Module 2: Mechanics and Actuators in Robotic Systems

Lecture Notes

1. Overview of Mechanical Systems and Actuators

What are Mechanical Systems?

• Mechanical systems in robotics refer to the physical components and mechanisms that
enable movement and perform tasks. These systems usually involve a combination of
actuators, gears, motors, joints, and structural elements that work together to carry
out specific operations.
• The mechanical design of a robotic system defines how the robot moves, interacts with
objects, and manipulates its environment. The robot's mechanical structure must be
designed to optimize factors like strength, stability, flexibility, and precision.

Key Components of a Robotic Mechanical System:

1. Actuators: The muscles of the robot, responsible for providing motion (linear or rotary).
2. Sensors: Devices that gather information (e.g., vision, force, proximity).
3. Control Systems: Logic that commands the actuators and processes sensor data.
4. Structural Components: The physical framework, including arms, legs, or chassis.
5. Power Supply: Provides the necessary energy to drive the actuators and sensors.

Role of Actuators in Mechanical Systems:

• Actuators convert energy (electrical, pneumatic, hydraulic) into mechanical motion.

They are the key components that allow robots to perform actions like lifting, rotating,
translating, or performing tasks like welding or assembly.
• Actuators can be categorized into three main types: electrical, hydraulic, and
pneumatic.

2. Types of Actuators: Electrical, Hydraulic, Pneumatic

Electrical Actuators:

• Overview: These actuators use electrical energy to produce mechanical motion. The most
common types of electrical actuators are DC motors, stepper motors, and servo
motors.
• Types:
1. DC Motors: Provide continuous rotation. Common in robotic arms and mobile
robots for driving wheels.
 2. Stepper Motors: Provide precise control over position with discrete steps, ideal
for applications requiring fine movement control, such as in 3D printers.
3. Servo Motors: Offer precise position control and are used in robotic joints, such
as in humanoid robots, where accurate and controlled motion is critical.
• Advantages:
o Precise control: High accuracy in positioning.
o Energy efficiency: Relatively energy-efficient when compared to hydraulic and
pneumatic actuators.
o Compact size: Can be made small for lightweight robots.
• Disadvantages:
o Limited force output (especially DC motors).
o Susceptible to wear over time due to friction and heat generation.
• Applications:
o Robotic arms
o Service robots
o CNC machines

Hydraulic Actuators:

• Overview: These actuators use pressurized hydraulic fluid to create mechanical force.
Hydraulic actuators provide high force output and are used where large amounts of force
are needed in a compact form.
• Types:
1. Hydraulic Cylinders: Produce linear motion; used in heavy-duty robots and
industrial automation systems.
2. Hydraulic Motors: Provide rotary motion; typically found in large industrial
robots or vehicles.
• Advantages:
o High force-to-weight ratio: Can provide significant force in a small size.
o Smooth, continuous motion: Useful for tasks requiring smooth, precise motion
(e.g., lifting heavy objects).
• Disadvantages:
o Complex and expensive system due to the need for pumps, reservoirs, and valves.
o Potential for leakage or maintenance issues.
• Applications:
o Industrial robots (e.g., welding robots)
o Heavy lifting or construction robots
o Autonomous vehicles

Pneumatic Actuators:

• Overview: Pneumatic actuators use compressed air to produce mechanical motion. They
are commonly used for applications that require quick, short bursts of movement and
moderate force.
• Types:
1. Pneumatic Cylinders: Provide linear motion.
 2. Pneumatic Motors: Provide rotary motion for smaller robots or tools.
• Advantages:
o Simple design: Pneumatic systems are easy to design and maintain.
o Fast response time: Can generate rapid movements, making them ideal for tasks
requiring speed.
• Disadvantages:
o Limited force output compared to hydraulic actuators.
o Air pressure needs to be carefully managed to avoid inconsistencies.
• Applications:
o Light-duty robotic arms
o Material handling robots
o Sorting robots in assembly lines

