forces & sensors on a quadcopter - eindhoven university of ... · •a quadcopter not only has...
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Forces & sensors on a quadcopter
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Forces & sensors on a quadcopter
• Definitions
• Pitch and roll
• Yaw
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Definitions
• Roll, pitch and yaw
• Lift, drag, trust and weight
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Pitch and roll
• Lift is always perpendicular to the flow direction, in our case the rotor plane.
• The gravity is always perpendicular to the ground.
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Pitch and roll
• Lift and gravity with pitch and roll.
– More lift required
– Horizontal lift component provides thrust
– When thrust overcomes drag the quadcopter moves.
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Pitch and roll
• Gyro can sense the rotation around the pitch or roll axis. – Unit is rotation in degrees per second. – Integrating this yields pitch or roll angle.
• Accelerometer can measure gravitational components. – Pitch and roll angle can be calculated with three axes,
two per angle (x,z) and (y,z).
• But….
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Pitch and roll
• Why will this not work well in practice? • Gyro’s (especially MEMS and Piezo):
– Have a noisy signal – Have an offset (temperature, other effects) – This results in accumulating error in the integration. – Gyro’s can be used for short corrections
• Accelerometers: – The forces described previously are only in uniform
linear motion. – In turns there are more forces due to horizontal lift
component.
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Pitch and roll
• When a plane/quadcopter turns it has to roll. • This results in a horizontal and vertical lift component. • The horizontal lift component gives a centripetal force. • The weight and centrifugal force result in cancel out the
lift (plane does not climb) • This is more commonly known
as g-force or load factor. • This can be estimated by:
For example 30 degree turn is 1.15g
• The accelerometer will measure this in the Z-axis, the other axes are 0.
)cos(
1
g
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Pitch and roll
• A quadcopter not only has this for roll but also for pitch in sideways movements.
• How to solve this:
– Combine both the gyro and accelerometer information using a Kalman filter.
– This also makes measurements insensitive to noise (for example due to vibration).
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Yaw
• Turning around the top-axis.
• Usually compensated with gyros (short term compensation). Again: drift.
• For absolute yaw control: use a magnetometer
– Basically an electronic compass.
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Altitude
• Can be determined with distance sensors (e.g. ultrasone or infrared)
• Can be determined with barometric sensors
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Some motivation…
• Best performing group can go gliding at Terlet near Arnhem.