Position-and-Velocity Sensorless Control for Starter Generator Electrical System Using Generator Back-Emf Voltage

The present invention relates generally to electromagnetic machines, and more particularly to a detector for detecting the rotor position of a brushless generator and a control system incorporating such a detector.

An auxiliary power unit (APU) system is often provided on an aircraft and is operable to provide auxiliary and/or emergency power to one or more aircraft loads. In conventional APU systems, a dedicated starter motor is operated during a starting sequence to bring a gas turbine engine up to self-sustaining speed, following which the engine is accelerated to operating speed. Once this condition is reached, a brushless, synchronous generator is coupled to and driven by the gas turbine engine during operation in a starting mode whereupon the generator develops electrical power.

As is known, an electromagnetic machine may be operated as a motor to convert electrical power into motive power. Thus, in those applications where a source of motive power is required for engine starting, such as in an APU system, it is possible to dispense with the need for the dedicated starter motor and operate the generator as a motor during the starting sequence to accelerate the engine to self-sustaining speed. This capability is particularly advantageous in aircraft applications where size and weight must be held to a minimum.

The use of a generator in starting and generating modes in an aircraft application has been realized in a variable-speed, constant-frequency (VSCF) power generating system. In such a system a brushless, three-phase synchronous generator operates in the generating mode to convert variable-speed motive power supplied by a prime mover into variable-frequency AC power. The variable-frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant-frequency AC power, which is then supplied over a load bus to one or more loads.

The generator of such a VSCF system is operated as a motor in the starting mode to convert electrical power supplied by an external AC power source into motive power which is provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator including a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it has been known to provide power at a controlled voltage and frequency to the armature windings of the main generator portion and to provide field current to the main generator portion field windings via the exciter portion so that the motive power may be developed. This has been accomplished in the past, for example, using two separate inverters, one to provide power to the main generator portion armature windings and the other to provide power to the exciter portion. Thereafter, operation in the generating mode may commence whereupon DC power is provided to the exciter field winding.

In order to properly accelerate the generator and prime mover during operation in the starting mode, it is necessary to properly commutate or switch the currents among the armature windings of the generator. In the past, proper commutation was achieved using an absolute position sensor, such as a resolver, a synchro, an optical encoder or hall effect devices. For example, in Lafuze, U.S. Pat. No. 3,902,073 three Hall sensors are mounted in an air gap of a PMG 120 electrical degrees apart with respect to permanent magnet rotor pole pairs. As the rotor of the PMG rotates, the voltage output of each Hall sensor switches on and off as a function of the rotor position thereby generating three square wave voltages spaced apart by 120 electrical degrees. The outputs from the Hall sensors are representative of the position of the PMG rotor. The output signals from the Hall sensors are used to control switching elements in cycloconverters to switch current to armature windings of a main generator portion.

Use of an external absolute position sensor entails considerable expense in the position sensor itself and associated electronics, and further results in extra wires and the need for an extra assembly step to install the components. Also, operational parameters often limit the accuracy of the sensor.

In view of the foregoing difficulties, other approaches have been taken in an effort to detect rotor position without the need for absolute position sensors.

In the case of a brushless DC motor control, a back EMF approach has been used to detect rotor position. The back EMF of the motor is defined by the following equation:

where K is a constant, .omega. is the angular speed of the motor and .alpha. is the electrical phase angle of the rotor. From the foregoing equation, it can be seen that if back EMF can be detected, rotor electrical phase angle can be determined and thus proper commutation of the armature windings of the motor can be achieved. The back EMF voltage can be detected using either of two methods, referred to as the direct method and the indirect method.

