In the process industry valves and actuators are historically used for controlling and diverting fluid flows. Increasingly the plant engineers want a feedback signal of the current position of valves, taps and actuators in the control system. This feedback is generally obtained by use of inductive sensors which are built into a switchbox, or double sensors which are mounted directly on actuators or the manual valve. This article gives an insight in to the type of sensors that are applied in the field and the emerging trends in the market. 在过程工业中，阀门和驱动器一贯用于液体流动的控制和转移。越来越多的工厂工程师希望在器控制系统中得到阀门、龙头、及驱动器当下位置的反馈信号。通常这种信号由内置于开关盒中的接近传感器，或直接安装在驱动器或手动阀门上的双传感器来获取。本文深入探讨在现场和市场新趋势中应用的传感器类型。
State of the Art with Regard to Standardisation 标准化现状
Industry standards can be viewed as both positive and negative toward technology development. On the one hand they limit creativity but on the other side they ensure that the apparatus of several manufacturers is compatible. The VDE/VDI 3845 standard describes, amongst other things, the important mechanical connection between pneumatic actuators and the apparatus which must be connected to it. The standard includes a defined mounting hole pattern of 30x80mm and of 30x130mm. Stem heights are 20, 30 and 50 mm. For stem diameter there is no fixed standard, therefore in the case of double sensors you must pay particular attention to this. 行业标准对于技术的发展而言利弊共存。一方面，它限制了创造性，但另一方面他们又确保了不同制造商的设备具备兼容性。VDE / VDI3845标准要求了诸多事项后，规定了气动驱动器机与所连接设备之间的重要机械连接。该标准包括了对安装孔的定义：30x80mm和30x130mm。安装杆的高度为20，30和50毫米。对于安装杆直径，则没有固定的标准，所以在双传感器的应用中，用户必须特别注意。
The term “switchbox” commonly describes the integration of sensors, cam and connectors in an enclosure, through which the mechanical connection is also made between the sensors and pneumatic actuator. This solution was designed so as not to expose the sensors to the immediate environment and influences, and to be able to use standard sensors (see fig 1). In these standard boxes, the following sensors are typically built in: 术语“开关盒”通常描述在一个封闭盒中传感器、凸轮和连接器的集成。传感器和气动执行器通过它实现机械连接。该解决方案旨在避免传感器在周边环境中的暴露或受到影响，同时能够使用标准传感器（见图1）。在这些标准开关盒中，典型内置以下传感器：
• Cylindrical sensors with a typical construction length of 40mm
• Slot sensors
• Rectangular sensors in V3 style (similar to the micro switch)
• Double sensors which are compact enough to be built in a box (see fig 2)
• PCB solution 印刷电路板解决方案
The box is used as a terminal compartment to connect the valve position sensor and solenoid. Encoded connections are already on the circuit board, as well as the necessary LED’s. The circuit board fits in most standard boxes.
The Open Solution开放式解决方案
The term “open solution” describes the direct mounting of double sensors on automated or manual valves (see fig 3). Two inductive sensors mounted one on top of another in one housing give a precise indication of the open and closed position of a valve. 术语“开放式解决方案”规定了自动阀或手动阀上的双传感器安装（见图3）。两个电感式传感器叠加安装于一个外壳中，精确显示了阀门的开关位置。
The design of this system takes into account the VDE/VDI 3845 standard. The sensors are activated by two stainless targets mounted at 90 degrees to each other on a “puck”. The puck itself is mounted by one screw on the actuator stem. 该系统的设计考虑了VDE / VDI 3845标准。这些传感器由两个90度垂直安装于“圆盘”上的不锈钢目标触发。圆盘本身通过一个螺丝钉安装在执行器上。
Apart from compact size and very simple assembly, we find the following advantages: 除了体积小巧，组装简单，我们还发现以下优点：
• Round mounting holes (not slots) in the sensor housing avoid incorrect mounting distances or movement through impact or vibration which otherwise may require maintenance.
• The different alternatives of pneumatic actuators are covered by only two switching elements (pucks). The only choice which must be made is the mounting pattern, e.g. 30x80 or 30x130 (see fig 4 and fig 5)
• Switching targets in stainless steel are already built in to the puck (see fig. 3).
