Abstract:
An adaptive, sensorless position sensing apparatus ( 250 ) derives rotor position of a synchronous machine ( 200 ). The apparatus ( 250 ) comprises a first rotor position deriving unit ( 300 ) for generating first rotor position values by applying a first sensorless rotor position calculation technique, which emulates a resolver; a second rotor position deriving unit ( 400 ) for generating second rotor position values by applying a second sensorless rotor position calculation technique; and a rotor position result output unit ( 450 ) for outputting rotor position results over a range of rotor speeds as a function of the first rotor position values, the second rotor position values, and rotor speed.

Description:
FIELD OF THE INVENTION 
   The present invention relates to synchronous machines, and more particularly to an adaptive, sensorless method and apparatus for detecting rotor position in a synchronous motor generator system over a full speed range of the motor generator. 
   BACKGROUND OF THE INVENTION 
   A conventional motor generator system, as utilized for example in the aerospace industry, includes a brushless synchronous machine that generates multi-phase AC power from a rotating shaft, e.g., coupled to a gas turbine engine, and DC excitation. In addition to operating in a generator mode, the brushless synchronous machine operates as a starter (motor) to start the aircraft engine. Following a successful engine start the system initiates the generator mode. 
   Conventionally, motor controllers for applications requiring a controlled torque use discrete sensors to determine rotor position in a rotating machine. This technique, however, increases system complexity and decreases system reliability. The electric machine must have a sensor built in or attached mechanically to the rotor. Interfaces and wiring must be added for control (excitation) and feedback signals between the controller and the sensor. Typical sensors include resolvers, encoders, and the like. The location of the rotating machine could be far from the controller, creating the need for unwanted extra wiring in the system. 
   A conventional motor control system having a position sensor is shown in  FIG. 1A . The primary components of the system include a power source  110 , a controller  120 , a motor generator  130  and a speed/position sensor  140 . The controller  120  includes inverter control  126  that receives signals from the sensor  140  (e.g., speed/rotor position) and the motor generator  130  (e.g., current, voltage). These signals are used to control the main inverter  122  and exciter inverter  124 , thereby providing a conventional closed loop system to regulate the current as a function of the speed of the motor generator  130 , as will be appreciated by those skilled in the art. 
     FIG. 1B  illustrates a block diagram of a sensorless system. As is apparent from the block diagram, the sensor and related signals to the controller  120  are absent. Those skilled in the art will appreciate that this requires the controller  120  to process the rotor position/speed of the motor generator  130  to allow closed loop current regulation or to execute certain control functions (e.g., current control) or operate in an open loop mode. 
   Sensorless motor control techniques can increase system reliability and eliminate the need for extra wiring in the system. In addition these techniques eliminate the need for a discrete position sensor and also reduce the system cost. A sensorless motor control technique is a more flexible/adaptable solution for a motor drive system than one that relies on a separate position sensor. It is particularly valuable for an aircraft system where increased reliability and reduction of weight (e.g., through elimination of the sensor and additional wiring) are extremely important. 
   Motor controller applications in systems with existing electrical machines can use a sensorless motor control scheme. For example, sensorless control systems are advantageous in retrofit applications, where a sensor and appropriate wiring may be unavailable and not easily installed. Some of these systems have synchronous generators that can be used as a motor generator but they do not have discrete sensors. Additional applications for this technique include motor controllers in the environmental control systems, electric power systems, industrial drive systems, and the like. 
   One known sensorless technique for determining rotor position observes back EMF voltage, which may be defined as E emf =k w sin α, where k is a constant, w is the angular speed of the motor, and α is the electrical phase angle of the rotor. 
   However, although observing back EMF to derive rotor position does not rely on a dedicated sensor, such a technique is not well suited for providing initial position sensing at standstill (zero back EMF) or low speed ranges (low signal to noise ratio), which is necessary at start-up under high load torque of the motor generator. 
   SUMMARY OF THE INVENTION 
   Aspects of the present invention include a method and an apparatus for adaptive, sensorless determination of rotor position of a brushless synchronous machine over a full speed range of the machine. In one aspect, the present invention is an adaptive, sensorless position sensing apparatus for deriving rotor position of a synchronous machine, the apparatus comprising a first rotor position deriving unit for generating first rotor position values by applying a first sensorless rotor position calculation technique, which emulates a resolver; a second rotor position deriving unit for generating second rotor position values by applying a second sensorless rotor position calculation technique; and a rotor position result output unit for outputting rotor position results over a range of rotor speeds as a function of the first rotor position values, the second rotor position values, and rotor speed. 