The direct method can be used to directly measure phase back EMF voltage only when the phase winding is not energized by the inverter connected thereto and when the winding is not short circuited either by closed switches in the inverter or by conducting flyback diodes in the inverter. Such conditions can be realized when a 120 degree commutation algorithm is utilized. In this case, a voltage reading is taken after a short delay following switching of the phase winding off to ensure complete current decay by the free-willing diodes. This direct technique is described in a paper entitled "Microcomputer Control for Sensorless Brushless Motor" by E. Iizuka et al., IEEE Transactions on Industry Application, Vol. IA-21, No. 4, May/June 1985.

The indirect method is based on estimating the back EMF from the motive terminal voltage and phase currents. This method is suitable for both 120 and 180 degree commutation algorithms. One technique that uses this method is described in a paper entitled "Position - and - Velocity Sensorless Control for Brushless DC Motor Using an Adaptive Sliding Mode Observer" by Furuhashi et al., IEEE Transactions on Industrial Electronics, Vol. 39, No. 2, April 1992.

Because the back EMF voltage of a motor is zero at standstill and the signal to noise ratio is small at lower speeds, the reliable determination of rotor position by detecting back EMF is limited at low rotor speeds.

A method of using a permanent magnet generator as a position sensor for motor/generator start is described in Stacey U.S. Pat. No. 5,140,245. A standard brushless generator is equipped with a PMG which is used as an emergency electric power source and as a source of control power during a normal or generating mode of operation. The PMG develops a multi-phase output which is supplied to a high resolution phase-locked loop having a binary counter which develops an output signal representing shaft position. This method, however, is limited to the situation where the number of PMG rotor poles is equal to or less than the number of poles on the main generator portion rotor so that ambiguous position readings are avoided.

In accordance with the present invention, a detector for detecting rotor position of brushless generator utilizes inexpensive components and operates in a simple and effective manner.

More particularly, a detector for detecting rotor position of a brushless generator having a motive power shaft and a main generator portion having a set of armature phase windings capable of receiving AC power thereon includes means coupled to the armature phase windings for deriving a number of interval pulses relating to the revolution of the motive power shaft from the AC power supplied to the armature phase windings. Means are coupled to the deriving means for measuring time periods between adjacent interval pulses and means are coupled to the measuring means for converting the measured time periods into an indication of the angular position of the motive power shaft.

Preferably, the measuring means comprises a counter which accumulates clock pulses during time periods between adjacent interval pulses. Also preferably, the counter is periodically reset a certain number of times during each revolution of the motive power shaft by the interval pulses.

Still further in accordance with the preferred embodiment, the converting means includes means for inverting a counter output signal developed by the counter to obtain an indication of the speed of the motive power shaft. The converting means also preferably includes an integrator coupled to the inverting means which develops the angular position indication.

In accordance with a further aspect of the present invention, a starting system control for operating a brushless generator in a starting mode to convert electrical power into motive power wherein the brushless generator includes a motive power shaft and a main generator portion having a set of armature phase windings which receive AC power during operation in the starting mode includes means coupled to the armature phase windings for deriving a number of interval pulses relating to the revolution of the motive power shaft from the AC power supplied to the armature phase windings. Means are coupled to the deriving means for measuring time periods between adjacent interval pulses and means are coupled to the measuring means for converting the measured time periods into indications of the speed and angular position of the motive power shaft. Preferably, means are coupled to the converting means for delivering AC power to the set of main generator portion armature phase windings in dependence upon the speed and angular position indications.

In accordance with yet another aspect of the present invention, a method of detecting rotor position of a brushless generator having a motive power shaft and a main generator portion having armature phase windings coupled to an exciter portion includes the steps of deriving a number of interval pulses relating to the revolution of the motive power shaft from the AC power supplied to the main generator portion armature phase windings and measuring time periods between adjacent pulses. The measured time periods are converted into an indication of the angular position of the motive power shaft.

The detector of the present invention does not require the use of an absolute position sensor, nor is it limited to use with any particular commutation algorithm.

Additional aspects of the invention will be apparent in view of the following detailed description of the preferred embodiment, made with reference to the drawings, a brief description of which is provided below.

