Abstract:
An x to pressure converter that uses valves oscillating between open and closed positions in a pulse width modulated mode, to force the output pressure to equal the input X.

Description:
This application claims the benefit, for purposes of priority under 35 U.S.C. § 119(e), of U.S. Provisional Patent Application No. 60/317,838 filed Sep. 7, 2001. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a novel converter and, more particularly, to a system incorporating a current to pressure (I/P) converter to control a valve using pressure as a control signal. 
     2. Related Information 
     It is desired in the art of automation to control fluid flow in an automation system by actuation of a valve. The typical control device is a valve positioner. 
     The typical system  100  is shown in  FIG. 1 , causes an actuator to actuate a valve  104 . The control mechanism, a valve positioner  106 , inputs a current signal  108  and outputs a pressure to control the actuator. Typically, 4-20 mA input signal is input to the valve positioner. 
     Notably, the system in  FIG. 1  controls the valve by reading the position of the valve and adjusting the output pressure until the position matches the input signal. The position of the valve is sensed typically by a physical connection to the actuator. 
     Many valve positioners in the market require that their input signals be pressure. Therefore, there is a great need to develop a system, where pressure is the controlled parameter, not valve position. 
     OBJECTS &amp; SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a converter for converting an input signal into an output pressure 
     According to the foregoing objective, there is provided an x to pressure P converter that inputs a value x and comprises a valve system to control pressure P based on input value x such that the valve system has discrete open and closed positions, wherein no significant pressure variation is sensed in switching between positions. 
     While the x to P converter includes a valve system, it should be understood that this valve system is different than the valve which is controlled for, for example, controlling the flow of fluid in an automation system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a prior system; 
         FIG. 2  is a schematic diagram of the present invention; 
         FIG. 3  is a schematic diagram of the valve system present invention; 
         FIG. 4  is a block diagram of the valve system; 
         FIG. 5  is a block diagram of the control system; and 
         FIG. 6  is a block diagram of the processor control. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There is shown in  FIG. 2  the novel system  200  of the invention. As shown, an input current  202  is input to an I/P converter  204 . A pressure (P) is utilized as a feedback signal  206  which is fed back to the I/P converter. The pressure is coupled to a valve positioner  208  to control the position of the actuator  210 , which, in turn, actuates the position of the valve  212 . 
     It shall be appreciated that the present invention, unlike the prior systems, utilizes pressure as the control signal. By contrast, the prior systems utilize a mechanical or electronic signal representative of the position of the valve. 
     In order to generate the control pressure, the present invention converts an input current into a pressure. Notably, the system utilizes the pressure output as a feedback, as contrasted with other systems, which use an electric signal as feedback. 
     The difference is not trivial. As indicated, many valve positioners receive a pressure input. Thus, the present invention represents a serious advantage that it has more flexibility in the marketplace to, for example, work with both existing and future positioner systems. 
     A system that utilizes pressure as the control signal, maintaining a high quality output, is a real challenge. As will be explained in more detail, the present invention does this by employing a pulse-width modulation technique to discretely position a valve system in an open and closed state. Unlike throttling, which maintains the valve in a state in between open and closed, the present invention ensures that no significant pressure variation is sensed between the discrete open and closed positions. 
     An advantage of this arrangement is that it reduces the effects of vibration, which are negated by the discrete open and closed positions. In a throttling type system, by contrast, vibration effects have a significant impact on valves that are maintained in an in between state because the valves are not fixedly closed or open. 
     Now with respect to  FIG. 3 , there is shown the I-P converter  300  of the present invention. A pressure regulator  302  provides a regulated supply pressure to a valve system  304 . The valve system is arranged in a relationship such that the pressure output is maintained at the desired level. This is done, such that the valve system is either in a discrete open or closed state such that no significant variation of pressure is sensed between the states. 
     In the particular embodiment shown in  FIG. 3 , there is show, for example, a dual-valve system. Of course, the valve system may have any number of valves, and may even be singular, as long as the system is capable of satisfying the condition that the valves are positioned in discrete open and closed states. To continue, the dual-valve system includes a first valve  304   a  and a second valve  304   b , wherein the first valve receives supply pressure from the pressure regulator and the second valve exhausts the pressure through vent  304   c . A common passage connects the valves  304   a, b  to provide for the relationship of discrete open and closed states. The pressure is output at  306  and sensed by a pressure sensor  308 . A microprocessor  310  controls the valve system. 
     As mentioned the prior converters utilized a pressure valve that throttled, that is, varied the size of a pressure opening, to control pressure input. In other words, the old method always held the pressure valve open to a certain degree. The old pressure valve was never fully closed or fully open at any time. In the present invention, there is provided a novel valve arrangement for an I/P that forces the primary valve to discrete, fully open and fully closed positions. The secondary valve relieves pressure from the primary and allows, indeed assists, in changing the valves system between discrete states. 
