ELECTRO-PNEUMATIC TRANSDUCER ASSEMBLIES

Abstract: A pnemuatic servo assembly (20) is provided for an electro-pneumatic control system (10) having closed loop control of a DC motor (12) operated from an error signal (56) developed from an electrical set point signal (40) and a pneumatic feedback signal (44). The feedback signal (44) is made compatible by a pressure transducer (26) monitoring a pressure output signal (22) of the pneumatic servo assembly (20) and establishing an electrical signal indicative thereof which is scaled, zeroed, and amplified by an amplifier unit (46).

Description: PNEUMATIC SERVO ARRANGEMENTS AND ASSEMBLIES FOR ELECTRO-PNEUMATIC CONTROL SYSTEMS This invention relates to pneumatic servo arrangements and assemblies for electro-pneumatic control systems and, more particularly, but not exclusively, to the control of electro-pneumatic converters. Control systems for electro-pneumatic converters are known. Usually, a 4 to 20 mA electrical signal is used to actuate a solenoid-like motor. The 4 to 20 mA electrical signal causes a proportionate displacement in a spring-loaded core of the solenoid-like motor, which displacement is used to control a restriction of an associated pneumatic valve producing a pressure change proportional to the motion of the core. An example of such a device may be found in U.S. Patent No. 3 334 642 (Borthwick). There are certain disadvantages to pneumatic servo assemblies of such electro-pneumatic control systems. Firstly, they are unable to hold positions on loss of power. Should power be removed from the coil, the core moves back to a position where it is in equilibrium with its associated spring. This causes the pneumatic output signal to go off scale, resulting in the movement of control devices actuated by the electro-pneumatic system to either fully-opened or fully-closed positions, which may be catastrophic under certain circumstances. Secondly, such pneumatic servo assemblies are vibration sensitive. Since the cores are suspended from springs which act as range and zero limiters, vibration of the core causes a variation in the pneumatic output signal. Also, there usually is no feedback signal of the pneumatic output signal to the input of the control systems. According to the present invention there is provided a pneumatic servo arrangement for an electro-pneumatic closed loop control system providing a pneumatic output signal in response to a corresponding input signal, the servo arrangement being characterised by: variable restriction means for producing different pneumatic output signals from the pneumatic servo arrangement; motor means for moving the variable restriction means to vary the pneumatic output from the pneumatic servo arrangement; means for actuating the motor means in response to a control signal; means for establishing a set point signal in response to the electrical input signal indicative of a desired pneumatic output signal; means for establishing a feedback signal indicative of the pneumatic output signal; and combining means for comparing the set point signal with the feedback signal to establish a control signal for the actuating means. The invention also provides a pneumatic servo assembly for an electro-pneumatic control system, the servo assembly being characterised by: a pneumatic backpressure nozzle for providing a pneumatic output signal in rsponse to a movable input restriction blocking the nozzle; electric motor means for moving the movable restriction with respect to the nozzle; and a bellows assembly connected to the nozzle to expand and contract in response to the pressure from the nozzle. A preferred embodiment of the present invention described hereinbelow overcomes or at least alleviates the above-mentioned problems of known electro-pneumatic control systems by providing a pneumatic servo assembly for such systems which is dependent upon an electrically-driven direct current (DC) motor to provide a variable restriction to a pneumatic nozzle, thereby providing a fail-safe device which will maintain the last electrical signal to a control system incorporating the pneumatic servo assembly upon a loss of electrical power since the motor will stop in its last driven position. The preferred pneumatic servo assembly utilises a DC motor-driven cam member to provide a variable restriction to a pneumatic backpressure nozzle thus allowing the nozzle to supply a spring-loaded bellows assembly which produces a 20.7 to 103.4 kPa (3 to 15 Ibf/in or "psi") pneumatic output signal also providing a feedback signal to the electrical input signal. The feedback signal is used, in a pneumatic servo arrangement incorporating the pneumatic servo assembly, to produce an error signal between a set point signal determined by a 4 to 20 mA electrical input signal and the feedback signal of the pneumatic output as sensed by a pressure transducer changing this pneumatic feedback signal to an electrically-equivalent signal. Advantages of the preferred pneumatic servo assembly for an electro-pneumatic control system are that it will maintain the last pneumatic output upon a loss of electric power, that it is insensitive to vibration of the pneumatic assembly, and that it provides a feedback signal to an electronic part of the control system producing an error signal for driving the restriction of the pneumatic assembly. