Abstract:
A control system for a permanent magnet motor having low-resolution speed measurement is taught. The control system is comprised of a speed regulator, voltage regulator and current regulator. The speed regulator uses a speed command and a measured speed for calculating a desired motor voltage; the voltage regulator uses a voltage command and an applied motor voltage for calculating a motor current command; and the current regulator uses a current command and a motor phase current for calculating a desired motor voltage for use by said voltage regulator; thereby providing a low resolution speed measurement to be used in a high performance application.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the field of electronics. More specifically, the present method and apparatus relate to methods and systems for controlling a permanent magnet motor having a speed sensor with poor or coarse resolution.  
           [0003]    2. Description of the Related Art  
           [0004]    Permanent magnet motors are generally regarded today as an interesting solution for a wide range of inverter-fed variable-speed drives. Advantages of these motors in comparison to state of the art asynchronous motors include lower losses and higher torque density.  
           [0005]    The motor control industry as a whole is a strong and aggressive sector. In order for members of the industry to remain competitive, they must not only reduce costs imposed by governments and power plant lobbies, but also answer to power consumption reduction and EMI radiation reduction issues. One consequence of these constraining factors is the need of enhanced control strategies for permanent magnet motors.  
           [0006]    Present permanent magnet motor control strategies typically require high-resolution sensors. A permanent magnet motor has available a number of various kinds of sensors that can be used, such as optical encoders which are subject to contamination, and resolvers which require a great deal of circuitry. Such sensors are expensive and are not very reliable. Accordingly, there is a need for a control strategy that provides excellent motor speed control without the use of high-resolution sensors.  
         BRIEF SUMMARY OF INVENTION  
         [0007]    In the disclosed embodiment, the present method and apparatus alleviate the drawbacks described above with respect to known control strategies for permanent magnet motors by employing algorithms that estimate motor speed. The present method and apparatus utilize a series of nested loops to give the power signals a uniform ground reference with associated signal stability and signal noise reduction.  
           [0008]    The control strategy of the present method and apparatus is useful for high performance equipment, including but not limited to fuel cells, compressors, blowers and the like. High performance equipment requires very accurate speed control with limited information and good dynamic response. Other motor control strategies do not allow such good performance with limited information. By providing good performance with limited information, cost is driven down, e.g., a very robust and very expensive sensor is no longer needed.  
           [0009]    In contrast to the prior art, the control strategy of the present method and apparatus allows the substitution of an estimated motor voltage for an actual measured speed of a permanent magnet motor. By utilizing an estimated motor voltage, a less expensive speed sensor having only coarse resolution may be utilized. Additionally, the estimated voltage is more reliable than the measured speed.  
           [0010]    Three nested control loops determine the estimated voltage. The outermost control loop is a speed regulator. Generally, an external speed command ω* (for rotational speed) is provided to the control system, e.g., from a fuel cell controller. This is compared to a speed measurement ω, which is very slow, creating a speed error. The speed regulator functions to drive this error to zero. This loop must operate at a low repetition rate due to the limitations of the available speed measurement.  
           [0011]    The output of the speed regulator is an outer loop voltage command V*, which is compared against a motor voltage V measured in the second control loop, a voltage regulator loop. The voltage regulator loop operates at a higher rate than the speed regulator loop because the feedback quantity is a higher bandwidth signal than the speed measurement. The voltage regulator likewise attempts to drive the error to zero, producing a current command I*.  
           [0012]    The last loop, a current regulator loop, takes the current command I* and measures the physical current and calculates the desired instantaneous motor voltage required to synthesize the commanded current. This instantaneous voltage is integrated or averaged and fed back as the measured motor voltage (inner loop voltage command feedback signal) that is used by the voltage regulator.  
           [0013]    Compensation for changing bus voltage is also provided in the form of a feed forward voltage function. The feed forward voltage function is provided to ensure a steady state and calculate the terminal voltage of the motor based upon the speed command. A pulse width modulator and inverter is provided which is comprised of a set of switches that will, over some period of time, average the value of the voltage that is applied. For example, suppose the system commands 50 volts. If the bus voltage is 200, a 25% duty cycle is commanded. With 25% of 200 and 75% of zero, the average over time is 50 volts.  
           [0014]    The system advantageously allows the use of a low resolution speed measurement in high performance applications. The voltage regulator loop provides a very good substitute for the motor speed measurement as it has relatively high bandwidth compared to the available physical measurement. The voltage regulator loop also has constant dynamics, unlike a speed sensor that typically has quantization difficulty at high speeds and delay problems at low speeds.  
           [0015]    The general beneficial effects described above apply generally to each of the exemplary descriptions and characterizations of the devices and mechanisms disclosed herein. The specific structures through which these benefits are delivered will be described in detail herein below.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0016]    In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn are not intended to convey any information regarding an actual shape of the particular elements, and are solely selected for ease of recognition in the drawings.  
         [0017]    [0017]FIG. 1 is an overall functional block diagram for a drive controller according to one illustrated embodiment of the present method and apparatus.  
         [0018]    [0018]FIG. 2 is a block diagram of one embodiment of the speed regulator according to one illustrated embodiment of the present method and apparatus.  
