Method for controlling motor and multi-channel motor circuit

Abstract

A method of controlling a motor and a multi-channel motor circuit are provided. A method of controlling an electric power steering system is provided that includes a multi-phase permanent magnet electric motor having a plurality of motor phases that, in normal operation, apply torque to a portion of the system connected to a wheel to control a steering angle of the wheel, the motor being driven by a bridge circuit, and a bridge circuit including a set of switches capable of opening and closing in response to a signal output from the bridge driver circuit to selectively couple each to a supply voltage of the power supply or ground, the method comprising: in the event of a fault that prevents the motor from applying a torque that enables the wheel to steer, starting the bridge driver circuit; a switch of the bridge circuit is operated independently of the bridge driver to short each phase of the motor to damp rotation of the motor and simultaneously turn off a power supply connected to the bridge circuit.

Description

Technical Field

[0001] The invention relates to a method of controlling a motor of an electric power steering system, to a motor circuit for use in an electric power steering system and to a vehicle comprising both front wheel steering and rear wheel steering, wherein one of the systems comprises the motor circuit. Background Art

[0002] Most current passenger and commercial vehicles intended for driving on highways have steerable front wheels. The driver can turn the front wheels by turning the steering wheel or yoke. With an electric power-assisted steering system, the steering wheel is directly connected to the wheels through a mechanical linkage, and movement of the steering wheel produces a corresponding change in the steering angle of the wheels. To reduce the effort required to turn the steering wheel, the torque applied by the driver is measured and fed into a control system that instructs the electric motor to produce the same torque as applied by the driver. This motor torque is then applied to the portion of the steering system connected to the wheels to assist the driver.

[0003] A recent trend has been to provide a steering system that does not have a direct connection from the steering wheel to the wheels, a system known as electric steering or steer-by-wire. This brings a number of benefits in terms of packaging and safety in the event of a frontal collision, as there is no longer a rigid steering column in front of the driver. To steer the vehicle, the torque applied by the driver at the steering wheel is measured and fed into a controller which causes an electric motor to apply torque to a steering rack connected to the wheels. This torque causes the motor to rotate and the rack to move, thereby turning the wheels. A secondary motor may also be connected to the steering wheel, which produces a torque that opposes the driver's turning of the wheel to give the desired steering feel and simulate the feedback that the driver would feel if there was a mechanical connection directly from the steering wheel to the wheels.

[0004] An increasing number of passenger and commercial vehicles also have steerable rear wheels. Rear wheel steering has long been available for specialist vehicles such as forklifts, but has only recently become common on road vehicles. Providing steerable rear wheels in addition to steerable front wheels can also help reduce the turning circle of the vehicle by turning the steerable rear wheels in the opposite direction to the front wheels. They can also be used to increase stability during high-speed manoeuvres, such as lane changes, by turning the rear wheels in the same direction as the front wheels.

[0005] In the event of a fault in a steering system (particularly a steer-by-wire system) which prevents the control system from driving the motor to apply torque to turn the wheel, the forces acting on the wheel may attempt to drive the motor in reverse, causing the steering angle of the wheel to change in an uncontrolled manner. The driver cannot resist this because there is no direct mechanical connection from the steering wheel to the wheel. In an emergency situation where such a fault has occurred and the vehicle is travelling at a high speed, the wheel may deviate from the straight ahead position by a considerable amount, causing the vehicle to drift laterally across the highway. This could put the driver in danger, for example by placing the vehicle on the wrong side of a two-way highway.

[0006] Since the steering system is considered a safety critical system, it is known to use a motor with dual channels. Each channel comprises a control circuit that generates a control signal for a driver circuit that varies the voltage applied to the motor to produce the required torque. In the case of a multiphase motor, each phase can be connected to a supply voltage or to ground by a pair of switches that form a drive circuit and are opened and closed according to the value of the control signal from the control circuit. This allows steering control to be maintained in the event of a failure in one of the channels. However, it is still possible that both channels fail and control of the front wheel steering is lost.

