Patent Publication Number: US-9899946-B2

Title: Sensorless control of switched reluctance machines

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to switched reluctance machines, and more particularly, to sensorless systems and methods for controlling switched reluctance machines. 
     BACKGROUND 
     An electric machine such as an electrical motor, power generation system, genset, or the like, is generally used to convert one form of energy into another and may operate in a motoring mode to convert electrical input into rotational or otherwise mechanical output, or operate in a generating mode to convert rotational or otherwise mechanical input into electrical output. Among the various types of machines available for use with an electric drive, switched reluctance (SR) machines have received great interest for being robust and cost-effective. While currently existing systems and methods for controlling such electric machines may provide adequate control, there is still room for improvement. 
     Among other factors, proper determination of the position and speed of the rotor of the SR machine during relatively low speed operations may have significant impacts on overall performance and efficiency. Some conventional control schemes rely on mechanically aligned speed wheels and sensors to detect and determine the position of the rotor relative to the stator at machine standstill or low speed operations. However, such sensor-based control schemes typically require costly implementations and are susceptible to error. For instance, an error of 2 degrees in the detected mechanical rotor position of an SR machine, caused by a skewed sensor, a mechanical misalignment of the speed wheel, or the like, may correspond to a 0.5% decrease in efficiency of the electric drive assembly at full load. 
     Although sensorless solutions also exist, conventional sensorless control schemes must implement two or more distinct processes for different ranges of operating speeds or operating modes. For instance, a conventional control scheme for low speed operations, such as that of U.S. Pat. No. 5,525,886 to Lyons, et al., may inject current signals and refer to lookup maps to estimate the rotor position, while a conventional control scheme for high speed operations may apply observers to phase currents to emulate and determine the rotor position. Such a need to simultaneously operate between distinct processes depending on the speed or mode of operation can be inefficient, cumbersome and unnecessarily waste computational resources. 
     In addition, although the lookup tables or maps used during low speed processes can quickly output the rotor position based on injected current signals, the accuracy of the rotor position at the output is only as good as the quality of the current signal that is read at the input. More specifically, because lookup tables or maps are not capable of sufficiently filtering out noise or distinguishing errors induced by noise from the targeted signal, the rotor position ultimately output can be based on noise-induced errors and thus susceptible to inaccuracies. Furthermore, while conventional systems typically derive rotor speed based on the rotor position, derivations or calculations based on noisy rotor position information can further compound noise-induced errors, output even noisier rotor speed information, and adversely impact the overall performance of the associated SR machine. 
     Accordingly, there is a need to provide control schemes for controlling SR machines that are not only less costly and easier to implement, but also more efficiently performed without compromising overall reliability. Moreover, there is a need to provide a simplified control system or a single unified control scheme that can operate across wider ranges of operating speeds or operating modes of an SR machine and consume less of the computational resources allocated for use with the SR machine. There is also a need to provide a solution that is more reliable and robust to noise, all without requiring additional computational resources or proximity sensors. The systems and methods disclosed herein are directed at addressing one or more of the aforenoted needs. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect of the present disclosure, a control system for a switched reluctance (SR) machine having a rotor and a stator is provided. The control system may include a converter circuit in electrical communication between the stator and a common bus, and a controller configured to monitor a bus voltage of the converter circuit and a phase current of the SR machine. The controller may include at least a phase voltage estimator module configured to determine a phase voltage based on one or more of main pulses and any diagnostic pulses, a flux estimator module configured to determine an estimated flux based on the phase voltage and an associated mutual voltage, a position observer module configured to determine a rotor position based at least partially on the estimated flux, and a main pulse control module configured to control the SR machine based on the rotor position and a desired torque. 
     In another aspect of the present disclosure, an electric drive is provided. The electric drive may include an SR machine having a stator and a rotor rotatably disposed relative to the stator, a converter circuit configured to electrically communicate with the stator and a common bus, and a controller in electrical communication with at least the converter circuit. The controller may be configured to monitor a bus voltage of the common bus and a phase current of the SR machine, receive main pulses and any diagnostic pulses, determine a phase voltage based on one of the main pulses and the diagnostic pulses, determine an estimated flux based on the phase voltage and an associated mutual voltage, engage a position observer to determine a rotor position based at least partially on the estimated flux, and control the SR machine based on the rotor position and a desired torque. 
