WO2003036249A1

WO2003036249A1 - Method and device for measuring forces

Info

Publication number: WO2003036249A1
Application number: PCT/US2001/030979
Authority: WO
Inventors: Norm Gitis; Michael Vinogradov; Vlad Dorfman
Original assignee: Center For Tribology, Inc.
Priority date: 2001-10-04
Filing date: 2001-10-04
Publication date: 2003-05-01

Abstract

The device of the invention is intended for measuring a loading force and a friction force in a tribological system. The device includes an assembly of two deformation sensitive sensors (200, 300) for simultaneous equal deformation in two opposite directions for eliminating misbalance created in the measurement system (100) when a single sensor is used. Each sensor (200, 300) comprises a deformable beam (212, 312) having through longitudinal slots (218, 220) extending in different and non-parallel directions and overlapped within the body of the beam (212, 312). Each sensor (200, 300) deforms in one direction under the effect of a loading force beam in another direction under the effect of a friction force measured by another two pairs of strain gauges (R1-2 and R3-2) located on opposite sides of the beam (212, 312) near the other end of the beam (212, 312). Two aforementioned sensors (200, 300) are sandwiched between two plates (102, 104) in diagonally symmetrical positions, so as to transmit forces between both plates (102, 104) and at the same time to ensure limited freedom of movement between both plates (102, 104) to allow deformations caused by the applied forces. One plate (102) is attached to the loading unit of the tribological system and another (104) supports an upper sample for engagement with the lower sample of the system.

Description

METHOD AND DEVICE FOR MEASURING FORCES

Field

The present invention relates to force measurement technique, in particular to a method and apparatus for measuring friction forces between contacting surfaces.

Background

Tribology is a science of friction, wear, and lubrication of contacting surfaces. The most common parameter widely used to characterize tribological properties of materials, devices and machine components is a coefficient of friction. This coefficient is defined as a ratio of a friction force, tangential to the contact, to an applied loading force, perpendicular to the contact.

Many different types of tribological systems, friction testers and other devices for measuring friction parameters are known. One such friction tester is disclosed in US Patent No. 5,795,990 issued to Norm Gitis, et al in 1998. A fragmental sectional view of this tester is shown in Fig. 1. The tester has a lower disc-like test specimen 21 and an upper rod-like or pin-like test specimen or probe 20. The latter one performs orbital motions while being in contact with a stationary lower specimen 21. As can be seen from Fig. 1 , a disadvantage of the sensor attachment device used in the aforementioned tester consists in that the upper specimen 20 has leverage with respect to the point of attachment, or center, of the lower specimen 21. As a result of the off-center loading, the loading force applied to the lower specimen 21 via the upper specimen 20, as well as the reaction force applied to the upper specimen 20 from the lower specimen 21 , create an unbalanced momentum and deformations in the force measurement system.

Regarding the known force measurement sensors themselves, it should be noted that they possess a number of disadvantages that make them inefficient for use in friction systems. U.S. Patent 4,785,673 issued in 1988 to Jean-Pierre Aumard describes a force measurement sensor for measuring at least two forces. This sensor comprises a beam with notches for imparting flexibility to the beam in the direction of action of forces to be measured. The aforementioned flexibility is required to increase reversible deformation of the beam under effect of the applied forces and thus to make it possible to measure these forces with sensing elements such as strain gauges attached to the respective deformable portions of the beam. In the aforementioned U S. Patent 4,785,673 the beam has a first pair of symmetrical transverse notches and second pair of symmetrical transverse notches at a distance from the first pair.

However, the above sensor measures two parallel forces, which are located in the same or in parallel planes, which limits its application. A disadvantage of this sensor is that the sensing elements, such as strain gauges, are arranged sequentially, so the sensor has an increased overall dimension in the longitudinal direction and therefore does not possess rigidity in a force transmission direction, which presents problems associated with excitation of the natural frequency resonance. The resonance, in turn, affects accuracy of measurements and limits the dynamic range of forces to be measured. Furthermore, an increased length of the sensor makes it unsuitable for measuring forced under increased temperature since the sensor is a subject to significant temperature deformations.