FIG. 1A comprises a combined block and schematic diagram of a brushless, synchronous generator;

FIG. 1B comprises a block diagram of an APU system together with a start converter;

FIG. 2 comprises a block diagram illustrating a rotor position detector and a starting system control according to the present invention;

FIG. 3 illustrates a number of inputs to and outputs from the system control of FIG. 2;

FIG. 4 comprises a diagram of the three-phase pulse-width modulated inverter of FIG. 2;

FIG. 5 comprises a block diagram of the rotor position detector of FIG. 2;

FIG. 6 illustrates the inputs to and the output from the logic circuit of FIG. 5;

FIG. 7 comprises a block diagram of the speed controller of FIG. 2;

FIG. 8 comprises a block diagram of the DC current estimator of FIG. 2; and

FIG. 9 comprises a circuit diagram of the phase converter of FIG. 2.

Referring now to FIG. 1A, a brushless, synchronous generator 10 includes a permanent magnet generator (PMG) 12, an exciter portion 14 and a main generator portion 16. The generator 10 further includes a motive power shaft 18 interconnecting a rotor 20 of the generator 10 and a prime mover 21, such as a gas turbine engine. In a specific application of the present invention, the generator 10 and the prime mover 21 together may comprise an aircraft auxiliary power unit (APU) 22, although the present invention is equally useful in other prime mover/generator applications.

The rotor 20 carries one or more permanent magnets 23 which form poles for the PMG 12. Rotation of the motive power shaft 18 causes relative movement between the magnetic flux produced by the permanent magnet 23 and a set of three-phase PMG armature windings including phase windings 24a-24c mounted within a stator 26 of the generator 10.

The exciter portion 14 includes a field winding 28 disposed in the stator 26 and a set of three-phase armature windings 30a-30c disposed on the rotor 20. A set of rotating rectifiers 32 interconnect the exciter armature windings 30a-30c and a main generator portion field winding 34 also disposed on the rotor 20. Three-phase main generator portion armature windings 36a-36c are disposed in the stator 26.

During operation in a generating mode, at least one, and preferably all three of the PMG armature windings 24a-24c are coupled through a rectifier and voltage regulator (not shown) to the exciter portion field winding 28. As the motive power shaft 18 is rotated, power produced in the PMG armature windings 24a-24c is rectified, regulated and delivered to the field winding 28. AC power is produced in the armature windings 30a-30c, rectified by the rotating rectifiers 32 and applied to the main generator portion field winding 34. Rotation of the motive power shaft 18 and the field winding 34 induces three-phase AC voltages in the main generator portion armature windings 36a-36c as is conventional. As seen in FIG. 1B, the AC voltages are supplied through a contactor set 37 to an APU power distribution network 38 and thence to one or more loads (not shown).

Often, it is desirable to use the brushless generator 10 as a motor to bring the prime mover 21 up to self-sustaining speed. This operation is accomplished by providing electrical power to the main generator portion field winding 34 via the exciter 14, providing AC power to the main generator portion armature windings 36a-36c and suitably commutating the currents flowing in the windings 36a-36c to cause the motive power shaft 18 to rotate. In a specific embodiment, the electrical power for the generator 10 is developed by an APU start converter 39, FIG. 1B, which receives external electrical power and which is connected by contactor sets 40a, 40b to the exciter field winding 28 and the armature windings 36a-36c, respectively. Various methods have been devised for controlling the power supplied to the main generator field winding 34 via the exciter 14. Inasmuch as the method of exciter control forms no part of the present invention, it will not be described in detail herein.

