GB2190748A

GB2190748A - Load measuring apparatus of vibration type with linear characteristic

Info

Publication number: GB2190748A
Application number: GB08711377A
Authority: GB
Inventors: Yuzuru Nishiguchi
Original assignee: Shinko Denshi Co Ltd
Current assignee: Shinko Denshi Co Ltd
Priority date: 1986-05-20
Filing date: 1987-05-14
Publication date: 1987-11-25
Also published as: JPS6345520A; GB8711377D0; DE3716786A1

Description

GB2190748A 1

SPECIFICATION

Load measuring apparatus of vibration type The present invention relates to a load measuring apparatus of vibration type comprising a 5 vibration type load sensor whose natural frequency is changed or shifted in accordance with a force applied in an axis direction of the load sensor, and particularly to a load measuring apparatus of vibration type in which a linearity error of a measured value can be eliminated.

With a vibration type load sensor having a vibration element such as vibrating string or tuning fork vibration element, pressure, load, weight, etc. can be measured by detecting the change in 10 vibration frequency, since a natural frequency of the vibration element has a predetermined relationship with respect to a force applied in an axial direction. However, said relationship between the force and the vibration frequency is not a linear one inherently, so that unless the non-linear relationship is compensated for by means of a micro processor, the measurement could not be effected practically. Therefore, the measuring apparatus is liable to be complicated 15 in construction and expensive in cost. Further, the application of the known measuring appara tuses is limited.

Fig. 1 shows a construction of a known load measuring apparatus having a tuning fork vibration element as the vibration element. The apparatus comprises a fixed post 1, a movable post 2, and a pair of horizontal links 3 and 4 connected between the posts 1 and 2 by means 20 of four elastic portions 5. These posts 1, 2 and links 3, 4 form a parallelogram link mechanism, and a weighing pan 6 is provided on the top of movable post 2. The fixed post 1 has an arm 7 formed integrally therewith and extending horizontally toward the movable post 2, and the movable post 2 has a projection 8 formed integrally therewith and extending horizontally toward the fixed post 1. Between the arm 7 and projection 8 is secured a tuning fork vibration element 25 9 by means of flexures 10, respectively. In the apparatus shown in Fig. 1, the posts 1, 2 and links 3, 4 are formed as an integral body by means of the cutting or the die casting, but the link mechanism may be composed of a plurality of parts. When a load W to be measured is applied on the weighing pan 6, an axial force F having the same magnitude as the load W is transferred to the tuning fork vibration element 9. When the tuning fork vibration element 9 is kept to 30 vibrate by a known method, its vibration frequency f is changed in accordance with the axial force F. Therefore, by measuring the vibration frequency f, it is possible to detect the axial force F, Le, the load W.

The vibration frequency f under the axial force F is represented by the following equation (1), while % is a frequency under no load and K is a constant determined by the tuning fo k vibration 35 element 9.

f =f,(1 + Kfil (1) Fig. 2(a) illustrates a relationship between the axial force F and the vibration frequency f. 40 When no load is applied, the tuning fork vibration element vibrates at the frequency fo, and when a tension force is applied to the vibration element the vibration frequency is increased, and when a compression force is applied, the vibration frequency is decreased. The above relation ship will be further explained by taking the case that the tension force is applied, because in many applications the tension force is applied to the vibration element. 45 Fig. 2(b) is a graph showing a hatched area in Fig. 2(a) in an enlarged scale. A chain line connecting a point at the maximum axial force Fm and the maximum variation Afm of the vibration frequency and a zero point at no-load condition represents an ideal linear relationship.

In practice, the frequency variation deviates from the ideal line by an error e in linearity. The maximum linearity error amounts to about 1.2% of the maximum frequency deviation Afm when 50 a dynamic range Afm/fo is set to 10%, and to about 2.3% of Afm when Afm/f, is selected to 20%. Therefore, the linearity error e could not be compensated for precisely unless rather complicated calculation is effected by means of a micro computer. Even after effecting the correction by the computation, there still remains an error which could not be ignored. There fore, it is practically impossible to make the dynamic range Afm/f, larger than 10%, so that the 55 measurable load range is limited.

