Abstract:
An apparatus is disclosed for simultaneously measuring the rotational speed and/or direction of a shaft, and providing control power in accordance with the shaft rotation. The apparatus includes a permanent magnet machine (PMM) having a multipole rotor and a stator. The rotor has a plurality of permanent magnet poles and connection to the rotating shaft; the stator includes a winding and electrical connections, so that motion of the rotor with respect to the stator causes a voltage signal at the electrical connections. The apparatus also includes a circuit including a power conversion portion and a speed/direction sensing portion. The circuit receives the voltage signal from the PMM, and simultaneously outputs control power from the power conversion portion and a signal indicating the rotational speed and/or direction of the shaft from the sensing portion.

Description:
FIELD OF THE DISCLOSURE 
     This disclosure relates to shaft speed sensors and starter/generator systems suitable for use in aircraft or automobiles, and more particularly to induction machines used in such systems. 
     BACKGROUND OF THE DISCLOSURE 
     In starter/generator systems (for example, in aircraft), it is highly desirable to use a machine where the excitation can be removed, to safely shut off operation of the generator function and thus prevent excess heat in failures or smoke generation. Removing excitation from the machine provides a positive shut-off of electrical power dissipation without the need to shut down the prime mover engine that also is used for propulsion. 
     A DC brushed machine is often used for this application, due to its ability to act as a motor or generator with minimal control electronics and its ability to remove most of the field excitation by reducing the field winding current to zero. Alternatively, an induction machine may be used. An induction machine has the advantage of low cost and does not employ brushes. In addition, it can control the excitation to ensure safety and maximize utilization of the interfacing power electronics. 
     The ability to remove excitation is also a disadvantage in that it is also highly desirable to be able to initiate generation with no external power source other than shaft power, so that each generator is failure independent of another. In addition, when an induction machine is used for a motor or a generator, a speed reference signal is often needed to provide control. 
     Accordingly, it is desirable to implement a combined sensor and power source which provides both the needed speed sensing and control power to allow an induction machine generation system to initiate generation with no external power input. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with the disclosure, a single small permanent magnet machine (PMM) simultaneously operates both as a speed sensor and as a source of control power. The permanent magnets provide magnetic field without any outside applied current. Mechanical rotation (for example, rotation of an engine shaft) causes a changing magnetic field, which induces a voltage in the machine. Since the sensor is small and the control power is low, permanent excitation (such as permanent coupling of the PMM to the engine shaft) is not a significant safety issue as is encountered when using a PMM for high power. The sensor/power source may be advantageously used with an induction machine, especially where the induction machine is used as a generator and power is not available to initiate generation. 
     According to an aspect of the disclosure, an apparatus is provided for simultaneously outputting a speed signal indicating the rotational speed of a shaft and control power for controlling another machine. The apparatus includes a permanent magnet machine (PMM) having a multipole rotor and a stator. The rotor has a plurality of permanent magnet poles and a connector for connection to the rotating shaft; the stator includes a winding and electrical connections, so that motion of the rotor with respect to the stator causes a voltage signal at the electrical connections. The apparatus also includes a circuit including a power conversion portion and a speed sensing portion. The circuit receives the voltage signal from the PMM, and simultaneously outputs control power from the power conversion portion and a speed signal indicating a rotational speed of the shaft from the speed sensing portion. In specific embodiments, the speed sensing portion includes a zero crossing detector, and the PMM voltage signal has two phases shifted by 90 degrees relative to each other; each phase may be rectified using a separate rectifier circuit, with the outputs of the rectifier circuits connected to a DC power bus. A switching device may be included on the DC power bus. 
     According to another aspect of the disclosure, a method includes the steps of providing a permanent magnet machine (PMM) coupled to a rotatable shaft; generating a voltage signal using the PMM, in accordance with a rotational speed of the shaft; and simultaneously generating from the voltage signal a speed signal indicating the rotational speed of the shaft and control power; the control power is effective to control a machine requiring external excitation for generation. 
