Abstract:
A flow controller having a volume, a pressure transducer coupled to said volume, a first valve coupled between a source of gas under pressure and said volume, a second valve coupled between said volume and a reference volume, a third valve coupled between said reference volume and a load wherein said valves are cyclically operated so as to permit gas to flow through said volumes to a load and wherein maximum and minimum pressures are measured with said first and second valves closed.

Description:
PRIOR ART 
     This invention is an improvement on one of the specie for controlling gas flow between a pressurized source and a load that is described in the U.S. patent application, Ser. No. 011,333, entitled &#34;Mass Flow/Pressure Control System&#34; which was filed on Feb. 12, 1979, for Roger A. Nalepa et al. now U.S. Pat. No. 4,373,549, issued Feb. 15, 1983, and which will be assigned to the same assignee as that patent application. 
     In the pertinent specie of the flow control system described in the above-identified patent application, gas from a pressurized source is cylically applied to a first chamber and coupled from the first chamber to a load via a second chamber. Means including a transducer coupled to the first chamber are provided for deriving signals representing the maximum pressure P A  and the minimum pressure P B  occurring therein during each cycle. The actual flow m was determined from the expression 
     
         [(P.sub.A -P.sub.B)V]/ΔtRT                           (1) 
    
     wherein V is the volume of the first chamber, Δt is the duration of a cycle, R is the universal gas constant and T is the absolute temperature of the gas. The flow was compared with a desired mass flow and the difference was used to control the flow of gas into the first chamber. Alternatively, either the maximum pressure P A  or the minimum pressure P B  could be controlled by respectively comparing them with desired values and using the difference to control the flow of gas into the first chamber. 
     In order to increase the accuracy of the signals representing P A  and P B , means were provided for the purpose of holding them constant while the signals representing them were derived. This was achieved for low cyclic frequencies by cutting off the first chamber from the gas supply and the second chamber while the signals representing P A  were being derived and by cutting off the series-coupled first and second chambers from the source of gas and load while the signals representing P B  were being derived. Whereas both P A  and P B  were constant at low cyclic frequencies, it was found that P B  varied during each cycle at the higher cyclic frequencies that are required to increase the resolution of the control. Such a change in P B  was an error and resulted in an erroneous control of mass flow or of pressure. This was surprising in view of the fact that the fixed volume of both chambers was coupled to the transducer. 
     BRIEF DISCUSSION OF THE INVENTION 
     Applicant has discovered that gas entering either chamber causes a change in temperature that gradually reverts to a steady value as heat passes through the walls of the chamber and that pressure signals derived before the steady value was reached would be in error. Because of the shape and small volume of the first chamber, the temperature attained a steady value before the signals representing the maximum value P A  were derived so that P A  was constant while it was being measured; but the shape of the second chamber caused the gas to be distributed farther from its walls and its volume was greater so that the temperature therein did not reach a steady value prior to the period when the signals representing the minimum pressure P B  were being derived. Because both chambers are coupled to the transducer during this period, the output valve between them being open, any error in pressure in the second chamber due to the fact that its temperature was not stabilized affected the pressure seen by the transducer and caused the signals representing the minimum pressure P B  to be in error. 
     In accordance with this invention, the errors are eliminated without reducing resolution by closing the output valve between the two chambers during the period when the signals representing the minimum pressure P B  are being derived. The first chamber is therefore coupled to the transducer, but the second chamber is not, so that any effect on pressure that the unstabilized temperature in the second chamber would otherwise have is eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a gas flow control system embodying the invention; and 
     FIG. 2 is a series of diagrams illustrating the operation of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, a supply 2 of pressurized gas is coupled to an input valve V I  that is connected by a tube 4 to an output valve V O . A tube 6 connects a pressure transducer 8 to the tube 4. The portion of the tube 4 between the valves V I  and V O , the tube 6 and the space inside the transducer 8 that is in communication with the tube 6 define a first chamber. A second chamber 10 is coupled to the output valve V O  and a load valve V L  connects the second chamber 10 via a tube 12 to a load which may be the column 14 of a gas chromatograph having a sample injector 16 and a detector 18. As illustrated, the valves V O  and V L  are comprised of opposing ends of the tubes 4 and 12 and a plate 20 that is mounted on a rod 22. The valve V L  is closed and the valve V O  opened by moving the rod 22 up with a driver 24 so that the plate 20 blocks the end of the tube 12 and leaves the end of the tube 4 open. The output valve V O  is closed and the valve V L  opened by moving the rod 22 down with the driver 24 so that the plate 20 blocks the end of the tube 4 and leaves the end of the tube 12 open. As is apparent, the valves V O  and V L  could be entirely separate. The input valve V I  is controlled by a valve driver 26. 
     Means including the pressure transducer 8 are provided for producing electrical signals representing pressure in the first chamber. Although the transducer 8 may be one of many types, it is shown as being comprised of a cylindrical metal tank 28 having a flexible bottom 30 that can bend up or down in response to the pressure of the gas in the tank 28. A disc 32 of insulating material is firmly attached to the bottom of the inside walls of the tank 28, and a metal disc 34 is adhered to the bottom of the disc 32 so as to form a variable capacitor having a capacitance depending on the distance between the bottom 30 of the tank 28 and the disc 32. By electrically coupling the tank 28 and the disc 34 to the tuned circuit, not shown, of an oscillator 36, fluctuations in the capacitance caused by the gas pressure in the first chamber vary the frequency of the oscillator. The output of the oscillator 36, which is generally sinusoidal in shape, as indicated by a wave 38, is coupled to a wave-shaping circuit 40 that amplifies and clips the output of the oscillator 36 so as to form square waves, as indicated at 42. 
     The square waves 42 are applied to a counter 44 that is turned on when a wave f 2  applied to its &#34;enable&#34; input is high. While enabled, the counter 44 counts the number of cycles in the square wave 42 and supplies a corresponding digital number to a latch 46. The average pressure occurring while the counter 44 is turned on is proportional to the number of cycles counted. The latch 46 acquires the digital number at the output of the counter 44 in response to a wave f 3  applied to its latch terminal. After this, the counter 44 may be cleared by a wave f 4  applied to its &#34;clear&#34; terminal. The digital output of the latch 46 is conducted to a computer 48 when the computer supplies a wave f 5  to the &#34;enable&#34; terminal of the latch 46. 
     The desired pressure or mass flow is introduced into the computer 48 via a SET input. In a manner to be explained, the computer 48 calculates a number N corresponding to the time the input valve V I  is to be open. The input of a downcounter 50 is coupled so as to receive the number N when a load pulse l is applied to its load terminal from the computer 48. As long as the count in the downcounter 50 is other than zero, its output is high so as to cause the valve driver 26 to which it is coupled to keep the input valve V I  open. The output of the downcounter 50 is connected to one input of an adder 52, pulses f 8  are applied to the other input, and the output is connected to the clock input of the downcounter 50. Thus, as long as there is a count in the downcounter 50, the adder 52 will go high at each pulse of f 8  and lower the count in the downcounter by one. The output of the downcounter 50 is also connected to the valve driver 26. As long as the output of the downcounter 50 is high, the input valve V I  will be fully open. The various waves, f 1 , f 1  &#39;, f 1 , f 2 , f 3 , f 4 , f 5  and f 8  are derived in any suitable manner by logic circuits 53. 
     OPERATION 
     Reference is now made to FIG. 2 wherein the timing of the open and shut positions of the input valve V I , the output valve V O  and the load valve V L  are respectively indicated by the waves DC, f 1  and f 1 . The variation in pressure in the first chamber that occurs when V I  is open for the entire first quarter of an interval Δt is illustrated by the solid line 54 in the pressure graphs P, and the corresponding variation in pressure in the second chamber 10 is illustrated by the solid line 56. If the input valve V I  is open for only a part of the first quarter Q 1  of an interval Δt, as indicated by the dashed lines 58 and 60 of the wave DC, the pressure variation in the first chamber is as indicated by the dashed line 62 of the pressure graphs P, and the pressure variation in the second chamber 10 is as indicated by the dashed line 64. As shown in the wave f 2 , the counter 44 is turned on during the quarters Q 2  and Q 4  of each interval Δt and the counts Ct#1 and Ct#2 are respectively attained at the ends of these quarters. Ct#1 is proportional to the maximum pressure P A   and Ct#2 is proportional to the minimum pressure P B  if the input valve V I  is open for the entire first quarter Q 4  of each interval; and the counts Ct#1 and Ct#2 are respectively proportional to the lesser pressures P A  &#39; and P B  &#39; if the input valve V I  closes at the times indicated by the dashed lines 58 and 60. 
     It is important to note that in accordance with this invention both V I  and V O  are closed when the counter 44 is counting so as to derive the counts Ct#1 from which the maximum pressure P A  is determined or when the counter 44 is deriving Ct#2 from which the minimum pressure P B  is determined. 
     If the computer 48 is an HP Model 21 MX, it can be made to perform the READ, FLOW and FEEDBACK functions indicated by the programs at the end of the specification. These functions are performed under the direction of a MAIN PROGRAM, also included at the end of the specification. In accordance with the READ program, Ct#1 is read into the computer 48 when the wave f 1  &#39; is decreasing and Ct#2 is read into the computer 48 when f 1  &#39; is increasing. 
     In accordance with the FLOW program, the pressures P A  and P B  are calculated in accordance with the following equations in which the constants A, B and C are calibration constants specific to the particular transducer. They may vary from transducer to transducer, but will remain the same for a given transducer. 
     
