Linear suction control system
This is a divisional of co-pending application Ser. No. 706,654 filed on 2-28-85, now U.S. Pat. No. 4,706,687. 
    
    
     BACKGROUND OF THE INVENTION 
     In the field of surgery as well as in many other technical disciplines there is a great need for a suction system in which the vacuum or negative pressure level can be highly controlled. In ophthalmic surgery, for example, many cutting instruments draw the tissue into the cutting edges by the use of suction. Indeed, the tissue removal rate is effectively controlled by the suction effect which is related directly to the negative pressure level. Thus controlling the negative pressure level to a fine degree is highly desirable to provide to the surgeon a concomitant degree of control of the tissue removal process. 
     However, prior art suction systems are generally deficient in their poor control of the vacuum level. Many systems employ a pressure delivery tank in which the vacuum level is controlled by selective connection to a lower pressure source or to a selectively valved &#34;leak&#34; from atmospheric pressure. These systems are characteristically underdamped pressure oscillators, in that the negative pressure level often swings wildly about the desired (and often changing) level. Furthermore, purposely introducing a leak into a vacuum system is inherently wasteful, and contradicts sound engineering practice. Also, the large volume of most systems causes a delay in their response, which may lead to poor user control and overshooting of the desired vacuum level. 
     Some suction systems deliver vacuum to a surgical handpiece, and provide a series of leak ports adapted to be blocked or unblocked by one or more of the fingers of the hand wielding the handpiece. The surgeon may slide a finger along the row of ports to unblock ports and thus set a limited number of vacuum suction levels. This pressure control is not continuous, being limited to a few discrete levels, and even these levels can be set only within a broad range. Here again, introducing a series of leaks is inherently inefficient and wasteful. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention generally comprises a vacuum or negative pressure delivery system which features precise, continuously variable, predictable control of the vacuum delivered. A significant feature is that the variation in vacuum may be adjusted electronically to follow any desirable functional mathematical relationship to a control signal. For example, the system may be arranged so that the vacuum level will vary linearly with respect to the position of a control lever, pedal, or the like. The vacuum does not fluctuate wildly, and the vacuum at any position of the control lever or pedal is predictable and reproducible. The system may be adapted to surgical suction/evacuation systems and the like, as well as to other technical disciplines. 
     The linear suction control system includes a linear solenoid valve which exhibits a flow through rate which varies with respect to the current through the solenoid in a known manner. Regulated fluid pressure is fed through the linear solenoid valve to a venturi-type pressure vacuum converter. The resulting vacuum is proportional to the flow rate through the valve and thus to a function of the current through the solenoid. A transducer samples the vacuum output and produces a signal fed to a differential amplifier. The signal is compared with a variable input voltage set by a potentiometer or other variable voltage source in negative feedback fashion, so that the transducer signal tracks the voltage source signal by actuating the valve to correct the flow rate so that the desired pressure will be attained. The voltage source may comprise a potentiometer actuated by a foot switch or the like, so that angular position of the foot switch relates directly and predictably to the vacuum output. The linear solenoid valve includes an electromagnetic coil arranged to drive a valve member, with an elastic member biasing the valve member closed with a generally linear restoring force. The circuitry of the differential amplifier may be arranged so that the overall relationship of the vacuum level to the controller position may follow any desired function. Thus the response to a control device may be tailored to suit the user, the process to which the vacuum is applied, or the like. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram of the vacuum control and delivery system of the present invention. 
     FIG. 2 is a cross-sectional elevation of the linear solenoid valve according to the present invention. 
     FIG. 3 is a schematic represenation of the linear solenoid valve depicted in FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention generally comprises a vacuum (negative gauge pressure) delivery system which features fine control of the vacuum which is generated and delivered. With reference to FIG. 1, the system includes a novel linear solenoid valve 11. The linear solenoid valve is an electrically actuated valve which exhibits a through-flow rate proportional to the current through the solenoid coil, as will be explained in the following. A regulated fluid pressure supply 12 is fed through the linear solenoid valve 11 to a converter 13 which converts positive fluid pressure to a corresponding vacuum (negative pressure). One such device, manufactured by Air-Vac Co., utilizes a venturi restriction in the flow path of the pressurized fluid to generate a partial vacuum. The partial vacuum (negative gauge pressure) is proportional to the flow rate of the pressurized fluid through the converter 13. The resulting partial vacuum output 14 may be connected to any device requiring negative pressure, such as a surgical cutting device, suction/evacuation device, or the like. 
