EP1827948A1

EP1827948A1 - Improvements relating to electrical power assisted steering assemblies

Info

Publication number: EP1827948A1
Application number: EP05804963A
Authority: EP
Inventors: Andrew James Stephen Williams
Original assignee: TRW Lucas Varity Electric Steering Ltd
Current assignee: TRW Lucas Varity Electric Steering Ltd
Priority date: 2004-12-21
Filing date: 2005-12-16
Publication date: 2007-09-05
Also published as: GB0427931D0; WO2006067394A8; WO2006067394A1

Abstract

An electric power assisted steering system comprising an electric motor (4); a reduction gearbox (5) operatively connecting the motor to a portion of a steering shaft (1) such that upon rotation of the motor (4) through a complete electrical revolution the shaft moves through an angle dependent upon the reduction ratio of the gearbox (5); and a steering column torque sensor (6) which is adapted to produce an output signal which is indicative of the torque carried by the steering shaft (1) and which output signal also varies periodically throughout a rotation of the shaft, the period of the signal being equal to the said angle.

Description

IMPROVEMENTS RELATING TO 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 motor in such a system.

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 road wheels 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.

For example, in a 3-phase brushless permanent magnet motor, three Hall effect sensors can be located around the rotor in such a manner that a crude measurement of rotor electrical position can be obtained.

In accordance with a first aspect, the invention provides an electric power assisted steering system comprising: an electric motor; a reduction gearbox operatively connecting the motor to a portion of a steering shaft such that upon rotation of the motor through a complete electrical revolution the shaft moves through an angle dependent upon the reduction ratio of the gearbox; and a steering column torque sensor which is adapted to produce an output signal which is indicative of the torque carried by the steering shaft and which output signal also varies periodically throughout a rotation of the shaft, the period of the signal being equal to the said angle.

The invention thus provides, in at least one advantageous embodiment, sufficient information to determine the angular position of the shaft within each period such that the electrical angle of the motor can be determined from the output signal. This allows the motor position sensor present in prior art systems to be eliminated.

The output of the sensor should, ideally, provide a signal from which the torque can be derived for all angular positions of the steering column. It should also, ideally, provide information from which the angular position of the shaft, at least within one repeating period, can be derived.

The measured position of the motor may, advantageously be used to control the timing of, or commutation, of the motor rotor windings.

The torque sensor may comprise an angular position sensor comprising at least two modulating members coupled at spaced locations to the steering column. A detecting means, such as a linear detector array, may be provided on one side of the modulating members which receives radiation that has passed through the members from a source of radiation. The source of radiation may comprise one or more light emitting diodes (LEDs) . The source of radiation may be provided on the opposite side of the modulating members, providing a sensor which works in a transmissive mode. Alternatively, it may be provided on the same side as the detecting means with a reflector provided on the other side such that the sensor works in a reflective mode. The sensor may comprise an optical sensor with the source comprising a light source or plurality of light sources.

The modulating members may comprise disks. They may be provided with an annular track of windows, each window allowing radiation to pass and spaced from an adjacent window by a blocking region which allows substantially less, or no, radiation to pass. The angular width of each window may correspond to the said angle. The windows may all be identical in width.

The output of the detector means will be periodic and repeat each time a window passes the detector. The repeat angle should be chosen to correspond to one electrical revolution of the motor.

By providing two sets of aligned annular tracks, coupled to the column at spaced locations, the relative alignment of the windows will change as torque is applied permitting a measurement of torque to be made.

The skilled man will appreciate that many arrangements of modulating members and tracks are possible. For example, each disk may carry two concentric tracks of windows. At least one detector may be associated with each track. Two detectors may be associated with each track, arranged on diametrically opposite portions of the disk. Each detector may comprise a detector array, perhaps a linear array. For further guidance on the design and implementation of a sensor based on this principle the skilled man is directed to the teaching of European patent publication EP0555987A2.

The reduction gearbox may, in a preferred arrangement have a reduction ratio of 33: 1 with the motor having 4 rotor poles and the modulating members each having 33 modulating regions. This ensures that rotation of the sensor through an angle equivalent to one region will correspond to the motor rotating through one whole electrical revolution. It will, of course, be necessary to determine an offset between the position of the motor and the sensor. This will depend on the, alignment of the motor relative to the shaft when gearbox components are introduced. The value of the offset may be determined during assembly and may be stored in a memory. This offset can then be used together with the output of the sensor to provide an absolute measure of motor electrical position. In an alternative, the gearbox, motor, column and sensor could be designed in such a way that they can only be assembled with the motor and sensor in one, preset, alignment.

It is to be noted that the motor will typically pass through more than one electrical revolution per complete mechanical revolution. Provided the electrical position is all that is required to perform motor control functions such a timing for commutation and the like. It may not be required to give a measure of the electrical angle of the motor at all times. For example, it may only be required to identify when the motor is at a given electrical angle which corresponds to a commutation point. In this case, it may be enough that the sensor only gives a measure of position at these points with no measure of position being available in between. In the case of a sinewave controlled motor, however, there are advantages to having a continuous position signal.

