Automation Switches Technical Information

Basics of... Automation Switch Terminology

Limit Switch: An information device that senses movement. Through an electrical contact, the limit switch provides information on the movement of an object. It performs this function by converting the mechanical motion of the object into an electrical signal. The information on object movement provided by the limit switch is used by machines in determining when to start, stop, reverse, slow down, speed up, or recycle.

Pressure Switch: An information device whose function is to limit pressure in a system so pressure neither exceeds nor drops below a certain limit. This type of switch opens or closes electrical circuits in response to the pressure of such media as water, air and oil. The pressure causes a piston, bellows or a diaphragm to move against a set of springs, which then open or close the contacts. If the pressure has dropped below a certain limit, the pressure switch will cause the machine or device to turn on; if the pressure exceeds a certain limit, the pressure switch causes it to turn off. Pressure switches are used to control such machinery as pumps and lubrication systems.

Photoelectric Switch: A switch which, when activated, sends out a signal to energize the control coil or solid state circuitry of an industrial or process control device. A photoelectric switch is activated when a beam of light is interrupted or changes intensity. There are two types of sensing styles: 1) Retroreflective types, the unit projects a beam of light to a specific target, which reflects the beam back to the unit. When this beam is interrupted, the unit is activated. 2) Diffuse reflection type; this unit projects a beam of light a given distance. When an object enters this beam and reflects it back to the unit, the unit is activated. Photoelectric switches have definite sensing ranges that vary with the type of material sensed.

Proximity Switches: A switch which functions like a limit switch. When activated, it sends out a signal which can energize such devices as motor starters, relays, solenoids, programmable controllers, or other industrial control devices. A proximity switch does not physically touch or move to detect the presence of an object, rather, it senses a disturbance in a magnetic field which radiates from the sensing face. This disturbance is caused by the presence of an object entering this field. Proximity switches have definite sensing ranges which vary with the type of material sensed.

Foot Switches: A switch which activates an electrical current, similar to a pushbutton. There are two types of foot switches: momentary and maintained. Momentary Foot Switch; press and hold, the switch is off when you release. Maintained Foot Switch; press on to activate, press again to deactivate.

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