11.7 - Friction and Actuator Dynamics
Enroll to start learning
You’ve not yet enrolled in this course. Please enroll for free to listen to audio lessons, classroom podcasts and take practice test.
Interactive Audio Lesson
Listen to a student-teacher conversation explaining the topic in a relatable way.
Understanding Friction Models
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Today, we'll explore the different models of friction that affect robotic motion. Can anyone tell me what static friction is?
Isn't static friction the force that keeps an object at rest until a certain force is applied?
Exactly, static friction prevents motion until the applied force exceeds a threshold. Now, what about viscous friction?
I think viscous friction is related to the movement speed; it opposes the motion as the surfaces slide against each other.
Correct! It's proportional to the velocity. Lastly, can anyone explain the Stribeck effect?
The Stribeck effect describes how friction decreases at very low velocities, right?
Well done! This effect is crucial for understanding friction in controlled environments. Remember, the total torque due to friction can be expressed as \( \tau_{f} = \tau_{c} \cdot sign(q̇) + b \cdot q̇ \).
That's interesting! So, the sign indicates direction while the second part accounts for viscosity?
Exactly! Summarizing, we've learned about static, viscous, and Stribeck friction, each plays a vital role in robotic dynamics.
Actuator Dynamics
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Now let's shift our focus to actuators. What do you think an actuator does in a robotic system?
Is it like a muscle for robots? It makes them move!
That's a great analogy! Actuators convert energy into motion. Could anyone explain what a torque-speed curve is?
It's a graph showing how much torque an actuator can produce at different speeds, right?
Exactly! The curve helps in understanding how an actuator will perform under various conditions. And what about the concept of time delay?
Time delay is when there is a lag between the control signal and the actuator's movement.
Spot on! This delay has to be accounted for in control strategies. Remember, actuator dynamics can be modeled as first or second-order systems to simplify control design.
How do gear ratios come into play in this context?
Gear ratios affect the torque output and speed of actuation. They are essential for maximizing efficiency. To summarize, actuators are the driving force in robots influenced by torque-speed curves, time delays, and gear ratios.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
Friction and actuator dynamics are crucial for understanding how robots interact with their environments and how their movements can be controlled effectively. The section outlines different friction models, including static, viscous, and the Stribeck effect, and discusses how actuator dynamics, such as motor torque-speed curves and time delays, impact robotic motion.
Detailed
Friction and Actuator Dynamics
In robotic systems, friction and actuator dynamics are essential factors that significantly influence a robot's performance and motion control capabilities.
Friction Models
Friction within robotic systems can be modeled primarily through three distinct types:
1. Static Friction (Coulomb): This model posits a constant opposing force at low velocities, which prevents motion until a threshold of driving force is overcome.
2. Viscous Friction: Generated due to the movement of robot parts, this type of friction is proportional to the velocity between surfaces in contact, manifesting as a damping effect.
3. Stribeck Effect: A unique characteristic where the friction decreases as the velocity approaches zero, often observed in lubricated contacts.
The total torque generated due to friction for a robot can be expressed as:
$$ \tau_{f} = \tau_{c} \cdot sign(q̇) + b \cdot q̇ $$
where \( \tau_{c} \) is the Coulomb friction, b is the viscous damping coefficient, and \( q̇ \) is the velocity.
Actuator Dynamics
Actuators, which include motors and hydraulic cylinders, are critical dynamic elements that significantly affect a robot's movement. Key elements associated with actuators include:
- Motor Torque-Speed Curve: Indicates the relationship between the torque an actuator can produce and its rotational speed.
- Time Delay: The delay that often occurs between the control command and the actuator response, crucial for control strategies in real-time applications.
- Gear Ratios: These ratios play an integral role in determining the output torque and speed characteristics of actuators.
In practice, actuator dynamics can be approximated as first-order or second-order systems, aiding in the design of controllers that can compensate for the inherent dynamics and maintain desired robot behavior.
Youtube Videos
Audio Book
Dive deep into the subject with an immersive audiobook experience.
