GB2443525A

GB2443525A - Operation of an angular position sensor in an electrically assisted power steering system

Info

Publication number: GB2443525A
Application number: GB0720926A
Authority: GB
Inventors: Russell Wilson-Jones
Original assignee: TRW Ltd
Current assignee: TRW Ltd
Priority date: 2006-10-31
Filing date: 2007-10-25
Publication date: 2008-05-07
Anticipated expiration: 2027-10-25
Also published as: DE102007052099A1; GB0720926D0; DE102007052099B4; GB0621613D0; GB2443525B

Abstract

A method of operating an angular position sensor for an electric power assisted steering system of the kind in which an eclectic motor applies an assistance torque to a portion of a steering mechanism comprises at least one angular position sensor. Said sensor is provided to produces an output signal having a plurality of discrete output states, the output changing between the states as a function of the angular position of <B>either the</B> steering mechanism or the motor rotor. The method includes steps of detecting the transitions between the output of the sensor and from this modifying a count value which gives a measure of the angular position of the sensor. The at least one sensor is alternately powered up and powered down at a frequency which is lower than the maximum expected frequency of transition between states of the sensor output.

Description

ELECTRICAL POWER ASSISTED STEERING ASSEMBLIES

This invention relates to improvements in electrical power assisted steering systems for vehicles of the kind in which an electric motor is operatively connected to a steering assembly via a gearbox to apply an assistance torque to the steering assembly. and in particular to apparatus for determining the angular position of a portion of a steering assembly.

It is well known to provide electric power assisted steering (EPAS) systems of the kind set forth. The steering assembly typically comprises a hand wheel connected to a steering shaft which is operatively connected to one or more roadwheels through a steering rack, although many different assemblies are in common use. Some EPAS systems use brushless motors in which the electric motor is provided with a motor position sensor to control the timing of switching, or commutation of windings of the motor. The motor position sensor typically comprises an electromagnetic type switch or switches which changes state whenever a magnet provided on the rotor passes the sensor. Alternatively, a magnetised disc can be mounted on the rotor shaft and the sensor may detect movement of the magnets on the disc.

It is known from our earlier European patent No EP1020344B1 an EPAS system in which a motor position sensing means is adapted to produce an output indicative of the angular position of the motor rotor. A counting means is provided which is adapted to count transitions in the output of the motor position sensing means to produce a count signal indicative of the angular position of the rotor relative to an arbitrary zero position, and a steering position sensing means is adapted to produce at least one position index signal indicative of a known angular position of a portion of the steering assembly. Finally a reset means resets the count signal produced by said counting means whenever said portion of said steering assembly is at said known angular position by monitoring said index signal.

In the assembly of EP1020344B1 it is proposed that the sensors are strobed at a rate that exceeds the maximum expected rate of change of the sensor outputs, meaning that they are only powered up to take readings and are powered down in between. In practice, the minimum rate is determined by the maximum motor speed and rate of rotation of the steering column, requiring a high strobe frequency. This will typically be at least 1000Hz for most practical steering systems.

According to a first aspect the invention provides a method of operating an angular position sensor for an electric power assisted steering system of the kind in which an electic motor applies an assistance torque to a portion of a steering mechanism and at least one angular position sensor is provided which produces an output signal having a plurality of discrete output states, the output changing between the states as a function of the angular position of either the steering mechanism or the motor rotor, transitions between the output of the sensor being detected and used to modify a count value which gives a measure of the angular position of the sensor, and in which the at least one sensor is alternately powered up and powered down at a frequency which is lower than the maximum eApected frequency of transition between states of the sensor output.

Alternating between powered up and powered down states is called strobing. By powered down we mean in a state of zero or minimal power consumption in which the output of the sensor is not being read. By powered up we mean the sensor is in a state where the output signal is being read. The strategy may therefore comprise periodically energising the sensors at all times and sampling the output of the sensing means when energised. The sensors may then be de-energised between samples 3.

and not sampled and that time. This minimises the average current drain compared to running the sensors continuously. It also enables the counter to track the steering shaft position even when the steering system is otherwise powered down by turning off the vehicle ignition without draining the vehicle battery excessively. A latch may be provided to latch the sampled value.

