1.4 PROXIMITY DETECTION
• At times we want to know when a physical object is present. This may be a non-contact detection.
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,
- target characteristics (transparent, reflective, diffuse, etc.)
• 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.
• 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.
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.