• Four LED’s indicate the output situation of the sensors, the presence of power and the output status of the solenoid valve.
• Thanks to the narrow detection area of the double sensor, there is no danger of a pinch point causing harm to the operator or allowing clogging from ice or dirt.
• The wiring of the solenoid valve can be routed through the sensor so that no separate cable needs to be run to the field.
Inductive Sensors Versus Mechanical Switch电感式传感器与机械开关
In all areas of industry where the mechanical switch was previously a much-used solution for a diversity of control problems, nowadays the approach has changed. The sensor is now the solution. This is a consequence of the ability of sensors to offer significant speed and reliability advantages: 在工业的所有领域中，机械开关曾经一度是各种控制问题的方案首选，而如今的方法已今非昔比，传感器毋庸置疑为首选。因为传感器的工作速度和可靠性的优势十分明显：
- Entirely solid state, as a result, no wear and tolerance modifications.
- Faster output switching
- Hysteresis and repeatability can be defined electronically much more successfully than in a mechanical device.
- No welding or corrosion of output contacts.
- Little or no influence of clogging or humidity.
- No contact bounce (output echo)
- LED indication on most sensors to show when the sensor has switched. This makes maintenance and commissioning simpler and quicker.
Typically the average life span of an inductive sensor is considerably longer than those of a mechanical switch. The fact that historically, very many mechanical switches were used for valve feedback is partly due to the difference in price and partly the fact that when using a switch polarity does not need to be considered. By use of modern techniques, Pepperl+Fuchs have succeeded in bringing sensors to the market which eliminate those remaining advantages of mechanical switches.
Possible Sensors for Valve Position Detection用于阀位检测的传感器
Choosing the correct sensor can be achieved in 5 simple steps: 选择正确的传感器可按以下5个简单步骤实现：
Step 1: Switchbox or open solution? 步骤1：开关盒或还是开放式解决方案？
Weigh the aforesaid advantages and disadvantages against each other and make the choice. You either require the protection that the switchbox gives, or the simplicity and convenience of an open solution. Most valve suppliers offer as a matter of course the two solutions. You will find that switchboxes come in all sizes and weights, and in several qualities and classes.
Step 2: Define electrically the final termination a sensor will have to the control system which regulates the process (AC, DC, Namur etc.). Of overriding importance is whether the system is used in an explosive area. We see the following electrical connection types. 步骤2：电气定义传感器的终端，传感器所连接的控制系统调节着工作过程（交流，直流，Namur等）。极为重要的一点是该系统是否在爆炸地区使用。我们看以下的电气连接类型。
• Three wire DC Sensors. These sensors have a separate wire beside the two wires for powering the sensor. The third wire is the output which is connected to the PLC input card or the control system. 直流三线传感器。这些传感器在两条供电线路外还有一条单独的线路。第三条线路连接到PLC输入卡或控制系统上的输出端。
• Two Wire DC sensors. Here the load is connected in series with the sensor. Many of these sensors are polarity tolerant which means that it does not matter how the wires are connected. For this reason, it is exactly the same as how you would connect a normal mechanical switch. In applications using these type of sensors, you must pay attention to the off state current in the circuit and the voltage drop of the sensor. 直流两线传感器。在这里，负载与传感器串联。许多传感器都具备极性容差，即无需再关心接线的极性。因此，它与连接正常的机械开关一样简单。实际应用这类传感器，您一定要注意电路的漏电流及传感器的电压降。
• AC voltage with Normally Open (NO) and/or Normally Closed contacts. (NC). Here the load is in series with the sensor. 带有常开（NO）和/或常闭输出的交流传感器。在这里，负载与传感器串联。
• Two Wire universal Voltage. The sensor is connected in series with the load. Either AC Voltage or DC Voltage can be connected. 交直流两线通用传感器。该传感器与负载串联。无论交流电压或直流电压均可连接。