The sensor may comprise a Hall effect sensor or a plurality of Hall effect sensors and when in the powered down state we mean that it is/they are supplied with insufficient power to function. Most preferably three Hall effect sensors are provided which are arranged such that a three bit output signal is provided.

Unlike the prior art where it is taught that the strobing must occur at twice the maximum change of output of the sensors to ensure that a transition is not missed, the present invention intentionally strobes the sensors at less than the maximum rate. This can in some cases provide a considerable saving in the power consumed by the sensors.

The strategy may comprise strobing the sensors at a rate less than the maximum rate of change of the sensor output when the vehicle to which the steering system is fitted is in a first condition and strobing the sensors at a higher rate when it is in a second condition. To determine whether the vehicle is the first condition of the second condition in this sense we may mean determining whether the engine of the vehicle is running or not, or whether the vehicle is moving, or whether the steering is moving or whether the motor is operating, or whether the vehicle is occupied or not. By strobing at the second higher rate when the condition of the vehicle indicates that the steering may be moving, or when the steering is known to be moving, it is possible to ensure that no transitions in the output are missed. The strobing may be performed at the first lower rate if the vehicle or motor or steering or engine are not moving, which gives a saving in power consumed.

The first, lower, rate may be selected as a function of the maximum expected or permitted rate of acceleration of the sensors when the steering column is rotated. Assuming that the steering starts at rest before accelerating, the strobe frequency may be chosen to equal to or greater than the time taken for the output of the sensors to move from one state to another under maximum acceleration from rest. This will also be a function of the number of changes in the output of the sensor for a revolution of the steering column, as well as a function of any gearing between the steering mechanism or motor and the sensors.

Depending on the maximum angular acceleration of the steering mechanism and the physical configuration of the steering system, the low rate may typically be chosen to be between 50 and 200 Hz which is substantially lower than the rate required to track the movement of the steering at its maximum velocity.

The second, higher, rate at which the sensors are strobed when the vehicle is running may be at least equal to the maximum expected rate of change of the sensor output. This is typically predetermined1 and will depend on the steering geometry and whether there is any gearing between the steering and the sensors. It may in fact be a continuous energising at the high state, i.e. not strobed at all when in the second condition.

The higher rate may be less than the maximum expected rate of change of the sensor output. To ensure in this case that no sensor state changes are missed, in the case where more than one sensor is provided which together produces a combined output signal or a signal that passes through more than two states, the strategy may comprise selecting states such that a double transition in the state of the output between strobes can be identified and the count accordingly changed by two.

By arranging for double changes in state to be detected, the frequency of strobing can be halved with no loss of performance.

To determine such a double change, the strategy may comprise identifying the state of the output of the sensors during a strobe, comparing this with the previous state and from this identifying whether the change corresponds to a single state change or a multiple state change.

The invention could of course extend to triple or quadruple state changes.

In this case, the strobing frequency could be further reduced. This would of course only be appropriate where the sensor output can pass through at least that many states and where each single, double or triple etc change in each direction gives a unique pair of current and previous states.

By counting transitions we mean, for example, incrementing the count signal when the sensing means output changes state corresponding to rotation in one direction, and decrementing the count signal when a change of state occurs corresponding to rotation in the opposite direction.

The sensor may determine the angular position of the motor rotor or the angular position of the steering mechanism.