[0007] In order to ensure the safety of the occupants of the vehicle, it has been proposed that in the event of a failure of the front wheel steering (where control of the steering angle of the wheels is lost), the vehicle can switch to an emergency steering mode. This emergency steering mode is intended to allow a separate and independent system to control the vehicle laterally. This is to allow the vehicle to be maneuvered into a safe position. This can be achieved using the process of brake steering, where individual brakes are applied to produce a yaw of the vehicle or by steering the vehicle from the rear using rear wheel steering. In this way, the vehicle can be safely moved to the side of the highway where the vehicle can be stopped outside the flow of traffic on the highway. The applicant has appreciated that the success of this emergency steering mode in safely steering the vehicle to safety depends on the behavior of the uncontrolled front wheels. Therefore, a mechanism or system is desired for locking the front wheels in a fixed steering orientation when in this failure mode, or at least providing a high resistance to movement.

[0008] Applicant is aware of the teaching of US, 128,241 B1, in which a motor control system short-circuits the motor windings of a motor so that the motor produces a braking torque, and all or some of the motor's electrical control units are disabled or ineffective. The control circuit includes a short-circuit circuit having a set of additional switches that short-circuit each phase of the motor at a point between an inverter of a drive axle and the motor phase windings. Summary of the invention

[0009] The object of the present invention is to provide a method and a system which limit the movement of the front wheels of a vehicle having an electric power steering system in the event of a fault in the steering system which prevents the wheels from turning.

[0010] According to a first aspect, the present invention provides a method for controlling an electric steering system, the electric steering system comprising a multiphase permanent magnet electric motor having a plurality of motor phases, the multiphase permanent magnet electric motor applying torque to a portion of the system connected to a wheel in normal operation to control the steering angle of the wheel, the motor being driven by a bridge circuit, the bridge circuit comprising a set of switches which can be opened and closed in response to a signal output from a bridge driver circuit to selectively connect each phase to a supply voltage of an electric power supply or to ground, the method comprising, in the event of a fault preventing the motor from applying a torque enabling the wheel to be steered, operating the switches of the bridge circuit independently of the bridge driver to short-circuit each phase of the motor so as to dampen the rotation of the motor and simultaneously shut off the power supply to the bridge circuit.

[0011] By shorting each phase of the motor to ground, the bridge driver is effectively disabled as it can no longer apply current to the motor phases, and by isolating the power supply this method ensures that the power supply will not short in the event of a fault causing a phase to be connected to the power supply.

[0012] The method may isolate the bridge circuit from the power supply by switching off the power supply or by switching off one or more isolation switches between the power supply and the bridge circuit.

[0013] Where the bridge circuit comprises an upper switch and a lower switch for each phase, the method may short-circuit each phase by closing the lower switch of each phase of the bridge circuit, wherein the upper switch connects the phase to the power supply when closed and the lower switch connects the phase to ground.

[0014] The method may override the operation of the lower bridge switch by the bridge driver, thereby preventing the bridge driver from opening the lower switch.

[0015] The switches may include transistors that are turned on and off by a bridge driver circuit applying a non-zero voltage drive signal to the gates of the switches, and the method may short-circuit the phases by connecting the gate or base of each switch to a non-zero voltage to turn the switches on.

[0016] The motor may include a control circuit that generates a control signal applied to the bridge driver circuit, the control signal indicating the torque to be applied by the motor. The bridge driver circuit may generate an appropriate switch mode waveform to apply to each switch of the bridge in response to receiving the control signal.

[0017] The method may include monitoring the control circuit and generating a bridge driver disable signal in the event of a control circuit failure, the method monitoring the signal to determine when to short circuit phases of the motor to dampen rotation of the motor.

[0018] The motor may include a dual channel motor having a first channel and a second channel, each channel including a set of phase windings, a bridge circuit, a bridge driver, a control circuit and a fault detection circuit, which monitors the control circuit and outputs a bridge driver disable signal in the event of a fault, and the method may monitor the two bridge driver disable signals and short each phase to ground to dampen rotation of the motor only when the value of the bridge driver disable signal indicates that a fault has occurred in both control circuits, while preventing any shorted bridge circuit from receiving power from the power supply, whether due to a failed switch or due to a failure of opening and closing switches in the control circuit or driver.

