Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P25/00—Arrangements or methods for the control of AC motors characterised by the kind of AC motor or by structural details
  • H02P25/02—Arrangements or methods for the control of AC motors characterised by the kind of AC motor or by structural details characterised by the kind of motor
  • H02P25/08—Reluctance motors
  • H02P25/086—Commutation
  • H02P25/089—Sensorless control

Definitions

• the present invention relates generally to motors/generators and, more particularly, to high speed switched reluctance machines capable of starting a prime mover as well as generating electrical power for use on aircraft.
• the aerospace industry has consistently driven the leading edge of technology with the requirement for lightweight, high efficiency, high reliability equipment.
• the equipment must be lightweight because each additional pound of weight translates directly into increased fuel burn, and therefore, a higher cost of ownership and shorter range.
• the need for high efficiency results from the fact that each additional cubic inch required for equipment displaces the amount of revenue- generating cargo and passengers that can be carried on an aircraft.
• High reliability is important because every minute of delay at the gate increases the cost of ownership, and likewise, increases passenger frustration.
• an engine starter is also typically installed on the aircraft engine. This component is used only during starting, which occupies only a very small fraction of each operational cycle of the aircraft. In effect, the starter becomes excess baggage during the remainder of the flight, increasing overall weight, fuel burn, and cost of ownership, and decreasing overall range. This problem has been recognized and efforts have been expended to combine the starter and generator into a single package, thus eliminating the need for an additional piece of equipment used only a fraction of the time. Unfortunately, using synchronous AC or permanent magnet generators for this purpose, in addition to crating new problems associated with the start function, does not eliminate the inherent problems with these machines as described above.
• a switched reluctance machine is an inherently low cost machine, having a simple construction which is capable of very high speed operation, thus yielding a more lightweight design.
• the rotor of the switched reluctance machine is constructed from a simple stack of laminations making it very rugged and low cost without the containment problems associated with rotor windings or permanent magnets. Further, the rotor does not require rotating rectifiers, which contribute to failures, as does the AC synchronous machine.
• sensoriess operational techniques have been developed.
• the most trivial solution to sensoriess operation is to control the switched reluctance machine as a stepper motor in the fashion disclosed in Bass, et al. U.S. Patent No. 4,611,157 and ac inn U.S. Patent No. 4,642,543.
• machine inductance or reluctance is detected and utilized to estimate rotor position.
• phase inductance of a switched reluctance machine varies as a function of angle from alignment of the stator pole for that phase and a rotor pole
• a measurement of instantaneous phase inductance can be utilized to derive an estimate of rotor position. See MacMinn, et al. U.S. Patent No. 4,772,839, MacMinn, et al. U.S. Patent No. 4,959,596, Harris "Practical Indirect Position Sensing for a Variable Reluctance Motor,” Masters of Science Thesis, MIT, May 1987, Harris, et al. "A Simple Motion Estimator for Variable Reluctance Motors," IEEE Transactions on Industrial Applications, Vol 26, No. 2, March/April, 1990, and MacMinn, et al. "Application of Sensor Integration Techniques to Switched Reluctance Motor Drives," IEEE Transactions on Industry Applications, Vol. 18, No. 6, November/December 1992.
• phase inductance can be determined using a frequency modulation approach whereby a non-torque producing phase forms part of a frequency modulation encoder. See Ehsani, et al. "Low Cost Sensoriess Switched Reluctance Motor Drives for Automotive Applications," Texas A&M Power Electronics Laboratory Report (date unknown), Ehsani, et al.
• a position estimation subsystem has been developed by the assignee of the instant application and includes a relative angle estimation circuit, an angle combination circuit and an estimator including a Kalman filter.
• the relative angle estimation logic is responsive to the phase current magnitudes of the switched reluctance machine and develops an angle estimate for each phase.
• the angle combination logic combines the phase angle estimates to obtain an absolute angle estimate which eliminates ambiguities that would otherwise be present.
• the estimator utilizes a model of the switched reluctance machine system as well as the absolute angle measurement to form a better estimate of the rotor position and velocity and, if necessary or desirable for other purposes, the rotor acceleration.
• the simplest approach is to utilize the estimated rotor position developed by the Kalman filter to directly control commutation.
• the time required to estimate rotor position limits the number of position estimates that can be developed per electrical cycle by the Kalman filter, and hence an instantaneous position generation circuit is provided to convert the output of the Kalman filter to a signal that can properly control commutation.