The present invention has for its object to provide a load measuring apparatus of vibration type which can remove the non-linear relationship between a load to be measured and a variation in frequency of a vibration element and can effect a precise measurement, while a substantially linear relationship can be attained between the load and the frequency variation over 60 a wide dynamic range.

According to the invention, in order to attain the above mentioned object, a load measuring apparatus of vibration type comprises a load sensor of vibration type having a vibration element whose vibration frequency is changed in accordance with a force applied there to; and a non linear spring arranged to transmit a load to be measured to said vibration element at a ratio 65 2 GB2190748A 2 which changes in accordance with the load to be measured such that a frequency variation of the load sensor is made proportional to the load to be measured.

Brief Description of the Drawings

Figure 1 is a side view showing the known load measuring apparatus of vibration type; and 5 Figures 2(a) and 2(b) are graphs showing characteristics of tuning fork vibration element.

Figure 3 is a side view showing an embodiment of the load measuring apparatus according to the invention; Figures 4(a) and 4(b) are schematic views for explaining the operation of the apparatus shown in Fig. 3; 10 Figures 5(a), 5(b) and 5(c) are graphs illustrating principal characteristics of the apparatus of Fig. 3; Figure 6 is a graph depicting a characteristic of a ring-shaped spring; Figure 7 is a graph representing a relationship between a load to be measured and an axial force applied to a vibration element in the apparatus shown in Fig. 3; 15 Figures 8(a) and 8(b) are side views illustrating another embodiment of the load measuring apparatus according to the invention; Figure 9 is a side view showing still another embodiment of the load measuring apparatus according to the invention constructed as a pressure measuring apparatus of vibration type; Figures 10 and 11 are side views illustrating still another embodiments of the load measuring 20 apparatus according to the invention constructed as load cells; Fig. 3 is a side view showing a construction of an embodiment of the load measuring apparatus according to the invention. In this embodiment, portions similar to those shown in Fig.

1 are denoted by the same reference numerals used in Fig. 1. In the present embodiment, a ring-shaped spring 11 is provided in the flexure 10 for supporting the tuning fork vibration 25 element 9. The other construction of the apparatus shown in Fig. 3 is the same as the known apparatus illustrated in Fig. 1.

Now the operation of the load measuring apparatus shown in Fig. 3 will be explained with reference to the principal construction illustrated in Figs. 4(a) and 4(b). Fig. 4(a) shows a condition in which a load is not applied to the apparatus. When a weight W is applied to the 30 weighing pan 6, the ring-shaped spring 11 is extended as shown in Fig. 4(b), and the horizontal links 3 and 4 each having a length L are tilted by an angle 0, so that the movable post 2 is moved downward by an amount y=L.O. The elastic reaction force of the link mechanism is given by the elastic portions 5, 5, 5, 5, and produces a counter torque T with respect to the load W.

In view of the fact that the tilting angle 0 and the vertical shift y are very small, it may be 35 assumed that the link mechanism generates equivalently the elastic reaction force of k-y in response to the vertical shift y of the movable post 2, where k is an equivalent spring constant of the link mechanism for the vertical shift y. As can be understood from Fig. 4(b), the vertical shift y of the movable post 2 is equal to the extended amount of the ring- shaped spring 11.

In the known load measuring apparatus shown in Fig. 1, when the load W to be measured is 40 applied, a slight part of the load W which is proportional to the shift y and amounts to k.y is absorbed by the elastic reaction force of the elastic portions 5 of the link mechanism and is not transferred to the tuning folk vibration element 9.

