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Importance of Thermal Management in Machine Vision

When selecting individual components for or investing in turn-key machine vision systems, acceptable performance, system longevity, and cost are always high priorities.  Particularly with the advent of ever-more compact components and higher brightness lighting, often-overlooked factors to consider in the earliest stages of designing a machine vision system are thermal management and heat dissipation.

Thermal Management Challenges in Machine Vision

Many modern machine vision systems are called upon to run in constant-on mode (100% duty cycle) over multiple shifts, sometimes 24/7 – in a wide variety of ambient temperature environments.  Despite the best design efforts of individual component manufacturers, this level of usage can sometimes create excessive heat.

If the cameras and lighting retain too much heat, not only can their internal components become damaged or destroyed – a costly investment to replace – but the resulting heat may affect the system longevity and reliability, along with the inspection quality, consistency, and robustness.   For example, excessive heat within a vision camera can adversely affect image clarity and the sensor’s signal-to-noise ratio, ultimately impacting the ability of the software to adequately analyze the resultant images – potentially resulting in a compromised inspection system.

Similarly, excessive heat remaining in the housing of a lighting source may also have a detrimental impact on your investment, causing output instability and drastically shortening the life of the light.  This in turn may require costly maintenance and even more damaging down-time.

Light sources experiencing high operating temperatures may be caused by several variables, including: the light source type; its ambient temperature environment; and its operating conditions.  For example, quartz halogen sources run very hot due to thermionic emission, or more specifically, thermal electron emission: the release of free electrons by virtue of a filament experiencing high temperatures and overcoming its work function. This ramp-up in temperature produces large bouts of thermal infrared in combination with visible light, and as such offers relatively high brightness at the expense of operational lifetime. The additional heat load from halogen light sources needs to be accounted for when operating the lights near inspection subjects and vision components that should not be exposed to high ambient temperatures.

On the other hand, LED lights are a more efficient alternative to quartz halogen, though they can also generate heat when ganged together into small enclosures and operated in a constant-on mode.  However, recent improvements in the efficiency and heat-tolerance of high-brightness LEDs have resulted in lighting options with higher intensity without sacrificing excessive heat build-up and subsequent damage.  Opting for LED lighting in your machine vision system will result in naturally lower heat output and a significant increase in component life expectancy, ultimately reducing lifetime cost and chance of failure.

There are still considerations that should be made to optimize lighting performance, stability, and longevity – even with the current higher-efficiency lighting options.  Incorporating the following mitigation methods to promote effective thermal management, which includes heat collection, transfer, and dissipation, will reduce the retained heat within and surrounding the machine vision components. Some of these methods are designed into the product by the lighting manufacturers, and some can be applied by the end user in their specific deployment environments.

Engineering Thermal Dissipation into Vision Systems

Quality Lighting Design and Thermal Management

Well-designed and implemented thermal management is engineered into quality lighting systems, increasing both the reliability and performance of a lighting product.  In designing lights, there are two effective means of providing heat dissipation: passive and active cooling.  Both methods result in higher light performance through more efficient heat transfer and dissipation.

Passive thermal management takes advantage of the naturally occurring thermodynamic properties of materials and their surrounding environment through natural convection, conduction, and radiation, without the use of a supplemental energy source.

Active thermal management involves introducing an external energy source to artificially accelerate the process of heat dissipation, typically through forced convection.

Passive Thermal Management in Lighting Design

Engineering a light with a housing that more readily collects and transfers heat is one way to achieve passive thermal management.  Aluminum is significantly more thermally conductive than stainless steel, conducting heat away from the LED/driver board junctions, and – if mounted to a conductive surface – passively conducts the heat away from the light itself.

One lighting family designed with passive thermal management in mind are the UltraSeal Washdown Lights from Ai, engineered with an aluminum housing that is 11x more thermally conductive than stainless steel.  As the LEDs generate the largest amount of heat within the die itself and the surrounding LED board, by mounting the LED board directly to the internal body of the aluminum housing via a thermally conductive paste or adhesive (also known as Thermal Interface Materials or TIMs), the LEDs are protected from overheating and irreversible damage because there is an effective conductive path to transfer the heat away efficiently.  Additional thermal transfer can be achieved by mounting the aluminum housing in considerable contact with more aluminum or other conductive metals, which can often be part of the mounting structure for the vision system.