• Two Wire NAMUR; with an associated intrinsically safe barrier, these can be used in areas with an explosive atmosphere (area 0, 1 and 2). Lead breakage and short circuit monitoring are possible as standard. 两线Namur传感器，与相关本安产品一起，用于防爆环境（区域0，1和2）。可实现断线及短路监测。
• Two Wire Failsafe sensors. As per two wire Namur but subject to use of a safety rated barrier, a SIL 3 approved circuit is obtainable. 两线故障安全传感器。按两线Namur传感器标准，但是使用满足安全级别的隔离栅，可以达到SIL3 安全等级。
Step 4: Connection possibilities第4步：连接的可能性
The manner of connection depends on the philosophy of the end-user. Such as, glands with cables or plug termination. For example, the chemical and pharma sector tend to choose cable connection by means of glanded entry to the box or sensor. 连接方式取决于最终用户。例如，带电缆的密封管或插头。化学和制药领域倾向于选择电缆连接，通过电缆密封管进入盒子或传感器。
Step 5: Operation of the solenoid valve步骤5：电磁阀操作
Just like with the switchbox, one also has the option with double sensors to wire the solenoid by means of the sensor. The advantage lies in the fact that there is then only one cable that must be laid to the actuator与开关盒一样，使用双传感器也可以经过传感器进行电磁阀的布线，其优势就在于执行器只需要一根电缆连接。
Finally, one must verify a number of general properties of the sensor to be certain the sensor will function perfectly in the application. 最后，您必须确认传感器的诸多特性以确保传感器在应用中完美运行。
Some examples of this are the off state current, voltage drop, environmental surroundings, maximum output current and operating voltage.
If possible one can verify by means of the specifications of the selected sensor also if quiescent current (disturbances), shock and vibration maximum values are not exceeded.类似的例子便是漏电流、电压降，周围环境，最大输出电流和工作电压。 如果可能，您可以通过选定传感器的规格来确认，保证在静态电流（干扰），冲击和振动最大值的允许范围内。
Trends and Future Developments未来的发展
Because installations are very often adapted to the newest technology in order to work more efficiently and reliably, all kind of bus systems have entered the market. 由于安装过程经常要适应最新的技术发展，以提高工作效率和稳定性，各种总线系统开始进入市场。
Of course producers of switchboxes and actuators want to anticipate this and we see that a number of these manufacturers have developed interfaces, in consultation with sensor manufacturers, to these bus systems. A good example of this is switchboxes with AS interface incorporated because this bus system is particularly suitable for digital information. If the end user wishes to use standard sensors, appropriate field mountable modules can be found for AS_interface, Profibus PA or foundation Fieldbus (see fig6).
On the other hand we see also electric actuators and positioners used with Profibus or foundation Fieldbus interface. It seems that more and more companies are realising the advantages of bus systems and particularly the combination of a field bus (for example Profibus) with AS interface is seen in installations more commonly.
We are increasingly seeing inductive sensors replacing the mechanical switch. In some markets, 60-70% of switchboxes are fitted with inductive proximity switches which have replaced mechanical forbearers. For valve position detection standardisation is still some way off. This is due to the original starting point from which switchboxes have been devised and therefore a large mixture of mechanical assemblies and switch technologies exist.
More commonly, end-users also frequently ask for solutions which lend themselves to greater reliability to allow for long term payback from the switchbox. 我们越来越多感受到电感式传感器取代机械开关的趋势。在某些市场，60-70%装有电感式开关的开关盒已取代了机械式开关。阀位检测的标准化还有一段路要走。这是因为原来开关盒设计的起点中机械装配和开关技术的大量混合存在着。更常见的是，最终用户也经常要求提供更为可靠的解决方案，着眼于开关盒的长期回报。
Sensor companies have continued development and innovation and it can be seen that an enormous mixture of sensors exist which can be assembled in all kinds of switchbox. 传感器公司的持续发展和创新使得大量传感器的混合存在，以便装配于各种开关盒中。
In certain industries a clear tendency to the open solution is noticeable due to a price that comes close to that of the switchbox solution with mechanical contacts. Fitting valve position feedback on manual valves now also becomes a more attractive option with the size and cost of the open solution.