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Method and apparatus for measuring piston position in a free piston compressor

  • US 5,342,176 A
  • Filed: 04/05/1993
  • Issued: 08/30/1994
  • Est. Priority Date: 04/05/1993
  • Status: Expired due to Term
First Claim
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1. An improved gas or vapor compressor including a control apparatus and a free piston linked to a spring and reciprocating in a cylinder in alternating suction and pressure phases, the piston during reciprocation having an alternating component of displacement, a velocity, an acceleration and an end displacement of the piston'"'"'s excursion in the cylinder, the piston being driven in reciprocation by an electromagnetic linear motor drivingly linked to the piston, the linear motor including a magnet and a winding having an associated resistance and inductance, the motor having input terminals and a characteristic electro/mechanical transfer constant, the motor being driven by an alternating voltage applied to and a current forced through the input terminals of the motor winding, wherein the improvement is a feedback control apparatus comprising:

  • (a) a voltage detector circuit connected to said winding input terminals for detecting the voltage applied to the winding as a function of time;

    (b) a current detector circuit connected to said winding for detecting the current through the winding as a function of time;

    (c) a command signal input for inputting a command signal representing a selected, required end displacement;

    (d) a computing circuit generating a signal representing a measured value of said end displacement and comparing said measured value signal to said command signal to generate an error signal by;

    (i) computing the velocity of the reciprocating piston as a function of time from the detected voltage and current in accordance with the equation;

    
    
    space="preserve" listing-type="equation">v=(1/α

    )(V-L(dI/dt)-IR);

    whereinα

    is said transfer constantV is said voltageI is said currentR is said winding resistanceL is said winding inductancet is time;

    (ii) integrating the computed velocity as a function of time to compute the alternating component of displacement of said piston as a function of time;

    (iii) differentiating the computed velocity as a function of time to compute the acceleration of the piston as a function of time;

    (iv) detecting the alternating component of displacement resulting from step (ii) when the computed velocity is zero;

    (v) simultaneously during said suction phase detecting the alternating component of displacement resulting from step (ii), the acceleration resulting from step (iii) and the current detected from said current detector;

    (vi) computing the displacement of the reciprocating piston at the end of its excursion in accordance with the equation;

    
    
    space="preserve" listing-type="equation">X.sub.c =x.sub.i -x.sub.o +(α

    /K)I.sub.o -(M/K) A.sub.o ;

    wherein;

    Xc is said end displacementxi is the alternating displacement when the velocity is zeroxo is the simultaneously detected alternating displacementAo is the simultaneously detected accelerationIo is the simultaneously detected currentM is the mass of the reciprocating bodyK is the spring constant of the spring;

    (vii) comparing said command signal to the computed end displacement signal Xc to generate an error signal; and

    (e) a motor voltage control circuit having an input connected to receive said error signal and having an output connected to said motor winding for changing the voltage applied to the motor winding in response to said error signal in a direction minimizing the error signal.

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