The present invention relates to a true position sensor for diaphragm valves and, more particularly, to the inclusion of a conductive member in parallel with the diaphragm to determine the true position of the diaphragm within the valve assembly.
Diaphragm valves are used extensively within the process industry and particularly within the biopharmaceutical process industry to control fluid flow for high purity water, steam and other process fluids. The diaphragm valve is the valve of choice since the design ensures minimal areas for trapped fluid, is freely drainable, and is easily cleaned. The typical design comprises a stainless steel valve body with two wells for the fluid inlet and outlet, similar to a double bowl kitchen sink. The diaphragm is typically a flat elastomer plate that seals against the top edge of the wall between the two wells when the valve is closed, and is lifted away from the top edge of the wall when the valve is open, so as to allow fluid to flow between the wells. An actuator, typically a compressed air operated piston, moves the diaphragm away from the valve body to open the valve, and a spring mechanism forces the diaphragm back against the valve body when the opening force is removed. The diaphragm is typically connected to the actuator with a threaded stud that screws into one end of a rod that is moved by the air-operated piston. The other end of the actuator rod protrudes out the top of the actuator when the valve is open, and retracts into the actuator when the valve is closed, to be used as an external visual indication of the state of the valve. A limit switch assembly is often attached to the top of the actuator and uses a cam rod that threads into the top of the actuator rod and moves with the actuator to activate switches that provide control systems with an electrical indication of the valve position.
With the critical importance of consistent control over, for example, pharmaceutical manufacturing, many of the diaphragm valves used are computer controlled and monitored. The typical method of monitoring the position of a diaphragm valve is using the limit switch assembly described above. However, a proven problem with the limit switch assemblies is that they indicate the position of the actuator rod, and not necessarily the true position of the diaphragm. Since the diaphragm is connected to the actuator rod by a threaded stud or other suitable mechanical attachment, the diaphragm can be mis-assembled or become separated from the actuator rod with no visible indication of any problem. In this case, the computer control system can command the valve to open and receive feedback from the limit switch that the valve is open, when in fact the diaphragm is still in the closed position.
Another problem with prior art limit switch indicators is that they must periodically be adjusted to correctly indicate the actuator position. This is both an extra step during the manufacturing process, as well as a continuing maintenance issue. Moreover, state-of-the-art limit switch assemblies have no way of indicating the presences of fluid within the valve body.
Thus, a variety of needs remain in the art for a diaphragm valve arrangement that can more accurately indicate the proper position and status of the diaphragm within the valve assembly.
The need remaining in the prior art is addressed by the present invention, which relates to a true position sensor for diaphragm valves and, more particularly, to the inclusion of a conductive member in parallel with the diaphragm, where a sensing circuit is coupled to the conductive member and utilizes changes in capacitive readings associated with the conductive member to determine the true position of the diaphragm within the valve assembly.
In accordance with the present invention, a thin conductive plate, electrically insulated from its surroundings and with dimensions similar to the diaphragm itself, is placed parallel to the diaphragm and disposed between the diaphragm and the actuator. The thin conductive plate and the actuator body are connected to a circuit that senses both the capacitance and resistance between the plate and the valve body, and uses the change in these electrical quantities as the valve is opened and closed to determine the true position of the valve.
An advantage of the true position sensor of the present invention is that if a fluid is in contact with the diaphragm when the diaphragm is in the open position, the capacitance and resistance will change relative to the open position with no fluid, thus providing an indication that fluid is present in the valve, as well as the type of fluid that is present.
Other and further advantages and embodiments of the present invention will become obvious during the course of the following discussion and by reference to the accompanying drawings.