U.S. Patent 4,628,745 issued in 1986 to Yotaro Hatamura describes a multi-axis load sensor, which has radial plates adapted to detect moments produced by forces acting in planes which neither coincide nor parallel to the standard axis of at least one of the parallel plate structure. The sensor is exemplified as a device for measuring loads produced by a mechanical robot having several degrees of freedom. This sensor is more universal than the one described in the earlier-mentioned U. S. Patent 4,785,673. However, it is still sensitive to temperature deformations and is subject to excitation of natural frequency resonance. This is because, though the load sensing elements are arranged in different non-coincident and non-parallel planes, they are still arranged in sequence and therefore the sensor has an increased length and entails the same disadvantages as the earlier described sensor. Furthermore, the beam of the sensor is significantly weakened by multiple cut-outs, extending inwardly from the external surface of the beam. This limitation is an essential disadvantage of the sensor of U.S. Patent 4,628,745, which makes it inapplicable for measuring large forces.

Summary of the Invention

The device of the invention for force measurement comprises an assembly of two deformation-sensitive sensors for simultaneous equal deformation in two opposite directions for eliminating misbalance created in the measurement system when a friction system with a single sensor is used. Each sensor comprises a deformable beam having two symmetrically shaped longitudinal through slots extending in different and non-parallel directions and overlapped within the body of the beam. Each slot has at its opposite ends notches which are wider than the slots, with the distance from an inner wall of the notch to the outer side surface of the beam shorter than the distance from an inner wall of the slot to the same outer beam surface. Each sensor deforms in one direction under the effect of a loading force measured, e.g., by two strain gauges located on opposite sides of the beam near one end of the beam and in another direction under the effect of a friction force measured by another two strain gauges located on opposite sides of the beam near the other end of the beam. Two aforementioned sensors are sandwiched between two plates in diagonally symmetrical positions so as to transmit forces between both plates and at the same time to ensure limited freedom of movement between both plates to allow deformations caused by the applied forces. One plate may be attached to a loading unit of a friction system and the other may support an upper specimen for engagement with a lower specimen.

Objects of the Invention

It is an object of the present invention to provide a method and a force measurement apparatus which eliminate an unbalanced momentum and deformation in the force measurement system of a friction system, improve accuracy of measurements, broaden the range of test conditions, and prevents such phenomena as parasitic vibrations. Another object of the present invention is to provide a bi-directional load measurement sensor, which allows for reduction in the overall length of the sensor, to increase its sensitivity, to reduce susceptibility to natural frequency resonance, to decrease temperature errors, to improve accuracy of measurement, and to increase the dynamic range of forces to be measured. Still another object is to improve compactness of the aforementioned sensor and to ensure high rigidity in the force transmission direction.

Brief Description of the Drawings

Fig. 1 is a view of a known friction tester with a non-balanced force measurement device.

Fig. 2 is a three-dimensional exploded view of a device of the invention for force measurement in a friction system.

Fig. 3 is a three-dimensional view of a sensor used in the device of Fig. 2.

Fig. 4 is an example of an electric circuit of double-force sensors.

Fig. 5 is a view of the sensor similar to Fig. 3 but with a beam of a round cross section.

Fig. 6 is a three-dimensional view illustrating two sensors in connection with forces applied to the sensors and measured by various strain gauges of both sensors.

Detailed Description

Fig. 2 is a three-dimensional exploded view of the device of the invention for force measurements. As can be seen from this drawing, the device, which in general is designated by reference numeral 100, consists of a lower plate 102 of a rectangular shape, an upper plate 104 which has substantially the same shape and dimensions as the plate 102, and a pair of sensors 200 and 300 sandwiched between the lower plate 102 and the upper plate 104. The upper plate 104 is .connected, e.g., to a loading unit of a friction testing apparatus (not shown), and the lower plate 102 supports, e.g., a stationary upper specimen, which during testing is maintained in contact with a moveable lower specimen. The specimens will be shown and described later in connection with operation of the device. Both sensors are spaced from each other and are arranged symmetrically diagonally opposite to each other. In other words, the sensor 200 is located in a position turned 180° with respect to the sensor 300. Since both sensors 200 and 300 are identical, for better understanding the principle of the present invention, the description of one of the sensors, e.g., the sensor 200, will now be given.

Fig. 3 is a three-dimensional view of the sensor 200 used in the device of Fig. 2. The sensor comprises a flexible beam 212 of a rectangular cross section with rigid solid end blocks 214 and 216 at both ends for securing the sensor to the upper and lower plates 104 and 102, respectively (Fig. 2). The beam 212 has two symmetrically shaped through slots 218 and 220 cut in mutually perpendicular directions X and Y, respectively. The slots 218 and 220 partially intersect within the body of the beam 212. Each slot has on its opposite ends a through hole or notch, i.e., notches 222, 224 on the opposite ends of the slot 218 and through notches 226, 228 on the opposite ends of the slot 220, respectively. The notches 222, 224 and 226, 228 are wider than the respective slots 218 and 220.