According to a second aspect the invention provides an electric power assisted steering system comprising a steering shaft and an electric motor connected to the steering mechanism for applying an assistance torque to the steering mechanism, the motor rotor and steering shaft having a maximum expected rate of rotation, an angular position sensor attached to either of the steering shaft or the motor rotor such as to measure the angular position of the steering shaft or motor, the angular position sensor being adapted to produce an output signal having a plurality of discrete output states, the output changing between the states as a function of the angular position of the motor rotor, a sensor drive circuit what is adapted to alternately power up and power down the at least one sensor at a frequency which is lower than the frequency of transition between states of the sensor output as at the maximum expected speed of rotation; processing means adapted to determine the state of the output of the sensor when it is powered up and to compare this with the previous detected state to determine if the state has changed, and a counter which holds a count value that gives a measure of the rotor position, the counter modifying the count value in the event that a change in the state of the sensor has been detected.

Preferably, the angular position sensor comprises a motor position sensor.

It may comprise one or more electromagnetic effect sensing elements adapted to detect the position of one or more rotor magnets or magnets fixed relative to the rotor of the motor. They may comprise Hall effect sensors.

The sensor drive circuit may power up and power down the sensor at a first frequency when in a first operating state, and at a second frequency when in a second operating state.

The sensor drive circuit may operate in the first state when the steering shaft is not rotating. The processing means may raise a flag in the event that a current sensor output state is the same as a previous state, indicating non movement. It may be lowered if the states differ. Of course, the reverse could be applied and the flag lowered to indicate non movement. The sensor drive circuit state may be determined by this flag.

In an especially advantageous arrangement, the motor comprises a brushless permanent magnet motor, and the magnetic-effect sensors detect the position of the rotor magnets. Of course, other types of sensor could be used as an alternative.

The output signal from the motor position sensor may, in addition to producing the count signal, advantageously be used to control the timing of, or commutation, of the motor rotor windings. The sensing means may comprise one or more Hall effect sensors. Preferably, it comprises three Hall effect sensors adapted to produce a three-bit output signal.

The counter may comprise a 16-bit counter although smaller or larger counters can be used. They may be binary counters.

The motor may comprise a three phase motor, which may pass through three electrical cycles per mechanical revolution. Three Hall effect sensors may be provided, giving an output that passes through six different states. The motor may be geared from the steering column through a gearbox which may have a gear ratio of 21:1. The maximum speed of rotation of the steering may be assumed to be about 3 revolutions per second (a measure of the fastest a driver could spin the steering wheel). This would give a maximum rate of change of the output state of the sensors of approximately 3 x 6 x 21 x3 revs/second which equates to 1134 counts per second. Alternatively, the maximum speed of the steering shaft may be assumed to be 8rev/s. This would give a maximum rate of change of the output of the state of the sensors of approximately 8 x 6 x 21 x 3 revs/second which equates to 3204 counts per second. The maximum may also lie in between these values.

In the arrangement of the present invention, a low state strobe frequency of 50Hz to 200Hz may be used (depending on steering mechanism geometry and maximum rate of acceleration) whilst ensuring that the strobe rate exceeds the rate of change of output when the column starts to rotate from rest.

Where a double transition can be detected, this low rate may be halved to 25Hz to 100Hz.

There will now be described, by way of example only, one embodiment of the present invention with reference to the accompanying drawings of which: Figure 1 is a schematic illustration of an electrical power assisted steering system in accordance with the invention; Figure 2 shows the arrangement of 3 Hall effect sensors around a 6-pole rotor of a brushless electrical motor incorporated in the system of Figure 1 and the sequence of alignment of the magnets relative to the sensors as the rotor rotates; Figure 3 (a) illustrates a set of representative output signals from the Hall effect sensors which can be combined (b) to produce a single set of 6 three-bit values per 360 degrees electrical rotation Of the rotor; Figure 4 illustrates the possible changes in state of the Hall sensors including both a single and double state change which will result in a double count value change; Figure 5 is a flow diagram showing the method of applying different operating modes to the sensor drive circuitry; and Figure 6 is an illustration of the sensor drive circuitry used to strobe the sensors and to read the sensor output values.

The system shown in Figure 1 comprises a steering shaft 1 operatively connected at one end to a steering wheel 2 and at its opposing end to a pair of road wheels 3, 4 through a rack and pinion gearbox 5, 6.