[0019] By damping only when the control circuits for both channels fail, this approach allows the system to continue to provide steering when one channel is functional, but safely damp the motor when both channels fail.

[0020] In the presence of two channels, each channel of the motor comprising a corresponding set of phase windings driven by a bridge circuit selectively connecting each phase to a positive supply or ground, the method may include, in the event of a fault, preventing the wheels from being turned, thereby shorting each phase in the two channels of the motor to ground to dampen rotation of the motor and simultaneously preventing both bridge circuits from receiving power from the power supply.

[0021] Short-circuiting the phases of both channels offers the possibility of increasing the braking torque compared to short-circuiting only one channel.

[0022] The method may include combining the bridge driver disable signals using logic such that both signals must indicate a fault for all phases of the motor channel to be shorted to ground.

[0023] According to a second aspect, the present invention provides a multi-channel motor circuit for driving a multi-channel permanent magnet motor of an electric power steering system, wherein each channel of the motor comprises a plurality of motor phase windings, the motor is a permanent magnet electric motor, and the motor circuit comprises:

[0024] The first motor channel includes:

[0025] a first bridge circuit comprising a set of top switches and a set of bottom switches, wherein each top switch selectively connects a corresponding phase of the motor to a supply voltage of a power supply and each bottom switch selectively connects a corresponding phase of the motor to ground;

[0026] a first bridge driver circuit that outputs a drive signal for each switch of the bridge, respectively, so as to control the voltage applied to each phase over time in response to a set of control signals, thereby controlling the current flowing through each phase of the motor,

[0027] a first control circuit configured to output a control signal to the bridge driver; and

[0028] a first fault detection circuit that outputs a bridge driver disable signal to disable the bridge driver associated with the channel if a fault occurs in the channel,

[0029] The second motor channel includes:

[0030] a second bridge circuit comprising a set of top switches and a set of bottom switches, wherein each top switch selectively connects a corresponding phase of the motor to a supply voltage of a power supply and each bottom switch selectively connects a corresponding phase of the motor to ground;

[0031] a second bridge driver circuit that outputs a drive signal for each switch of the bridge, respectively, so as to control the voltage applied to each phase over time in response to a set of control signals, thereby controlling the current flowing through each phase of the motor, and

[0032] a second controller configured to output a control signal to the bridge driver;

[0033] a second fault detection circuit that outputs a bridge driver disable signal to disable the bridge driver associated with the channel if a fault occurs in the channel,

[0034] And also includes a monitoring device and a safety damping circuit, wherein the monitoring device is arranged to monitor the bridge driver disable signal, and the safety damping circuit is configured so that when the monitoring device determines that the bridge driver disable signal indicates that the control circuits of both channels have failed, the safety damping circuit causes the bottom switch of the bridge driver to short-circuit each phase of at least one channel of the motor so as to dampen the rotation of the motor, and at the same time isolates the top switch of the bridge circuit from the power supply.

[0035] The safety damping circuit may include a pair of switches arranged in series between a positive supply voltage and a switch for each phase of at least one channel of the motor, the switches including a first switch and a second switch, wherein the first switch is normally open and is closed when a first bridge driver disable signal indicates disabling the first bridge driver circuit, and the second switch is normally open and is closed when a second bridge driver disable signal indicates disabling the second bridge driver circuit, and is configured such that a short circuit is applied to each phase of at least one channel to provide damping to the motor by generating electricity or braking torque only when both switches are closed.

[0036] The two switches in series perform a logical AND function, both switches need to be closed to apply damping.

[0037] Each of the two switches of the safety damping circuit may comprise a transistor such as a MOSFET, the safety damping circuit being adapted to close the switch by applying a positive voltage to the base of the switch to turn on the MOSFET, thereby allowing current to flow between the source and the drain.