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a functional block diagram of an electro-pneumatic control system; Figure la is an expanded view of a motor-driven cam and backpressure nozzle of a pneumatic servo assembly of the system of Figure 1; Figure 2 is a functional block diagram of a pneumatic servo arrangement of the control system of Figure 1; Figure 3 is a detailed side view of a bellows and spring assembly of Figure 2; Figure 4 is a detailed end view of the bellows and spring assembly of Figure 3; and Figure 5 is a detailed end view of a cam assembly of Figure 2. Referring now to the drawings, Figures 1 and la show an electropneumatic control system 10 wherein a DC motor 12 is controlled by an electronically-controlled motor servo circuit or assembly 14 which is powered by a power supply 16 operated from a 4 to 20 mA input control signal connected to the power supply 16 along a line 18. The DC motor 12 is mechanically constrained to a pneumatic servo assembly or mechanism 20 which has a backpressure nozzle 21 variably restricted by a cam assembly 23 connected to and driven by the DC motor 12 to thus provide a variable backpressure output along an output line 22 of the pneumatic servo assembly 2 20, normally in a 20.7 to 103.4 kPa (3 to 15 lbf/in ) output range. This 20.7 to 103.4 kPa (3 to 15 lbf/in2) output is linear and corresponds to the linear 4 to 20 mA electrical input provided along the input line 18. The same 20.7 to 2 103.4 kPa (3 to 15 lbf/in ) output is also sent along a line 24 to a pressure transducer 26 which provides a feedback signal used in determining control of the DC motor 12 as will be described more fully hereinbelow. To allow the DC motor 12 to be operated bi-directionally without the need for dual polarity voltages, the power supply 16 establishes dual voltages V+ and Vref along power lines 28 and 30, respectively. The V+ voltage is in the range of 6.4 volts nominal and powers the motor servo circuit 14, as well as powering a desired position amplifier 38 and an amplifier circuit 46, along input lines 31 and 33, respectively. The Vref voltage from the power supply 16 is transmitted along the line 30 to bias up the motor servo circuitry 14 and power the pressure transducer 26 along a line 32. To establish a set point from the 4 to 20 mA input signal to which the 2 appropriate 20.7 to 103.4 kPa (3 to 15 lbf/in2) output will have to be supplied from the output line 22, the particular electrical input signal is sent along a line 34 to a 10 ohm precision resistor located between the circuit common at a line 36 and an input to the desired position amplifier 38. The precision 10 ohm resistor senses the particular current level and establishes a voltage drop across itself. That voltage drop provides a counterpart voltage input to the position amplifier 38. The position amplifier 38 raises the input signal level to a predetermined level and sends it along a line 40 to a difference amplifier 42 as a set point signal compatible with the level of a feedback signal also provided to the difference amplifier 42. The difference amplifier 42 is the first stage of the motor servo circuit 14. The second input to the difference amplifier 42 is the feedback signal, which is provided on a line 44 from the amplifier circuit 46 which scales and zeroes the pressure signal provided by the pressure transducer 26 which acts as a pneumatic-to-electric converter for the 20.7 to 103.4 kPa (3 to 15 lbf/in ) output signal established at the output line 22. The difference amplifier 42 senses any deviation of the feedback signal on the line 44 from the established set point signal on the line 40 and establishes an error signal on a line 48, which error signal is an amplified difference signal so long as such difference between the set point signal and feedback signal is maintained. This amplified error signal is supplied to an input of a proportional and integral (P & I) controIler 50 where it is integrated and scaled up or down with respect to Vref Thus, bi-directional rotation of the motor 12 is achieved by the voltage output of the proportional plus integral controller 50 rising or falling below the voltage reference Vref When the output signal is above the voltage reference Vref, the current through the motor 12 will drive the motor 12 in a first rotational direction. When the voltage output of the proportional plus integral controller 50 is equal to Vref, no current flows through the motor and the motor is stationary. If the output voltage drops below Vref, the rotation of the motor 12 will reverse to a second rotational direction due to the voltage level applied to it crossing the V Vref point. The amplifier circuit 46 has both a zero adjustment 60 and a span adjustment 62. The zero and span adjustments 60 and 62 allow the feedback signal to be adjusted to respond over a variety of ranges. The predominant pressure range and pressure starting or zero point that the feedback will be adjusted for is the 20.7 to 103.4 kPa (3 to 15 lbf/in2) signal, which is a standard for pneumatic instrumentation as 4 to 20 mA is a standard for electrical instrumentation. Other ranges are also available and may be set, including any 50 per cent split range desired (i.e. 0 per cent is 20.7 kPa (3 lbf/in2) and 100 per cent is 62.1 kPa (9 lbf/in2 . Should an electrical failure occur in the system 10, the DC motor 12 would stop with the cam assembly 23 remaining in its last position to provide the same backpressure