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.
Aircraft have typically used synchronous brushless AC generators or permanent magnet generators for electric power generation needs.
In addition to an electrical generator, 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.
As an alternative to the use of the synchronous AC or the permanent magnet generator for this combined starter/generator function, a switched reluctance machine can be used. 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.
In order to properly operate a switched reluctance machine, it is necessary to determine the rotor position in order to properly commutate the currents flowing in the phase windings of the machine. Resolvers are used, particularly in high speed systems, or sometimes encoders in low speed systems, to obtain a measure of rotor position. However, resolvers and required associated apparatus (chiefly, a resolver-to-digital converter and an excitation circuit) are expensive and both resolvers and encoders are a source of single point failure.
In order to obviate the need for position sensors, such as resolvers or encoders, sensorless operational techniques have been developed. The most trivial solution to sensorless operation is to control the switched reluctance machine as a stepper motor in the fashion disclosed in Bass, et al. U.S. Pat. No. 4,611,157 and MacMinn U.S. Pat. No. 4,642,543. In an alternative technique, machine inductance or reluctance is detected and utilized to estimate rotor position. Specifically, because the 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. Pat. No. 4,772,839, MacMinn, et al. U.S. Pat. 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.
In a further technique, 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 Sensorless Switched Reluctance Motor Drives for Automotive Applications," Texas A&M Power Electronics Laboratory Report (date unknown), Ehsani, et al. "An Analysis of the Error in Indirect Rotor Position Sensing of Switched Reluctance Motors," IEEE Proceedings IECON '91, Ehsani "A Comparative Analysis of SRM Discrete Shaft Position Sensor Elimination by FM Encoder and Pulsed Impedance Sensing Schemes," Texas A&M Power Electronics Laboratory Report, (date unknown) and Ehsani, et al. "New Modulation Encoding Techniques for Indirect Rotor Position Sensing in Switched Reluctance Motors," IEEE Transactions on Industry Applications, Vol. 30, No. 1, January/February, 1994.
A model-based approach to rotor position estimation has been developed by General Electric Company and is disclosed in Lyons, et al. "Flux/Current Methods for SRM Rotor Position Estimation," Proceedings of IEEE Industry Applications Society Annual Meeting, Vol. 1, 1991, and Lyons, et al. U.S. Pat. No. 5,097,190. In this technique, a multi-phase lumped parameter model of the switched reluctance machine is developed and utilized.
A position estimation subsystem for a sensorless switched reluctance machine control 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 sampled 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. An instantaneous position generation circuit converts the output of the Kalman filter to a signal that can properly control commutation.
An object of the present invention is to provide an apparatus for acquiring data representing a machine operating condition.
It is further an object to provide an apparatus which obtains high reliability data concerning machine operation.
Yet another object is to obtain data for a sensorless switched machine control and, more particularly, to obtain high reliability data for the relative angle estimation logic described above.
It is a still further object to obtain high reliability data using circuitry which is simple, reliable and low in cost.
These and other objects and advantages are attained by a circuit which synchronizes sampling instants to machine rotor position and/or velocity. Specifically, synchronization circuitry is responsive to rotor position and velocity estimates and develops a sampling signal which controls sampling of machine phase current magnitudes.
The apparatus of the present invention is effective to obtain data representing machine rotor position in a fashion which enhances the reliability of such data.
These and other objects, advantages and novel features of the present invention will become apparent to those skilled in the art from the drawings and following detailed description.
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