     A transducer 16 is connected to the output 14 to sample the negative pressure level of the output. The transducer 16 generates a signal which is fed to a differential amplifier 17. The differential amplifier 17 is also fed a reference signal generated by a variable voltage device, such as the potentiometer 18. The potentiometer is actuated by a control device, such as a foot pedal controller 19 having a continuously variable range of angular position settings which are directly related to the potentiometer setting. It may be appreciated that other reference voltage sources may be employed, such as a microprocessor system in which the pedal position of the device 19 is read by a linear shaft position encoder, and the pedal position is correlated with the desired voltage level by a look-up table stored in the system memory. 
     The differential amplifier 17 is configured to compare the transducer signal with the reference voltage level, and to produce a difference output which is provided to adder 20. Further the original reference voltage signal from foot pedal controller 19 is provided to adder 20. These two signals are added together by adder 20 and drive the linear solenoid valve in typical feedback loop fashion. If the signal from transducer 16 is different from that from controller 19, the difference will be combined with the signal from controller 19 to cause the vacuum at output 14 to track the level set by controller 19. If the difference between the signals provided to differential amplifier 17 is zero, only the signal from controller 19 is provided to adder 20 and the vacuum output at 14 is at the desired level set by controller 19. Due to the fact that the relationship between the flow rate through the valve 11 varies in generally linear proportion to the current through the solenoid of valve 11, and that the partial vacuum output level from the converter is related directly to the flow rate therethrough, the differential amplifier circuitry is configured to produce a system in which the vacuum level is directly related to the position of the pedal of the controller 19. This overall system response may be configured as linear, in which incremental change in pedal position divided by incremental change in negative pressure level results in a constant number. On the other hand, the system of the present invention may be configured to have a particular non-linear response, as may be required by the end user of the system. 
     It should be noted that the negative pressure delivery system of the present invention does not employ the standard pressure accumulator tank, and thus has no large volumes which typically slow the response of a partial vacuum system. Thus the prior art problem of overshooting the desired pressure level in reaching steady state in the accumulator tank is obviated. 
     With regard to FIGS. 2 and 3, the construction of the linear solenoid valve 11 includes a solenoid coil 32 wound about a hollow tubular member 33. One end of the tubular member 33 is sealed by a plug 34, and the stem of a valve member 36 is slidably received in the other end portion of the member 33. A significant feature of the invention is the provision of an elastic member 35 disposed between the plug 34 and the valve stem 36 to bias the latter member outwardly of the tube 33. The member 35 may comprise elastomeric rubber, foamed plastic, or the like, and it exhibits a restoring force which varies linearly with the amount of compression applied thereto. 
     A valve head member 37 is secured at the open end of the tube 33, and includes a valve seat 38 adapted to receive the distal end of the valve stem 36. A collar 39 is secured to the member 37 by threads, and an O-rinq seal 21 retains pressure in the assembly. A valve port member 22 is joined to the collar, and includes an inlet port 23 and an outlet port 24. The flow path between the inlet and outlet includes an orifice 26 which is selectively and variably opened by the valve member 36. The magnetic force generated by the solenoid coil 32 drives the valve stem 36 toward the plug 34, thus opening the orifice 26. However, this translation is opposed by linear restoring force of the elastic member 35, so that an incremental increase in the fluid flow through the orifice 26 requires an incremental increase in the magnetic force driving the valve stem and thus in the current fed through the coil 32. The elastic member also provides a damping effect to the motion of the valve stem, and thus is superior to a metal spring or the like. The material of the elastic member is generally serviceable through approximately one billion operating cycles.