In order to provide torque assistance to the driver, the system further includes an electric motor 7 connected to the steering shaft 1 through a reduction gearbox 8 having a ratio of 21:1. The motor 7 comprises a 3-phase permanent magnet brushless motor and a sensing means 9 comprising three Hall effect sensors A, B, C is arranged around the motor 7 to detect the electrical angle of the rotor by measuring the position of the rotor magnets 10. This is shown in Figure 2.

The motor has 3 electrical cycles per mechanical revolution and is connected to the steering column through a gearbox having a ratio of 21:1. Of course, motors having 4 or 5 or more electrical cycles per mechanical revolution could be used, as could different gearbox ratios.

As shown in Figure 3(a), each of the sensors A, B, C produces an output which is either zero or non-zero depending on whether a north pole of a rotor magnet or a south pole of a rotor magnet is within the detection range of a respective sensor. By suitable spacing of the sensors an incremental output having six 3-bit values can be produced as shown in Figure 3(b). Importantly, the sensor output is a measure of electrical angle and not absolute mechanical angle. The sensor output sequence will therefore repeat after every 120 degrees of mechanical movement.

The output from all three Hall effect sensors is passed to a first processing stage. Whenever a Hall effect sensor changes state, the first processing stage produces either a count-up or a count-down signal.

A count-up signal is produced whenever the transition in Hall-effect state is indicative of movement of the motor rotor in a first direction. The count down signal is produced whenever the transition is indicative of movement of the motor rotor in a second, opposite direction. For instance, the first direction may correspond to transitions of the Hall sensor output from S1-S2, S2-S3, S3-S4, S4-S5, S5-S6, S6-S1. The second direction may correspond to transitions S1-S6, S6-S5, S5-S4 S2-S1 etc. This is shown in the state diagram of Figure 4 of the accompanying drawings.

The output of the first processing stage is passed to a 16-bit counter. If a count-up signal is received, the count value held in the counter is incremented by 1. If a count-down signal is received, the value stored in the counter is decremented by 1.

For the example, the maximum rate of change of the output of the sensors, and hence maximum count rate, at a steering wheel speed of 3 revs/second is 3x21x3 = 189e-revs/second which corresponds to 1134 counts/second.

The value in the counter is calibrated or otherwise set to zero when the steering shaft is in the straight ahead position. For continuous turning of the wheel in one direction, the value held in the counter will increase in steps until such time as the steering shaft returns in the opposite direction to the straight ahead position. The counter counts up in one steering direction and down in the other. At the straight ahead point, the counter is reset. Thus, any errors in the count are continuously reset as the steering wheel is moved through the straight ahead position.

A problem may arise if the counter is not updated when the steering system is powered down. For instance, the electrical power assisted steering system may be placed in an inoperative state when the vehicle ignition is switched off. This may be intentional to prevent battery drain due to operation of the motor when the engine is not running. In such a case, however, it may still be possible to turn the steering wheel. The value held in the counter would not be updated (as the power is switched off) and on restarting would provide an erroneous position signal.

To overcome this problem, the Hall effect sensors must still be interrogated even when the system is otherwise powered down, e.g. when the engine is not running.

A strategy employed by the sensor drive circuitry for powering the sensors when the engine is not running is set out in the flow diagram of Figure 5. The sensors are strobed, by which we mean repeatedly switching them on for a short period spaced by periods in which they are switched off. This allows a position reading to be made and the counter updated during the switched on time. The sensors are thus switched off between readings.

It has previously been understood that it is essential that the frequency of sampling the sensor output should exceed at least the maximum rate of change of the sensor output. A limit is imposed by the physical limitation of the maximum speed of the motor. This would ensure that no samples are missed.

