What is a quadrocopter?
• Like a helicopter with 4 blades.
• 2 blades spin clockwise, 2 spin
counterclockwise = zero net torque
• Motion Control Unit
– Keeps the copter stable without human
• Power system
– Based on lithium polymer battery
Outrunner Brushless Motors
Motion Control Unit
• Miniature computer (Arduino microcontroller)
– Runs at 16Mhz (Your computer runs about 200 times
– Takes electrical signals in, does math, spits out more
• IMU – Inertial Measurement Unit – 3-axis gyroscope and
Accelerometers or Gyros?
• What do you use to measure the angle of
the copter relative to the ground?
• 3-axis, measures acceleration
– When incidental acceleration is
small enough, it measures just the
acceleration of gravity.
– Absolute angle relative to earth.
• Highly susceptible to noise:
∆0.11V = ∆30°
10 bit ADC = 1024 steps of
3.3V/1024 = .00322V/step
3-axis, measures angular velocity (w)
Integrate w over time to get angle
Good at instant angle measurements
Bad at absolute angle measurements
because it drifts.
• Small error in w leads to big error over
• You want best of both with
– High speed low noise measurements
– Accuracy over long periods of time
• Combines accelerometer and gyro to
produce a better angle measurement.
• High pass filter on gyro and low pass filter
• Aka only long term for accelerometer and
only short term for gyro.
• Need to go from angle to motor output.
• Based on idea of error from setpoint.
• The ideal method is PID control –
Proportional Integral Derivative.
• Proportional – Controls the now
• Integral – Controls by looking at the past
• Derivative – Controls by looking forward to
Error = θdesired-θcurrent
Low Pass Filter
High Pass Filter
Motor Output (200hz)
How does it move?
Hint: r X F
• Many people add other fancy things to
– Altitude Sensor (Barometer)
– FPV (First Person View)
– Tracking Systems to do crazy stuff like flips
and multiple copter coordinated dances.