Sensors are devices that sense the presence or absence of objects. The sensors gather information and convert it into a signal that can be read and/or seen by an observer or a piece of equipment.
Sensors are used in everyday applications such as cars, machines, manufacturing and robotics, and perform several functions.
A sensor can be thought of as an automatic switch. In a factory, a sensor can be used to detect a problem on the production line and stop the process automatically. No human intervention is necessary.
Limit switches |
Proximity sensors |
Photoelectric sensors |
|
| Method of detection | Physical contact | Electromagnetic field | Light beam |
| Sensing range | Physical contact | Close: with 1” (25.44 mm) | Distance: can be 800 ft (243.8m) |
| Target material | The target must be able to withstand physical contact | Inductive: metallic only Capacitive: metallic and non-metallic |
Can be affected by target surface, for example, if the target is shiny or transparent |
| Object markings | Not able to detect | Not able to detect | Able to detect |
| Cost | Low | Low | Low to high depending upon sensing method |
| Sensor size | Tend to be large | Small to large | Very small (fiber optic) to large |
| Environmental sensitivity | Affected by debris | Inductive: electrical interference Capacitive: humidity |
Light interference |
| Response time | Milliseconds | Milliseconds | Microseconds |
A limit switch is a mechanical device that requires the physical contact of an object with the switch’s actuator to make the contact change state (open/closed).
As the object or target contacts the operator of the switch, it eventually moves the actuator to the “limit” where the contacts change state.
In a Normally Closed (NC) circuit, this mechanical action opens the electrical contacts and in a Normally Open (NO) circuit, it closes them. The contacts then start or stop the flow of current in the electrical circuit.
Proximity sensors use an electromagnetic field to detect when an object is near.
There is no physical contact between the object and the sensor. This type of sensor is generally used to sense at distances less than one inch. There are two types of proximity sensors.
Inductive proximity sensors are useful for precise detection in the following applications:
Capacitive proximity sensors are ideally suited for liquid level control and for sensing powdered or granulated material. They are designed to:
Photoelectric sensors apply fiber optic technology to applications with space restrictions.
A fiber optic cable can detect objects in locations too crowded for a standard sensor. Because light, rather than current, travels down these cables, the signal is unaffected by electromagnetic interference (EMI) and vibration.
However, fiber optics have their disadvantages. They are limited to a 15-inch maximum sensing distance for the thru-beam design. Also, these sensors have a small sensing area.
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