FIG. 2 illustrates the PMG 12, the main generator portion 16 and the motive power shaft 18 of the generator 10 together with a starting system 41 for providing electrical power to the armature windings 36a-36c during operation of the generator 10 in the starting mode to convert electrical power into motive power. While not shown, power may also be provided to the exciter field winding 28, and thus to the exciter armature windings 30a-30c and thus to the main generator portion field windings 34 by any suitable means during operation in the starting mode. The application of power to the exciter field winding 28 forms no part of the present invention and will not be described in detail herein. The starting system 41 includes a rotor position detector 44 which receives via lines 43a-43c the voltages developed on a series of lines 45a-45c coupled to the main generator portion armature windings 36a-36c of the main generator portion 16 and three synchronization signals developed on a series of lines 46a-46c by the starting system 41 as described in detail below. The rotor position detector 44 develops a signal representing the angular position of the motive power shaft 18 on a line 47 and a signal representing the speed of the motive power shaft 18 on a line 49.

The speed controller 48 develops a ramp signal on a line 50 representing the mechanical position of the motive power shaft 18 based on the angular position signal on the line 47. The signal on the line 50 is summed with a phase advance signal by a summer 54. The phase advance signal is developed by a function generator 56 and is dependent upon the speed of the motive power shaft 18 as detected by the rotor position detector 44. The function generator 56 provides increasing phase advance as speed increases in a high speed range. The summer 54 develops an electrical angle command signal on a line 58 which is supplied to first and second functional blocks 60, 62 which generate a cosine waveform signal and a sine waveform signal, respectively, each of which has the same frequency as the electrical angle command signal on the line 58.

The cosine signal and the sine signal are supplied via a pair of lines 61, 63 to a 2-to-3 phase converter 64 which converts these signals into three-phase signals which are in turn supplied to three zero crossing detectors 66a-66c via lines 67a-c, respectively. Each of the zero crossing detectors 66a-66c detects the zero crossings of one of the three-phase signals produced by the phase converter 64 to produce square-wave commutation signals S.sub.a, S.sub.b, and S.sub.c, respectively, which are shown in FIG. 3 and which are separated in phase by 120.degree.. A controller 68 responds to the signals S.sub.a, S.sub.b, and S.sub.c and to a power control signal delivered on a line 70 to produce on lines 72a-72f pulse-width modulated (PWM) inverter control waveforms T.sub.1, T.sub.3, T.sub.5 and commutation signals T.sub.2, T.sub.4, T.sub.6 which are shown in FIG. 3.

The controller 68 also produces the synchronization signals on the lines 46a-46c. Referring to FIG. 3, the voltage provided to the main generator portion armature winding 36a is shown as waveform V.sub.a ; the current provided to the winding 36a is shown as waveform I.sub.a ; and the synchronization signal associated with the winding 36a is shown as waveform SYNC.sub.a. The voltage, current and synchronization signal waveforms associated with the windings 36b and 36c are identical to those shown for the winding 36a, except that they are phase-shifted by 120.degree.. The synchronization signal SYNC.sub.a for winding 36a has a high value when neither of the inverter control waveforms T.sub.1 and T.sub.2 (which control TR.sub.1 and TR.sub.2 to control the current provided to the winding 36a) has a high value. Thus, SYNC.sub.a has a high value when TR.sub.1 and TR.sub.2 are both off, which coincides with the period during which the voltage V.sub.a makes a zero crossing. The synchronization signals selectively enable the comparators 102 to detect the phase voltages only when their associated windings are not in conduction so that the winding phase voltage detected is equal to the back EMF voltage. In detecting rotor position, it is assumed that the rotor 20 moves in accordance with the phase voltages provided to the main generator portion 16 by the inverter 74.

Referring again to FIG. 2, the inverter 74 receives DC power over a DC link 76. The DC link 76 may receive DC power from a three-phase rectifier 78 which is in turn coupled to an external AC source 79 or any other type of DC source.

