Logistics

Homework submission details

• If you submit an assignment, code, a video or whatever
• We may very well grade it
• If you want to submit a revision before the cutoff
• You need to tell us because we don't get a notification
• Your assignment will be considered late and so cannot get an "exceeds"

Team Standups

Introduction to Behavior Trees

• Some similarity to FSM, however…
• BTs are specifically trees, with one root and leaves
• BTs have different kinds of nodes with very different semantics
• The leaves represent the actual control actions
• All the other nodes are part of the control logic

py_trees

• This example uses this library: py_trees
• As we are using ROS1, you need to use py_trees 0.7 of this library
• Latest version is not compatible with the current version of ROS

Behavior Tree Execution

• “Tick” oriented control flow
• A tree is “ticked”, which “ticks” the top node
• Each node type has specific semantics about what happens on a tick
• Nodes can return one of: SUCCESS, FAILURE, RUNNING

Node Types

• These control flow
  • Sequence - perform each child node in turn as long as they return SUCCESS
  • Selector - perform the first child node that returns SUCCESS
  • … several mode
• Behavior nodes actually DO somerthing

Behavior Tree Returns

• SUCCESS: The action is complete and was successfull
• FAILURE: The action is done but failed

• RUNNING: The action is incomplete and needs to be ticked again

• You will discover that the semantics of these are a little subtler than they seem initially

Behavior Nodes - Example

• Behavior nodes are the leaf nodes.
• They execute code which actually “do something” to outside of the tree
• They could cause an action or read a sensor or anything else.
• Simple example: blink light once would:
  1. On Update: Tell the hardware to turn the light on and off; Return SUCCESS (because it always works.)
• Simple example: turn light on with a parameter: n seconds, would:
  1. On Init: set initial time <- current time; turn the light on
  2. On Update: if current time > initial time + n: turn off the light; return SUCCESS; else return RUNNING

Behavior Nodes

• Life Cycle of a behavior node
• Setup (optional): one time initiatlization of state when first instantiated
• Initialize (optional): called each time that the node is entered anew (i.e. when its not RUNNING)
• Update (required): called each time the node is ticked
• Terminate (optional): called whenever the node goes to not RUNNING

Composites

• The non-leaf nodes are all known as composites
• Generally the leftmost leaf is ticked first
• Depending on Composite type and returned status of that node, different next nodes may be ticked
• Core (traditional) set of Composites are: Sequence, Selector, Parallel
• But depending on the library, there may be other variations (which are all extensions of this set.

Sequence

• Most common
• Tick from left to right
• Each SUCCESS result leads to the next node
• Any non-SUCCESS ends the sequence and returns that same status up the chain

Selector

• Also very useful, kind of the reverse
• Tick from left to right as long as the child returns FAILURE
• The first non-failure makes the whole Composit3e return

Parallel

• Returns FAILURE if any child returns FAILURE
• Otherwise keeps ticking all the children
• Depending on policy it will return when one or all the children have returned SUCCESS

Thank you. Questions?  (random Image from picsum.photos)