Limit Switches vs. Proximity Sensors

An object can be used to activate a switch directly, producing an ON or OFF signal to indicate the presence of that particular object, alternatively we can use the presence of a nearby object to activate a sensor giving a signal which is either ON or OFF. A limit switch is a mechanical switch in the form of an actuator mechanically linked to a set of contacts. When an object comes into contact with the actuator, electrical contacts are opened or closed, so making or breaking electrical circuits. Limit switches can therefore be used to determine whether an object is passing the actuator and contacting it, and thus at a specific time is in a particular position. Limit switches are also used to determine whether an object has at the end of its travel reached the actuator. They are primarily employed in applications where the switch actuation and wear are minimal, otherwise other sensors are used like non-contact presence sensors that are not impacted by wear. Proximity sensors, on the other hand, are non-contact presence sensors that consists of an element that changes either its state or an analog signal, when it is close to, but often not actually touching an object. Magnetic, optical, electrical capacitance, inductance and eddy currents methods are some of the techniques used in the design of proximity sensors.

Due to wear and the potential eventual failure of the contact sensors i.e. limit switches, they are often avoided in the development of automated systems. Automated systems are increasingly being built with non-contact sensors (proximity sensors). The common proximity sensors in use include: inductive proximity sensor, the capacitive proximity sensor and the optical proximity sensor.

The choice of proximity sensor is determined by the range and the type of the object being sensed and can be summarized as follows:

1. If contact with the target is possible and acceptable, then a limit switch can be used.
2. If the sensing distance is less than 65 mm and the target is metal then an inductive sensor can be used, otherwise if the target is non-metal a capacitive or photoelectric sensor can be employed.
3. If the target is transparent an ultrasonic or photoelectric sensor can be employed.
4. If the target is a solid object a photoelectric sensor ultrasonic sensor can be used. If the sensing distance is greater than about 6 meters, the photoeletric sensor is used.
5. If the target is a liquid and can be sensed from its top surface, an ultrasonic sensor can be used, otherwise a capacitive sensor is used.

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