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page 154

inductive coil

metal

+V

oscillator

output

and level

switching

detector

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note: these work by setting up a high frequency field. If a target nears the field will induce eddy currents. These currents consume power because of resistance, so energy is in the field is lost, and the signal amplitude decreases. The detector examines filed magnitude to determine when it has decreased enough to switch.

• The coils can be shielded to make them more selective to the front of the coils. Unshielded coils have larger fields and sensitivity to the sides.

• Clearly ferrous targets will work well, but other metals can be used also.

12.4.4 Ultrasonic

• The reflection of sound waves can be used to detect parts or distances.

page 155

These normally use frequencies above 20KHz which is above the normal human hearing threshold of 16KHz.

These sensors are commonly,

electrostatic - uses capacitive effects. It has longer ranges and wider bandwidth, but is more sensitive.

piezoelectric - based on charge displacement during strain in crystal lattices. These are rugged and inexpensive.

Good for sensing distances to most materials with surfaces perpendicular to the beam.

Applications of these are similar to optical sensors.

12.4.5 Hall Effect

These sensors use a magnetic field in a semiconductor to change current flow.

As a result these sensors are designed to switch in the presence of a magnetic field.

these are typically used for no contact limit/home detectors in machines.

12.4.6 Fluid Flow

• A specific fluid flow rate can be detected using a float in a tapered channel.

fluid flow out

float

 

proximity sensor

 

 

 

fluid flow in

As the fluid flow increases the float is forced higher. A proximity sensor can be used to detect when the float reaches a certain height.

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