         [0019]    [0019]FIG. 3 is a block diagram of one embodiment of the voltage regulator according to one illustrated embodiment of the present method and apparatus.  
         [0020]    [0020]FIG. 4 is a block diagram of one embodiment of the current regulator according to one illustrated embodiment of the present method and apparatus. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with dispensing apparatus, actuators, motors, motor controllers, processors and automated systems and devices have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.  
         [0022]    Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
         [0023]    The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention.  
         [0024]    [0024]FIG. 1 illustrates an overall control strategy for a drive controller system  10 . A controller area network (CAN)  20  accepts commands, including commands turning the controller ON or OFF, from an external device (not shown), such as a fuel cell control unit. The ON/OFF command is passed to a state machine function  25  that supplies an enable signal to an optional slew rate limiter  30 . The slew rate limiter  30  processes a speed command  26  from the external device and passes the slew rate limited speed command  35  to a speed regulator function  40 . The output of the speed regulator function  40  is an outer loop voltage command  45  that is then passed to a voltage regulator function  50 . The output of the voltage regulator function  50  is a current command  55  that a current regulator function  60  uses to generate pulse width modulator (PWM) commands (upper and lower phases) for driving an inverter  75  (FIG. 4). The commutation of the PWM to the inverter switches may be done in a Programmable Logic Device (PLD) in order to minimize the lag between positioned measurement and commutation that may otherwise result in reduced drive system capability and increased torque ripple. These PWM commands then set the motor drive  80 .  
         [0025]    As indicated above, the CAN  20  receives commands from an external device and passes the commands to the state machine function  25  that controls the overall operation of the drive controller system  10 . The CAN  20  may also receive feedback data from other auxiliary modules, package the data into a single feed back message, and send the message back to the external device. Also, the CAN  20  may be responsible for generating diagnostics relative to the operation of the CAN  20 , e.g., missed messages. In one embodiment, the CAN  20  may function at a rate in the range of 10 ms to 100 ms (10 Hz to 100 Hz), with one or more inputs and one or more calibratable parameters.  
         [0026]    The state machine function  25  controls the overall sequential operation of the motor drive  80 . As such, it takes inputs from many other functions. The main output of the state machine function  25  is an enable signal that both turns on the PWM outputs to the inverter  75  (FIG. 4) and turns on the speed regulator  40  via the slew rate limiter  30 .  
         [0027]    While the various functions may be implemented using discrete components, the drive controller system  10  may employ an integrated circuit for some or all of the functions, such as a microprocessor, application specific integrated circuit (ASIC), programmable logic array (PLA) or other integrated device.  
         [0028]    [0028]FIG. 2 shows one illustrated embodiment of a speed regulator  40  which may take the form of a subsystem of the drive controller system  10 . The speed regulator  40  sums an actual measurement of speed (i.e., speed feedback  41 ) and the speed command (speed command  26  from the external device  26  or slew rate limited speed command  35 ) to calculate a speed error  46 . The speed feedback  41  may, for example, be the actual drive speed of a compressor. The speed regulator determines the outer loop voltage command  45  from the speed error  46  and provides the outer loop voltage command  45  to the voltage regulator  50 . Since motor voltage is generally proportional to the motor speed, the speed regulator  40  may employ a Proportional/integral (PI) speed controller  43  to generate the outer loop voltage command  45 . The speed regulator  40  may also sum a feedforward term  42  with the output of the PI speed controller  43  to reduce the effort required by the PI controller  43 . The speed regulator  40  may further generate diagnostic information.  
         [0029]    Examples of inputs into the speed regulator include speed feedback  41 , slew rate limited speed command  35  (e.g., speed ramp), and voltage feedforward  42 . Speed feedback  41  represents the actual measured speed. Voltage feedforward  42  represents the feed forward voltage. Outputs from the speed regulator  40  include outer loop voltage command  45 , which represents the reference or command voltage. Preferably, the speed regulator  40  functions at a rate slower than the voltage regulator  50 .  
         [0030]    [0030]FIG. 3 shows one illustrated embodiment of a voltage regulator  50  which may take the form of a subsystem of the drive controller system  10 . The voltage regulator  50  controls the motor voltage. In general, motor speed is proportional to motor voltage. Though there is no motor voltage sensor, motor voltage may be estimated by averaging the inner loop voltage command feedback  51  of the current regulator  60 , preferably in the voltage measurement function  62  (FIG. 4). As an intermediate step, the voltage regulator  50  sums the outer loop voltage command  45  and inner loop voltage command feedback  51  to determine a voltage error  54 . The output of the voltage regulator  50  is the current command  55 . The voltage regulator  50  employs a PI voltage controller  53  to generate the current command  55 . Due to the topology of the inverter and commutator there is only one phase current, which is always considered to be positive.  
