Motor-driven power steering system for a vehicle

Claim 2

A motor-driven power steering system for a vehicle having a steering wheol operatively connected with steerable road wheels so that the steering wheel is turned by an operator to appropriately steer said steerable road wheels, said motor-driven power steering system comprising: a torque sensor for detecting steering force imparted to said steering wheel by an operator to generate an output signal representative of the detected steering force; a vehicle-speed sensor for detecting the vehicle speed to generate an output signal representative of the detected vehicle speedy a motor adapted to be energized by a source of electric power for power assisting the steering motion caused by the operator through said steering wheel; a clutch means adapted to be switched on or off for selectively establishing or interrupting the transmission of the power-assisting force from said motor toward said steerable road wheels in accordance with the vehicle speed: and a control unit adapted to receive output signals from said torque sensor and said vehicle-speed sensor for controlling the operations of said motor and said clutch means in a manner such that fluctuation in the steering torque due to time lags in operation during transmission of the power-assisting force from said motor to said steerable road wheels is substantially reduced.

Claim 3

A motor-driven power steering system for a vehicle as set forth in claim 1, wherein said control unit comprises: a steering-torque measuring means adapted to receive the output signal from said torque sensor for measuring the operator-induced steering torque; a vehicle-speed measuring means adapted to receive the output signal from said vehicle-speed sensor for measuring the vehicle speed; a ateering-torque change-rate calculating means adapted to calculate the rate of change per unit time of the steering torque on the basis of the presently measured steering torque value and the precedingly measured steering torque value, and generate an output signal representative of the calculated steering-torque change rate; a first memory means for storing motor-current multipliers which correspond to varying steering-torque change rates; a second memory means for storing motor-current values which correspond to varying steering torque values; a motor-current determining means adapted to receive the output signals from said steering-torque measuring means, said vehicle-speed measuring means, said steering-torque change-rate calculating means, and said first and second memory means and read out from said first and second memory means an appropriate motor-current multiplier corrresponding to the calculated steering-torque change rate and an appropriate motor-current value corresponding to the measured steering torque to multiply the motor-current value by the motor-current multiplier so as to determine an appropriate motor-current value supplied to said motor; a motor-current controlling means for controlling the current supplied to said motor in accordance with the output from said motor-current determining means; and a clutch controlling means for switching on or off said clutch means on the basis of the output from said motor-current determining means.

Claim 4

A motor-driven power steering system for a vehicle as set forth in claim 2, wherein said motor-current determining means determines whether or not the measured vehicle speed is less than a first reference speed level and a second reference speed level which is greater than the first reference speed level, and controls the operations of said motor and said clutch means in a manner such that if it is determined that the measured vehicle speed is less than the first reference speed level, the current, calculated by multiplying the read-out motor-current value by the read-out motor-current multiplier, is supplied to said motor and said clutch means is switched on; and if it is determined that the measured vehicle speed is equal to or greater than the first reference speed level and less than the second reference speed level, the current supplied to said motor is reduced to an offset current level and said clutch means is switched on; and if it is determined that the measured vehicle speed is equal to or greater than the second reference speed level, the current supplied to said motor is made to be zero and said clutch means is switched off.

Claim 5

A motor-driven power steering system for a vehicle as set forth in claim 1, wherein said control unit comprises: a steering-torque measuring means adapted to receive the output signal from said torque sensor for measuring the operator-induced steering torque; a vehicle-speed measuring means adapted to receive the output signal from said vehicle-speed sensor for measuring the vehicle speed; a steering-torque change-rate calculating means adapted to calculate the rate of change per unit time of the steering torque on the basis of the presently measured steering torque value and the precedingly measured steering torque value, and generate an output signal representative of the calculated steering-torque change rate; a first memory means for storing motor-current constants which correspond to varying steering-torque change rates; a second memory means for storing motor-current values which correspond to varying steering torque values; a motor-current determining means adapted to receive the output signals from said steering-torque measuring means, said vehicle speed measuring means, said steering-torque change-rate calculating means, and said first and second memory means and read out from said first and second memory means an appropriate motor-current constant corrresponding to the calculated steering-torque change rate and an appropriate motor-current value corresponding to the measured steering torque to add the motor-current constant to the motor-current value so as to determine an appropriate motor-current value supplied to said motor; a motor-current controlling means for controlling the current supplied to said motor in accordance with the output from said motor-current determining means; and a clutch controlling means for switching on or off said clutch means on the basis of the output from said motor-current determining means.

Claim 6

5 . A motor-driven power steering system for a vehicle as set forth in claim 4, wherein said motor-current determining means determines whether or not the measured vehicle speed is less than a 'first reference speed level and a second reference speed level which is greater than the first reference speed level, and controls the operations of said motor and said clutch means in a manner such that if it is determined that the measured vehicle speed is less than the first reference speed level, the current, calculated by adding the read-out motor-current constant to the read-out motor-current value, is supplied to said motor and said clutch means is switched on; and if it is determined that the measured vehicle speed is equal to or greater than the first reference speed level and less than the second reference speed level, the current supplied to said motor is reduced to an offset current level and said clutch means is switched on; and if it is determined that the measured vehicle speed is equal to or greater than the second reference speed level, the current supplied to said motor is made to be zero and said clutch means is switched off.

Claim 7

A motor-driven power steering system for a vehicle, the system comprising sensor means (3) for providing a signal indicative of torque applied to the steering system by an operator, control means (9) for providing a control signal in dependence upon the torque signal and the speed of the vehicle, a motor (13) for power steering the steering system, and clutch means (16) for interrupting transmission of power from the motor (13) to the steering system in accordance with the vehicle speed, characteriaed in that the motor (13) and the clutch means (16) are controlled in dependence upon the control signal, and means (109f,109g) is provided for varying the control signal in dependence upon predetermined coefficients thereby to compensate for fluctuations in the steering torque due to time lags in operation of the steering system.

Claim 8

A motor-driven power steering system for a vehicle, the system comprising sensor means (3) for providing a signal indicative of torque applied to the steering system by an operator, control means (9) for providing a control signal in dependence upon the torque signal and the speed of the vehicle, a motor (13) for power steering the steering system, and clutch means (16) for interrupting transmission of power from the motor (13) to the steering system in accordance with the vehicle speed, characteriaed in that the motor (13) and the clutch means (16) are controlled in dependence upon the control signal, and means (109f,109g) is provided for varying the control signal in dependence upon predetermined coefficients thereby to compensate for fluctuations in the steering torque due to time lags in operation of the steering system.

Claim 9

A motor-driven power steering system for a vehicle, according to claim 6, wherein memory means (109c,109d) is provided for storing the predetermined coefficients, and the predetermined coefficients are selected from the memory means in dependence upon changes in the torque signal.

Claim 10

A motor-driven power steering system for a vehicle, according to claim 7, wherein the predetermined coefficients represent current values to be supplied to the motor (13), which coefficients correspond to measured changes in steering torque.