Motor control circuit

Abstract

A motor control circuit for an electric motor of an electric power assisted steering system comprises a switching circuit comprising a plurality of electrical switches, a current demand signal generator which converts the torque demand signal into a current demands signal; and a fault mode motor current controller that is responsive to an error signal that represents the difference between the current demand signal and the actual current flowing in the motor and is operable in the event of a fault where one phase is open-circuit to drive the remaining two phases as a single combined phase by generating a single voltage demand signal that is representative of the voltage to be applied across the combined phases, the voltage signal being in turn fed into a drive circuit for the switches that generates pulse width modulated switching signals for the switching circuit required to apply the voltage across the combined phases.

Claims

1. A motor control circuit for an electric motor of an electric power assisted steering system in which a measurement of torque carried by a part of the steering system is used to produce a torque demand signal indicative of a torque to be applied to the steering system by the motor, the motor control circuit comprising: a switching circuit comprising a plurality of electrical switches, a motor current controller that generates voltage demand signals to be passed to a drive circuit for the switches that in turn generates pulse width modulated switching signals for the switching circuit that cause the switches to selectively connect the phases to a power supply so as to cause current to flow through the phases of the motor, in which the current controller is responsive to a d-q axis error signal that represents the difference between the value of a d-q axis current demand signal that is derived from the torque demand signal and an actual d-q axis current flowing in the motor, whereby the current controller converts the d-q axis error signal into three phases voltages using a converter which receives as input the d-q axis error signal and a motor position signal, and in which in a normal mode of operation the motor position signal varies according to the position of the rotor of the motor and in a fault mode when one phase is open circuit the motor position signal is fixed according to the position of the faulted phase.

2. The motor control circuit according to claim 1 in which, in the fault mode, the motor position signal is fixed at the angle of the faulted phase such that when the three voltage signals are calculated the voltage of the faulted phase will be zero and the voltages of the two other phases will be equal and opposite.

3. The motor control circuit according to claim 2 in which the current controller in use converts the error signal into the three phase voltages using the equations that define a Park transform to first convert from the d-q frame to an alpha-beta frame or to the fixed frame, the current controller subsequently converting from the alpha-beta frame, or fixed frame, to the three phased voltages using the following equations: Phase U voltage demand=Alpha axis voltage demand Phase V voltage demand=(Alpha axis voltage demandBeta axis voltage demand3) Phase W voltage demand=(Alpha axis voltage demand+Beta axis voltage demand3).

4. The motor control circuit according to claim 1 which includes two current controllers, one controlling the q-axis current and the other the d-axis current.

5. The motor control circuit according to claim 4 in which in the event of a fault where one phase in open circuit, the d-axis controller is disabled to force the output of the d-axis controller to zero, control of the current then being performed solely by the q-axis controller.

6. The motor control circuit according to claim 1 which is arranged so that each of the currents demanded for the two non-faulted phases are waveshaped to reduce the torque ripple as the motor rotates, the shaping comprising varying the magnitude of the current demand signal as a function of motor position.

7. An electric power assisted steering system comprising a steering mechanism that connects a steering wheel to a road wheel, a torque sensor that produces a torque signal indicative of a torque carried by a part of the steering mechanism, an electric motor that is connected to the steering mechanism so that torque produced by the motor is transferred to the steering mechanism, wherein a measurement of the torque carried by the part of the steering system is used for generating a torque demand signal indicative of a torque to be applied to the steering system by the motor that is a function of the torque measured by the torque sensor, and a motor control circuit that is responsive to a signal dependent on the torque demand signal, the motor control circuit comprising a switching circuit comprising a plurality of electrical switches, a motor current controller that generates voltage demand signals to be passed to a drive circuit for the switches that in turn generates pulse width modulated switching signals for the switching circuit that cause the switches to selectively connect the phases to a power supply so as to cause current to flow through the phases of the motor, in which the current controller is responsive to a d-q axis error signal that represents the difference between the value of a d-q axis current demand signal that is derived from the torque demand signal and an actual d-q axis current flowing in the motor, whereby the current controller converts the d-q axis error signal into three phases voltages using a converter which receives as input the d-q axis error signal and a motor position signal, and in which in a normal mode of operation the motor position signal varies according to the position of the rotor of the motor and in a fault mode when one phase is open circuit the motor position signal is fixed according to the position of the faulted phase.

