WO2008132449A1

WO2008132449A1 - Electric power steering

Info

Publication number: WO2008132449A1
Application number: PCT/GB2008/001446
Authority: WO
Inventors: Connel Brett Williams; Christopher David Dixon
Original assignee: Trw Limited
Priority date: 2007-04-26
Filing date: 2008-04-24
Publication date: 2008-11-06
Also published as: DE112008001109T5; GB0708061D0

Abstract

An electromagnetic apparatus, such as an electric power assisted steering system (1), comprising: a mechanical component (5) having a limited range of travel having at least one end position and a motor (6) arranged to apply a force to the mechanical component (5) to cause it to move along its range of travel, in which the motor comprises at least three phases each comprising a phase winding, a rotor and drive circuitry to supply drive currents (12, 13, 14) to each of the phases dependent upon the position of the rotor relative to the phase windings, and in which the apparatus is arranged such that when the mechanical component (5) is at an end position in use, the magnitude of the drive current supplied to at least one phase of the motor (6) is substantially at a minimum, and in which the drive circuitry is arranged so as to provide, in use, cyclic drive signals to each phase as the rotor rotates, where the drive signals applied to each phase are out of phase with the drive signals provided to other phases.

Description

ELECTRIC POWER STEERING

This invention relates to electromagnetic apparatus, and particularly, but not exclusively, to electric power assisted steering apparatus.

In electromechanical apparatus comprising a torque -applying motor and a mechanical component that have a finite range of travel, there exists a point at which it is not physically possible for the apparatus to move the mechanical component any further. Applying full torque at this point is inefficient and the power applied is dissipated as heat. An example of such an apparatus is an electric power assisted steering (EPAS) apparatus, where a torque-applying motor is arranged to generate an assistance torque to the steering apparatus of a vehicle, where the steering apparatus is limited to travel between two "lock" positions.

As with all apparatus, the control and drive electronics for such an apparatus can only operate within a finite temperature range. Should the temperature of the electronics exceed a safe limit, the electronics are likely to degrade and stop working, possibly permanently. To maintain the apparatus in the finite temperature operating range the available power can be reduced as the apparatus approaches the safe limit.

To maximise the availability of full power and to limit the need to limit the power available, temperature, increases need to be kept to a minimum. One solution that has been proposed is to limit the available power at the end of travel of the mechanical component. However, this means that full power is not available to move the mechanical component away from the end of travel.

According to a first aspect of the invention, there is provided an electromagnetic apparatus, comprising: a mechanical component having a limited range of travel having at least one end position and a motor arranged to apply a force to the mechanical component to cause it to move along its range of travel, in which the motor comprises at least three phases each comprising a phase winding, a rotor and drive circuitry to supply drive currents to each of the phases dependent upon the position of the rotor relative to the phase windings, and in which the apparatus is arranged such that when the mechanical component is at an end position in use, the magnitude of the drive current supplied to at least one phase of the motor is substantially at a minimum.

By keeping at least one phase at minimum current, the peak phase current at the end position, and hence the peak phase power at that position, can be reduced. This is important, as the rate of temperature increase in any one phase and in the drive circuitry for that phase will depend on the power through that phase. By reducing the peak phase power at the end position, the temperature rise in any phase or its drive circuitry is reduced, and so the probability that any one phase or its drive circuitry will exceed a safe temperature limit will be reduced. Continued safe, full-power, operation of the apparatus may therefore be possible for longer than otherwise would be the case.

Preferably, the minimum drive current applied to the phase at the end position is substantially zero.

The invention is particularly applicable to apparatus where drive circuitry is arranged so as to provide, in use, cyclic drive signals to each phase as the rotor rotates, where the drive signals applied to each phase are out of phase with the drive signals provided to other phases. In the preferred embodiment, the cyclic drive signals are substantially sinusoidal. Typically, the motor will also comprise an angular position sensor for sensing in use the angular position of the rotor relative to the windings and providing in use a rotor angular position signal indicative of that position. Preferably, the drive circuitry determines the drive currents to be applied to the phase dependent on the rotor angular position signal.