Referring now to FIG. 4, the inverter 74 includes controllable power switches TR.sub.1 -TR.sub.6 and associated flyback diodes D1-D6 connected in a three-phase bridge configuration across DC link conductors 76a, 76b. Referring again to FIG. 3, the control waveforms T.sub.1, T.sub.3 and T.sub.5 include PWM notches in the positive-going positions thereof. These signals control the power switches TR.sub.1, TR.sub.3 and TR.sub.5, respectively, of the inverter 74. The control waveforms T.sub.2, T.sub.4 and T.sub.6 control the inverter power switches TR.sub.2, TR.sub.4 and TR.sub.6, respectively. The widths of the PWM notches of the control waveforms T.sub.1, T.sub.3 and T.sub.5 are controlled by a power control circuit 80.

Referring back to FIG. 2, the power control circuit 80 includes a summer 82 which subtracts a signal I.sub.dc representing the DC link current provided on a line 81 (which in turn represents the power delivered by the inverter 74 to the armature windings 36a-36c) from a power reference signal. The I.sub.dc signal on the line 81 is generated by a DC current estimator 83 based upon three current signals I.sub.A, I.sub.B, and I.sub.c generated by three current transformers coupled to the outputs of the AC power source 79. As shown in FIG. 8, the DC current estimator may comprise a three-phase rectifier 85 coupled to a conversion factor circuit 87 such as a multiplier or scaled amplifier.

Referring back to FIG. 2, the power error signal generated by the summer 82 is conditioned by a conditioner 84 which, preferably, comprises a proportional-integral type compensator but could, alternatively, comprise any type of gain and compensation unit. The output of the conditioner 84 is delivered to a controlled or adjustable limiter 86 which produces the power control signal on the line 70.

The limiter 86 is controlled in accordance with a limiter control signal. The speed indication produced by the rotor position detector 44 is delivered to a multiplier 88 and multiplied by a constant volts-per-hertz signal to produce a speed dependent voltage signal. The speed dependent voltage signal is summed with a constant boost voltage by a summer 90, which in turn produces the limiter control signal. The constant boost voltage signal is proportional to the IR voltage drop in the main generator portion control windings and allows the power control signal to overcome these losses at initial start-up. Thereafter, as the speed indication developed by the rotor position detector 44 increases, the multiplier 88 produces a ramping signal which, when added to the boost voltage, increases the adjustable limit of the limiter 86 so that increasing power magnitudes can be delivered to the armature windings 36a-36c by the inverter 74.

Referring now to FIG. 5, the rotor position detector 44 is shown in greater detail. The voltage waveforms provided to the main generator armature windings and sensed via the lines 43a-43c are supplied through level shifting amplifiers 100a-100c and zero crossing detectors 102a-102c to a logic circuit 104. Each zero crossing detector 102a-102c is enabled for the period of time during which its respective synchronization signal provided on one of the lines 46a-46c has a high value.

FIG. 6 illustrates the phase currents I.sub.a, I.sub.b, I.sub.c and three waveforms S.sub.d, S.sub.e, and S.sub.f representing the outputs of the zero crossing detectors 102a-102c, respectively. From the waveforms S.sub.d -S.sub.f, the logic circuit 104 develops a signal S.sub.g consisting of brief pulses separated by 60 electrical degrees as shown in FIG. 6. Referring back to FIG. 5, the waveform S.sub.g is provided by the logic circuit 104 to a delay circuit 106. The delay circuit 106 periodically provides a reset signal to a counter 108 which accumulates clock pulses produced by a clock 110. The counter 108 is reset every 60.degree. with respect to the waveforms provided to the main generator portion 16, and thus every 60.degree. of rotation of the motive power shaft 18.

The output of the counter 108 represents the time that elapses between each pulse in the waveform S.sub.g. The falling edge of each pulse comprises a write command to a latch 112 which latches the output of the counter 108. The output of the latch 112 is inverted, i.e. the reciprocal thereof is calculated, by a circuit 114 to generate a speed signal indicative of the speed of the motive power shaft 18. Since the latch 112 is activated once every 60.degree. period, the speed signal generated has a constant value for each 60.degree. period.

