Line scan imaging is used extensively for high-speed, continuous inspection of materials such as sheet steel, paper, textiles and packaging film. Line scan systems lends themselves to versatile imaging techniques by utilising high line resolutions, multi-line configurations, or using dedicated controllers to provide timing and lighting control. A typical system might look like figure 1 which utilises a lighting controller and a timing controller. Lighting controllers can help overcome many of the shortcomings of traditional line scan imaging systems and provide some significant benefits, including:
• Reduced cost
• Higher performance
• More reliable inspections
• Novel imaging configurations
• More compact systems
Line scan cameras will require precise control in most applications. For example, the camera line rate must be synchronised to the speed of the moving object to avoid image distortion, and precise control of lighting intensity is essential since the fast line rates permit each pixel very little light.
OPTIMISING LIGHTING INTENSITY
Stable lighting is a crucial factor in all machine vision systems which need reliable, reproducible inspections. All LED lighting deteriorates in efficiency over time due to aging, so the ability of lighting controllers to compensate for this is of critical importance. Most line scan applications use continuous illumination and aging will be most problematic if the light is being run continuously at 100% of its rating. The solution is to use an LED with a higher rating than necessary and drive it via a lighting controller. When the system is first set up, the controller can be used to set the brightness to 70% or 80% of maximum. As the light gets older, the brightness can be adjusted upwards to compensate for the loss of efficiency and extend the useful lifetime of the light. With Ethernet connectivity, these adjustments can be carried out remotely. Lighting controllers can also be used to ensure uniform illumination over the full length of the line sensor to compensate for geometric variations. For line lights with a uniform brightness, images tend to be darker at the ends for geometrical reasons. This can be overcome by splitting the light into segments and using a lighting controller to modify the brightness of each segment to create uniform illumination across the length of the light. This gives true flat field correction, which will perform better than a softwarebased compromise.
REDUCING THE NUMBER OF CAMERA STATIONS
Since line scan cameras run at increasingly high line frequencies, there are more opportunities to be creative in their use. Applications that previously required two or more camera stations can now often be combined into a single camera station. This can lower costs as well as reducing the overall size of the system. Typically, a combined station will involve topologies where a controller drives multiple lights in a sequence. Images from each light are acquired on consecutive lines using a single camera and the separate images are extracted using software.
The example in figure 2 shows a two-light system, where the object is illuminated by both dark field and bright field light sources. Bright field is often used to generate high contrast to reveal topographic detail whereas dark field lighting can emphasise surface imperfections. The controller drives the bright field and dark field illumination sources in synchronisation with alternative lines on the camera sensor. In the resulting interleaved image, the odd lines of the sensor correspond to a dark field image of the object and the even lines to the bright field image. The two individual images can then be extracted with software. Accurate timing control is necessary to ensure that the lines don’t get one out of step. Different combinations of light sources could be used in this approach with different geometries or wavelengths, making this a very versatile technique. Accurate trigger timing can also be used to eliminate the possibility of random noise disrupting multi-light sequences. This technique can also be used with 3 or more lights to save a lot of cost without losing resolution, by driving the lights hard at a high enough frequency, making the use of a high quality controller to do this accurately and safely. For example, an eight-light system can be used for the inspection of printed film. Four independent images can be created using a single line scan camera to check for print quality, surface defects from different directions and for holes.
HIGH PERFORMANCE TRIGGERING
Line scan cameras require accurate, high-speed trigger signals. Many machine vision systems use a frame grabber to generate these signals. However, not all systems feature a frame grabber, and if they do it may be located a long way from the camera, especially if GigE Vision or CoaXPress connectivity is being used. In these cases, a very fast and accurate trigger timing controller is needed. For example, Gardasoft’s CC320 Machine Vision Timing Controller offers the functionality of a high-performance
PLC in a format ideal for machine vision applications and provides an easy and complete solution for precision triggering of not only of the camera but also of the lighting controller. This can be particularly useful when an encoder is used to generate the basic timing, since it is generally not possible to for the encoder to directly trigger each line of the camera.
LIGHTING CONTROL FOR LINE SCAN IMAGING
High-frequency line scan cameras require consistent and accurate pulsing of the LED output in close synchronization with the line rate, particularly in multi-light systems. It is important for the lighting to be pulsed with a fast and consistent rise and fall time. Poor lighting control can give an inconsistent pulse shape which may have overlapping pulses or doesn’t utilise the full exposure time which will compromise the resulting images. Timing repeatability is also an essential requirement.
CHOOSING THE BEST CONTROLLER
Using dedicated lighting controllers and a specialist trigger timing controllers can significantly improve both the performance of a line scan machine vision system and the scope of applications. However, the best controller for a particular system depends very much on the application. Gardasoft has a reputation for technology innovation in lighting control and has been solving lighting problems for over 20 years.
For more information about the advanced control of line scan systems, see http://www.gardasoft.com/how-controllers-improve-line-scan-systems/
AUTHOR: Jools Hudson, Gardasoft Vision Ltd, UK