3. Actuator Selection Procedure: Criteria, Performance Metrics, and

Applications

Key Criteria for Actuator Selection:

Selecting the right actuator for a robotic system is a critical decision. Several factors must be
considered when choosing the appropriate actuator:

1. Force and Torque Requirements:

o Linear Force: If the task involves pushing, pulling, or lifting, you need to choose
an actuator that can produce the required linear force.
o Torque: For tasks involving rotation, such as rotating joints in a robotic arm or
turning wheels on a mobile robot, torque output is crucial.
2. Speed and Precision:
o Speed: For fast movements, electrical actuators like DC motors or pneumatic
actuators are often preferred.
o Precision: For high-precision tasks, such as surgical robots or robotic arms used
in assembly, servos or stepper motors are ideal because they can provide fine,
repeatable motion.
3. Energy Efficiency:
o Electrical actuators tend to be more energy-efficient than hydraulic or pneumatic
systems.
o Consider the energy cost for long-term operations, especially in mobile robots
where battery life is a constraint.
4. Environment:
o Harsh Environments: Hydraulic actuators are often preferred for industrial
environments where robots are required to work under heavy loads and extreme
conditions (e.g., welding robots).
o Clean Rooms: For environments where cleanliness is a priority, such as
semiconductor manufacturing, pneumatic actuators are often used due to the lack
of lubricants.
 5. Size and Weight:
o The actuator must fit within the design constraints of the robot without making
the system too large or too heavy. This is especially important in mobile robots or
drones.

Performance Metrics:

1. Force-to-Weight Ratio:
o Indicates how much force the actuator can generate compared to its weight.
Hydraulic actuators often have the highest force-to-weight ratio.
2. Speed:
o Measured in the time required to complete a full cycle or the rate of movement.
3. Precision and Accuracy:
o Measured in terms of angular position or linear displacement and how
consistently the actuator can achieve the desired position.
4. Reliability and Maintenance:
o Hydraulic and pneumatic actuators may require more maintenance compared to
electrical actuators due to the complexity of fluid or air systems.

4. Mechanical Components: Gears, Pulleys, Bearings, and Their Selection in

Design

Gears:

• Overview: Gears are mechanical components used to transmit rotational motion and
torque between machine parts. They come in many types, such as spur gears, bevel gears,
and planetary gears.
• Types of Gears:
1. Spur Gears: Straight-cut gears used for transferring motion between parallel
shafts.
2. Helical Gears: Gears with angled teeth, used for smoother, quieter operation than
spur gears.
3. Planetary Gears: Provide high torque output in a compact design, often used in
robotic arms for rotational movements.
• Selection Criteria:
o Load capacity
o Gear ratio (for speed and torque requirements)
o Noise and efficiency considerations

Pulleys:

• Overview: Pulleys are used to transmit rotational motion using belts. Pulleys can change
the direction of motion and are often used for lifting or translating motion.
• Types:
 1. Fixed Pulley: Used to change the direction of motion.
2. Movable Pulley: Provides mechanical advantage by reducing the amount of force
needed to lift an object.
• Selection Criteria:
o Size of the pulley
o Load capacity
o Type of belt used (timing belts, V-belts)

Bearings:

• Overview: Bearings support rotating parts, reducing friction and wear. They are essential
in actuators and joints of robots.
• Types:
1. Ball Bearings: Provide low friction and are used in high-speed applications.
2. Roller Bearings: Used in applications requiring heavier load capacities.
3. Linear Bearings: Used for smooth, low-friction linear motion, important in
robotic arms.
• Selection Criteria:
o Load rating
o Speed capability
o Environmental conditions (e.g., clean rooms, high temperatures)

5. Practical Examples of Actuator Selection for Industrial Robots

Example 1: Industrial Robotic Arm (Welding)

• Actuators:
o Motors (servo motors or stepper motors) for precise joint control.
o Hydraulic actuators for powerful lifting of heavy materials.
• **Reasoning