A modulated bias unit, for controlling the direction of drilling of a rotary drill bit when drilling boreholes in subsurface formations, comprises a number of hydraulic actuators spaced apart around the periphery of the unit. Each actuator comprises a movable thrust member which is hydraulically displaceable...http://www.google.com.au/patents/US5582259?utm_source=gb-gplus-sharePatent US5582259 - Modulated bias unit for rotary drilling
