I finally got my AR Drone last week! I have to say, I was a little apprehensive based on some of the reviews I read. It really comes down to how you personally classify it. Is it a toy? Is it an RC model? Is it a highly sophisticated device that is very well engineered? For me the answer to all these questions is yes. When you ask is it a RTR FPV setup? Does it deliver professional quality video and sound? No.
AR Drone Nose
I have to say, it’s the most fun toy I’ve had in a long time. It is easy to fly and still very dynamic under human control. There’s a weird overlap between the copter flying and the person flying. It’s a combination of the two. It takes off and hovers at about 3 feet completely automatically. Landing is the inverse. Hover somewhere close to the ground (and hopefully a flat surface) and hit the land button and it does the rest. This has been hard for me to get used to. After takeoff it will stay in the same spot as long as you want, or until the battery runs out, at which point it will land on it’s own.
Ready for Takeoff!
There’s a video camera pointing out the front, and one of lesser resolution pointing down. It uses the video from the downward facing camera to maintain position while hovering. I haven’t been able to tell if it has an effect under other circumstances. It’s also got a downward facing ultrasonic sensor for altitude information at altitudes less than ~15 ft. Above that it does not know it’s altitude. Some have reportedly ‘flown away’ when the sensor got blocked or failed (up, up, and away!). It’s obviously stabilized with gyros, and I’m pretty sure it has an accelerometer. It will let you do almost anything short of going upside down but is good at keeping you from doing some stupid things. Whenever you get into trouble all you have to do is let go of the controls and it will stop and hover in place. It’s counter intuitive, especially if you’ve flown RC.
Parts Exploded
The controls can be tuned easily and there are a number of settings it takes into account such as, indoor/outdoor. It’s meant to be controlled by the iPhone but can also be piloted with an iPad or by a PC. They have released an API for it and seem to support public development efforts. There are a number of apps for it that all do slightly different things, some cool, some gimmicky. I prefer flying with the iPhone. The iPad just seems wrong. All the apps can show video from either camera. The video quality isn’t anything to write home about. Sometimes it’s choppy and the quality is never great. It’s a pretty risky setup for FPV flying. It is fun to play with though. At some point I’ll add another camera with a separate TX to solve that problem.
I think the thing is engineered pretty well all in all. All the parts are available, including the main boards ($99). However, they’re only available as assemblies for the most part. This makes them slightly more expensive but I’ve been pretty hard on it already and after several good crashes bent a gear and shaft. It’s strong. It is effortless to work on. The main board and the ‘nav’ board are centered just below the battery. The motor controllers are actually built into an assembly that sits around the motors. They’re tiny little boards, I wouldn’t have thought about doing it that way but it does simplify things a lot.
Their marketing sucks but I guess you have to appeal to a mass audience, not just geeks.
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A fact often overlooked by newbies and some ‘experts’ to the RC world is the importance of and science behind tires in the performance of your vehicle. Sure, it’s fun to do donuts endlessly in the driveway, but if you really want to go anywhere fast, your tires will have a big impact on how you get there.
The only part of your car touching the track most of the time are the tires. Think about how tiny an area of the tire actually touches the ground is. Vehicles under 1/8 scale may not even have 2 square inches of contact patch! You have to maximize your use of this contact patch as much as possible, sometimes it can make a bigger difference in track times than any other single modification.
Why is contact patch important you might ask? Since it’s the only part of your car touching the ground, it’s the only place where force can be transferred from the ?track to the vehicle. The amount of force that can be transmitted determines how fast a certain tire can accelerate a given vehicle in a given direction. A tire with a lot of grip always = faster. Unless you’re going for style points, a la drifting, you want as much grip as possible.
The force a tire can generate is comprised of several factors which may help you decide what tire to run for a certain track condition. Rubber tires generate this force in three different ways, adhesion, deformation, and wear.
