Unmanned aerial vehicles (UAVs) are playing increasingly important roles in our everyday lives. On the basis of gross weight (1,000 lb to 0.1 lb), operational altitude above ground level (10,000 ft to 100 ft), and mission endurance (six hours to 0.5 hours), UAVs may be classified broadly into large, mini, small, micro, and nano classes. Across all UAV classes, applications are expected to require more autonomy, i.e., the ability to operate without the need for sustained human supervision. The development of effective flight control algorithms that are computationally inexpensive, though fully accommodating limitations on control surface type, size, and mode of actuation, is a challenging problem. In the future, these UAVs will operate as a system, so all levels of autonomous operation will need to be developed.
Quadrotors are among mankind's oldest flying machines (the 1922 Flying Octopus) and are ideal platforms for autonomous flight in unknown and complex environments. The picture below shows a Stanford University quadrotor UAV with autonomous attitude and altitude control.
Their small size and maneuverability are conducive to operating in confined spaces and avoiding obstacles... A quadrotor's small size and maneuverability also create problems for making them autonomous. The same fast dynamics that make quadrotors maneuverable require accurate and frequently updated position, orientation, and velocity state estimates to enable autonomous control. Additionally, the amount of energy and payload available limits the available sensors and processing capability.
The quadrotor's fixed-pitch rotors are independently speed-controlled and can manipulate two forces, thrust and torque. Roll, pitch, and yaw can be controlled by varying rotor speed. They are simpler, both mechanically (i.e., they do not require complex mechanical control linkages for rotor actuation) and in handling, than a helicopter, which has two rotors. Also, as another research team wrote in a 2007 paper:
The use of four rotors ensures that individual rotors are smaller in diameter than the equivalent main rotor on a helicopter, relative to the airframe size. The individual rotors, therefore, store less kinetic energy during flight, mitigating the risk posed by the rotors should they entrain any objects.
Conventional quadrotors are what's called underactuated robots, which means that they can move in more ways than they have independent control over. For example, they can happily yaw around to any angle you want while otherwise stationary, but if you ask them to pitch or roll, they can’t do it without also changing their position...
Having controls coupled together like this places some restrictions on what you can do with quadrotors, but a new design presented yesterday at the 2013 IEEE International Conference on Robotics and Automation (ICRA) gets around all of that with propellers that tilt. This level of control turns the quadrotor into a fully-actuated robot: you have complete control over its position and orientation.
The conventional quadrotor requires motor control and electronic stabilization using a three-axis gyroscope, accelerometer, and magnetometer (an electronic compass), and a barometer (altitude determination). Sensor fusion is used with the accelerometer and gyroscope, just as is done on a self-balancing transporter, like the Segway. As Mouser Electronics tells us: "The purpose of sensor fusion is to take each sensor measurement data as input and then apply digital filtering algorithms to compensate each other and output accurate and responsive dynamic attitude (pitch/roll/yaw) results."
As is always the case, none of this can be done without a model of the quadrotor. Improvements in control result from more accurate models, as well as from more effective control algorithms. The role of control as an enabling technology has never been more visible in everyday life.
I certainly agree with you naperlou on needing a collision avoidance system for quadrotor drones and other remote controlled drones. I also agree that drones can provide a remarkable service when used in difficult and dangerous environments. I feel the mechanical aspects of developing a system is fairly straightforward--its' the control aspects that become the tripping point. I feel that as these drones find more and more domestic uses; the FAA will step in to regulate them in some fashion, not to mention Homeland Security. Great post Kevin.
At least one of the drones was hijacked by "spoofing" the GPS system, using fake sattelite signals to mislead the system into believing it was in a different location. My preferred solution would involve a small nuclear device rigged as a booby trap to detonate upon unauthorized access. It would allow us to know just exactly where the drone had been taken for dismantling. And it would help protect the drone technology. It might possibly deter additional hijackings.
If you have been viewing the latest 24 episodes, you will relize that hi-jacking the USAF X47 is the major plot line. I really don't believe it is possible, right now, to hi-jack an operational military drone. In fact, it is almost impossible to take control of any drone. Having said that, I will state that it maybe possible to interrupt the comm link with a drone such that it goes out of control and likely crashes. In case of the Iranian drone; the Russians likely over powered the drone link causing it to lose comms. I can only guess that the fail-safe program that should be installed in these drones, failed and it subsequently crashed. Some small scale, non-military quadcopters (multirotors) also have a "return to home" function in case the comm link is lost.
Again, only guessing because the actual military contro system is classified, I must assume that it is a satellite based and highly encrypted. There is simply too small of a time window available to intercept, decode and decrypt the real-time digital control signals to hi jack a military drone, no matter how powerful the enemy's computer system.
In a shameless plug, please take a look a my recently published book, "Build Your Own Quadcopter" to understand why I can make these statements with confidence. I go through the comm link technology that controls civilian drones pointing out how resistent they are to interference.
I agree with you naperlou. I remember when the Russians hacked that drone flight and even Iran (probably a lie) claimed to have taken control of the X-47B. It's a scary thought that they can be hijacked, especially when they are outfitted with munitions.
Kevin, from the title I thought you were going to talk about something else.
I was judging senior projects at DeVry University in Chicago as part of an IEEE group and one of the projects was a quadrotor.
Frankly, with the microcontrollers available today, at very low cost, implementing these control algorigthms is surely feasible,
On the other hand, I think we are putting too much emphasis on them in the military. Against an enemy like the Taliban, they are great. On the other hand, the Russians have already taken control of and crashed (mostly intact) one of our military drones. In a conflict with a sophisticated enemy the only useful ones will be the micros that front line troops can use to take a quick look ahead.
As for civilian use, collision avoidance needs to be required. I don't want one crashing in my back yard.
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