     In yet another aspect of the present disclosure, a method for determining rotor position of an SR machine being operated through a converter circuit is provided. The method may include monitoring a bus voltage of the converter circuit and a phase current of the SR machine; receiving main pulses and any diagnostic pulses; determining a phase voltage based on one of the main pulses and the diagnostic pulses; determining an estimated flux based on the phase voltage and an associated mutual voltage; engaging a position observer to determine a rotor position of the SR machine based at least partially on the estimated flux; and controlling an output torque of the SR machine based on the rotor position and a desired torque. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of one exemplary embodiment of an electric drive with a control system for controlling a switched reluctance (SR) machine in accordance with the teachings of the present disclosure; 
         FIG. 2  is a diagrammatic view of one exemplary embodiment of a control scheme to be implemented by a controller for operating an SR machine; and 
         FIG. 3  is a flow diagram of one exemplary method for determining the rotor position and rotor speed of an SR machine. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , one exemplary electric drive  100  that may be employed to communicate power between a primary power source  102  and one or more electrical loads  104  is schematically illustrated. The primary power source  102  may include a diesel engine, a gasoline engine, a natural gas engine, or any other source of mechanical or rotational energy commonly used in association with mobile tools, industrial machines, and the like. The primary power source  102  may alternatively include power sources commonly used in conjunction with stationary applications, such as windmills, hydro-electric dams, batteries, fuel cells, or any other suitable source of energy. The electrical loads  104  may include one or more devices or components which consume and/or employ electrical power provided thereto by the electric drive  100 . For example, with respect to industrial work machines or mobile work vehicles, the electrical loads  104  may include one or more motors for operating tools of the machine and/or one or more traction motors for causing motion of the associated vehicle. 
     Mechanical energy that is supplied by the primary power source  102  may be converted into electrical power by the electric drive  100  for use by the connected electrical loads  104 . Conversely, any electrical power that may be supplied by the electrical loads  104  and/or the electric drive  100  may be supplied to drive mechanical power to the primary power source  102 . As shown in the particular embodiment of  FIG. 1 , for instance, the electric drive  100  may communicate with the primary power source  102  through a switched reluctance (SR) machine  106 , or the like. As is well known in the art, the SR machine  106  may include a rotor  110  that is rotatably disposed within a fixed stator  112 . The rotor  110  of the SR machine  106  may be rigidly and rotatably coupled to an output of the primary power source  102  via a coupling  108 , or in other embodiments, via a direct crankshaft, a gear train, a hydraulic circuit, or the like. Each phase or phase winding of the stator  112  of the SR machine  106  may be electrically coupled to a common bus  114  of the electric drive  100  via a converter circuit  116 . 
     During a generating mode of operation, as the rotor  110  of the SR machine  106  is rotated within the stator  112  by the primary power source  102 , electrical current may be induced within the stator  112  and supplied to the converter circuit  116 . The converter circuit  116  may in turn convert the electrical signals into the appropriate direct current (DC) voltage for distribution to the electrical load  104  and/or any other device via the common bus  114 . The common bus  114  may provide terminals  118 , such as positive and negative or ground lines, across which the common bus  114  may communicate a bus voltage or DC link voltage between one or more electrically parallel devices of the electric drive assembly  100 . The electrical loads  104  may include circuitry for converting the DC voltage supplied by the converter circuit  116  into the appropriate electrical signals for operating any one or more devices associated with the electric drive  100 . Additionally, during a motoring mode of operation, or when the electrical loads  104  become the sink of electrical power, the SR machine  106  may be enabled to cause rotation of the rotor  110  in response to electrical signals that are provided to the stator  112  from the common bus  114 . 
     As shown in  FIG. 1 , the converter circuit  116  may include a series of transistors or gated switches  120  and diodes  122  for selectively enabling one or more phase windings or phases of the SR machine  106 . A three-phase SR machine  106 , for example, may be driven using a converter circuit  116  with six switches  120  and six diodes  122  for selectively enabling or disabling each of the three phases of the SR machine  106 . Each of the switches  120  may further be enabled or disabled via gate signals while an external or secondary power source  124  provides power across the positive and negative lines  118  of the common bus  114  to force current through the respectively enabled switches  120  and diodes  122 . The electric drive  100  may also be provided with an exemplary control system  126  configured to, among other things, determine the position of the rotor  110  of the SR machine  106  relative to the stator  112  and control operation of the SR machine  106  based on the determined rotor position. 
