- Published on Monday, 21 July 2008 08:16
- Written by Greg Merrifield
The success of automated industrial processes depends on many individual components working together flawlessly to provide a high-quality end product. This article focuses on position sensors, a small but important component in the world of industrial automation.
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
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.