Friction Models
Chapter 1 of 2
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
• Static Friction (Coulomb): Constant opposing force at low velocity
• Viscous Friction: Proportional to velocity
• Stribeck Effect: Drop in friction near zero velocity
Total torque due to friction:
τ = τ_f·sign(q˙) + b·q˙
Detailed Explanation
This chunk introduces three models of friction that affect robotic movement:
- Static Friction (Coulomb): This type of friction occurs when the robotic part is not moving, presenting constant resistance that must be overcome for motion to start. Think of it as the force that keeps a parked car from rolling down a hill.
- Viscous Friction: This occurs when there is relative motion and is proportional to the velocity. It's similar to how thick syrup resists movement; the faster you move your spoon through it, the more resistance you feel.
- Stribeck Effect: At very low velocities, friction can drop off, making it easier for motion to begin. This is like how it's easier for a sled to start sliding on snow when prodded gently, rather than starting with a full shove.
The total torque from friction combines these effects, suggesting that friction's impact is based on both the state of motion and the speed at which the motion occurs.
Examples & Analogies
Consider a bicycle on a flat road. When you first start pedaling (static friction), there is noticeable resistance until you apply enough force to start moving. Once you’re moving, the resistance you feel is like viscous friction, which grows as you pedal faster, making it harder to maintain high speeds. Finally, when you slow down near a stop, you may feel less resistance (Stribeck Effect) as the bike gears down to a halt.
Actuator Dynamics
Chapter 2 of 2
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
Actuators (like electric motors or hydraulic cylinders) have dynamics that affect the system. Common elements:
• Motor torque-speed curve
• Time delay
• Gear ratios
In real robots, actuator dynamics are modeled as a first-order or second-order system.
Detailed Explanation
This chunk discusses the dynamics of actuators, which are crucial components in robotics that create movement.
- Motor Torque-Speed Curve: This illustrates how the torque produced by a motor varies with its speed. It’s essential to understand this relationship as it defines how effectively the motor can perform at different speeds.
- Time Delay: Actuation doesn’t happen instantaneously; there is often a delay from when a command is issued to when it is executed. This is due to the internal processing and physical response time of the actuator.
- Gear Ratios: These affect the output speed and torque of the actuators, allowing for a trade-off between how fast an actuator can move and how much weight it can carry. When you have a high gear ratio, you might get more torque but less speed.
Overall, these dynamics are represented using either first-order or second-order system models in robot dynamics simulations or designs.
Examples & Analogies
Think of driving a car. The engine is like an actuator—it generates torque to make the car move. The torque-speed curve explains how fast you can accelerate based on the power of the engine at different speeds. Just like there’s a delay in feeling the response when you push the gas pedal, actuators also have delays in their response times. Gear ratios are like changing gears in a manual transmission: low gears give you lots of torque for starting and climbing, while high gears let you cruise at high speed. Understanding your vehicle’s dynamics helps you drive efficiently, just as it does in robotics.
Key Concepts
-
Friction Models: Types of friction affecting robot movement include static, viscous, and Stribeck friction.
-
Actuator Dynamics: Essential for robot movement, characterized by the torque-speed curve, time delay, and gear ratios.
Examples & Applications
Static friction is what prevents a robot arm from moving when a load is applied until sufficient force is available to move.
In a robotic gripper, viscous friction allows for precise control when holding objects, with higher speeds leading to increased resistance.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Friction static, hold on tight, / Viscous flows, speed feels right.
Stories
Imagine a car at a stop light. It doesn't move until you press the gas—this is like static friction at work. Then, when you're cruising down the highway, viscous friction helps guide smooth movement.
Memory Tools
Remember 'SAVe' for types of friction: Static, Actuator, Viscous.
Acronyms
FAS
Friction (Static)
Actuator dynamics
Speed (Torque-Speed Curves).
Flash Cards
Glossary
- Static Friction
The constant opposing force that must be overcome to initiate movement in a stationary object.
- Viscous Friction
A frictional force that is proportional to the velocity of the moving surfaces.
- Stribeck Effect
The phenomenon where friction decreases as the velocity approaches zero.
- Actuator
A component of a machine that is responsible for moving or controlling a mechanism or system.
- TorqueSpeed Curve
A graphical representation of the relationship between torque and speed in an actuator.
- Time Delay
The lag between the input signal and the actuator's response.
- Gear Ratio
The ratio of the output speed of a gear to the input speed, affecting the torque and speed output of an actuator.
Reference links
Supplementary resources to enhance your learning experience.