View Full Version : grip force ideas

Stephen M. Gatesy
09-06-1994, 12:41 AM
Thanks to all who responded to my plea for help with measuring grip force
in owls. Please don't hesitate to pass on any new inspirations.

-I have a student interested in measuring force output in owl feet. She's
-hoping to quantify both toe pad (crushing) and claw (puncturing) force in
-different species of owls.

-Does anyone have any suggestions for tackling such a complex system? Are
-there transducers for measuring human grip force? If so, how do they
-operate? We've been tossing around ideas about pressure sensitive films
-wrapped around rods of different diameter for them to perch on. Would
-these be quantifiable? Are there materials that can be penetrated with a
-sharp point that will measure force at the claw tip? Would the
-deformation of a clay-like material accurately show force and where it
-was applied? This is all assuming that we can convince live owls to
-really grab our transducer.

Steve Gatesy
Dept. of Biology
Wake Forest University
Winston-Salem, NC 27109



From: james a ashton-miller

One approach you might consider is to use a similar idea to
that used in a bite force transducer described by David S.
Carlson and a Ph.D student of his. They published a Technical
Note in J. Biomech. in the late 1980's. Basically you could
take an AL rod, split it down the middle longitudinally until
you get near the near end where it is to be gripped by a support.
The rod now looks sort of like a tuning fork. You machine flats
on the outer surface of the rod at the base of each fork and mount
two strain gages a fixed distance apart on those flats. The gages
are then wired so that they measure the grip force applied by the owl
feet so as to bend the two forks together. This is acturate the
device with known loads. The beauty of this arrangement is that it will
essentially measure the grip force independent of where the owl
is perched along the rod. You can twist the rod relative to the foot
to see whether the resultant grip force varies with orientation. If
you vary the distance between the forks you can examine the effect of
object size on grip force.



You might be interested in a paper by Lee, J.W. and Rim K. (1991) in
J. Biomed. Eng, 13, 152-162 where they use pressure sensitive film to
measure finger grip forces.



The most common approach adopted by Biomechanics people
would be to have two cantilevers, (i.e. rods, bars or beams)
held parallel close to each other by being held in a block
at one end.
You would then need to strain gauge one of them in two sections.
at each section you will have two on top and two exactly on the
bottom surface of the beam. This is commonly referred to as a
full bridge configuration.
At some distance from this you will have the same arrangement
on the same beam, thereby forming two full bridges.
You connect these to two strain gauge amplifiers and put the
output of these amplifiers through a differential amplifier which
will in turn give you one single output proportional to the force
applied to bring the two beams together.
with such a system your signal will be sensitive to the force
and not its point of application on the beam. So your Owl could
grip the beams at any point.


From: Duane Knudson

I have had experience in measuring forces on human hands with Pizeofilm,
Sensing Resistors, and strain gage transducers. I believe that if you are
really interested in forces (or pressures) localized in small areas of a
foot you need a rigid sensing surface. I would suggest miniature strain gage
transducers. I had some custom made in 1989 for $250 by Precision
Meassurement Company PO Box 7676 Ann Arbor MI 48107. They were half-bridge
sensors with a 10 mm sensing face. Using sensors like these you do not have
nonlinearity problems, problems of changing areas of contact, or problems of
the claw damaging the system.


From: rkram@garnet.berkeley.edu

For grip: How about a liquid filled tube with a pressure tranducer.
or a perch with a C cross section could be strain gaged and calibrated.

For puncture... hmmm. Perhaps there are materials of known puncture
resistance and you could put a nice dead mouse inside of a bag made of
material x and see if the hungry owl can get the mouse.

i remembered where i saw the pressure thing. it was a paper by wendy silk
at davis who measured the forces applied by climbing vines. it looks like
the cost would be minimal.