Device For Transducing Force Into Pneumatic Signal

Abstract

A device for transducing a force into a pneumatic signal comprises an air obstructing body composed of a rigid ball and supporting disc against which a force to be transduced is applied, and an air nozzle including an opening facing the rigid ball, the air nozzle being connected to a compressed air supply unit. Compressed air from the opening of the air nozzle flows away along the surface of the rigid ball. As a result the above, the air obstructing body moves upward or downward by the difference between a compressed air force acting upon the rigid ball compressed and the applied force to be transduced. When both forces balance, the air obstructing body settles in an equilibrium position and at that time, back air pressure of the nozzle has a relationship which is proportional to the applied force to be transduced. Back air pressure is measured by an air pressure measuring instrument connected to the air nozzle as a pneumatic signal.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a device for transducing a force into a pneumatic signal. The invention provides a force to a pneumatic transducing device which converts the amount of applied force to be transduced into a corresponding air pressure value used as a conventional industrial instrumentation technology pneumatic signal.

Referring to the accompanying drawing, the following is a description of one of the most popular transducing devices that has been used for more than 20 years for a similar purpose as the present invention.

In FIG. 1, the basic components comprise an air nozzle 1, an opening 2, an air restrictor 3, air tubings 4, 5, 6 and 7, an air bellows 8, an air pressure measuring instrument 9, an air supply unit 10, a rod 11, a balancing beam 12 and a fulcrum 13. The air nozzle 1 having an opening 2 for flowing compressed air is connected to the air supply unit 10 through the air tubing 4 having an air restrictor 3 therein. The air bellows 8 is connected to the air pressure measuring instrument 9 through the air tubings 6 and 7, and is connected to the air nozzle 1 thhrough the air tubings 5 and 7. One end of the balancing beam 12 is rigidly connected to the air bellows 8 by the rod 11, and the other end of the balancing beam 12 is supported by the fulcrum 13. The balancing beam 12 is placed adjacent the opening 2 of the air nozzle 1. The principle of operation is as follows.

A force (F) to be transduced is applied on one end of the balancing beam 12. When the force (F) increases, the balancing beam 12 tends to incline around the fulcrum 13 in a direction to plug the outcoming air flow from the air nozzle 1 (known as the so-called "nozzle and flapper mechanism"), causing the back air pressure of air nozzle 1 to increase to such an extent that a force produced by the inflation of air bellows 8, with inside air pressure equal to nozzle back air pressure, in the opposite direction to compensate, or balance, the applied force (F) so that the balancing beam movement settles at a new equilibrium position.

When the applied force (F) to be transduced decreases, the balancing beam 12 and relating components act in reverse of the above operation.

Such conventional transducing devices have some difficulties with respect to the severe performance requirements often encountered in industrial applications. Some of these are high measuring accuracies, more reliable and stable operations, improved stability under ambient temperature variations, adjustment free construction, simplicity and compactness of mechanisms, and zero drift free characteristics.

Careful observation during the performance evaluating test of the typical conventional transducing device mentioned above reveals the following points of consideration.

1. The angular displacement of one end of th balancing beam during the full scale span range change of applied force was observed as more than 100 to 200 .mu. (micron). This means the conventional "force-balance" beam "moves" an amount that should not be ignored from the standpoint of complete elimination of the undesirable effects generally expected from a mechanism operating on the deflection method.

2. The conventional device has many metal constructed parts, such as bellows, springs andd hinges, each of which is subjected to the influence of ambient temperature variations, etc.

3. The conventional device has many component parts, none of which can be eliminated without sacrificing the guaranteed performance, and each of which should be insured by accurate and elaborate fabricating technology.

As a result, the overall performance characteristics can easily deviate from the limits of specification performance by even insignificant carelessness in assembly and adjustment.

4. One method of suppressing the undesirable results indicated above is to adopt a relatively heavy order of force levels around the balancing beam; the resulting larger dimensions of the main component parts through miniaturization is the recent trend in instrumentation technology.

Most of the above mentioned problems suggest we look over at the basic conventional force balance mechanism traditionally used during the past 20 years.

The invention we have developed solved most of those problems with a minimum of component parts and with outstanding simplicity and compactness.

The invention has no air bellows, balancing beam, fulcrums or hinges. Nevertheless, the accuracy of force to air pressure conversion performance was improved considerably.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects are accomplished by the parts, improvements, combinations and arrangements comprising the invention, a preferred embodiment of which is shown and described in detail herein.

Various modifications and changes in details of construction are comprehended within the scope of the appended claims.

FIG. 1 is a principle illustration in cross-section of the device for transducing the force into the pneumatic signal of the prior art.

FIG. 2 is a schematic cross-sectional view illustrating one embodiment of the present invention.

FIG. 3 is a schematic cross-sectional view illustrating the basic system composition of the present invention.

FIG. 4 is a schematic cross-sectional view showing the force to air pressure transducing element.

FIG. 5 is a graph relating to an applied force to be measured and the pneumatic signal output.

