EP2243002A1

EP2243002A1 - A method and device for measuring a property depending on mass

Info

Publication number: EP2243002A1
Application number: EP08700532A
Authority: EP
Inventors: Benno Bucher; Adam Dombi
Original assignee: Hochschule Rapperswil HSR
Current assignee: Hochschule Rapperswil HSR
Priority date: 2008-01-28
Filing date: 2008-01-28
Publication date: 2010-10-27
Also published as: WO2009094787A1

Abstract

The device comprises a liquid (13) placed in a container (10). The liquid (13) has a configuration that depends on the mass of a body external to the device. The device further comprises a detector assembly (6) for monitoring the configuration. The detector assembly (6) can e.g. comprise a light source (16), whose light is diffracted by a surface (15) of the liquid (13).

Description

i

A method and device for measuring a property depending on mass

Technical Field

The invention relates to a method and device for measuring a property depending on a mass based on a configuration change of a liquid.

Background Art

It has been known to measure the presence or mass of a body by means of Cavendish scales, which detect the motion of a pair of masses on a horizontal bar in the present of the body. Scales of this type are, however, difficult to set up because they require an evacuated or at least carefully controlled, large chamber to receive the pair of masses and because they are highly sensitive to vibrations.

Disclosure of the Invention

Hence, the problem to be solved is to provide a method and device of this type that is more robust and easier to set up.

This problem is solved by the method and device according to the independent claims .

Accordingly, a liquid is placed in a container and has a configuration that depends on the masses around it. The term "configuration", in this respect, is used to designate the distribution of the liquid within the container. This configuration will primarily depend on the gravitational field of the earth, but (to some minor extent) also on the gravitational field of the masses around it. According to the invention, at least one parameter depending on this configuration is measured, and the measured parameter is used to derive the property.

For clarity, the term "property" is used for the property of the mass of the body that is to be measured. The property can e.g. be:

- the presence or absence of the body,

- the mass (e.g. in kg) itself, and/or

- the location of the body, such as its dis- tance from the container or its direction from the chamber.

The term "parameter", on the other hand, is used for the parameter depending on the configuration of the liquid that is determined by the device's detector. The parameter can e.g. be a light intensity affected by the configuration, a magnetic or dielectric or electric property that depends on the configuration, or any combination thereof.

Advantageously, when the measured parameter changes, the container is tilted for bringing back the measured parameter to its original value. This allows to bring the container back a defined operating position where the parameter has a given value and the resulting gravitational force at the location of the container has a given direction.

Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein: Fig. 1 is a schematic illustration of a first embodiment of the device, Fig. 2 is a first embodiment of the container and detector,

Fig. 3 is a diagram of a circuit for processing the signals of the detector of Fig. 2, Fig. 4 is a second embodiment of the container and detector,

Fig. 5 is a third embodiment of the container and the detector,

Fig. 6 is a fourth embodiment of the con- tainer and the detector,

Fig. 7 is a view of a second embodiment of the device, and

Fig. 8 is an fifth embodiment of the container.

Modes for Carrying Out the Invention

A first embodiment of the device is schematically shown in Figs. 1 - 3. It comprises a static support 1, which e.g. rests on the floor of a laboratory. Static support 1 carries a movable carrier 2. Movable carrier 2 rests at two locations 3 and 4 on support 1.

At location 3, it is supported in tiltable fashion and can be tilted in respect to support 1 about a tilt axis perpendicular to the drawing plane of Fig. 1. The tilt axis is horizontal, i.e. perpendicular to the gravitational field of the earth.

At location 4, which is at a distance from location 3, movable carrier 2 comprises an actuator 5, such as a piezo-electric actuator, which rests against support 1. Movable carrier 2 can be tilted by operating actuator 5.

Close to location 3, advantageously vertically above (or below) it, movable carrier 2 carries a detector assembly 6. Fig. 2 shows a sectional view of an embodiment of this assembly. Detector assembly 6 comprises a container 10, which is held in fixed position in respect to movable carrier 2. It has, advantageously, a curved, convex top wall 11 and a bottom wall 12 (curved or flat) opposite to top wall 11. At least top wall 11 and bottom wall 12 are made of a transparent material, such as glass.

A liquid 13 is located in container 1. It fills all of container 10 except for a region 14, which is either field by a fluid different from liquid 13 or which is empty, i.e. under vacuum. The term ^λΛa fluid different from liquid 13" also encompasses a gaseous phase of liquid 13. Liquid 13 forms a surface 15 bordering region 14.

The device is equipped with a detector meas- uring a parameter depending on the configuration of surface 15. This detector can be of various nature, and some embodiments thereof are explained in the following.