   In another aspect, the present invention is an adaptive, sensorless position sensing method for deriving rotor position of a synchronous machine from signals output from the machine, the method comprising generating first rotor position values by applying a first sensorless rotor position calculation technique, which emulates a resolver; generating second rotor position values by applying a second sensorless rotor position calculation technique; and outputting rotor position results over a range of rotor speeds as a function of the first rotor position values, the second rotor position values, and rotor speed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1A  is a general block diagram of a conventional motor generator system using a position sensor to determine rotor position for motor generator control; 
       FIG. 1B  is a general block diagram of a conventional sensorless motor generator system; 
       FIG. 2  illustrates a synchronous motor generator system to which principles of the present invention may be applied to determine rotor position over a full speed range; 
       FIG. 3  is a block diagram of an adaptive, sensorless position deriving apparatus according to an embodiment of the present invention; 
       FIG. 4  is a block diagram of a position deriving unit used to derive rotor speed at standstill and lower speeds according to an embodiment of the present invention; 
       FIGS. 5A–5B  illustrate an adaptive technique for sensorless determination of rotor position over a full range of machine speeds in accordance with an embodiment of the present invention; and 
       FIGS. 6A–6B  further illustrate aspects of an adaptive, sensorless technique for determining rotor position in accordance with principles of the present invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention are more specifically set forth in the following description, with reference to the appended drawings. In the following description and accompanying drawings like elements are denoted with similar reference numbers. Further, well-known elements and related explanations are omitted so as not to obscure the inventive concepts presented herein. 
   U.S. patent application Ser. No. 10/244,496 (“the &#39;496 application”), filed Sep. 16, 2002 and titled “Position Sensor Emulator for a Synchronous Motor/Generator,” which discloses embodiments for deriving rotor position information from phase voltage signals output by main generator stator windings (stator phase windings) of a synchronous motor generator, is incorporated herein by reference in its entirety. 
     FIG. 2  illustrates a synchronous motor generator system  200  to which principles of the present invention may be applied to derive rotor position from signals output from stator phase windings. The synchronous motor generator system  200  includes the following main components: a brushless synchronous motor generator  210 ; a generator control unit  202 ; an exciter power supply  204 ; and a start converter  206 . The motor generator system  200  further includes motor generator switching units  203 ,  208 , for switching between a generator mode and a motor (starter) mode for the synchronous motor generator  210 . 
   The synchronous motor generator  210  includes a rotating unit  212 , including three-phase exciter windings  211 , a rectifier bridge  213 , and a main generator field winding  215 , mounted on a rotatable shaft, e.g., coupled to a gas turbine engine of an aircraft. The synchronous motor generator  210  further includes stator components, including an exciter field winding  220  and three-phase main generator windings  216 . The exciter field winding  220  of the stator and the three-phase exciter windings  211  of the rotor constitute an exciter generator and the field winding  215  of the rotor  212  and the three-phase windings  216  of the stator constitute a main generator. 
   In generator mode, the motor generator switching units  203 ,  208 , which may be for example well known switching elements, are positioned so the generator control unit  202  is connected to supply DC current (“DC excitation”) to the exciter field winding  220  (via switch  203 ) and the outputs, A, B, C, of the three-phase generator windings  216  are connected to an AC bus  218  (via switch  208 ). In an exemplary embodiment, when DC excitation is supplied to DC winding  220 , rotation of the generator shaft (not shown) by the aircraft engine causes the generation of a polyphase voltage in the armature winding  211  that is rectified by the rectifier assembly  213  and coupled to the winding  215 . This rectified voltage sets up a DC field in the main rotor field winding  215  which causes a rotating magnetic field in the main stator coil  216  that produces output power with regulated voltage at a point of regulation (POR)  208  (prior to the bus contact switch) for delivery to AC bus  218  via terminals A, B, C, and switch  208 . The DC current flowing through the exciter field winding  220  may be varied in amplitude to achieve the desired AC power on the AC bus  218 . In generate mode, rotor position information is not required by the system  200 . 
