Electric power assisted steering systems

Details Electric power assisted steering systems Electric power assisted steering systems

2000-02-02

2002-03-12

Hurley, Kevin (Department: 3619)

Motor vehicles

Steering gear

With electric power assist

Reexamination Certificate

active

06354396

ABSTRACT:

This invention relates to improvements in electrical power assisted steering systems, and in particular to an improved apparatus for measuring the absolute steering angle of the road wheels.
Typical electric power assisted steering systems comprise a steering shaft operatively connected at a first end to a steering wheel and at its opposite end operatively connected to the roadwheels of a vehicle. An electric motor is provided which can apply torque to the steering shaft through a reduction gearbox. The gearbox may be of the worm and wheel, or other, type.
A steering gear is provided between the steering shaft and the steered wheels. This steering gear typically provides a substantial gearing between angular movement of the steering shaft (and hence hand wheel) and the movement of the roadwheels. For a typical road vehicle, more than one turn a of the handwheel (i.e. complete revolution of the steering shaft) is needed to move the roadwheels from lock to lock.
It is desirable to be able to measure the steering angle of the roadwheels. This can be used to influence a number of sub-systems in the vehicle such as suspension damper control systems, vehicle stability control systems and vehicle lane guidance.
One solution to the problem would be to provide an angular position sensor on the steering shaft to measure the angle of rotation of the steering shaft. However, as more than one fill revolution is needed to turn from lock to lock such a measurement would not unambiguously describe the angle of the roadwheels.
The problem of measuring multiple turns of the steering shaft can be overcome in several ways, each with its own disadvantage.
In one proposal, the steering shaft angular position sensor can be driven by the steering shaft through a step down gear, reducing the total number of turns of the sensor to less than one full revolution. This overcomes the problem of ambiguity, but unfortunately reduces the resolution which can be obtained from the sensor. To produce a high resolution system is therefore expensive.
According to the present invention, we provide an electric power assisted steering system comprising: a steering shaft operatively connected at a first end to a handwheel and at its other end operatively connected to at least one roadwheel, an electric motor having a rotor operatively connected to the steering shaft through a gearbox having a non-integer reduction gear ratio, a first sensing means adapted to produce an output dependent on the angular position of the steering shaft; a second sensing means adapted to produce an output dependent on the angular position of the rotor, and processing means adapted to process both output signals to produce an angular position signal indicative of the angular position of the steering shaft over a range of greater than one complete revolution.
The invention thus employs outputs from two sensors, one monitoring the position of the steering shaft and the other monitoring the position of the motor rotor to provide, if desired, an unambiguous measurement of steering shaft angle over a range of angles in excess of one full revolution.
Preferably, both sensors are adapted to produce a cyclic output signal dependent upon angular position which repeats after a complete revolution, or perhaps a fraction of a full revolution. The cycle may repeat upon a complete rotation of the associated steering shaft or motor rotor, i.e. 1 cycle corresponds to 360° of rotation. For example, one sensor may produce an absolute angular position value which varies substantially linearly over the range 0-360 degrees of rotation between a value of 0 and 1. The sensor will therefore produce the same output value for shaft or rotor positions of 90°, 90°+360°, 90°+720° etc. Alternatively, it may have a range of 0-180°, and thus the cycle will repeat itself once within a single revolution.
At least one of the sensors may comprise an absolute angular position sensor. By this we mean that the sensor produces a signal that represents the absolute angular position of the shaft or rotor within a complete revolution (or part of a revolution). Examples of sensors of this kind include potentiometers, a resolver, a synchro and an optical angle encoder. For clarity, it is assumed that the absolute sensor produces an output that varies substantially linearly between 0 and 1 over its range of output values.
Alternatively, at least one of the sensors may comprise an index sensor. By this we mean a sensor which is adapted to produce an output signal dependent on angle which is indicative of the position of the shaft within a small fraction of a revolution. Such a sensor may, for example produce a short pulse as the shaft rotates past its index position, and zero output in all other positions. Again, more than one index pulse may be produced within a single revolution, i.e. two equal-spaced pulses per complete revolution.
Preferably, the sensors are driven directly from the steering shaft or motor rotor without intermediate gearing. Thus, with a sensor having a cycle of 360 degrees, for one turn of the shaft the sensor measures one full revolution.
Preferably, the gear ratio may be expressed as p/q whereby the motor turns through p/q revolutions for each revolution of the steering shaft, p is greater than q, q is greater than unity, and the greatest common integer factor of p and q is also unity.
By gearbox ratio, we mean the ratio between the rotation of the two sensors. Thus, if each sensor produces an output value over a range corresponding to one full revolution, the gear ratio is the turns ratio between the input side and output side of the gearbox. If one sensor produces an output which cycles or repeats n times within one revolution of its respective shaft or rotor, the gearbox ratio will be np′/q where np′=p as herein before.
By selecting a non-integer ratio, the outputs of the two sensors will drift out of synchronisation as the steering shaft rotates. Eventually, after a predetermined number of revolutions, the output will return into synchronisation This “beating” enables an unambiguous measurement of rotation over a range greater than one revolution to be achieved from sensors which produce an output over a range of one revolution or less.
In one arrangement, the first sensing means comprises an absolute handwheel position sensor and the second sensing means comprises an index sensor adapted to produce an index signal at a known angular position of the motor rotor, said processing means being adapted to sample the output of the first sensing means corresponding to the position when the second sensing means produces an index signal;
multiply the sampled value by p;
round the multiplied value to the nearest integer to produce a reference value and
use the reference value to access the corresponding entry in a look-up table, said entry being indicative of the number of revolutions of the steering shaft from an arbitrary zero position.
In another arrangement, the first sensing means may comprise an index sensor adapted to produce an index signal at a known angular position of the handwheel with the second sensing means comprising an absolute position sensor.
In yet a further alternative, both sensing means may comprise absolute position sensors. Again, the processing means is adapted to exploit the way in which the outputs of the sensors drift out of synchronisation and back into synchronisation after a number of revolutions to obtain a measure of the number of rotations of the steering shaft from an arbitrary zero angular position. A benefit of using two absolute position sensors is that it is no longer necessary to wait until one of the sensors passes an index, allowing a more regular estimate of position to be made.
In the event that both sensors comprise absolute position sensors, the processing means may be adapted to estimate the angular position of the motor rotor from a measurement of the angular position of the steering shaft assuming it is on its “zero” revolution. This estimate may then be compared with the actual output signa

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