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Editor's Note: In the process industry, valves and actuators are used for the control and transfer of liquid flow. More and more plant engineers hope to get the feedback signals of the current positions of valves, faucets and actuators in the control system. Usually this signal is obtained by a proximity sensor built into the switch box or a dual sensor mounted directly on the drive or manual valve. This article explores in depth the types of sensors used in the new trends at the site and in the market. Standardization industry standards for the development of technology pros and cons of co-existence. On the one hand, it limits creativity, but on the other hand they ensure the compatibility of devices from different manufacturers. VDE / VDI3845 standard requires a lot of things, the provisions of the pneumatic actuator and connected equipment between the important mechanical connection. The standard includes the definition of mounting holes: 30 × 80mm and 30 × 130mm. The height of the mounting bar is 20, 30 and 50mm. There is no fixed standard for the diameter of the mounting rod, so the user must pay particular attention to the dual sensor application. Switch boxes "Switch boxes" usually describe the integration of sensors, cams and connectors in a closed box. The mechanical connection is achieved by sensors and pneumatic actuators. The solution is designed to avoid sensor exposure or exposure in the surrounding environment while using standard sensors (see above). In these standard switch boxes, the following sensors are typically built in: Cylindrical sensors, typically 40 mm in length; Channel sensors; Rectangular V3 style sensors (like microswitches); Dual sensors in a compact package housed in a box; Printed circuits Board solution. Switch boxes are used as terminal equipment to connect valve position sensors and solenoid valves. The coded connector and the necessary LEDs are already on the circuit board. The size of the board fits most standard boxes. Open Solution "Open Solution" provides for dual sensor mounting on automatic or manual valves. Two inductive sensors are stacked in a single housing and show exactly where the valve is to be opened and closed. The system is designed with the VDE / VDI 3845 standard in mind. These sensors are triggered by two stainless steel targets mounted 90 ° vertically on the "disc". The disk itself is mounted on the actuator by a screw. In addition to its compact size and ease of assembly, we have also found the following advantages: Round mounting holes (non-grooved) in the sensor housing to avoid service impacts due to impact or vibration due to incorrect mounting distances or movement. Two switching elements (discs) can be adapted to a wide range of pneumatic actuators. The only choice that needs to be made is the installation mode, for example: 30 × 80mm or 30 × 130mm. Stainless steel target has been designed on the disc. Four LED lights indicate the sensor output, power supply and solenoid valve output status. The detection area of ​​the dual sensor is narrow enough that there is no blockage caused by operator injury or ice or dust at the crush point. Solenoid valve wiring, through the sensor wiring, the scene without a separate cable. Inductive Sensors and Mechanical Switches In all industrial fields, mechanical switches were once the preferred solution for a variety of control problems, and today's methods are no longer the case. Sensors are no doubt the first choice. Because the advantages of sensor speed and reliability are obvious: All solid state, so no wear or error correction; Fast output switching state; Hysteresis and repeatability can be electronically defined more than mechanical equipment; No welding or output corrosion ; Little or no clogging or humidity effects; no contact jitter (output echo); most sensors switch LED display. Make maintenance and debugging easier and faster. Typically, the average life expectancy of an inductive sensor exceeds the mechanical switch. In fact, many mechanical switches were used for valve feedback in the past because of the low price, but also because we did not need to consider the polarity problems when using switches. With modern technology, Pepperl + Fuchs has successfully brought the sensor to the market, eliminating the mere advantage of a mechanical switch. Sensors for Valve Position Detection Selecting the right sensor can be done in 4 easy steps: Step 1: Switch box or open solution? Weigh the above advantages and disadvantages, make a choice. You may need to switch box protection, or open the solution simple and convenient. Most valve suppliers offer two solutions. You will find the switch box size and weight are different, the number and type are very different. Step 2: Electrically define the terminal of the sensor and the control system to which the sensor is connected regulate the process (AC, DC, Namur, etc.). It is extremely important that the system is used in flammable and explosive areas. We look at the following types of electrical connections. DC three-wire sensor. These sensors have a separate line outside the two power lines. The third line is connected to the output on the PLC input card or control system. DC two-wire sensor. Here, the load is connected in series with the sensor. Many sensors have polar tolerances, which eliminate the need to worry about the polarity of the wiring. Therefore, it is as simple as connecting a normal mechanical switch. Practical application of these sensors, you must pay attention to the circuit leakage current and sensor voltage drop. AC sensor with normally open (NO) and / or normally closed output. Here, the load is connected in series with the sensor. AC and DC two-wire general sensor. The sensor is connected in series with the load. Either AC or DC voltage can be connected. Two-line Namur sensors, together with the relevant intrinsically safe products, are used in explosion-proof environments (zones 0, 1 and 2). Can realize the disconnection and short circuit monitoring. Two-wire fail-safe sensor. According to the two-wire Namur sensor standard, but with the use of safety barrier to achieve SIL3 safety level. Step 3: The possibility of connection The connection depends on the end user. For example, a sealed tube or plug with a cable. Chemicals and pharmaceuticals tend to choose the cable connection through which the cable is sealed into the box or sensor. Step 4: Solenoid Valve Operation Like a switch box, dual sensors can also be used to route solenoid valves through sensors. The advantage of this is that the actuator requires only one cable connection. Finally, you have to confirm the many features of the sensor to ensure the sensor runs perfectly in your application. A similar example is leakage current, voltage drop, ambient, maximum output current, and operating voltage. If possible, you can confirm it with the specifications of the selected sensor, guaranteed to be within the permissible range of static current (disturbance) shock and vibration maxima. Future developments As the installation process often has to adapt to the latest technological developments in order to improve work efficiency and stability, various bus systems are beginning to enter the market. Manufacturers of switch boxes and actuators certainly want to join. We see that many such manufacturers have already negotiated with sensor manufacturers to develop these bus system interfaces. A switch box with AS-i is a good example because this bus system is particularly suitable for digital information. If the end user wants to use a standard sensor, a suitable field installation module, such as an AS-Interface, Profibus PA or FOUNDATION fieldbus, is required. On the other hand, we also see the use of electric actuators and positioning systems with Profibus or FOUNDATION fieldbus interfaces. Obviously more and more companies are aware of the advantages of the bus system, especially the fieldbus combination with AS-i (such as Profibus) is very common. Conclusion: We are increasingly feeling the trend of inductive sensors replacing mechanical switches. In some markets, 60% to 70% of switch boxes with inductive switches have replaced mechanical switches. Standardization of valve position detection is still some way to go. This is because the original switch box design starting point in a large number of mechanical assembly and switching technology exist. More often, end-users often demand more reliable solutions that focus on the long-term return on switchgear. Open solutions are an obvious trend in some industry segments because of their close proximity to mechanically-accessible switchgear solutions. Now, thanks to the size and cost advantages of open solutions, the assembly of position feedback on manual valves has also become more attractive. This article Keywords: valve sensor
May 12, 2021