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An apparatus for creating pressure pulses in the fluid of a bore hole is described. The preferred embodiment takes the form of a mud pulser apparatus having a signalling valve controlled by a variable pilot valve. The forces on the signalling valve are balanced and controlled by the flow of mud thro

An apparatus for creating pressure pulses in the fluid of a bore hole is described. The preferred embodiment takes the form of a mud pulser apparatus having a signalling valve controlled by a variable pilot valve. The forces on the signalling valve are balanced and controlled by the flow of mud through the variable orifice of the pilot valve. The arrangement is such as to act like a hydraulic amplifier, and results in the signalling valve being compensated for variable flow rates. In the preferred embodiment, the pilot valve has rotary vanes that allow it to be self-cleaning.

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1. A device for creating pressure pulses in the fluid of a bore hole, the device comprising: a housing for deployment in a bore hole, the housing having a chamber, and a piston mounted within the chamber for reciprocal motion along a longitudinal axis of the device, wherein the piston has first and

1. A device for creating pressure pulses in the fluid of a bore hole, the device comprising: a housing for deployment in a bore hole, the housing having a chamber, and a piston mounted within the chamber for reciprocal motion along a longitudinal axis of the device, wherein the piston has first and second opposing faces and forms a first variable volume chamber between the first opposing face and a first end wall of the chamber, and a second variable volume chamber between the second opposing face and a second end wall of the chamber;a hollow valve linkage member, mounted on the second opposing face of the piston, and extending out of the second end wall of the chamber towards a fluid flow restriction in the bore hole, the hollow valve linkage member being in fluid communication with the bore hole fluid in the vicinity of the restriction via an opening in the hollow valve linkage member, and wherein the end of the hollow valve linkage member outside of the chamber forms a valve tip arranged to cooperate with the fluid flow restriction to create a pressure pulse in the fluid according to the position of the piston;a control port in the piston providing a fluid communication path between the hollow valve linkage member and the first variable volume chamber; biasing means, located in the chamber for biasing the piston away from the first end wall of the chamber, towards the fluid flow restriction;a port in the chamber wall providing fluid communication between the bore hole and the second variable volume chamber, wherein the pressure of fluid pressure in the second volume variable chamber acts against the biasing means;a pilot valve in the first end wall of the chamber, which when open provides a fluid communication path between the bore hole and the first variable volume fluid chamber, and when closed shuts the fluid communication path; anda controller for controlling the pilot valve;wherein the pilot valve comprises a valve seat and a valve member, wherein the valve seat comprises one or more valve ports through which fluid can flow, each valve port having an opening, and wherein the valve member is mounted for movement in a direction across the openings of the one or more valve ports to respectively reveal or block the one or more valve ports. 2. The device of claim 1, wherein the valve member is arranged for translational motion in the plane of the valve port openings. 3. The device of claim 1, wherein the pilot valve is a rotary pilot valve having a rotary valve member arranged for rotational motion in the plane of the valve port openings. 4. The device of claim 3, wherein the rotary valve member is a disc having a plurality of lobes and voids for blocking or revealing the plurality of valve ports. 5. The device of claim 4, wherein the disc has four lobes and four voids for covering or revealing respective valve port openings located in the valve seat. 6. The device of claim 4, wherein the controller is arranged to spin the disc to transition from an open state to a closed state by an angle that is greater than the angular displacement between two successive lobes. 7. The device of claim 4, wherein the controller is arranged to spin the disc through one or more complete revolutions in a cleaning cycle. 8. The device of claim 4, wherein the controller is arranged to spin the disc continuously, and encode information by varying the speed of rotation of the disc. 9. The device of claim 1, wherein a cross-sectional area of each respective valve port in the valve seat is less than the cross-sectional area of the control port in the piston. 10. The device of claim 1, wherein a cross-sectional area of the interior of the ports in the valve seat is larger than the cross-sectional area of the valve port opening. 11. The device of claim 1, wherein the area A2 of the first opposing face of the piston, the area A1 of the second opposing face of the piston, and the hydraulic impedances k1 and k2 of the valve ports and the control port respectively satisfy the inequality A1>A2·k2/(k1+k2) 12. The device of claim 1, wherein the pilot valve is a rotary vane valve, rotary or linear sleeve valve, or any slide valve, arranged for variable opening.