2. Robotics Sharif In the name of Allah

3. Robotics Sharif Introduction to Robotics o Leila Sharif o l_sharif@sharif.edul_sharif@sharif.edu o Lecture #4: The Big Picture

4. Robotics Sharif Last time we saw:  Controller  Reactive  Delibrative  Hybrid  Behaviour  A brief history of robotics  Feedback control  Cybernetics  Artificial Intelligence (AI)  Early robotics  Robotics today

5. Robotics Sharif Lecture Outline  Why is robotics hard?  Degrees of Freedom (DOF)  holonomicity, redundancy  Legged locomotion  stability (static and dynamic)  polygon of support  Wheeled locomotion  Trajectory/motion planning

6. Robotics Sharif Why is Robotics hard?

7. Robotics Sharif Why is Robotics hard?  Sensors are limited and crude  Effectors are limited and crude  State (internal and external, but mostly external) is partially- observable  Environment is dynamic (changing over time)  Environment is full of potentially- useful (and useless) information

8. Robotics Sharif Key Issues  Grounding in reality: not just planning in an abstract world  Situatedness (ecological dynamics): tight connection with the environment  Embodiment: having a body  Emergent behavior: interaction with the environment  Scalability: increasing task and environment complexity

9. Robotics Sharif Definition of Effector  An effector is any device that has an effect on the environment.  A robot’s effectors are used to purposefully effect the environment.  E.g., legs, wheels, arms, fingers...  The role of the controller is to get the effectors to produce the desired effect on the environment, based on the robot’s task.

10. Robotics Sharif Definition of Actuator  An actuator is the actual mechanism that enables the effector to execute an action.  E.g, electric motors, hydraulic or pneumatic cylinders, pumps…  Actuators and effectors are not the same thing.  Incorrectly thought of the same; “whatever makes the robot act”

11. Robotics Sharif Degrees of Freedom  Most simple actuators control a single degree of freedom (DOF)  Think of DOFs as ways in which a motion can be made (e.g., up-down, left-right, in-out)  E.g., a motor shaft controls one rotational DOF; a sliding part on a plotter controls one translational DOF.

12. Robotics Sharif Counting DOF  A free body in space has 6 DOF  3 are translational (x, y, z)  3 are rotational (roll, pitch, and yaw)  Every robot has a specific number of DOF  If there is an actuator for every DOF, then all of the DOF are controllable  Usually not all DOF are controllable  This makes robot control harder

13. Robotics Sharif Example: DOF of a Car  A car has 3 DOF: position (x,y) and orientation (theta)  Only 2 DOF are controllable  driving: through the gas pedal and the forward-reverse gear  steering: through the steering wheel  Since there are more DOF than are controllable, there are motions that cannot be done, like moving sideways (that’s why parallel parking is hard)

14. Robotics Sharif Actuators and DOFs  We need to make a distinction between what an actuator does (e.g., pushing the gas pedal) and what the robot does as a result (moving forward)  A car can get to any 2D position but it may have to follow a very complicated trajectory  Parallel parking requires a discontinuous trajectory, velocity, i.e., the car has to stop and go