Angular displacement sensing system and method using brushless DC motor commutation hall effect sensors

A method and system for sensing angular displacement is provided. More specifically, the system includes a brushless DC motor, three Hall effect sensors, an angular displacement signal processor, and an output state observer. The Hall effect sensors provide three electrical angles of the motor rotor which are used in an angular displacement signal processor that is implemented in hardware and software. The angular displacement signal processor provides an improved resolution angular displacement output. Based on the angular displacement output from angular displacement signal processor and the motor control command signal from the motor drive, an output state observer is applied to generate a high resolution angular displacement signal.

TECHNICAL FIELD

The present invention generally relates to a system and method for sensing an angle or angular displacement. More specifically, the invention relates to a system and method for sensing an angle or angular displacement with improved resolution in a system with a brushless DC motor using Hall effect sensors.

BACKGROUND

Conventional sensing technologies employed to determine the angle or angular displacement of a shaft of a motion control system include encoders, resolvers, and potentiometers. These sensing technologies are readily available, however, they require a fair amount of space to connect to the shaft. Further, these technologies can also be quite costly when implemented in a high volume product.

However, less costly prior art technology for measuring angular displacement of a shaft of a motion control system is available. This technology utilizes three Hall effect sensors mounted in a brushless DC motor to provide motor commutation signals. The three Hall effect sensor signals are indicative of three electrical angles of the motor rotor. Brushless motors produce motion according to the commutation logic based on these three electrical angles of the motor rotor measured by the Hall effect sensors.

The use of the Hall effect sensors in conjunction with a brushless motor provides a much more cost effective method of sensing the angular displacement of the shaft of a motion control system. The Hall effect technology also employs no moving parts resulting in higher reliability. The disadvantage of using Hall effect sensors in the brushless motor to sense angular displacement of a shaft of a motion control system, is that the angular measurement resolution is typically lower than that of an encoder, resolver, or potentiometer.

Additionally, known techniques of processing Hall effect sensor signals to provide an angle or angular displacement have included the use of an index position signal to track the number of shaft resolutions. The index position requires an additional sensor and additional processing hardware. The additional components increase the cost and complexity of the angle sensing system for applications which only require a relative angle measurement and a continuous angular output. The resolution of angle measurement is also limited by the resolution of the hardware devices. In view of the above, it is apparent that there exists a need for a system and method for sensing the angle and angular displacement of a shaft using low cost Hall effect sensors with improved resolution.

SUMMARY

In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the prior art, in an embodiment of the present invention a system and a method for sensing the angle and angular displacement in a system with a DC brushless motor using Hall effect sensors is provided. More specifically, three Hall effect sensors are mounted in the DC motor. The Hall effect sensors provide three electrical angles of the motor rotor which are used in an angular displacement signal processor that is implemented in hardware and software. The angular displacement signal processor provides an improved resolution angular displacement output. Based on the angular displacement output from the angular displacement signal processor and the motor control command signal from the motor drive, an output state observer is applied to generate a high resolution angular displacement signal referred to as the observed angular displacement.

In an embodiment of the present invention, the Hall effect sensor signals are communicated to a angular displacement signal processor. The angular displacement signal processor includes three functional blocks. The first functional block determines the direction of the angular displacement using two of the Hall effect sensor signals. The second functional block combines the three Hall effect sensor signals to provide a pulse signal which has a frequency that is six times the frequency of each Hall effect sensor signal. The pulse signal from the second functional block and the angular direction signal from the first functional block are provided to a third functional block. The third functional block is a counter block that provides a continuous output corresponding to the angular displacement.

The angular output of the angular displacement signal processor can then be provided as an input to an output state observer. The output state observer receives the motor control command signal from the motor drive and the angular displacement signal from the angular displacement signal processor to generate an angular displacement signal with a high resolution termed the observed angular displacement.

In another embodiment, the Hall effect sensor signals are communicated to three encoder interface circuits. The two of the three Hall effect sensor signals are distributed to the inputs of each encoder interface circuit. The output from the encoder interface circuits are provided to a mathematical operation unit which generates an angular displacement signal with six times resolution to each of the Hall effect sensor signals.

In another aspect of the invention the output of the mathematical operation unit is provided to an output state observer. The output state observer receives the angular displacement signal from the mathematical operation unit and the motor control command signal from the motor drive to generate the observed angular displacement signal.

Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention related from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

Now referring toFIG. 1, an angular displacement sensing system12is provided according to an embodiment of the present invention. The angular displacement sensing system12includes a brushless DC motor14, a motor drive16, an angular displacement signal processor18and output state observer19. The brushless DC motor14uses three Hall effect sensors to provide motor commutation signals and uses a motor drive16to implement control for motor torque output. The angular displacement signal processor18receives the Hall effect sensor signals13from three Hall effect sensors mounted in the brushless DC motor14, and generates an improved resolution angular displacement output15. Based on the angular displacement output15from angular displacement signal processor18and motor drive input signal47from the motor drive16, an output state observer19generates a high resolution angular displacement signal17which is referred to as the observed angular displacement17.

Now referring toFIGS. 2-5, an angular displacement sensing system20and Hall effect sensor signal output plots embodying the principles of the present invention are illustrated.FIG. 2shows the angular displacement sensing system including a brushless DC motor22with three Hall effect sensors26, the motor drive16, an angular displacement signal processor72and an output state observer19. The three Hall effect sensors26are mounted into the motor22and motor drive16is in communication with motor22to control motor movement according to commutation logic based on the signals of three Hall effect sensors26. Three Hall effect sensors26generate three sensor signals40,42and44indicative of the angular displacement of a rotor in motor22. The first sensor signal40as shown in FIG.3(a) is a series of pulses. FIG.3(b) shows the second sensor signal42. The second sensor signal42is shifted 120° out of phase with the first sensor signal40. FIG.3(c) shows the third sensor signal44. The third sensor signal44is 240° out of phase with the first sensor signal40. The three sensor signals40,42and44are received by a buffer circuit28. The buffer circuit28transmits the sensor signals to the angular displacement signal processor72.

The angular displacement signal processor72, shown inFIG. 2, includes an I/O port30, a direction processing functional block32, a pulse processing functional block34, and a counter functional block36. The angular displacement signal processor72can be implemented using a software program in an embedded system or using a hardware circuit. The software program implementation for an embedded system is described below.

The output signals of the buffer circuit28, Hall effect sensor output signals40,42and44, are received by the I/O port30and read into the embedded system. I/O port30transmits two of the three Hall effect sensor signals40,42, and44to the direction processing functional block32. The direction processing functional block32indicates the direction of rotation of the brushless DC motor22according to the phase information using two of the Hall effect sensor signals40,42and44.

For example, Hall effect sensor signals40,42are transmitted to direction processing functional block32. The value 0 is generated for output signal45by the direction processing functional block32to indicate clockwise rotation of motor22when Hall effect sensor signal40indicates phase A is in a high logic state (signal value 1) and Hall effect sensor signal42indicates phase B is in a low logic state (signal value 0). The value 1 is generated for output signal45by the direction processing functional block32to indicate counter-clockwise rotation of motor22when Hall effect sensor signal40indicates phase A is in a low logic state and Hall effect sensor signal42indicates phase B is in a high logic state.

The I/O port30also transmits the three Hall effect sensor signals40,42,44to the pulse processing functional block34in FIG.2. The pulse processing functional block34combines the three Hall effect sensor signals40,42,44to create a continuous pulse signal46by a logical operation as shown in FIG.4. The frequency of continuous pulse signal46is six times of the frequency of the Hall effect sensor signals40,42,44.

The counter function block36combines the output signal45from the direction processing functional block32and the continuous pulse signal46from the pulse processing functional block34to generate an angular displacement signal48with direction indication. The angular displacement signal48is a continuous signal, as shown inFIG. 5, created by incrementing or decrementing the count for each pulse of the continuous pulse signal46provided by the pulse processing functional block34. Thus, the angular displacement signal processor72inFIG. 2generates a continuous angular displacement output signal with improved resolution relative to the pulse signals of Hall effect sensors26. The angular displacement signal48can achieve a resolution of 1/(6×pole number of the motor22).

Now referring toFIG. 6, another embodiment of the angular displacement sensing system provides for a angular displacement signal processor74including three encoder interface circuits50,52,54and an integrated processing unit58for performing a mathematical operation56. Encoder interface circuits50,52, and54are known devices which receive two phase-shifted digital signals from an incremental encoder to produce a continuous angle signal with indication of direction. One such encoder interface circuit is commercially available on the DS 3001 incremental encoder interface board manufactured by dSPAC Gmbh of Paderborn, Germany.

As shown inFIG. 6, three Hall effect sensor signals40,42,44are communicated to the encoder interface circuits50,52,54in the angular displacement signal processor74. The first Hall effect sensor signal40and the third Hall effect sensor signal44are connected to the first encoder interface circuit50. The first encoder interface circuit50combines Hall effect sensor signals40,44to generate the first output signal60. The first Hall effect sensor signal40and the second Hall effect sensor signal42are connected to the second encoder interface circuit52to generate the second output signal64. The second Hall effect sensor signal42and the third Hall effect sensor signal44are connected to the third encoder interface circuit54. The third encoder interface circuit54combines the Hall effect sensor signals42,44to generate the third output signal65. Output signals60,64,65from encoder interface circuits50,52,54are received by the integrated signal unit58and provided as input signals to the math operation56.