Adhesion is the rubber ‘sticking’ to the track. Chemical bonds constantly form and break and these bonds create the adhesion force. The amount of this force is dependent only on the force pressing down on the tire, and the coefficient of friction between the two surfaces. Coefficient of friction (Cf) is a dimensionless number that describes how well the two surfaces stick together. This value is different when sliding and when not sliding. The Cf drops off as the tire starts to slide, which means less grip when you’re sliding around. This is not necessarily the case on dirt though as sometimes there’s not much to adhere to. Some dirt tracks will wear in a ‘groove’ and it will be smooth and stable enough to form adhesive bonds. A green track that hasn’t been run on in a while will not have this groove and your tires will have to rely on the following two forces.
Deformation is a result of the surface of the tire bending to match up with the rough track surface, usually on a very small scale. Even very smooth paved tracks have tiny bumps if you look very closely. When it’s wet on a hard track, deformation is the only grip you’ll have, as even a thin layer of water will prevent adhesion.
The last way in which a tire generates grip is through wear. If the forces acting on the tire surface in any one spot exceeds the strength of the rubber, it will begin to tear on a microscopic level. This tearing eventually causes tire wear, and in extreme cases on a very sticky track can cause graining on a tire. As the tire wears, tiny pieces of rubber are constantly being removed. The energy it takes to split these pieces off of the rest of the tire generates force.
The sum of these three forces is the total friction force of the tire. All three forces are directly related to how much contact patch there is, the surface and material characteristics of the two surfaces, and the coefficient of friction. More contact patch always means more grip. This is not to say that tread pattern doesn’t matter, it does matter greatly, but more area of the tire touching the ground for a given type of tire, the more grip.
I’ll leave it up to you to draw your own conclusions from this information for now. We’ll discuss the specifics of tire selection later on.
-RC Scientist
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Tech, tires Tags:
racing, tires
I thought this tip was pretty interesting. If you’ve ever looked closely at a lipo that doesn’t have a hard case, you probably saw each cell in a little silver ‘pouch’. Surprisingly, or not, these are called pouch cells and is the form that most Lithium ion polymer cells are made. One of the main advantages of this is weight, as the pouches weigh much less than the case of say, a AA battery. The downside to this is that the batteries can swell when discharged or charged at high current. Some swelling isn’t anything to be too alarmed about, but if it gets out of hand, your cells can delaminate and stop working properly. Keep an eye on your batteries to make sure you don’t have one cell that appears drastically different than the others. Always use a balance charger to make sure all your cells are being properly charged.
A Single Lipo Cell. Notice the insulating yellow foil around the fused conductive edges.
Another thing to watch out for is to make sure you electrically insulate the battery as the foil cell pouches on some cells can be conductive. If they come in contact with metal or even carbon fiber, they can short and be damaged. Most batteries come wrapped in shrink wrap so you never have to worry about it, but some leave parts of the foil exposed. This is one nice thing about hard case batteries for RC cars, and some planes. Having the hard plastic case around the battery eliminates any possibility of shorting between the pouches and your vehicle. It also prevents damage from the battery wiggling slightly 0n the chassis and wearing a hole in the pouch, or taking a sharp impact and tearing a hole in the pouch.
The last interesting thing I noted in this article was the idea of containing the expansion of the cells to prevent delamination and obtain the full potential of the pack. I had always assumed that containing the expansion would be a bad idea, but maybe there is another advantage to hard case Lipos.
Wikipedia has some good information on lipo batteries as well.
-RC Scientist
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Batteries, Safety, Tech Tags:
battery, charge, damage, danger, explode, fire, foil, lipo, lithium polymer, Safety, short, swelling
Radio is the beating heart of RC. It does all the work behind the scenes and we rarely give a thought to how it all works. It’s so important in fact, that RC didn’t exist until radio was invented. The first patent for radio control was number 613,809 “Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles” and was awarded to Nikola Tesla in 1898. The two boats he built for exhibition at the worlds fair that year were guided by Tesla’s early method of remote control and used radio waves to control the direction and speed of the boats as well as turning on and off lights and other tasks.