         [0031]    The voltage regulator  50  preferably functions at a rate faster than the speed regulator  40  but slower than the current regulator  60 . The inputs to the voltage regulator  50  include, but are not limited to, a inner loop voltage command feedback  51 , the outer loop voltage command  45 , and, in one embodiment, a current limiter (not illustrated). The inner loop voltage command feedback  51  is the estimated motor voltage provided by the voltage measurement function  62 . The outer loop voltage command  45  is the reference or command voltage provided by the speed regulator  40 . Current limit is the instantaneous phase current limit provided by a dynamic current limiter function. Outputs of the voltage regulator  50  include, but are not limited to, a current command  55 , the reference or current command that is sent to the current regulator function  60 .  
         [0032]    [0032]FIG. 4 shows one illustrated embodiment of the current regulator  60  which may take the form of a subsystem of the drive controller system  10 . The current regulator  60  is responsible for the regulation of the motor current  61 . Permanent magnet motors typically have low inductance. Accordingly, the current regulator  60  should have a very high bandwidth and therefore should execute very rapidly. In its simplest sense, the current regulator  60  is a proportional regulator where the gain is increased with speed in order to compensate for reduced forcing voltage, or bus voltage  62  less back emf. Preferably, a feed forward term  44  is included in order to eliminate the need for an integral in the loop. As an intermediate step the current regulator  60  sums the measured motor phase current  61  and the current command  55  to determine a current error  66 . The inner loop voltage command  56  of the current regulator  60  is preferably integrated for use as inner loop voltage command feedback  51  in the voltage regulator  50 . The bus voltage  62  should be greater than the dynamic motor voltage limit, thereby ensuring that the motor voltage calculation is accurate. The portion labeled  63  for the modulation index serves to drop small pulses when the modulation index is almost equal to 1.  
         [0033]    The current regulator  60  preferably functions at a rate that is faster than the voltage regulator  50 . Inputs into the current regulator  60  include, but are not limited to, the reference or phase current command  55  provided by the voltage regulator  50 ; the measured motor phase current  61  provided from the current measurement; the feedforward motor voltage  44  provided by the feedforward voltage function  42 ; a dynamic motor voltage limit from a dynamic voltage limiter; the filtered, scaled inverse of the bus voltage  62  provided by the bus voltage measurement function; the measured motor speed feedback  41 ; a signal to reset the voltage integrator; and the drive enable command provided by the state machine  25 . Outputs from the current regulator  60  include, but are not limited to, the modulation index  63  that is sent to the PWM output function  75 , preferably having a range of 0.5 to 1.0, and the integrated inner loop voltage command feedback  51 , which is supplied to the voltage regulator  50 .  
         [0034]    In operation, the above-described components provide a process for speed control for a motor drive  80  with limited position information. Each of the above subsystems or regulators  40 ,  50 ,  60  function as a respective control loop, with the current regulator loop  60  nested inside the voltage regulator loop  50 , and the voltage regulator loop  50  nested inside the speed regulator loop  40 .  
         [0035]    Beginning with the speed regulator loop, a speed command  35  and measured speed (i.e., speed feedback  41 ) are input into the loop in order to calculate a reference voltage or outer loop voltage command  45 . The speed command  35  is provided from an external device such as a fuel cell unit. The measured speed (i.e., speed feedback  41 ) is provided from the motor drive  80 . Both inputs are provided to a proportional/integral controller  43  in order to calculate the outer loop voltage command  45 . A feedforward voltage function  42  may provide a feedforward motor voltage  44  which is summed with the output of the PI controller  43  in order to reduce the effort required by the controller  43 . In one embodiment, the feedforward voltage function  42  may also provide diagnostics.  
         [0036]    The outer loop voltage command  45  from the speed regulator loop is then input to the voltage regulator loop along with an estimated inner loop voltage command feedback  51  in order to determine a reference current or phase current command  55 . As there is no motor voltage sensor, inner loop voltage command feedback  51  is estimated by averaging the inner loop voltage command  56  of the current regulator  60 . Utilizing another proportional/integral controller  53 , the two inputs provide the reference current or phase current command  55 .  
         [0037]    The phase current command  55  from the voltage regulator loop is combined with the current measurement  61  in a proportional controller  64 . The output from the proportional controller  64  is added with the feedforward motor voltage  44  in order to eliminate the need for an integral within this control loop. The output may then be integrated in order to provide the inner loop voltage command feedback  51  for use in the voltage regulator loop. A divider  70  divides the output by the voltage measurement function  62  for providing a modulation index value  63  for input into the pulse width modulator  75  (upper and lower phases), which is then commutated to the inverter switches of the motor drive. The commutation is preferably performed within a programmable logic controller.  
         [0038]    Although specific embodiments of an examples for the motor control apparatus and methods are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the invention, as will be recognized by those skilled in the relevant art. The teachings provided herein of the invention can be applied to other motor control apparatus and methods, not necessarily the permanent magnet motor control apparatus and method generally described above.  
         [0039]    The various embodiments described above can be combined to provide further embodiments. All of the above U.S. patents, U.S. patent applications and publications referred to in this specification are incorporated herein by reference, in their entirety. Aspects of the invention can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments of the invention. These and other changes can be made to the invention in light of the above detailed description. In general, in the following claims, the terms used should not be construed to limits the invention to the specific embodiments disclosed in the specification and the claims, which should be construed to include all motor control apparatus that operate in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.