Proximity Switches

3.2.1.3 Proximity Switches

Proximity switches are used to detect the position of an object. While photoeyes are also sometimes referred to as proximity switches, here we are discussing inductive, capacitive, limit switch and hall effect types. Proximity switches are often called proxes for brevity.

Inductive

Inductive proximity switches are used to detect metal objects. A coil of thin wire is energized with a weak current which is connected to an oscillating circuit. When a large enough piece of metal enters the field created by the current flowing through the coil, the oscillator is stalled and a discrete signal is generated signaling the presence of an object. The type of metal being detected strongly influences the range of an inductive proximity switch. Metals such as steel containing iron make the best target, while aluminum reduces the sensing range by about 60%.

Inductive proximity switches are categorized as being shielded or unshielded. Shielded proxes have a metal housing all the way up to the sensing face of the switch. This reduces the range but allows the sensor to be mounted flush with a metal surface without detecting the metal to the side. Unshielded proxes have a longer range since the field extends out from the sides of the prox, but is more susceptible to damage or interference.

Inductive proximity switches are available in a threaded barrel style, a flat surface mount or various other styles. Barrel proxes typically have a metal housing with a plastic covered sensing surface, but can be entirely stainless steel for ruggedness.

Capacitive

Capacitive proximity switches use a capacitive sensing surface which discharges when an object is placed close to it. As such it can be used to detect solid or liquid objects. A common use of capacitive proxes is to detect a liquid through the sides of a plastic or fiberglass vessel. As long as the vessel walls are fairly thin the prox can be set to detect the difference in mass between an empty and full vessel. Capacitive proxes are also sometimes used as operator pushbuttons for ergonomic purposes since they take less pressure to activate than mechanical pushbuttons.

Like the inductive proximity switch, capacitive proxes have a very small detection range. They are usually larger than their inductive cousins.

Hall Effect

Hall effect sensors are transducers that vary their output voltage in response to changes in magnetic field. Hall sensors are used for proximity switching, positioning, speed detection, and current sensing applications. In its simplest form, the sensor operates as an analog transducer, directly returning a voltage. With a known magnetic field, its distance from the Hall plate can be determined. Using groups of sensors, the relative position of the magnet can be deduced. Hall Effect sensors are often used to detect the position of a magnet carried by the piston of a pneumatic cylinder. Hall sensors and Inductive proximity switches are the most common sensors used in detecting cylinder or actuator position.

Limit

Limit switches are mechanically activated devices that open or close electrical contacts when an object contacts it. There are wide variety of configurations, sizes and degrees of ruggedness for limit switches. Roller limit switches have a metal or plastic roller which allows an object such as a cam to slide along the contact point. Lever arm and “whisker” style switches extend the reach of the switch. Precision limit switches are used to precisely control the actuation point of a switch for positioning or measurement.

Analog Sensors