     The foregoing has outlined, rather broadly, the preferred features of the present disclosure so that those skilled in the art may better understand the detailed description of the disclosure that follows. Additional features of the disclosure will be described hereinafter that form the subject of the claims of the disclosure. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present disclosure and that such other structures do not depart from the spirit and scope of the disclosure in its broadest form. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a permanent magnet machine (PMM) having a 12-pole permanent magnet rotor installed in a stator, according to an embodiment of the disclosure. 
         FIG. 2  is a block diagram illustrating an embodiment in which the PMM of  FIG. 1  is configured as a dual purpose shaft speed sensor and power converter. 
         FIG. 3  is a block diagram of a system including the dual purpose shaft speed sensor and power converter of  FIG. 2 . 
         FIG. 4  is a block diagram of a combined AC sensor circuit and DC control power generation circuit connected to a PMM, according to an embodiment of the disclosure. 
         FIGS. 5A and 5B  illustrate output waveforms from the circuit of  FIG. 4 , showing how direction of rotation may be sensed in the forward and reverse directions respectively. 
         FIG. 6  is a circuit diagram illustrating an embodiment of the combined circuit of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     A PMM configuration according to an embodiment of the disclosure includes a permanent magnet rotor that generates a rotating magnetic field and a stator assembly that produces an alternating current (AC) output voltage proportional in amplitude and frequency to the rate of change of the magnetic field generated by the rotor.  FIG. 1  illustrates a rotor/stator assembly  1  including stator  2  and rotor  3 . The rotor has a central fitting  4  with an opening  5  for receiving a drive shaft perpendicular to the plane of the drawing. As shown in  FIG. 1 , the rotor has twelve pole pieces  6 , with six sets of permanent magnets. The pole pieces are held in place by a retaining ring  7 , which is separated from the stator by an air gap  8 . Stator  2  has a twelve-pole winding. Increasing the number of magnetic poles offers an advantage in sensor performance, since an increased number of poles allows better resolution in timing at low shaft speeds. Assembly  1  is capable of about 72 V peak output voltage at 12,000 RPM. In this embodiment, the stator winding is two-phase, in order to provide two independent and redundant speed output signals. The two signals are shifted in phase by 90 degrees and are rectified for the generation of control power. 
     In general, the electrical output of assembly  1  permits a direct measurement of the shaft rotational speed and direction. The PMM can be implemented with a dual-phase or multi-phase output of various pole/winding configurations as long as the output of the PMM is proportional in frequency to the speed of the rotor&#39;s drive shaft. In other embodiments, the PMM may be designed to support a large number of proportionality relationships for shaft speed to PMM output voltage and/or frequency. 
       FIG. 2  illustrates a dual-mode permanent magnet speed sensor and power converter, according to an embodiment of the disclosure. The PMM rotor/stator assembly  1  is powered by rotating shaft  111 , which turns rotor  3  relative to stator  2 ; in this embodiment there are two output signals  21 ,  22  corresponding to the two-phase stator winding. Mechanical power from shaft  111  is converted to electrical DC power by power converter  12 . The PMM output is simultaneously used to obtain an AC signal corresponding to the rotation speed of shaft  111 . Each of the output signals from the PMM is fed into a power converter  12  and AC speed sensor  13 . Power converter  12  outputs DC power  18  for operating another unit (e.g. drive electronics unit  170 , an induction machine, control equipment, etc.). In this embodiment, the AC speed sensor  13  includes sensor circuits  14 ,  16  for each of the two phases, to produce the two separate and redundant rotation speed signals  15 ,  17 . 
     The output impedance of the PMM can also be designed for high reactance to provide self protection from overloads or shorted wires, or low reactance to maximize electrical efficiency and minimize size and weight. 