   Additionally, the system  200  may use the starter/generator  210  as a motor to start the aircraft engine. An external power source (exciter power supply—EXPS)  204  is coupled to the generator  210  using the exciter field winding  220 . The coupled power from EXPS  204  induces AC power through transformer effect in the polyphase winding  211  of the rotor  212  because no relative motion between rotor and stator exists at zero speed. The AC power established in winding  211  may be rectified by rectifier assembly  213  to generate DC power in the main field winding  215 . Additionally, a start converter  206  is used to supply controlled AC power to main stator coil  216  such that sufficient torque is produced by the starter/generator  210 . This torque is produced by the interaction between the flux in the main rotor winding  215  and the current (flux) established in coil  216 . The frequency of the controlled AC power supplied to the main stator is increased from 0 Hz (0 RPM) to a predetermined frequency corresponding to the generated torque for starter/generator  210  at the end of start. The phase of the current for the supplied AC power input is controlled as function of rotor position to develop the desired torque for starter/generator  210 . To create sufficient torque for efficiently moving the rotor via electromagnetic force, commutation of stator phase windings  216  requires that the position of the rotor be known. 
   Instead of using a resolver or some other dedicated position-sensor to determine rotor position and control commutation, embodiments of the present invention rely on a combination of sensorless techniques to adaptively determine rotor position over a full speed range of the synchronous motor generator system  200 . 
     FIG. 3  illustrates an adaptive, sensorless rotor position determining apparatus  250  according to an embodiment of the present invention. The position determining apparatus  250  may be implemented as a component of the start converter  206 . In this embodiment, the rotor position determining apparatus  250  includes: a first position deriving unit  300 ; a second position deriving unit  400 ; a position value selecting and combining unit  450 ; and a control unit  475 . Although the elements of the position determining apparatus  250  are shown as discrete elements in  FIG. 3 , such an illustration is for ease of description, and it should be recognized that the functions performed by these elements may be combined in one or more devices, e.g., implemented in software, hardware, and/or application-specific integrated circuitry (ASIC). In accordance with principles of the present invention, the control unit  475  controls elements of the position determining apparatus  250  to adaptively operate in multiple control phases over a full range of machine speeds. In these control phases, the value selecting and combining unit  450  outputs rotor position values generated by the first position deriving unit  300  (rotor angle θ 1 ), rotor position values generated the second position deriving unit  400  (rotor angle θ 2 ), or a weighted combination of θ 1  and θ 2 . An embodiment of the present invention is described below, with reference to  FIGS. 5–6 , in which the rotor position determining apparatus  250  executes four control phases from rotor standstill to full speed during start up to determine rotor position. It should be recognized that the scope of the present invention is not limited to such a four control phase embodiment, since various alternatives are possible, including additional control phases. 
   1. Control Phase 1 
   At standstill and low rotation speeds (e.g., up to 5% of maximum speed, as shown in  FIGS. 5A and 6A ), the position value selecting and combining unit  450  selects the output of the first position deriving unit  300 . This is referred to as control phase 1. In one implementation, the first position deriving unit  300  takes advantage of the fact that signals output by the main generator windings  216  when AC current is applied to the exciter field winding  220  contain a rotor position-dependent component that can be extracted and processed to emulate voltages induced in sine and cosine pickup windings of a resolver. The &#39;496 Application discloses multiple embodiments that emulate a resolver by extracting and processing such signals for controlling a synchronous motor generator system, such embodiments being incorporated herein by reference. 
     FIG. 4  illustrates a block diagram of the first position deriving unit  300  according to an embodiment of the present invention. As seen in  FIG. 4 , the first position deriving unit  300  includes: a bandpass filter  322 ; a converter  324 ; and a position processor  326 . The position sensor emulator  300  of this embodiment further includes a rectifier  328  and a second bandpass filter  322 A. Operation of the first position deriving unit  300  according to this implementation will be described below. As described above, when the starter mode is initiated, controlled AC power from the exciter power supply  204  is applied to the field winding  220  of the motor generator  210  resulting in the main rotor behaving as a position sensor, due to its salient construction. This reaction is similar to a resolver-type position sensor. Accordingly, the first position deriving unit  300  continuously detects the voltages on the main stator phase windings  216 , V A , V B , V C , to produce a set of signals that define the rotor position. The position information is obtained from the stator phase voltages of the motor generator after being filtered by the bandpass filter  322  and converted by converter  324 . For example, converter  324  can use the well known Clarke transformation (a,b,c/α,β) that converts three-phase quantities (a,b,c) into balanced two-phase quadrature quantities (α,β). The output signals from the converter  324  are fed to a position processing block  326  for determining rotor position by relying on the fact that the extracted two-phase quadrature quantities emulate the voltages induced in sine and cosine pickup windings of a resolver. The exciter voltage from the exciter power supply  204  is passed through rectifier  328  and bandpass filter  322 A to provide a reference signal, as described for example in the &#39;496 Application. 
