A photoelectric sensor is another type of position sensing device. Photoelectric sensors, similar to the ones shown below, use a modulated light beam that is either broken or reflected by the target.
Modulated light increases the sensing range while reducing the effect of ambient light. Modulated light is pulsed at a specific frequency between 5 and 30 KHz. The photoelectric sensor is able to distinguish the modulated light from ambient light. Light sources used by these sensors range in the light spectrum from visible green to invisible infrared. Light-emitting diode (LED) sources are typically used.
CLEARANCE
It is possible that two photoelectric devices operating in close proximity to each other can cause interference. The problem may be rectified with alignment or covers. The following clearances between sensors are given as a starting point. In some cases it may be necessary to increase the distance between sensors.
Many environments, particularly industrial applications, include dust, dirt, smoke, moisture, or other airborne contaminants. A sensor operating in an environment that contains these contaminants requires more light to operate properly. There are six grades of contamination:
1. Clean Air (Ideal condition, climate controlled or sterile)
2. Slight Contamination (Indoor, nonindustrial areas, office
buildings)
3. Low Contamination (Warehouse, light industry, material
handling operations)
4. Moderate Contamination (Milling operations, high
humidity, steam)
5. High Contamination (Heavy particle laden air, extreme
wash down environments, grain elevators)
6. Extreme/Severe Contamination (Coal bins, residue on lens)
Excess gain represents the amount of light emitted by the transmitter in excess of the amount required to operate the receiver. In clean environments an excess gain equal to or greater than 1 is usually sufficient to operate the sensor’s receiver. If, for example, an environment contained enough airborne contaminants to absorb 50% of the light emitted by the transmitter, a minimum excess gain of 2 would be required to operate the sensor’s receiver. Excess gain is plotted on a logarithmic chart. The example shown below is an excess gain chart for an M12 thru-beam sensor. If the required sensing distance is 1 m there is an excess gain of 30. This means there is 30 times more light than
required in clean air hitting the receiver. Excess gain decreases as sensing distance increases. Keep in mind that the sensing distance for thru-beam sensors is from the transmitter to the receiver and the sensing distance for reflective sensors is from the transmitter to the target.
Photoelectric sensors have a switching zone. The switching zone is based on the beam pattern and diameter of the light from the sensor’s emitter. The receiver will operate when a target enters this area.
SCAN TECHNIQUES
A scan technique is a method used by photoelectric sensors to detect an object (target). In part, the best technique to use depends on the target. Some targets are opaque and others are highly reflective. In some cases it is necessary to detect a change in color. Scanning distance is also a factor in selecting a scan technique. Some techniques work well at greater distances while others work better when the target is closer to the sensor.
THROUGH BEAM
Separate emitter and receiver units are required for a thru-beam sensor. The units are aligned in a way that the greatest possible amount of pulsed light from the transmitter reaches the
receiver. An object (target) placed in the path of the light beam blocks the light to the receiver, causing the receiver’s output to change state. When the target no longer blocks the light path
the receiver’s output returns to its normal state. Thru-beam is suitable for detection of opaque or reflective objects. It cannot be used to detect transparent objects. In addition, vibration can cause alignment problems. The high excess gain of thru-beam sensors make them suitable for
environments with airborne contaminants. The maximum sensing range is 300 feet.
The effective beam of a photoelectric sensor is the region of the beam’s diameter where a target is detected. The effective beam on a thru-beam sensor is the diameter of the emitter and receiver lens. The effective beam extends from the emitter lens to the receiver lens. The minimum size of the target should equal the diameter of the lens.
REFLECTIVE OR RETROEFLECTIVE SCAN
Reflective and retroreflective scan are two names for the same technique. The emitter and receiver are in one unit. Light from the emitter is transmitted in a straight line to a reflector and returns to the receiver. A normal or a corner-cube reflector can be used. When a target blocks the light path the output of the sensor changes state. When the target no longer blocks the light path the sensor returns to its normal state. The maximum sensing range is 35 feet.
