Choosing a Sensor

This article gives a brief overview of inductive sensors, capacitive sensors and ultrasonic sensors. Sensing technologies are used within a wide variety of industries including Oil and Gas, Pharmaceutical and Food and Beverage. In order to select the most suitable sensor considerations should include; environment, size, mounting type, housing and connection options, but here we will focus on sensor types.

Inductive Sensors

Solid state devices using the induced magnetic field principle to sense the presence of metal objects.

Operating principle:

There are four basic elements within the construction of an Inductive sensor these are; the coil, oscillator, trigger circuit and output. 

An inductive sensor creates a high frequency field from the oscillator; the coil radiates this at the sensor tip producing a sensing field. When a metal object enters the sensing field, eddy currents will be induced within the metal, this in turn reduces energy from the oscillator. When the reduction in energy is sufficient the oscillator stalls. The trigger circuit on sensing that oscillation has stopped changes the state of the switching device in the output.

Oscillation amplitude decreases the closer the metal object is to the tip of a sensor, and increases as it moves away. This in essence is the operating principle of an inductive sensor.

 

 

As the metal object gets closer to the sensor, oscillation decreases and will ultimately stall.

 

 

Detects ferromagnetic metallic material, suitable for use in dirty and wet areas, immune to dust, but limited sensing distances (up to 50mm)

Advantages

  • Suitable for use in dirty environments, not effected by moisture or dust.
  • No moving parts, means no mechanical wear and tear
  • Less surface dependent than other sensing technologies
  • No blind zones
  • Detection is not colour dependant

Considerations

  • Will only sense metallic objects
  • Strong electromagnetic fields can effect operation

Applications

  • Assembly lines
  • Machine Tools
  • Metal Detection in substances/objects
  • Parts sorting
  • Identification of ferrous and non-ferrous metals (Ferrous selective sensors only)

 

Capacitive Sensors

Solid state devices using alterations in electrostatic fields for detection of metal and non metals, liquids and solids.

Operating Principle:

There are four basic elements within the construction of a capacitive sensor, these are; the plate, oscillator, trigger circuit and output.

The target object and the plate form the oscillators feedback circuit. Feedback capacitance occurs when the object and plate move closer, when there is significant capacitance, oscillation will begin. The trigger circuit senses the level of oscillation and controls the trigger circuit, which in turn changes the state of the switching device in the output.

Oscillation amplitude decreases the further the object is from the tip of a sensor, and increases as it moves close. This in essence is the operating principle of a capacitive sensor.

 

 

As the object gets closer capacitance increases between the plate and target object, thus causing oscillation.

 

 

Advantages

  • Detects metals non-metals, solids and liquids
  • Can see through certain materials (product boxes).
  • Solid-state, no wear and tear thus long life operation
  • Many mounting configurations

Considerations

  • Shorter sensing distance, varies according to target material, (25mm or less)
  • Sensitive to environment, humidity can effect sensing output
  • Not at all selective for its target, control of what comes close to the sensor is essential, (false triggering).

Applications

  • Liquid level sensing
  • Product filling lines
  • Plastic parts detection
  • Pallet Detection for materials handling
  • Irregularly shaped objects 


Ultrasonic Sensors

Ultrasonic sensors use the constant velocity of sound to detect the presence of objects.

Based on the principle that sound travels at a relative constant velocity, measuring the time it takes for the ultrasonic sound wave to leave the sensor and return after striking the target object is directly proportion to the distance of the target object.

A sound pulse is emitted by the sensor that reflects off objects entering the wave field. The reflected sound is received by the sensor, this sound echo generates an output signal when the object is within the specified range. This in essence is the operating principle of an ultrasonic sensor.

 

 

Time measurement of pulse and echo.

Advantages

  • Not dependent on the colour or optical reflectivity of the object. 
  • Background objects can be ignored.
  • Detects solids and liquids
  • Linear response with distance, means indications of distance can be given when interfaced with measurement device.
  • Large sensing range up to 15m

Considerations

  • Loud noises can give false reading
  • Sound absorbent objects can be difficult to sense at longer ranges
  • Ensure square Surface alignment

Applications

  • Distance & height measurement
  • Industrial level control
  • Glass and transparent object detection
  • Detection of missing parts or objects