In one embodiment, the relationship between the position of the mechanical component and the position of the rotor relative to the phase windings is consistent, such that the end position of the mechanical component corresponds to the position in the cyclic drive signal applied to one phase that results in substantially minimum current magnitude being applied to that phase.

Alternatively, in use, when the mechanical component is at an end position, the rotor angular position signal may be indicative of a rotor position corresponding to a minimum current magnitude position in one of the cyclic drive signals irrespective of the actual position of the rotor. This may thus allow a standard motor control method to be used, with the only change possibly being the output of the angular position sensor.

The angular position sensor may be arranged so that, over a range of positions of the mechanical component from the end position to a first position corresponding to the next position of the rotor corresponding to a minimum current magnitude position in one of the cyclic drive signals, the rotor angular position signal is indicative of the first position.

In one alternative, the rotor angular position signal outside the range may be indicative of the angular position sensor of the rotor, as is normal. However, in another alternative, the angular position sensor is arranged such that, over a second range of positions of the mechanical component from the end position to a threshold, the rotor angular position signal is quantised such that it only outputs values corresponding to positions of the rotor corresponding to points in the cyclic drive signals where one of the phases would be provided with minimum current magnitude. This reduces the accuracy needed to define when the mechanical component is near the end position.

The mechanical component may have two end positions; the apparatus may be arranged such that when the mechanical component is at either end position in use, the drive current supplied to at least one phase of the motor is substantially at a minimum. All of the above optional features may apply to the end position at either end of the travel of the mechanical component.

Where the angular position sensor is arranged to give an rotor angular position signal that is not directly indicative of the position of the rotor, this may be achieved by use of a limiting circuit, which limits the rotor angular position signal in the desired manner. This may form part of the angular position sensor itself, or may be comprised within the drive circuitry. The limiting circuit may be implemented in a microprocessor or an Application Specific Integrated Circuit (ASIC) .

In a preferred embodiment of the invention, the motor comprises three phases.

The motor may be a linear motor; in such a case, the "rotor" comprises the part that moves relative to the phases.

According to a second aspect of the invention, there is provided an electric power assisted steering apparatus, comprising a steering mechanism for a vehicle and an electric motor coupled to the steering mechanism to provide assistance to a driver of the vehicle in operating the steering mechanism in use, in which the apparatus forms an electromechanical apparatus of the first aspect of the invention, in which the mechanical component is or forms part of the steering mechanism and the motor is the electric motor.

In such a case, the end position may comprise a "lock" position of the steering mechanism, being the limit of steering travel in one direction. Where there are two end positions, the end positions may comprise the two locks of the steering mechanism.

Where the steering mechanism comprises a handwheel, the threshold may be less than 10° from an end position when considering rotation of the handwheel.

According to a third aspect of the invention, there is provided a method of controlling a motor in a electromechanical apparatus comprising the motor and a mechanical component having a limited range of travel having at least one end position, the motor being arranged to apply a force to the mechanical component to cause it to move along its range of travel, in which the motor comprises at least three phases each comprising a phase winding, and a rotor, in which the method comprises controlling the current applied to the phases of the motor such that, when the mechanical component is at an end position in use, the magnitude of the drive current supplied to at least one phase of the motor is substantially at a minimum.

Preferably, the minimum current applied to the relevant phase of the motor at the end position is substantially zero. The invention is particularly applicable to methods comprising the application of cyclic drive signals to each phase as the rotor rotates, where the drive signals applied to each phase are out of phase with the drive signals provided to other phases. In the preferred embodiment, the cyclic drive signals are substantially sinusoidal.

Typically, the method will also comprise the use of an angular position sensor for sensing the angular position of the rotor relative to the windings and providing in use a rotor angular position signal indicative of that position, the method comprising determining the drive currents to be applied to each phase dependent on the rotor angular position signal.