RETROREFLECTIVE SCAN EFFECTIVE BEAM
The effective beam is tapered from the sensor’s lens to the edges of the reflector. The minimum size of the target should equal the size of the reflector.
Reflectors are ordered separately from sensors. Reflectors come in various sizes and can be round or rectangular in shape or reflective tape. The sensing distance is specified with a particular reflector. Reflective tape should not be used with polarized retroreflective sensors
RETROREFLECTIVE SCAN AND SHINY OBJECTS
Retroreflective scan sensors may not be able to detect shiny objects. Shiny objects reflect light back to the sensor. The sensor is unable to differentiate between light reflected from a shiny object and light reflected from a reflector.
A variation of retroreflective scan is polarized retroreflective scan. Polarizing filters are placed in front of the emitter and receiver lenses. The polarizing filter projects the emitter’s beam in one plane only. This light is said to be polarized. A corner-cube reflector must be used to rotate the light reflected back to the receiver. The polarizing filter on the receiver allows rotated light to pass through to the receiver. In comparison to retroreflective scan, polarized retroreflective scan works well when trying to detect shiny objects.
DIFFUSE SCAN
The emitter and receiver are in one unit. Light from the emitter strikes the target and the reflected light is diffused from the surface at all angles. If the receiver receives enough reflected
light the output will switch states. When no light is reflected back to the receiver the output returns to its original state. In diffuse scanning the emitter is placed perpendicular to the target. The receiver will be at some angle in order to receive some of the scattered (diffuse) reflection. Only a small amount of light will reach the receiver, therefore, this technique has an effective range of about 40”
The specified sensing range of diffuse sensors is achieved by using a matte white paper. The following correction values may be applied to other surfaces. These values are guidelines only
and some trial and error may be necessary to get correct operation.
Test Card (Matte White) 100%
White Paper 80%
Gray PVC 57%
Printed Newspaper 60%
Lightly Colored Wood 73%
Cork 65%
White Plastic 70%
Black Plastic 22%
Neoprene, Black 20%
Automobile Tires 15%
Aluminum, Untreated 200%
Aluminum, Black Anodized 150%
Aluminum, Matte (Brushed Finish) 120%
Stainless Steel, Polished 230%
DIFFUSE SCAN WITH BACKGROUND SUPPRESSION
Diffuse scan with background suppression is used to detect objects up to a certain distance. Objects beyond the specified distance are ignored. Background suppression is accomplished
with a position sensor detector (PSD). Reflected light from the target hits the PSD at different angles, depending on the distance of the target. The greater the distance the narrower the angle of the reflected light.
The effective beam is equal to the size of the target when located in the beam pattern.
There are two operating modes: dark operate (DO) and light operate (LO). Dark operate is an operating mode in which the load is energized when light from the emitter is absent from the
receiver.
Fiber optics is not a scan technique, but another method for transmitting light. Fiber optic sensors use an emitter, receiver, and a flexible cable packed with tiny fibers that transmit light. Depending on the sensor there may be a separate cable for the emitter and receiver, or it may use a single cable. When a single cable is used, the emitter and receiver use various methods to distribute emitter and transmitter fibers within a cable. Glass fibers are used when the emitter source is infrared light. Plastic
fibers are used when the emitter source is visible light.
Lasers are sometimes used as sensor light sources. Siemens uses Class 2 lasers which have a maximum radiant power of 1 mW. Class 2 lasers require no protective measures and a laser
protection officer is not required. However, a warning notice must be displayed when laser sensors are used. Laser sensors are available in thru-beam, diffuse scan, and diffuse scan with background suppression versions. Lasers have a high intensity visible light, which makes setup and adjustment
easy. Laser technology allows for detection of extremely small objects at a distance. The Siemens L18 sensor, for example, will detect an object of 0.03 mm at a distance of 80 cm. Examples
of laser sensor applications include exact positioning, speed detection, or checking thread thickness of 0.1 mm and over
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