     As illustrated in  FIG. 1 , the control system  126  may generally include the converter circuit  116 , at least one controller  128  in communication with the gated switches  120  of the converter circuit  116 , as well as a memory  130  in communication with the controller  128  that is provided within and/or external to the controller  128 . More particularly, the controller  128  may be electrically coupled to the switches  120  in a manner which enables the controller  128  to selectively engage the switches  120  and source current through the different phases of the SR machine  106 , as well as in a manner which enables the controller  128  to monitor electrical characteristics of the SR machine  106  and the bus or DC link voltage of the common bus  114  during operation of the SR machine  106 . The memory  130  may retrievably store one or more algorithms, machine data, predefined relationships between different machine parameters, preprogrammed models, such as in the form of lookup tables and/or maps, or any other information that may be accessed by the controller  128  and relevant to the operation of the SR machine  106 . 
     The controller  128  of  FIG. 1  may be implemented using one or more of a processor, a microprocessor, a microcontroller, a digital signal processor (DSP), a field-programmable gate array (FPGA), an electronic control module (ECM), an electronic control unit (ECU), or any other suitable means for electronically controlling functionality of the control system  126 . The controller  128  may be configured to operate according to predetermined algorithms or sets of instructions for operating the electric drive  100  and the SR machine  106  based on the rotational speed and/or position of the rotor  110  relative to the stator  112  or other operating characteristics of the electric drive  100 . Such algorithms or sets of instructions may be preprogrammed or incorporated into memory  130  that is associated with or at least accessible to the controller  128  as is commonly used in the art. Moreover, the algorithms or instructions implemented by the controller  128  may be categorized into modular arrangements such as that schematically shown for example in  FIG. 2 . 
     As shown in  FIG. 2 , the controller  128  may be configured to include a speed control module  132 , a torque control module  134  and a main pulse control module  136 , combinations of which are often employed in conventional SR machine controls. Specifically, the speed control module  132  may be configured to determine the desired or target speed of the SR machine  106  based on any combination of operator input, machine input parameters or constraints, automated controls, and the like. In turn, the torque control module  134  may be configured to determine the target output torque of the SR machine  106 , which corresponds to achieving the target speed determined by the speed control module  132 . The main pulse control module  136  may be configured to generate or use the switching command, or the sequence of pulses or current signals designed to selectively enable the switches  120  of the converter circuit  116 , to operate the SR machine  106  according to the target output torque or speed. 
     While the main pulse control module  136  may be suited for use with high speed operating modes or relatively high operating speeds of the SR machine  106 , low speed operating modes or relatively low operating speeds of the SR machine  106  may be managed by a diagnostic pulse control module  138  as shown in  FIG. 2 . In particular, high speed operating modes may involve operating speeds that are higher than or inclusive of a nominal or base speed, while low speed operating modes may involve operating speeds that are zero or lower than the base speed. Although base speeds may vary per application, base speeds can generally be defined as the maximum speed at which the SR machine  106  is able to output constant torque and before torque output begins to decrease proportionally in relation to the operating speed. 
     Still referring to  FIG. 2 , the diagnostic pulse control module  138  of the controller  128  may be disposed in parallel to the main pulse control module  136 . The diagnostic pulse control module  138  may be configured to generate and inject diagnostic or test pulses into each idle phase of the stator  112  or phases that are not controlling the SR machine  106  at a given instance. Moreover, the diagnostic pulses may selectively enable the corresponding switches  120  of the converter circuit  116  and drive a phase current with a substantially constant current height through each idle phase of the stator  112 , so as to facilitate computations to be performed later. Furthermore, the diagnostic pulse control module  138  may be configured to generate and inject the test pulse into idle phases according to the most recently assessed or estimated rotor position. 
     As demonstrated by the architecture of the controller  128  in  FIG. 2 , processes that are applied to either the main pulses or the diagnostic pulses are streamlined into a single algorithm or sequence of instructions. Moreover, the entire range of operating speeds of the SR machine  106 , for instance, both high speed and low speed operating modes, which previously required two or more distinct algorithms or process groups, may be managed by the unified algorithm or set of processes of  FIG. 2  to not only reduce complexity, but also to spare significant computational resources. For example, either the main pulse during relatively high speed operations, or the diagnostic pulse during relatively low speed operations can be processed by the controller  128  in substantially the same manner to assess rotor position and/or rotor speed. At even higher or very high speeds, the main pulse control module  136  will likely shift between a discontinuous conduction mode (DCM) and a continuous conduction current mode (CCM) to satisfy certain power requirements. Based on the sensorless architecture and techniques disclosed herein, rotor position and speed estimation may also be possible at such high speeds and during CCM modes of operation. 