The method may comprise the step of, when the mechanical component is at an end position, limiting the angular position sensor to output a signal indicative of a rotor position corresponding to a minimum current magnitude position in one of the cyclic drive signals irrespective of the actual position of the rotor. This may thus allow a standard motor control method to be used, with the only change possibly being the output of the angular position sensor.

The method may comprise the step of limiting the angular position sensor to output, over a range of positions of the mechanical component from the end position to a first position corresponding to the next position of the rotor corresponding to a minimum current magnitude position in one of the cyclic drive signals, a signal indicative of the first position.

The method may also comprise the step of outputting a signal from the angular position sensor, when the mechanical component is outside the range that is indicative of the angular position sensor of the rotor, as is normal. However, in another alternative, the method may comprise limiting the output of the angular position sensor, over a second range of positions of the mechanical component from the end position to a threshold, such that it is quantised such that it only outputs values corresponding to positions of the rotor corresponding to points in the cyclic drive signals where one of the phases would be provided with minimum current magnitude.

There now follows, by way of example, embodiments of the invention described with reference to the accompanying drawings, in which:

Figure 1 shows an electric power assisted steering apparatus according to embodiments of the present invention;

Figures 2a and 2b shows graphs of the current and power of the drive signals of the motor of the electric power assisted steering system of Figure 1 with rotor position;

Figure 3 shows a graph of the output of the angular position of the rotor of the motor and of the position indicated by the angular position sensor of Figure 1 in a first embodiment against steering mechanism position;

Figure 4 shows a flowchart showing how the angular position sensor of Figure 3 is operated;

Figure 5 shows an equivalent graph to that of Figure 3, with an angular position sensor of a second embodiment; and

Figure 6 shows a flowchart showing how the angular position sensor of Figure 5 is operated. An electric power assisted steering apparatus 1 forming an electromechanical apparatus according to embodiments of the present invention is shown in Figure 1 of the accompanying drawings.

The apparatus 1 comprises a steering shaft 2 co-operatively connected at one end to a handwheel 3 and at its other end to a pair of road wheels 4 through a rack and pinion 5. The handwheel 3 is adapted to rotate the steering shaft, in turn to displace the rack and eventually to turn the roadwheels. The amount of movement permissible for the handwheel between end stops (so-called "turns for lock to lock") is determined by the road wheel geometry and suspension design which supports the wheels, but in all cases exceeds one complete revolution, two to four revolutions being typical.

An electric motor 6 is connected to the steering shaft through a reduction gearbox 7. A control circuit 11 provides current to the motor 6 in response to the output of a torque sensor 8 mounted on the steering shaft. The torque sensor 8 measures the torque demanded by the driver and from this the motor current is calculated to provide more or less assistance as demanded.

In addition to the torque sensor 8, an angular position sensor 10 is provided on the motor rotor. The sensor produces an output signal, and these signals are fed to the control circuit 11 to produce a signal indicative of the position of the rotor 9 of the motor 6.

The motor 6 is a star wound, three phase permanent magnet motor. As such, it is necessary to know the angular position of the motor in order to know the correct current to apply to each phase in order to operate the motor. An example graph of the currents applied to each phase of the motor can be seen in Figures 2a of the accompanying drawings. Each of the traces 12, 13, 14 represents the current applied to one phase of the motor 6, normalised such that the maximum current is +1.

These cyclic drive signals are sinusoidal, with the waveform for each phase being offset by 120° from the next. The graphs in Figure 2a and 2b of the accompanying drawings are depicted in radians, and the angle is the electrical angle of the motor; the period of the drive signals in electrical angle is therefore 2π radians (360°) . For a given physical angle of the rotor relative to some datum and a number of pairs of permanent magnet poles n in the rotor, the electrical angle will be n times the physical angle, relative to the same datum. Accordingly, for a rotor with two pairs of poles, the electrical angle will be twice the physical angle, and so the period of the waveforms compared to the physical angular position of the rotor will be 180° .