• estimator lock will be indicated even when the estimator is running at an integer submultiple of the actual rotor electrical velocity.
• the estimator will appear to be in lock when in fact it is not. For example, assume that the check for lock detection is to be performed when the estimated angle is midway between unalignment and alignment (i.e., at a position of 3 ⁇ /2 radians). If the estimated velocity developed by the estimator is half the actual velocity, and there is a ⁇ /4 position shift, agreement will be obtained even though the estimator is not in lock.
• the prior art approaches discussed in the above-identified '244 patent may not be feasible at high rotor speeds when the number of samples per electrical cycle is limited and data are not taken at the necessary angles.
• Kalman filter innovation i.e., the difference between an absolute angle estimate and a predicted estimate. If the innovation comprises a white noise ⁇ like signal of small magnitude having substantially zero mean, then the estimator is determined to be operating properly. If the estimator loses lock with actual operating conditions, the innovation loses randomness. Such an occurrence is detected to develop an indication of a loss of accurate position estimate.
• Fig. 1 comprises a block diagram of a starting/generating system for an aircraft
• Fig. 2 comprises a block diagram of a prior art inverter control, inverter and switched reluctance machine
• Fig. 3 comprises a block diagram of an inverter control incorporating the present invention together with an inverter and a switched reluctance machine;
• Fig. 4 comprises a block diagram of a portion of the inverter of Fig. 3 together with the position estimation circuit of Fig. 3;
• Figs. 5A and 5B when joined at the similarly lettered lines, together comprise a flowchart illustrating programming for initializing the estimator 68 of Fig. 4;
• Fig. 6 comprises waveform diagrams of control signals for the switches of the inverter of Fig. 4 during initialization
• Figs. 7A and 7B when joined at the similarly lettered lines, together comprise a flowchart illustrating programming for detecting lock of the estimator 68 of Fig. 4 according to the present invention.
• a power conversion system 10 is provided on ⁇ board an aircraft (shown diagrammatically at 12) or other aerospace, land or water vehicle and includes a prime mover, for example, a gas turbine engine 14, which is coupled by a motive power shaft 16 to a switched reluctance machine 18.
• the machine 18 includes phase windings which are coupled to an inverter 20 operated by an inverter control 22.
• DC power is supplied to the inverter 20 and the inverter control 22 develops control signals for switches in the inverter 20 to cause the switched reluctance machine 18 to operate as a motor and supply motive power via the shaft 16 to the jet engine 14 for starting purposes.
• motive power is supplied by the gas turbine engine to the switched reluctance machine 18 via the shaft 16 and the resulting electrical power developed by the switched reluctance machine 18 is converted by the inverter 20 into DC power for one or more loads.
• the inverter 20 could be modified to develop constant frequency AC power for one or more AC loads.
• a prior art inverter control for operating the switched reluctance machine 18 includes a resolver 30, which is coupled by a motive power shaft 32 to the rotor of the switched reluctance machine 18. Excitation is provided by a resolver excitation circuit 34.
• the resolver 30 develops first and second signals over lines 36, 38 that have a phase quadrature relationship (also referred to as sine and cosine signals).
• a resolver-to-digital converter 40 is responsive to the magnitudes of the signals on the lines 36 and 38 and develops a digital output representing the position of the rotor of the switched reluctance machine 18.
• the position signals are supplied along with a signal representing machine rotor velocity to a control and protection circuit 42.
• the rotor position signals are also supplied to a commutation and current control circuit 44 having an input coupled to an output of the control and protection circuit 42.
• the circuits 42 and 44 further receive phase current magnitude signals as developed by the inverter 20.
• the circuits 42 and 44 develop switch drive signals on lines 46 for the inverter 20 so that the phase currents flowing in the windings of the switched reluctance machine 18 are properly commutated.
• the resolver 30 is expensive and inherently a source of single point failure. Further, the resolver-to-digital converter 40 is also an expensive component and, hence, it is desirable to eliminate these and other components (including the excitation circuit 34), if possible.
• Fig. 3 illustrates an inverter control 50 that incorporates the present invention together with the inverter 20 and the switched reluctance machine 18.
• a position estimation circuit 52 is responsive to the phase current magnitudes developed by the inverter 20, switch control or drive signals for switches in the inverter 20 and DC bus voltage magnitude to develop position and velocity estimate signals for a control and protection circuit 54.