FIG. 6 is a graph relating to output air pressure change/supply air pressure change and a radius of supporting disc/radius of the rigid ball.

FIG. 7 is a schematic illustration of some variations of the supporting disc. FIGS. 8, 9 and 10 are graphs relating to hysteresis, linearity or error and a radius of the rigid ball/radius of the opening.

FIGS. 11, 12 and 13 are graphs relating to hysteresis, linearity or error and radius of the opening.

FIGS. 14, 15 and 16 are graphs relating to hysteresis, linearity or error and edge angle of the opening.

FIG. 17 is a graph relating to output air pressure change and an air resistance of the air restrictor/an air resistance of the air tubing.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, a force (F) to be transduced is applied downward on the lower end of the vertical rod 25 which is connected with circular suspension ring 21 by an over-load safety spring 24. Force (F) is conveyed in sequence from vertical rod 25, circular suspension ring 21, and supporting disc 22 to air obstructing ball 23.

On the other hand, compressed air is supplied from air supply unit 28 through capillary air restrictor 26 and air reservoir chamber 27 to air outflow nozzle 29.

The difference of the force (F) to be transduced and a force developed in the opposite direction by a static back pressure within the nozzle opening and an outcoming air flow causes the air obstructing ball 23 to move upward or downward, depending on whether the sign of the difference of the forces is plus or minus, until the air ostructing ball 23 reaches an equilibrium position.

In this state, the air pressure between air nozzle 29 and air restrictor 26 shows an exact one-to- one correspondence with the force to be transduced, and can be used as a measured pneumatic signal of force (F).

Eventually, the change in the amount of force to be transduced can be converted to the corresponding change in output air pressure at the output air tubing 30 and measured by the air pressure measuring instrument 31.

Further detailed explanation of the invention follows.

FIG. 3 shows the basic system composition of the invention, and FIG. 4 shows the force to air pressure transducing element, the most essential part of the invention.

Each component part is shown as: compressed air supply unit 1, air tubing 2, air restrictor 3, air reservoir 4, air nozzle opening 5, air obstrucing body 6 composed of rigid ball 6A and supporting disc 6B, air pressure outlet 7, air tubing 8, pneumatic signal indicating instrument 9 and the applied force to be transduced (F).

Referring to FIG. 3, compressed air is supplied from a compressed air supply 1 and passed through air restrictor 3 and air tubing 2, reaching air reservoir 4 and flowing out to the atmosphere through nozzle opening 5.

In the upper outside position of the central vertical axis of air reservoir 4, the air obstructing body 6 is placed for free movement, especially in a vertical direction.

The air obstructing body 6 is composed of a rigid ball 6A and a supporting disc 6B with one side of rigid ball 6A facing closely the nozzle opening 5 and a supporting disc 6B is contacted with rigid ball 6A on the other side of the nozzle opening 5.

The air obstructing body 6 reaches a certain equilibrium position depending on the relationship between two forces oppositely exerted. One force is developed on one side of rigid ball 6A by the outflowing air from the air nozzle opening 5, and the other force to be transduced is exerted on the supporting disc 6B.

When a force to be transduced changes the outgoing flow rate of air, a corresponding change results, with air restrictor 3 causing an air reservoir pressure change.

Consequently, the air pressure in the air reservoir 4,

by indicating instrument 9 through air tubing 8, has a reproducible relation with force (F) to be transduced.

The edge shape of the end of the air nozzle opening 5 assures stable and smooth air.

The data from observations made during a performance evaluating test of the invention using the actual prototype shown in FIGS. 3 and 4 are explained as follows.

Referring to FIG. 5, a force to be transduced as a pneumatic output signal relation is shown. The scale unit of the abscissa axis is in grams and that of the ordinate axis is in percentage. Each dimension of this example is as follows. The radii of nozzle opening, ball and supporting disc are 5.1 mm, 5.1 mm and 14.75 mm respectively. Compressed air pressure is 1.6 kg/cm.sup.2 G. 100 percent of output pneumatic signal corresponds to 1.0 kg/cm.sup.2 G. FIG. 5 shows that the invention assures highly linear conversion characteristics. An interesting dimensional relationship exists between the radii of the supporting disc and rigid ball and the ratio of output air pressure change and supply air pressure change under constant applied force. FIG. 6 shows this relationship as observed in the test using the prototype shown in FIGS. 3 and 4.

This data indicates that the effects of the variation in supply air pressure on the measured value of output could be neglected when the radius ratio of supporting disc to ball exceeds 1.5.

Generally, it is desirable to select a supporting disc having a larger projection area in the perpendicular plane against the central axis of air outflow nozzle than the ball has.

In the case explained above, the air obstructing body was composed of a complete sphere and a disc. However, another experiment indicated that the rigid ball need not be completely spherical in shape although the side facing against the air nozzle should have a spherical surface.

Similarly, the supporting disc need not necessarily be completely circular in shape. In this case, the projection area of the component parts of the air obstructing body should be defined as the value giving maximum projection diameter considered as having complete circular cross sectional shapes.

