Method and Apparatus for Determining Weight and Mass

This invention pertains to weight and/or mass measuring, and more particularly to a novel method and apparatus for determining the weight and/or mass of an object.

Heretofore many vibratory methods have been devised to measure the weight and/or mass of an object. Many of the prior vibratory schemes rely upon the object to cause a change in frequency of an introduced signal, or upon measuring a change in the natural frequency of the system when the object is added into it. Some of these prior systems can be seen with reference to the following U.S. Pat. Nos.: G. BRADFIELD Pat. No. 3,332,506 Issued: July 25, 1967; H. PATASHNICK Pat. No. 3,926,271 Issued: Dec. 16, 1975; J. L. CHRISTMANN Pat. No. 3,566,678 Issued: Mar. 2, 1971; J. W. FOGWELL Pat. No. 3,572,098 Issued: Mar. 23, 1971; A. WIRTH et al Pat. No. 3,621,713 Issued: Nov. 23, 1971; M. GALLO Pat. No. 3,612,198 Issued: Oct. 12, 1971; and J. L. CHRISTMANN Pat. No. 3,608,359 Issued: Sept. 28, 1971 W. E. THORTON Pat. No. 3,555,886 Issued: Jan. 19, 1971.

The present invention does not rely upon a change in frequency of the system to determine the weight of an object. Rather, the invention contemplates obtaining an output signal, whose frequency is essentially the same as the input signal frequency. The invention measures a change in amplitude of the output signal, the amplitude being a function of the mass. The inventive system uses a piezoelectric crystal to excite the mass into vibrating. Another piezoelectric crystal is used as a detector of the force necessary to accelerate and decelerate (vibrate) the mass. The force necessary to vibrate the mass is proportional to the output voltage of the crystal. A mass determination can be made by knowing the average acceleration and the force (F = ma).

This invention relates to a method and apparatus for determining the weight and/or mass of an object. The object is supported upon a platform or pan. The object and the pan is in turn supported by a first piezoelectric crystal means. This crystal means, the pan, and the object receive a vibratory signal from a second piezoelectric crystal means, that is in vibratory contact with them. The second piezoelectric crystal means is caused to produce a vibratory signal in response to a voltage impressed thereon. The first and second crystal means each vibrate at essentially the same frequency, but the voltage output or amplitude of the signal from the first crystal means is a function of the mass, and/or the supported weight. If the pan and first crystal means are vibrated without the object, a reference signal is obtained. This reference signal can be subtracted from the total output signal to determine the mass and/or weight of the object.

The oscillating output voltage is rectified to provide an average voltage reading.

The present invention provides many advantages over the prior art. The apparatus does not need to rely on resonant frequencies (although resonant frequencies can be employed). This allows for a greater versatility in operating energies, voltages, and frequencies. Because a piezoelectric crystal(s) is used as the excitation means, the apparatus can vibrate at a high frequency, thus appearing to be motionless. In addition, the crystals and pan can be made very flat, giving rise to a very compact structure. This method and apparatus can supply the accurate mass of an object, regardless of changes in the gravitational constant (g).

It is an object of the invention to provide an improved method and apparatus for determining the weight and/or mass of an object;

It is another object of this invention to provide a weight and/or mass determination for an object by means of vibrationally exciting a piezoelectric crystal(s) and measuring its voltage output or amplitude of its signal as a function of the mass or weight being supported;

It is a further object of the invention to provide a compact, versatile, and accurate apparatus for determining the weight and/or mass of an object .

These and other objects of this invention will be better understood and will become more apparent with reference to the following detailed description taken in conjunction with the attached drawing, in which:

FIG. 1 is a front view of the weight and/or mass determining apparatus of the invention;

FIG. 2 is a typical circuit for rectifying the output voltage obtained from the apparatus of FIG. 1; and

FIG. 3 is a schematic diagram of the output voltage signal before and after rectification.

Now referring to FIG. 1, a pan or platform 10 is shown supporting an object 10 having a mass "M" to be measured.

At least one piezoelectric crystal 12 (two shown) supports, or otherwise is in vibrational contact with the pan 12.

At least one piezoelectric crystal 13 (two shown) supports or otherwise is in vibrational contact with piezoelectric crystals 12. An electrically isolating member 14 may be provided between the two types of crystals 12 and 13.

An excitation voltage "E.sub.i " is impressed (arrow 15) upon crystal 13, causing it to vibrate at a given frequency.

Because crystal 12 is in vibratory contact with crystal 13, crystal 12 will likewise be caused to vibrate. Additionally, pan 10 and mass 11 will be caused to vibrate, since they are also in vibratory contact with crystals 12 and 13.

Crystal 12 will vibrate at a frequency essentially the same as crystal 13.

Crystal 12 will produce an output voltage "E.sub.o " (arrow 16) in response to the frequency signal being imparted to its by crystal 12. However, the amplitude of the output voltage "E.sub.o " will vary in response to the weight (mass) being supported by crystal 12. When no object 11 is resting on platform 11, the output voltage "E.sub.o " will only be a function of the "tare" or platform weight (mass).

When an object 11 to be measured is placed upon the pan 10, and an input voltage "E.sub.i " is impressed upon crystal 13, an output voltage signal "E.sub.o " will be produced by crystal 12. The output voltage "E.sub.o " will have an amplitude which is proportional to the force necessary to accelerate and decelerate the supported weight or mass, in accordance with the Newtonian formula:

where:

"F" is the average force "m" is the mass of the supported weight; and "a" is the average acceleration and deceleration (vibration) of the mass "m".

For a sinusoidal impressed voltage on crystal (13) E.sub.i the mass acceleration will be

where:

x = amplitude of vibration (inches) (a function of E.sub.i)

w = frequency of E.sub.i (radians)/sec (a constant frequency)

If the acceleration "a" is known, then the mass "m" can be determined from: F = mE.sub.i

Therefore for crystal 12

where:

"E.sub.m " is the average "E" output voltage. and K is a constant value for crystal 12.

The average output voltage E.sub.m is obtained by rectifying the output voltage E.sub.o. This rectification can be accomplished by using a circuit such as is shown in FIG. 2.

The crystal 12 produces an output voltage E.sub.o as illustrated. A full wave bridge rectifier 17 receives the "E.sub.o " signal on lines 18 and 19, and produces the average "E.sub.m " signal over lines 20 and 21.

FIG. 3 depicts the diagrammatic shapes for the signals "E.sub.o " and "E.sub.m ", respectively.

Weight may be determined from the value of the mass by the formula:

where:

W is weight;

m is mass;

g is the gravitational constant.

In order to eliminate the tare weight, a reference output voltage is first obtained with no weight (mass) 11 being supported on pan 10. This reference voltage is subtracted from the subsequent readings, or the read-out instrumentation (not shown) is zeroed at this reference voltage level. In any event, this reference voltage is subracted from the final weight readings.

Having described the invention, it is deemed that all obvious improvements be considered as falling within those limits defining the full scope and spirit of the invention. For example, means may be devised to preventing inaccuracies due to temperature effects or changes in frequency of the crystals. The input voltage may have to be carefully regulated to prevent drift in the output frequency signal, etc.

The invention should be interpreted and defined with reference to the appended claims.