FEATURE MACHINE VISION SYSTEMS FIVE REASONS TO USE A LIGHTING
CONTROLLER IN MACHINE VISION I
llumination is a critical component of machine vision systems since it is lighting that determines how an object appears to the camera. LEDs are the most widely used illumination source and precise control of lighting can be crucial, with factors such as the direction, wavelength and intensity of illumination all having a key role. Here we look at five reasons why a dedicated lighting controller can be an essential part of a machine vision system.
1. CONSISTENT LIGHT LEVELS Machine vision systems are used to monitor, regulate, check, analyse, sort and classify, with high precision and repeatability. Consistent light levels are essential to guarantee repeatable results. In many industries, only one in 100,000 reject products can be tolerated which means that measurement reliability must be of the same order or better. Yet even with a simple caliper measurement, a 10 per cent reduction in light level can cause a change of 0.5 per cent in the measured values, which means that light intensity must be regulated extremely closely to meet required tolerances. A number of factors can affect the
illumination intensity. These include both the age and temperature of the LED, effects of ambient light, and external factors such as dust and dirt which may reduce the amount of light reaching the object. In one recent example, after a year in use, a machine vision system began rejecting good parts and accepting bad parts because, as the light had aged, the light intensity had dropped. Fortunately, the system had been equipped with a lighting controller that supported ethernet connectivity, so the brightness was easily increased via remote access to maintain long term performance without an on-site visit. Although LEDs are specified by voltage, their light output is actually directly
proportional to current, which means that if the current changes by one per cent the intensity will also change by one per cent. However, a very small change in voltage causes a much larger change in the current through the LED, and that large change in current will cause a large change in brightness. For example, a one per cent decrease in voltage will cause a 10 per cent reduction in current and hence a 10 per cent reduction in light output. Thus, to achieve accurate control of lighting levels it’s essential to provide stable current through the LED. A lighting system that is driven by a voltage source will inherently be susceptible to electrical interference or other variations in power supply.
2. PULSING/STROBING THE LIGHT In many applications, it is necessary to pulse the lighting. One example is when there is insufficient light intensity to obtain sufficient detail in the image when lit by a continuously-driven LED. In this case, image quality can usually be enhanced by overdriving the LED to increase the light output intensity. Large increases in brightness can be obtained by overdriving, but this should be done in short pulses to prevent damage to the LED, and requires precise control of pulse frequency, duration and amount of overdriving. Pulsing, (or strobing) a light source is often used to freeze an image of moving objects, particularly at high speeds, where overdriving may also be essential to deliver sufficient light intensity for the short camera exposure times needed. The flexibility a dedicated lighting
controller brings is illustrated by a vision system that was designed with the lighting controller operating the LED continuously at 100 per cent brightness. During commissioning ambient light was found to interfere with the images, so more light from the LED was needed. It was a simple matter to change the controller configuration from continuous to overdrive pulsing for 200 per cent brightness and shorten the camera exposure, meaning that the problem could be solved quickly without hardware changes.
3. MULTIPLE IMAGES FROM A SINGLE CAMERA A number of different measurements may be needed on an individual product at a particular point in the production process. This can be achieved by acquiring multiple images with a single
16 DECEMBER/JANUARY 2018 | AUTOMATION
camera using several lights operating under different lighting schemes. A dedicated lighting controller can trigger the lights at different intensities and for different durations in a defined sequence. This can eliminate the need for multiple cameras, saving both on cost and space. This is also possible to use multiple lighting schemes with line scan imaging, where information from different illumination sources can be captured on sequential lines and individual images for each illumination source extracted using image processing software.
4. ETHERNET COMMUNICATIONS Lighting controllers offering Ethernet communication provide great flexibility. They can be easily configured using a web interface and allow the lighting to be changed for different batches. Lighting status information can be acquired and any issues can be fixed remotely. Consider adding new products requiring different light levels to an existing production line equipped with a machine vision system. The system can be changed to send a different configuration for each batch change. Operators can select the new product from a menu and the light levels automatically change.
5. REDUCE COSTS Increased machine vision measurement accuracy reduces waste and customer returns, while careful lighting control can increase LED lifetime reducing cost of ownership of a vision system. The flexibility to provide multiple lighting schemes can reduce the need for additional hardware such as extra cameras. Ethernet communications can provide accurate system status information for preventative maintenance and changes can be made to a system remotely, reducing system downtime and removing engineer call-out costs.
CHOOSING THE RIGHT CONTROLLER LED lighting controllers from Gardasoft are available in 1, 2, 4, 8 and 16 channel modules. They offer precise current control in strobe, pulse or continuous modes to ensure stable and repeatable illumination for machine vision lighting and image recognition systems.
Gardasoft Vision
www.gardasoft.com /AUTOMATION T: 01954 234970
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