     The invention of forcing the primary valve into the fully open or fully closed position is advantageous. The previous throttling method was subject to error arising from external vibration effects. Due to the fact that the prior valve was not in the extended position, the old valve concept was based on situating the opening to precise degrees. Obviously, external vibration translated to the valve and effected the opening of the old valve, thereby making it difficult, and introducing substantial error, to the resultant measured pressure in the previous method. 
     By contrast, the present invention alleviates the effect of vibration by ensuring that the primary valve is in a discrete open or closed state. This alleviates the vibration effect because the valve is physically supported by the structure of the valve. When the valve is fully closed for example, the leaf or diaphragm responsible for regulating pressure through the valve is biased against the structure of the valve itself. There is, thus, no possibility that vibration plays an effect on the valve. 
     Similarly, in the dual-valve system, when the first valve of the present invention is fully open, vibration has no chance to effect the valve opening status. In addition, the second valve relieves pressure in order to open the primary. In combination with the action of the primary, the dual valves act together to force the first valve leaf into the extended positions, thereby increasing the strength of the opening and closing of the valve and, further, reducing the effect of vibration. 
     In addition, another novel feature of the present invention is that it employs a pressure feedback system to boost the reliability of the pressure converted. Particularly in an input device such as the I/P converter, it is important that the input pressure created by the invention matches as precisely as possible the pressure indicated by the input current. This is not as critical in the prior technique, because the pressure produced is not the input pressure, but is the pressure that drives the actuator. The present invention creates an input pressure. Therefore, the pressure must be more accurate than ever before. As mentioned, the present invention employs the pressure as the feedback signal. 
     The microprocessor may also control the valve system to include dither. By controlling the timing of the supply and exhaust valves, small high frequency pressure variations can be created. These variations will not upset the state of the output system, but they introduce a margin of movement that reduces the error in mechanical systems due to inertial effects of causing a stationary object, to move. This mechanical error may take several forms such as dead band, the delay in moving the valve system toward the other state due to inertia. Also, backlash may occur, which is a violent change from one state to the other. Hysteresis may form, which is a difference in performance between one change of direction in comparison to the other. Also, static friction may occur, which is the sticking of the tendency of the valve to stick in one state. 
     In addition, the valve system may improve efficiency as follows. The valves appear electronically as capacitors. Most prior systems simply reverse the polarity of the valve. This prior practice is horribly inefficient because current is required to first reverse bias the valve and additional, requiring additional power, even before the valve can be switched. In the prior systems, this was normal because power was not a consideration. However, in the applications in which the present invention may be applied, power conservation would translate into a market advantage. 
     In the present invention, the microprocessor may drive the valve by pre-discharging the valve by shorting the effective terminals of the capacitor and the valve itself dissipates the excess current through the short circuit. In this manner, the system reduces the power consumption, allows a higher switching rate and reduces dead time. In addition, the system effectively decreases the size of the power supply, thereby reducing the cost of the system. 
       FIG. 4  illustrates a block diagram of the system  300  ( 400 ) shown in  FIG. 3 . As shown, the input current  402  is input to the microprocessor  404 . The microprocessor generates a control signal thereon. Level shifters  406   a ,  406   b  may be provided to provide the correct signal level to the valves  408   a  and  408   b  which control the pressure as previously described. An output pressure  410  is sensed by a pressure sensor  412  and fed back to the microprocessor. A temperature input  414  which inputs a temperature and a calibration control input  416  may be provided to the microprocessor for adjusting the microprocessor signals according to the temperature or the calibration, respectively. 
       FIG. 5  illustrates a block diagram of the control system  500  of the invention. There is shown an input voltage  502  that is supplied to the summer. A gain  506  inputs the result of the summer and outputs the gain to the pulse width modulators  508   a  and  508   b , respectively. Valves  510   a  and  510   b  receive the pulsed signals, which respectively receive high pressure  512   a  and low pressure  512   b . The output pressure  514  is sensed by pressure sensor  516  and subtracted by the negative terminal of summer  504  to provide the feedback to the system. 
       FIG. 6  illustrates a block diagram for a software application  600  An input current  602  is input to a summer  604 . Based on the output from the summer, a gain  606  controls the gain of a control signal that is sent to the pulse width modulators  608   a, b . A temperature input  610  inputs the temperature to a temperature controller  612  and a pressure sensor  614  inputs the pressure sensed, via a linear compensation unit  616 . The temperature and/or pressure are input to the negative terminal of the summer  604  which adjusts the result sent to the gain accordingly. 
     It shall be appreciated that the invention encompasses any number of embodiments that meet the objective of the invention so described. Although the invention has been described as a dual-valve, for example, it is within the scope of the invention to provide any number of valves in the system.