Electricity: motive power systems – With particular motor-driven load device – Power- or motion-transmitting mechanism
Reexamination Certificate
1999-12-09
2001-04-03
Martin, David (Department: 2837)
Electricity: motive power systems
With particular motor-driven load device
Power- or motion-transmitting mechanism
C180S444000
Reexamination Certificate
active
06211631
ABSTRACT:
This invention relates to an electrical power-assisted steering system for a vehicle of the kind comprising an electric motor connected through a gearbox to act on a steering mechanism of the vehicle. For example, the gearbox may provide a connection between the motor and a steering column shaft, or directly onto a portion of a rack and pinion mechanism forming part of the steering mechanism.
The electric motor is used to assist a driver in applying torque to the steering mechanism, by applying an assistance torque of the same sense, to make it easier to turn the steering wheel, for example during parking manoeuvres. Thus, operation of the motor may assist in rotating the steering column shaft, or moving a portion of the steering rack mechanism. Of course, the motor may be connected to any part of any typical steering mechanism as long as it can provide an assistance torque to aid the driver in turning the steering wheel.
The motor, which may be a multi-phase brushless star-connected permanent magnet motor, is controlled by motor control means comprising control and drive circuits, which is operative to supply a current from a power supply to the motor phase windings. The phase windings of the motor are connected at a star point. Each phase is connected to a positive terminal of the power supply by a top transistor, and to a negative terminal by a bottom transistor, the two transistors defining an arm of a multiple arm bridge. This bridge forms the drive circuits, while the control circuits are provided by a microprocessor or digital signal processor or analogue signal processing or some combination thereof. The microprocessor is operative in response to signals from a torque sensor provided on the steering column to measure the torque applied by the driver, from a motor rotor position sensor providing information about motor speed and direction and optionally from signals corresponding to current flowing in the motor bridge or power supply. This information can be used in combination with the torque sensor signal and/or column position sensor signal to determine which phase winding should be energised and when. The microprocessor produces control signals which energise the transistors of the drive circuits to cause current to flow in a desired motor phase.
A problem with this electrical power-assisted steering system is that a fault occurring in the motor drive or control circuits can cause an error condition which is unacceptable in a vehicle steering system, where safety is critical.
For example, suppose that a top transistor in one arm of the bridge is energised in error while a bottom transistor in another arm of the bridge is also energised in error. This fault would result in a phase of the motor becoming permanently energised and cause the motor to become permanently attracted to a particular position, tending to clamp the steering column in position and resist rotation. This would be readily apparent to the driver, and is clearly undesirable. This situation could occur if the microprocessor is at fault, or if a short circuit occurs across the transistors.
One known solution to this problem is to provide a clutch between the motor and the steering column. The clutch, typically a dog or friction clutch, is normally engaged but in the event of a fault being detected, the clutch is operated to disengage the motor from the steering column. However, the clutch is costly and bulky, and additional test procedures must also be incorporated to check that the clutch can still be disengaged should it be necessary to do so, which again adds to the cost and is time consuming.
Another solution is to provide a means for isolating the motor drive circuitry from the power supply in the event of a failure, for example by providing a relay between the supply positive terminal and the drive circuitry. This removes the drive current from the motor but is not without its problems. Because the phase windings are still connected together and connected to the drive circuitry, an electrically conducting path could still be present around the bridge (i.e. through two short circuited transistors and two phase windings). Then, on rotation of the motor shaft drive due to rotation of the steering column, a back EMF is produced in one or more of the windings. As a result of this EMF and the complete electrical path around the bridge, a substantial current can flow through the motor phase windings which produces a torque in the opposite sense to the rotation of the motor shaft. This resisting torque is highly undesirable because in this case, not only is steering assistance lost due to disconnection of the power supply, but a resisting torque is then applied which makes turning the steering column difficult.
In accordance with a first aspect of the present invention, in an electrical power-assisted steering system for a vehicle of the kind set forth, the electric motor is a brushless motor having a plurality of phase windings connected at a star point, and a switching means is provided in at least one phase of the motor, the switching means being movable between a closed position in which current is able to flow in the phase winding, and an open position which prevents current flowing in the respective phase winding of the motor.
The switching means ensures, if a fault condition arises, that the motor does not provide a resisting torque to the movement of the steering column because no current flows in the phase winding. This means that no clutch is required. Secondly, the switching means, when open, prevents the back EMF generated by the movement of the motor rotor from producing a current in the winding as the phase is connected to an open circuit and so no current can flow. This is true regardless of the nature of the fault in the motor or the drive circuitry.
Preferably, the switching means comprises a switch or relay which is connected in series between an end of the phase winding and the motor drive circuitry. Alternatively, the switching means may comprise a switch or relay which is provided at the star point of the motor and opening of the switching means isolates the phase winding from the star point.
A separate switching means may be provided in each phase of the motor to allow each phase to be isolated from the star point independently. The switching means may comprise a solid state relay, but any suitable isolating switch could be used.
In a preferred embodiment, the electric motor comprises a 3-phase motor and the switching means comprises a pair of relay contacts operated by a single relay coil so that two of the three phases can be isolated from the third phase which is common to both relay contacts.
A further problem with electrical power-assisted steering systems of the kind set forth arises due to the need to connect the motor phase windings to the motor control means and power supply.
In order to reduce bulk, cost and EM radiation emissions an electrical power-assisted steering system requires that the drive circuitry, motor and gearbox are housed in close proximity to each other. In one known system the drive circuitry is mounted onto a printed circuit board (PCB) which is then fixed near to the motor, for example inside the gearbox housing. The motor windings are then connected to the PCB using flying leads. However, flying leads are undesirable because they can become tangled up with a rotary part of the motor or gearbox due to the confined space, and they are time consuming to connect. The provision of the relay to isolate phase windings, where required, also imposes problems of mounting and adds to assembly time as it requires extra leads.
In accordance with a second aspect of the present invention, in an electric power assisted steering system of the kind set forth, a connector assembly connects the electric motor to an electronic circuit, the connector assembly comprising a substantially rigid frame which is adapted to support a plurality of conducting tracks which provide a connection between phase windings of the motor and the electronic circuit.
By providing the rigidly s
Appleyard Michael
Horton Steven John
Ironside John Michael
Wilkes Mark Anthony
Wilson-Jones Russell
Christensen O'Connor Johnson & Kindness PLLC
Lucas Industries Public Limited Company
Martin David
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