What is a drone? According to the dictionary, it’s when your weird friend won’t stop talking about Bitcoin. According to the Aerospace industry, however, it’s an Unmanned Arial Vehicle (UAV). All high school Call of Duty flashbacks aside, UAVs can take a lot of different forms. They can look like a plane, with wings and an engine, or they can look like mini helicopters, with multiple rotors. [1]
Fixed-wing drones, like the one above, takeoff, fly, and navigate just like a regular plane with the exception that they don’t have a pilot. Drone operators can often be hundreds of miles away controlling them via a satellite connection. These have impressive ranges, can carry sizeable payloads, but most importantly carry a substantial price tag. These days an RQ-1 Predator drone will run you somewhere in the ballpark of $40 million. [2] This is clearly out of my price range, so today we’re going to focus on the ones you might have some direct experience with—quadcopters. These little fellas start at a much more reasonable ~$100 for personal use. [3]
Having established a clear value proposition for this week’s newsletter, let’s take a more detailed look at how these quadcopters work. The “quad” specifically means four propellers. To hover normally, all four propellers must produce the same upwards force. If the sum of this upwards force equals the weight of the quadcopter, it will hover in place. Mechanically, there is a delicate dance when designing these propellers. The larger the blades, the more power you need to spin them, but you get more lift. So you need to balance the power you can get from your power source (batteries) with the number and size of your propellers. I would give an example of this design process, but I don’t think you’d be interested in that. No. You’re classy. You strike me as more of a controls type of person. First of all, I commend your interest and second of all, I think you’re absolutely crazy. Let’s dive in.
Controlling a quadcopter is very important. After all, you probably want your UAV to actually go somewhere, and not just hover in one place. With a fixed prop, this is done by changing how much power you send to each of your propellers. For example, if you bump up the power in your rear propellers, it’s going to tilt your quadcopter forward. Once it’s angled forward, the lift of the propellers has both an upwards and forwards component, which pushes the craft ahead.
Naturally, this is must be controlled, you can’t just hit the gas on the rear props and hope for the best. We must increase it slightly to tilt it forward, then adjust so that it remains angled enough to travel at the right speed. But how does it know its orientation? And how can it adjust if it’s off? Great question—I’m glad you asked.
Contrary to popular belief, there are a lot of things going on behind the scenes when operating a quadcopter. It’s similar to how in your car when you want to speed up, you just step on the gas. You don’t care about the amount of additional fuel that is needed to provide the power, or the gear changes needed to do it efficiently (if you’re in an automatic), and if you have a hybrid, you probably don’t care about how much power needs to come from the engine vs the batteries. All you want is to go faster. You’re late. It’s the same idea with quadcopters, when you tell it to go forward at a certain speed, it must adjust its position and maintain it to achieve what you want. This control is done in part with accelerometers. These are sensors that measure your orientation relative to some baseline. The baseline is typically steady, level flight. Anytime you tilt one way or accelerate forward, the quadcopter can adjust its power distribution and measure the changes to know if it’s meeting your desired output. If it’s not, it will adjust.
This is the basic idea behind controls not just in quadcopters, but in general. Use sensors to measure the current state of the object, and if is meeting the desired output, maintain it. If it’s not, adjust yourself. There is a myriad of clever ways that Engineers have come up with to improve controls, make them respond more quickly and accurately, etc. In quadcopters, these can take the form of variable pitch and/or counter-rotating propellers. If you get your controls good enough, you can take them to the next level and make your UAV autonomous. Couple your control logic and sensors with computer code that can provide the correct inputs and boom! You’re off and running (in this case flying) with a self-operating vehicle. This has been pretty heavily explored in drones small and large. Companies from Amazon, who want to autonomously deliver packages to customers, to startups like Joby, who want to advance the next generation of urban air mobility, use these controls and power management principles to succeed in their goals.
This is a pretty high-level summary, so if you’re interested in controls in aviation and specifically drones, check out this really interesting YouTube video.
Thanks for reading this week’s edition of It’s Not Rocket Science, have a great Wednesday!
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[2] https://www.militaryfactory.com/aircraft/detail.php?aircraft_id=46