Abstract:
A flow control valve satisfies the operational requirements of a cooling machine for hot metal strips or plates. The valve is operated by a pneumatic actuator which is controlled by use of a high performance pneumatic proportional valve in conjunction with an electronic controller receiving reference and feedback signals.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention is concerned with the accurate and fast control of fluid flow by a valve. It is particularly useful for the control of water flow during cooling of hot metal strips or plates but finds application in other areas where accurate and fast control of fluid flow is required. 
         [0003]    2. Description of the Prior Art 
         [0004]    A very common type of valve that is used in industry is the butterfly type valve illustrated in  FIG. 1 . The body of the valve  1  is clamped between flanges in a pipe. A rotatable barrier, known as a butterfly,  2  is rotated by the shaft  3  to control the flow through the valve and hence the pipe. 
         [0005]    The most common method of actuating the shaft  3  is using pneumatics. A typical installation is illustrated in  FIG. 2 . The shaft of the butterfly valve is connected to the pneumatic actuator  4  by a coupling  5 . 
         [0006]    There are many different types of pneumatic actuator available in the market but a typical design is illustrated in  FIG. 3 . The pistons  6  and  7  are moved by air pressure in chambers  8 ,  9  and  10 . If the force on the pistons due the air pressure in chambers  9  and  10  is greater than the force on the pistons due to the air pressure in chamber  8  then the pistons tend to move towards the closed position and vice versa. Movement of the pistons is transferred to the shaft which operates the valve by the racks  11  and  12  and pinion  13  (teeth on pinion omitted for clarity). 
         [0007]    If the valve is only going to be used for simply on-off control of the flow then the air pressure to the actuator chambers  8 ,  9  and  10  can be switched using, for example, solenoid operated pneumatic valves. 
         [0008]    However, in order to use the valve for flow control it is necessary to control the position of the butterfly. The most common method of controlling the position of the butterfly is using an electro-pneumatic device known as a positioner. There are many different designs of positioner available in the market but a typical design is illustrated in  FIGS. 4 and 5 .  FIG. 4  shows how the positioner  14  is usually mounted on the actuator  4 . 
         [0009]      FIG. 5  illustrates a typical internal arrangement of a positioner. The unit contains an I/P convertor (i.e. current to pressure) convertor (not shown). This takes an electrical current input signal—typically 4-20 mA—and produces an air pressure which is proportional to the electrical current signal. The air pressure from the I/P convertor acts upon a diaphragm  15 . This is the control force. On the other side of the diaphragm is a feedback spring  16  and balance arm  17  which is attached to spool  18 . An increase in air pressure from the I/P convertor causes the diaphragm  15  to move down against the feedback spring  16 . This moves the balance arm  17  and hence the spool  18 . The movement of the spool opens ports in such a way that air is allowed to flow into one side of the actuator and out of the other side of the actuator. 
         [0010]    The spindle  19  and cam  20  provide a feedback mechanism. The spindle  19  is connected to an extension of the actuator shaft so that when the actuator moves the spindle moves as well. Rotation of the spindle causes the cam  20  to rotate which in turn moves the lower arm  21  up which increases the force onto the diaphragm from the feedback spring. The lower arm  21  is biased against cam  20  by spring  31  or other suitable biasing means. When the feedback force balances the control force then the diaphragm and balance arm move back to the zero position in which the spool is closed and the movement of the actuator stops. Different cams  20  can be used to give different characteristic curves for the position of the valve versus the reference signal. 
         [0011]    Whilst the detailed design of positioners varies between different suppliers the general principles are similar in that they use mechanical-pneumatic force amplification to operate the main spool  18  and they use mechanical feedback of the valve position. 
         [0012]    For many purposes the prior art designs described above are satisfactory but numerous problems include:
       The response time of the valve tends to be very non-linear; it is generally OK on large movements but very poor for very small movements.   The response time tends to be different for opening and closing movements of the valve.   The position control of the valve tends to have significant hysteresis because of the mechanical feedback arrangement and the poor response for small movements.   The zero and span adjustments of the positioner are time consuming and they tend to drift over time.       
 
         [0017]    Another problem with the prior art designs is that the requirements for speed of response and for accuracy conflict with each other. To achieve a fast response from the valve a small actuator (i.e. an actuator with relatively small diameter pistons) has to be used so that a given air flow from the positioner produces fast movements of the valve. Unfortunately the use of a small actuator also means that only a limited actuator force is available and it is difficult to achieve accurate movements. Conversely, to achieve accurate positioning of the valve, a large actuator can be used so that the air flow from the positioner only results in small movements of the valve. 
         [0018]    In the prior art there are many example of positioners which have electronic position feedback units built into them or available as an add-on. However these electronic position feedback units are not used for closed loop position control of the valve; they are generally used for monitoring purposes only. 
         [0019]    In the prior art the only solutions to these problems with pneumatically actuated and positioned valves have been to use either electro-mechanical actuation or hydraulic actuation. 
         [0020]    Electro-mechanical actuation using solenoids or stepper motors or similar techniques can achieve good performance on small valves but in general it is not possible or would be very expensive on large valves because the forces that can be generated by reasonably sized solenoids or stepper motors are small compared with the forces that can be generated by pneumatic actuators. 
         [0021]    Hydraulic actuation of valves can achieve very fast response and high accuracy because the hydraulic fluid is much less compressible than air. However the cost of a hydraulic pump set plus hydraulic control valves and actuators is significantly higher than for pneumatics. 
