[Dprglist] Subsumption expressed as a behavior tree

Rud Merriam rudmerriam at gmail.com
Thu Nov 4 12:39:30 PDT 2021


Maybe now I don't need to do a presentation next Tuesday? Kidding. I'll 
still do one. The articles you found are good and I've seen them 

One of the unfortunate aspects of BT discussions in non-standard 
terminology. The "fallback" node is often called "selector", for 
instance. Adding the term "reactive" to a node type is one I've not seen 
previously. Also encountered "nonreactive" in another article that is 
new. <sigh>

A response of "RUNNING" is returned up through the tree. I need to look 
more at "Reactive" and "NonReactive" (seen in another article) to 
understand what they do. In your diagram, the "RepeatNode" would return 
running and passed upwards.

Yes, your diagram is a good design with what I understand so far about 
the BTCpp implementation. With more classic terminology the 
BumperPressed sequence would be a SequenceStar which remembers what 
nodes return SUCCESS and does not visit them again. There may be another 
problem but I need to look at the library to be sure. The question is 
how to you reset the SequenceStar remembering BumperPressed once the 
RepeatNode completes?

Good find and analysis.

-73 -
*Rud Merriam K5RUD*
/Mystic Lake Software/ <http://mysticlakesoftware.com/>

On 11/4/21 12:38 AM, Paul Bouchier via DPRGlist wrote:
> David, Rud,
> At Tuesday's RBNV you guys seemed to have agreed that the Subsumption 
> architecture was a special case of behavior trees. I wanted to 
> understand behavior trees, and to understand whether your assertion is 
> correct. I found a basic explanation of behavior trees here:
> https://towardsdatascience.com/designing-ai-agents-behaviors-with-behavior-trees-b28aa1c3cf8a 
> <https://towardsdatascience.com/designing-ai-agents-behaviors-with-behavior-trees-b28aa1c3cf8a>
> It's a good overview, but is missing sufficient detail to design a 
> subsumption stack as a behavior tree. For example it's description of 
> a sequence node is "A node that returns success if all children return 
> SUCCESS" but it doesn't specify what happens if a child returns 
> RUNNING - does it retry all previous nodes next tick time? It only 
> describes one kind of decorator node (invert). It doesn't specify 
> whether a Fallback node re-ticks all nodes next tick-time if a child 
> after the first returns SUCCESS.
> After some searching, I found this set of pages
> https://www.behaviortree.dev/ <https://www.behaviortree.dev/>
> They describe an implementation by David Faconte (of PlotJuggler fame) 
> of a behavior tree library that is general purpose, and doesn't depend 
> on, but works well with ROS. In the "Learn the basics" section they 
> describe variations on the sequence and fallback nodes which seem 
> sufficient to design a subsumption robot behavior. The attached image 
> is that design. I'd be interested in your thoughts on whether I'm on 
> the right track here.
> The following description of the design assumes the reader has read 
> the Sequence Nodes and Fallback Nodes and Decorators Nodes pages, and 
> it offers commentary on how the behavior tree implements the 
> subsumption stack, including non-ballistic behavior.
> ReactiveFallback always ticks all nodes below it until it gets the 
> first SUCCESS. Thus, if Action ReverseTurn was running owing to a 
> bumper press, and then BatteryLow becomes active (SUCCESS), the 
> ReactiveFallback node calls HaltAllChildren() which propagates down 
> the tree and halts (subsumes) the ReverseTurn action. (This 
> implementation produces an abort behavior, but it's included to show 
> that BumperPressed->ReverseTurn is subsumable, not ballistic.)
> The subsumption behavior arises from the way ReactiveFallback stops 
> ticking lower-priority (more rightward) nodes when any preceding nodes 
> return SUCCESS. If no bumper is pressed, no IR detection is made, and 
> battery isn't low, then the last action, "Drive toward goal" gets to 
> run. But as soon as bumper or IRDetection is true (SUCCESS), the 
> corresponding behavior is executed, and DriveTowardGoal is subsumed 
> (doesn't run) until the corresponding Sequence node is done and 
> returns FAILURE
> If IRDetection returns SUCCESS (IR sensor detects object), Action 
> "SteerAway" runs, and steers the robot away from the sensor with the 
> detection. Once the sensor clears, IRDetection condition returns 
> FAILURE, Action SteerAway doesn't run (is halted), and no longer 
> subsumes DriveTowardGoal, which can then start running again. This 
> behavior relies on ReactiveSequence ticking all nodes (starting at the 
> beginning of its list of children), so the sensor gets checked each tick.
> By contrast, BumperPressed is called by a regular Sequence, which 
> doesn't tick all its children; if a child returns RUNNING (which will 
> happen while ReverseTurn action is active), it will only tick the 
> running child. Thus BumperPressed can become false as the robot backs 
> away from the obstacle, but it won't be ticked, so it won't affect the 
> "ReverseTurn" action that should run for some time after a bumper 
> contact clears. The RepeatNode decorator runs ReverseTurn up to 40 
> times (2 seconds at 20 Hz tick rate) as long as ReverseTurn returns 
> SUCCESS, then returns FAILURE (maybe - the documentation isn't clear 
> on this point - it might not work as described). When it returns 
> FAILURE the subsumption of lower priority actions ends.
> The tree shown is a DriveToWaypoint behavior tree - it could be used 
> as an element in a higher-level sequence that did something else after 
> achieving the waypoint, like seek a cone.
> This is of course all theoretical - may not work in practice - but it 
> looks to me as if your assertion that Subsumption can be represented 
> as a behavior tree is correct, and that a richer set of control-node 
> behaviors is beneficial. The Fallback node acts like David's arbiter, 
> running the highest priority behavior, as assessed by traversing the 
> list of nodes and not running any beyond the first to report SUCCESS. 
> It relies on its children to return FAILURE if they don't want to 
> control the robot, so that arbitration can pass on down the line.
> A very nice aspect of this library is it works with GROOT which 
> apparently allows realtime and after-the-fact replay of state 
> transitions (active behaviors).
> Thanks for an interesting exploration down an avenue I've wondered 
> about. I feel more knowledgeable now than I was.
> Regards
> Paul
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