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Autonomous Mobile Robots CPE 470/670

This lecture provides an overview of the expression of behaviors in autonomous mobile robots, including stimulus-response, finite state acceptor, and behavioral encoding. It also explores the coordination of behaviors through competitive and cooperative strategies. Examples of schema-based robots and emergent behavior are discussed. The lecture concludes with a discussion on the design and execution of emergent behavior in the real world.

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Autonomous Mobile Robots CPE 470/670

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  1. Autonomous Mobile RobotsCPE 470/670 Lecture 11 Instructor: Monica Nicolescu

  2. Review • Expression of behaviors • Stimulus Response • Finite State Acceptor • Situated Automata • Behavioral encoding • Discrete: rule-based systems • Continuous: potential fields, motor schemas CPE 470/670 - Lecture 11

  3. Examples of Schemas • Obstacle avoid and stay on corridor schemas CPE 470/670 - Lecture 11

  4. The Role of Gains in Schemas • Low gain • High gain CPE 470/670 - Lecture 11

  5. Schema-Based Robots • At Georgia Tech (Ron Arkin) • Exploration • Hall following • Wall following • Impatient waiting • Navigation • Docking • Escape • Forage CPE 470/670 - Lecture 11

  6. Behavior Coordination • Behavior-based systems require consistent coordination between the component behaviors for conflict resolution • Coordination of behaviors can be: • Competitive: one behavior’s output is selected from multiple candidates • Cooperative: blend the output of multiple behaviors • Combination of the above two CPE 470/670 - Lecture 11

  7. Competitive Coordination • Arbitration: winner-take-all strategy  only one response chosen • Behavioral prioritization • Subsumption Architecture • Action selection/activation spreading (Pattie Maes) • Behaviors actively compete with each other • Each behavior has an activation level driven by the robot’s goals and sensory information • Voting strategies (DAMN architecture, Rosenblatt) • Behaviors cast votes on potential responses CPE 470/670 - Lecture 11

  8. Cooperative Coordination • Fusion: concurrently use the output of multiple behaviors • Major difficulty in finding a uniform command representation amenable to fusion • Fuzzy methods • Formal methods • Potential fields • Motor schemas • Dynamical systems CPE 470/670 - Lecture 11

  9. Emergent Behavior The resulting robot behavior may sometimes be surprising or unexpected  emergent behavior CPE 470/670 - Lecture 11

  10. Wall Following • A simple wall following controller: • If too close on left-back, turn left • If too close on left-front, turn right • Similarly for right • Otherwise, keep straight • If the robot is placed close to a wall it will follow • Is this emergent? • The robot has no explicit representations of walls • The controller does not specify anything explicit about following CPE 470/670 - Lecture 11

  11. Emergence • A “holistic” property, where the behavior of the robot is greater than the sum of its parts • A property of a collection of interacting components • Often occurs in reactive and behavior-based systems (BBS) • Typically exploited in reactive and BBS design CPE 470/670 - Lecture 11

  12. Flocking • How would you design a flocking behavior for a group of robots? • Each robot can be programmed with the same behaviors: • Don’t get too close to other robots • Don’t get too far from other robots • Keep moving if you can • When run in parallel these rules will result in the group of robots flocking CPE 470/670 - Lecture 11

  13. Emergent Behavior • Emergent behavior is structured behavior that is apparent at one level of the system (the observer’s point of view) and not apparent at another (the controller’s point of view) • The robot generates interesting and useful behavior without explicitly being programmed to do so!! • E.g.: Wall following can emerge from the interaction of the avoidance rules and the structure of the environment CPE 470/670 - Lecture 11

  14. Components of Emergence • The notion of emergence depends on two components • The existence of an external observer, to observe the emergent behavior and describe it • Access to the internals of the controller, to verify that the behavior is not explicitly specified in the system • The combination of the two is, by many researchers, the definition of emergent behavior CPE 470/670 - Lecture 11

  15. Unexpected & Emergent Behavior • Some argue that the description above is not emergent behavior and that it is only a particular style of robot programming • Use of the environment and side-effects leads to the novel behavior • Their view is that emergent behavior must be truly unexpected, and must come to a surprise to the external observer CPE 470/670 - Lecture 11

  16. Expectation and Emergence • The problem with unexpected surprise as property of behavior is that: • it entirely depends on the expectations of the observer which are completely subjective • it depends on the observer’s knowledge of the system (informed vs. naïve observer) • once observed, the behavior is no longer unexpected CPE 470/670 - Lecture 11