The distances "f_t" and "f₃" from the inner walls of the respective notches 222 and 226 to the outer side surfaces 230 and 232 (only the edge of the surface 230 is seen in Fig. 3) of the beam are shorter than the distances to the surface 230 and 232 from the inner walls 234 and 236 of the slots 218 and 220. The thinned portions of the beam 212 impart anisotropic flexibility to the beam required for increasing sensitivity of the sensor. The aforementioned anisotropic flexibility is ensured in the direction perpendicular to the direction of a respective slot and thus coincides with the direction of the force to be measured. In other words, for a force acting in the direction of axis X flexibility will be provided only in the direction of axis X by the notches 226 and 228 of the slot 220, and for a force acting in the direction of axis Y flexibility will be provided only in the direction of axis Y by the notches 222 and 224 of the slot 218.

In fact, the beam 212 with the slots 218 and 220 cut through the body of the beam in two different intersecting directions, which in the embodiment shown in Fig. 3 are two mutually perpendicular directions, can be compared with a pair of mutually overlapped parallelograms combined in one body which will be described later in connection with the operation of the sensor. Force sensing elements, such as strain gauges 238a, 238b and strain gauges 240a, 240b are attached to opposite sides of the beam on mutually perpendicular surface areas at the ends of the beam which are flexible enough (due to provision of the notches) to comply with sensitivity of strain gauges used for measuring deformations and registering the measured deformations with appropriate electronic instrumentation (not shown). Only one strain gage of each pair, i.e., the strain gauges 238a and 240a, are seen in Fig. 3, while strain gauges 238b and 240b are not seen and their reference lines reach the edges of their respective sides.

The dimensions of the slots 218, 220 and the notches 222, 224 and 226, 228 are chosen in connection with the material of the beam so that deformations caused by the measured forces are reversible without residual deformations and directly proportional to the aforementioned forces. It is understood that the strain gauges 238a, 238b, 240a, 240b, should be chosen so as to respond to mechanical deformations caused by measured forces within the entire possible range of the forces.

Examples of sensing elements suitable for the above purposes are strain gauges of N2AQ-XX-S061 P-350 type produced by Measurement Group VISHAY, Raleigh, NC, USA. Such a sensing element normally comprises a thin-film serpentine-type resistor, which can be connected to one arm of a bridge-type or a potentiometric electric measurement circuit.

The sensor 300 is identical to the aforementioned sensor 200. Therefore only end blocks 314, 316 and a beam 312 of the sensor 300 are designated in Fig. 2.

The end block 214 of the sensor 200 is positioned with respect to the upper plate 104 by means of set pins 250 and 252 inserted into openings 250a and 252a of the upper plate 104 through openings 250b and 252b of the lower plate 102 and openings 250c and 252c of the end block 214 (Fig. 2). The end block 214 is attached to the upper plate 104 by means of bolts 254 and 256 inserted through openings 254a and 256a of the lower plate 102, openings 254b, 256b of the end block 214, and screwed into threaded openings 254c, 256c of the upper plate 104.

The end block 216 of the sensor 200 is positioned with respect to the lower plate 102 by means of set pins 258 and 260 inserted into openings 258a and 260a of the lower plate 102 through openings 258b and 260b of the upper plate 104 and openings 258c and 260c of the end block 216. The end block 216 is attached to the lower plate 102 by means of bolts 262 and 264 inserted through openings 262a and 264a of the upper plate 104, openings 262b, 264b of the end block 216, and screwed into threaded openings 262c, 264c of the lower plate 102.

The end block 314 of the sensor 300 is positioned with respect to the lower plate 102 by means of set pins 266 and 268 inserted into openings 266a and 268a of the lower plate 102 through openings 266b and 268b of the upper plate 104 and openings 266c and 268c of the end block 314. The end block 314 is attached to the lower plate 102 by means of bolts 270 and 272 inserted through openings 270a and 272a of the upper plate 104, openings 270b, 272b of the end block 314, and screwed into threaded openings 270c, 272c of the lower plate 102.