       FIG. 3  illustrates a system  100  in which an engine is connected via a drive shaft to an induction machine and to a dual-purpose permanent magnet (PM) speed sensor and power converter, according to an embodiment of the disclosure. In this embodiment, engine  101  (e.g. an aircraft engine) including an auxiliary gearbox  102  drives an induction machine  105  (e.g. a brushless starter/generator) by shaft  103 . The induction machine has a power connection  125  to a drive electronics unit  170 , which is connected to a power bus  176 . Mechanical power is delivered via shaft  111  to the dual-purpose PM sensor/power source  20 . Outputs  21 ,  22  from sensor/power source  20  are connected to PM sensor electronics unit  60 , which serves as an interface between sensor/power source  20  and the drive electronics unit  170 . In this embodiment, electronics unit  60  delivers two redundant speed signals  15 ,  17  to the electronics unit  170 , and also outputs control power  18  to the drive electronics unit. Drive electronics unit  170  is connected to power bus  176  (for example, 28 VDC). Power from the power bus is input to electronics unit  170 , and then to induction machine  105 , for starting engine  101 ; power is output to the power bus when induction machine  105  operates as a generator. 
     A block diagram showing details of electronics unit  60  according to an embodiment, including an AC rotation speed/direction sensor and a power converter, is shown in  FIG. 4 . 
     AC Rotation Speed/Direction Sensor 
     In this embodiment, a zero crossing detection method is used to measure the rotation speed and direction of shaft  111 . Differential amplifiers  29 ,  30  with filters  27 ,  28  are used to measure the differential voltage across each of the two sensor windings. The outputs of amplifiers  29 ,  30  are input to zero crossing detectors  32 ,  33  respectively. Signals from the zero crossing detectors  32 ,  33  are transferred to outputs  15 ,  17 . Voltage signals at  15  and  17  relative to ground (the same ground as for the vehicle control unit) thus may be used to determine rotation speed and direction, as described in more detail below. 
     The amplifier frequency response is used to optimize the low speed signal amplitude and compensate for the increasing sensor output voltage with increasing shaft speed. Comparators with hysteresis are used to provide noise immunity in the zero cross detectors  32 ,  33 . The time between zero crossings is used to measure the period T of the sensor output voltage signal; the signal frequency Freq=1/T. The shaft speed in RPM is then Freq*60/N, where N is the number of sensor pole pairs (6 for the PMM shown in  FIG. 1 ) and 60 is the conversion factor from seconds to minutes. 
     The signal period T may be measured by two methods, suitable for low and high rotation speeds respectively. At low speed, the period may be expressed as a multiple of a control cycle time Tc; the number Y of control cycles between successive sensor zero crossings is counted, and the period T is then T=TcY. This method gives acceptable resolution provided that the period T is long compared to Tc. At higher speeds, when T approximates Tc, the resolution is too coarse for an accurate rotation speed measurement. In a further embodiment, a processor capture module is used at higher rotation speeds to measure the period T. The capture module is a hardware counter timer included in a processor as part of the vehicle control unit. Typically the processor has a 16-bit counter that will overflow if used at low speeds. 
     As shown in  FIG. 4 , in this embodiment independent speed sensing is provided for each of the two stator windings with independent outputs  15 ,  17 , for an increased level of fault tolerance. 
     In applications where the rotation direction must be sensed in addition to the rotation speed, the two independent outputs  15 ,  17  are coupled to direction detector  36 . In detector  36 , one of the outputs  15 ,  17  is used as a reference and is compared with the other output. The waveforms  115 ,  117  of the two outputs in one rotation direction (defined here as forward) are shown in  FIG. 5A ; the waveforms  215 ,  217  of the two outputs when rotation is in the other direction (reverse) are shown in  FIG. 5B . Waveform  115  has a rising edge while  117  is low; waveform  215  has a rising edge while  217  is high. The result of comparing  15  and  17 , sampled just after a rising edge in output  15 , thus can indicate the rotation direction; a direction signal is output at  19 . In this embodiment, rotation direction sensing depends on the phasing of assembly  1 ; accordingly, changing the phasing changes the apparent direction of rotation. The comparator function for rotation direction can be performed using either hardware or software. 
     Power Conversion to DC 
     The voltage signals  21 ,  22  from the two output phases of the PMM are rectified to create a DC power output  18 . Each of the rectifier circuits  25 ,  26  has a full wave rectifier with four diodes. As shown in  FIG. 4 , the rectified outputs are combined and connected to a power bus  31 . 
