If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

A crash course on indoor flying robots

Learn the physics behind how quadrotors fly and find out how they can by themselves without human help. Created by MIT+K12.

Want to join the conversation?

  • leafers sapling style avatar for user Peter Collingridge
    If the quadrotors fly up by pushing air downward, how is it possible for them to fly upside down like at ""?
    (150 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user Buddy Michini
      Glad you picked that up in the video! What we didn't tell you was that the quadrotor flying upside-down is actually a variable-pitch version, i.e. the blades themselves change pitch (they are actuated by a servo) so that even though the motor is only spinning in one direction, the direction and magnitude of the thrust can be changed very quickly. Here's the full video explaining the variable-pitch quadrotor and some neat things it can do: http://www.youtube.com/watch?v=VIkqqVr_u9U
      (147 votes)
  • female robot grace style avatar for user KangK
    About how much does it cost to build a quadrotor?
    (44 votes)
    Default Khan Academy avatar avatar for user
  • orange juice squid orange style avatar for user R.T.G.
    What is C++ code?
    (25 votes)
    Default Khan Academy avatar avatar for user
  • purple pi purple style avatar for user TheFourthDimension
    At , to determine the quadrotor position, what is the "slightly more complicated math" that they are talking about?
    (11 votes)
    Default Khan Academy avatar avatar for user
    • leafers tree style avatar for user Dinesh
      As the video showed, assuming you have a camera whose view is parallel to a wall of a cubic room (lets say define this wall as the x axis), they can obtain the height (the z axis) and x coordinate. Using a camera on parallel to the ceiling would give the x and y coordinates of the quadcopter, the information of height is lost. Therefore, from both the views, the position of the quadcopter (x,y,z coordinates) can be accurately obtained. However, its generally very hard to get a camera exactly parallel to a wall/an axis, and generally a projection of the 3rd axis slips into the image. So, to find out the accurate position, they use a technique called Direct Linear Transformation (DLT), which solves for the actual x,y,z coordinates, from the two camera projections of the room. This, I believe, is the 'slightly more complicated math' they are referring to.
      (9 votes)
  • hopper cool style avatar for user hcheng1
    Where can you get quadrators?
    (7 votes)
    Default Khan Academy avatar avatar for user
  • hopper jumping style avatar for user Hope
    Do more propellers equal higher lift? Also do more motors make it go faster?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • hopper cool style avatar for user Yosef
      Simple answer - Yes. Well it all depends on how much thrust each one must provide. For example: a 6lb quadcopter with 4 motors, each weighing 1lb: comes out to 10lbs total. Times by 2, the number of pounds of thrust you'll want is 20lb. Which divided amongst 4 motors would be:
      5lb thrust per motor.
      With 5 motors, total weight is 11lb. Times by 2 is 22. Divided into 5 motors, you'll need:
      22/5 - 4.4 lb thrust per motor.
      It really depends on the weight of everything. Do the calculations yourself.
      Sorry if too complicated.
      (4 votes)
  • male robot hal style avatar for user Christopher Jennings
    Ok, so if you have variable-pitch propeller, are the RPMs constant? For example, a traditional prop gains or loses altitude as based on a variable speed motor, with a fixed pitch, so is the inverse true?
    (5 votes)
    Default Khan Academy avatar avatar for user
  • leaf orange style avatar for user ☢Arjun Peddireddy☢
    At how do they make it stop perfectly on the battery changing machine without falling, does it have like a camcorder on the front of it or something?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • female robot grace style avatar for user M. Wancewicz
    Are there any parts you can use in place of those required that would cost less money?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Charles
    what does the man say in the song at the end?
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