restriction from the nozzle 21 to the pneumatic servo assembly 20, and the last conforming pressure output signal would be maintained along the output line 22 by the pneumatic servo assembly 20. Referring now to Figure 2, the pneumatic servo assembly or mechanism 20 is seen to include a regulator 64 which is connected to an air supply of unregulated high pressure air and acts to reduce the air supply pressure to a constant clean, low pressure of 151.7 + 13.8 kPa (22 + 2 Ibf/in ). The filtered regulated air from the regulator 64 is piped to the backpressure nozzle 21 by way of an orifice 66. As is known to those skilled in the pneumatic control art, the size of the orifice 66, in conjunction with the opening of the backpressure nozzle 21, act to determine the air consumption as well as response time of the pneumatic servo assembly or mechanism 20. As was previously described, the motor servo circuit 14 causes the DC motor 12 to be rotated in either a clockwise or anticlockwise direction, which direction is dictated by a comparison of the set point and the feedback signals supplied to the control circuit 14 which acts to thus control the DC motor 12. The rotation of the motor 12 causes the cam assembly 23, which may best be seen from Figure 5, to rotate with respect to the backpressure nozzle 21, causing a relative blockage or opening in the backpressure nozzle 21. With particular reference to Figure 5, it will be seen that the cam 2 assembly 23, shown as the typical 20.7 to 103.4 kPa (3 to 15 lbflin ) output cam assembly 23, is formed as a spiral-generated plane or plane section 68 having a notched portion 70 with a hub 72 located in the centre of the spiral plane 68. The spiral is formed to produce a linear function pressure output from the output line 22 from the backpressure nozzle 21. By way of example, when the nozzle 21, which will always seek the edge of the plane 2 68 aligned therewith, is positioned at a point A, a 20.7 kPa (3 Ibf/in ) output will be produced. Similarly, at a point B, a 103.4 kPa (15 Ibflin2) output will be produced. Angularly linear outputs will be produced between the points A and B. Thus, the height of the notch 70 is the range of the output signal. The hub 72 is used to mount the cam assembly 23 to the shaft of the DC motor 12. Referring now to Figures 2 to 4, it will be seen that restricting the backpressure nozzle 21 causes an increase in backpressure which is piped to a bellows spring assembly 74 via a line 76. There is a directly-proportional relationship between the pressure in the bellows spring assembly 74 and the height to which it will expand. This is determined by the construction of a bellows 78 of the assembly 74, as well as a spring 80 which is mounted in parallel with the bellows 78. The spring 80 acts to limit the motion of the bellows 78, thereby limiting the output range of the pneumatic output signal along the line 22 to a desired range which is determinable by adjusting the spring pressure of the spring 80 by either extending or loosening the spring 80 and setting it in that particular position by way of adjusting nuts 82. Thus, the nuts 82 may be used to provide fine adjustment to the particular output pressure range desired. Should a different pressure range be desired, 2 such as 20.7 to 186.2 kPa (3 to 27 lbf/in ), the spring 80 may be replaced by a different spring having a different spring coefficient. With particular reference to Figures 3 and 4, it will be seen that the backpressure nozzle 21 is rigidly mounted to a bracket assembly 84 to which the bellows 78 and the spring 80 are also mounted. The bracket assembly 84 is then mounted to a stationary frame member 85 through a hinge 88 to thus allow rotational motion of the backpressure nozzle 21 and the bellows 78 and spring 80 around a pivot point constituted by the hinge 88. This mounting of the backpressure nozzle 21, bellows 78, and spring 80 as a single unit makes the pneumatic servo assembly 20 impervious to vibration induced errors by allowing the entire assembly to move as a single unit in response to any vibration induced by external sources into the pneumatic servo assembly 20. In operation, it will be seen that as the motor 12 and the cam assembly 23 rotate to variably restrict the backpressure nozzle 21, the bellows 78 will expand causing a pivoting of the previously-mentioned assembly around the pivot point 88 until the backpressure nozzle 21 reaches a position along the edge of the cam plane 68 producing a backpressure feedback signal which will balance the set point signal and stop the rotation of the motor 12 and cam assembly 23. This will result in an output pressure signal along the line 22 which is proportional to the electrical input signal which originally had caused the DC motor 12 to rotate. As may be seen, there is no mechanical contact between the backpressure nozzle 21 and the cam assembly 23. Therefore, to rotate, the DC motor 12 need only overcome its own internal friction along with a small amount of drag on the cam assembly 23 which may be caused by a nozzle clamp 86. The nozzle clamp 86 rides on the cam assembly 23, loading it away from the motor 12 to thus take up any end play in the shaft of the motor 12. This force is minimal and there are no forces developed which will tum the motor 12 off. If the motor 12 and the cam assembly 23 do not turn clearly the backpressure output along the line 22 will not change.