As you may or may not know, ‘radio’ is really just electromagnetic waves. The electromagnetic spectrum (shown above) contains all the different frequencies from gamma rays to VLF or very low freqency radio waves. Imagine these waves like the waves in a pond when a stone is thrown in. The type of antenna on your RC transmitter is refferred to as omnidirectional as it spreads it’s waves more or less evenly in all directions. The waves it transmits extend in every direction and are reflected and absorbed differently by different materials and shapes. The radio waves from your transmitter can also be reflected off of and absorbed by things around you, buildings, trees, power lines, etc. Most RC vehicles are operated within ‘line of sight’, i.e. we can always see the model from where we’re standing. Losing sight of a plane or helicopter can end in disaster, as would losing control of it at any point. Radio waves generally travel well through the atmosphere and as long as radio waves only pass through atmosphere (line of sight), their strength and signal quality isn’t degraded enough to be noticible.
To transmit the users intentions from the transmitter to the receiver it is necessary to convert the movement of sticks, knobs, and buttons into electrical information which can then be transmitted via radio waves. This information tells the receiver what we want the model to do. The receiver translates the radio signals into electrical signals, and finally into servos or speed controls. This information can be encoded and decoded in many different ways depending on the application. This encoding is called modulation, the decoding is called demodulation. Transmitters vary characteristics of the waves they output based on the command given. The receiver is able to transfer this information into electrical commands which control servos or speed controls on the model.
Early radios primarily relied on AM or amplitude modulation. In AM, a carrier wave is transmitted at the listening frequency, the same frequency the receiver is tuned to. The signal to be transmitted is converted to electrical impulses which tell the transmitter to vary the amplitude or ‘height’ of the waves as shown below. If your radio uses a crystal with a color code on it, this is an AM radio. There are 6 different AM frequencies used by RC hobbyists in the US, all around 27MHz.
FM or frequency modulation is another way to send information via radio waves. FM radios actually change the frequency of the wave up and down from the carrier frequency. You would think that the receiver might not be able to hear the transmitter if it’s transmitting at a different frequency, but as we saw above, every frequency can ‘carry’ a different amount of information at any one time. This is called the bandwidth. Low frequencies have low bandwidth, high frequencies have high bandwidth. There are exceptions with the use of modern digital communications, but until recently, this was the case. It becomes more and more difficult to detect changes in the waves when they pass at lower frequencies than at higher ones. Imagine a telegraph operator transmitting Morse code over a telegraph line. The sending operator plays the role of the transmitter, encoding the information to be sent into a form that can be transmitted electrically and understood on the other end. The receiving operator does the opposite on the other end. He associates the dots and dashes, or electrical impulses creating the dots and dashes, as letters forming a message. Hopefully the same message that was sent! If you have a radio with a crystal having a number on the side like 75.77, this is the base frequency your radio uses to communicate. There are 79 different channels used by RC modelers in the 75MHz band. About half of those are reserved for flying models, and half for surface models.
Modern RC radios use DSM and PCM to modulate and demodulate their signals. PCM stands for Pulse Code Modulation. Instead of representing our data as an analog signal that can vary continuously and fluidly, we represent it as 1′s and 0′s. This is called binary and is the same mathematical mechanism at work in the computer you’re reading this post on. The analog signal is converted to different discrete values (digitized) which are then transmitted as binary information from the transmitter to the receiver. The method for transmitting this data is unique in that it employs Spread Spectrum (SS). DSM is a proprietary technology developed by Spektrum. It stands for Digital Spectrum Modulation and is an optimized version of a technique called Direct Sequence Spread Spectrum modulation. This is an entire article unto itself and we’ll address it in detail later. Suffice to say, the new 2.4GHz radios use digital information to transmit the users commands to the receiver. The advantage of this is that there is no interference from other radios. The transmitter is ‘bound’ to the receiver and they will only talk to each other. This eliminates all the headaches of making sure you’re not on the same frequency as someone else nearby.
