Force-based power steering system

1. A power steering system for a vehicle having dirigible wheels, comprising:

a steering wheel;

a power steering gear comprising a double-acting power cylinder having first and second cylinder ports;

an accumulator;

a reservoir;

an electronically controlled closed-center control valve assembly having an input pressure port fluidly connected to the accumulator, a return port fluidly connected to a reservoir, and first and second output ports respectively fluidly connected to first and second ports of the double-acting power cylinder, the electronically controlled closed-center control valve assembly comprises first and second electronically controlled three-way control valves, each having an input pressure port fluidly connected to the accumulator, a return port fluidly connected to the reservoir and an output port fluidly connected to a respective power cylinder port;

a steering wheel torque transducer for providing a steering wheel torque signal indicative of torque applied to the steering wheel;

a controller, wherein the controller provides a control signal to the electronically controlled closed-center control valve assembly for operating the control valve assembly in response to at least the steering wheel torque signal so as to substantially provide the desired instant differential pressure value between the first and second ports of the power cylinder, wherein the controller selects the one of the electronically controlled three-way control valves fluidly connected to the one of the first and second pressure transducers issuing the higher valued pressure signal, and operates the selected electronically controlled three-way control valve in response to at least the magnitude of the steering wheel torque signal so as to substantially provide the desired instant differential pressure value between the first and second ports of the power cylinder; and

first and second pressure transducers fluidly counted to the first and second ports of the double-acting power cylinder, respectively, wherein the first and second pressure transducers respectively issue pressure signals indicative of fluid pressure values instantly present at the first and second ports of the double-acting power cylinder to the controller.

2. A method for enabling a power steering system utilizing an accumulator, a reservoir, and an electronically controlled closed-center control valve assembly for selectively fluidly coupling the accumulator and reservoir to a double-acting power cylinder of a power steering system, to function in the manner of a force-based power steering system, wherein the method comprises the steps of:

measuring values of torque applied to the steering wheel of a host vehicle;

determining desired instant differential pressure values to be applied between first and second ports of the power cylinder in response to a selected function of the instant applied torque values, vehicle speed and any other desired system input factors;

measuring instant differential pressure values actually present between the first and second ports of the power cylinder;

subtracting actual instant differential pressure values from the desired instant differential pressure values to form an error signal;

amplifying the error signal to form a control signal; and

operating the electronically controlled closed-center control valve assembly in response to the control signal so as to continually reduce the error signal and thus provide the desired instant differential pressure values between the first and second ports of the power cylinder.

3. A method for enabling a power steering system utilizing an accumulator, a reservoir, a two-position three-way valve for fluidly coupling the cylinder port having the lowest pressure value to the reservoir, and first and second electronically controlled three-way closed-center control valves, to function in the manner of a force-based power steering system, wherein the method comprises the steps of:

measuring values of torque applied to the steering wheel of a host vehicle;

determining polarity of and measuring instant differential pressure values actually present between the first and second ports of the power cylinder;

determining desired polarity and instant differential pressure values to be applied between first and second ports of the power cylinder in response to a selected function of the instant applied torque value, vehicle speed and any other desired system input factors;

subtracting actual instant differential pressure values from the desired instant differential pressure values to form an error signal;

amplifying the error signal to form a control signal;

selecting an appropriate one of the first and second electronically controlled three-way closed-center control valves in accordance with the polarity of the actual instant differential pressure value between the first and second ports of the power cylinder; and

operating the selected electronically controlled three-way closed-center control valve in response to the control signal so as to continually reduce the error signal and thus provide the desired instant differential pressure values between the first and second ports of the power cylinder.