The mathematical operation56performs a real time processing on the three output signals60,64,65from encoder interface circuits50,52,54, shown inFIG. 6, to generate an angular displacement signal66. For example, the output signals60,64,65are processed according to the relationshipα=∑i=13⁢θi/3;
where α corresponds to the angular displacement signal66and θicorresponds to the encoder interface circuit output signals60,64,65.

As one skilled in the art will appreciate, many mathematical operations may be effectively used to generate an angular displacement signal66from output signals60,64,65of the encoder interface circuits50,52,54.

As mentioned above, the output state observer19shown inFIG. 1can be used in cooperation with either embodiment of the present invention. As shown inFIG. 2, the output state observer19receives the angular displacement signal48from the angular displacement signal processor72and the motor control command signal47to generate a high resolution angular displacement signal49. Alternatively, as shown inFIG. 6, the angular displacement signal66from the angular displacement signal processor74, and the motor control command signal47from motor drive16are received by the output state observer19. Output state observer19generates a high resolution angular displacement signal68which is referred to as the observed angle or observed angular displacement.

Now referring toFIGS. 7 and 8to describe the general implementation of the output state observer19with reference toFIG. 1, the output state observer19receives the angular displacement signal (θdisp)15from angular displacement signal processor and the motor control command signal (uc)47from the motor drive to generate the observed angular displacement (θobs)17. The state space equation description of the output state observer inFIG. 7is given as follows:x.^=A⁢⁢x^+Buc+K⁡(θdisp-C⁢⁢x^)(1⁢a)θobs=C⁢⁢x^(1⁢b)
where θdispis the input angular displacement signal from the angular displacement signal processor; ucis the motor control command signal from motor drive16; θobsis the resultant observed angular displacement signal17generated by the output state observer19; {circumflex over (x)} is the state to be observed;is a derivative of state {circumflex over (x)}; K is the observer gain which is determined by the output state observer design; A, B, and C are matrices indicative of the controlled plant model between the motor control command signal ucand angular displacement signal θdisp. To provide additional background information on the functioning of a signal state observer “Parameterization of observers for time delay systems and its application in observer design”, IEE Proceedings: Control Theory and Applications 143, 3 May 1996, IEE p 225-232 1350-2379 by Yao, Y. X.; Zhang, Y. M.; Kovacevic, R; is incorporated herein by reference.

In this embodiment, the output state observer provides angular displacement θobsbased on angular displacement signal θdispand motor control command signal ucby using the system described in Equations (1a) and (1b) and shown in the block diagram of FIG.7. The output state observer receives the angular displacement signal θdispfrom the angular displacement signal processor and subtracts θdispwith the observed angular displacement θobs=C{circumflex over (x)} to generate error signal (θdisp−C{circumflex over (x)}). The output state observer uses the observer gain K to minimize the error (θdisp−C{circumflex over (x)}) in Equations (1a) and (1b). The observed angular displacement signal θobsis smoothed to improve the resolution relative to the angular displacement signal θdisp.

The controlled plant model between the motor control command signal ucand angular displacement signal θdispis modeled as follows:
{dot over (x)}=Ax+Buc(2a)
θdisp=Cx  (2b)
where θdispis the angular displacement signal from the angular displacement signal processor, ucis the motor control command signal from the motor drive, x are states of the controlled plant, {dot over (x)} is derivative of state x, and A, B, and C are constant matrices. The output state observer in Equations (1a) and (1b) provides an observation for the angular displacement signal θdispbased on the controlled plant description (2a) and (2b). The constant matrices A, B, and C determine the order and parameters of the controlled plant.

The output state observer19provides a higher resolution output signal. More specifically, the output state observer19inFIG. 7acts as a signal predictor for the angular displacement signal15. The observer19minimizes the error between the angular displacement signal15and observed angular displacement signals17.

FIG. 8shows a plot of the angular displacement signal15as an input of the output state observer19(shown inFIG. 7) and the observed angular displacement as an output17of the output state observer19(shown in FIG.7). The plot inFIG. 8illustrates that the output state observer19(shown inFIG. 7) provides smooth observed angular displacement signal17with the minimal acceptable time delay relative to the angular displacement signal15.

The present invention has many advantages and benefits over the prior art. For example, the present invention provides a high resolution signal suitable for steer-by-wire applications. Further, the present invention has high reliability and a significantly lower cost as compared to alternatives offered by the prior art.