In the load measuring apparatus according to the invention, the shift y is not proportional to the load W to be measured, but a relative shift y/W is decreased in accordance with the 45 increase in W. That is to say, a ratio of k.y to the load W, i.e. k-y/W is decreased in accordance with the increase in W. This results in that when the load W gradually increases, the ratio of the axial force F actually applied to the tuning folk vibration element 9 to the load W, i.e. F/W is gradually increased such that the relationship between the load W and the frequency variation Af is made linear. In order to decrease the relative shift y/W of the load measuring 50 apparatus in accordance with the increase in the load W, the ring-shaped spring 11 having a non-linear characteristic such that its spring constant is increased in accordance with the in crease in the tension is arranged between the tuning folk vibration element 9 and a fixed portion or in the force transmission path.

Next, the principle of the present invention will be analyzed theoretically. The equation (1) may 55 be modified into the following equation (2), flf,= 1 +Af/fo=(1 +K-F)A (2) In the present invention, the force transfer ratio F/W is increased in accordance with the 60 increase in W as represented by the following equation (3), while in the known load measuring apparatus the axial force F actually applied to the vibrating element 9 is made equal to or has constant ratio to the load W.

F/W=(a/K).[1+(a/4)-W] (3) 65 3 GB2190748A 3 In the equation (3), a is a constant which can be selected in accordance with the practical applications.

After modifying the equation (3), a modified equation is applied to the equation (2), the following equations (4), (5) and (6) are derived. 5 1 + K.F = (1 + a.W/2)2 (4) (1 + Kfil = 1 +a-W/2 (5) Af/fo=a.W/2 (6) 10 As can be seen from the equation (6), the frequency variation Af is precisely proportional to the load W, so that the linearity error e can be removed completely as long as the condition prescribed in the equation (3) is satisfied, and the object of the invention can be achieved.

Figs. 5(a), 5(b) and 5(c) are graphs showing characteristics of the load measuring apparatus according to the invention. Fig. 5(a) shows a relationship between the load W and the axial 15 force F. The axial force F actually applied to the tuning folk vibration element 9 is increased as a quadratic function of the load W. Fig. 5(b) shows a relationship between the load W and the ratio F/W of the axial force F to the load W. The ratio F/W is increased linearly in accordance with the increase in W.

According to the invention, the characteristic line of the ratio F/W is not limited to that 20 illustrated in Fig. 5(b), but may be selected at will. This will be explained herein below.

When the equation (3) is rewritten by using the equation (6), the following equation (7) may be obtained.

F/W=(a/K).[1 +Af/(2.f,))] (7) 25 Fig. 5(c) illustrates the relationship between the Af/f, and the F/W for various values of F/W=a/K at a zero point Af=0. For instance, a measuring range is set to 20% of Af/fo and the force transfer ratio F/W=a/K near the zero point is set to 80%, F/W at the maximum load is set to 88% and at the middle point the value may be selected on a line connecting said two 30 points. In general, as can be understood from the equation (7), when F/W at the zero point is taken as a reference, F/W may be increased by 5% in accordance with the increase in Af/f, by 10%.

As explained above, the force distribution in the load measuring apparatus according to the invention can be expressed as follows. 35 Load W=(force k-y absorbed by the link mechanism) +(axial force F applied to the tuning fork vibrating element) Fig. 6 is a graph showing the relation between the axial force F and the extension amount y of the ring-shaped spring 11. At first, the spring 11 extends easily, but the spring 11 is hardly stretched in the direction of the axial force F in accordance with the increase in F, so that the 40 extension ratio y/F becomes smaller. Similarly the relative shift y/W is decreased. This is shown in Fig. 7. That is to say, the increasing ratio of the force k.y absorbed by the link mechanism is decreased in accordance with the increase in the load W. In other words, the transfer ratio F/W to the tuning fork vibration element 9 is increased in accordance with the increase in W. The F W curve shown in Fig. 7 corresponds to the theoretical characteristic curve illustrated in Fig. 45 5(a), so that when these characteristics are made coincident with each other, the linearity error e becomes zero.

After various experiments, it has been confirmed that the linearity error e/Afm in the embodi ment shown in Fig. 3 amounts only to 0.0 1 %, when the dynamic range Afm/fo is set to 10%.

this error is smaller than that of the known apparatus by a hundred times. 50 In the load measuring apparatus according to the invention, the nonlinear spring may be formed in various shapes instead of the ring-shaped ring 11 used in the above embodiment. In short, the spring constant of the non-linear spring in the apparatus may increase in accordance with the extension.