         [0022]    High performance pneumatic proportional valves are known in the art—for example the Norgren VP60, described at norgren.com/virtualpresscentre/pressreleases/itemdetail.asp?ItemID=239 
         [0023]    These valves produce an air flow which is roughly proportional to the input electrical signal. Typically, they are used for rotational speed control, velocity control etc. where the fact that the air flow is proportional to the electrical signal means that the speed of the pneumatic motor or actuator will also be roughly proportional to the electrical signal. 
       SUMMARY OF THE INVENTION 
       [0024]    An object of the invention is to provide a flow control valve offering advantages over the prior art in terms of inter alia speed of response and accuracy. 
         [0025]    The above object is achieved in accordance with the present invention by a flow control valve that has a housing defining a fluid conduit, a barrier movable between a first position offering maximum impedance to fluid flowing in the conduit and a second position offering minimum impedance to fluid flowing in the conduit, a pneumatic actuator that is arranged and operable to drive the barrier to positions intermediate of and including the first and second positions via a shaft, a pneumatic proportional valve that provides an air flow to the pneumatic actuator, the air flow being proportional to an input electrical signal and being switchable between at least two flow paths, a feedback generator that generates and emits a feedback signal indicative of the position of the barrier, and a controller that receives the feedback signal, and a reference signal dependent on a required barrier position, and that provides a control signal to the pneumatic proportional valve, as the input electrical signal. 
         [0026]    In a preferred embodiment the feedback generator includes (or is) a position measurement device that measures the signal of the barrier position and generates an electrical signal indicative thereof. 
         [0027]    In another preferred embodiment, the position measuring device is attached to the barrier side of a coupling between the pneumatic actuator and the shaft. 
         [0028]    The controller may advantageously arranged to provide a control signal which is a non-linear function of at least one of the feedback signal, the reference signal and the difference between said control signal and said reference signal. 
         [0029]    The valve may be a rotary valve, wherein the barrier is rotatable between the first and second position and wherein the feedback generator generates a feedback signal indicative of the angular position of the barrier. 
         [0030]    The invention is especially useful for the control of water for the cooling of hot metal strips or plates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  illustrates a ‘butterfly’ type flow control valve according to the prior art. 
           [0032]      FIG. 2  represents an installation of a flow control valve including its pneumatic actuator and coupling. 
           [0033]      FIG. 3  illustrates the mechanism of a typical pneumatic actuator according to the prior art. 
           [0034]      FIGS. 4 and 5  illustrate a typical positioner device used to control the position of the butterfly. 
           [0035]      FIG. 6  illustrates a flow control valve according to the invention. 
           [0036]      FIG. 7  illustrates some output characteristics of a controller used in a flow control valve according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]    The invention is described herein with reference to the ‘butterfly’ type valve that is well known in the art. Nevertheless, it will be apparent to those skilled in the art that the invention is applicable to other types of valve, for example a ‘gate’ type valve, also well known in the art, or other types of valve which operate by movement of a barrier between positions in and out of a fluid path. 
         [0038]    Referring to  FIG. 6 , the valve  1  is operated by a conventional pneumatic actuator  4  which is mechanically connected to the valve shaft either directly or by the coupling  5 . The mounting bracket for the actuator is omitted for clarity. 
         [0039]    The pneumatic actuator  4  is connected by pneumatic pipes to a pneumatic proportional type valve  22 . The details of this valve will vary between manufacturers and model types but the general characteristic is that a positive control signal  23  causes the valve spool to move such that air flows from P to A while at the same time air is allowed to flow from B to the exhaust ports. The control signal  23  is typically derived from an error signal, which in turn is derived from the difference between a reference signal and a feedback signal. The feedback signal is representative of the barrier position. A negative control signal  23  causes air to flow from P to B while at the same time allowing air from A to flow to the exhaust ports. Furthermore the opening of the valve is roughly proportional to the control signal  23  so that the larger the magnitude of the signal the greater the air flow. 
         [0040]    An angular position measurement device such as an encoder is also attached to the valve as shown at  24 . In an alternative arrangement the angular position measurement device could be mounted on an extension of the actuator shaft as shown at  25  or it could be mounted between the valve and the actuator as shown at  25   a . Positions  24  and  25   a  have the advantage that any clearances in the coupling  5  or other connection between the valve and the actuator do not affect the angular position measurement. 
         [0041]    The angular position measurement feedback is connected to the controller  26 . In the controller the reference position is compared with the feedback position and a control signal  23  is generated to move the valve  22  to remove any error between the reference and the feedback position. 
         [0042]    A significant advantage of the invention is that the controller  26  is electronic—typically a PLC or other micro-processor based controller—and therefore it can contain non-linear functions to improve the performance of the valve. A simplified diagram is shown in  FIG. 7 . This simplified version of the controller includes a proportional gain  27  which is modified by the non-linear function  28  which is a function, in this example, of the sign and the magnitude of the error signal. Using this one can have, for example, much higher gain for small errors (discrepancies between the required and actual valve setting) and lower gain for large errors and also, for example, have different gains for positive and negative errors. This example controller also contains a damping term  29  which is a function of the rate of change of the feedback signal. This damping term  29  is also modified by a non-linear function  30  which is a function, in this example, of the size and sign of the feedback signal. 
         [0043]    In summary, the invention uses electronic feedback of the valve position together with an electronic controller and a pneumatic proportional type flow control valve to control the position of a butterfly type flow control valve. The use of non-linear terms within the controller to improve the performance of the valve—in particular the response for different size movements and for different directions of movement. The mounting of the electronic feedback unit for the valve position at positions  24  or  25   a  so that any clearances in the coupling between the valve and the actuator do not effect the feedback position measurement. 
         [0044]    The advantages offered include more consistent response times for large and small movements and for movements in opposite directions; reduced hysteresis; easier set-up (the zero and span adjustments are made electronically) and no drift of the zero or span. 
         [0045]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.