[0038] When the two switches of the safety damping circuit are closed, the safety damping circuit outputs a voltage at or close to the supply voltage, which is used to turn on the bottom switch. This voltage can be fed to the gate driver, which outputs the appropriate level voltage to the gate of the bottom switch.

[0039] The motor circuit may further include an isolation circuit comprising a set of isolation switches, one switch being located in series between the bridge circuit and each motor phase, each isolation switch being normally closed. The main function of the switches is to allow a circuit breaker to be placed in each phase, thereby preventing a short circuit from being formed in the event of a fault in one channel of the motor and the other channel being used to control the steering of the wheels.

[0040] In the event of a two-channel fault, the isolation circuit can keep the isolation switch closed to allow for intentional introduction of damping.

[0041] The motor circuit may include a replica safety damping circuit comprising a pair of switches arranged in series between a positive supply and the end of each phase of a second channel of the motor, wherein a first switch is normally open and is closed when a first bridge driver disable signal indicates disabling of the first channel, and a second first switch is normally open and is closed when a second bridge driver disable signal indicates disabling of the second channel, both switches need to be closed to apply a positive voltage to the end of each phase of the second channel.

[0042] Providing duplicate safety damping circuits enables damping to be maximised as both channels can be used to provide damping.

[0043] The two safety monitoring circuits described above can be combined to form a single circuit that can damp both channels. However, having separate switch pairs with monitoring circuits for each channel provides a high level of redundancy and improves safety in the event that one of the switches fails to close when instructed.

[0044] The bridge disable signal can be input to the power supply isolation block, which isolates the top switch of the bridge from the power supply to prevent the phase from receiving a positive voltage from the power supply. This ensures that the positive voltage cannot pass through the closed top switch of the bridge directly to ground through the closed bottom switch.

[0045] The power supply isolation block can shut off the power supply for the bridge circuit, removing the positive voltage. The motor circuit can be arranged so that this power supply is independent of the power supply to the control circuit so that when it is shut off, the control circuit can continue to operate, and the safety monitoring circuit can continue to operate to appropriately turn on the bottom bridge switch to produce the desired motor damping effect.

[0046] Alternatively, it may open the switch to isolate the top switch of the bridge from the positive voltage of the power supply.

[0047] The safety damping circuit or each safety damping circuit (when two are provided) may include a pair of switches arranged in series between a positive voltage of the power supply and the gate of each lower switch of the bridge circuit, wherein a first switch is normally open and is closed when a first bridge driver disable signal indicates that the first channel is disabled, and a second first switch is normally open and is closed when a second bridge driver disable signal indicates that the second channel is disabled, both switches need to be closed to apply a positive voltage to the gate of each phase of the channel.

[0048] The multi-channel motor may have only a first channel and a second channel, a so-called two-channel motor. However, the invention can be applied to motors with more than two channels. Damping can be provided on at least one or any number of the total motor channels.

[0049] According to a third aspect, the invention provides a front axle assembly for a steer-by-wire vehicle, the front axle assembly comprising the motor circuit of the second aspect and a multi-phase dual channel permanent magnet electric motor operable to steer the front wheels of the vehicle.

[0050] According to a third aspect, the present invention provides a vehicle comprising a front wheel steering assembly and a rear wheel steering assembly, wherein the front wheel steering assembly comprises the motor circuit of the second aspect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] An embodiment of the present invention will now be described, by way of example only, with reference to and as shown in the accompanying drawings, in which:

[0052] Figure 1is a plan view of a vehicle including steerable front wheels and steerable rear wheels according to an embodiment of the present invention, wherein the front wheels are steered by a steer-by-wire assembly including a motor circuit.

[0053] Figure 2 is a block diagram of key circuit blocks of a motor circuit for a steering system for front wheels;

[0054] Figure 3 is a schematic diagram of one of the bridges supplying current to the phase of one channel of the motor;

[0055] Figure 4 Can be used as Figure 3 Representation of a typical FET switch in a bridge circuit.