Another design element that improves thermal conductivity is the incorporation of heat sinks.  Heat sinks increase the surface area of the housing, enabling an increase of radiative heat transfer into the air surrounding the light (keeping in mind the housing material should be highly heat emissive).  When considering applications that require higher-intensity illumination, a quality high-performance light will incorporate a large heat sink to conduct and then dissipate the considerable heat generated by the LEDs.

One such example is the SL246 High Intensity Spot Light from Advanced illumination.  The SL246 is engineered for applications requiring long-throw, high-brightness illumination, and provides even illumination at distances beyond 300mm.  To protect the LEDs within the light and deliver reliable illumination, the Ai Engineering Team designed the SL246 with an integrated heat sink for efficient thermal dissipation.

Active Thermal Management in Lighting Design

For most applications, the LED lighting design can passively reduce enough generated heat for the light to continue functioning through its intended lifetime.  However, there may be circumstances which require additional active cooling methods.

Air Circulation & Forced Air

Inspection operations in high ambient temperature locations may sometimes benefit by incorporating local area fans – often applied by the end user in the general inspection area. Utilizing fans to move air around the machine vision environment is effective in assisting radiative and convective transfer of heat away from the light by continually exchanging warm air with cool air, actively reducing the temperature in the immediate inspection space.

To meet the demand for high brightness line lights, Ai incorporates active thermal management into the design of the LL230 Ultra High Intensity Line Light Series. Engineered to provide the most intense continuous illumination in the industry for high-speed line scan applications, the LL230 integrates built-in cooling fans to take advantage of forced air convection using a patented thermal design. In addition, the fans are engineered to be hot-swappable for convenient replacement in dusty environments.  Furthermore, the LL230 control boards incorporate special circuitry and components to automatically shut the light down when the housing reaches 60°C, to protect the light head from damage (only occurring in extremely rare cases where light is placed in an abnormally high ambient temperature environment with little to no local air circulation).

Refrigeration & Liquid Coolants

An additional active cooling method for thermal dissipation includes the use of liquid coolants.  Liquid coolants are advantageous as they absorb and transfer heat from the housing more efficiently than by air movement alone, particularly if the ambient temperature is high.

However, utilizing liquid cooling introduces other challenges, including increased footprint component considerations such as refrigeration/pumping hardware and cooling lines.  For those refrigerated liquid cooling applications, particularly in humid environments, it is also important to monitor the vision system carefully for condensation, as this could pose a serious risk to the function and longevity of the light and other components, specifically electronics.  There is also always the risk of liquid leaks, which can be catastrophic.

Adjusting Light Mode to Facilitate Thermal Regulation

The inclusion of manufacturer designed thermal management functionality is an important consideration to control unwanted and potentially damaging heat; however, end users and system developers can often adjust the operation and deployment of the light itself to minimize heat generation during operation.  To better understand the potential heat generated from a light in their application, end users should work with an engineer to fully characterize and describe the machine vision application, ultimately developing a lighting solution most effective for that application.

At Advanced illumination, our team has developed a rigorous and thorough thermal testing method for quality lighting designs to increase LED lifetime and reliability for our customers’ applications.  This includes:

  1. Establishing the forward voltage and forward current that the LED within the light can safely handle.
  2. Calculating the LED density to heat sink size ratio. This may involve specific thermal design software.
  3. Utilizing the ratio to develop the safe current at which the light could theoretically function within the environment at 100% duty cycle.
  4. Performing internal testing by powering the light with its current controlled power supply, allowing it to run until it has reached the maximum safe operating temperature.
  5. Continuing to operate the light, ensuring it maintains an appropriate safe operating temperature, and adjusting the output power of the light based on the end user’s unique ambient operating temperature.

In some inspection applications, users can achieve significantly higher intensity with lower operating temperatures by choosing to strobe the LEDs.  By strobing the lights rather than operating them continuously, the LEDs can operate at a higher intensity in shorter bursts (typically at very low duty cycles less than 5%).  The “off-time” between strobe pulses allows the LEDs time to dissipate heat, effectively transferring the generated heat load within the light head.  While this may not be a solution for every inspection application, it may be a cost-effective solution for some.

Developing an Inspection Solution to Last

With the various solutions to improve thermal management in machine vision, there is likely one or a combination of options that will ensure your investment in your machine vision application is not short-lived due to overheating.  The important takeaway is to consider thermal management, and ultimately thermal dissipation, into the early stages of your vision system design process.  Understand how the lighting and other vision components may be affected by your inspection environment and work closely with your integrators to develop a long-lasting, reliable solution.