1.4.1 Optical (Photoelectric) Sensors

 

• Optical sensors can detect part presence using a light source and detector.

 

• Emitters generate light in visible and infrared light bands. These are usually LEDs or laser diodes.

 

• Detectors are designed to vary electrically as light intensity varies. The most common used is the phototransistor.

 

• Ambient light can interfere with a simple optical beam. As a result most sensors now use a modulated pulse with a frequency up to the low KHz range. This allows better detection at longer distances with lower power.

 

 

• The relative locations of the source and detectors, as well as surface conditions have a major impact on the selection of sensor types. These include,

- distance to target

- target characteristics (transparent, reflective, diffuse, etc.)

- target size

 

• The simplest form uses a detector only with ambient or radiated light.

- ambient light requires care in scene lighting

- radiated light requires some sort of photometric phenomenon such as a hot part will radiate infrared light.

 

• Optic sensors can often be separated for space and other constraints.

- fiberoptics allow the lens to be separated from the LED or phototransistor.

- the phototransistors and LEDs can be separated from the other circuitry to fit the sensors into smaller parts.

 

• When the emitter and detector are separated and the beam is interrupted this is known as opposed mode.

 

 

• When the emitter and detector are in a single unit this is known as retroreflective.

 

 

• Polarized light can be generated using filters.

 

 

• Diffuse sensors are like the retroreflective type, except that the returning light does not need to be polarized.

 

 

• Alignment of the emitter is necessary, and can be a problem if the sensors are separated by a large distance and the beam intensity decreases.

 

 

• The beam of emitted light should generally be less than the width of the detected part.

 

 

• Separated sensors can detect reflective parts using specular reflection. This needs a reflective surface.

 

 

• By focussing emitters and detectors optics we can sense presence at a specific distance. This is known as convergent beams sensing.

 

 

• Fixed field sensors use a physical setting.

 

 

• Opposed beams can also be for a large range light curtains.

 

 

• Typical reflectivity values are given below [Banner Handbook of Photoelectric Sensing]

 

 

• Many sensors have sensitivity adjustments that will need to be adjusted to the materials.

 

 

1.4.2 Capacitive Sensors

 

• Uses changes in capacitance to detect part presence. Recall the basic equation.

 

 

• Works well with most materials, very good for plastics.

 

 

• If the part is conductive it acts as added surface area for the capacitive plates and increases capacitance. If the part is nonconductive it acts like a dielectric and increases the capacitance. In total the changes are normally in the order of pF.

 

 

• In the sensors the electrodes are normally round rings.

 

 

• Different materials have various dielectric properties. The list below is a sample from [Turck Proximity Sensors Guide].

 

 

 

 

1.4.3 Inductive Sensors

 

• These sensors work for all metals (conducting materials).

 

• These use an oscillating magnetic field.

 

 

• 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.

 

 

1.4.4 Ultrasonic

 

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

 

• 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.

 

 

1.4.5 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.

 

 

1.4.6 Fluid Flow

 

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

 

 

 

1.4.7 Other Types

 

• Other sensors types include,

- color detection