"8. The containment system of claim 4, wherein the shock collar has a
leash sensor for detecting the pet in proximity to the pet owner.
9. The containment system of claim 8, wherein the shock collar is deactivated when the leash sensor detects the pet in proximity to the pet owner."
(
https://www.google.com/patents/US20080272920 )
"Although Seeing Eye dogs are widely
used, current artificial intelligence and robotics
technology may be able to create a better guide
dog. In order to simulate a guide dog robot, SID
has a suite of sensors to detect various danger
conditions in a world consisting of a sidewalk
and crosswalk.
[…]
6.2
Leash sensorAs a guide robot is useless if its partner
is unable to keep up with it, SID must have a
sensor to detect when it is getting too far from
the person it is leading. This is accomplished by
a leash attached to a Jameco flex sensor (cat.
#150551) (figure 2)."
(
http://www.mil.ufl.edu/publications/fcrar02/lavi_sidshort_fc... )