Abstract:
An electro-pneumatic transducer converts an electrical input signal into a proportional (direct or indirect) pneumatic output signal. A circuit will sense the magnitude of the input signal, monitor the output fluid pressure and adjust a piezoelectric beam voltage until a desired output fluid pressure is achieved. The output fluid pressure is controlled by the deflection of a piezoelectric beam which is polarized and will deflect with an applied voltage. The output fluid pressure is monitored by a pressure sensor.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a current to pressure transducer, and more particularly to an electro-pneumatic transducer which converts a current input to a proportional pneumatic output. 
     Current designs, such as known from U.S. Pat. No. 4,481,967, use a piezoelectric beam driven with a single polarity voltage or a magnetically susceptible element suspended in a magnetic field. The devices which have an element suspended in a magnetic field use a stationary coil to create a net force on the element, moving it toward a nozzle to create a back pressure. Devices which have a piezoelectric beam use a single polarity voltage to drive the beam. A piezoelectric beams&#39; position will creep over time with the applied voltage. The direction of the creep is toward the neutral position. In the case of a single polarity voltage, the beam will eventually creep until the drive voltage can no longer control the beam to achieve the specified output range. Another problem with the piezoelectric beam is the creep due to temperature fluctuations. The polarization of the beam will cause break down when a specified voltage, per mil of piezoelectric beam thickness, is exceeded. The single polarity drive will allow a limited amount of excess voltage to compensate for temperature creep of the piezoelectric beam. 
     Accordingly, the problems of the prior art are overcome by the invention which incorporates a dual polarity drive voltage. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an electro-pneumatic transducer which converts an electrical input to a proportional pneumatic output. 
     According to the broader aspects of the invention, a piezoelectric beam is driven in two directions with a dual polarity voltage, which allows any creep (drift), of the piezoelectric beam to the neutral or unbiased position, and toward the middle of the driving voltage as opposed to the rail of a single polarity drive voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiment, the appended claims and the accompanying drawings in which: 
     FIG. 1 is block diagram of the apparatus according to the invention; and 
     FIGS. 2, 2a and 2b is circuit diagram for implementing the block diagram of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a block diagram of the apparatus according to the invention. A 4-20 or 10-50 mA DC input current is coupled to terminals 11,12. A voltage regulator 14 coupled to terminal 11 holds a constant DC voltage for the circuits of the apparatus and couples excess current to circuit common ground. In order to determine what the output fluid pressure should be, a DC current sensor (command circuit) 16 monitors the input DC current and sends a DC command signal via line 18 to a comparator 20. A pressure sensor (feedback circuit) 22 is coupled by lines 21,23 to a pressure sensor DC current source 24 which determines the sensitivity, and thus the span, of the pressure sensor. Therefore, the pressure sensor DC current source 24 is used as the manual span adjustment. The pressure sensor 22 is coupled by fluid line 25 to monitor the output fluid pressure in fluid line 26 and to send a proportional DC feedback signal 28 to the comparator 20. 
     The comparator 20 monitors the DC command signal 18 and the DC feedback signal 28 and provides a DC control signal 30. When the DC command signal 18 is greater than the DC feedback signal 28, the DC control signal 30 increases. The DC control signal 30 decreases when the DC command signal 18 is less than the DC feedback signal 28. An oscillator 32 converts the DC command signal 30 to a proportional AC signal 34, and an AC amplifier 36 increases the value of the oscillators&#39; AC signal 38 proportionally. A rectifier 40 converts the AC amplifiers&#39; signal 38 to a DC signal 42. The DC output signal 42 of the rectifier 40 is proportionally larger than the DC control signal 30. 
