Active vibration isolation system

Abstract

An active force actuated vibration isolation system uses a velocity sensor in the form of a velocity-sensitive geophone to sense payload velocity and modifies the geophone signal to control the force applied to oppose the vibration. The modified signal effectively reduces the inherent resonant frequency of the geophone and avoids instability.

I claim: 
     
       1. A vibration isolation system comprising: a payload mass;   a support;   a passive isolation system for supporting said payload mass on said support;   a velocity transducer for sensing vibrational movement of said payload mass .[.relative to said support.]. and for producing a velocity transducer output signal that varies as a function of the velocity of said payload mass as it undergoes vibrational motion .[.relative to said support.].;   control circuit means responsive to said velocity transducer output signal for modifying said velocity transducer output signal so that the effective resonant frequency of said velocity transducer is substantially lower than the resonant frequency of said passive isolation system;   a force transducer for applying a force to said payload mass in response to an input signal; and   signal applying means for applying said modified velocity transducer output signal as said input signal to said force transducer so as to cause said force transducer to apply a force to said payload mass in a direction opposite to the direction of movement of said mass as it vibrates.   
     
     
       2. A vibration isolation system according to claim 1 wherein said control circuit means modifies said velocity transducer output signal so that the effective resonant frequency of said velocity transducer is lower than the resonant frequency of said passive isolation system by at least one decade. 
     
     
       3. A vibration isolation system according to claim 1 wherein said control circuit means and said signal applying means form a feedback loop having a feedback gain of substantially zero at dc. 
     
     
       4. A vibration isolation system according to claim 3 wherein said feedback loop has a relatively small feedback gain at frequencies above 200 Hz. 
     
     
       5. A vibration isolation system according to claim 1 wherein said control circuit means and said signal applying means form a negative feedback loop between said velocity transducer and said force transducer, and said loop is characterized by a gain of substantially zero at dc and at frequencies above about 200 Hz. 
     
     
       6. A vibration isolation system according to claim 1 wherein said control circuit means comprises at least two series-connected lag-lead stages for controlling signal gain and phase as a function of frequency. 
     
     
       7. A vibration isolation system according to claim 6 wherein each lag-lead stage comprises an operational amplifier having first and second input terminals and an output terminal, a lag-lead feedback network coupling its output terminal to its first input terminal, and further including means for applying the output signal of said velocity transducer to the second input terminal of the operational amplifier of the first in series of said lag-lead stages; and further wherein said signal applying means is arranged to couple the output signal appearing on the output terminal of the operational amplifier of the last in series of said lag-lead stages to input terminal of said force transducer. 
     
     
       8. A vibration isolation system according to claim 7 wherein said control circuit means comprises three of said lag-lead stages. 
     
     
       9. A vibration isolation system according to claim 1 wherein said velocity transducer is a geophone. 
     
     
       10. A vibration isolation system according to claim 9 wherein said geophone comprises a case, a proof mass suspended in said case, and means for sensing the velocity of said proof mass relative to said case and for producing said velocity transducer output signal, and further including means for applying said velocity transducer output signal to said control circuit means, and a resistor connected between said last-mentioned means and ground for damping said velocity transducer output signal, said resistor having a value such that the overall fraction of critical damping is unity. 
     
     
       11. A vibration isolation system according to claim 1 wherein said force transducer is an electromagnetic device and comprises a speaker magnet voice coil assembly having a first magnetic structure defining an annular air gap and a second coil-supporting structure mounted in said air gap so as to permit substantially frictionless relative motion between said first and second structures in response to said modified velocity transducer output signal. 
     
     
       12. A vibration isolation system comprising: a payload;   a support;   a passive isolation system for supporting said payload on said support;   a geophone disposed so as to sense vibrational movement of said payload .[.relative to said support.]., said geophone comprising a case, a proof mass suspended within the case, and means for measuring the relative velocity of the proof mass relative to the case and for producing an output signal that is proportional to said relative velocity by the expression S(X-Z), where S is the Laplace operator, X is the displacement of the case relative to said support, and Z is the displacement of the proof mass as the payload undergoes vibrational movement;   circuit means coupled to said geophone for modifying said geophone output signal so that it is more nearly proportional to the actual velocity shown by the expression SX of said case and payload;   a force transducer for applying a force to said payload in response to an input signal; and   means for applying said modified geophone output signal as an input signal to said force transducer so as to cause said force transducer to apply a force to said payload in a direction in opposition to the direction of movement of said payload as it vibrates.   
     
     
       13. A vibration isolation system according to claim 12 wherein said circuit means modifies said geophone output signal so that the effective resonant frequency of said geophone is lower than the resonant frequency of said passive isolation system by at least one decade. 
     