[MUSIC PLAYING] This is a quadrotor. It's called a quadrotor because it has four propellers that spin and generate thrust. More on that in a second. This is the pilot controlling the vehicle with a radio transmitter. That's pretty neat. But if we take a short trip across the street-- of course looking both ways before we cross-- we come to a place where this quadrotor can fly by itself, without any human help at all. We don't even need a pilot. This flying robot can operate with extreme precision in tight indoor spaces, and can do some other pretty neat stuff as well. So if you're wondering how to make robots fly, you've come to the right place. [MUSIC PLAYING] Maybe crash course isn't the right term. [MUSIC PLAYING] To figure out how to make robots fly, we'll need to understand the basic physics of quadrotors, how humans pilot them, how we can use a computer to achieve the same task, and why the resulting flying robots can do more complex things. First, let's take a quick look at the physics behind how the quadrotor flies. When the propellers spin, they push downward on the air around them. Newton's third law tells us that the air applies an equal and opposite reaction force on the propeller. When this lifting force equals that of gravity, the quadrotor achieves hover flight. In order to bank, one propeller spins slightly faster than the opposite one. This introduces a horizontal force, in addition to the one opposing gravity. And the vehicle moves sideways. That's great. But it doesn't tell us how the quadrotor can rotate about its vertical axis. It turns out that Newton's third law also applies to rotational force, called torque. When these two propellers spin, they apply a torque to the air in the clockwise direction. The air applies an equal and opposite reaction torque, pushing the vehicle in a counterclockwise direction. Meanwhile, the other two motors spin in the other direction, plus the reaction torque pushes the vehicle clockwise. When all four motors are turned on, the rotational forces-- remember they're called torques-- balance each other. In flight, the vehicle turns by spinning two motors ever so slightly faster than the other two. Now we know the basic physics of how a quadrotor flies, but before we can make it fly robotically, we need to know how to control it. First, let's figure out a human would do this. The task can be broken down into four key steps. First, the pilot uses his eyes to observe the vehicle and figure out where it is, and in which direction it's pointing. In this example, let's say that the pilot sees that the quadrotor is sinking. Next, the pilot has to decide what control commands to give the vehicle. In this case, the pilot has to stop the vehicle from sinking, and thus decides to increase the speed of all four propellers. To tell the quadrotor what he's decided on, the pilot uses a radio transmitter, which is basically a fancy remote control. Finally, the quadrotor listens for the radio commands, and adjusts the speed of each motor accordingly. Now let's see how each of these four steps changes in order to make the quadrotor fly robotically. In the first step, we use specialized cameras for vision, instead of the pilot's eyes. The cameras shine infrared light, which bounces off of small reflective markers on the vehicle, and go back to the camera. A camera from this side point of view can tell how far the marker is in the vertical direction, and one horizontal direction. And a camera from this top point of view can tell how far the marker is in both horizontal directions. Using some slightly more complicated mathematics, we can use the points of view from 12 different cameras mounted along the ceiling to determine the exact three-dimensional position of the markers. This process is executed many times per second to check the position of the markers, and plus the quadrotor, in real time. In step two, we use a computer to calculate the control commands, instead of the pilot's brain. The computer program consists of a couple hundred lines of C++ code, written by grade students who really don't get out much. It does essentially the same thing as the pilot, using the observed position of the quadrotor to calculate control commands, only it does so in a much faster and less dramatic fashion. In step three, the system uses a similar radio transmitter, except a smaller one without any switches or control sticks. Step four is exactly the same as before. The quadrotor listens for radio commands, and adjusts the speed of each motor accordingly. So we've updated all four steps in order to make the quadrotor to fly entirely by itself. Now all we need is for our grad student to press the go button, and voila. One of the reasons the robots fly more precisely than the human pilot is because this loop of information-- called a feedback control loop-- can be executed much more quickly by computers. In this case, 200 times per second. This allows researchers to do cool things with these indoor flying robots. For instance, fly six of them at once. Or why not 10? They can teach the vehicles how to switch out their old batteries for new ones automatically. Or stop a payload from swinging. They can even do flips like this one. Or this one. Or this one. And the fun doesn't stop with quadrotors. The same technology can be applied to weirdly shaped three-winged maneuvers. Or more conventional fixed-wing vehicles like this one, this one, and this one that can even fly loops. Well hopefully you've learned the basics of how to make robots fly. This concludes the crash course-- I mean, expedited learning experience. [MUSIC PLAYING]