• the position estimate signals are supplied to a commutation circuit 56.
• a current control circuit 58 is responsive to the phase current magnitudes developed by the inverter 20, as well as phase enable output signals developed by the commutation circuit 56 and a reference current signal developed by the control and protection circuit 54.
• the current control circuit 58 produces the switch control or drive signals on lines 60 for the inverter 20.
• a relative angle estimation logic circuit 62 includes N individual phase relative angle estimate circuits 63A, 63B....63N, each of which is associated with one of the N phases of the switched reluctance machine 18.
• the phase relative angle estimate circuit 63A is associated with phase A of the machine 18 and receives a current magnitude signal developed by a current sensor 64 adapted to sense the current flowing in a phase A winding WA of the machine 18.
• the winding WA is connected in a phase A leg 65A of the inverter 20 having a pair of diodes D1 and D2 and a pair of controllable power switches Q1 and Q2.
• the switches Q1 and Q2 receive switch control signals CS1 and CS2 (Fig. 6) from a pulse generator 66, and thereafter receive the control signals on the lines 60 from the current control circuit 58 of Fig. 3. Furthermore, during initialization, the phase relative angle estimate circuit 63A also receives the switch control signals CS1 and CS2 and thereafter receives the control signals on the lines 60 from the circuit 58. During initialization, and subsequently, during operation of the circuitry of Figs. 3 and 4 to control the machine 18, the circuit 63A develops a signal * representing ⁇ A an estimate of instantaneous angle from rotor/stator alignment for phase A of the machine.
• each of the remaining phase relative angle estimate circuits 63B,...,63N is responsive to an associated phase current magnitude signal and is further responsive to switch control signals either identical to the signals CS1 and CS2 (during initialization) or control signals developed by the circuit 58 (after initialization) for switches in the associated inverter phase.
• Each circuit 63B,...,63N develops a signal & ,... * , respectively, representing an estimate of instantaneous angle from rotor/stator alignment for the associated phase of the machine, both during initialization and thereafter.
• Each angle estimate signal * , & ,... » represents two possible ⁇ A ⁇ B ⁇ N solutions for estimated rotor position, either phase advanced with respect to (i.e., moving toward) the respective phase pole or phase delayed with respect to (i.e., moving away from) the respective phase pole.
• This ambiguity is removed by an angle combination circuit 67 which combines the signals ⁇ , ⁇ ,... & , to obtain an
• the angle estimate Q is provided to an estimator 68, e e preferably including a Kalman filter, which improves the estimate of rotor position to obtain a value Q .
• the estimator 68 develops a velocity estimate and further develops an estimated acceleration signal representing the estimated acceleration of the machine rotor.
• the acceleration signal may be used by other circuits (not shown).
• the signals Q and are supplied to an instantaneous position generation circuit 70 which converts the coarse sampled output of the Kalman filter into a signal having position update intervals which are sufficiently fine to properly control commutation.
• the signal is further supplied to a scaling circuit 72, which in turn develops a velocity estimate signal in the correct units (e.g., rpm's) for the control and protection circuit 54 of Fig. 3.
• a scaling circuit 72 which in turn develops a velocity estimate signal in the correct units (e.g., rpm's) for the control and protection circuit 54 of Fig. 3.
• the estimator 68 further receives an initialization command signal and develops a trigger signal for the pulse generator 66 in the fashion noted in greater detail hereinafter.
• Figs. 5A and 5B illustrate a portion of the operation of the estimator 68 in flowchart form.
• the estimator 68, as well as the relative angle estimation circuit 62, the angle combination circuit 67, the instantaneous position generation circuit 70 and the scaling circuit 72 of Fig. 4 may be implemented by a suitably programmed digital signal processor (DSP).
• DSP digital signal processor
• any of these circuits may be implemented by different circuitry, for example, discrete logic circuits or other hardware, or may be implemented by a combination of hardware and software, as desired.
• the programming illustrated in Figs. 5A and 5B is executed once per program cycle and is repeated a particular number of times (e.g., 100) to develop initial condition values ⁇ , and ⁇ representing initial machine rotor position and i Ji initial machine rotor speed, respectively, for the Kalman filter of the estimator 68.
• an initial condition is established for the Kalman filter indicating zero acceleration of the rotor of the machine 18 by the DSP.