In the embodiment of Fig. 2, the detector comprises a light source 16 placed on one side of con- tainer 10 as well was two light sensors 17, 18 placed on the other side of container 10. One of the detectors 17, 18 is closer to location 4 than the other.

As can be seen, the light from light source 16 enters chamber 10 and passes through liquid 13 and through surface 15. At surface 15 the light is diffracted by some degree, then exits container 10 through top wall 11 and arrives at the light sensors 17 and 18.

The circuit processing the signal from the light sensors 17, 18 is shown in Fig. 3. It comprises an operational amplifier 20, which measures the difference of the signals from light sensors 17 and 18. The output signal is fed to a feedback controller 21, such as a PID- , PI- or PD-controller, which tries to control actuator 5 in such a manner that the signal from amplifier 20 goes to zero.

The device is further equipped with an output device 22, which e.g. displays the voltage used to con- trol actuator 5 or some other signal that is indicative of the tiling angle of movable carrier 2 in respect to support 1. Output device 22 can, in addition or alternatively to a display, e.g. comprise a digital interface for transferring the signal to an external system.

Prior to a measurement, the device is set up such that the signals of both light sensors 17, 18 are approximately equal. Then, controller 21 is activated and tilts movable carrier 2, e.g. until the signals are ex- actly equal. In this position, light source 16, light sensors 17, 18 and surface 15 are symmetrical in respect to a vertical plane parallel to the device's tilt axis.

When a body 24 is brought to a position as e.g. indicated in Fig. 1, the gravitational field of its mass will pull the liquid 13 towards it, thereby changing the configuration of surface 15. This leads to a non-zero output at amplifier 20, whereupon controller 21 will change the tilting angle of movable carrier 2 until the output signal of amplifier 20 goes back to zero. The cor- responding change of the signal at output device 22 allows to calculate the tilting angle α.

As can be shown, tilting angle a depends on the mass M of body 24 and its distance r from chamber 10, in approximation, as follows:

with g being the gravitational acceleration of the earth at the site of the device and G the gravitational con- stant.

Hence, by measuring tilting angle α, it is e.g. possible to determine the mass M or the distance r.

A second embodiment of detector assembly 6 is shown in Fig. 4. It again comprises a light source 16, whose light this time enters top wall 11 of container 10 and is then reflected, at least partially, by surface 15. The two light sensors 17, 18 are positioned symmetrically in respect to the axis of light source 16 and measure two light beams reflected at opposite parts of surface 15. The signals from the light sensors 17, 18 are again proc- essed by a circuit as shown in Fig. 3.

In this embodiment, the device is again set up such that the signals of both light sensors 17, 18 are approximately equal and controller 21 is activated to tilt movable carrier 2 until the signals are exactly equal. When the position of body 24 is changed (or if body 24 is brought from a remote place to a location close to container 10) , the configuration of surface 15 changes, which leads to (opposite) changes in the signals from the light sensors 17, 18. This can again be used by controller 21 to readjust the device by tilting movable carrier 2.

By using two light sensors measuring different light beams passing through or being reflected by surface 15, it is possible to make a relative measure- ment, which e.g. does not depend on the level of light yielded by light source 16, which makes the measurement more robust. However, a measurement with only one light sensor is possible as well.

In general, detector assembly 6 can comprise at least one light source that illuminates at least part of the liquid and at least one light sensor that detects light influenced by the liquid. The light can e.g. be diffracted or reflected at surface 15, or it can be partially absorbed or scattered by the liquid. The term "light", in this respect, is to be understood in broad manner, and, in particular, encompasses infrared, visible and ultraviolet light.

In addition or alternatively to this, detector assembly 6 can, however, also carry out non-optical measurements as well. An example of a non-optical measurement is shown in Fig. 5. Here, detector assembly 6 comprises two capacitors Cl, C2, each straddling container 10. Each capacitor comprises a first electrode mounted to top wall 11 of container 10 and a second electrode mounted to bot- torn wall 12 of container 10. Surface 15 is located at least partially within each capacitor, i.e. it extends into the space between the electrodes of each capacitor.

If the liquid has a dielectric constant ε^ that is not equal to the dielectric constant 82 of the fluid or vacuum in region 14, the capacitance of each capacitor Cl, C2 depends on the configuration of the liquid 10.

Advantageously, the capacitors Cl, C2 are arranged symmetrically to a vertical plane parallel to the device's tilt axis.

Fig. 5 also shows a circuit for feeding a signal to controller 21. It comprises an ac-driven bridge circuit with two nominally equal reference capacitors Crefl, Cref2, each of which is in series to one of the capacitors Cl, C2. The voltages of over the two capacitors Cl, C2 are fed to the inputs of an AC amplifier 20, and controller 21 strives to keep the output of amplifier 20 zero (i.e. C1/C2 = Crefl/Cref2) .