In the strategy of this embodiment, movement of the motor rotor is detected and if it indicates that the motor is not rotating the strobe state is set to a first condition called a low speed mode 200. In this condition it is assumed that the vehicle is not moving because the steering is also not moving. The strobe mode is interrogated by the sensor drive circuitry and if it is in the low speed mode 200 the sensors are strobed at a relatively low rate of say 100Hz. The detection of whether the rotor is moving is performed by monitoring a change in the Hall sensor state each time the sensors are strobed. If it has not changed, it is assumed not to have moved.

This 100Hz value is derived by assuming that the steering is initially not moving. Then, assuming a constant angular acceleration of the motor rotor gives the change of angle as d = 1/2 a t2 where a is the acceleration and t is the time since the acceleration was imposed. Then the period of the strobe mode needs to be selected to ensure that a change in the Hall sensor state will always be detected at the maximum angular acceleration.

For example, by setting d to the angular distance between adjacent Hall states, the equation can be re-arranged to calculate the maximum time interval between strobe pulses for an appropriate level of motor acceleration. Consequently, the low speed strobe rate will depend on the gear ratio of the steering mechanism, the maximum angular acceleration of the motor rotor, the number of Hall states per motor revolution and the initial offset of the motor rotor from a Hall sensor change angular position.

Whilst in this low speed mode 200, the motor rotor position is continuously checked. If it is detected that the rotor is moving then the strobe mode is switched to a second, different, condition called a high-speed mode 210. In this mode the sensors may be energised continuously or they may be strobed at a higher rate. This is because in this mode it can be assumed that the steering could be moving at a higher rate than can be monitored by the low strobe rate as it could be accelerating from a non-rest speed, and the strobing must be faster to ensure that no changes in state are missed.

If strobing is used in the high speed mode 210, it may be selected to be higher than the maximum transition rate. A value of 2400 Hz may be

chosen for example.

The strobing of the sensors is achieved by an oscillator as shown in Figure 6. This is powered from a twelve volt vehicle battery through a 5 volt regulator 103. This also powers the Hall effect sensors through the switch 110, a latch 104 which samples and holds the output of the sensors, decode logic 105 and a transition counter 106 which processes the output of the Hall effect sensors. The output of the counter is passed to a microprocessor 107 which converts the count into an angle 108. The microprocessor can cause the counter to be reset to zero or some other value by sending a reset signal. By this means the counter can be initialised when the system is calibrated, or when the steering angle is reacquired after the vehicle battery is disconnected.

As an additional safety feature, a battery disconnect latch 109 is provided which detects when the vehicle battery has been disconnected or drained when the system is powered down. The latch 109 raises an error flag if power is removed. Another latch within the decode logic 105 raises an error flag if an invalid transition between the Hall states is detected; such transitions are shown in Figure 4. On start-up, these flags are interrogated by the microprocessor 107. If any flag is raised, the value held in the counter can be treated as erroneous. An alternative means can then be used to determine the straight ahead position where upon the counter can be reset and the error flag cleared.

The applicant has appreciated that the strobe frequency used can be reduced still further provided that a double change in the output of the sensors between strobes (between energised periods of the sensor) can be detected. If this is possible, the rates can be halved.

The decode logic used to detect changes in sensor state to do this needs to be modified to enable a double change of state between strobes to be detected. When detected, the count is increased or decreased by two. This enables the high strobe rate (or normal strobe rate if only a single rate is used) to be reduced by half whilst retaining the same performance as a strategy in which every single transition must be counted. This is shown in the state diagram of Figure 6. As can be seen, any double change in state results in a unique pair of values which can be identified and used to change the count value. This uniqueness is achieved by careful selection of the outputs of the sensors for each state.

Where the double counting is employed, a low strobe rate of, say, 40Hz can be used.

In a further refinement also shown in Figure 5, a timer is provided which is triggered when the movement is detected and which indicates the time at which the strategy switched from the low speed mode to the high speed mode. When the timer reaches a predefined value, perhaps 30 seconds or a minute, the mode will be switched back to the low speed mode. The intention here is to put the system in the low speed mode as much as possible to minimise the power that is consumed.

