Sensors

A sensor is a transducer used to measure a physical variable by responding to stimuli such as heat, light, sound, pressure, or motion and transmitting a corresponding signal. While sensors and transducers are similar, sensors do not always involve energy conversion but rather a change in their properties. For instance, a temperature sensor may alter its resistance in response to temperature variations. Similarly, a photocell, commonly used in position-measuring transducers, changes its electrical resistance based on the intensity of light falling on it. When a voltage is applied across the photocell, this voltage fluctuates with light intensity, allowing it to function like a transducer, even though no direct energy conversion occurs.


Limit Switch

A limit switch is an electromechanical device with ON/OFF characteristics, typically featuring a pressure-sensitive mechanical arm. When an object applies pressure to the arm, the switch circuit is energized, or alternatively, a magnet attached to the object can trigger the switch. Limit switches can be normally open (NO) or normally closed (NC) and may have single or multiple poles, allowing one or multiple circuits to be controlled. While widely used, limit switches have drawbacks, including susceptibility to mechanical failure and relatively low operational speed.

 Limit switches can be either 
  • Normally open (NO) 
  • Normally closed (NC) and may have multiple poles. 



Proximity sensors 

  • Proximity sensors are electronic, non-contact devices that detect the presence or absence of an object.
  • They are available in three types: inductive, capacitive, and magnetic.
  • Initially, mechanical limit switches were employed for object detection in industrial applications.
  • Inductive proximity sensors have replaced mechanical limit switches for detecting metallic objects without contact.
  • Capacitive proximity sensors can identify both metallic and non-metallic objects.
  • Both inductive and capacitive sensors operate as limit switches with detection ranges of up to 100mm.
  • They provide reliable, wear-free operation across various industrial settings

Inductive Proximity Sensors

  • Inductive sensors are used to detect the presence of metallic objects.
  • These sensors require DC or AC voltage to power the circuitry, generate fields, and produce an output signal.
  • An inductive proximity sensor consists of four basic elements:
    1. Sensor coil and ferrite core
    2. Oscillator circuit
    3. Trigger/Detector circuit
    4. Solid-state output circuit

 
  • The oscillator circuit generates a radio-frequency electromagnetic field that radiates from the ferrite core and coil assembly.
  • The ferrite core shapes and directs the field at the sensor face.
  • When a metal target enters the field, eddy currents are induced on its surface.
  • These eddy currents cause a loading effect, reducing the amplitude of the oscillator signal.
  • The detector circuit senses this change in amplitude and switches ON at a specific threshold.
  • This ON signal activates the solid-state output, referred to as the damped condition.
  • As the target moves away, the system responds accordingly, returning to its normal state.



Capacitive Proximity Sensors

  • Capacitive sensing is based on dielectric capacitance.
  • Capacitance is the property of insulators to store an electric charge.
  • A capacitor consists of two plates separated by an insulator (dielectric).
  • When the switch is closed, a charge is stored on the two plates.
  • The distance between the plates determines the capacitance and can be calibrated for ON/OFF switching.

Basic Elements of a Capacitive Proximity Sensor:

    1. Sensor (the dielectric plate)
    2. Oscillator circuit
    3. Detector circuit
    4. Solid-state output circuit




  • The oscillator circuit includes feedback capacitance from both the external target plate and the internal plate.
  • In a capacitive switch, oscillation begins when sufficient feedback capacitance is detected.
  • As the target approaches, the capacitance increases until it reaches a threshold.
  • At this threshold, the trigger circuit activates the output switching device.
  • The output modules can function as normally open (NO), normally closed (NC), or changeover switches.

Advantages of Capacitive Proximity Sensors:

  • Can detect non-metallic targets.
  • Suitable for detecting lightweight or small objects that mechanical limit switches cannot detect.
  • Provide a high switching rate for rapid response in object counting applications.
  • Can detect liquid targets through non-metallic barriers like glass or plastic.
  • Have a long operational life with virtually unlimited operating cycles.
  • The solid-state output provides a bounce-free contact signal.

Magnetic Proximity Sensors

  • Magnetic proximity sensors consist of the following components:
    1. LC oscillating circuit
    2. Signal strength indicator
    3. Switching amplifier
    4. Strip of magnetically soft-glass metal


  • The soft-glass metal strip attenuates the oscillating circuit.
  • When a magnet is brought closer, the oscillation de-attenuates.
  • Unlike inductive proximity sensors, where power consumption decreases as the target approaches, magnetic proximity sensors consume more power as the magnet gets closer.
  • A key advantage of magnetic proximity sensors is their ability to achieve large sensing ranges even with small sensor sizes.