     
       14. A vibration isolation system comprising: a payload mass;   a support;   a passive isolation system supporting said payload mass on said support;   a geophone for sensing vibrational movement of said payload mass .[.relative to said support.]. and for producing an output signal in response to said movement, said geophone comprising a case adapted to vibrate with said payload mass, a proof mass suspended in said case so as to be capable of displacement relative to said case as said case undergoes vibrational movement, and means for measuring said relative displacement and providing in response thereto a geophone output signal that is represented approximately by the following equation using Laplacian notation: ##EQU11## where E v  (s) represents the geophone transfer function, β=gain in units of volts per inch per second, S=the Laplace operator, X is the displacement of the geophone case, Z is the displacement of the proof mass, S(X-Z) is the relative velocity of the proof mass relative to the geophone case, ξ g  is the geophone fraction of critical damping, and w g  is the proof mass resonant frequency;   control circuit means responsive to said geophone output signal for modifying said geophone output signal so that the effective resonant frequency of said geophone is lower than the resonant frequency of said passive isolation system by at least one decade;   a force transducer for applying a force to said payload mass in response to an input signal; and   means for applying said modified geophone output signal as an input signal to said force transducer so as to cause said force transducer to apply a force to said payload mass in a direction in opposition to the direction of movement of said payload mass as it vibrates.   
     
     
       15. A vibration isolation system according to claim 14 wherein said control circuit means has a transfer function which is approximately as follows: ##EQU12## where: g(s) is the transfer function of said control circuit means, w g  is the geophone resonant frequency,   S is the Laplace operator and   T g  is a time constant= ##EQU13##   
     
     
       16. A vibration isolation system according to claim 15 wherein said circuit means modifies said geophone output signal so that the effective resonant frequency of said geophone is lower than the resonant frequency of said passive isolation system by at least one decade. 
     
     
       17. A vibration isolation system according to claim 15 wherein said circuit means comprises a plurality of leg-lead amplifier stages adapted to provide substantially zero gain at dc and high gain greater than unity at the frequencies of said vibrational motion except that the gain is substantially less than unity for all frequencies at open loop phase angles of ±180 degrees. 
     
     
       18. A vibration isolation system comprising: a payload mass;   a support means supporting said payload mass so that said payload mass is subject to vibrational motion and will respond to application of a force opposing said vibrational motion, said support means comprising a passive isolation system;   a geophone for sensing vibrational movement of said mass .[.relative to said support.]. and for producing a transducer output signal that varies as a function of the velocity of said payload mass as it undergoes vibrational motion .[.relative to said support.].;   circuit means responsive to said geophone output signal for modifying said geophone output signal;   an electromagnetic force transducer for applying a force to said payload mass in response to an input signal; and   means for applying said modified geophone output signal as an input signal to said force transducer so as to cause said force transducer to apply a force to said payload mass in a direction in opposition to the direction of movement of said payload mass as it vibrates;   said circuit means being adapted to modify the output signal of said geophone so as to as to provide a compensated geophone transfer function represented as follows: ##EQU14## where: F(s) is the geophone transfer function, g(s) is the transfer function of said circuit means, T g  is the geophone time constant, and S is the Laplace operator.   
     
     
       19. An active vibration isolation system comprising: a payload mass supported so as to be subject to vibrational motion and to respond to application of a force opposing said vibrational motion;   a geophone for producing a velocity signal in response to vibrational motion of said payload mass;   an electrically operated force transducer for applying a force to said payload mass in response to an input signal;   circuit means responsive to said velocity signal for producing a control signal that provides a compensated geophone transfer function as follows: ##EQU15## where F(s) is the transfer function of the geophone, g(s) is the transfer function of the circuit means, T g  = is the time constant for the geophone, and S is the Laplace operator; and   means for applying said control signal as an input signal to said force transducer.   
     
     
       20. A system according to claim 19 wherein said payload mass is supported by a passive vibration isolation means. 
     
     
       21. A system according to claim 20 wherein said circuit means modifies said velocity signal so as to lower the effective resonant frequency of said geophone to approximately a decade below the resonant frequency of said passive vibration isolation means. 
     
     
       22. A system according to claim 19 wherein said circuit means modifies said velocity signal so that effectively the response of said geophone is equivalent to that of a geophone having a resonant frequency about 10 times lower than the resonant frequency of said first-mentioned geophone. 
     