• Figs. 5A and 5B automatically begins upon startup of the system including the position estimation circuit 52, at which point an initialization period is begun.
• the programming of Figs. 5A and 5B may be invoked when estimator lock is lost, (i.e., where the estimator has lost synchronism with actual machine operating conditions) or when initialization is otherwise commanded by the initialization command signal.
• the pulse generator 66 develops and provides the control signals CS1 and CS2 to the switches Q1 and Q2. Control signals identical to the signals CS1 and CS2 are simultaneously provided to the switches in the remaining phases of the inverter 20. As seen in Fig.
• the control signals including the signals CS1 and CS2 repetitively turn on and turn off all of the switches in the inverter 20 a certain number of times (preferably 100) during the initialization period.
• the inverter switches are operated together such that they are rendered conductive at the same time and are turned off at the same time, with a period equal to ⁇ t.
• the widths of the pulses in CS1 and CS2 are such that the phase current magnitude does not become excessive and such that the phase current magnitude decays to zero before application of the next pulse.
• the estimator 68 analyzes the samples or estimates Q from the angle combination circuit 67 to derive the initial values.
• the programming begins at a block 86, which checks to determine whether the current pass through the programming of Figs. 5A and 5B is the first since the initialization period was begun. If so, a block 88 sets various values and counters equal to zero. A block 90 then triggers the pulse generator 66 so that each phase winding of the machine 18 receives a single current pulse and a block 92 increments a pulse counter CNTO. Thereafter, a block 94 checks to determine whether the output of the counter CNTO indicates that each phase winding has been pulsed at least three times since the beginning of the initialization period. If not, control exits the programming of Figs. 5A and 5B to other programming executed by the estimator 68 for the balance of the current program cycle. Alternatively, if the output of the counter CNTO is equal to or greater than three, control passes to a block 100.
• the block 100 checks to determine whether the angle estimate Q from the angle combination circuit 67 is a valid estimate. This is determined by checking to determine whether Q is within an expected range of values. If the
• a e current estimate Q IS valid and a block 102 determines that the immediately e ⁇ preceding estimate Q was likewise valid using the same validity testing criterion, a e ⁇ block 104 calculates a value ⁇ Q which is equal to the difference between the e current and immediately preceding estimates developed by the circuit 67.
• a block 106 increments a counter CNT1 which indicates the number of valid estimates developed by the circuit 67 since the beginning of the initialization period.
• a block 108 (Fig. 5B) checks to determine whether the contents of the counter CNT1 exceed a certain threshold THR1.
• the value of THR1 is set equal to ten so that the first ten valid instantaneous velocity estimates developed by the programming of Figs. 5A and 5B are ignored owing to the possibility of rotor pulsing effects, such as jerk, when a low inertia rotor and load are first pulsed by currents resulting from application of the control signals.
• a block 110 checks to determine whether the value of ⁇ Q is less than a second threshold value THR2.
• the second threshold THR2 establishes a maximum expected change in Q from one estimate to the next, and, in the preferred e embodiment, is set equal to ⁇ radians.
• a block 112 calculates a value equal to ⁇ ⁇ / ⁇ t, where ⁇ t is the e e period of the control signals, as noted previously. The block 112 thus develops an instantaneous velocity estimate ⁇ for the rotor of the machine 18. Following the e ⁇ block 112, a block 114 calculates the value ⁇ representing the estimated rotor i ⁇ speed as an average of all of the instantaneous speed estimates ⁇ . e
• a block 116 then establishes the value of the initial position estimate Q equal to the current value of . Thereafter, a block 118 establishes the value of a variable LAST_VALID_ ⁇ , also equal to the current value of . A block 120 then e clears a counter CNT2 and control passes to other programming of the estimator 68 for the balance of the program cycle.
• a block 122 increments the counter CNT2 and a block 124 calculates a value for ⁇ according to the following equation:
• Control from the block 124 then exits the programming of Figs. 5A and 5B to other programming executed by the estimator 68 for the balance of the program cycle.
• a position e ⁇ estimate is obtained from the sum of the last valid reading of Q incremented by a e rotor position change value determined from the product of the average velocity and the time since the last reading of ⁇ was validly developed.
• the sensing pulses in the control signals must occur at a repetition rate sufficient to ensure that at least two pulses occur during each electrical cycle of the machine 18 so that aliasing effects are eliminated.
