Method and system for controlling torque in permanent magnet brushless electric motors

A novel method is proposed for controlling the torque of a PM brushless motor with sinusoidal back-emfs without current sensors by computing the required input phase voltages with measured rotor position and speed and known machine parameters. These voltages are fed to the machine at an angle computed in terms of input parameters and the phase voltage with respect to their back-emfs so that phase currents are aligned with their back-emfs to exactly mimic the performance of the current mode controller.

TECHNICAL FIELD

This invention relates to torque control in automotive permanent magnet (PM) brushless electric motors.

BACKGROUND OF THE INVENTION

It is known in the art of permanent magnet brushless electric motors to control torque by aligning phase currents with back-emf. The torque delivered by the electric motor is then directly proportional to the phase current and is therefore easily controlled by simply controlling the aligned phase currents. This is commonly referred to as current mode control. The drawback is that current sensors are required to determine what the currents are. The sensors necessarily have a finite, though small, dc voltage drops that induce torque ripple into the motor.

SUMMARY OF THE INVENTION

An exemplary embodiment is a method of controlling torque in electric motors that is analogous to traditional current mode control methods, but which requires no current sensors to determine what the current is in any particular phase. Instead, phase current information is calculated from knowledge of the rotor's position and rate of rotation. The information is used to calculate a required voltage and electrical angle offset needed to obtain a user-specified torque. The calculated voltage and offset is added to the input power of the electric motor by known means, usually an inverter.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A description of the preferred embodiment of the present invention will now be had with reference to FIGS. 1 and 2 of the drawings.

Referring to FIG. 1 there is shown a phasor diagram. In permanent magnet (PM) synchronous machines with sinusoidal back-emfs, torque control is accomplished by regulating the phase currents and aligning them with respective back-emfs, typically by adding a phase voltage to the motor input at a load angle . Under these conditions, the torque is directly proportional to the phase current. This is shown in the phasor diagram of FIG. 1 , wherein:

E stator phase back-emf,

V stator phase terminal voltage,

I stator phase current,

R stator phase resistance,

X e L a stator phase reactance,

L a stator phase inductance,

load angle between back-emf and stator phase terminal voltage, and

f magnet flux linkage.

The electromagnetic torque of the machine is given by

T e 3 E I/ m (1)

where m P e mechanical angular frequency in rad/sec, and P number of pole pairs. Equation (1) may be written in terms of a back-emf constant, K e as

T e 3 K e I (2)

Where K e E/ m , V/(mech rad/s). Because torque is directly proportional to the phase current in current mode controller, it requires two current sensors for measuring phase currents.

The drawback of current mode control is the production of low frequency torque ripple caused by dc offset in the current measurements of the current sensors. This is undesirable in the case of column-assisted electric power steering where the sinusoidal PM motor is coupled directly into the steering column to provide torque assistance.

The phasor diagram of FIG. 1 is used to compute required phase voltages for a given torque command, T cmd :

V ( E I R ) j I X (3)

where j is the imaginary square root of 1, therefore

From Equation (2),

I T cmd /3 K e (5)

Substituting Equation (5) into Equation (4) gives

v 2 E ( T cmd R/ 3 K e ) 2 T cmd P L a m 3 K e 2 (6)

Equation (6) can then be simplified as

where K 1 R/3K e and K 2 (P L a /3K e ) 2 . The phase voltages can also be obtained from a V 2 vs. V look-up table to reduce computational time.

From FIG. 1 , the load angle, , is obtained in terms of known parameters as

V sin T cmd m K 3 (8)

and therefore,

where K 3 P L a /3K e

The load angle, , can be calculated from a V sin vs look-up table and V 1 can be obtained from a V 2 vs V 1 look-up table to reduce computational time. Therefore, the input phase voltages are

V a V sin( ) (10)

Referring to FIG. 2 , the system in an exemplary embodiment includes a rotor position encoder 110 coupled to a PM motor 108 . The encoder 110 is operative to measure the angular position, , of the rotor of the motor and provides as output a position signal 112 indicative thereof. A speed measuring circuit 114 is connected to the position encoder 110 for determining the angular speed, d /dt m and providing as output therefrom a speed signal 116 indicative thereof. The position and angular speed signals 112 , 116 as well as a torque command signal, T cmd , 118 , indicative of a desired motor torque, are applied to a controller 200 . The controller 200 generates the input phase voltages 214 and motor voltage command signals 218 in response to the position and angular speed signals 112 , 116 and the torque command signal 118 . Blocks 202 , 208 . 212 , and 216 indicate processing performed by controller 200 . An inverter 104 is coupled between a power source 102 and the controller 200 for applying phase voltages 106 across the motor 108 in response to the motor voltage command signals 218 in order to develop the desired motor torque. In order to generate phase voltages 106 with an average sinusoidal shape, switching devices indigenous to the inverter 104 , must be turned on and off for specific durations at specific rotor angular positions, . Control of the inverter 104 to generate phase voltages 106 with an average sinusoidal shape can be implemented by way of any appropriate pulse width modulation (PWM) scheme 216 . Because space vector modulation (SVM) has advantages in higher output voltage, low harmonic distortion, low switching power loses and easy microprocessor implementation, SVM-based control may be preferred.

An exemplary method includes sensing the angular position, , of the rotor and determining the angular speed, d /dt m , thereof. In response to the angular position, , and angular speed, m , of the rotor and to the torque command signal, T cmd , 118 , the controller 200 generates motor voltage command signals 218 indicative of the voltage required to produce the desired motor torque. Phase voltages 106 are applied across the motor windings in response to the motor voltage command signals 218 to develop the desired motor torque. In particular, in response to the torque command signal, T cmd , 118 , the angular speed signal 116 and known parameters, at 202 the controller 200 calculates V according to Equation (7) above. Alternatively, V may be determined by using V 2 vs V or V 2 vs V 1 look-up tables. Based upon the calculated value for V and known parameters, at 208 the controller 200 calculates according to Equation (9) above. Alternatively, may be determined by using a sin vs look-up tables. Based upon the calculated value for V, the calculated value for and the angular position, , at 212 the controller 200 calculates the input phase voltages, V a , V b , and V c , 214 . At 216 , input phase voltages, V a , V b , and V c , 214 are subject to a pulse width modulation (PWM) scheme. Motor voltage command signals 218 , indicative of the voltage required to produce the desired motor torque and in the form of pulse width modulated signals having an average sinusoidal shape, are applied to the inverter 104 wherein the necessary switching is performed for application to the motor 108 . By controlling torque without using current sensors, torque ripple is reduced resulting in smoother application of torque.

While the invention has been described by reference to a preferred embodiment, it should be understood that numerous modifications can be made thereto. Accordingly it is intended that the invention not be limited to the disclosed embodiment, but that it retain the full scope and spirit permitted by the language of the appended claims.