     
       23. An active vibration control system for a payload that is supported relative to a supporting structure and is subject to undesirable vibration in a plurality of degrees of freedom comprising: a plurality of geophones mounted on said payload, each of said geophones being oriented to sense a velocity component of said payload in a predetermined one of several degrees of freedom and adapted to produce a geophone electrical output signal that varies as a function of the velocity component in said predetermined one degree of freedom;   a plurality of force actuators each operable in response to an individual input signal for applying a force to said payload in opposition to vibrational motion of said payload in one of said degrees of freedom;   a plurality of channel circuit means each adapted to produce a channel electrical output signal in response to an input signal;   a plurality of means for applying said geophone output signals as input signals to said channel circuit means; and   means for applying said channel output signals to said force actuators;   said channel circuit means being adapted to modify said geophone output signals so that each of said channel circuit means output signals provides a compensated geophone transfer function that is represented approximately as follows: ##EQU16## where F(s) is the transfer function of one of said geophones, g(s) is the transfer function of the channel circuit means to which the output signal of said one geophone is applied, T g  = is the time constant for said one geophone, and S is the Laplace operator.   
     
     
       24. A system according to claim 23 wherein each of said channel circuit means effectively lowers the natural resonant frequency of one of said geophones to approximately a decade below the natural resonant frequency of said passive vibration isolation means. 
     
     
       25. A system according to claim 23 wherein each of said channel circuit means modifies the output signal of one of said geophones so that it is equivalent to the signal produced by a geophone having a resonant frequency approximately ten time less than the actual resonant frequent of said one geophone. 
     
     
       26. A system according to claim 23 wherein each of said channel circuit means comprises a plurality of lag-lead amplifier stages for controlling gain and phase as a function of frequency. 
     
     
       27. A system according to claim 23 wherein said payload is supported on said base by a plurality of passive vibration isolation systems so that said payload has said several degrees of freedom. 
     
     
       28. A system according to claim 27 wherein said channel circuit means effectively lower the natural resonant frequency of said geophones to approximately a decade below the natural resonant frequency of said passive vibration isolation means. 
     
     
       29. A system according to claim 23 wherein said force transducers are electromagnetic devices. .Iadd. 
     
     
       30.  A vibration isolation system according to claim 1 wherein said force transducer is an electrically-controlled pneumatic device. .Iaddend. .Iadd. 
     
     
       31.  A vibration isolation system according to claim 12 wherein said force transducer is an electrically-controlled pneumatic device. .Iaddend. .Iadd.32. A vibration isolation system according to claim 19 wherein said force transducer is an electrically-controlled pneumatic device. .Iaddend. .Iadd.33. A vibration isolation system according to claim 23 wherein said force actuators are electrically-controlled pneumatic devices. .Iaddend. 
     
     
        .Iadd.34.  An active vibration isolation system comprising: a payload mass;   a piston attached to said payload mass;   first pneumatic means, connected to said piston and having a first pressure chamber, for urging said piston in a first direction in accordance with an increase in fluid pressure in said first pressure chamber and for permitting said piston to move in a second opposite direction in accordance with a decrease in fluid pressure in said first pressure chamber;   second pneumatic means, connected to said piston and having a second pressure chamber, for urging said piston in said second opposite direction in accordance with an increase in fluid pressure in said second pressure chamber and for permitting said piston to move in said first direction in accordance with a decrease in fluid pressure in said second pressure chamber;   pressure sensor means for measuring the fluid pressures in said first and second pressure chambers and for providing a differential pressure signal which is representative of the difference in pressure between said first pressure chamber and said second pressure chamber;   a velocity transducer for sensing vibrational movement of said payload mass and for providing an electrical velocity signal that varies as a function of the velocity of said payload mass as it undergoes vibrational motion;   control circuit means for generating an electrical flow command signal that varies as a function of said differential pressure signal and said velocity signal; and   pneumatic control means for adding fluid to, or exhausting fluid from, said first and second pressure chambers so as to vary the fluid pressures in said first and second pressure chambers in response to said flow control signal so as to cause said first and second pneumatic means to cause said piston to move in a direction opposite to the direction of movement of   
     
     
        said mass as it vibrates. .Iaddend. .Iadd.35.  A system according to claim 34 wherein each of said first and second pneumatic means comprises a flexible diaphragm that is coupled to said piston so as to apply forces to said piston in accordance with the fluid pressures in said first and 
     
     
        second pressure chambers. .Iaddend. .Iadd.36.  A system according to claim 34 further including a support, a passive isolation system for supporting said payload mass on said support, and electronic means for modifying said velocity signal so that the effective resonant frequency of said velocity transducer is substantially lower than the resonant frequency of said passive isolation system, and said control circuit means comprises a differential summing device for comparing said modified velocity signal and said differential pressure signal and producing an output error signal that varies in accordance with the difference between said modified velocity signal and said differential pressure signal, and means for deriving said flow command signal from said output error signal. .Iaddend. .Iadd.37. A system according to claim 34 wherein said velocity transducer 
     