The angular position of the motive power shaft 18, and thus the rotor 20, is determined by an integrator 116 comprising a multiplier 116a coupled to an accumulator 116b. During each 60.degree. period, the multiplier 116a multiplies the speed signal, which is constant for each period, with the output of the counter 108, which increases during the period, to generate a ramp signal on the line 117. The magnitude of that ramp signal falls to zero every 60.degree. since the counter 108 is reset every 60.degree..

The periodic signal S.sub.g is also provided to a staircase generator 118 which generates a staircase signal that increases a constant amount each 60.degree. period. The magnitude of the staircase signal at each 60.degree. period represents the rotor position at the start of that period. Every 360.degree., corresponding to one revolution of the rotor 20, the magnitude of the staircase signal falls back to zero.

The accumulator 116b continuously determines the rotor position by adding the current magnitude of the ramp signal generated by the multiplier 116a with the magnitude of the staircase signal for the current 60.degree. period. The resulting position signal generated by the accumulator 116b is a ramp signal having a magnitude representing rotor position. The period of the ramp signal corresponds with 360.degree., one full revolution of the rotor 20. Other integrative techniques could also be used.

FIG. 7 illustrates the speed controller 48 in greater detail. A speed command signal may be developed on a line 130 which is coupled to a non-inverting input of a summer 132. The speed command may comprise a step voltage from a first voltage to a second, higher voltage or may comprise any other type of waveform as desired. The output of the summer 132 is coupled to a function generator 134 which develops an acceleration command signal which is, in turn, integrated by an integrator 136 to produce a speed reference signal. The speed reference signal is fed back to an inverting input of the summer 132, and hence the elements 132, 134, and 136 comprise a closed-loop circuit. The speed reference signal is integrated by a further integrator 138 to develop a position reference signal which is, in turn, provided to a controllable switch 140 and to a non-inverting input of a summer 142. The angular position indication comprising the position signal from the integrator 116 of FIG. 5 is also provided to the controllable switch 140 and is further provided to an inverting input of the summer 142. The summer 142 produces a position error signal indicative of the error between the derived position reference signal and the actual rotor position as developed by the integrator 116. The position error signal is provided to an error comparator 144 which compares the position error signal to an error reference and produces a high state signal on a line 146 when the position error signal is less than the error reference.

Furthermore, the speed reference signal produced by the integrator 136 is provided to a comparator 150 that compares the speed reference signal with a valid speed reference signal indicative of the value at which the speed reference signal becomes a reliable representation of the speed of the motive power shaft. If the speed reference signal equals or exceeds the valid speed reference signal, the comparator 150 produces a high state output signal on a line 152 at such time. The signals on the lines 146 and 152 are delivered to an AND gate 156 having an output which is coupled to and controls the controllable switch 140.

At the initiation of a start-up sequence, at which time the speed of the motive power shaft 18 is zero, the controllable switch 140 is set to a position which passes the output of the integrator 138 to the output of the switch 140, and thus to the summer 54 of FIG. 2. Also at this time, power is applied to the exciter portion 14, and hence to the main generator portion field winding 34 of FIG. 1A, and power is also applied to the main generator armature windings 36a-36c. The motive power shaft 18 is thus accelerated. When the error between the rotor position, as measured by the integrator 116 of FIG. 5, and the position reference signal, as derived by the integrator 138, is less than the error reference and when the speed reference signal produced by the integrator 136 is equal to or greater than the valid speed reference, the high-state signals on the lines 146 and 152 cause the AND gate 156 to move the controllable switch 140 to the position shown in FIG. 7. The controllable switch 140 is latched in such position so that the output of the accumulator 116b of FIG. 5 is thereafter provided to the summer 54 of FIG. 2. The switch 140 remains latched until a new start-up sequence is initialized.

FIG. 9 illustrates in greater detail the phase converter 64 shown schematically in FIG. 2. The converter 64 includes three operational amplifiers 160, 162, 164 and associated biasing circuitry connected to the lines 61, 63 which generate outputs on the lines 67a-67c.

Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.