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; and

Figure 2 is a schematic illustration of a torque sensor for use in the system of Figure 1. A first embodiment of a system in accordance with the present invention is shown in Figure 1. It 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 (not shown) through a rack and pinion gearbox 3.

In order to provide torque assistance to the driver, the system further includes an electric motor 4 connected to the steering shaft 1 through a reduction gearbox 5. The gearbox comprises a worm and gear wheel in which the worm has 2 starts and the gear wheel has 33 teeth giving a reduction ratio of 16.5: 1. The motor 4 comprises a 3-phase permanent magnet brushless motor having 4 rotor poles, e.g. a 6:4 or a 12:4 topology.

In order to provide accurate control of the motor 4, a combined torque/ position sensor 6 is provided which measures the torque applied to the steering shaft by the driver acting on the steering wheel. A typical torque sensor is shown in Figure 2 of the accompanying drawings. The output signal 7 from the torque sensor also provides a measure of the angular position of the steering column. In the embodiment shown, the sensor 6 comprises a pair of spaced modulation members 6a, 6b in the form of annular disks which are coupled to the column at spaced locations. In fact, the cross section of the column between the disks is greatly reduced such that application of torque to the shaft causes a relative rotation of the disks.

Each disk 6a, 6b carries an annular track (not shown) comprising 33 equi- sized and spaced transparent windows separated by equi-sized opaque blocking regions. The disks 6a, 6b are aligned such that the two tracks are in register. On one side of the disks are two light source 6c' and 6c" arranged on diametrically opposite sides of the shaft, and on the other is a complimentary pair of linear detector arrays 6d' and 6d" . The detectors 6d pick up light that has passed through the windows from their respective light sources. As the disks rotate, the 33 windows pass across the detectors to give a periodic change in pattern of light and shade that falls upon the detector arrays, and accordingly a periodic change in the signal output from the detectors . This periodic change provides a measure of the angular position of the column. Also, as torque is applied the disks move angularly relative to one another, altering the overlap of the windows on the two disks. This alters the pattern of light and shade that falls upon the detector arrays and also modulates the output of the detector allowing a measure of torque to be produced.

The reduction gearbox in this embodiment is chosen such that the motor passes through one complete electrical revolution every 10.91 degrees of revolution of the column. This is the same as the angle that corresponds to each of the 33 windows on the column torque/position sensor. Accordingly, a knowledge of the angular position of the column within one window gives information about the electrical position of the motor within a revolution. The output of the torque/position sensor can therefore be used to measure motor electrical angle and form the basic information for a motor control strategy. This allows the traditional motor position sensor to eliminated.

The sensor is powered by an output signal from an ECU 8. The ECU receives the signal (s) 7 from the sensor and processes the signals to determine the position of the motor. From this the required motor drive signals are produced and supplied to the motor 4.

In an alternative arrangement the embodiment of Figure 1 may be modified for use with a different motor construction. For example, in an alternative a motor having 6 poles could be used, i.e. a 9:6 topology. Staying with a column sensor having 33 windows, a reduction gearbox with a reduction ratio of 22: 1 will ensure that there is the same correlation between the window position and the motor electrical position. A reduction gearbox comprising a worm with 3 starts and a 66 tooth gear wheel could be used in this arrangement. Again, the need for the motor position sensor of the prior art is eliminated.

Claims

1. An electric power assisted steering system comprising: an electric motor 4; a reduction gearbox 5 operatively connecting the motor 4 to a portion of a steering shaft 1 such that upon rotation of the motor through a complete electrical revolution the shaft 1 moves through an angle dependent upon the reduction ratio of the gearbox 5; and a steering column torque sensor 6 which is adapted to produce an output signal which is indicative of the torque carried by the steering shaft 1 and which output signal also varies periodically throughout a rotation of the shaft 1 , the period of the signal being equal to the said angle.

2. An electric power assisted steering system of claim 1 in which the measured position of the motor 4 is used to control the timing of, or commutation, of its motor rotor windings.

3. An electric power assisted steering system according to claim 1 or claim 2 in which the torque sensor 6 comprises an angular position sensor comprising at least two modulating members 6a, 6b coupled at spaced locations to the steering shaft 1 , a source of radiation 6c ' ,6c" and a detecting means 6d, provided on one side of the modulating members 6a, 6b which receives radiation that has passed through the members from the source of radiation.

4. An electric power steering system according to claim 3 in which the modulating members comprise disks.

5. An electric power assisted steering system according to claim 1 , 2, 3 or 4 in which the reduction gearbox 5 has a reduction ratio of 33: 1 with the motor having 4 rotor poles and the modulating members each having 33 modulating regions.