VOXL 2 Thermal and Performance

Table of contents

Note: this is written with a soft tongue-in-cheek demeanor to keep the topic light and fun.

Thermal is a huge challenge, especially in tight spaces and with enormous compute power. We have decades of experience with thermal management, and the summary that best explains how to approach thermal design is this 4-step process:

1. Do not solve a problem you do not have.
   • This is the biggest mistake we see designers do. Prior experiences and peer led discussions and legacy designs lead many designers into instinctively wanting to add heat-sinks and thermal mass to a device that generates heat. This is a mistake.
   • Yes, there are times when heatsinks are needed, however do not forget what they do.
   • • All a heatsink is trying to do is provide a good thermally conductive path for heat energy to be removed from the heat source and put someplace else benign.
   • • Good heatsinks have a ton of surface area to aid in dissipation into the ambient/surrounding air.
   • • Even a great heatsink needs airflow to move the heat energy off it. If there is no airflow, the ambient air provides very little thermal conductance, and the heatsink will effectively become a large thermal capacitor. So, it will certainly delay the onset of thermal performance degradation, but it will also sustain that degradation even when no new thermal energy is being added and take much longer to return to an ambient state.
   • In many instances, designers end up chasing their tail solving self-induced issues brought on by adding thermal mass (or any engineering “solution” needlessly added). One such item is EMC performance, especially if the heatsink was grounded. We know of instances where folks added a heatsink and then they failed FCC. Why? It was grounded incorrectly, and then became a noise radiator. Don’t be that designer.
   • So, how do you know if you need any thermal solution… item #2:
2. Test your use-cases first!
   • You must understand what your system is trying to achieve, and you need to know “what does success look like”. There are many use cases using our Voxl ecosystem and components that do not require any additional thermal mitigation techniques at all. It would be a shame to add more SWaP-C elements (size, weight, power, and cost) for no benefit whatsoever.
   • Our Voxl tools have comprehensive logging and monitoring features that will report temperatures, frame rates, RSSI, etc. You must log the important factors relevant for your design goals, and create the overlap of temperature changes to these performance metrics. You need to confirm if you are indeed experiencing thermal mitigation, or some other artifact, such as sub-optimal data paths (having overflows or re-tries due to pushing too much data into a slow data channel, such as 500Hz+ IMU data over I2C, or trying to push 1Gb links into USB2, and so forth), or over-constrained and/or buggy algorithms for your AI processing.
   • Get comfortable running tests. Just do it. A 10-20 minute test can open your eyes much more than weeks of design and anaylsis. Work with your SW team to automate and log results you need.
3. Give yourself options in your design.
   • Whether you need more thermal mass (in the form of a heatsink) or a fan (preferred), make sure you think through your design and give yourself the option to incorporate these elements later on, after you have tested your use-case and observe performance limitations brought upon by thermal throttling, and try them out one at a time and re-test.
   • Plan on a fan, where it can fit, and how to plug into the Voxl board.
4. Airflow, airflow, airflow (4th in this list of info, but this is “the” first way to solve it!)
   • Airflow is the single biggest factor for improved thermal performance, which is why every Voxl platform has a dedicated fan connector, and we sell a very convenient small 25mm x25mm 5V fan on our website that will plug directly into Voxl, Voxl-Flight, Voxl2, and Voxl2-Mini. https://www.modalai.com/collections/accessories/products/voxl-cooling-fan
   • If you want to add a heatsink into your design to improve thermal mass, you still should have a fan to blow the heat off of the fins.
   • You are making a drone, right? Use the prop-wash!!! Recall step #2 to test your use case? Add into your monitoring the overlap with active flight. You should try to add into your design the ability to deflect some of the drone’s prop-wash into the circuit areas, and you will find a tremendous boost in thermal performance.
   • • We have a story about a very experienced thermal team, using the latest and greatest thermal modeling techniques ($100k+ software license add-ons, months of cad development, then months of simulations that took days to converge on each pass, etc.) and after 8-12 months of modeling and physical testing, they found that simply using the drone’s prop wash the most effective solution compared to high-end vapor chambers, heatsinks, or any passive thermal material, by many degrees C. We unfortunaly cannot share that data, but it was an eye opener for many on that project that were convinced otherwise from their decades of experience in the mobile space. It took them over $1M to come to that conclusion. And now we all get to benefit from it. This is why ModalAI will always first push the claim of airflow before using any passive metal brick that WILL negatively impact flight time and your payload capability.
   • • We understand that ground-based robots, or drones that may “perch look and listen” potentially in a solar loaded environment, will not benefit from active prop-wash. Therefore, know your use case, test for it, and be clever with ways to avoid that. Maybe reduced performance in that simple “perched” use case is OK? Then, you fly a bit to regain performance and perch back again.
   • Re-test and improve on your airflow. If you really think you still need a heatsink or even a heat spreader, then that’s OK. But, make sure you do not create additional issues, such as an EMC violation. Keep your thermal solutions passive, do not make them electrically conductive.

ModalAI will help ensure you have all the 3D data you need to accurately design your thermal solution. If we do not have a 3D step file for a specific board on our website, please reach out to us on the Forum https://forum.modalai.com/ and we will address it.