  17. Emergent Behavior and Execution • Emergent behavior cannot always be designed in advance and is indeed unexpected • This happens as the system runs, and only at run-time can emergent behavior manifest itself • The exact behavior of the system cannot be predicted • The real world is filled with uncertainty and dynamic properties • Perception is affected by noise • Would have to consider all possible sequences and combinations of actions in all possible environments • If we could sense the world perfectly, accurate predictions could be made and emergence would not exist! CPE 470/670 - Lecture 11

  18. Desirable/Undesirable Emergent Behavior • New, unexpected behaviors will always occur in any complex systems interacting with the real world • Not all behaviors (patterns, or structures) that emerge from the system's dynamics are desirable! • Example: a robot with simple obstacle avoidance rules can oscillate and get stuck in a corner • This is also emergent behavior, but regarded as a bug rather than a feature CPE 470/670 - Lecture 11

  19. Sequential and Parallel Execution • Emergent behavior can arise from interactions of the robot and the environment over time and/or over space • Time-extended execution of behaviors and interaction with the environment (wall following) • Parallel execution of multiple behaviors (flocking) • Given the necessary structure in the environment and enough space and time, numerous emergent behaviors can arise CPE 470/670 - Lecture 11

  20. Architectures and Emergence • Different architectures have different methods for dealing with emergent behaviors: modularity directly affects emergence • Reactive systems and behavior-based systems exploit emergent behavior by design • Use parallel rules and behaviors which interact with each other and the environment • Deliberative systems and hybrid systems aim to minimize emergence • Sequential, no interactions between components, attempt to produce a uniform output of the system CPE 470/670 - Lecture 11

  21. Deliberative Systems • Deliberative control refers to systems that take a lot of thinking to decide what actions to perform • Deliberative control grew out of the field of AI • AI, deliberative systems were used in non-physical domains, such as playing chess • This type of reasoning was considered similar to human intelligence, and thus deliberative control was applied to robotics as well CPE 470/670 - Lecture 11

  22. Shakey (1960) • Early AI-based robots used computer vision techniques to process visual information from cameras • Interpreting the structure of the environment from visual input involved complex processing and required a lot of deliberation • Shakey used state-of-the-art computer vision techniques to provide input to a planner and decide what to do next (how to move) CPE 470/670 - Lecture 11

  23. Planning Planning: • Looking ahead at the outcomes of possible actions, searching for a sequence that would reach the goal • The world is represented as a set of states • A path is searched that takes the robot from the current state to the goal state • Searching can go from the goal backwards, or from the current state to the goal, or both ways • To select an optimal path we have to consider all possible paths and choose the best one CPE 470/670 - Lecture 11

  24. SPA Architectures • Deliberative, planner-based architectures involve the sequential execution of three functional steps: • Sensing (S) • Planning (P) • Acting (A), executing the plan • SPA has serious drawbacks for robotics CPE 470/670 - Lecture 11

  25. Drawback 1: Time-Scale • It takes a very long time to search in large state spaces • The combined inputs from a robot’s sensors: • Digital sensors: switches, IRs • Complex sensors: cameras, sonars, lasers • Analog sensors: encoders, gauges + representations  constitutes a large state space • Potential solutions: • Plan as rarely as possible • Use hierarchies of states CPE 470/670 - Lecture 11

  26. Drawback 2: Space • It may take a large amount of memory to represent and manipulate the robot’s state space representation • The representation must be as complete as possible to ensure a correct plan: • Distances, angles, landmarks, etc. • How do you know when to stop collecting information? • Generating a plan that uses this amount of information requires additional memory • Space is a lesser problem than time CPE 470/670 - Lecture 11

  27. Drawback 3: Information • The planner assumes that the representation of the state space is accurate and up-to-date • The representation must be updated and checked continuously • The more information, the better • Updating the world model also requires time CPE 470/670 - Lecture 11

  28. Drawback 4: Use of Plans Any plan is useful only if: • The representation on which the plan was based is accurate • The environment does not change during the execution of the plan in a way that affects the plan • The robot’s effectors are accurate enough to perfectly execute the plan, in order to make the next step possible CPE 470/670 - Lecture 11

  29. Departure from SPA • Alternatives were proposed in the early 1980 as a reaction to these drawbacks: reactive, hybrid, behavior-based control • What happened to purely deliberative systems? • No longer used for physical mobile robots, because the combination of real-world sensors, effectors and time-scales makes them impractical • Still used effectively for problems where the environment is static, there is plenty of time to plan and the plan remains accurate: robot surgery, chess CPE 470/670 - Lecture 11

  30. SPA in Robotics • SPA has not been completely abandoned in robotics, but it was expanded • The following improvements can be made: • Search/planning is slow  saved/cache important and/or urgent decisions • Open-loop execution is bad  use closed-loop feedback and be ready to re-plan when the plan fails CPE 470/670 - Lecture 11

