In the process industry, valves and actuators have always been used for liquid flow control and transfer. More and more factory engineers hope to get feedback signals of the current position 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 directly mounted on the actuator or manual valve. This article discusses in depth the types of sensors used in new trends in the field and in the market.

Standardization status

Industry standards have both advantages and disadvantages for the development of technology. On the one hand, it limits creativity, but on the other hand, it ensures the compatibility of devices from different manufacturers. After the VDE / VDI3845 standard requires many items, it specifies the important mechanical connection between the pneumatic drive machine and the connected equipment. The standard includes the definition of mounting holes: 30 × 80mm and 30 × 130mm. The height of the mounting rod is 20, 30 and 50 mm. There is no fixed standard for the diameter of the mounting rod, so users must pay special attention to the application of dual sensors.

The term "switch box" generally describes the integration of sensors, cams and connectors in a closed box. Sensors and pneumatic actuators are mechanically connected through it. This solution aims to avoid the sensor being exposed to the surrounding environment or being affected, while being able to use standard sensors. In these standard switch boxes, the following sensors are typically built in:

● Cylindrical sensor with a typical length of 40 mm;

● Slot sensor;

● Rectangular V3 style sensor (similar to micro switch);

● Double sensors, compact structure in a box.

The switch box is used as a terminal device to connect the valve position sensor and the solenoid valve. The coded connector and the necessary LED indicators have been placed on the circuit board. The size of the circuit board fits most standard boxes.

The "open solution" specifies the installation of dual sensors on automatic or manual valves. Two inductive sensors are superimposed and installed in a housing, which accurately shows the switch position of the valve.

Figure 1. Two inductive sensors are superimposed and installed in a housing, which accurately shows the switching position of the valve. >

The design of the system considers the VDE / VDI3845 standard. These sensors are triggered by two 90-degree stainless steel targets mounted vertically on the "disc". The disc itself is mounted on the actuator with a screw.

In addition to its small size and simple assembly, the following advantages can also be found:

● The round mounting hole (non-groove) on the sensor housing can avoid maintenance caused by collision or vibration caused by wrong installation distance or movement.

● Two switching elements (disc) can adapt to many different pneumatic actuators. The only choice you need to make is the installation mode, for example: 30 × 80 or 30 × 130

● The target of stainless steel has been designed on the disc.

● 4 LEDs indicate the output status of the sensor, the power supply and the output status of the solenoid valve.

● The detection area of ​​the dual sensor is narrow enough, so that there will be no squeeze point causing injury to the operator, or clogging caused by ice or dust.

● The wiring of the solenoid valve can be wired through the sensor, no separate cable is needed on site.

Inductive sensors and mechanical switches

In all fields of industry, mechanical switches were once the first choice for various control problems, and today's methods are not the same as before, and sensors are undoubtedly the first choice. Because the advantages of sensor working speed and reliability are obvious:

● All solid state, so there is no wear or error modification;

● Quick output switch status;

● Hysteresis and repeatability can be more electronically defined than mechanical equipment;

● No welding or corrosion at the output end;

● Little or no influence from clogging or humidity;

● No contact vibration (output echo);

● Most sensors have LED display when switching, which makes maintenance and debugging easier and faster.

Generally, the average life of an inductive sensor greatly exceeds the mechanical switch. In fact, many mechanical switches used to be used for valve feedback in the past, because of the low price, but also because the polarity of the switch does not need to be considered. Through modern technology, Pepperl + Fuchs has successfully brought sensors to market, eliminating the only advantage of mechanical switches.

Sensor for valve position detection

Selecting the correct sensor can be achieved in the following 5 simple steps:

Step 1: Switch box or open solution?

Weigh the above advantages and disadvantages and make a choice. You may need the protection of a switch box, or the simplicity and convenience of an open solution. Most valve suppliers will provide two solutions. You will find that the volume and weight of the switch box are different, and the number and type are also very different.

Step 2: The terminal of the sensor is defined electrically, and the control system connected to the sensor regulates the working process (AC, DC, Namur, etc.). It is extremely important whether the system is used in explosive areas. Please see the following electrical connection types.

● DC three-wire sensor. Are these sensors one outside of the two power lines?

Separate line. The third line is connected to the PLC input card or the output on the control system.

● DC two-wire sensor. Here, the load is connected in series with the sensor. Many sensors have polarity tolerances, which means that there is no need to care about the polarity of the wiring. Therefore, it is as simple as connecting a normal mechanical switch. In practical application of this type of sensor, you must pay attention to the leakage current of the circuit and the voltage drop of the sensor.

● AC sensor with normally open and / or normally closed output. Here, the load is connected in series with the sensor.

● AC and DC two-wire universal sensor. The sensor is connected in series with the load. Both AC voltage and DC voltage can be connected.

● Two-wire Namur sensors, together with related intrinsically safe products, are used in explosion-proof environments (zones 0, 1 and 2). It can realize disconnection and short circuit monitoring.

● Two-wire fail-safe sensor. According to the two-wire Namur sensor standard, but using an isolation barrier that meets the safety level, it can reach the SIL3 safety level.

Step 3: Possibility of connection

● The connection method depends on the end user. For example, sealed tubes or plugs with cables. The chemical and pharmaceutical fields tend to choose cable connections and enter boxes or sensors through cable glands.

Step 4: Solenoid valve operation

Like the switch box, the dual sensor can also be used to route the solenoid valve through the sensor. The advantage is that the actuator requires only one cable connection.

Step 5: Finally, you must confirm the many characteristics of the sensor to ensure that the sensor works perfectly in the application.

Similar examples are leakage current, voltage drop, surrounding environment, maximum output current and operating voltage. If possible, you can confirm by selecting the specifications of the sensor to ensure that it is within the allowable range of the maximum value of static current (interference), shock and vibration.

future development

As the installation process often has to adapt to the latest technological developments to improve work efficiency and stability, various bus systems have begun to enter the market.

Manufacturers of switch boxes and actuators certainly want to join. Many such manufacturers have negotiated with sensor manufacturers to develop interfaces for these bus systems. A switch box with AS-i is a good example, because this bus system is particularly suitable for digital information. If end users wish to use standard sensors, they will need suitable field-installed modules, such as AS-interface, ProfibusPA or FOUNDATION fieldbus.

On the other hand, you can also see the use of electrical 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 field bus combinations with AS-i (such as Profibus) are very common.

Conclusion of this article

More and more people feel the trend of inductive sensors replacing mechanical switches. In some markets, 60% -70% of switch boxes equipped with inductive switches have replaced mechanical switches. Standardization of valve position detection still has a way to go. This is because in the original starting point of the switch box design, a large number of mechanical assembly and switch technology coexist. More commonly, end users often demand more reliable solutions, focusing on the long-term return of the switch box.

The continuous development and innovation of the sensor company makes a large number of sensors coexist in order to be assembled in various switch boxes.

In some industries, open solutions are an obvious trend, because its price is very close to the mechanical contact switch box solution. Now due to the advantages of the size and cost of the open solution, the assembly position feedback on the manual valve has become more attractive.