So we’ve covered the science and a little history behind the development of radio control. Next up we’ll talk about how the receiver actually makes things happen.
-RC Scientist
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History, Radio, Tech Tags:
AM, DSM, DSSS, electromognetic, FM, PCM, Radio, spectrum, tesla
The term RC stands for Radio Control. It’s a method of controlling an object or machine without physical contact. While RC now stands for ‘Radio Control’ it really began with ‘remote control’.
Remote Control has been around as long as vehicles have. The two are intertwined. It’s easy for a child to play house, or GI Joes, manipulating the miniature figures directly with their own hands. A vehicle however, is a different matter. They aren’t about where you are, who you’re pretending to be. Vehicles are about the journey. How you get there. To pretend to go somewhere, it’s easier to detach yourself from your reality in a fixed location and ‘travel’ to another place if you don’t have to use your hands to ‘move yourself’.
The first remote control vehicles in widespread use were almost certainly electric trains. While wind up toys did exist before this time, they can’t really be considered remote control because once released, there is no control! Electric trains weren’t self powered but instead ran using electrical current supplied to them through the metal rails which were connected to a transformer and then the mains. The only control afforded was for speed, and occasionally on larger setups, a few track switches.
As soon as radio burst onto the world stage, radio control followed closely behind. At the 1898 Worlds Fair, Nicola Tesla demonstrated a radio controlled boat. It was claimed that the boat responded to verbal commands, but really Tesla held the remote controller and secretly steered the craft. Regardless, it was but one in a series of inventions by Tesla that would change the world and one giant leap for RC.
The first military application of the new devices occurred over a decade later in 1909. It was a radio controlled torpedo developed by a French inventor. RC got it’s wings in 1917 with the flight of the first RC plane; another military venture. In the 30′s the soviets developed RC tanks for actual use in combat against Finland. In the 40′s RC ships began being used for target practice as well as RC ‘Drones’ for aerial target practice.
Radio control models didn’t really come into hobby use until the 1950′s with the advent of electronics hobbyists. They quickly realized they could create homebuilt, single channel radios. This was quickly developed into multi channel radios, and more recently, PCM digital radios. These first models were usually airplanes. RC cars were built by hard core hobbyists in the early 60′s but kits for the average person to build didn’t come about until about 1967.
The first companies to release kits were WEN, Model Car Enterprises, Dynamic Models, and Associated. If you’re of my generation, you’ll recognize the Associated name. It’s hard to believe they’ve been building RC cars for over 40 years! Electric cars didn’t hit the scene until 1974. In 1976 Tamiya sold it’s first RC car and has been around since. These early cars, both electric and gas were run mostly on smooth pavement and were simple pan cars. In 1979 the Tamiya Rough Rider revolutionized the industry with the first real off-road capability of any RC.
The RC hobby exploded in the ’80s as large races were organized, and world championships crowned in a number of on-road and off-road classes. The 80′s and 90′s were considered by many to be the heyday of RC. These decades saw huge leaps in technology and the support of large companies both here and abroad. These advances continue today and RC is as popular as ever.
RC vehicles are everywhere now and range from tiny 1/64 scale cars up to 1/5 scale off road buggies and stock cars. The price of technology continues to go down which forces constant new development. RC thrives in the present and will continue to do so in the future, both for fun, and for more serious applications.
-RC Scientist
Hello and welcome to RC Science! We hope to be your source for up-to-date and relevant information regarding the science of RC. In this blog we will cover a wide variety of topics regarding the scientific and engineering principles behind your RC gear. We’ll give tips along the way that have been learned from years of racing, bashing, and flying. We’ll talk about RC cars, boats, airplanes, helicopters, tanks, robots, etc. We’ll start off with a few of our own topics to get things going, but if there’s something specific you’re interested in or have been wondering about, drop us an email. We’d love to hear your comments and questions.
-RC Scientist