[0056] Figure 5 is a schematic diagram of a portion of a motor circuit for one channel showing the interaction of a monitoring circuit and a safety damping circuit with a bridge of the channel;

[0057] Figure 6 is with Figure 5 The same schematic diagram, but showing the interaction of the monitoring circuit and the safety damping circuit for the second channel;

[0058] Figure 7 It is shown Figures 2 to 6 a flow chart of the behavior of the circuit under various conditions including normal function of the control circuits and a fault condition in one or both control circuits; and

[0059] Figure 8 Represents a dual motor having two independent sets of phase windings, each set defining three phases and driven by a separate bridge circuit. DETAILED DESCRIPTION

[0060] Figure 1 is a schematic plan view of a passenger vehicle 10 having both steerable front wheels and steerable rear wheels. The two front wheels 11, 12 are connected together by a steering rack 13, which can be translated laterally through the body of the vehicle to turn the wheels. The steering rack 13 is moved by an electric actuator assembly including a motor 14. The motor 14 responds to control signals from a motor circuit that converts a required wheel steering angle into an appropriate position of the steering rack and drives the motor to ensure that the required rack position is achieved. These control signals can be generated using a PI control strategy, whereby a measurement of the wheel steering angle is compared to the required steering angle to generate an error signal that the PI control strategy attempts to drive to zero.

[0061] The vehicle 10 includes a steering wheel 15 or yoke which can be turned by the driver to steer the vehicle. It is within the scope of the invention for the steering wheel to completely omit steering until it is performed autonomously by the vehicle.

[0062] Where a steering wheel 15 is provided as shown, it can be turned by the driver and this, together with the torque applied by the driver, is detected by the sensor. A second motor is provided which outputs a torque opposite to the torque applied by the driver in order to provide the driver with a sense of the forces acting on the front wheels. In this example, there is no direct mechanical connection from the steering wheel to the front wheels.

[0063] It should be noted that it is within the scope of the present invention to omit the steering wheel entirely, wherein control of the front wheel steering angle is performed autonomously by the vehicle.

[0064] The two rear wheels 16, 17 of the vehicle are connected to a very similar steering rack 18, which is displaced by a third motor (not shown). The rack is moved by the third motor in response to a control signal from a rear wheel steering control circuit (not shown). At low speeds, the rear wheels 16, 17 can turn in the opposite direction to the front wheels, and at high speeds can turn in the opposite direction. During normal driving, the rear wheels can be steered according to the steering wheel position and the vehicle speed, so that the driver has direct control. In an emergency or fault mode, the rear wheels can be steered independently of the driver's movement of the steering wheel.

[0065] The first motor 14 associated with the front steering rack forms Figure 2 . In the figure, the center block with a dashed outline represents a motor having multiple channels, each channel including a set of three-phase windings. Above the box is the portion of the first channel of the motor circuit, and below the box is the portion of the second channel of the motor circuit. The motor in this example is a dual channel motor, meaning that it has two independent sets of phase windings. Current can be supplied to either set of phases to produce motor torque. In normal operation, the two channels can be used together so that both contribute to the total torque applied to the front steering rack by the first motor, or only one can be used at a time, as determined by the control strategy of the steering system. As shown Figure 8 As shown, the motor has two sets of three phases, each set of phases being connected at the start point in this example, but other configurations are possible within the scope of the invention.

[0066] Each channel includes a PI or PID control circuit. The circuit's job is to compare the motor current demand with the actual current flowing in the motor to produce an error signal that is fed to the PI or PID controller. The PI control circuit outputs a control signal representing the motor's required current, which is selected so that the actual motor current closely follows the required current over time. These control signals are fed to a bridge driver circuit that converts the signals into a set of PWM voltage waveforms, one for each phase of the channel's motor. These signals are then fed to Figure 3The gates of the switches of the bridge circuit shown. The bridge circuit includes three pairs of MOSFET switches. Each pair of MOSFET switches includes a top switch connected in series with a bottom switch, and the center tap between the two switches is connected to the motor phase. For each motor phase, the top switch 20a, 20b, 20c in a pair of switches selectively connects the phase to the positive voltage of the power supply, and the bottom switch 21a, 21b, 21c of the same pair of switches selectively connects the phase to ground. In normal use, the bridge driver circuit produces a PWM waveform that ensures that the top switch and the bottom switch of a pair of switches are turned on at the same time, as this will short the power supply to ground. The use of PWM to control motors in this way is well known and will not be described in detail here.

