Position Sensor Fundamentals

The Function of Position Sensors

In manufacturing and processing plants, position sensors help monitor and control plant processes by confirming that critical activities are completed as intended. More specifically, their primary function is to detect the presence, or absence, of a moving object, or “target.”

When the 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 moves away from the position sensor, the position sensor signals that the system should stop performing that preprogrammed function or switch to a new function.

Theoretically, the target could be just about anything, but for the sake of simplicity, only metal targets and “mainstream” technologies that sense the presence of metal targets will be discussed in this article. These technologies include mechanical limit switches, inductive proximity sensors, magnetic reed switches and leverless limit switches

Key Terminology

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 activated and release points of the switch
• Repeatability: A switch’s ability to detect the same target at the same range repeatedly during the life of the switch
• Response Time: The amount of time between the detection of a target and the generation of the 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 do not require power to operate and can handle high current loads. Since mechanical switches use dry contacts, they are not polarity or voltage sensitive and are immune to many electrical pitfalls such as electrical noise, radio frequency interference, leakage current and voltage drops.

These switches typically consist of multiple moving parts (lever arm, push button, body, base, head, contacts, terminals, etc.) that can require maintenance. Because mechanical limit switches make physical contact with the target to operate, their repeatability can be poor. The physical contact causes wear and tear on the lever arm and even the target itself. In addition, there are unsealed openings that provide poor defense against moisture, dust and corrosion. Sealed contacts and hazardous area approvals often cost significantly more due to this problem.

Reed Switches

Reed switches are electromechanical devices that detect the position of a magnetic target by the attraction of the target’s magnetic field. Inside the switch are two small metal prongs, hermetically sealed in a glass tube. This is called the “reed element.” The reed element is magnetically sensitive and activates when a magnetic target passes close. The reed switch has almost all the advantages of the mechanical switch and avoids wear and tear issues by not relying on physical contact from the target to operate.

With reed switches, a normal ferrous metal target cannot be used; a magnetic target is required. The reed element is fragile, the glass tube is breakable and the small metal prongs are subject to fatigue from bending, causing the reed switch to become undependable. Due to low contact pressure, applications with high vibration can cause contact chatter and false signals from the reed.

Inductive Proximity Sensors

Inductive proximity sensors are solid-state electronic devices that detect the position of metal targets via the disturbance of their energy fields. No physical contact is required, and there are no moving parts to jam, wear or break, resulting in less maintenance. No moving parts also mean it is not affected by dust or dirt. A large selection of shapes and sizes are available, so the inductive proximity sensor is highly customizable for a wide variety of applications.

Inductive proximity sensors require external power (electricity) to operate and are incapable of handling high current loads. Further, they can be susceptible to the effects of electrical noise, radio frequency interference, leakage current and voltage drops. Inductive proximity sensors are sometimes adversely affected by extreme temperature fluctuations and moisture ingression.

Leverless Limit Switches

Leverless limit switches use a unique, hybrid technology to detect the position of a ferrous target via an electromagnetic field. Rugged and with a long life expectancy, leverless limit switches are very dependable in harsh environments and difficult applications. No physical contact or outside power source is required, so there is nothing to jam, bend, break or wear out, and high current loads are possible. Like the mechanical switch, they are not polarity or voltage sensitive and are immune to electrical noise, radio frequency interference, leakage current and voltage drops.

Leverless switches are not affected by dust, dirt, moisture, physical contact or most caustics or chemicals. Most models are inherently intrinsically safe and have a wide operating temperature range. With their sealed contacts and all-metal enclosures, leverless limit switches are ideal for explosion-proof and waterproof applications.

Conclusion

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 choose the right type of sensor for the application in order to achieve the reliability and performance required.

Greg Merrifield is GO Switch Business Manager at TopWorx (www.topworx.com), a manufacturer of valve control and position sensing solutions.