8. The electric power assisted steering system according to claim 7 in which the motor comprises a three phase brushless DC motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram showing an motor control circuit for an electric motor of an electric power steering system, the motor control circuit comprising an embodiment with the scope of an aspect of the present invention;

(2) FIG. 2 is a more detailed block diagram showing the elements of the closed loop current controller of FIG. 1;

(3) FIG. 3 shows the three phase currents for a working motor which vary sinusoidally to provide a smooth motor toque;

(4) FIG. 4 shows the three phase currents for a motor with a single phase open circuit fault when a fault mode strategy is not used;

(5) FIG. 5 shows the variation in phase currents applied by the motor control circuit of FIG. 2 when operating in a fault mode;

(6) FIG. 6 shows the motor torque corresponding to the current waveforms shown in FIGS. 3, 4 and 5;

(7) FIG. 7 shows a typical three phase motor switching circuit that is used to modulate the voltages applied to the ends of each phase;

(8) FIG. 8 is a block diagram of an alternative motor control circuit that include a normal mode current controller and a fault mode current controller;

(9) FIG. 9 is a plot of the voltage applied to the combined phases in the fault mode;

(10) FIG. 10 is a diagram showing the PWM signals applied to the bridge in a first mode of operation when in the fault mode;

(11) FIG. 11 is a corresponding diagram showing the PWM signals applied to the bridge in a second mode of operation when in the fault mode; and

(12) FIG. 12 shows an alternative current waveform that can be applied to the motor phases in the fault mode which has been waveshaped using a combination of a cosecant function and linear ramp of the current demand.

DETAILED DESCRIPTION OF THE INVENTION

(13) Referring to FIG. 7 a three phase brushless motor 1 comprises three motor windings 2, 4, 6, generally designated as phases U, V and W. The phases are connected in a star network so that one end of each phase is connected to a corresponding end of each of the other phases at the star point. The free ends of each phase are connected to a switching circuit arranged as an H-bridge.

(14) The switching circuit comprises a three phase bridge 8, one for each phase of the motor. Each arm 10, 12, 14 of the bridge comprises a pair of switches in the form of a top switch (transistor 16) and a bottom switch (transistor 18) connected in series between a DC voltage supply rail 20 and a ground line 22. The motor windings 2, 4, 6 are each tapped off from between a respective complementary pair of transistors 16, 18. The switches can be open (OFF) or closed (ON).

(15) The transistors 16, 18 are turned on and off in a controlled manner by a motor controller 21, which is shown in detail in FIG. 2 of the drawings, to provide pulse width modulation of the potential voltages applied to each of the phase windings, thereby to control the potential difference applied across each of the windings 2, 4, 6 and hence also the current flowing through the windings. This in turn controls the strength and orientation of the magnetic field produced by the windings, which sets the torque produced by the motor.

(16) A current measuring device in the form of a resistor 24 is provided in the ground line 22 between the motor 1 and ground so that the controller 21 can measure the total current flowing through all of the windings 2, 4, 6. In order to measure the current in each of the windings the total current has to be sampled at precise instances within the PWM period where the voltage applied to each terminal of the winding (and hence the conduction state of a particular phase) is known. If preferred a separate current sensor could be provided for each phase.

(17) The controller in this example uses a Space Vector Modulation (SVM) algorithm although any modulation technique can equally be used within the scope of the present invention and this should not be construed as limiting.

(18) Each winding 2, 4, 6 in a three phase system can only be connected to either the supply rail 20 or the ground line 22 and there are therefore eight possible states of the switches of the control circuit. Using 1 to represent one of the phases being at positive voltage and 0 to represent a phase connected to ground, state 1 can be represented as [100] indicating phase U at 1, phase V at 0 and phase W at 0, State 2 is represented as [110], state 3 as [010], state 4 as [011], state 5 as [001], state 6 as [101], state 0 as [000] and state 7 as [111]. Each of states 1 to 6 is a conducting state in which current flows through all of the windings 2, 4, 6, flowing in one direction through one of them and in the other direction through the other two. State 0 is a zero volt state in which all of the windings are connected to ground and state 7 is a zero volt state in which all the windings are connected to the supply rail.