         P.sub.A =A+B(Ct#1).sup.2 +C(Ct#1).sup.4                    (2) 
    
     
         P.sub.B =A+B(Ct#2).sup.2 +C(Ct#2).sup.4                    (3) 
    
     In accordance with the FEEDBACK program, the mass flow m for an interval of a duration Δt is calculated as indicated by the following expression ##EQU1## wherein V is the volume of the first chamber, R is the universal gas constant, T is the absolute temperature indicated by a digital thermometer mounted in the tube 4 and Δt is the length of an interval in seconds. 
     The computer 48 then calculates the time t during which the valve V I  is to be open in accordance with the following well-known proportional integral control algorithm: ##EQU2## wherein the constants K 1 , K 2  and K 3  are such as to provide for optimum performance with the particular combination of mechanical components involved. 
     The computer 48 outputs the digital number N representing the time t during which V I  is to be open. When this is done, the computer 48 provides a loading pulse l to the load terminal of the downcounter 50, and the number N is transferred to it. The manner in which the open time of the input valve V I  is controlled has already been explained. The maximum time that V I  is to be open is one-quarter of a cycle, and the number N corresponding to this time is such that it equals the number of cycles of f 8  occurring during one quarter-cycle. 
     If a lower flow or pressure is called for, the number N will be less so that the output of the downcounter 50 ceases to be positive at times indicated by the dashed lines 58 and 60 of the graph DC. The valve V I  closes at this time, so that the pressure in the first chamber follows the dotted line 62 in the graph P. The pressure in the chamber is therefore constant for the last part of Q 1  as well as for all of Q 2 . ##SPC1## ##SPC2##