The apparatus 1 therefore forms an electromechanical apparatus. The steering mechanism, and particularly the rack 5, can be considered to the a mechanical component of that electromechanical apparatus, which can be driven between its end positions by the motor 6. The "lock" positions therefore define limits to the travel of the rack 5.

All of the embodiments of the invention described with reference to Figure 1 have the same feature that, when the rack is at one of its end positions, the motor is controlled such that the drive signal applied to one of the phases is at substantially zero current. By making the current applied to one phase substantially at a minimum magnitude - zero being the ultimate minimum magnitude - the peak phase current over all the phases is reduced.

The control circuit will typically comprise a set of switches for each phase. By reducing the peak phase current, the maximum resistive power loss for a set of switches - as determined by I²R, where I is the current flowing in that switch and R is the resistance - is reduced.

This can be seen in Figure 2b of the accompanying drawings. Zero magnitude current in Figure 2a is equivalent to zero power in Figure 2b when considering resistive I²R losses. From Figure 2b, one can consider the maximum phase power; this would be the maximum of each of phases 12, 13, 14 at a given angle. The maximum is the value of the power in whichever of the phases that has the most power flowing through it according to I²R at a given angle. It can be seen that the minima of the maximum power occur when one of the phases is at zero current and hence power. The two other phases will have equal power. Moving away from a zero current point will lead to one of the non-zero current phases losing more power (and hence heating up more) than the other non-zero current phase. This can be unfortunate, as it could lead to that set heating up quicker than the others and so malfunctioning, taking the entire apparatus out of service.

It can be seen from Figure 2b that the total I²R power loss 15 is constant, independent of rotor angle. The overall power dissipated is constant; the invention resides in apportioning the power dissipated so as to minimize the dissipation's deleterious effects.

In order to implement the invention, it is possible to modify the angular position sensor 10 so as to give a modified output as the rack reaches the end of its travel. Whilst we refer herein to the output of the angular position sensor 10 having certain values, we also include the output of the angular position sensor 10 being modified in the control circuit 11 before it is used to calculate the drive signals for the motor 6. Either is equally applicable; by modifying the output of the angular position sensor, a standard algorithm for determining the cyclic drive signals can be used, but "tricked" into operating according to the invention.

In the first embodiment shown in Figures 3 and 4 of the accompanying drawings, the control unit 11 monitors the position of the mechanical component - that is, the rack 5. This monitoring can make use of a preexisting Absolute Steering Position (ASP) signal already provided in the vehicle by known means, or by counting the rotations of the motor rotor. When the rack 5 moves past the last phase current zero position before the end position, the control unit 11 limits the output of the angular position sensor such that it is fixed to a value indicative of that last phase current zero position. Thus, as the rack 5 approaches lock, the current in one of the phases of the motor 6 will be zero.

This is depicted in Figure 3, which shows the output of the angular position sensor 10 in solid lines and the output as limited by the control unit 11 in dotted lines. The y-axis represents the angle indicated in degrees of electrical angle. The zero phase current positions 20, 21 are marked on the graph; these correspond any zero-crossing in Figure 2a or any zero value in Figure 2b. As the rack approaches end lock, the limited output (dotted line) is restricted to the last zero current position 21 before the end lock position.

This can also be seen in the flow chart of Figure 4. The control unit 11 initialises (step 30) in a normal operation mode 32. In this mode, the control unit does not modify the output of the angular position sensor 10.

Should the rack 5 move within END LOCK PROTECT ENABLE

THRESHOLD of the end position, then the control unit operates in the end lock protection mode 34. In this mode, the position will be limited to a maximum or minimum (depending from which the lock position is being approached) of the END LOCK LIMITED POSITION, that is, the last zero phase current position 21. Once the rack moves back out of the END LOCK PROTECT DISABLE THRESHOLD (typically roughly the same as the END LOCK PROTECT ENABLE THRESHOLD) then the control unit reverts to the normal operation mode 32.

