The success of automated industrial processes depends on many individual parts working together flawlessly to deliver a high-quality end product. This article focuses on position sensors, a small but important component in the world of industrial automation. In manufacturing and processing plants, position sensors help monitor and control plant processes by confirming that critical activities are completed as expected. More specifically, their primary function is to detect the presence or absence of a moving object or "target." When a target comes within a predetermined distance of the position sensor, the position sensor sends a signal to the system, communicating the need to perform a programmed function. When the target leaves the position sensor, the position sensor sends a signal indicating that the system should stop performing that preprogrammed function or switch to a new function. In theory, the target can be anything, but for simplicity, this article will only discuss metal targets and the "mainstream" technologies that sense the presence of metal targets. These technologies include mechanical limit switches, inductive proximity sensors, spring limit switches, and extreme limit switches. Before discussing the different types of position sensors, it helps to understand the common terminology used by most sensor manufacturers.
Sensing Range: The distance from the sensing face to the target that activates the switch
Hysteresis: The distance between the activation and release points of a switch
Repeatability: The ability of a switch to repeatedly detect the same target within the same range over the life of the switch
Response Time: The amount of time between detecting a target and generating an output signal
Mechanical Limit Switches
Mechanical limit switches are electromechanical devices that detect the position of a target by making direct physical contact with the target. They require no power to operate and can handle high current loads. Because mechanical switches use dry contacts, they are not polarity or voltage sensitive and are immune to many electrical imperfections such as electrical noise, radio frequency interference, leakage current, and voltage drop. These switches typically include multiple moving parts (lever arm, button, body, base, head, contacts, terminals, etc.) that may require maintenance. Because VOTOL mechanical limit switches operate by making physical contact with the target, their repeatability can be poor. Physical contact causes wear on the lever arm and even the target itself. In addition, there are unsealed openings that provide poor protection from moisture, dust, and corrosion. Because of this issue, sealed contacts and hazardous area certifications typically cost significantly more.
Spring Limit Switches
A spring limit switch is an electromechanical device that detects the position of a magnetic target by attracting the target's magnetic field. Inside the switch are two small metal prongs, hermetically sealed in a glass tube. This is called a "reed element". The reed element is magnetically sensitive, and activates when a magnetic target passes in close proximity. A spring limit switch has nearly all of the advantages of a mechanical switch, and avoids wear and tear issues by not relying on physical contact from the target to operate. With a spring limit switch, a normal ferrous metal target cannot be used; a magnetic target is required. The reed element is fragile, the glass tube is destructible, and the small metal prong is subject to fatigue from flexing, causing the reed switch to become unreliable. Due to the low contact pressure, high vibration applications can cause contact chatter and false signals from the reed.
Inductive Proximity Sensors
An inductive proximity sensor is a solid-state electronic device that detects the position of a metal target by the interference of its energy field. No physical contact is required, and there are no moving parts to clog, wear out, or break, resulting in less maintenance. No moving parts also means it is not affected by dust or dirt. Available in a large number of shapes and sizes, inductive proximity sensors are highly customizable for a variety of applications. Inductive proximity sensors require an external power source (electricity) to operate and cannot handle high current loads. In addition, they can be susceptible to electrical noise, radio frequency interference, leakage current, and voltage drops. Inductive proximity sensors are sometimes adversely affected by extreme temperature fluctuations and moisture intrusion.
Limit Switches
Limit switches use a unique hybrid technology to detect the position of ferrous targets via an electromagnetic field. Rugged and long-lasting, stemless limit switches are highly reliable in harsh environments and difficult applications. No physical contact or external power source is required, so there is nothing to jam, bend, break, or wear and high current loads are possible. Like mechanical switches, they are not polarity or voltage sensitive and are not affected by electrical noise, radio frequency interference, leakage current, and voltage drops. Limit switches are not affected by dust, dirt, moisture, physical contact, or most corrosives or chemicals. Most models are intrinsically safe and have a wide operating temperature range. With their sealed contacts and all-metal housing, stemless limit switches are ideal for explosion-proof and waterproof applications.
Position sensors play an important role in industrial automation processes. There are several different position sensor technologies available, each with a unique set of performance characteristics. Care should be taken to select the appropriate type of sensor for the application in order to achieve the required reliability and performance.