December 2021

32 | Plant & Works Engineering www.pwemag.co.uk Annual Buyers’ Guide 2022 Process, Controls & Plant Focus on: Sensors & Measurement colour space. Depending on the colour, the human perception is limited at 0.5 to 1. The automotive industry requires Delta-E < 0.1. Metrological challenge When the observation geometry is determined, colour is (in physical terms) a reflected intensity spectrum in the visible wavelength range. This reflection spectrum depends on the object colour and the illumination. Illumination is defined by different light sources, for example light bulbs, daylight, fluorescent lamps or cold white LEDs. A colour sensor detects the reflected spectrum and imitates the principle of the human eye. For measurements, the reflected light is refracted into its spectral parts. The easiest method is to use filters where only one part of the spectrum can pass through. Most conventional CCD cameras whose sensors are divided into green, red and blue pixels are also based on this technique. A more accurate principle is used by colour sensors for industrial applications. The specimen is illuminated by a light source, which in most cases is a white light LED. The light reflected by the probe hits the sensor where the light passes through different filters and eventually reaches a light-sensitive sensor element. The absorption spectra of the filters used should overlap. The filters divide the light into long-wave (X), medium-wave (Y) and short-wave (Z) parts. The individual signals are then transformed into L*a*b* colour values. This is how measurement values are generated which enables the assignment of colours according to the colour perception of the human eye. Therefore, we refer to these as perceptive colour sensors or true colour sensors. They are ideal for recognition of colour deviations. A teach-in function enables the user to program the desired colour on the sensor and to determine the maximum permissible colour deviation. Next, the sensor compares the colour of the products during operation and can for example send a signal via a digital output if the colour of the specimen is within the tolerance limits. Another functional principle is applied by colour measurement systems. This divides the spectrum of the incident light via refraction on a grid into 256 parts, which are imaged behind the grid onto a CCD sensor line. This is how the complete visible spectrum can be accurately measured with a spectral resolution of 5nm. The colour measurement system not only compares the colours to reference values, but also identifies and outputs these as coordinates in the colour space. As well as the sensor system, the white light LED is incorporated into the housing of the system, enabling different sensor heads to be connected via optical-fibre cables. In the ACS1 standard sensor for conventional measurement tasks, illumination and receiver are either arranged at an angle of 30°x: 0° or 45°x: 0°. This leads to observation distances of 50mm or 38mm respectively. For more complex measurement tasks, for example, with structured, highly reflective or shiny metallic surfaces, the ACS2 circular sensor with 24 circular arranged lighting optics at a measuring angle of 45°c:0° is used. This ensures continuous, homogeneous lighting for measurements, regardless of the angular position of the target object. Even with smaller measurement objects or on curved surfaces, reliable colour measurements are possible. For colour measurements of transparent objects such as film or glass, the ACS3 transmission sensor is required where illumination and receiver are arranged at an angle of 180°:0° to each other. This receiver sensor also measures colours of self-luminous objects. No lighting unit is required. Ease of use Although ‘colour measurement’ is a complex issue, colour sensors and colour measurement systems from Micro-Epsilon are easy to use. Colour teaching is carried out using a teach-in button; LEDs on the colour sensor or the measurement system signify the mode of operation. In most cases, configuration is performed via an integrated web server. Settings such as illumination and measurement frequency for perfect colour recognition and measurements can be set up easily. The output signals are also set up directly using the web interface. In order to ensure high quality standards, measurement of colour is an invaluable technique in numerous industrial applications. A typical example is the automotive industry: most bumpers, door handles and exterior mirrors are painted the same colour as the car chassis. Furthermore, distance sensors and headlight cleaning systems are integrated in the bumpers. These components are painted separately but must precisely match the colour of the body. During the assembly process, the measurement system verifies if these colours are identical because the human eye recognises even the smallest of deviations. Also, in the passenger compartment, the colours must be correct. Here, structured, curved and reflecting surfaces are additional challenges for colour measurement technology. Another example is in the production of facade panels made from zinc whose surface colour can change after being coated. During production, the colour must be inspected continuously in order to ensure early recognition of any deviations. Otherwise, facade panels with slightly deviating colours would stand out, particularly in large facade areas. In tablet production, the slightest colour deviations must be detected. In transparent blister packs, different colour shades would give a poor impression. * Glenn Wedgbrow is Business Development Manager at Micro-Epsilon UK