Group Transport by Ants and Robots

This talks summarizes material from:

C. Ronald Kube and Eric Bonabeau. Cooperative Transport By Ants and Robots. [1] Robotics and Autonomous Systems,:85--101, 2000.

A group of 100 ants Pheidologeton diversus was able to transport a 10cm earthworm weighing 1.92.g (5000 times the weight of a single ant) at .41 cm/s.
Ants in solitary transport can only carry 5 times their wight at 1cm/s.

Is there an advantage to group transport?
When and how does an ant know that it cannot carry an item alone?
How are nestmates recruited when help is needed?
How do several ants cooperate and coordinate their action to actually transport the item?
How do ants ensure that there is the right number of individuals involved in carrying the item?
How does a group of transporting ants handle deadlock (when something is stuck)?

A large piece of cereal which, if broken down, would have required 498 solitary ants was carried collectively by just 14 ants.
The weight that can be carried by N workers is proportional to N^2.044.
Increased efficiency seems to come from the group's ability to overcome rotational forces.
An ant will first try solitary transport. If the object cannot be lifted the ant will try to drag it. If it will not budge then the ant tries at a different position. After some time the ant gives up and starts recruiting helpers.

In Short-Range Recruitment (SRR) a scout releases a poison gland secretion in the air immediately after discovering a large prey item; nestmates less than 2m away are attracted.
If SRR does not attract enough nestmates the ant lays a chemical trail with a poison gland secretion from the prey to the nest; nestmates leave the nest and follow the trail toward the prey. This is Long-Range Recruitment (LRR).
It is effective. In a series of experiments with 20 freshly killed cockroaches placed randomly the were discovered within several minutes (avg. 8.05min). 5-8 ants were quickly recruited with SRR.
When LRR was used more ants showed up than was needed. The extra ones returned to the nest. The ants do not try to estimate the size.

Coordination occurs through the item being transported. A movement of one ant engaged in group transport is likely to modify the stimuli perceived by the other group members, possibly producing orientational or positional changes in them.
This is an example of stigmergy--the coordination of activities through indirect interactions.
It suggests that a group of robots can cooperate without direct communication.
The number of ants engaged in transporting is proportional to the weight of the object. The group size is affected by the items' characteristics.
A prey item that resists stimulates the ants to recruit other ants. Success in carrying a prey item in one direction is followed by another attempt in the same direction. Recruitment ceases as soon as a group of ants can carry the prey in a well-defined direction.
When the pray will not move the ants randomly rearrange themselves until the prey starts to move.
Experiments show that the prey item will not move at all until, at some point, it starts moving at the final speed.

Robots were built to push a box. They used the subsumption architecture:
1. FIND: move randomly.
2. FOLLOW the nearest agent.
3. SLOW down.
4. GOAL: move towards the box.
5. AVOID other objects.
This had the effect of all robots getting on a cluster then heading towards the box and pushing it. However, the system would deadlock when an equal number of robots surrounded the box (everyone pushing against each other).
To solve this the robots were modified to detect stagnation if the box did not move after t seconds of pushing. When stagnated, a robot would reposition itself. This completely eliminated deadlock for large number of robots.

They found that pushing a box involved a series of phases or steps, often executed in a repetitive manner.
A termites nest is constructed through a series of building steps. Each construction phase is thought to be governed by a building program with step transition specified as stimulus cues (i.e., communication through the environment---stigmergy).

Transport Controller

Box Detected Box Contact Subtask Controller

0 X Find-Box

1 0 Move-To-Box

1 1 Push-To-Goal

Find-Box

Touch? Contact? Avoid? Action

0 0 0 Random-Walk

0 0 1 Avoid

0 1 X Contact

1 X X Back-Off

Move-To-Box

Touch? Contact? Avoid? Box? Action

0 0 0 1 Seek-Box

0 0 1 X Avoid

0 1 X X Contact

1 X X X Back-Off

Push-To-Goal

See-Goal? Action

0 Reposition

1 Push-Box

1 Group Transport by Ants