Patent ID: 7503409

Claim:
An earth drilling rig having components including a drill head for rotating a hollow drill pipe, an elongated, tiltable, mast for supporting the drill head and a hollow drill pipe supported by, and rotatable by, the drill head, and a hoist for moving the drill head longitudinally along the mast, the drilling rig also comprising: a hydraulic pump mechanism for supplying hydraulic fluid under pressure for driving at least one of said components of the drilling rig; an air receiver for storing air under pressure, said air receiver being connected to the drill head for delivery of compressed air to the hollow drill pipe supported by the drill head; an air compressor, having an air inlet port, and an air outlet port, for supplying air, through the outlet port, to the air receiver, an engine arranged to drive both the air compressor and the hydraulic pump; a valve having a variable aperture, the valve being arranged to throttle the flow of air through the inlet port of the compressor; an actuator, connected to the valve, for opening and closing of the aperture of the valve, the actuator being capable of maintaining each of a plurality of discrete valve apertures between limits of a range of valve apertures; a sensor responsive to the pressure of air within the air receiver; and an electronic controller for operating said actuator, said controller having a manually selectable input for selecting a compressor outlet pressure, and a feedback input, the feedback input being responsive to said sensor, for controlling said valve through said actuator and thereby maintaining the compressor outlet pressure at a level corresponding to the pressure selected through said manually selectable input; in which said electronic controller comprises a first comparison device, responsive to said manually selectable input and said feedback input, for producing an error signal corresponding to the difference between the manually selected pressure and the pressure of air within the air receiver as sensed by said sensor, a target rate of change generator, responsive to the error signal, for generating an output having a predetermined relationship to the magnitude of the error signal, a differentiator, responsive to the sensor, for producing a signal proportional to the time rate of change of the air pressure in the receiver, and a second comparison device, responsive to said output of the target rate of change generator and the signal produced by the differentiator, for producing a control output, the actuator being responsive to said control output of the second comparison device.