Fig. 8(a) is a side view illustrating another embodiment of the load measuring apparatus 55 according to the invention. In this embodiment, use is made of the vibration type sensor described in U.S. Patent 4,544,858. A tuning fork vibration element 9 is formed integrally with lever 12, base portion 13 and pulling strip 14. The base portion 13 is secured to the arm 7, and a lower end of the pulling strip 14 is connected to the projection 8. The ring-shaped spring 11 having the non-linear characteristic is provided at a middle of the pulling strip 14. The 60 operation and performance of the load measuring apparatus of the present embodiment are same as those of the previous embodiment, so that a detailed explanation there of is omitted. It should be noted that the ring-shaped spring 11 may be provided in series with the tuning fork vibration element 9 in the same manner as the embodiment depicted in Fig. 3. In general, the ring-shaped spring 11 may be provided between the tuning fork vibration element 9 and the 65 4 GB2190748A 4 fixed portion or between the vibration element 9 and the movable portion.

Fig. 8(b) is a side view showing still another embodiment of the load measuring apparatus. In this embodiment, a leaf spring 11' having the non-iinear characteristic is used instead of the ring shaped spring 11. A root portion of!he leaf spring 11' is secured to the lever 12, and the pulling strip 14 is secured to a free end of the leaf spring 11'. At least one of opposing 5 surfaces of the lever 12 and leaf spring 11' is curved such that a distance between these surfaces becomes wider non-linearly toward the free ends of the lever and leaf spring. In such a construction, a contact point there between is moved in accordance with the increase in the shift y, so that an effective length of the leaf spring 11' becomes shorter and the spring constant becomes larger in accordance with the increase in the tension force. This characteristic 10 is entirely the same as that shown in Fig. 6 of the ring-shaped spring 11, and thus the load measuring apparatus illustrated in Fig. 8(b) can attain the linearity in the similar manner to the embodiment shown in Fig. 8(a).

Fig. 9 is a side view showing still another embodiment of the load measuring apparatus according to the invention. This embodiment is to measure the pressure. The apparatus com- 15 prises a base plate 20 and a bellows 21 provided on the base plate. To the bellows 21 is introduced a pressure to be measured via a hole 22 formed in the base plate 20. With the base plate 20 is integrally formed a post 23, an elastic portion 24 and a lever 25. To a free end of the lever 25 is connected a tuning fork vibration element 9 whose lower end is coupled with the base plate 20 by means of a ring-shaped spring 11. When the pressure is increased, the 20 bellows 21 is extended and the ring-shaped spring 11 is also extended. At the same time, the lever 25 is rotated slightly about the elastic portion 24. As in the previous embodiments, since the extension of the ring-shaped spring 11 is not proportional to the increase in the pressure, a ratio of a force absorbed by the bellows 21 and the elastic reaction force of the elastic portion 24 to the pressure is decreased in accordance with the increase in the pressure. Therefore, the 25 extension force applied to the tuning fork vibration element 9 is increased at a greater ratio than the increasing ratio of the pressure, so that the frequency variation proportional to the applied pressure can be obtained.

Figs. 10 and 11 illustrate still another embodiments of the load measuring apparatus according to the invention. In these embodiments, the apparatus is constructed in the form of a load cell 30 comprising the combination of a proving ring and a tuning fork vibrating element. Fig. 10 shows a tension type load cell comprising a proving ring 30 and a tuning fork vibration element 9 arranged within the ring by means of flexures 10 and an arcuate spring 31. When forces W are applied to the proving ring 30 to extend it, the force and displacement are transferred to the tuning fork vibration element 9 not directly, but via the arcuate spring 31. The arcuate spring 31 35 has a non-linear relationship between an axial force F and a shift y similar to the ring-shaped spring in the previous embodiments. Therefore, the linearity of the frequency variation Af of the tuning fork vibration element 9 can be improved to a great extent.

