signal having an on-state and an off-state. As shown in the flow chart of Fig.5, the Hall-effect sensors, as previously mentioned, send TAC signals back to the controller (block 50) and, more specifically, TAC signals are accumulated as they occur every 62.5 ms in a TAC timer (block 52). The resulting count from the TAC counter is processed into a tachometer signal which is processed by the controller and fed back to the processor 16 over line 22, and is indicative of the actual speed of the motor. In accordance with a preferred feature, a watchdog counter (block 54 in Fig.5) has a pre-set count of, for example, 500 ms. Upon receipt of the TAC timer interrupt, the watchdog counter counts down. If, as determined in block 56, the 500 ms has elapsed, then the entire system is shut down (block 58). If, however, the watchdog time has not elapsed, then the command data from the processor 16 is sent to the controller over line 20 as denoted in block 60. It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above. While the invention has been illustrated and described as embodied in an all-digital speed control system for brushless three-phase DC motors, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the present invention as defined in the following claims.

Claims: An all-digital control system for a brushless, three-phase, DC motor having an armature, comprising: (a) a main digital signal processor (16) for supplying a digital input command signal indicative of a desired motor operation; (b) a drive controller (18) in direct digital communication with the main processor, for generating, for each phase, and in response to the command signal, a digital commutation signal to rotate the armature, and a digital pulse width modulated signal having a duty cycle established by the input command signal; (c) switching means (26) in digital communication with the controller, for generating, for each phase, and in response to each commutation signal and each pulse width modulated signal, a digital two-state control signal having an on-state which lasts for said duty cycle; (d) Hall-effect sensors (14) in direct digital communication with the controller, for generating, for each phase, a digital tachometer signal indicative of armature position; and (e) said controller being further operative for processing the tachometer signal, to generate a digital output signal indicative of the actual armature speed or position, and for directly digitally communicating the output signal to the main processor; (f) said main processor and said controller being interconnected by and digitally communicating through a plurality of optical fibres. The system according to claim 1, wherein the main processor and the drive controller are interconnected by, and digitally communicative through, parallel or serial optical fibres. The system according to claim 1, wherein the plus width modulated signal has two states, and wherein the controller includes timer means having a timer output signal whose duration is established by the state of the command value. The system according to claim 1, wherein the controller includes a look-up table having commutation bit patterns, each corresponding to a different armature position. The system according to claim 1, wherein the controller includes watchdog timer means having a predetermined watchdog time, and wherein the controller includes shutdown means for ceasing generation of the commutation signals upon elapse of said watchdog time without receipt of the input command signal. An all-digital speed control system for a brushless, three-phase, DC motor having an armature, comprising: (a) a main digital signal processor (16) for supplying a digital input speed signal indicative of a desired armature speed; (b) a drive controller (18) in direct digital communication with the main processor, for generating, for each phase, and in response to the input speed signal, a digital commutation signal to rotate the armature, and a digital pulse width modulated signal having a duty cycle established by the input speed signal; (c) switching means (26) in digital communication with the controller, for generating, for each phase, and in response to each commutation signal and each pulse width modulated signal, a digital two-state speed control signal having an on-state which lasts for said duty cycle; (d) Hall effect sensors (14) in direct digital communication with the controller, for generating, for each phase, a digital tachometer signal indicative of armature position; and (e) said controller being further operative for processing the tachometer signal, to generate a digital output speed signal indicative of the actual armature speed, and for directly digitally communicating the output speed signal to the main processor; (f) said main processor and said controller being interconnected by, and digitally communicating through, a plurality of optical fibres. The system according to claim 6, wherein the duty cycle of the pulse width modulated signal lies in a range between 5.47% - 94.53%. The system according to claim 6, wherein the pulse width modulated signal is carried on a signal carrier having a frequency of 3.90625 kHz. The system according to claim 6, wherein the pulse width modulated signal has two states, and wherein the controller includes timer means having a timer output signal whose duration is established by the state of the pulse width modulated signal. The system according to claim 6, wherein the controller includes a look-up table having commutation bit patterns, each corresponding to a different armature position. The system according to claim 6, wherein the controller includes watchdog timer means having a predetermined watchdog time, and wherein the controller includes shutdown means for ceasing generation of the commutation signals upon elapse of said watchdog time without receipt of the input speed signal.

Inventors: KRAUSE KENNETH W, KRAUSE, KENNETH W.
Applicants: SMITH & NEPHEW INC, SMITH & NEPHEW, INC.
Application year: 1993
Main classification: H02P6
IPC codes: H02P6, H02H7