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Colour and contrast sensors – tools for effective quality control

Author : Pierre-Yves Dénervaud is International Product Manager at Contrinex

03 July 2017

Subjectivity has no place in a modern manufacturing plant. High-volume production needs effective in-process quality control for consistent, defect-free throughput.

Photoelectric colour sensors and contrast sensors in secondary packaging processes

Rigorous scrutiny is nothing new, but expectations have never been higher. Photoelectric sensors from Contrinex ensure that quality engineers have the tools they need.

Every technological advance brings new applications for industrial sensors. Modern devices from Contrinex provide solutions for some of the most demanding tasks in high-volume manufacturing. These include reliable detection of a product’s optical characteristics, including contrast and colour.

Contrast sensors use advanced optical sensing technology to detect the presence, absence or position of product features. Process engineers automate tasks that would otherwise require manual intervention or complex, costly mechanical solutions.

Colour sensors go one step further, identifying specified colours with a very high degree of accuracy. Colour changes between product variants, or minor variations in shade between batches no longer require the human eye as a trigger for action.

Contrast depends on degrees of difference

Successful applications for contrast sensors utilise good product design. RGB tri-colour LED technology equips Contrinex KTS-4155 contrast sensors to detect changes in product contrast and to trigger action accordingly. Auto-selection of the optimum light colour for sensing an expected mark ensures reliable detection of even small differences between the contrasting shades. An IO-Link interface offers the option of remote setup, operation and diagnostics.

Applications for contrast sensors are everywhere. Incorporating subtle colour contrasts into product design, whether to assist orientation or to identify a specific feature, carries little or no cost penalty; the payback is typically rapid. The use of product branding or packing labels in carefully chosen, contrasting colours simplifies inspection significantly. 

Checking label alignment and printing 

In the food-processing industry, packaged consumer products require labelling – both for identification and for batch control – before final packing. During confectionery production, sealed cartons of bagged candy travel by conveyor to a label printer-applicator, before passing to the final packing area. 

A contrast sensor, mounted beside the conveyor, checks the label alignment and confirms the presence of print markings as each carton leaves the labelling area. If a label is blank, illegible or wrongly positioned, the carton diverts to a holding area for investigation. 

Ensuring correct product alignment

Inspecting bottled beverages by sensing cap colour

In a consumer environment, poor presentation detracts from perceived product quality; contrast sensors are widely used during packaging operations to ensure uniformity. In a typical application, cans of paint must be uniformly aligned on pallets so that branding is clearly visible for display purposes.

A contrast sensor, mounted on the pick-and-place head that transfers cans to pallets, offers a highly effective solution. The can rotates in flight, allowing the sensor to detect a known, contrasting feature of the product branding; halting the rotation at the detection point ensures consistent product alignment on the pallet.

Detecting print-mark position

Print-mark detection is a common application for contrast sensors. For manufacturers of high-quality printed products correct positioning and alignment of continuously fed sheet material is essential. Handling a range of surface finishes, from plain paper to reflective metallic film, requires extremely versatile sensors.

During continuous production of pharmaceutical products in blister packs, manufacturers rely on print-mark detection for accurate pack positioning. Contrast sensors, mounted immediately prior to punching stations, detect the presence of print marks on the surface of the packaging film, ensuring each tray is punched accurately.

Colour sensors pinpoint an exact shade

Colour sensors also use RGB technology. Sensors illuminate the target, either with pulses of white light or with separate pulses of red, green and blue light. Analysing the intensity of each of the three components within the reflected beam(s) allows the sensor to identify the target’s exact shade and to compare it with one or more pre-set values. 

For a highly accurate match, the sensor performs a greater number of analyses; the trade-off here is a reduced switching frequency. With typical response times ranging from 200 to 400 µs, process engineers select the optimum balance between detection tolerance and process throughput without compromising performance. 

Identifying beverage product variants by sensing cap colour

Beverage manufacturers often produce product variants that differ only in respect of capping and labelling. Changing between variants without interrupting production is highly desirable; typically, an upstream change in cap colour triggers a corresponding move to an alternative labelling scheme. 

In-process colour-checking of continuously manufactured textiles

A Contrinex FTS-4155 colour sensor, mounted immediately after the capping station, provides real-time detection of cap colour. Sensors detect a change to a new product variant and communicate the cap colour to a central control system for action. Alternatively, the presence of an incorrectly coloured cap where no variant is expected may trigger a local rejection routine. 

Rejecting incorrect confectionery product by sensing bag colour

After filling, confectionery manufacturers typically convey individual bags of candy to secondary-packaging stations. It’s essential that only the required type of candy is packed; incorrect alternatives must be detected and removed from the conveyor. Bag colour is a common indicator of product type. 

A colour sensor, mounted beside the conveyor, checks each bag as it approaches the packing station and rejects it if the colour is incorrect. Rapid response times ensure reliable operation at high throughput rates. 

In-process colour-checking of continuously manufactured textiles 

During continuous manufacture of textiles and fabrics, effective quality control is essential; in-process colour checks are required across the full width of textile webs. Inspecting material immediately after its formation is highly desirable to allow real-time adjustment to the production process. 

A transverse-axis carriage lies immediately above the moving web; a sensing head, mounted on the carriage, traverses the width of the web as it passes below the head. Mounting a colour sensor on the sensing head provides an ideal means of non-contact inspection. High-frequency measurement accommodates the speed and width of the moving web. 

Seeing is believing

No two products are identical; application-specific characteristics, including colour variation, surface finish, target speed, size and sensing distance, determine whether colour or contrast sensors are right for real-time process control. What’s not in doubt is the ability of modern photoelectric devices to deliver reliable, accurate information in a timely fashion, simplifying the task of automating production across a broad spectrum of industries.

The enhanced performance of KTS-4155 and FTS-4155 colour and contrast sensors from Contrinex offers designers and process engineers a highly cost-effective means of verifying a product’s visual characteristics. Sensing accuracy is now at levels previously only available from more expensive vision systems; forward-thinking manufacturers have been quick to recognise the opportunity to steal a march on their competitors.

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