TechArchive

Original article date: November 1998

Photoelectric sensors are one of the key elements of virtually all factory automation systems and there is a tendency to take the advances for granted says KEITH WILLIAMS of IMO Precision Controls.

While it is true that the fundamental technology of photoelectrics has not changed much since Kennedy was President what they can achieve has changed enormously. And the advances continue.

Perhaps not surprisingly the low profile of photoelectric sensors has resulted in a lack of awareness of some of the key differences between the various types of sensor even amongst people using systems dependent upon them. Manufacturer’s data sheets will provide data on the detecting range of each of their sensors. This is the maximum operating distance over which the device will work. As a general rule the shorter the maximum range of a sensor the more precisely it detects.

So the first thing to do is to decide how close the device can be to the object to be detected. You then choose a photoelectric with a range just beyond this distance. Do not be tempted to choose a sensor with plenty of spare range. This will reduce the accuracy and reliability of the system. if an object can be detected reliably at a range of. say l0mm with no risk of the object touching the photocell at any time choose a suitably low power photocell. Too much power will sometimes “see” right through an object or in the case of a diffuse sensor can reduce effective resolution or cause spurious background reflections.

The minimum object size which can be safely detected is also shown on data sheets. In high speed applications it is important to think about the time for which the object to be detected will be in front of the photoelectric. The minimum object size as defined in the data sheet should be in front of the sensor for the whole of the minimum response time as shown in the data sheet otherwise the object may not be detected.

With through-beam and retroreflective photoelectric switches the so-called “standard detectable object” is one which interrupts a sufficient part of the light beam to cause detection. For diffuse-reflection sensors a piece of white matt paper is used of a size defined by the data sheet for that particular switch.

There have been considerable advances in the sensitivity of photoelectric sensors and switches during the last couple of years to meet the increasingly stringent requirements in inspection processes. Some extremely sensitive devices have been developed able to identify extremely small differences in the colour and size of objects.

When a user is attempting to resolve a very small difference of this sort there is a temptation to set the sensitivity of the photoelectric too close to the switching point. This can lead to false results and to nuisance tripping caused by changes in ambient light temperature or atmospheric condition. Increasing false alarms can also arise as the system ages or if inadequate provision has been made for keeping photoelectric receivers clean.

To avoid these problems the more sensitive photoelectrics incorporate a green stability indicator LED to help with setting up. The LED lights when the object to be detected is within the reliable operating range of the photoelectric switch and should always be orn while the switch is in operation.

The stability indicator can also be useful when the photoelectric is being used in a dusty environment since as the dust builds up on the lens the stability LED will go out indicating that it is time to clean the lens.

Designing a system to use photoelectric detectors efficiently is one of those tasks for which the obvious solution is not always the best. If you have any doubts at all it is best to ask someone with experience as a system designer before you commit time and money.

What kinds of photoelectric switch are available and how do you select the right type for any particular task?

Through-beam detectors: This type of photoelectric has a separate transmitter and receiver and detects any object large enough to break the beam between the two. It is suitable only for opaque objects which are always large enough to break the beam. Avoid using it if there is any likelihood of needing to detect transparent objects.

Diffuse reflective detectors: The diffuse detector has its transmitter and receiver in a single head. It works on the principle of transmitting a beam of light and detecting reflections from the objects to be detected. Since all colours will reflect some light diffuse detectors can be used to detect almost all types of object although at closer range than separate types. It is particularly suitable for transparent objects such as bottles. Avoid using this type of detector where there is a background close to or more reflective than the object since the background will also be detected.

Fixed focus detectors: These are similar in operation to diffuse detectors. The most advanced models will ignore both background and foreground objects. IMO’s SAT also has a choice of focusing lenses for different detecting distances as well as a sensitivity adjustment. Fixed focus detectors are particularly useful for applications where two reflections must not be confused such as detecting the contents of a transparent tunnel or detecting the position of a meter needle.

Background suppression detectors: Similar to fixed focus detectors but using a triangular principle to ignore background objects background suppression detectors have the advantage that they can operate at a constant distance regardless of the colour and glossiness of the objects to be detected. Background suppression detectors also sense the presence of objects over greater distances than fixed focus models.

Retroreflective detectors: These operate on essentially the same beam-break principle as through-beam detectors but with the transmitter and the receiver in the same unit. The beam is bounced off a prismatic reflector which sends the beam back in the direction whence it came. Retroreflective sensors can be used for broadly the same applications as separate through-beam sensors but are easier to set up and wire since only one unit has to be connected and securing correct alignment is much more straightforward. However retroreflective sensors should not be used to detect objects which will reflect light such as polished metal cellophane or shrink-wrapped packages since they cannot distinguish between the two types of reflections.

Retroreflective detectors with polarised light: For detecting shiny objects it is best to use a retroreflective photoelectric switch in conjunction with polarising filters one aligned vertically in front of the transmitter lens the other horizontally in front of the receiver. The prismatic reflector used with the photoelectric is designed to re-orientate polarised light through 90deg so uninterrupted vertically polarised light from the transmitter reaches the receiver as horizontally polarised light. If an object is between the transmitter and receiver the light reflected from the object is returned still with vertical polarisatlon and does not pass through the horizontally aligned polarising filter so is not registered by the receiver.

This technique makes it possible to obtain reliable detection under difficult conditions such as sensing shrink-wrapped products or kegs whose characteristics would normally prevent the use of a retroreflective detector.

Optical fibre detectors: Available as through-beam or diffuse detectors optical-fibre sensors use a fibre-optic light guide to carry light from a remotely mounted amplifier unit to an object to be detected. The principle offers several advantages over conventional detectors:

Small size: the detecting head can be fitted into very confined spaces.

Sensitivity: Objects as small as 0.1mm diameter or even less can be reliably detected.

Noise immunity: The fibre can be installed close to high power cables without interference problems.

Optical fibres can also be taken into areas where ordinary photoelectrics cannot normally be used for safety reasons such as aerosol filling machines and areas containing paint fumes. The amplifier can be mounted remotely from the hazardous area and only the fibre is taken into the area of risk.

• IMO Precision Controls
• Keith Williams
• 0181 452 6444

November 1998