     
        is a geophone. .Iaddend. .Iadd.38.  A system according to claim 37 wherein said electronic means modifies said velocity signal so that the effective resonant frequency of said velocity transducer is lower than the resonant frequency of said passive isolation system by at least one decade. .Iaddend. .Iadd.39. A system according to claim 36 wherein said electronic means modifies said velocity signal so that the effective resonant frequency of said velocity transducer is lower than the resonant frequency of said passive isolation system by approximately one decade. .Iaddend. .Iadd.40. A system according to claim 34 wherein said pneumatic control means comprises a servovalve for adding fluid to or exhausting fluid from said first and second chambers whereby to vary the fluid pressures in said chambers, said servovalve being adapted to add fluid to one chamber while exhausting fluid from the other chamber in accordance with changes in said flow command signal, with said servovalve being arranged so as to vary the fluid pressures in said chambers responsively to said flow command signal so as to urge said piston to move in a direction opposite to the direction of movement of said pay load mass as it vibrates. .Iaddend. .Iadd.41. A system according to claim 40 wherein said fluid is air. .Iaddend. 
     
     
        .Iadd.     A system according to claim 40 wherein said first and second pneumatic means comprise first and second flexible diaphragms respectively that are coupled to opposite ends of said piston, each of said diaphragms being arranged to apply a force to said piston with the magnitude of the forces applied by said first and second diaphragms varying in accordance with the fluid pressures in said first and second chambers respectively. .Iaddend. .Iadd.43. An active vibration isolation system comprising: a payload mass;   a support;   passive isolation means for supporting said payload mass on said support;   a piston connected to said payload mass so that a force applied to said piston will be transmitted to said payload mass;   a velocity transducer for sensing vibrational movement of said payload mass and for providing a velocity signal that varies as a function of the velocity of said payload mass as it undergoes vibrational motion;   means for modifying said velocity signal so as to provide a modified velocity signal such that the effective resonant frequency of said velocity transducer is substantially lower than the resonant frequency of said passive isolation means;   first pneumatic means for applying a first force to said piston, said first pneumatic means comprising a first pressure chamber and a first displaceable means coupled to said first chamber so as to be moveable responsively to changes in the fluid pressure in said first chamber, with said first displaceable means (a) moving in a direction to increase the volume of said first chamber when a fluid under pressure is injected into said first chamber and (b) moving in a direction to decrease the volume of said first chamber when a fluid under pressure is exhausted from said first chamber;   second pneumatic means for applying a second force to said piston, said second pneumatic means comprising a second pressure chamber and a second displaceable means coupled to said second chamber so as to be moveable responsively to changes in the fluid pressure in said second chamber, with said second displaceable means (a) moving in a direction to increase the volume of said second chamber when a fluid under pressure is injected into said second chamber and (b) moving in a direction to decrease the volume of said second chamber when a fluid under pressure is exhausted from said second chamber;   said first and second displaceable means being disposed in force-coupling relation with said piston, with said first and second pneumatic means being disposed so that said second force is applied to said piston in opposition to said first force, whereby said first and second displaceable means can urge said piston to move in a first or second direction so as to apply a directional force to said payload mass; and   pneumatic control means comprising electrically operable valve means operable responsively to a flow command input signal for (a) injecting a fluid under pressure into said first chamber when said flow command signal undergoes a change in a first selected direction and exhausting fluid under pressure from said first chamber when said flow command signal undergoes a change in a second selected direction opposite to said first selected direction, and (b) exhausting a fluid under pressure from said second chamber when said flow command signal undergoes a change in said first selected direction and injecting fluid under pressure into said second chamber when said flow command signal undergoes a change in said second selected direction;   pressure sensor means for measuring the fluid pressures in said first and second pressure chambers and for providing a differential pressure signal which is representative of the difference between the fluid pressure in said first pressure chamber and the fluid pressure in said second pressure chamber;   electrical circuit means for generating a flow command signal that varies as a function of said differential pressure signal and said modified velocity signal; and   means for applying said flow command signal as the input to said valve means so as to cause said valve means to inject fluid into one and exhaust fluid from the other of said first and second chambers according to changes in said flow command signal, whereby said first and second pneumatic means cooperate to urge said piston to move in a direction opposite to the direction of movement of said mass as it vibrates. .Iaddend.