Claims: CLAIMS 1. A pneumatic servo assembly for an electro-pneumatic control system, the servo assembly (20) being characterised by: a pneumatic backpressure nozzle (21) for providing a pneumatic output signal in rsponse to a movable input restriction blocking the nozzle; electric motor means (21) for moving the movable restriction with respect to the nozzle (21); and a bellows assembly (74) connected to the nozzle (21) to expand and contract in response to the pressure from the nozzle. 2. A pneumatic servo assembly according to claim 1, wherein the bellows assembly (74) includes a bellows (78) and an adjustable spring (80) connected in parallel with the bellows to adjust the output from the bellows assembly. 3. A pneumatic servo assembly according to claim 2, wherein the backpressure nozzle (21) is rigidly attached to the bellows assembly (74) to move simultaneously with the bellows assembly. 4. A pneumatic servo assembly according to claim 3, wherein the backpressure nozzle (21) and bellows assembly (74) are attached as a unit to a single pivot point (88) allowing the rotation of both as a unit around the pivot point in response to bellows expansion or contraction. 5. A pneumatic servo assembly according to any one of claims 1 to 4, wherein the motor means is a DC motor (12) and a cam assembly (23) is connected to the motor for variably restricting the pneumatic servo assembly (20) depending upon the position of the cam assembly. 6. A pneumatic servo assembly according to claim 5, wherein the cam assembly (23) comprises a flat, substantially spiral-generated plane (68) having a spiral generating centre mounted hub (72) for mounting the cam assembly to the motor means. 7. A pneumatic servo arrangement for an electro-pneumatic closed loop control system providing a pneumatic output signal in response to a corresponding input signal, the servo arrangement being characterised by: variable restriction means for producing different pneumatic output signals from the pneumatic servo arrangement; motor means for moving the variable restriction means to vary the pneumatic output from the pneumatic servo arrangement; means for actuating the motor means in response to a control signal; means for establishing a set point signal in response to the electrical input signal indicative of a desired pneumatic output signal; means for establishing a feedback signal indicative of the pneumatic output signal; and combining means for comparing the set point signal with the feedback signal to establish a control signal for the actuating means. 8. A pneumatic servo arrangement according to claim 7, wherein the feedback establishing means comprises a pressure transducer (26) operative to measure the pneumatic output from the pneumatic servo arrangement and establish an electrical signal representative thereof. 9. A pneumatic servo arrangement according to claim 8, wherein the feedback establishing means includes an amplifier circuit (46) having a span and zero adjustment (62, 60) to allow the pneumatic output from the pneumatic servo arrangement to be scaled to a desired range of output pressures from a desired starting pressure. 10. A pneumatic servo arrangement according to claim 7, claim 8 or claim 9, wherein the motor means is a DC motor (12) and a cam assembly (23) is connected to the motor for variably restricting the pneumatic servo arrangement depending upon the position of the cam assembly. 11. A pneumatic servo arrangement according to claim 10, including a backpressure nozzle (21) mounted proximately to the cam assembly (23) to be variably restricted by the cam assembly. 12. A pneumatic servo arrangement according to claim 11, wherein the cam assembly (23) comprises a flat, substantially spiral-generated plane (68) having a generating centre mounted hub (72) for mounting the cam assembly (23) to the motor means. 13. A pneumatic servo arrangement according to claim 12, wherein the backpressure nozzle (21) is connected to a bellows assembly (74) to pressurise the bellows assembly in response to the pressure from the b ackpressure nozzle. 14. A pneumatic servo arrangement according to claim 13, wherein the bellows assembly (74) has an output connected to the feedback signal establishing means. 15. A pneumatic servo arrangement according to claim 14, wherein the bellows assembly (74) includes a bellows (78) and an adjustable spring (80) connected in parallel with the bellows to adjust the output from the bellows assembly. 16. A pneumatic servo arrangement according to claim 15, wherein the backpressure nozzle (21) is rigidly attached to the bellows assembly (74) to move simultaneously with the bellows assembly. 17. A pneumatic servo assembly according to claim 16, wherein the backpressure nozzle (21) and bellows assembly (74) are attached as a unit to a single pivot point (88) allowing the rotation of both as a unit around the pivot point in response to bellows expansion or contraction.

Inventors: BARNES, CHRISTINE BROBST
Applicants: THE BABCOCK & WILCOX COMPANY
Application year: 1984
Main classification: F15C3
IPC codes: F15B5, F15C3, G05D16, F15B1, F15B3