     To determine rotor position or rotor speed, the controller  128  of  FIG. 2  may initially provide a phase voltage estimator module  140  configured to determine the voltage of the phase or phase voltage based on either the main pulse or the diagnostic pulse and known electrical properties between phase voltage and phase current for the given SR machine  106 . The controller  128  may further include a mutual voltage estimator module  142  configured to determine the associated mutual voltage, for instance, with reference to one or more preprogrammed lookup tables, maps, or the like, which predefine relationships between mutual voltage values, phase current values, estimated rotor position values, and the like. Furthermore, the controller  128  may apply the phase voltage and the mutual voltage, and any suitable calculation, computation, derivation and/or manipulation thereof, as inputs to a position observer module  144  to determine rotor position and to a speed observer module  146  to determine rotor speed as shown in  FIG. 2 . 
     While other manipulations or derivations based on the phase voltage and the mutual voltage will be apparent to those of skill in the relevant art, the controller  128  of  FIG. 2  may provide a flux estimator module  148  configured to determine an estimated flux based on the phase voltage and the mutual voltage. For example, the flux estimator module  148  may use known electrical properties to determine a total flux based on the determined phase voltage and to determine a mutual flux based on the determined mutual voltage. The flux estimator module  148  may then calculate the estimated flux based on a difference between the total flux and the mutual flux. As shown, the controller  128  may further employ a current estimator module  150  which determines an estimated phase current based on the estimated flux, and a current error synthesis module  152  which determines the error between the estimated phase current and one or more phase currents of the SR machine  106 . The current error may then be fed into each of the position observer module  144  and the speed observer module  146  to determine the rotor position and the rotor speed, respectively. 
     The position observer module  144  of  FIG. 2  may employ a state observer system which emulates the internal state of a real system, as well as receives input parameters and generates output parameters much like a real system. In the present case, the position observer module  144  may be configured to at least partially emulate a real SR machine  106 , receive current error as input, and generate rotor position as output. While the position observer module  144  shown in  FIG. 2  may be configured to determine rotor position based on current error, it will be understood that the position observer module  144  may be modified to determine rotor position using other inputs, such as the phase voltage, mutual voltage, estimated flux, phase current, or any other suitable parameter adapted by the controller  128 . Optionally, the controller  128  may also provide a position processing module  154  configured to process the output of the position observer module  144  as needed to further refine and/or calibrate the estimated rotor position. 
     Similar to the position observer module  144 , the speed observer module  146  may employ a state observer system to at least partially emulate the internal state of a real SR machine  106 , receive current error as input, and generate rotor speed as output. Additionally, although the speed observer module  146  may be configured to determine rotor speed based on current error, the speed observer module  146  may be modified to employ other inputs, such as the phase voltage, mutual voltage, estimated flux, phase current, or any other suitable parameter adapted by the controller  128  to assess rotor speed. In other modifications, the speed observer module  146  may be omitted entirely, and derivations of the rotor position with respect to time may be used to determine rotor speed. However, it will be understood that such indirect estimations of rotor speed may magnify any noise or other errors untreated by the position observer module  144 . Furthermore, the controller  128  may optionally include a speed processing module  156  configured to process the output of the speed observer module  146  as needed to further refine and/or calibrate the estimated rotor speed. 
     INDUSTRIAL APPLICABILITY 
     In general, the foregoing disclosure finds utility in various applications relating to switched reluctance (SR) machines or any other suitable electric machine being employed as a motor and/or generator. In particular, the disclosed systems and methods may be used to provide more efficient control of SR machines that are typically employed in association with the electric drives of power generation machines, industrial work vehicles, and other types of machines commonly used in the art. The present disclosure may also be implemented with other variable-speed drives commonly used in association with industrial and consumer product applications. The present disclosure may further be used with integrated starters, generators, or the like, commonly associated with automotive, aerospace, and other comparable mobile applications. 