In use, the operation of the embodiment of Fig. 5 is therefore equivalent to the one of Fig. 2 or 4.

Detector assembly 6 can also be adapted to measure a magnetic parameter depending on the configuration of the liquid. For this purpose, detector assembly 6 can be equipped with one or more coils. A corresponding embodiment is shown in Fig. 6.

In the embodiment of Fig. 6, container 10 is a symmetrical, bent tube with two coils 25, 26 wound around opposite halves thereof. Surface 15 is located at least partially within each coil, i.e. it extends into the space enclosed by the coil's windings.

If the liquid has a dielectric constant μ_]_ that is not equal to the dielectric constant μ£ of the fluid or vacuum in region 14, the inductance of each coil 25, 26 depends on the configuration of the liquid 10. Fig. 6 also shows a circuit for feeding a signal to controller 21. It comprises two oscillator cir- cuits 28, 29, whose frequencies fl and f2 are controlled by coils 25 and 26, respectively. A frequency comparator 30 generates a signal proportional to the difference fl - f2 and feeds the same to controller 21. Controller 21 strives to keep the output of comparator 30 zero (i.e. fl - f2) .

In use, the operation of the embodiment of Fig. 6 is therefore again equivalent to the one of Fig. 2 or 4.

The present device can also be used to carry out spatially resolved measurements if movable carrier 2 is movable about two tilt axes 32, 33 and if detector assembly 6 is equipped to detect rotations about each of these axes. Such an embodiment is shown in Fig. 7. It comprises a first actuator 5a for tilting carrier 2 about first tilt axis 32 and a second actuator 5b for tilting carrier 2 about second tilt axis 33. Tilt axes 32, 33 are horizontal and perpendicular to each other.

Detector assembly 6 can e.g. comprise two chambers and detectors of the type shown in Figs. 2 - 6, or it can contain a single chamber with detectors for detecting a change of configuration in both directions. In other words, detector assembly 6 measures a first and a second parameter depending on the tilting of carrier 2 about the first and the second tilt axis 32 and 33, re- spectively

As shown, support 1 can be equipped with screws 36, 37, which allow a tilting of support 1 about axes parallel to the tilt axes 32 and 33, respectively. They can be used for a zero position adjustment of the device. A similar screw or set or screws can also be used in the embodiment of Fig. 1. As mentioned, the device advantageously measures two parameters depending differently (e.g. oppositely) on the configuration of liquid 13, and the feedback loop controller 21 strives to keep the parameters equal. This design allows to compensate for any drift that affects both measured parameters .

The liquid 13 should have low viscosity. It can e.g. be an alcohol of low viscosity.

In an advantageous embodiment, liquid 13 can be a superfluid, in which case liquid 13 has to be kept at a suitable temperature .

In the embodiments shown in Figs. 2 and 4 - 6, top wall 11 of container 10 was curved and convex. Top wall 11 can, however, also be flat and extend horizon- tally, in which case the location of region 14 becomes unstable and is highly sensitive to changes of the mass distribution around detector assembly 6.

In yet another embodiment, as shown in Fig. 8, container 10 can be shaped such that two spaced-apart regions 14 of a second fluid or vacuum are formed, each having its own surface 15. For example, cavity 10 can comprise two domes 40a, 40b interconnected by a duct 41, with the regions 14 formed at the top of each dome 40a, 40b. Duct 41 can have a comparatively small diameter for damping the motions of liquid 13.

In the absence of a gravitational force along the direction between the two regions 14, the two surfaces 15 are at equal height. If a mass is placed close to one of the domes 40a, 40b, the levels, i.e. the con- figurations, of the two surfaces 15 will change in opposite directions, which can be measured by a suitable sensor.

In the embodiment of Fig. 8, a capacitive sensor is proposed, which comprises a float 42 floating on each of the surfaces 15. Each float 42 carries a floating electrode 44 mounted on its top side. At the top of each dome 40a, 40b, a pair of electrodes 46 is pro- vided. Each pair of electrodes 46 forms, together with floating electrode 44, a capacitor Cl or C2, respectively. When the levels of the two surfaces 15 change in opposite directions, the capacitances of capacitors Cl and C2 will also change oppositely, which can e.g. be measured with circuitry of the type shown in Fig. 5.

In the embodiment of Fig. 8, the capacitors Cl, C2 each comprise an electrode (namely the floating electrodes 46) , whose position changes when the configu- ration of the liquid changes. If fluid 14 is electrically conductive, the one of the electrodes of each capacitor Cl, C2 can be formed by the fluid itself if the fluid is set to a defined electrical potential.

