Re: Ultrasonic sensor interactions

The problem here is the management of a single shared resource, i.e. the air
through which the ultrasonic signals travel. The lack of a Bluetooth broadcast
mechanism makes the implementation of a conventional resource locking system
difficult, but not impossible.

A similar situation exists in Ethernet communication where a set of
communication channels must be implemented over a single physical cable, optical
fibre etc. This is achieved without using a separate communication mechanism to
achieve locking.

When an Ethernet interface wishes to transmit a packet, it first listens to see
if the “Ether” is in use. If it is, it continues to listen until the “Ether” is
quiet. If the “Ether” is not in use it transmits the packet but listens to see
if the packet is corrupted by a “collision” with a another packet from a
different interface that has started to transmit at the same time. If there is a
collision, it waits for a randomly selected interval and then repeats the whole
procedure until the packet is transmitted successfully. All packets are
transmitted in full weather or not a collision occurs to ensure that all senders
detect collisions.  In this environment “collisions” are a normal part of
network operation. This is called “carrier sense multiple access with collision
detection” (CSMA/CD).

I wonder is the characteristics of the ultrasonic sensor might permit something
similar. It must be able to listen to see if the “Ether” (the air) is quiet, but
could it detect “collisions”? If this is possible, the next problem concerns the
relatively low bandwidth of the ultrasonic channel compared to even the slowest
Ethernet. It might be that, in order to avoid deadlock, the collision back-off
times would need to be quite long, e.g. several seconds.  If this is the case,
the rate at which an individual robot could sample its ultrasonic sensor data
might be too low to be useful for navigation purposes. [I suspect that this last
problem may be the limiting factor for any implementation of a suitable locking
mechanism.]

Perhaps someone with access to a detailed sensor spec, or the real thing, could
investigate.

Arthur Clarke