TechArchive

Original article date: March 1999

Two methods of detecting contrasting colour for production systems are described in detail in a new technical guide from OMRON.

Colour mark sensors are a type of diffuse sensor designed to detect a mark by the contrast between the mark and the background.

A diffuse sensor incorporates emitter and receiver in the same body much like a retro-reflective sensor but without the reflector to return the beam to the receiver. Instead the target itself reflects the light back to the sensor and the target object is detected. This principle has the advantage of only requiring wiring and fitting in one place making it especially useful for applications where there is only access to one side.

The lens arrangement of colour mark sensors is slightly different to that of a standard diffuse type in that they are focused The amplifier is also more sensitive to changes of colour. The sensitivity or mark sensors is also influenced by the light source used. A red light source (which is the normal light source for most diffuse sensors) provides reasonable sensing distances but is more limited in its response to colour combinations and in particular red on white as the red content is reflected.

If a green LED is used it means a smaller sensing distance but it is much more sensitive across the colour range. It is more suitable for like colours (those with less of a contrast between mark and background).

Which colour light sources should be considered when wishing to detect a mark? If the marks (such as shades of the same colour) are to be detected then sensitivity adjustment can be difficult if not almost impossible if the shades have minute differences. If this is the case a different type of sensor called the RGB sensor can be used.

An RGB sensor works not on the amount of light received but on what the light actually consists of recognising that light is actually made up of many different parts visible light being only a small part.

So to detect colours accurately it is possible to analyse the content of the light which is reflected. Firstly it is necessary to know the exact nature of light that is transmitted. Omron’s E3MC sensor has three light sources which are red green and blue LEDs. They emit lens via the same lens and this is reflected from the target object. Depending upon the colour of the target object different quantities of light are absorbed and reflected.

The reflected light is detected by a photodiode and is then analysed by the sensor for the content of R (red) G (green) and B (blue) . The operation of the sensor is then dependent upon which of the two operation modes the user has selected.

If C mode has been used the ration between R G and B is analysed. Regardless of the sensing distance the colour ratio of the light will not change although the intensity will. This gives the advantage of stable detection if the target is mechanically fluctuating.

In I mode the detection of minute colour differences is possible. This mode uses the intensity of the RGB reflected light. Using this method the detection of minute colour differences is achievable but unlike the C mode the sensor is influenced by the mechanical position of the target.

Whichever method is used a teach function is used to teach the sensor the target colour required. The light received is indicated by a bar graph and the threshold level can be set to determine the operation point.

• Omron
• Tel: 0181 450 4646
• Web:http://www.omron.co.uk

March 1999