FEATURE MACHINE VISION SYSTEMS
LED PULSING EXTENDS SCOPE OF MACHINE VISION APPLICATIONS
Pulsing LED lighting is a well-established technique for machine vision. The two most common applications are to ‘freeze’ motion for inspecting fast-moving parts, and to overdrive the LED to obtain short pulses of increased brightness
U
sing a dedicated lighting controller not only provides the stability and
flexibility needed for these tasks, but also enables a range of versatile lighting applications. This includes controlling multi-lighting and multi-intensity schemes such as sophisticated line scan applications, using a short pulse of bright light to overcome interference from ambient light, improving depth of field and enabling computational imaging. In the simplest applications, pulsing may be useful simply to extend LED lifetime, thereby reducing costs.
PULSING CONTROL Machine vision systems need consistent, stable illumination in order to deliver accurate, repeatable measurements. Since LED output is directly proportional to current, lighting controllers should regulate the current, not the voltage, to provide illumination that is stable, tightly controlled and highly repeatable. The easiest lighting configuration is for an LED to be run continuously. However, LED lifetime is dependent on how long the light is on for, so by only switching it on when it is needed, the lifetime can be extended. This is generally achieved by pulsing the light so that, for example, if the LED is only on for 10 per cent of the time, the lifetime is increased by a factor of 10. In this example, a trigger signal is used to synchronise the camera exposure to the pulsed light from the lighting controller. The delay between the trigger signal and the output pulse, the length of the pulse and the brightness of the pulse can all be precisely configured within the lighting controller. Fast-moving objects such as those on a
high speed production line can be frozen in time by appropriate pulsing of the light, thereby eliminating motion blur in the images. In some cases, motion can be
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frozen by using a defined camera exposure time, but exposure resolution is likely to be much better if a strobed light is used instead. The fine adjustment of the pulse timing will usually be far more accurate than the internal camera timing.
OVERDRIVING By taking advantage of the precise control of the pulse width, frequency and intensity provided by a lighting controller, the light can also be ‘overdriven’. This technique can boost light output to more than 100 per cent of the light rating, making it an incredibly powerful technique for applications that require high brightness. Overdriving will allow, in the best case, up to 10x brightness to be obtained for very short periods. Care must be taken when overdriving LEDs to avoid damage from overheating, so Gardasoft controllers are equipped with patented SafePower and SafeSense technology, which allow overdriving to be carried out without risk to the light. Overdriving can also potentially allow the whole system to be run faster because exposure times can be reduced as a result of the increased light intensity, and higher light output also offers the possibility of reducing the aperture to give better depth of field.
AMBIENT LIGHT CONTROL The lighting environment in which a machine vision system is installed may be subject to variation. For example, increased ambient light might be needed for human operators or there might be light interference from nearby equipment. Pulsing and overdriving the system’s LED
could overcome such problems. For example, overdriving the LED by 200 per cent would double the light intensity and enable the camera exposure to be halved. Double the light and half the exposure will reduce the effects of ambient light by a factor of four.
MULTI-LIGHTING SCHEMES Lighting controllers also make it possible to individually trigger multiple lights at different intensities and durations in a predefined sequence to highlight different features in different images. This allows multiple measurements to be made using a single camera station instead of needing multiple stations. This can reduce the mechanical complexity and saves money by needing less equipment. In the example shown, the lighting controller is used to sequentially trigger three different lights for the inspection of a single component using a single camera. The image optimised for the barcode is
produced using a ring light; the surface defect image is produced using dark field illumination and, finally, an on-axis, bright light is used for a dimensional check on the component. The technique of computational imaging, which is used mainly on processed metal components, also benefits from sequential lighting pulses. The component is illuminated sequentially by four different lights from different directions. The resultant images are processed and combined to remove the effect of random reflections that may arise from different surface treatments such as milling, turning, grinding, punching or stamping. It 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.
Gardasoft Vision
www.gardasoft.com /AUTOMATION T: 01763 262621
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