Variable frequency stress transducer

A magnetostrictive mechanical, variable frequency stress transducer is disclosed in which the frequency of the output signal is proportional to the applied stress. This device consists of a magnetic stress actuating member, to which the stress is applied, and a transistor-magnetic oscillator constructed so the stress sensitive magnetic characteristic of the stress actuating member is the principal frequency determining element of the oscillator.

This invention relates to load cells, stress transducers, dynamometers and 
methods of measuring mechanical stress. This invention discloses means for 
measuring tensile and compressive stresses. It is an object of this 
invention to provide a stress transducer which is constant in operation 
and which can be used for accurately determining the values of rapidly 
varying stresses as well as stresses involving only microinches of 
displacement. A further object is to provide a rugged stress transducer 
which will resist shifting its characteristics due to mechanical shock or 
vibration. Further objects are to provide a stress transducer with several 
volts output signal, low power consumption, and a simple, reliable design. 
A further object is to provide a stress transducer in which the frequency 
of the output signal is proportional to the applied stress. The variable 
frequency output signal provides a high signal-to-noise ratio in 
power-limited applications and is suitable for remote data monitoring. 
The invention will be described referring to the sketches of FIGS. 1, 2 and 
3 and the typical characteristic of FIG. 4. The objects of the invention 
are attained by providing a stress actuating member 1 of a magnetic 
material with magnetic permeability highly susceptible to mechanical 
stress, a series of coils, 5-6-7, 8-9, 10-11, 12-13, wound around the 
stress actuating member to facilitate detecting the magnetic permeability 
of that member and to aide in generating the variable frequency output 
signal, a transistor-magnetic oscillator using the series of coils around 
the actuating member, and a magnetic return path 2 for the external coil 
flux through an unstressed magnetic material.

Referring to FIG. 1, the stress actuating member 1 has tensile or 
compressive force applied along its axis between coupling ends 4. The 
force causes a change in the permeability of member 1 which causes the 
frequency of the transistor-magnetic oscillator to shift in a manner which 
correlates with the magnitude and direction of the force. Notice from FIG. 
4 that each force is measured as a unique frequency and that there is a 
continuum of unique frequencies for a continuum of loads. The frequency of 
oscillation is principally dependent upon the magnetic characteristics of 
the stress actuating member, the DC battery voltage, the cross sectional 
area of the stress actuating member, the number of wire turns in coils 
5-6-7, the magnetic coupling between said coils and the stress actuating 
member, the magnetic reluctance between the stress actuating member and 
the external coil flux magnetic return path, and the magnetic 
characteristics of the external coil flux magnetic return path. The 
invention is preferably configured as a stress transducer so the stress 
actuating member magnetic characteristic is the major determinant of the 
oscillating frequency. This is done by holding the DC battery voltage 
constant, fixing the number of wire turns for any one device, making the 
magnetic coupling between the coils and the stress actuating member as 
tight as possible by using a very thin bobbin which closely but freely 
fits over the stress actuating member, and by making a large area with a 
small gap between the stress actuating member and the external coil flux 
magnetic return path. The external coil flux magnetic return path 2 is 
preferred to be laminated as shown in FIG. 1. The output signal may be 
directly coupled from the oscillator transistor collectors or from one 
collector to ground; however, the separate coil 12-13 is provided for that 
purpose and is preferred. 
The transistor-magnetic oscillator may be any of numerous configurations 
(described in the literature) based on the concept described by Uchrin and 
Taylor in "A New Self-Excited Square-Wave Oscillator," Proceedings of the 
Institute of Radio Engineers, Volume 43, page 99, (1955). FIG. 1 shows the 
oscillator with NPN transistors 14 and 15, temperature conpensation diode 
17, transistor over-voltage protection zener diodes 18 and 19, 
self-starting resistor 16, DC battery 20, and coils 8-9, 5-6-7, 10-11, 
12-13 wound on bobbin 3. The transistor-magnetic oscillator may be 
constructed using PNP transistors with the appropriate battery and diode 
polarity changes.