(19) During normal operation when the switching circuit is being controlled by the controller 21 to produce pulse width modulation, each of the phases 2,4,6 will normally be turned on and off once in each PWM period. The relative lengths of time that are taken up in each state will determine the magnitude and direction of the magnetic field produced in each winding, and hence the magnitude and direction of the total torque applied to the rotor.

(20) The motor control circuit of FIG. 2 can be used in many applications, and in this embodiment forms part of an electric power steering system as shown in FIG. 1. The steering system includes a torque sensor 26 that measures the torque in a steering column, and feeds this to a torque controller. The torque controller takes as an input an assistance torque signal that is indicative of the amount of torque that the motor is to produce and to apply to the steering system to help the driver turn the steering wheel. The value of the assistance torque demand signal is set as a function of the torque applied to the steering system by the driver, hence the need to input the measured torque to the controller.

(21) The torque demand signal is fed into the current controller, which converts the torque demand into a current demand according to known characteristics of the motor. The current demand signal is set as a d-q axis current value, which comprises a vector in a frame of reference that is fixed relative to the motor rotor.

(22) The current controller compares the current demand signal with the actual current flowing in the motor, also in the dq frame of reference, to produce an error signal. The current controller then produces voltages to be applied to each phase that will drive the error to a minimum, thereby ensuring that the actual current matches the demanded current.

(23) The current in the motor causes the motor to generate the demanded assistance torque, and this torque makes it easier for a driver to turn the steering wheel. The system therefore has two closed loopsthe torque controller loop which sets the assistance torque, and the current controller loop which drives the current error to zero and so sets the current in the motor.

(24) As shown in FIG. 2, the error signal is produced by subtracting the actual current from the demanded current (although the demanded current could be subtracted from the actual current). This error signal is fed into the PI controller that applies one or more gains to the error to produce a voltage signal in the dq frame of reference.

(25) The d-q frame voltage is then converted into the three voltages needed for the three phases by first converting it into two voltages in the alpha beta frame using a Park transform as follows:
Alpha axis voltage demand=d axis voltage demandCOS(motor position)q axis voltage demandSIN(motor position)
Beta axis voltage demand=d axis voltage demandSIN(motor position)+q axis voltage demandSIN(motor position)

(26) where the motor position is a value indicative of the position of the rotor or some other position within the motor.

(27) The two voltages are the converted into the three voltages using the following equations:
Phase U voltage demand=Alpha axis voltage demand
Phase V voltage demand=(Alpha axis voltage demandBeta axis voltage demand3)
Phase W voltage demand=(Alpha axis voltage demand+Beta axis voltage demand3)

(28) These three voltages are then applied to the respective phases using any known PWM strategy by a PWM converter 38. The controller constantly attempts to drive the error signal to zero, which ensures that the current in the motor phases matches the demanded current.

(29) To apply the Park transform, the current controller block requires a measured or estimated motor position signal as an input parameter. This is provided by the position calculator block shown in FIG. 2.

(30) In normal use, the position calculator outputs a position signal that is representative of the actual position of the motor rotor. This can be derived from the output of a position sensor. Alternatively, in a position sensorless system this may be derived from an estimate of position obtained from observing the currents flowing in the motor.

(31) In a fault mode where one phase is faulted as an open circuit, the calculator block does not output a position signal indicative of the motor rotor position. Instead, it outputs a signal indicative of the position of the faulted open circuit. This may take one of three discrete values, each 120 degrees apart, for a typical three phase motor. The position calculator switches between normal and fault modes in response to an output from a fault indication block (give example of how this block functions).

(32) By fixing the angle when a fault occurs, the same converter and equations used in the normal mode can be used in order to produce two voltage signals, one for each of the unfaulted phases, as shown in FIG. 5 for the case of phase U being at fault. Because the converter sees the rotor as fixed, the voltage will be of a constant value regardless of the position of the rotor and the voltage of the failed phase will be set to zero.