However, this requires knowing to a resolution of 60 electrical degrees when the rack is approaching the end position. By implementing a second embodiment of the invention depicted in Figures 5 and 6 of the accompanying drawings, the determination of the end positions need not be so accurate.

In this embodiment, once the rack moves within a certain threshold of an end position, the control unit 11 constrains the output of the angular position sensor 10 such that it only indicates values indicative of zero phase current points; as the rotor moves the limited output will indicate different zero phase current points, which still enables operation of the motor.

This can be seen in Figure 5 of the accompanying drawings, which is depicted equivalently to Figure 3. Outside the threshold, the control unit does not modify the output of the angular position sensor, as illustrated by the coincidence of the dotted and solid lines. Once the rack 5 moves within the threshold, which can be significantly larger than the threshold in the first embodiment, the control unit 11 quantises the output of the angular position sensor such that it can only have values 20, 21 indicative of zero phase current positions.

When the rack 5 reaches lock, the limited output of the angular position sensor 10 will indicate the last zero current position 21 before the actual lock position. Thus, the current in one of the phases will be zero throughout the threshold and particularly when the rack 5 reaches the lock position.

The operation of the control circuit 11 can be seen in Figure 6 of the accompanying drawing. The circuit 11 initialises at step 40 into normal operation mode 42. In this mode, the circuit does not modify the output of the angular position sensor 10. However, once the rack 5 moves within END LOCK PROTECT ENABLE THRESHOLD of a lock position, the control circuit 11 enters end lock protection mode 44. In this mode, the control unit 11 quantises the output of the angular position sensor, so as to always output a value indicative of a zero-current position; the zero current position returned is the preceding zero-current position as the rack 5 approaches lock. The example algorithm shown takes the next lowest integer number of 60° rotations completed and multiplies that by 60° to produce a quantised output.

Once the rack 5 moves away from the lock position outside of a END LOCK PROTECT DISABLE THRESHOLD, roughly equal to the END LOCK PROTECT ENABLE THRESHOLD, the control circuit 11 reverts to normal operation 42.

The thresholds should be set fairly close to the end position, as the quantised operation of the angular position sensor output could lead to noticeable torque ripple in use by a driver. A threshold of approximately 10° measured at the handwheel 3, given a gearbox 7 ratio of 20: 1, corresponds to a rotation of the rotor of 200° mechanical of the motor rotor 9. For a three pole-pair magnet motor 6, this equates to 600° electrical. This is an achievable resolution, with an acceptable range over which slight torque ripple is unlikely to be noticed by a driver. The thresholds for enabling and disabling the limitations or quantisations of the signal may be slightly different provide some hysteresis to prevent repeated cycling between the modes should the rack position be close to the end position.

In a third embodiment, not shown, the apparatus is arranged such that the position of the rotor 9 of the motor 6 relative to the position of the rack 5 are consistent; that is, the rotor 9 will be in a given angular position for any particular position of the rack 5. In such a case, the lock positions of the rack 5 can correspond directly to zero-current positions of the motor. However, this is not always feasible.

Claims

1. An electromagnetic apparatus, comprising: a mechanical component having a limited range of travel having at least one end position and a motor arranged to apply a force to the mechanical component to cause it to move along its range of travel, in which the motor comprises at least three phases each comprising a phase winding, a rotor and drive circuitry to supply drive currents to each of the phases dependent upon the position of the rotor relative to the phase windings, and in which the apparatus is arranged such that when the mechanical component is at an end position in use, the magnitude of the drive current supplied to at least one phase of the motor is substantially at a minimum, and in which the drive circuitry is arranged so as to provide, in use, cyclic drive signals to each phase as the rotor rotates, where the drive signals applied to each phase are out of phase with the drive signals provided to other phases.