• the value ⁇ g representing the change e in rotor position cannot exceed ⁇ .
• there is a minimum pulse repetition rate for the signals CS1 and CS2 which is determined by the highest expected rotor velocity.
• the block 104 calculates ⁇ Q using a function e modulo 2 ⁇ such that a value (in radians) is developed in a range between negative - ⁇ and positive + ⁇ .
• Kalman filter in the estimator 68 continues to operate during initialization; however, the output of the Kalman filter is overwritten during this time and not used.
• the initialization period must be kept small enough so that the velocity change during maximum acceleration is not significant.
• Fig. 7A there is illustrated programming also executed by the estimator 68 to determine whether the Kalman filter is in lock or synchronism with actual machine operating conditions.
• the programming of Fig. 7A and 7B is executed once per program cycle so that lock detection is continuously effected.
• the programming begins at a block 140 which checks to determine whether a new sample of a signal INNOV developed by the Kalman filter has been obtained. Each sample or value of INNOV represents the difference between an absolute angle estimate ⁇ sample received from the angle combination circuit 67 of e
• Fig. 4 a predicted angle value ⁇ p ⁇ taken modulo 2 ⁇ in the range from - ⁇ to + ⁇ and developed by the Kalman filter. If the value of INNOV has not been updated, control pauses at the block 140 until such value is updated, whereupon control passes to a block 142.
• the block 142 calculates a value m ⁇ - ⁇ , according to the following equation:
• m innov(n) is the mean of the most current value of INNOV and the N-1 immediately preceding samples thereof.
• N equals 32, although any other suitable value of N could alternatively be used.
• a block 144 then calculates the mean m innov(ntk) of a second sequence of samples of INNOV displaced k samples in time from the first sequence of samples used by the block 142 according to the following equation:
• k 8 8 and hence the block 144 calculates the mean of the 9th through 40th preceding samples of the signal INNOV prior to the current sample.
• the required innovation storage array in the preferred embodiment, is equal to the sum of n + k, resulting in a size of 40 elements.
• a block 1 6 calculates the covariance ⁇ ,. ⁇ according to the following equation:
• a block 148 checks to determine whether the innovation covariance ⁇ is greater than a threshold THR4, and, if this is the case, a counter CNT3 is incremented by a block 150.
• the threshold THR4 is set to a value representing 0.1 radian 2 , although any other suitable value could alternatively be used. If the innovation covariance is not greater than THR4, the block 150 is skipped and the counter CNT3 is not incremented.
• a block 152, Fig. 7B increments a counter CNT4 and a block 154 checks to determine whether CNT4 is greater than a threshold THR6. If this is found not to be the case, then the lock detection programming of Figs. 7A and 7B is exited inasmuch as a determination has been made that the estimator 68 is in lock.
• a block 156 checks to determine whether the value of the counter CNT3 is greater than a threshold THR5.
• a block 158 resets the values of the counter CNT3 and CNT4 to 0 and control exits the lock detection programming of Figs. 7A and 7B.
• the thresholds THR5 and THR6 are established so that, if a first number of samples, for example, 100, of the covariance ⁇ , exceed the threshold THR4 out of a second, larger number of such samples, for example 400, then the reinitialization routine of Figs. 5A and 5B is invoked.
• a first number of samples for example, 100
• the threshold THR4 out of a second, larger number of such samples, for example 400
• the block 148 determine whether the absolute value of the covariance is greater than the threshold THR4.
• the Kalman filter innovation INNOV when the estimator 68 is in lock, the Kalman filter innovation INNOV will be a small, white-noise like signal with substantially zero mean. If the estimator loses lock with actual operating conditions, the signal INNOV will lose this randomness characteristic.
• the calculation of the covariance of INNOV is performed for each new innovation sample using a sliding window N samples long. In order to avoid calculating the complete sum of the mean of the innovation for each sample, a running sum is kept with the newest innovation sample added and the oldest innovation sample subtracted. Computation time is further reduced by multiplying by the inverse of N.

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• Control Of Electric Motors In General (AREA)

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• 1997
  • 1997-03-17 JP JP53622797A patent/JP2002516058A/ja active Pending
  • 1997-03-17 EP EP97919894A patent/EP0893006A1/en not_active Withdrawn
  • 1997-03-17 WO PCT/US1997/004576 patent/WO1997038483A1/en not_active Application Discontinuation

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