A good image is the core requirement for image processing and a strobe controller will drive machine vision lights so the camera always finds a well-illuminated target.
For automated image processing, shifting illumination conditions can create massive problems and it’s a core requirement to achieve consistent illumination. Strobe controllers will guarantee perfect illumination of the object and help improve speed and reliability of the process. They can also save costs.
Achieving Correct Illumination
Perfect illumination of the camera image should be our goal and requires the following principles:
- No shadows or reflections
- All important areas visible at medium intensity
- The highest brightness value should be just below the maximum pixel value, e.g. around 245 in an 8-bit system
- The lowest brightness should be just above zero, e.g. around 5 in an 8-bit system
To avoid motion blur with fast-moving objects, choose a short exposure time, in the range of a few milliseconds or less.
Characteristics of LED Lighting
These days, nearly all machine vision illumination is based on LEDs. They reach maximum luminous flux very quickly after power-on and provide long life, high efficiency, and small form-factor. In addition, the emitted light has a consistent spectral curve which does not vary with power or during its lifespan.
The brightness of LED sources is proportional to electrical current over a wide range of operating conditions. However, LEDs do have significant differences in maximum parameters and low-power switching thresholds. Therefore, it makes sense to regulate LEDs via current and not via voltage. This is where the strobe controller shows its real value.
Main Features of a Strobe Controller
The principle job of a strobe controller is to switch power to the light and to regulate the brightness of the light. The light must be powered synchronously with the camera exposure time and the exposure signal from the camera can be used for this. Some lighting controllers include sophisticated timing capabilities which enable the controller to become the timing hub for the machine vision system and trigger devices such as reject gates. Some controllers also provide features such as adjustable current ramps and recognition of the connected LED head.
Overdriving LED Lighting
An important benefit of a strobe controller is the ability to overdrive to achieve much more brightness than the manufacturer’s specification. LEDs are sensitive to heat and if power dissipation causes them to overheat, they may be damaged. However, the dissipated thermal power is an integral over time so the LED can be safely driven with a significantly higher current than the manufacturer’s specification so long as it’s done in a controlled way. While an exposure is not occurring, the LED can be turned off and allowed to cool. The result is higher light intensity during the exposure when the light is needed. Overdrive is particularly useful when exposure times are short because brighter intensity will be available.
Current Drive Controllers and PWM Controllers
Since LEDs produce an illumination that is proportional to current, the most logical method to control LED lighting is via a variable current lighting controller. Current control has many benefits but sometimes requires careful management of power dissipation. An alternative is to use a Pulse Width Modulation (PWM) controller which uses an entirely different principle and is based on Voltage Drive. With PWM strobe controllers, the maximum current stays constant and the intensity is managed by pulsing the LED on and off several times during the exposure. The choice of current control or PWM control for a strobe controller is an important one.
Machine Vision Lighting Controllers, Strobe and lens Controllers.
You can read more about the differences between current drive and PWM drive at:
www.gardasoft.com/voltage-drive-current-drive/.