   Those skilled in the art will appreciate that the first position deriving unit  300  may include additional elements. The position sensor emulator  300  of the present invention extracts signals that emulate signals of a resolver. Therefore, the position processing block  326  can be a conventional position processing block. The position information is then used to control the start converter  206 . The inventors of this application have found that, although the above-described resolver emulation technique is well suited for standstill and low speed operations, it is less suited for higher rotation speeds typically experienced during start up of a brushless synchronous machine. Therefore, the present invention adaptively utilizes at least one alternate sensorless rotor position determining technique at increasing rotor speeds. 
   2. Control Phase 2 
   As machine speed increases, a second control phase (control phase 2, e.g., at 5%–7% of full speed, as seen in  FIGS. 5A ,  6 A) is initiated, in which the position value selecting and combining unit  450  continues to output rotor angle θ 1 , while at the same time the second position deriving unit  400  calculates rotor angle θ 2 . The control unit  475  performs a phase-locked loop function to adjust/adapt the calculation performed by the second position deriving unit  400  to minimize the error between θ 1  and θ 2 . For example, the second position deriving unit  400  may implement a known technique for deriving rotor position based on monitoring back EMF, and the control unit  475  may change a variable in this calculation (e.g., the theoretical value of the mutual inductance between the rotor and stator of the machine) to minimize the error between θ 1  and θ 2 . This phase-locked loop operation reduces transient conditions upon switching to θ 2  as the output of the position value selecting and combining unit  450 . 
   3. Control Phase 3 
   As machine speed increases further, a third control phase (control phase 3, e.g., at 7%–10% of full speed, as seen in  FIGS. 5A ,  6 A) is initiated. During control phase 3, the position value selecting and combining unit  450  combines θ 1  and θ 2  in a weighted manner, to ensure a smooth transition from θ 1  and θ 2 . More specifically, the position value selecting and combining unit  450  may calculate:
 
θ=θ 1   ·k   1 +θ 2   ·k   2 ,
 
where k 1 +k 2 =1. The value k 1  steadily decreases as rotation speed increases during control phase 3 so that a gradual transition from θ 1  to θ 2  is made. This transition phase is illustrated in  FIG. 5B . This type of weighting operation has been described, for example by Aihara et al., Sensorless Torque Control of Salient Pole Synchronous Motor at Zero Speed Operations, IEEE Transactions on Power Electronics, Vol. 14, No. 1, January 1999.
 
4. Control Phase 4
 
   As machine speed increases further, a fourth control phase (control phase 4, e.g., 10%–100% of full speed, as seen in  FIGS. 5A ,  6 A) is initiated. In control phase 4, the position value selecting and combining unit  450  outputs rotor angle θ 2  as the rotor position value. 
   As illustrated in  FIG. 6B , during slow down, a transition back to θ 1  is performed, although it is not necessary to implement control phase 2 during slow down. 
   In accordance with principles of the present invention described above, a first sensorless rotor position deriving technique is applied at standstill and low machine speeds to take advantage of the suitability of such a method for deriving rotor position under such conditions and a second sensorless rotor position deriving technique is applied at higher speeds to take advantage of the suitability of such an alternative technique at such speeds. Although an embodiment described above applied back EMF as the second sensorless rotor position deriving technique, other sensorless techniques may be applied, such as a floating-frame-based technique as disclosed by Huggett et al. in U.S. Pat. No. 6,301,136, which is hereby incorporated herein by reference. 
   The foregoing illustrates the principles of the invention. It will be appreciated by those skilled in the art will that various arrangements of the present invention can be designed. For example, the band of the bandpass filter and converter can be implemented in analog, digital, or hybrid configurations. Those skilled in the art will appreciate that the bandpass filter, converter, etc., can be implemented in software and/or hardware or in a single device such as an application specific integrated circuit (ASIC). Those skilled in the art will appreciate that the ranges can be optimized based upon the specific requirements of the system and design of the synchronous machine (e.g., number of poles, and the like), which form the basis for calculating the appropriate passband frequencies. Therefore, the scope of the invention is not limited by the foregoing description but is defined solely by the appended claims.