2. The apparatus of claim 1, in which the drive current applied to the phase at the end position is substantially zero.

3. The apparatus of claim 1 or claim 2, in which the cyclic drive signals are substantially sinusoidal.

4. The apparatus of any preceding claim, comprising an angular position sensor for sensing in use the angular position of the rotor relative to the windings and providing in use a rotor angular position signal indicative of that position.

5. The apparatus of claim 4, in which the drive circuitry determines the drive currents to be applied to the phase dependent on the rotor angular position signal.

6. The apparatus of any preceding claim, in which the relationship between the position of the mechanical component and the position of the rotor relative to the phase windings is consistent, such that the end position of the mechanical component corresponds to the position in the cyclic drive signal applied to one phase that results in substantially minimum current magnitude being applied to that phase.

7. The apparatus of any of claims 1 to 5, in which, in use, when the mechanical component is at an end position, the rotor angular position signal is indicative of a rotor position corresponding to a minimum current magnitude position in one of the cyclic drive signals irrespective of the actual position of the rotor .

8. The apparatus of claim 7, in which the angular position sensor is arranged so that, over a range of positions of the mechanical component from the end position to a first position corresponding to the next position of the rotor corresponding to a minimum current magnitude position in one of the cyclic drive signals, the rotor angular position signal is indicative of the first position.

9. The apparatus of claim 8, in which the rotor angular position signal outside the range is indicative of the angular position sensor of the rotor.

10. The apparatus of claim 8, in which the angular position sensor is arranged such that, over a second range of positions of the mechanical component from the end position to a threshold, the rotor angular position signal is quantised such that it only outputs values corresponding to positions of the rotor corresponding to points in the cyclic drive signals where one of the phases would be provided with minimum current magnitude.

11. An electric power assisted steering apparatus, comprising a steering mechanism for a vehicle and an electric motor coupled to the steering mechanism to provide assistance to a driver of the vehicle in operating the steering mechanism in use, in which the apparatus forms an electromechanical apparatus according to any preceding claim, in which the mechanical component is or forms part of the steering mechanism and the motor is the electric motor.

12. A method of controlling a motor in a electromechanical apparatus comprising the motor and a mechanical component having a limited range of travel having at least one end position, the motor being arranged to apply a force to the mechanical component to cause it to move along its range of travel, in which the motor comprises at least three phases each comprising a phase winding, and a rotor, in which the method comprises controlling the current applied to the phases of the motor such that, when the mechanical component is at an end position in use, the magnitude of the drive current supplied to at least one phase of the motor is substantially at a minimum, in which the method comprises the application of cyclic drive signals to each phase as the rotor rotates, where the drive signals applied to each phase are out of phase with the drive signals provided to other phases.

13. The method of claim 12, in which the minimum current applied to the relevant phase of the motor at the end position is substantially zero .

14. The method of any of claims 12 or 13, in which the method comprises the use of an angular position sensor for sensing the angular position of the rotor relative to the windings to provide a rotor angular position signal indicative of that position, the method comprising determining the drive currents to be applied to each phase dependent on the rotor angular position signal.

15. The method of claim 14, in which the method comprises the step of, when the mechanical component is at an end position, limiting the angular position sensor to output a signal indicative of a rotor position corresponding to a minimum current magnitude position in one of the cyclic drive signals irrespective of the actual position of the rotor.

16. The method of claim 14, comprising the step of limiting the angular position sensor to output, over a range of positions of the mechanical component from the end position to a first position corresponding to the next position of the rotor corresponding to a minimum current magnitude position in one of the cyclic drive signals, a signal indicative of the first position.

17. The method of claim 16, comprising the step of outputting a signal from the angular position sensor, when the mechanical component is outside the range that is indicative of the angular position sensor of the rotor, as is normal.

18. The method of claim 16 comprising limiting the output of the angular position sensor, over a second range of positions of the mechanical component from the end position to a threshold, such that it is quantised such that it only outputs values corresponding to positions of the rotor corresponding to points in the cyclic drive signals where one of the phases would be provided with minimum current magnitude.