     One exemplary algorithm or controller-implemented method  158  for operating an SR machine  106  is diagrammatically provided in  FIG. 3 . As shown in block  158 - 1 , the controller  128  may initially be configured to monitor the bus voltage or the DC link voltage of the common bus  114  and/or the converter circuit  116  associated therewith, as well as the phase current of the SR machine  106 . The controller  128  per block  158 - 2  may further be configured to generate or use main phase current pulses, such as via the main pulse control module  136  of  FIG. 2 , for example, in accordance with a switching command designed to selectively enable switches  120  of the converter circuit  116  to operate the SR machine  106  according to a desired or target speed or torque. Furthermore, during low speed operations, the controller  128  according to block  158 - 3  may be configured to generate diagnostic pulses, such as via the diagnostic pulse control module  138  of  FIG. 2 . More specifically, the diagnostic pulses may be generated and injected into each idle phase of the stator  112  and designed to selectively enable the corresponding switches  120  of the converter circuit  116 . 
     Additionally, the controller  128  according to block  158 - 4  of  FIG. 3 , may be configured receive either the main pulses or the diagnostic pulses such as via the phase voltage estimator module  140  of  FIG. 2 . More particularly, the controller  128  may accept the main pulses or the diagnostic pulses without discrimination and without regard to the operating mode or speed of the SR machine  106 . Furthermore, the controller  128 , such as via the phase voltage estimator module  140 , may then be configured to determine the phase voltage associated with the received pulses according to block  158 - 5 . The controller  128 , such as via the flux estimator module  148  of  FIG. 2 , may additionally be configured to determine an estimated flux based at least partially on the phase voltage in block  158 - 6 . In one example, the flux estimator module  148  may compute a total flux based on the determined phase voltage, compute a mutual flux based on a mutual voltage associated with the received pulse, and ultimately calculate the estimated flux as the difference between the total flux and the mutual flux. In alternative embodiments, the controller  128  may determine the estimated flux using other techniques known in the art. 
     Still referring to  FIG. 3 , the controller  128  may further be configured to engage a position observer, such as with the position observer module  144  of  FIG. 2 , to determine the rotor position based on the estimated flux according to block  158 - 7 . As described with respect to  FIG. 2 , the controller  128  may determine an estimated phase current based on the estimated flux, determine the error between the estimated phase current and one or more phase currents of the SR machine  106 , and use the current error as an input to the position observer to output the estimated rotor position. In other embodiments, the controller  128  and/or the position observer may employ any one of a variety of other techniques to similarly derive the estimated rotor position directly or indirectly from the estimated flux. In still further modifications, the controller  128  and/or the position observer in block  158 - 7  may derive the estimated rotor position based on parameters other than the estimated flux, such as the phase voltage, mutual voltage, phase current, and the like. 
     Furthermore, the controller  128  may be configured to engage a speed observer, such as with the speed observer module  146  in  FIG. 2 , to determine the rotor speed based on the estimated flux according to block  158 - 8 . Similar to block  158 - 7 , the controller  128  in block  158 - 8  may use the speed observer to emulate a real SR machine  106  and to estimate the rotor speed based on current error. The controller  128  and/or the speed observer may alternatively employ other techniques to arrive at the estimated rotor speed. In addition, as illustrated in  FIG. 3 , the controller  128  may also be configured to perform the functions of block  158 - 8  in parallel with or independently of those of block  158 - 7 , so as to minimize any spread of corrupted data between the position observer and the speed observer. The controller  128  in block  158 - 9  may additionally perform any post-processing that may be needed to further refine and/or calibrate the estimated rotor position and/or rotor speed. Still further, the controller  128  in block  158 - 10  may be configured to control the SR machine  106  based on the estimated rotor position and/or the estimated rotor speed, as well as based on the desired torque and/or speed of the SR machine  106 . 
     Based on the foregoing, the present disclosure provides a simplified and yet robust solution for operating an SR machine across a much wider range of operating speeds. More particularly, the present disclosure provides a control architecture which streamlines the processes used for determining the rotor position of an SR machine to conserve computational resources and excess costs associated therewith. The present disclosure also employs independent position and speed observers which naturally filter and/or correct for noise-induced errors to provide for more reliable results. The present disclosure thereby provides a sensorless solution that eliminates the need for costly position or proximity sensors without compromising performance. It will be appreciated that while only certain embodiments have been set forth for the purposes of illustration, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.