The end block 316 of the sensor 300 is positioned with respect to the upper plate 104 by means of set pins 274 and 276 inserted into openings 274a and 276a of the upper plate 104 through openings 274b and 276b of the lower plate 102 and openings 274c and 276c of the end block 316. The end block 316 is attached to the upper plate 104 by means of bolts 278 and 280 inserted through openings 278a and 280a of the lower plate 102, openings 278b, 280b of the end block 316, and screwed into threaded openings 278c, 280c of the upper plate 104.

Reference numeral 281 designates a protective shield, which prevents access to the sensors from outside when the device shown in Fig. 2 is in an assembled state.

An example of a bridge-type circuit for strain gauges 238a, 238b, 240a, 240b is shown in Fig. 4. In this circuit, R1-1 designates a pair of resistors corresponding to the strain gauge 238a of the sensor 200 shown in Fig. 3, whereas R1-2 designates a pair of resistors corresponding to the strain gauge 238b which is located on the side of the sensor 200 opposite to the side of the strain gauges 238a and which is not seen in the drawing. The pairs of resistors R1-1 and R1-2 form a first bridge. R1-3 and R1-4 designate balancing resistors for the first bridge. R2-1 designates a pair of resistors corresponding to the strain gauge 240a of the sensor 200 shown in Fig. 3, whereas R2-2 designates a pair of resistors corresponding to the strain gauge 240b which is located on the side of the sensor 200 opposite to the side of the strain gauge 240a and which is not seen in the drawing. The pairs of resistors R2-1 and R2-2 form a second bridge. Resistors R2-3 and R2-4 are balancing resistors for this second bridge.

R3-1 and R3-2 designate pairs of resistors, which form a third -bridge and which are located on the sensor 300 similarly to the pairs of resistors R1-1 and R1-2 of the beam 200. R3-3 and R3-4 are balancing resistors of the third bridge. R4-1 and R4-2 designate pairs of resistors, which form a fourth bridge and which are located on the sensor 300 similarly to the pairs of resistors R2-1 and R2-2 of the sensor 300. R4-3 and R4-4 are balancing resistors for the fourth bridge.

In the electric circuit of Fig. 4, reference numeral 400 designates a power source, which is connected to each bridge formed by respective strain gauges and balancing resistors in both sensors 200 and 300. As shown in Fig. 4, both output leads of the first bridge formed by the resistors R1-1 and R1-2 are connected to a positive and a negative inputs 402a and 402b, respectively, of the first adder amplifier 402. Similarly, both output leads of the second bridge formed by the resistors R2-1 and R2-2 are connected to a positive and negative inputs 404a and 404b, respectively, of the second adder amplifier 404. Similarly, both outputs of the third bridge formed by the resistors R3-1 and R3-2 are connected to a positive and a negative inputs 406a and 406b, respectively, of the third adder amplifier 406. Both outputs of the fourth bridge formed by the resistors R4-1 and R4-2 are connected to a positive and negative inputs 408a and 408b, respectively, of the fourth adder amplifier 408.

Outputs of the first amplifier 402 and of the third amplifier 406, which produce output signals corresponding to force F_x acting in the direction of axis X and measured by both sensors 200 and 300, respectively, are supplied to a first output amplifier 410, whereas outputs of the second amplifier 404 and of the fourth amplifier 408, which produce output signals corresponding to force F_y acting in the direction of axis Y and measured by both sensors 200 and 300, respectively, are supplied to a second output amplifier 412. An output of the amplifier 410 is connected to channel 1 and an output of amplifier 412 is connected to channel 2 of the measurement and registration apparatus (not shown).

Attached to the lower plate 102 (Fig. 2) is a specimen mounting plate 282 for attaching an upper specimen 283, which is secured in a chuck 284 connected to mounting plate 282. Mounting plate 282 is connected to lower plate 102 by bolts (not shown) which are screwed into threaded opening 287a, 287b, 287c, and 287d of the lower plate 102 via openings 288a, 288b. The upper specimen 283 is located in the geometrical center of the lower plate. It is assumed that the loading force and the reaction force pass through this point.

