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Choosing power sources for machine vision lighting


Achieving reliable images needs careful design of lighting control E


xperienced designers know that to get great results from a machine vision system


you must get a great image. No amount of software processing can recreate data that hasn’t been captured on your camera sensor, and to get that perfect image, time after time, you must have accurate and consistent lighting control. Most machine vision systems use an LED illumination source with one of three drivers: a simple voltage drive, voltage-drive with pulse width modulation (PWM) or a constant- current drive. Simple voltage drive can be the cheapest option but it has significant practical disadvantages due to the way LEDs function. Although LED lights are often specified by voltage, it is actually the current flowing through the light that dictates the brightness obtained and not the supply voltage. As you can see in diagram 1, a tiny change in voltage creates a much bigger change in the current flowing, and the change in current creates a large change of brightness. Tis important effect means that simple voltage drive cannot be used to accurately adjust lighting intensity. In addition, any fluctuations in the system supply voltage may cause fluctuations in lighting levels. PWM voltage controllers provide


limited intensity control by pulsing the light on and off so that the average intensity is approximately what’s needed. To increase brightness, the light is simply pulsed on for longer. However, PWM also


Relative Output Flux vs. Forward Current 20ms, single pulse, Tj-25C


600% 500% 400% 300% 200% 100% 0%


0 2468 10 12 14 16 18 20 If (A)


10 12 14 16 18 20


0 2 4 6 8


01 23 4 Vf (V)


Diagram 1: Relationship between voltage, current and intensity of LED lighting www.imveurope.com | @imveurope


has practical disadvantages. Te speed of the PWM cycles dictates how many intensity steps are available for adjustment and it’s usually limited to about 100 coarse levels. Te PWM pulses are usually independent of exposure timing which can cause intensity variations between exposures, particularly for fast systems. Diagram 2 shows an example where the timing of the camera exposure and PWM pulses results in the first exposure receiving more than 30 per cent more light than the second exposure. A faster internal PWM clock can reduce this effect, but these high-speed voltage pulses can cause EMC interference in neighbouring systems. Constant-current controllers are


usually the best choice of lighting drive for LED lighting. Tey directly adjust illumination levels by accurately controlling the current flowing through the light. Tere is no fundamental limitation on the number of adjustment steps and a typical Gardasoft controller will provide 3,600 steps of brightness with no risk of EMC interference.


Strobing and overdriving Continuous illumination can be adequate for some applications. Usually, a light will be set up to below 100 per cent brightness so that illumination can be increased later if needed. Both voltage-drive and constant-current controllers can drive continuous illumination. However, one well-known effect is that as an LED light heats up,


Forward Current vs. Forward Voltage 20ms, single pulse, Tj-25C


Diagram 2: How PWM can cause variable brightness


the illumination intensity will fall. Tis effect is often minimised by using the ability of LEDs to be pulsed on briefly so that the light is only illuminated while the image is captured. Pulsing the light prolongs the life of the light, which will degrade with on-time. Pulsing is also an important technique to prevent image blur on fast production lines. Effective strobing relies on obtaining consistent and repeatable square light pulses with rapid rising and falling edges. Sharp edges are more difficult to achieve with a PWM controller, particularly if there is a long cable between the controller and the light. If the pulse edges have a poor profile, the intensity of the illumination is likely to vary between exposures. Constant-current lighting controllers are inherently able to create accurate, repeatable square pulses with sharp rising and falling edges. A powerful extension of strobing


is overdriving, where a current higher than the maximum rating is driven through the LED light. Tis short pulse can produce much brighter illumination than the maximum specified by the device manufacturer and will not damage the light when a good strobe controller is used. A quality strobe controller will carefully manage the frequency, pulse duration and current to prevent heat build-up that could damage the light. Only constant-current controllers can achieve a safe and accurate overdrive. Voltage-based lighting


FEBRUARY/MARCH


controllers including PWM versions that supply a standard 24V cannot overdrive lights and those that offer the option of an additional, higher voltage suffer from an unpredictable discontinuity in brightness between the two voltage levels. Most constant-current controllers can operate with complete flexibility in either continuous, strobe or overdrive modes.


High-speed lighting control Certain applications must have very short camera exposures. High-speed inspection lines and systems using a single imaging station to capture multiple images will need short exposures. Line scan applications also use short exposure times which need precision lighting control. Some more advanced PWM systems offer the ability to synchronise the PWM cycle with the camera shutter to avoid the issues shown in diagram 2, but this can be difficult to implement in practice. Constant- current lighting controllers offer much more flexibility at high speed because they allow the intensity and duration of lighting to be precisely adjusted and are not tied to a PWM cycle. O


Author: Jools Hudson, Gardasoft Vision Ltd, Trinity Court, Buckingway Business Park, Swavesey, Cambridge, CB24 4UQ, UK Tel: +44 1954 234970 E-mail: vision@gardasoft.com Internet: www.gardasoft.com


JUNE/JULY 2020 IMAGING AND MACHINE VISION EUROPE 33


Relative Lumens (%)


If (A)


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