(33) In practice, the circuit of FIG. 2 may include two current controllers that respectively control the q-axis and d-axis currents. When the fault mode is detected the output of the d-axis controller may be set to zero by changing the gains of the d-axis controller. This ensures that only the q-axis controller is used to control the current in the motor, which gives enhanced performance in the fault mode.

(34) The embodiment of FIG. 2 therefore uses as much as possible of the normal current control circuitry to operate the motor in a fault mode. This can be advantageous during design and manufacture as there are minimal changes relative to a system that does not function in the fault mode. Indeed, the only changes needed may be to the block that outputs the motor position signal to the two phases to three phase converter and a block that detects when a fault has occurred.

(35) In a modification, shown in FIG. 8, the motor control circuit is provided with a separate current controller for use in the fault mode, the normal current controller used in a non-fault mode being arranged as described above. The circuit may therefore include thee controllers is separate q-axis and d-axis controllers are provided for use in a normal mode. As with the first embodiment, the fault mode is considered to be a mode in which one phase is faulted as an open circuit, the normal mode being when all three phases are unfaulted.

(36) The fault mode controller is simpler than the normal current controller in that it drives the two unfaulted phases as a single combined phase. The applicant has appreciated that in the fault mode the current flowing into one of the unfaulted phases can only be equal to the current flowing out of the other phase. As such the end of one phase can be fixed to the DC supply rail or ground and the other phases PWM modulated, which means that the controller only needs to calculate a single voltage demand signal from the error signal. This single voltage demands signal can then be applied across the combined phases using any known PWM technique.

(37) To determine the single voltage demand signal, the torque demand is first converted into a current demand value that is not in the d-q frame, but is simply a current that is proportion to the torque (or related in some other manner according to the motor characteristics). The controller then compares the current flowing in any of the two unfaulted phases with the current demand signal to produce an error signal, and this is fed into a PI controller with appropriate gains. The output of the PI controller is then converted into a single voltage, or may be used directly as the single voltage. Finally this single voltage is applied across the combined phases using a PWM strategy.

(38) In one advantageous PWM strategy, shown in FIG. 9, the voltage applied to the combined phases is switched in polarity each time the rotor has passed through 180 degrees, the changeover corresponding to the position of the faulted phase.

(39) The switch means may be driven in a first mode to apply the voltage demanded by the controller across the single combined phase by clamping a first end of the combined phase to the positive supply and applying a PWM signal to the second end of the combined phase over one half of an electrical rotation of the motor rotor. For example, with the U phase faulted, the V phase may be clamped to the positive supply by closing the top switch and opening the bottom switch and modulating the W phase switches.

(40) During the other half of the electrical rotation of the rotor the switch means are driven in a second mode by clamping the second end to ground and applying a PWM signal to the first end, the changeover between the two modes corresponding to the position of the failed open circuit phase. In the example of the preceding paragraph, this will comprise turning on the bottom switch of the W phase and turning off the top switch of the W phase, then modulating the switches of the V phase.

(41) The PWM modulation is therefore only applied to one phase of the combined phases, but is set to control the overall voltage applied across the combined phases.

(42) The signals applied to each of the switches of the bridge are shown in FIG. 10 for the first mode and FIG. 11 for the second mode when the U phase is at fault. Note that during the fault mode the switches of the faulted phase are held open (OFF) at all times.

(43) In both embodiments there will still be some ripple present in the motor torque when operating in the fault mode. This is because there are two points in each full rotation of the rotor where the torque generated by the motor will be zero. Also, if the demanded current is held constant the torque will still vary with rotor position.

(44) To reduce this, the current demand signal may be waveshaped so that it varies with angular position of the rotor. A suitable waveshape may comprise applying a linear or non-linear ramp gain to the current demands signal as a function of rotor position. For example a non linear function such as a cosine of cosecant curve could be applied. An example of a shaped waveform is shown in FIG. 12. This shaping will, of course, cause the voltages applied to the motor to vary with rotor position in both the normal and fault modes.

(45) Note that the waveform in FIG. 12 varies in polarity every 180 degrees. It could, of course, have the same polarity for all rotor positions with the PWM strategy operating in the two modes described in relation to the first embodiment taking care of the reverse in polarity.

(46) In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.