Fig. 5 illustrates a sensor 510, which is similar to the one shown in Fig. 3 and differs from it in that a deformable beam 512 has a round cross section. Similar to the embodiment of Fig. 3, the beam 512 has through slots 518, 520 with through notches 522, 524 and 526, 528 on the ends of the respective slots. In addition to the notches 522, 524 and 526, 528, the deformable portions of the beam are defined by flats S₅ and S₆ and by another pair of flats that are invisible in Fig. 5 and are located on the sides of the round beam opposite to the flats S and S₆. In particular, the flat S₅,S₆ and two other flats impart to the beam flexibility additional to that provided by the notches 526, 528 for deformation under effect of a force acting in the direction of axes X and Y. Another function of the flats is to serve for convenient attachment of strain gauges 538 and 540. The round cross-section of the beam simplifies the construction and manufacturing of the sensor 510 and reduces its cost, whereas provision of the flats increases sensitivity of the beam to forces being measured. The remaining elements of the sensor 510 and principle of operation are the same as in connection with the sensor 200 of Fig. 3, therefore, their description is omitted

Operation of the device of the invention will now be described with reference to Fig. 6, which is a three-dimensional view illustrating both sensors in connection with forces applied to the sensors and measured by various strain gauges of both sensors.

In the course of testing, the moveable lower specimen 289 is brought into movement, e.g., into rotation, and then a loading force F_x (Fig. 2) is applied to the sensor assembly from the loading unit of the friction system (not shown) via the stationary upper plate 104. Strictly speaking, the upper specimen is not stationary, as it moves slightly together with the lower plate 102 when the beams of the sensors deform. However, for the sake of simplicity these movements of the upper specimen 283 are not taken into consideration and in the context of the present patent application the upper specimen 283 is considered as stationary.

When the upper specimen 283 comes into contact with the moving lower specimen 289, application of force F_x causes interaction between the upper specimen 283 and the lower specimen 289. The aforementioned interaction generates a reaction force F_R and a friction force F R (Fig. 2). These forces deform the beams 212 and 312 of both sensors and hence the strain gauges. Since the sensors are sandwiched between the upper plate 104 and the lower plate 102 and are attached to both plates in manner shown and described in connection with Fig. 2, both sensors 200 and 300 (Fig. 6) are subject to simultaneous equal deformations in two opposite directions for eliminating misbalance created in the measurement system when a single sensor is used. Each sensor deforms in the direction of axis X under the effect of a loading force F_x measured in each sensor by two strain gauges located on opposite sides of the beam near one end of the beam and in direction of axis Y under the effect of a friction force measured in each sensor by another two strain gauges located on opposite sides of the beam, which are perpendicular to the sides of strain gauges for the loading force. Since both sensors 200 and 300 are sandwiched between two plates in diagonally symmetrical positions, they transmit forces between both plates and at the same time ensure limited freedom of movement between the plates to allow deformations caused by the applied forces.

More specifically, due to the provision of the slots 218 and 220 (Fig. 6) with notches 222, 224 and 226, 228, respectively, in the sensor 200 and of the slots 318 and 320 with notches 322, 324 and 326, 328, respectively, in the sensor 300 , the beams 212 and 312 are deformed under effect of force F_x in a X-Z plane as a first parallelograms so that the materials of the beams are stretched on the sides of the resistors R1-1 and R3-1 and are compressed on the side of the resistors R1-2 and R3-2 (Fig. 6).

Friction force F_FR acts in the direction of axis Y (Fig. 6) and deforms the beams 212 and 312 as second parallelograms in Z-Y plane so that the materials of the beams are stretched on the sides of resistors R2-1 and R4-1 and are compressed on the sides of resistors R2-2 and R4-2 (Fig. 6). As the beams deform, the strain gauges also deform. These deformations change resistances of the resistors in the aforementioned bridges of the electric circuit shown in Fig. 4. As a result, resistors R1-1 , R1-2 and R3-1 , R3-2 measure force F_x, whereas resistors R2-1 , R2-2 and R4- 1 , R4-2 measure force F_y.

Due to the fact that each bridge is formed by pairs of resistors one of which always increases in its resistance while the other decreases, or vice verse, the electric circuit shown in Fig. 4 improves the sensitivity of the measurement system approximately by a factor of 2. Furthermore, output signals that correspond to the same force, e.g., force F_x, also are summed to form an electric signal of a doubled magnitude. Thus, electric signals of increased amplitude corresponding to respective forces are supplied to the measuring and registering system (not shown).

