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Correct selection of a Photoelectric Switch Type

Three sensing modes are available with photoelectric switches - through beam, retro reflection and diffuse reflection. This application note explores the determination of the best sensing mode for a given application.

The best sensing mode is that one which yields the highest sensing contrast i.e. the greatest amount of difference in the intensity of light falling on the receiver between the conditions of target presence and target absence. For the through beam and retro reflective modes, the intensity of light at the receiver is the highest in the absence of a target. The entry of the target into the light path will either block out or diminish the intensity of the light at receiver. The difference between these two intensities is the contrast.

For the diffuse reflection mode, the intensity of light at the receiver is the lowest in the absence of a target. The presence of the latter diverts some of the transmitted light to the receiver, thus increasing the intensity of light at the receiver. The difference between these two intensities is the contrast. To determine which sensing mode will yield the highest contrast the following application details must be evaluated: target size, shape, opacity and reflectivity. Each of the three modes has its strengths and weaknesses, which are summarised below.

THROUGH BEAM

Strengths

Provided the object is opaque (i.e. it completely blocks out the transmitted light from the receiver), this mode is the most reliable one. In any application its use should therefore be always considered first.

For a given size this mode offers the highest sensing distance. Since the receiver is across from the transmitter and aligned with the latter, the highest amount of transmitted light energy (as compared with the other two modes) is available at the receiver, thus enabling higher distances.

Since the highest intensity level is available at the receiver, this mode works reliably in highly contaminated environments i.e. those where a lot of dust, dirt, smoke, moisture etc may be present.

Since the light path of this mode is the best defined of the three modes, it is best suited for parts counting applications. Where the size of the part is smaller than that of the light path, the size of the latter can be reduced by adding apertures to the two sensors (contact factory for details).

Since this mode is unaffected by the target's colour and reflectivity, it is best suited for applications where targets have varying colours and reflectivities.

Weaknesses

Where transparent or translucent objects need to be detected, this mode should be avoided. Small targets (i.e. smaller than the effective beam width) cannot be reliably detected by this mode without the addition of apertures to the two sensors.

When used at ranges much closer than those rated, this mode may actually sense through thin-walled opaque targets such as paper boxes, cloth, plastic bottles etc. the use of apertures over the two sensors will also help here. Where mounting space on both sides of the detection path is unavailable, this mode cannot be used.

Retro Reflection

Strengths

Where mounting space on both sides of the detection path is unavailable, this mode proves useful.

This mode is particularly useful in conveyor applications, more so when the conveyor runs along a wall.

Weaknesses

Given the relatively large width of the effective beam, this mode should not be used for the detection of small objects. For detection anywhere on the detection path, a target atleast as large as the provided reflector must be used. For closer-than-specified detection, smaller targets are acceptable.

Where transparent or translucent objects need to be detected, this mode should be avoided.

This type may malfunction where a shiny target enters the light path perfectly parallel to the sensor (in which case the light returned by the shiny object is parallel to the light incident on it; the sensor may thus confuse the shiny object for the reflector and may not respond). One solution is to position the sensor such that the target enters the light path angled (contact factory for details).

This mode should not be used for detection very close to the sensor body. The switch specifications generally specify a minimum sensing distance.

Diffuse Reflection

Strengths

Mounting is required only on one side of the detection path. Where mounting space is unavailable on both sides of the detection path and where relatively lower sensing distances are required, this mode is most useful.

The convenience of single side mounting and relatively lower cost.

This mode is best suited for the detection of transparent objects.

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This mode can be used for the detection of very small objects albeit at closer distances (consult factory for details).

This mode can be used for the detection of shiny targets (consult factory for details).

Weaknesses

As the reflectance of the target changes so does the sensing distance (the rated sensing distance is for a white Kodak reference card). Where targets of varying colours need to be detected the sensor should be positioned so as to detect the least reflective colour.

This mode should not be used in contaminated environments; even a small build-up of dirt, moisture etc may cause the switch to malfunction.

In summary

The first choice should always be through beam; where this mode is unsuitable for reasons given above, use of the other two modes should be considered.

In highly contaminated applications use of the through beam or retro reflective modes is advisable; use of the diffuse reflection mode should never be considered.

For parts counting applications, especially where the parts fall at random distances along the detection path, the through beam mode is best used.

Where adequate mounting space is unavailable on both sides of the detection path, use of the retro reflective or diffuse reflection modes should be considered.

For detection of small objects use of the diffuse reflection type can be considered. The through beam mode may also be used, if necessary by the addition of apertures.

For the detection of transparent objects the diffuse reflection mode is best suited.