FIG. 7 shows some variations of supporting disc shapes tested in the experiments. The typical data relating to displacement of the air obstructing body observed in the experiments shows displacement of the air obstructing body to be less than 20 .mu., and reproducibility of the output signal better than .+-. 0.12 percent and the effect of ambient temperature variation of .+-. 30.degree. C on the output signal variation to be better than .+-. 0.2 percent.

The dimensional relation of the radius of a spherical air obstructing body and the air nozzle opening, using the prototype shown in FIGS. 3 and 4, was observed with regard to several performance characteristics.

FIGS. 8, 9 and 10 show the manner by which the radius ratio of spherical air obstructing body to air nozzle opening affects the performance characteristics with regard to hysteresis, linearity, and error caused by variations in the supply of air pressure respectively.

FIGS. 8, 9 and 10 illustrate that the performance error could be suppressed within 1 percent of full scale span if the range in ratio falls between 1 and 10.

Better performance characteristics can be obtained when the radius of air nozzle opening is selected as more than 1.75 mm and the ratio of the radius of ball to air nozzle opening is also selected within the range of 1.5 -5.0 . Then the three performance parameters mentioned above show better than .+-. 0.25 percent of full scale span.

The three performance parameters explained above, (hysteresis, linearity and error caused by variation in supply of air pressure) also depend on the dimension of the radius of the air nozzle the relationship of which are shown in FIGS. 11, 12 and 13.

This data indicates that in order to maintain the performance error at less than 1 percent of full scale span, then it is necessary to select the dimension of radius of air nozzle of at least 1 mm. By selecting the diameter in a proper value, performance better than .+-. 0.25 of full scale span was obtained.

AS to the relation of the edge angle (.theta.) of the air nozzle, shown in FIG. 4, to the performance characteristics, experimental results are shown in FIGS. 14, 15 and 16.

The abscissa axis of these figures express the angle (.theta.) of the edge shape of the air nozzle.

This data indicates that when the value of (.theta.) exceeds 180.degree. the performance characteristics suddenly show a steep increase in errors and near the 180.degree. value the data suggests unstable performance.

Consequently, the desirable edge angle is less than 180.degree..

A relation also exists between performance characteristics and the air path resistivity of the air restrictor 3 (abbreviated as resistance (A)), and the air path resistivity of the air path beginning from the air restriction outlet via the air reservoir to the air nozzle opening (abbreviated as resistance (Ao).

FIG. 17 shows this relation using the prototype shown in FIGS. 3 and 4. Abscissa axis is the ratio of (A) vs. r,in logarithmic scale, and ordinate axis is output signal error readings in kg/cm.sup.2.

These data were plotted at each (.sup.A /Ao) value by changing air supply pressure by 0.4 kg/cm.sup.2 from the stationary value, and the force to be measured in four cases, namely Og, 10g, 20g, 25g, 25g. The radii of spherical obstructing body, air nozzle, and supporting disc were 5.6 mm, 5.1 mm, and 7.5 mm, respectively. Air supply pressure was 1.6 kg/cm.sup.2 G. FIG. 17 indicates that below an (.sup.A /Ao) ratio value of 80,the output signal showed unstable performance with considerable errors. These results suggest that the ratio value should be more than 80 for the purpose of the invention.

In this experiment, the changing amount of air supply was selected as 0.4 kg/cm.sup.2 based on the certain criteria popular in the industrial instrumentation field: (A) can be properly selected by adjusting air restricting element.

Claims

What is claimed is:

1. Device for transducing force into pneumatic signal which comprises:

1. means for supplying compressed air,

2. an air nozzle having an opening connected to said means for supplying compressed air through an air tubing, the opening forming an edge angle at a tip of the opening and the air tubing containing an air restrictor,

3. an air obstructing body composed of a member having a curved surface and a supporting disc, the member having the curved surface facing said opening of said air nozzle and the supporting disc contacting the opposite side of said curved surface, said body having a member for transmitting an applied force to be transduced; wherein said air obstructing body moves upward and downward by the difference in force between a force acting upon the curved surface by the compressed air from the air nozzle and the applied force to be transduced, and

4. means for measuring back air pressure in said air nozzle, wherein:

the radius of the opening of the air nozzle, the radius of curvature of the curved surface, the radius of the supporting disc, the air resistance of the air tubing and the edge angle of the outer side of the opening of the air nozzle have the relationship defined by the expressions of

R1/r = 1.about.10

R2 > r1

a/ao .gtoreq. 80

.theta. < 180

r .gtoreq. 1 where (r) is the radius of said opening, (R1) is the radius of curvature of said curved surface, (R2) is the radius of said supporting disc, (A) is the air resistance of said air restrictor (Ao) is the air resistance of said air tubing and (.theta.) is the edge angle of said opening.

2. A device as described in claim 1, wherein the member having a curved surface is composed of a part of sphere.

3. A device as described in claim 1, the member having a curved surface is a spherical ball.

4. A device as described in claim 1, wherein the projected area of the supporting disc is circular.