     The PMM preferably has high enough output inductance to limit the short circuit current to a low enough level to prevent wiring damage in the event of a short circuit. To increase the output power capability of the PMM, resonant capacitor circuits  23 ,  24  are used to cancel out the inductive reactance of the PMM. A short beyond the resonant capacitor will cause high currents; a fuse is therefore preferably connected to circuits  23 ,  24  to protect against this failure mode. 
     In this embodiment, a switch  35  is used to connect the output of the power converter  12  to the load, in accordance with an external control signal  34 . 
     Sensor/Power Converter Combined Circuit 
     A schematic diagram of a circuit according to the above-described embodiment, suitable for an aircraft application, is shown in  FIG. 6 . This circuit has a rotation speed sensing portion with redundant outputs and a power conversion portion with a switched DC power output. 
     Independent inputs  41 ,  42  are 90 degrees out of phase, corresponding to the two phases of the stator windings of the PMM. As noted above, the PMM preferably has high series inductance that provides a current limiting function to protect the aircraft wiring from shorts. To allow higher power transfer, the power portion includes capacitor circuits  43 ,  44  (sets of parallel capacitors) that resonate with the leakage inductance allowing higher power transfer than the leakage inductance would otherwise allow. Capacitor circuits  43 ,  44  are connected to rectifier circuits  45 ,  46  through fuses  61 ,  62 . 
     The outputs of the rectifier circuits  45 ,  46  feed the DC power bus  63 . A switching circuit, including MOSFET  55  and controlled by switching signal  54 , is used to disconnect the power from output terminal  56 . This is useful when the control equipment is intended to be powered from other sources. 
     In the speed sensor portion, the inputs  41 ,  42  are connected to filters  47 ,  48  and amplified with differential amplifiers  49 ,  50 . The filter and differential amplifier provide controlled gain over the speed range. The frequency of output voltages  57 ,  58  are directly proportional to the shaft rotation speed and the corresponding frequency of the signals  21 ,  22  from the PMM. By having high gain at low frequencies it is possible to have maximum performance at low speeds where the sensor output is low. At high frequencies the filtering can roll off the voltage to achieve better noise immunity. The speed sensor portion in  FIG. 6  is able to detect rotation speeds from about 70 RPM up to the maximum speed required of 14,000 RPM. Comparators  51 ,  52  are used to sense the zero crossing of the conditioned signals. Hysteresis is used to reduce sensitivity to noise corruption. The rotation speed is determined by measuring the period of the comparator output signal, with the electrical frequency being equal to 1/period. To convert to a shaft rotation speed in RPM the frequency is multiplied by 60/N, where N is the number of pole pairs in the sensor (6 for a 12 pole sensor). 
     Application to Induction Machines 
     As noted above, the PMM and combined circuit may be advantageously connected to an induction machine, such as a brushless starter/generator. The PMM is small enough so that it may have permanent excitation (for example, permanent mechanical connection between shaft  111  and rotor  3 ) without raising safety concerns. At the same time, the power output is effective to excite an induction machine and/or provide control power to a drive electronics unit. In the embodiments described above, the power output is typically in the range of about 30 W to about 70 W. 
     Vector Control 
     In applications where the PMM and combined circuit are connected to an induction machine, vector control may be advantageously used for starting control of the induction machine, as is understood by those skilled in the art. Since the above-described speed sensor is not able to provide a true speed signal below about 70 RPM, an input representing a fictitious speed is supplied when the true shaft speed below 70 RPM; the supplied input is ramped up with time. This is done to ensure that the vector control “slip” (proportional to the Iq command divided by the Id command) is high enough in this mode to avoid machine saturation. Too low a slip causes torque producing currents to be steered into magnetization, causing saturation of the motor steel. As the shaft speed increases past the point where a valid speed sensing signal is available, the control switches to using the actual speed signal from the PMM speed sensing circuit. 
     A single PMM/sensor device, configured with circuits as described above, may provide both control power and a speed signal. This arrangement is especially useful for an induction machine but is applicable to other rotating machines such as a switched reluctance machine. The PMM/sensor is rugged and will work in extreme environments. This sensor is also less expensive and has been shown to provide superior performance compared to a resolver which calculates speed from shaft position change information. 
     While the disclosure has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the disclosure is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the disclosure and the following claims.