     The DC output signal 42 of the rectifier 40 is applied to the middle of the piezoelectric plates forming the piezoelectric beam 44. A DC offset voltage 46 from circuit 48 is applied to both the top and bottom of the piezoelectric plates forming the piezoelectric beam 44 to allow dual polarity to be applied to the piezoelectric beam. Combined, the DC output signal 42 of the rectifier 40 and the DC offset voltage 46 become a DC drive voltage for the piezoelectric beam 44. The DC drive voltage causes the piezoelectric beam to defect toward and away from a nozzle 50 with respect to the neutral or unbiased position of the beam. When the output 42 of the rectifier circuit increases, the piezoelectric beam 44 is driven toward the nozzle 50 creating a back pressure via fluid line 58 in a pneumatic booster 52 thus causing a net increase in output fluid pressure in fluid line 26 via fluid line 60. When the output 42 of the rectifier circuit decreases, the piezoelectric beam 44 is driven away from the nozzle 50 allowing the fluid in the pneumatic booster 52 to escape and, therefore, the output fluid can exhaust by fluid line 54 thus causing a net decrease in output fluid pressure. An orifice 56 causes a pressure drop between a controlling chamber and output chamber of the pneumatic booster 52. The fluid in the controlling chamber feeds the nozzle by fluid line 58; and the fluid in the output chamber is coupled by fluid line 60 to the output fluid pressure line 26. A pneumatic booster arrangement of this type is known from model type GT300 Electro-Pneumatic Transducers (ITT Conoflow, St. George, South Carolina). 
     Referring now to FIG. 2, the input current is received at terminal T1. Capacitor C2 prevents errors and instabilities due to noise on the input current. Diodes D1 and D3 are to protect the circuit from misapplication of a large voltage or reversal of the input leads. Capacitors C1 and C4 also help eliminate any AC noise from affecting the operation of the circuit. Capacitor C7 prevents the output of the circuit from drifting due to EMI. Op amp U22 is an unused op amp biased to not consume current. 
     A voltage regulating circuit 14 is connected between conductors 1 and common ground to maintain a voltage that will power the circuits and dump excess current from conductor 1 and Common Ground. The voltage regulating circuit consists of op amp U12, transistor Q1, diode D2, and resistors R1, R2, R3, R4, R5, R6, and SIP2(E). 
     The current sensor (command circuit) 16 monitors the input current and sends a proportional command signal to the comparator circuit 20. A voltage drop is formed across the monitoring resistors and is amplified to a desired level for the command signal. Jumper J3 allows for either 4-20 mA or 10-50 mA input current. The command circuit 16 consists of op am U21, jumper J3, capacitor C5, and resistors R7, R8, R9, R25, R26, R27, SIP1(A), SIP1(B), and SIP1(C). 
     The pressure sensor (feedback circuit) 22 monitors the output fluid pressure and sends a proportional feedback signal to the comparator circuit 20. A piezoresistive sensor bridge U4 (I C Sensors, Model 1210A, Milpitas, California) changes the resistance across various legs of the bridge as the pressure changes; therefore, giving a proportional output voltage due to the monitored fluid pressure. The feedback circuit amplifies the voltage output of the piezoresistive bridge to a desired feedback signal. Potentiometer VR2 is used to zero the feedback signal when the monitored fluid pressure is zero (0). The feedback circuit 22 consists of op amp U13, pressure sensor U4, potentiometer VR2, and resistors R14, R15, R17, SIP1(D), and SIP1(E). 
     A pressure sensor current source 24 supplies supply current to the piezoresistive bridge U4. Potentiometer VR4 is a manual adjustment for the magnitude of current supplied to the piezoresistive bridge, and therefore, determines the magnitude of the feedback signal for a given output fluid pressure. Potentiometer VR4 manually adjusts the voltage to the non-inverting input of op amp U11 causing the current supplied to the piezoresistive bridge and the monitoring resistors R19 and R20, to be at a magnitude that will stabilize the voltage at the inverting input of op amp U11 to be equivalent to that at the non-inverting input. Jumper J1 allows for full scale output fluid pressure adjustment for one half (1/2) of full scale input current adjustment. The pressure sensor current source 24 consists of op amp U11, potentiometer VR4, jumper J1, and resistors R19, R20, R21, and R22. 
     The comparator circuit 20 monitors the command signal and feedback signal to output a control signal that will maintain a predetermined relationship between the command signal and feedback signal for a given command signal. Potentiometers VR1 and VR3 are manually adjusted to set the predetermined relationship between the command signal and feedback signal. Potentiometer VR3 is used to center potentiometer VR1 for adjustment of the predetermined relationship of the command signal and feedback signal solely using VR1 regardless of the positions of user adjustment jumpers J1, J2, and J3. Potentiometer VR6 adjusts the internal response of the control signal, due to a change in the command signal or feedback signal, to alleviate the output fluid pressure from getting into a resonant oscillation with the control circuit by changing the RC value of C3, R12, and VR6. Jumper J2 allows the output fluid pressure to be manually changed from directly proportional to inversely proportional, or vice versa, with respect to the input current. The comparator circuit 20 consists of op amp U14, capacitor C3, jumper J2, potentiometers VR1, VR3, and VR6, and resistors R10, R11, R12, R16, and R18. 