  31. Summary of Deliberative Control • Decompose control into functional modules: sense-world, generate-plan, translate-plan-to-actions • Modules are executed sequentially • Require extensive and slow reasoning computation • Encourage open-loop execution of generated plans CPE 470/670 - Lecture 11

  32. Hybrid Control • Idea: get the best of both worlds • Combine the speed of reactive control and the brains of deliberative control • Fundamentally different controllers must be made to work together • Time scales: short (reactive), long (deliberative) • Representations: none (reactive), elaborate world models (deliberative) • This combination is what makes these systems hybrid CPE 470/670 - Lecture 11

  33. Biological Evidence • Psychological experiments indicate the existence of two modes of behavior: willed and automatic • Norman and Shallice (1986) have designed a system consisting of two such modules: • Automatic behavior: action execution without awareness or attention, multiple independent parallel activity threads • Willed behavior: an interface between deliberate conscious control and the automatic system • Willed behavior: • Planning or decision making, troubleshooting, novel or poorly learned actions, dangerous/difficult actions, overcoming habit or temptation CPE 470/670 - Lecture 11

  34. Hybrid System Components • Typically, a hybrid system is organized in three layers: • A reactive layer • A planner • A layer that puts the two together • They are also called three-layer architectures or three-layer systems CPE 470/670 - Lecture 11

  35. The Middle Layer The middle layer has a difficult job: • compensate for the limitations of both the planner and the reactive system • reconcile their different time-scales • deal with their different representations • reconcile any contradictory commands between the two • The main challenge of hybrid systems is to achieve the right compromise between the two layers CPE 470/670 - Lecture 11

  36. An Example • A robot that has to deliver medication to a patient in a hospital • Requirements: • Reactive: avoid unexpected obstacles, people, objects • Deliberative: use a map and plan short paths to destination • What happens if: • The robot needs to deliver medication to a patient, but does not have a plan to his room? • The shortest path to its destination becomes blocked? • The patient was moved to another room? • The robot always goes to the same room? CPE 470/670 - Lecture 11

  37. Bottom-up Communication Dynamic Re-Planning • If the reactive layer cannot do its job  It can inform the deliberative layer • The information about the world is updated • The deliberative layer will generate a new plan • The deliberative layer cannot continuously generate new plans and update world information  the input from the reactive layer is a good indication of when to perform such an update CPE 470/670 - Lecture 11

  38. Top-Down Communication • The deliberative layer provides information to the reactive layer • Path to the goal • Directions to follow, turns to take • The deliberative layer may interrupt the reactive layer if better plans have been discovered • Partial plans can also be used when there is no time to wait for the complete solution • Go roughly in the correct direction, plan for the details when getting close to destination CPE 470/670 - Lecture 11

  39. Reusing Plans • Frequently planned decisions could be reused to avoid re-planning • These can be stored in an intermediate layer and can be looked up when needed • Useful when fast reaction is needed • These mini-plans can be stored as contingency tables • intermediate-level actions • macro operators: plans compiled into more general operators for future use CPE 470/670 - Lecture 11

  40. Universal Plans • Assume that we could pre-plan in advance for all possible situations that might come up • Thus, we could generate and store all possible plans ahead of time • For each situation a robot will have a pre-existing optimal plan, and will react optimally • It has a universal plan: • A set of all possible plans for all initial states and all goals within the robot’s state space • The system is a reactive controller!! CPE 470/670 - Lecture 11

  41. Domain Knowledge • A key advantage of pre-compiled systems • domain knowledge (i.e., information that the designer has about the environment, the robot, and the task), can be embedded into the system in a principled way • The system is compiled into a reactive controller  the knowledge does not have to be reasoned about (or planned with) explicitly, in real-time CPE 470/670 - Lecture 11

  42. Applicability of Universal Plans • Examples have been developed as situated automata • Universal plans are not useful for the majority of real-world domains because: • The state space is too large for most realistic problems • The world must not change • The goals must not change • Disadvantages of pre-compiled systems • Are not flexible in the presence of changing environments, tasks or goals • It is prohibitively large to enumerate the state space of a real robot, and thus pre-compiling generally does not scale up to complex systems CPE 470/670 - Lecture 11

  43. Reaction – Deliberation Coordination • Selection: Planning is viewed as configuration • Advising: Planning is viewed as advice giving • Adaptation: Planning is viewed as adaptation • Postponing: Planning is viewed as a least commitment process CPE 470/670 - Lecture 11

  44. Readings • M. Matarić: Chapters 17, 18 CPE 470/670 - Lecture 11

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