Claims

1. A method of operating an angular position sensor for an electric power assisted steering system of the kind in which an electric motor applies an assistance torque to a portion of a steering mechanism and at least one angular position sensor is provided which produces an output signal having a plurality of discrete output states, the output changing between the states as a function of the angular position of either the steering mechanism or the motor rotor, transitions between the output of the sensor being detected and used to modify a count value which gives a measure of the angular position of the sensor, and in which the at least one sensor is alternately powered up and powered down at a frequency which is lower than the maximum expected frequency of transition between states of the sensor output.

2. The method of claim 1 which further comprises strobing the at least one sensor at a rate less than the maximum rate of change of the sensor output when the vehicle to which the steering system is fitted is in a first condition and strobing the at least one sensor at a higher rate when it is in a second condition.

3. The method of claim 2 which comprises determing whether the vehicle is the first condition or the second conditionby determining whether the engine of the vehicle is running or not, or whether the vehicle is moving, or whether the steering is moving, or whether the motor is operating, or whether the vehicle is occupied or not.

4. The method of claim 3 in which the strobing is performed at the first Lower rate if the vehicle or motor or steering or engine are not moving.

5. The method of any one of claims 2 to 4 in which the first, lower, rate is selected as a function of the maximum expected or permitted rate of acceleration of the sensors when the steering column is rotated such that the steering starts at rest before accelerating, the strobe frequency is equal to or greater than the time taken for the output of the sensors to move from one state to another under maximum acceleration from rest.

6. The method of any one of claims 2 to 5 in which the low rate is between 50 and 200 Hz.

7. The method of any one of claims 2 to 6 in which the second, higher, rate is at least equal to the maximum expected rate of change of the sensor output.

8. The method of any precending claim in which more than one sensor is provided which together produce a combined output signal or a signal that passes through more than two states, the strategy comprising selecting states such that a double transition in the state of the output between strobes can be identified and the count accordingly changed by two.

9. An electric power assisted steering system comprising a steering shaft and an electric motor connected to the steering mechanism for applying an assistance torque to the steering mechanism, the motor rotor and steering shaft having a maximum expected rate of rotation, an angular position sensor attached to either of the steering shaft or the motor rotor such as to measure the angular position of the steering shaft or motor, the angular position sensor being adapted to produce an output signal having a plurality of discrete output states, the output changing between the states as a function of the angular position of the motor rotor, a sensor drive circuit what is adapted to alternately power up and power down the at least one sensor at a frequency which is lower than the frequency of transition between states of the sensor output as at the maximum expected speed of rotation; processing means adapted to determine the state of the output of the sensor when it is powered up and to compare this with the previous detected state to determine if the state has changed, and a counter which holds a count value that gives a measure of the rotor position, the counter modifying the count value in the event that a change in the state of the sensor has been detected.

10. An electric power assisted steering system according to claim 9 in which the angular position sensor comprises a motor position sensor.

11. An electric power assisted steering system according to claim 9 or in which the sensor comprises one or more electromagnetic effect sensing elements adapted to detect the position of one or more rotor magnets or magnets fixed relative to the rotor of the motor.

11. An electric power assisted steering system according to any one of claims 9 to 10 in which the sensor drive circuit powers up and powers down the sensor at a first frequency when in a first operating state, and at a second different frequency when in a second operating state.

12. An electric power assisted steering system according to claim 11 in which the sensor drive circuit operates in the first state when the steering shaft is not rotating.

13. An electric power assisted steering system according to claim 11 or claim 12 in which low state strobe frequency is between 50Hz and 200Hz.

14. An electric power assisted steering system in which the processor is adapted to detect a double change in the state of the sensor output and the counter is adapted to increase or decrease the count by two when such a change in detected.

15. A method of operating an angular position sensor for an electric power assisted steering system substantially as described herein with reference to and as illustrated in the accompanying drawings.

16. An electric power assisted steering system substantially as described herein with reference to and as illustrated in the accompanying drawings.