     An oscillator DC to AC circuit 32 transforms the DC control signal into a proportional AC signal. Potentiometer VR5, in combination with capacitor C6, is for manual adjustment of the AC frequency to minimize losses due to heating of the components in the AC portion of the control circuit. The oscillator DC to AC converter consists of multivibrator U3, transistors Q2, Q3, Q4, and Q5, capacitor C6, potentiometer VR5, and resistors SIP2(A), SIP2(B), SIP2(C), and CIP2(D). 
     An AC amplifier circuit 36 amplifies the AC output signal from the oscillator DC to AC converter 32. The AC amplifier circuit is transformer XFMR1. 
     A rectifier AC to DC converter circuit 40 transforms the AC signal from the AC amplifier circuit to a proportional DC signal. The rectifier AC to DC converter circuit 40 is a voltage doubler circuit consisting of diodes D4 and D5, capacitors C8 and C9, and resistor R24. The output of a DC offset circuit 48 is combined with the output of the rectifier AC to DC converter circuit 40 to form a dual polarity drive voltage at terminal T2. The DC offset circuit consists of transistors Q6 and Q7, transformer XFMR2, diode D6, capacitor C10, and resistors R28, R29, and R30. A piezoelectric beam is connected to terminal T2 and is driven toward and away from a nozzle to create a desired back pressure. 
     A supply fluid pressure is supplied via fluid line 62 to a pneumatic booster 52. The back pressure created by the piezoceramic beam is connected to a control chamber via fluid line 58 in the pneumatic booster 52 that opens a pilot valve and allows the supply fluid to enter an output chamber of the pneumatic booster. There is a pressure drop across the pilot valve that allows different output pressures due to the amount of opening in the pilot valve. There is an orifice 56 between the output chamber and the control chamber to allow fluid to flow to the control chamber and nozzle 50, as well as, create a pressure drop between the output chamber and control chamber. When the piezoelectric beam moves away from the nozzle, the fluid in the control chamber is able to escape causing a reduction in the magnitude of pressure in the control chamber. The pressure reduction in the control chamber causes the fluid in the output chamber to escape through the exhaust port 54 causing a reduction in the output fluid pressure. When the piezoelectric beam moves toward the nozzle, the fluid in the control chamber escapes at a slower rate causing an increase in the magnitude of pressure in the control chamber. The pressure increase in the control chamber causes the pilot valve to open letting input pressure to flow into the output chamber causing an increase in the output fluid pressure. 
     Typical values for the components shown in the circuit diagram of FIG. 2 are as follows: 
     C1, C4, C7--4.7uf/35V 
     C2--0.01uf 
     C3, C5--0.33uf 
     C6, C8, C9, C10--0.0022uf 
     D1, D3--IN4737A 
     D2--ICL8069CMSQ 
     D4, D5, D6--IN4007 
     Q1, Q2, Q4, Q6--2N2222A 
     Q3, Q5, Q7--2N2907A. 
     R1--12.1K ohms 
     R2--107K ohms 
     R3--27.4K ohms 
     R4--5.23K ohms 
     R5--23.2K ohms 
     R6--221K ohms 
     R7, R8, R25, R26, R27--10K ohms 
     R9--750K ohms 
     R10, R16--499K ohms 
     R11, R18--1M ohms 
     R12--2M ohms 
     R14--3.01M ohms 
     R15--2.43M ohms 
     R17--56.2K ohms 
     R19, R20--1K ohms 
     R21--34.1K ohms 
     R22--294K ohms 
     R24--6.49M ohms 
     R28, R29--100K ohms 
     R30--4.02M ohms 
     SIP1, SIP2--100K ohms 
     U1--LP324N 
     U2--TLC27L2IP 
     U3--CD4047BE 
     U4--1210A 
     VR1, VR3--20K ohms pot 
     VR2--1M ohms pot 
     VR4--50K ohms pot 
     VR5--100K ohms pot 
     VR6--2M ohms pot 
     XFMR1--20:1 
     XFMR2--10:1 
     While the present invention has been disclosed in connection with a preferred embodiment thereof, it should be clearly understood that there may be other embodiments which fall within the spirit and scope of the invention as defined by the following claims.