Proximity switches are made to open or close an electrical circuit when they either make contact or come within a certain distance of an object. They detect objects in close “proximity.” Proximity switches come in four basic types: infrared, acoustic, capacitive, and inductive.
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How Proximity Switches Work
Infrared proximity switches work by sending out beams of infrared invisible light. A photodetector on the switch determines if there are any objects nearby by detecting any reflection of the beam of light sent out. However, a switch with only a light source and photodiode is susceptible to inaccurate readings due to background light. For that reason, more complex models modulate the transmitted light at a specific frequency and have receivers that only respond to said frequency. Even more complex models can use the light reflected from an object to compute its distance from the sensor. In principle, acoustic proximity switches are similar to infrared switches, but instead of light, they use sound. Acoustic switches work by using a transducer to transmit inaudible sound waves at various frequencies in a preset sequence. Then, they measure the length of time the sound takes to hit a nearby object and then return to a second transducer on the switch. Essentially, acoustic switches employ echolocation, a practice similar to sonar, used in nature by bats and dolphins. Capacitive proximity switches sense distance to objects by detecting changes in surrounding capacitance. A radio-frequency oscillator is connected to a metal plate and its frequency changes when the plate nears an object. When this change takes place, the frequency detector sends a signal telling the switch to open or close. Finally, inductive proximity switches sense distances to objects by generating magnetic fields, similar in this way to metal detectors. To function, a wire coil is charged with an electrical current, which is measured by an electronic circuit. If a metallic part gets close enough to the coil, the current will increase, and the switch will open or close accordingly.
Applications of Proximity Switches
Proximity switches are used primarily in manufacturing processes and equipment, robotics, and security systems. For example, proximity switches may measure the position of machine components, detect the opening of a door or monitor a robot or its component's proximity to an object, and steer it accordingly.