Diffuse optical scanning sensors - Part 1

There are many myths about sensors, in particular the world of diffuse scanning sensors. With many users preferring the ‘make/break’ philosophy of retro-reflective and thru-beam sensors, we thought it might be important to take a look at what diffuse scanning sensors are all about, what’s hot and what’s not, and show you how you might be able to choose a more reliable sensor from the diffuse scanning options available.

It is often said in the automotive world that “oils ain’t oils”. In the same way, in the industrial automation world “diffuse sensors ain’t diffuse sensors”. There are various technologies available in diffuse scanning sensors, each with their own advantages and challenges.

General operating principle

Like retro-reflective photoelectric sensors, the transmitter and receiver are contained in a single device. However, the transmitter light is not reflected by a reflector but by the object to be detected itself. In this case relatively little light gets back to the receiver, which means shorter operating ranges (scanning ranges) can be realised than with the reflection principle.

The achievable scanning range will depend on several variable factors, including the distance of the object, the colour of the object, the object’s size and surface structure, and the angular pitch of the object surface.

Object distance

The intensity of the reflected light (known as remission) is highly dependent on the distance of the object. The closer an object is to the scanner, the greater the amount of reflected light on the receiver.

Object colour

Dark surfaces reflect significantly less light than lighter-coloured surfaces. For this reason, the achievable scanning ranges also largely depend on the colour of the target object to be detected.

Object size

The light emitted by the scanner forms a light spot on the object to be detected. If not all of this light spot strikes the object, only a lower amount of light can also be reflected, which in turn results in shorter scanning ranges.

Surface structure of the object

Object surfaces can also have different structural properties - the range extends from ‘smooth as glass’ up to rough surfaces such as raw timber logs. Smooth surfaces reflect the light back highly aligned, due to the law of reflection. Rough surfaces, in contrast, cause wide diffusion of the light. Consequently, greater scanning ranges can be realised with smooth surfaces than with rough ones.

Angular pitch of the object

Although only low scanning ranges can be realised with rough, diffusely reflecting surfaces, pitching of the object does not significantly affect the function of the sensor. With smooth reflecting surfaces, however, minimal tilting of the object can lead to false signals.

Black/white behaviour

A key parameter in diffuse reflection scanners is the black/white behaviour. This refers to the relationship of the scanning ranges to black and white target objects.

Figure 1: Black/white behaviour

The black/white behaviour can be graphically illustrated in the diagram shown in Figure 1. Here, the scanning range is on the x-axis and the reduction of the scanning range is on the y-axis. The scanning range is set with a white (90%) object as reference, so that the scanning range reduction for white objects is equal to 0 and the white curve is identical with the x-axis. Shorter scanning ranges are achieved with grey (18%) and black (6%) objects. In other words, if a white object is sensed at a distance of 200 mm, a grey or black one will be detected at 200 mm minus the y-value (resulting in, say, 150 mm for black). This variation in the scanning range is known as the ‘black/white error’.

Diffuse scanning classes

Diffuse reflection scanners can be divided into the following classes according to their black/white behaviour:

• Energetic diffuse reflection light scanner: ~70% b/w error
• Scanner with fading suppression: ~50% b/w error
• Scanner with background suppression: <10% b/w error
• Scanner with foreground suppression: <10% b/w error

Energetic diffuse scanner construction

Figure 2: Energetic diffuse scanner

In addition to the operating principle discussed above, the optical construction of an energetic diffuse reflection light scanner is also similar to that of a dual lens retro-reflective photoelectric sensor. The transmitter and receiver always have their own respective optics - there is no single-lens scanner (see Figure 2). If the distance between the object and sensor is too small, the light beam reflected by the object no longer strikes the receiving lens. This means energetic diffuse reflection light scanners are ‘blind’ at short ranges. The so-called ‘blind zone’ varies according to the colour of the object. With a white object the blind zone is very small, but it is noticeably larger with grey or black objects. Typical areas of application include detection of objects of the same colour and surface structure on conveyors, simple collision protection on overhead conveyors, differentiation of strong contrasts and plate glass detection (with no background present).

Fading scanner construction

The way the scanner with fading works essentially corresponds to that of the energetic diffuse reflection light scanner. But by placing a mechanical diaphragm against the receiving element (see Figure 3), a specific suppression of the background is achieved. Objects are reliably detected by scanners with fading suppression as long as sufficient reflected light strikes the receiving element through the hole in the diaphragm.

Figure 3: Fading scanner

If the dark object in Figure 3 is now removed, the effect is the same as with energetic diffuse reflection light scanners - the light background previously directly behind the object continues to be detected.

Figure 4: Diaphragm adjustment in a fading sensor

So that the light background does not trigger a signal at the sensor, the diaphragm must be adjusted so that its reflected light must be projected onto the lower edge of the diaphragm - outside of the diaphragm hole (see Figure 4). The extreme situation of a dark object directly against a light background cannot therefore be resolved with this type of diffuse scanner, because the angle of reflection has not changed sufficiently and there is not enough light reflected back from the dark object.

Typical areas of application of fading scanners include object detection on conveyor tracks with a defined background or short scanning range and price-sensitive project applications.

In Part 2

As you can see from the above discussion, the differentiation between the foreground object and the background can present difficulties in some situations. In Part 2 of this article we will look at the technologies used to achieve background suppression and conclude with how to choose the right diffuse optical scanning sensor for your application.

By Tony Barnett, National Product Manager, Leuze Electronic