Fig. 11 shows a compression type load cell comprising proving ring 30, tuning fork vibration element 9, flexures 10 and arcuate spring 31. Also in the present embodiment, the improvement 40 of the linearity can be attained in the same manner as the embodiment shown in Fig. 10.

It should be noted that the non-linear arcuate spring 31 may be combined not only with the proving ring 30 as shown in Figs. 10 and 11, but also with other spring mechanisms. For instance, the ring-shaped spring 11 in the embodiment illustrated in Fig. 8(a) may be deleted and the arcuate spring may be provided between the pulling strip 14 and the projection 8. 45 As explained above, the load measuring apparatus according to the invention can provide the following advantages.

(1) By inserting the non-linear spring of a simple construction such as ring-shaped spring and arcuate spring between the vibration type load sensor and the fixed portion or in the load transmission path, the non-linearity of the vibration type load sensor can be substantially re50 moved, and the cheap and precise load measuring apparatus can be obtained.

(2) The load can be measured linearly without using a micro processor. Further, when the non linearity is compensated for by means of the computation by a micro computer or other electronic circuits, the compensation can be carried out by simple devices and the linearity can be highly improved. 55 (3) In the known apparatus, the measuring range of frequency Afm/fo is limited to about 10% due to the non-linearity of the vibration type load sensor. In the present invention, the dynamic range can be increased up to 20% or more due to the fact that the limitation for the linearity is mitigated. This means that errors due to drift and creap of the zero point, hysterisis and non reproducibility can be reduced relatively. In this manner the precision other than the linearity can 60 also be improved.

Claims

1. A load measuring apparatus of vibration type comprising a load sensor of vibration type having a vibration element whose vibration frequency is 65 GB2190748A 5 changed in accordance with a force applied thereto; and a non-linear spring arranged to transmit a load to be measured to said load sensor at a ratio which changes in accordance with the load to be measured such that a frequency variation of the load sensor is made proportional to the load to be measured.

2. An apparatus according to claim 1, wherein said non-linear spring is constructed such that 5 said ratio (F/W) of the force (F) applied to tha load sensor with respect to the load (W) to be measured changes linearly in accordance with a change in said load (W).

3. An apparatus according to claim 1, wherein said non-linear spring has such a construction that its spring constant is increased or decreased in accordance with the increase in tension force or compression force. 10

4. An apparatus according to claim 3, wherein said non-linear spring is formed by a ringshaped spring.

5. An apparatus according to claim 3, wherein said non-linear spring is formed by an arcuate spring.

6. An apparatus according to claim 3, wherein said non-linear spring is formed by a leaf 15 spring whose effective length is shortened in accordance with the increase in the applied load.

7. An apparatus according to claim 1, wherein said load sensor is arranged between a link mechanism to which the load to be measured is applied and a fixed portion, and said non-linear spring is arranged between the load sensor and the fixed portion.

8. An apparatus according to claim 7, wherein said link mechanism is formed by a parallelo- 20 gram link mechanism.

9. An apparatus according to claim 1, wherein said load sensor is arranged between a fixed portion and one end of a lever, and said non- linear spring is arranged between the other end of the lever and a link mechanism to which the load to be measured is applied.

10. An apparatus according to claim 9, wherein said link mechanism is formed by a parallelo- 25 gram link mechanism.

11. An apparatus according to claim 1, wherein said load sensor is arranged between a free end of a lever and a fixed portion via the non-linear spring, and bellows is arranged between the lever and fixed portion, whereby the load to be measured is applied to the bellows as a pressure. 30

12. An apparatus according to claim 1, wherein said load sensor is arranged in a proving ring via the non-linear spring.

13. An apparatus according to claim 12, wherein said non-linear spring is formed by an arcuate spring.

14. An apparatus according to claim 1, wherein said load sensor is formed by a tuning fork 35 vibration element.

15. Load measuring apparatus substantially as herein described with reference to any of Figs. 3 to 11 of the accompanying drawings.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987. Published at The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies may be obtained.