2. Autonomous Mobile Robots CPE 470/670 Lecture 11 Instructor: Monica Nicolescu

3. CPE 470/670 - Lecture 112 Review Expression of behaviors –Stimulus Response –Finite State Acceptor –Situated Automata Behavioral encoding –Discrete: rule-based systems –Continuous: potential fields, motor schemas

4. CPE 470/670 - Lecture 113 Examples of Schemas Obstacle avoid and stay on corridor schemas

5. CPE 470/670 - Lecture 114 The Role of Gains in Schemas Low gain High gain

6. CPE 470/670 - Lecture 115 Schema-Based Robots At Georgia Tech (Ron Arkin) Exploration Hall following Wall following Impatient waiting Navigation Docking Escape Forage

7. CPE 470/670 - Lecture 116 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

8. CPE 470/670 - Lecture 117 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

9. CPE 470/670 - Lecture 118 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

10. CPE 470/670 - Lecture 119 Emergent Behavior The resulting robot behavior may sometimes be surprising or unexpected  emergent behavior

11. CPE 470/670 - Lecture 1110 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

12. CPE 470/670 - Lecture 1111 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

13. CPE 470/670 - Lecture 1112 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

14. CPE 470/670 - Lecture 1113 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

15. CPE 470/670 - Lecture 1114 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

16. CPE 470/670 - Lecture 1115 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

17. CPE 470/670 - Lecture 1116 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

18. CPE 470/670 - Lecture 1117 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!

19. CPE 470/670 - Lecture 1118 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

20. CPE 470/670 - Lecture 1119 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

21. CPE 470/670 - Lecture 1120 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

22. CPE 470/670 - Lecture 1121 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

23. CPE 470/670 - Lecture 1122 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)

24. CPE 470/670 - Lecture 1123 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

25. CPE 470/670 - Lecture 1124 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

26. CPE 470/670 - Lecture 1125 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

27. CPE 470/670 - Lecture 1126 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

28. CPE 470/670 - Lecture 1127 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

29. CPE 470/670 - Lecture 1128 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

30. CPE 470/670 - Lecture 1129 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

31. CPE 470/670 - Lecture 1130 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

32. CPE 470/670 - Lecture 1131 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

33. CPE 470/670 - Lecture 1132 Hybrid Control Idea: get the best of both worlds Combine the speed of reactive control and t he 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

34. CPE 470/670 - Lecture 1133 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

35. CPE 470/670 - Lecture 1134 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

36. CPE 470/670 - Lecture 1135 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

37. CPE 470/670 - Lecture 1136 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?

38. CPE 470/670 - Lecture 1137 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

39. CPE 470/670 - Lecture 1138 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

40. CPE 470/670 - Lecture 1139 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

41. CPE 470/670 - Lecture 1140 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!!

42. CPE 470/670 - Lecture 1141 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

43. CPE 470/670 - Lecture 1142 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

44. CPE 470/670 - Lecture 1143 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

45. CPE 470/670 - Lecture 1144 Readings M. Matarić: Chapters 17, 18