[0067] Each channel also includes a fault detection circuit and a safety damping circuit. The fault detection circuit monitors the control circuit of the channel to ensure that the control signal that can be used is being output to the bridge driver circuit. The output of the fault detection circuit is a bridge driver disable signal. When the bridge is not disabled, the value of the signal is maintained low and the bridge driver will be disabled, and the output is maintained at a non-zero voltage, such as a supply voltage or a value close to the supply voltage. In this example, the power supply is configured to monitor itself and the control circuit, and the control circuit is configured to monitor itself, monitor the power supply to the control circuit, and also monitor the bridge circuit and the bridge driver circuit. The bridge driver is configured to monitor the bridge circuit and itself. When any of these determines that there is a fault, they switch the common / SOFF (safety off) signal for the channel containing the faulty circuit. If both channels set / SOFF to low, the safety damping will be triggered.

[0068] exist Figure 5 and Figure 6 The safety damping circuit can be seen in more detail in Figure 1. The safety damping circuit comprises a pair of switches Q1 and Q2 connected in series, with one side of the pair of switches connected to the power supply and the other side connected to the gate of each bottom switch of the bridge through a network of diodes and resistors, which limit the magnitude of the current drawn from the power supply through Q1 and Q2 when both Q1 and Q2 are open. The gate of each switch Q1 and Q2 is connected to a gate driver, which in turn is fed by the bridge driver disable signal of each channel. These switches Q1 and Q2 are normally off, so no current can flow from the source to the drain, but each switch will turn on when their associated bridge driver disable signal goes high. When both Q1 and Q2 are turned on, all the bottom FETs are provided with a voltage high enough to turn them on, which shorts all three phases of the motor channel to ground potential. Note that the actual voltage applied will depend on the voltage drop across Q1 and Q2 and the voltage drop across the resistors and diodes and on the supply voltage of the power supply.

[0069] Figure 5 This arrangement is shown for the first channel. Figure 6 It is shown that the same arrangement can also be provided for the second channel. It is within the scope of the invention to provide a safety damping circuit on only one channel, but providing a safety damping circuit for both channels will allow them to both provide damping to the motor, thereby increasing the resistance to the motor turning. If the motor channels are identical, the braking torque achieved using both channels will be twice the braking torque that can be achieved from one channel.

[0070] Figure 5 and Figure 6 A power supply isolation circuit is also shown in FIG. This circuit receives a signal from the safety damping circuit when switches Q1, Q2, or Q3 and Q4 are all turned on, and is used to shut off the power supply to the bridge of the damped channel. This ensures that if a fault causes the top switch on the bridge to accidentally open or fail in an open conductive state where current can flow between the source and drain, there will be no accidental shorting of the power supply to ground through the turned-on bottom switch of the bridge.

[0071] In addition, Figure 5 and Figure 6 The optional damping circuit test block can be seen in . The optional test block performs the tests necessary to ensure that there are no faults in the damping circuit that could cause the damping circuit to not operate when needed or to trigger when not wanted.

[0072] To run the test in a channel, the test block will ensure that the bridge driver is disabled and the bridge power supply is off and turn on Q1 while Q2 is off to check that the bridge bottom FET is not switched on. It will then turn Q1 off and turn on Q2 and check that the bridge bottom FET remains off. If necessary, it will then force both Q1 and Q2 to switch on to check that damping is triggered. This can be performed with a single SSPIR FET closed to avoid damping and prove that each bridge bottom FET is switching.

