Patent ID: 8312958

Claim:
A vehicle power steering unit comprising: an engine driven positive displacement hydraulic pump; a rotary valve coupled to a steering shaft for receiving hydraulic fluid under pressure in a rotary valve inlet and providing a differential pressure to a steering rack piston for a steering boost responsive to a torque exerted on the steering shaft; a two-position three-way valve coupled between an output of the positive displacement hydraulic pump, the rotary valve inlet and an input of the positive displacement hydraulic pump for controllably coupling the output of the positive displacement hydraulic pump to the rotary valve inlet or the input of the positive displacement hydraulic pump; an engine speed sensor; and a controller coupled to the engine speed sensor and the two-position three-way valve to control the two-position three-way valve to control flow rate of hydraulic fluid coupled to the rotary valve to maintain a flow rate of hydraulic fluid coupled to the rotary valve by pulse width modulation of the two-position three-way valve.