Top 10 Hints For Designing Drones #7

Too often, we see UAV and Drone manufacturers that, when the time comes to test their product, assume that because they’ve done a bunch of flying everything is fine. Unfortunately, there are many assumptions built into how a drone manufacturer operates their drones that their customers may not share. Their customers often won’t operate their drone in the same way as the manufacturer and this can expose design flaws or cause the drone to enter unexpected states that are not handled well. There are also many assumptions built into where you test. The particular location where the manufacturer tests has a certain minimum and maximum temperature. The location determines the elevation, what sorts of winds are encountered, humidity, and other environmental conditions. The UAV manufacturer’s customers are operating the UAV at different locations and unless the manufacturer has tested their product through its entire operating range it’s likely that problems will arise, problems that result in customer dissatisfaction and loss of reputation.

One of the best ways to test the correct operation of a UAV is to start with the Drone’s requirements and develop tests based on those requirements. Unfortunately, we see all too many UAV manufacturers that don’t bother to develop a set of requirements at the beginning of their development. Requirements are not that hard to develop and are a great way to keep management, the development team, and the test team on the same page. By testing each of a drone’s requirements the test team can be confident that they haven’t missed testing one or more of the UAV’s important features.

It is also important to pay attention to temperature. Many things change in mechanical systems as the temperature changes. When it’s cold, many materials shrink and stiffen. This can cause problems in the operation of electro-mechanical systems. Things that move become much slower than at more normal temperatures. Servos are noticeably slower at temperatures below zero than they are at room temperature. Vibration dampers can become much stiffer which reduces their ability to dampen vibrations and this can have a big effect on the operation of a UAV. Most sensors sense what you want them to sense plus temperature and unless this temperature effect is calibrated out there will be temperature induced errors.

High temperatures can also cause problems. Virtually all power supply electronics have a built-in mechanism that shuts off power if the temperature gets too high. The temperature inside a black UAV sitting under the sun is going to be much higher than the ambient temperature. A power regulator that is supplying a few amps of current is going to be operating at a much higher temperature than the temperature found inside the UAV.

Elevation, wind and humidity can also have a significant effect on a Drone’s operation. There are locations where there is almost never any wind. A UAV manufacturer in such a location likely doesn’t know how their vehicle will perform in wind. Elevation can also have an effect. The air gets thinner as you get higher and thicker as you get lower and this will affect the effectiveness of control surfaces as well as propeller. As the air gets thin the temperature of electronics will increase because the thin air does not carry temperature away from electronics as effectively as thick air. Humidity, especially near salt water, can cause connectors and electronics to corrode. We’ve seen badly corroded circuit boards that are less than a year old but have been operated in a high humidity environment near the ocean.

It’s also important to have a safety margin. The drone should be tested and shown to work at temperatures above and below the maximum. At wind speeds above the maximum published wind speed. There should be a safety margin for all the conditions the vehicle operates under and these safety margins should be chosen with care.

RF testing is also crucial. All electronics are RF generators and they can interfere with one another. This is especially true for GNSS receivers and radio modems. GNSSS receivers are attempting to receive incredibly weak signals and the various RF generators on board a UAV can interfere with this signal. In the worst case, they can block the signal but RF interference will also decrease the accuracy of the position generate by the GNSS receiver. This goes double if you are using an RTK GNSS receiver. On board RF interference can also significantly reduce the range of radio modems. At short distances on board RF interference has almost no effect on radio modem reception. However, the strength of the signal received by the radio modem decreases with the square of distance. The on board RF noise on a UAV is often the limiting factor in range as its strength remains the same regardless of distance and eventually is stronger than the signal coming from the radio modem on the ground.

An “iron bird” is also an excellent test tool. “Iron bird” is a term from the aviation industry and refers to a simulation where all of electrical and/or hydraulic systems are connected together in the same fashion they would be in the aircraft. Usually an “iron bird” is built on a table (or tables depending on the complexity) and gives test engineers better access to all of the systems. There are many tests that can be accomplished more easily on an “iron bird” than a UAV in flight.

Finally, accelerated life testing is an important product test tool. There are standardized methods of subjecting products to extreme vibration, temperature change and other conditions that speed up the aging of a product and expose weak points in a products design.

Product testing is not a perfect process. No drone manufacturer can possibly find all of the weak points or faults in their product prior to first shipment. A comprehensive test strategy can find many problems resulting in dramatically less stress for the manufacturer and happier customers

 

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