[0073] A check can also be performed to detect if there is an error from the optical isolator shown in the figure. If the other channel is operational, the safe shutdown signal will be communicated via the inter-channel bus so that the SPI reported status can be compared to the signal in the channel. If the optical isolator signal is low, the power-up test will be adjusted accordingly as it will exceed the in-channel microcontroller power supply.

[0074] Figure 7 It is shown Figure 2 A flow chart of the behavior of the motor circuit during normal operation in which there is no fault in either of the two control circuits, and a flow chart of the behavior in the event of a fault in one or both of the motor's control circuits.

[0075] At the first step, two bridge driver disable signals are monitored. If both indicate that the control circuit is not faulty, the control signals from the two control circuits are fed to the corresponding bridge drivers, and the motors are operated to steer the wheels.

[0076] If the check indicates that one is faulty, one of the switches of the fault detection circuit is turned on and the control signal from the control circuit is not fed to the bridge driver of that channel. The other channel is used to control the motor.

[0077] If the check indicates that both are faulty, the second switch of the fault detection circuit is also turned on, thereby applying the power supply voltage to the base of each bottom FET of the bridge circuit.

[0078] The power supply to the bridge is also turned off so that the top FET is not connected to the positive supply voltage.

[0079] Will Figure 7 to the second channel, so that both channels are protected.

[0080] By damping one or both channels of the motor that are not capable of providing steering control, the motor will produce high braking torque in situations where the forces applied to the wheels will not rapidly change the steering angle of the wheels while the vehicle is moving. This prevents rapid, unexpected changes in the direction of the vehicle from occurring. In such situations, the rear wheels can be steered to safely move the vehicle to one side of the highway before stopping. This rear wheel steering may be performed autonomously by the vehicle, or may be performed in response to the driver turning the steering wheel of the vehicle.

Claims

1. A method of controlling an electric power steering system comprising a multiphase permanent magnet electric motor having a plurality of motor phases which, in normal operation, apply torque to a portion of the system connected to a wheel to control the steering angle of the wheel, the motor being driven by a bridge circuit comprising a set of switches capable of opening and closing in response to signals output from a bridge driver circuit to selectively connect each phase to a supply voltage of a power supply or to ground, The method comprises: In the event of a fault preventing the motor from applying torque enabling the wheels to be steered, switches of the bridge circuit are operated independently of the bridge driver to short-circuit each phase of the motor to dampen rotation of the motor and simultaneously shut off the power supply connected to the bridge circuit.

2. The method according to claim 1, wherein: The bridge circuit comprises, for each phase, an upper switch, which when closed connects the phase to the power supply, and a lower switch, which connects the phase to ground, The method includes short-circuiting each phase of the bridge circuit by closing the lower switch of each phase.

3. A method according to claim 1 or 2, comprising overriding the operation of the lower bridge switch by the bridge driver, thereby preventing the bridge driver from opening the lower switch.

4. A method according to any preceding claim, wherein: The switch includes a transistor, the transistor being turned on and off by applying a non-zero voltage drive signal to a gate of the switch through the bridge driver circuit, and The method includes short-circuiting the phases by connecting the gate or base of each switch to a non-zero voltage to turn the switch on.

5. A method according to any preceding claim, comprising Monitoring control circuit, generating a bridge driver disable signal in the event of a fault in the control circuit, and The bridge driver disable signal is monitored to determine when to short circuit the phases of the motor to dampen rotation of the motor.

