2.1. Detection of friction using strain gauges
We attach two pairs of two strain gauges close
to the right and left frames supporting the cross
strings at the middle, as shown in Fig. 1. One pair
(half bridge with two active strain gauges in
opposite arms) is used to obtain the force normal to
the racket from the strain (tension) in the string
caused by impact of the ball. If forces parallel to
the racket surface occurs when the ball impacts, i.e.
if friction occurs, one cross string expands and the
other contracts. The difference in strain (tension) is
picked up by the other pair of gauges (half bridge
with two active gauges in adjacent arms) to tabulate the friction of the ball parallel to the racket surface.
Two rackets (KTA105A from Gosen Co., Ltd.) with different string tension are used; racket A with
35DynamicTension (about 55lbs.) and racket B with 26DynamicTension (about 42lbs.).