Measuring and testing – Dynamometers – Responsive to force
Patent
1989-06-21
1991-04-16
Ruehl, Charles A.
Measuring and testing
Dynamometers
Responsive to force
73DIG1, G01L 510
Patent
active
050072943
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an improved method and device for measuring the string tension of rackets such as tennis, squash and racket ball rackets.
BACKGROUND OF THE INVENTION
It is already known from European patent application No. 0 141 976 to evaluate the string tension indirectly by finding the natural frequency of the strings. Due to the known basic correlation between the string tension and the corresponding frequency, the string tension can be calculated from the frequency.
To evaluate the frequency of the membrane which is formed by mounting the strings in a crosswise manner onto the racket frame, the conventional methods employ acoustical or mechanical devices to induce a forced oscillation. The amplitude value of the membrane oscillation is determined by a transmitter (transducer), such as a piezo crystal, for example, affixed to the membrane and the inducing frequency is then varied in order to find the point of resonance which is identical to the frequency of the membrane.
As conventional methods recommend the use of a variable frequency generator to induce an oscillation, these methods are complicated and expensive.
It is therefore an important object of this invention to provide an improved method and device for measuring the tension of racket strings which are simple to use, give reliable results and which can be employed with a reduced amount of equipment.
SUMMARY OF THE INVENTION
According to the invention, the membrane of the racket is aperiodically (non-periodically) induced to oscillate, for example, by tapping a finger to the frame or by dropping a ball of the related game, e.g. a tennis ball, onto the membrane. The free oscillation of the membrane after the aperiodical induction is then evaluated. The present method and device provides a simple, quick and reliable measurement of the string tension even in the extremely short period of time between the beginning of the induced oscillation and the complete dampening of this oscillation.
To measure the string tension according to the invention, it is only necessary to employ a simple frequency counter which is triggered by the oscillation signal itself. The complete test circuit according to the invention can therefore be assembled into one small unit which can be attached to the membrane of the racket for completing the measurement. The weight of the unit which is affixed to the membrane is preferably made substantially identical to the weight of a ball of the related game, e.g. a tennis ball, so that the mechanical conditions under which the test is carried out are comparable to the real playing situation when hitting a ball.
BRIEF DESCRIPTION OF THE DRAWING
In the accompanying drawing:
FIG. 1 is a perspective view of a tennis racket having affixed thereto a string tension measurement device according to one embodiment of the invention;
FIG. 2 is a circuit diagram showing the electrical components of the device of FIG. 1;
FIG. 3 is a time chart showing the shape and relative sequence of the various signals produced when operating the device of FIG. 1.
FIG. 4 is similar to FIG. 2 showing another embodiment of a tension measurement device according to the invention;
FIG. 5 is a time chart showing the signals produced by the device of FIG. 4;
FIG. 6 is similar to FIG. 2 showing a further embodiment of a tension measurement device according to the invention;
FIG. 7 is similar to FIG. 2 showing still another embodiment of a tension measurement device; and
FIG. 8A and 8B are time charts showing the signals produced by the device of FIG. 7.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawing, there is shown in FIG. 1 a tennis racket 10 having a frame 12, a handle 14 and a membrane 16 formed by a plurality of strings 18 which are mounted in a crosswise manner onto the frame 12. A tension measurement device 20 is affixed to the center of the membrane 16 by small clamps 22 and has a display 24 to indicate the string tension.
The housing of the device 20 contains a battery and a transmitter (not shown) which receives
REFERENCES:
patent: 2792707 (1957-05-01), Anderson
patent: 3040565 (1962-06-01), Church et al.
patent: 3862569 (1975-01-01), Lucas et al.
patent: 4114454 (1978-09-01), Ernye et al.
Doherty John R.
Mauve Karl-Eberhard
Ruehl Charles A.
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