6. A multi-channel motor circuit for driving a multi-channel permanent magnet motor of an electric power steering system, each channel of the motor comprising a plurality of motor phase windings, a permanent magnet electric motor, the motor circuit comprising: A first motor channel, the first motor channel comprising: a first bridge circuit comprising a set of top switches and a set of bottom switches, wherein each top switch selectively connects a corresponding phase of the motor to a supply voltage of a power supply and each bottom switch selectively connects a corresponding phase of the motor to ground; a first bridge driver circuit that outputs a drive signal for each of the switches of the bridge, respectively, so as to control the voltage applied to each phase over time in response to a set of control signals, thereby controlling the current flowing through each phase of the motor, a first control circuit configured to output a control signal to the bridge driver; and a first fault detection circuit that outputs a bridge driver disable signal to disable the bridge driver associated with the channel if a fault occurs in the channel, A second motor channel, the second motor channel comprising: a second bridge circuit comprising a set of top switches and a set of bottom switches, wherein each top switch of the second bridge circuit selectively connects a corresponding phase of the motor to a supply voltage of a power supply and each bottom switch of the second bridge circuit selectively connects a corresponding phase of the motor to ground; a second bridge driver circuit that outputs a drive signal for each of the switches of the bridge, respectively, so as to control the voltage applied to each phase over time in response to a set of control signals, thereby controlling the current flowing through each phase of the motor, and a second controller configured to output a control signal to the bridge driver; a second fault detection circuit that outputs a bridge driver disable signal to disable the bridge driver associated with the channel if a fault occurs in the channel, and It also includes a monitoring device and a safety damping circuit, wherein the monitoring device is arranged to monitor the bridge driver disable signal, and the safety damping circuit is configured so that when the monitoring device determines that the bridge driver disable signal indicates that the control circuits of both channels have failed, the safety damping circuit causes the bottom switch of the bridge driver to short-circuit each phase of at least one channel of the motor so as to dampen the rotation of the motor, and at the same time isolates the top switch of the bridge circuit relative to the power supply.

7. The multi-channel motor circuit according to claim 6, wherein: The safety damping circuit comprises a pair of switches arranged in series between a positive supply voltage and a switch of each phase of at least one channel of the motor, the switches comprising a first switch, the first switch being normally open and closing when the first bridge driver disable signal indicates disabling the first bridge driver circuit, and a second switch, the second switch being normally open and closing when the second bridge driver disable signal indicates to disable the second bridge driver circuit, The first switch and the second switch are configured such that only when both switches are closed, a short circuit is applied to each phase of at least one channel to provide damping to the motor by generating power or braking torque.

8. The multi-channel motor circuit according to claim 6 or 7, wherein: Each of the two switches of the safety damping circuit comprises a transistor such as a MOSFET, The safety damping circuit is adapted to close the switch by applying a positive voltage to the base of the switch to turn on the MOSFET, thereby allowing current to flow between the source and the drain.

9. The multi-channel motor circuit according to any one of claims 6-8, further comprising an isolation circuit, the isolation circuit comprising a set of isolation switches, one switch being connected in series between the bridge circuit and each motor phase, each isolation switch being normally closed.

10. A multi-channel motor circuit according to any one of claims 6 to 9, further comprising a replica safety damping circuit comprising a pair of switches arranged in series between a positive supply and an end of each phase of a second channel of the motor, The first switch is normally open and is closed when the first bridge driver disable signal indicates disabling the first channel, The second first switch is normally open and is closed when the second bridge driver disable signal indicates disabling the second channel, Both switches need to be closed to apply a positive voltage to the ends of each phase of the second channel.

11. The multi-channel motor circuit according to any one of claims 6 to 10, wherein: One or more of the bridge disable signals are input to a power supply isolation block which isolates the top switch of the bridge relative to the supply to prevent the phase from receiving the positive voltage from the power supply.

12. The multi-channel motor circuit according to any one of claims 6 to 11, wherein: the safety damping circuit or each safety damping circuit when two safety damping circuits are provided comprises a pair of switches arranged in series between a positive voltage of a power supply and a gate of each lower switch of the bridge circuit, The first switch is normally open and is closed when the first bridge driver disable signal indicates disabling the first channel, The second first switch is normally open and is closed when the second bridge driver disable signal indicates disabling the second channel, Both switches need to be closed to apply the positive voltage to the gate of each phase of the channel.

13. A front axle assembly for a steer-by-wire vehicle, the front axle assembly comprising: A motor circuit according to any one of claims 6 to 12, and A multi-phase dual-channel permanent magnet electric motor is operable to steer the front wheels of the vehicle.

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