Aerial Robotics

Actuation

C. Papachristos

Robotic Workers (RoboWork) Lab

University of Nevada, Reno

CS-791

Actuation

Electromechanical Motion

How forces & torques can be generated and controlled, in order to create coordinated motion.

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IEEE Spectrum

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Actuation

Different Actuation & Propulsion Designs enable Diversity

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Actuation

• Different locomotion & propulsion systems designs are employed for different robotic configurations.

• Miniaturization of locomotion & propulsion systems –in combination with good efficiency– is among the reasons for the success of small robotics.

Focus on:

• DC Motors
• DC Brushless Motors
• Propelled-systems
• Wheeled-systems
• Servomotors

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Electromechanical

DC Motors

• Stationary permanent magnet.
• Electromagnet on axis induces torque.
• Split ring + brushes (commutators) switch�direction of current.�
• Easy to build and control.

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Electromechanical

DC Motors

• More power means faster rotation.�Need a convenient method to control power in a) Analog or b) Digital approach.

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• How to modulate power using a digital signal?�Digital-equivalent functionality allowing to directly control power at the input.

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• Fixed voltage input, pulse-modulated.

Pulse Width Modulation (PWM)

Duty cycle is the proportion of “ON” time vs. period.

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Electromechanical

AC Motors

• Electromagnets are stationary.

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• Two or Three coils (or more).

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• No brushes /commutators

(less maintenance, higher efficiency).

Understanding RMF | The driving force behind every AC machine�https://www.youtube.com/watch?v=wqrGHeuxUvI&t=3m23s

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Electromechanical

Brushless Motors

• Electromagnets are stationary (stator part)

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• Permanent magnets on the rotating axis (motor part)

(either inside or outside).

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• Three coils (or more).

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• No brushes

(less maintenance, higher efficiency).

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• Brushless motors come with high torque, eliminating the need for gearboxes in case of multirotor aerial robots.

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4-pole

Inrunner

High-pole

Outrunner

Image Copyright: mpoweruk.com

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Electromechanical

Brushless Motors

• Typically one microcontroller per motor.

Phase-switching handled by high-priority Interrupt Service Routines.

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• Called Electronic Speed Controller (ESC).

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• Generates PWM signal for the three (3) motor phases.

• Switching MOSFETs to convert DC input power.

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• Phase-switching handled based�on rotor position!

Measured directly (sensored) or� estimated using back-EMF (sensorless).

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Communication Protocols

Analog

• PWM-based modulation of an analog voltage corresponding to a command signal magnitude.

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Digital

• PWM-based digital encoding of commands.
• Convention of “counting single/multiple pulse width times” within a given frame period (e.g. 20ms).
• Serial & I2C
• More advanced digital protocols used with ESCs, sensors, etc.
• Communication between motor controller and autopilot.
• I2C allows for multiple devices-per-line or bi-directional communication).
• Serial data line (SDA) + serial clock line (SCL)
• Specific encoding/decoding allows master/slave communication.
• Multiple devices connected in parallel.
• 7-10 bit address, 100-3400 kbit/s speed.

Image Copyright: Wikipedia

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Mechanical

Wheeled Vehicles

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Mechanical

The Wheel

Topfuel dragster race slow motion�https://www.youtube.com/watch?v=Lt6iltuxD48&t=2m

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Mechanical

The Wheel

• Simplified model – Circular motion rotational formulas:

• Angular Velocity

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• Angular Velocity and Acceleration

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• Angular Displacement

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• Angular Acceleration

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• Angular Momentum or Torque

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Control

DC Motor Modeling

• Electromechanical Model
• Frequency Domain modeling

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DC Motor Model

(A low-pass filter)

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Speed Control

Motor Feedback Speed Control

• Linearized Model
• Linear Control
• Output Feedback - driven

DC Motor

Model

Control

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Position Control

Motor Feedback Position Control

• Linearized Model
• Linear Control
• Output Feedback - driven

DC Motor

Model

Control

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Position Control

The Servomotor

• Position Feedback Control is crucial and widely used in actuation of mobile robots

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Articulated Systems

The Manipulator

• Construction of Multi-DoF Kinematic Chains by daisy-chaining actuated Joints & rigid Links
• Powerful brushless servomotors with position feedback to support high loads while maintaining positioning accuracy

Base Joint

Shoulder Joint

Elbow Joint

Wrist 1 Joint

Wrist 2 Joint

Wrist 3 Joint

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Articulated Systems

• Principle of Hydraulic actuation – but with compressed air
• e.g. move a piston by pressurizing chamber
• Hydraulic industrial automation without fluid leaks/spills

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“Soft” Robotics

• Utilize tube-like / compartmentalized / bellows-based robot body designs, (de-)pressurized by air

Pneumatic Actuation

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Articulated Systems

• Principle of “Push / Pull” actuation
• Push / Pull can be accomplished via traditional actuators …
• … or “smart” materials, e.g. Shape Memory Alloys (SMAs)

Tendon/Linkage-Driven

Uikyum Kim, et al., “Integrated linkage-driven dexterous anthropomorphic robotic hand”, Nature Communications, vol 12, number: 7177 (2021)

Xuanang Chen, “The actuation and control of a bioinspired origami manipulator”, Springer, Soft Computing (2023)

“Origami” Robots

• Create compact (“packable”) robot bodies that expand

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Aerodynamics

Rotorcraft

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Aerodynamics

The Rotor

Video of airflow and vortex patterns with propellers.�These tests were conducted at NACA, now NASA Langley Research Center.

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Aerodynamics

• Rotor modeling is a very complicated process.

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• A Rotor is different than a propeller.
• Not rigid and contains more motion degrees of freedom. Among them blade flapping allows the control of the rotor tip path plane and therefore control the helicopter.

• Used to produce thrust.
• Propeller plane perpendicular to shaft.
• Assumed rigid blades - No flapping.
• Fixed blade pitch angle or collective changes only.

• Used to produce lift and directional control.
• Elastic element between blade and shaft.
• Blade flapping used to change tip path plane.
• Blade pitch angle controlled by swashplate.

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Aerodynamics

Aerodynamic Force

• “Force” generated due to the airflow separation around the airfoil�which causes a difference in upper/lower streamline velocities,�and thus pressures.

By J Doug McLean, CC BY-SA 3.0

By Kraaiennest, CC BY-SA 3.0

By רונאלדיניו המלך - , CC BY-SA 4.0

Resultant Force

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Aerodynamics

Aerodynamic Force & Moment

v

Note: CoP location changes with AoA

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Aerodynamics

Aerodynamic Coefficients

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Aerodynamics

The Propeller Blade

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Aerodynamics

The Propeller

• Simplified model forces and moments:

• Thrust Force: the resultant of the vertical forces acting on all the blade elements.

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• Hub Force: the resultant of all the horizontal forces acting on all the blade elements.

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• Drag Moment: This moment about the rotor shaft is caused by the aerodynamic forces acting on the blade elements. The horizontal forces acting on the rotor are multiplied by the moment arm and integrated over the rotor. Drag moment determines the power required to spin the rotor.

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Bioinspired Principles

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Time for Questions !

CS-791

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