Quadrocopter

Kyle Bartholomew

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 control.

• Power system– Based on lithium polymer battery

Power System

LiPo Battery

Motor Controllers

Motors

+ -

5VRegulator Arduino

Outrunner Brushless Motors

AC Power

Motion Control Unit• Miniature computer (Arduino microcontroller)

– Runs at 16Mhz (Your computer runs about 200 times faster)

– Takes electrical signals in, does math, spits out more electrical signals.

• IMU – Inertial Measurement Unit – 3-axis gyroscope and accelerometers.

RF ReceiverArduino

IMU

Motor Controllers

Accelerometers or Gyros?

• What do you use to measure the angle of the copter relative to the ground?

Accelerometers• 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

measurement3.3V/1024 = .00322V/step

(30°/0.11V)*(.00322V/step) =

0.88°/step

3.3V

1.65V

0V

0g1g

1.65V

1.95V

0g

1g

30deg

Gyroscopes

• 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

time.

Combining measurements

• You want best of both with– High speed low noise measurements– Accuracy over long periods of time

Complimentary Filter

• Combines accelerometer and gyro to produce a better angle measurement.

• High pass filter on gyro and low pass filter on accelerometer.

• Aka only long term for accelerometer and only short term for gyro.

PID Control• 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 the future.

Error = θdesired-θcurrent

Integral = Integral + (Error•CycleTime)Control = P•(Error) + I•(Integral) - D•(GyroRate)

Complimentary Filter

Accelerometer Low Pass Filter

GyroscopeHigh Pass FilterIntegration

Angle

P Coefficient I Coefficient D Coefficient

DerivativeIntegration

Motor Output (200hz)

PID

How does it move?

CCW CCW

CW

CW

Hint: r X F

I•ɑ

Going further

• Many people add other fancy things to their quadrocopters:– GPS– Altitude Sensor (Barometer)– Camera– FPV (First Person View)– Tracking Systems to do crazy stuff like flips

and multiple copter coordinated dances.