Author:
Andrew Dereka, Product Director at Laird Thermal Systems
Date
05/04/2020
PDF
Machine vision technology has traditionally been used for vision inspection, quality control, robotics, wire bonding and down-hole applications where machine vision systems obtain data from analyzing images of a specific process or activity. Leveraging advanced hardware and software systems, machine vision technology allows manufacturers to increase accuracy and throughput by replacing human examination and decision-making with more reliable and efficient machine vision systems.
However, machine vision technology has evolved beyond simple inspection systems, empowering machines to perform essential tasks. Today, machine vision technology is enabling sophisticated object detection, recognition and collision avoidance systems for next-generation autonomous vehicles, drones and robotics. Machine vision also plays a role in many artificial intelligence and machine learning applications such as facial recognition.
Nothing can be achieved in a machine vision application without the successful capture of a very high-quality image. Utilizing temperature-sensitive imaging sensors and cameras, machine vision systems require active cooling to deliver optimal image resolution independent of the operating environment.
Thermal Challenges
Machine vision applications use two types of sensors to convert photons to electrons for digital processing; charge-coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) sensors. As the temperature of the sensor assembly increases, image resolution is negatively affected by thermal noise. This can occur in many machine vision applications where environmental conditions or the heat generated by surrounding electronics causes the temperature to rise above the sensors maximum operating temperature.
A rough estimation below shows that dark current doubles for every 6°C rise in temperature. A 20°C drop in temperature could reduce the noise floor by 10dB, thereby improving dynamic range by 10dB. In outdoor environments, temperatures can exceed 40°C. To prevent image quality from deteriorating, solid-state thermoelectric coolers can be used to reduce the sensor's temperature below its maximum operating temperature and maintain a high image resolution.
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Figure 1. Thermal noise impact with increasing temperature
Spot cooling CCD and CMOS sensors in machine vision applications can present significant challenges. The addition of thermoelectric cooling devices will increase the size, cost, weight and complexity of the imaging system. Cooling of the imaging sensors can result in condensation due to surfaces in contact with thermoelectrics going below dew point. Therefore, many vision systems are contained in a vacuum environment. Outside surfaces that are exposed to temperatures below dew point are typically insulated with a closed cell foam to keep condensation from occurring.
Passive cooling can be quite inadequate for CMOS sensor applications. For electronics in an outdoor environment, the heat generated inside a compartment can easily increase the temperature of the imaging device above 60⁰C. Passive cooling cannot cool below ambient temperatures, and the thermal resistance between ambient and the sensor will at best keep the devices a few degrees above ambient. In many cases, this exceeds the maximum operating temperature of the imaging sensor. When using an active cooling solution such as a thermoelectric device, the sensor can be cooled far below ambient temperature, minimizing thermal noise and assuring a high-resolution image will be captured.
An efficient heat exchanger is needed to dissipate heat into the surrounding environment as it will need to absorb the heat generated by the imaging sensor and thermoelectric cooler. The coldest temperature a thermoelectric cooler can achieve is based on how efficient the hot side heat exchanger is. If the temperature of the heat exchanger increases, then the coldest temperate achieved by the thermoelectric cooler will also increase. Although fan cooling is a better heat exchanger to ambient than a heat sink with natural convection, liquid cooling is the best option. For demanding applications, a liquid heat exchanger is the most efficient solution. This offers the lowest hot side thermal resistance and provides the maximum coldest temperature that can be achieved by a thermoelectric cooler.
It is recommended to use thermal interface material on either side of a thermoelectric cooler during assembly to maximize thermal conductivity. However, standard thermal interface materials, such as greases, can outgas and coat the imaging sensor. Special thermal epoxies and phase change materials with low outgassing characteristics exist, but to make sure all outgassing has been released, they should be baked in oven prior to installation. An alternative mounting method is to specify thermoelectric coolers with metallized exteriors and low temperature solder. Here, InSn solder is typically used, which melts at a temperature below the solder construction of the thermoelectric cooler. The one concern with soldering is to minimize flux residue as this too can outgas and contaminate imaging sensors.
In addition to thermal requirements, design considerations are needed to minimize thermal shorting. This is the result of the cold side surfaces coming in contact with hot side surfaces, which cause the thermoelectric cooler to draw more current in order to deliver the same cooling performance. An example of thermal shorting would be the mounting screws holding the cold and hot side together, allowing heat to travel from the hot to the cold side. Another example is lead wire attachments to the thermoelectric cooler. Heat can travel via the lead wire to the thermoelectric cooler and reduce thermal performance. Limiting thermal shorting paths during the design phase can optimize the overall thermal solution and make it more efficient.
Thermal Solutions
The image quality of CCD and CMOS sensors degrade at temperatures typically in the 50ºC to 60ºC range depending on the quality of the sensor. For passive cooling solutions, if the hot side heat sink is in the same temperature range then the cooling will be inadequate. To reduce the overall thermal resistance, it is important to design a heat sink with maximum surface-to-air contact. Typically, space constraints make it difficult to accommodate a properly sized heat sink and forced air is required to keep the temperature just a few degrees above ambient.For most outdoor applications, these thermal solutions will not be enough. Heat from surrounding electronics can exceed the maximum temperature limit of the CMOS sensor and therefore a thermoelectric cooler will be required as it can lower the temperature of the critical sensor well below ambient. Single stage thermoelectric coolers can achieve a temperature differential of more than 40⁰C below the hot side temperature of the hot side heat sink. If the hot side heat sink is at 80ºC, then the Peltier can cool the CCD or CMOS sensor down to 40ºC and keep it below its maximum operating temperature.
Thermoelectric Coolers
Thermoelectric cooler modules use the Peltier effect to create a temperature differential ("T) to transfer heat from one side of the sensor to the other. Thermoelectric coolers, often called Peltier coolers, are integrated directly into CCD and CMOS sensor assemblies to provide temperature control. A standard single-stage Peltier module can achieve temperature differentials of up to 72°C, while a multi-stage Peltier device can achieve much higher "Ts. These thermoelectric modules typically require a heat sink or other heat exchanger to dissipate heat into the surrounding environment
Supporting machine vision applications operating in temperatures ranging from 80°C to 150°C, the single-stage HiTemp ET Series offers a cooling capacity of more than 300 Watts in a compact form factor.To meet a broad range of emerging applications, the HiTemp ET Series offers more than 50 models with a wide variety of heat pumping capacities, form factors and input voltages. In high temperature environments, enhanced module construction prevents copper diffusion and degradation in performance, which can occur on standard grade thermoelectric coolers.
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Figure 2. The HiTemp ET Series thermoelectric cooler is designed for higher current and larger heat-pumping applications offering precise temperature control accuracy
Conclusion
Machine vision systems now operate in several environments for both industrial and non-industrial applications. Although used in different ways, all machine vision systems require CCD or CMOS sensors to create high quality images that computer hardware and software systems can evaluate and analyze for the decision-making process. Active cooling solutions keep mission-critical sensors below their maximum operating temperature to ensure the highest image resolution for object detection, recognition and collision avoidance systems in next-generation autonomous vehicles, drones and robotics. Thermoelectric coolers offer precise spot cooling to keep critical sensors below their maximum operating temperature to assure high image resolution. Integrating thermoelectric cooling systems into machine vision applications reduce risks and accelerates time-to-market for this emerging application.