The embodiment of Fig. 8 can also be combined with inductive or optical sensors as described in the other embodiments .

Chamber 10 has been shown as a hermetically closed chamber in all the embodiments. A hermetically closed chamber is advantageous because it prevents an evaporation of liquid 13. However, chamber 10 can also be in communication with the environment.

In the embodiments described so far, chamber 10 is substantially rigid. However, chamber 10 can also be flexible or elastic. For example, it can be formed at least partially by a flexible or elastic membrane, e.g. a balloon, whose shape changes when the gravitational field varies. In this case, the position of the surface of the membrane can be measured. For such embodiments, chamber 10 can be fully filled by the liquid. While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

1. A method for measuring a property depending on the mass of a body, said method comprising providing a container (10) at least partially filled by a liquid (13) , wherein said liquid (13) has a configuration that depends on said mass as well as on a gravitational field not generated by said mass, measuring at least one parameter depending on said configuration, and deriving said property from said parameter.

2. The method of claim 1 further comprising tilting said container (10) for keeping said parameter constant.

3. A device for measuring a property depending on the mass of a body, in particular a presence, an absence, a position or the mass of said body, said device comprising a container (10) partially filled by a liquid (13) , wherein said liquid (13) has a configuration in said container (10) that depends on said mass, a detector assembly (6) located at said container (10) and measuring at least one parameter depending on said configuration as well as on a gravitational field not generated by said mass, an output device (22) for outputting said property.

4. The device of claim 3 wherein said liquid (13) has at least one surface (15) bordering vacuum or a fluid, and wherein said detector assembly (6) measures a parameter depending on the configuration of said surface (15) .

5. The device of claim 4 wherein said detector assembly (6) detects light reflected from a said sur- face (15) .

6. The device of any of the claims 4 or 5 wherein said detector assembly (6) detects light passing through said surface (15) and diffracted at said surface (15) .

7. The device of any of the claims 5 or 6 wherein said detector assembly (6) comprises at least two

₅ light sensors (17, 18) detecting different light beams passing through or being reflected by said surface (15) .

8. The device of claim 7 wherein said detector assembly (6) calculates a difference between signals from said at least two light sensors. o 9- The device of any of the claims 4 to 8 wherein said liquid has two spaced apart surfaces (15), and in particular wherein said container (10) comprises at least two domes (40a, 40b) interconnected by a duct

(41) with one surface (15) in each of said domes (40a,s 40b) .

10. The device of any of the preceding claims wherein said detector assembly (6) comprises at least one light source (16) illuminating at least part of said liquid (13) and at least one light sensor detecting lighto influenced by said liquid (13) .

11. The device of any of the preceding claims wherein said detector assembly (6) comprises a capacitor

(Cl, C2) whose capacitance depends on said configuration.

12. The device of the claims 4 and 11 wherein5 said surface (15) is located at least partially within said capacitor (Cl, C2) .

13. The device of claim 11 wherein each capacitor comprises an electrode (44) whose position changes when said configuration changes, and in particu-o lar wherein said electrode (44) is arranged on a floater

(42) .

14. The device of any of the preceding claims wherein said detector assembly (6) is adapted to measure a magnetic parameter depending on said configuration.5

15. The device of the claims 4 and 14 wherein said detector assembly (6) comprises at least one elec- trical coil (25, 26) wherein said surface (15) is located at least partially within said coil.

16. The device of any of the preceding claims further comprising a static support (1), a movable carrier (2) tiltable in respect to said support about at least one tilt axis, wherein said container (10) is mounted to said movable carrier, an actuator (5, 5a, 5b) for tilting said mov- able carrier about said axis, and a feedback loop controller (21) operating said actuator for keeping said parameter constant.

17. The device of claim 16 wherein said movable carrier (2) is tiltable in respect to said support (1) about a first and a second tilt axis (32, 33) and comprises a first and a second actuator (5a, 5b) for tilting said movable carrier (2) about said first and said second tilt axis (32, 33), respectively, and wherein said detector assembly (6) measures a first and a second parameter that depend on a tilting of said carrier (2) about said first and said second tilt axis (32, 33) , respectively.

18. The device of any of the claims 16 or 17 wherein said detector assembly (6) measures at least two parameters depending differently on said configuration and wherein said feedback loop controller (21) operates said actuator (5, 5a, 5b) for keeping said parameters equal .

19. The device of any of the preceding claims wherein said fluid is a superfluid.

20. Use of the device of any of the claims 3 to 19 for measuring a presence, absence or a mass of a body.