Pressure feedback servovalve for a seismic vibrator

• US 5,128,908 A
• Filed: 11/05/1990
• Issued: 07/07/1992
• Est. Priority Date: 04/09/1990
• Status: Expired due to Term

• Current Assignee: INOVA Limited
• Original Assignee: Pelton Co., Inc. (ION Geophysical Corporation)
• Inventors: Reust, Dennis K.
• Primary Examiner(s): Lobo, Ian J.
• Application Number:
  US07/608,834
• Time in Patent Office: 610 Days
• Field of Search: 367/189, 367/190, 91/461, 181/119, 137/625.62, 137/625.64
• US Class Current: 367/190
• CPC Class Codes: G01V 1/155 using reciprocating masses
• CPC Class Codes: G05D 16/2024 the throttling means being a multiple-way valve
• CPC Class Codes: G05D 16/2097 using pistons within the main valve
• CPC Class Codes: Y10T 137/86614 Electric

Abstract

An apparatus for reducing harmonic distortion and causing a more linear response in a hydraulic vibrator apparatus which generates a seismic signal into the surface of the earth is disclosed. The force developed in the actuator of a hydraulic vibrator is directly related to pressure applied to the hydraulic piston. Conventional vibrators use flow control type servovalves to develop this pressure. Changes in loading due to earth surface conditions reflect back as changes in pressure across the hydraulic piston. Pressure can be better controlled in the actuator such that it varies only as a function of the input signal, and the natural response of a vibrator can be made more linear, and the harmonic distortion can be reduced by using a pressure control type servovalve. Apparatus for changing a flow control type servovalve into what is essentially a pressure control type servovalve includes hydraulic feedback passageways provided from the output ports of the servovalve main stage to the input ports of the servovalve main stage. The passageways are constucted to provide differential negative pressure feedback around the servovalve main stage. Further apparatus damps the servovalve output to increase stability, and further tunes the valve to its load. It consists of a bypass passageway to port hydraulic fluid between the two output ports of the servovalve main stage. Orifices in the pressure feedback and bypass passageways determine the feedback gain and the amount of damping.

Claims

1. Apparatus for improving linearity and reducing undesirable harmonic distortion of a seismic vibrator having a baseplate for contacting the earth, a hydraulically operated vibrator actuator having a piston and piston rod connected to said baseplate, and a reaction mass in sliding coupling with said piston rod, a servovalve including a pilot servovalve operatively coupled to said actuator and means for controlling said servovalve, an improved servovalve comprising:

(a) a housing having a central bore with first and second means for closing said central bore at its opposite ends;

(b) spool means including a piston rod positioned in said central bore for movement axially along said central bore, said spool means having a first pair of pistons mounted on the ends of said piston rod and at least an additional piston means having a first and second side and attached intermediate said first pair of pistons to said piston rod;

(c) first and second port means coupled to said central bore to communicate a controlled high pressure hydraulic fluid from said pilot servovalve to said central bore between said first and second means and said first pair of pistons in response to an electrical control signal causing said spool means to move axially in said central bore in response to a pressure differential in said hydraulic fluid at said first and second port means;

(d) hydraulic pressure input port means communicating with said central bore between said first pair of pistons;

(e) third and fourth port means communicating with said central bore and located on each side of said additional piston in a manner to provide pressurized hydraulic fluid alternately to first and second sides of said vibrator actuator piston means as said spool means is moved

(f) hydraulic pressure feedback means communicating between said third and fourth port means and said first and second port means, to provide continuous predetermined quantity of hydraulic fluid communication between said third and fourth port means and said first and second port means respectively;

(g) hydraulic bypass means communicating between said third and fourth port means to provide continuous and controlled fluid communication between said third port means and said fourth port means;

wherein said harmonic distortion is substantially reduced and linearity of said vibrator actuator operation is improved.

2. Apparatus as described in claim 1 wherein an orifice means is mounted in said hydraulic feedback means and in said hydraulic bypass means for determining flow resistance.

3. Apparatus as claimed in claim 1 wherein said spool means comprises a second pair of spaced pistons and wherein said high pressure input port is located in said central bore between said second pair of spaced pistons.

4. Apparatus as claimed in claim 1 wherein said piston means comprises a single piston mounted over said high pressure input port to block said high pressure input port when said spool is in a centered or neutral position intermediate said first and second closing means.

5. In a hydraulic vibrator apparatus for generating a seismic signal into the surface of the earth, said hydraulic vibrator having a baseplate, a vibrator actuator having a piston and piston rod connected to said baseplate, and a reaction mass in sliding mechanical coupling with said piston rod, said vibrator actuator hydraulically operated by a servovalve having input ports, and having output ports coupled to said hydraulically operated vibrator actuator, and a pressure hydraulic input port to said servovalve, an apparatus for improving linearity and for reducing harmonic distortion of said seismic vibrator when transmitting a signal into said surface of the earth comprising:

(a) hydraulic feedback means coupled between said output ports and said corresponding input ports;

(b) hydraulic bypass means coupled from one output port of said servovalve to the remaining output port;

(c) means in said hydraulic feedback means for predetermining the flow resistance to hydraulic fluid passing from said output ports to said corresponding input ports; and

,

(d) means in said hydraulic bypass means for predetermining the flow resistance to hydraulic fluid passing from one output of said servovalve to the remaining output port.

6. Apparatus as described in claim 5 wherein said means for predetermining the flow resistance to hydraulic fluid is an orifice means.

7. Apparatus as described in claim 5 wherein said servohydraulic valve comprises a housing having a longitudinal cylinder therein with a spool valve slidably positioned therein, said spool valve having a pair of end pistons and an intermediate piston means, each of said input ports communicating with each of said end pistons;

said input pressure port communicating with said longitudinal cylinder between said pair of end pistons; and

wherein said hydraulic feedback means includes passageways formed in said servohydraulic valve between said output port and its corresponding input port, said passageways having means for limiting the quantity of hydraulic fluid passing through said passageways.

8. Apparatus as described in claim 7 wherein said means for limiting the quantity of hydraulic fluid comprises an orifice.