The invention has been shown and described with reference to a specific embodiment, which should be construed only as an example and does not limit the scope of practical applications of the invention. Therefore, any changes and modifications in materials, shapes, electric diagrams and their components are possible, provided these changes and modifications do not depart from the scope of the patent claims. For example, the electric bridge circuits shown in Fig. 4 can be circuits operating on a.c. current or on d.c. current. The a.c. bridge can be a resonance type bridge circuit. The strain gauge resistors can be represented by a part of a potentiometric circuit. The resistor-type strain gauges can be replaced by either capacitance or conductance or another type or strain gauges. The sensing elements of the capacitance type can be a part of a dilatometric measurement circuit in which deformation of the gauge proportionally changes capacity of the sensing element. Although the sensors were mentioned for use in measuring a load force and a friction force, it is understood that they can be used for other purposes, such as measuring bending moments in two directions. In association with known masses properly attached to the sensors, the latter can be used as acceleration and velocity sensors. The solid end blocks 214, 216 and 314, 316 can be fixed and attached to fixation and actuating elements in a variety of modes. The lower specimen can perform reciprocating movements. The measurements can be carried out in oil, vacuum, and/or at elevated temperatures. The beams may have an elliptical, or any other cross-section and the direction of applied forces may not necessarily be mutually perpendicular. The slots and notches may have shapes different from those shown in the drawings. The beams themselves can be assembled from several parts. The beams can be formed without notches, i.e., only with two pairs of slots. The deformable beams not necessarily should be solid bodies with the notches and slots and can be formed by four or more deformable rods, plates, or tubes, which together may form two mutually perpendicular parallelograms. The strain gauges contained in one bridge circuit may have any other suitable location, e.g., on the same side of the beam instead of opposite sides of the beam.

Claims

CLAIMS:

1. A device for measuring a first force acting in one direction and a second force acting in a direction which is different from said one direction and is not parallel thereto, said device comprising: a first mounting member; a second mounting member; a first flexible member, one end of which is attached to said first mounting member and the opposite end is attached to said second mounting member; a second flexible member, which is identical to said first flexible member, is arranged parallel thereto and has one end, which corresponds to said one end of said first flexible member, attached to said second mounting member, and the opposite end, which corresponds to said opposite end of said first flexible member, attached to said first mounting member; and deformation sensitive means for measuring deformations of said first flexible member and of said second flexible member in terms of said first force and said second force respectively, said first flexible member and said second flexible member being deformed simultaneously by equal amounts and in mutually opposite directions.

2. The device of Claim 1 , wherein said first mounting member is a first plate, said second mounting member is a second plate, said first flexible member and said second flexible member comprising deformable beams sandwiched between said first plate and said second plate in diagonally symmetrical positions with respect to each other so that one end of each of said deformable members is attached to one of said plates and the opposite end to the other of said plates.

3. The device of Claim 2, wherein each of said deformable beams comprises: a first pair of deformable portions for deforming said deformable beam in said one direction and a second pair of deformable portions for deforming said deformable beam in said direction which is different from said one direction; a first pair of deformation sensitive elements attached to opposite sides of one of said deformable portions of said first pair and a second pair of deformation sensitive elements attached to opposite sides of one of said deformable portions of said second pair; said first pair of deformable portions and said second pair of deformable portions being formed in said deformable beam by a first pair of through holes with a first through slot which interconnects said first pair of through holes, said first pair of through holes and said first through slot passing through said deformable beam in said direction different from said one direction, and by a second pair of through holes with a second through slot which interconnects said second pair of said through holes, said second through holes and said second through slot passing through said deformable beam in said one direction; each through hole of said first pair of through holes and of said second pair of through holes having a longitudinal axis; said first pair of said through holes comprising a first through hole located close to one end of said deformable beam and having its respective longitudinal axis in said direction different from said one direction and a second through hole located close to the end of said deformable beam opposite to said one end and having its respective longitudinal axis in said direction different from said one direction; said second pair of said through holes comprising a third through hole located close to one end of said deformable beam and having its respective longitudinal axis in said one direction and a fourth through hole located close to the end of said deformable beam opposite to said one end and having its respective longitudinal axis in said one direction; said second through hole and said third through hole being located between said first through hole and said fourth through hole; said second through hole being located between said third through hole and said fourth through hole; said third through hole being located between said first through hole and said second through hole.

4. The device of Claim 3, wherein each of said deformable beams further comprises a first beam attachment means connected to said one end of said deformable beam and a second attachment means connected to said end opposite to said one end.

5. The device of Claim 1 , wherein each of said deformation sensitive means comprises a strain gauge.

6. The device of Claim 3, wherein each of said deformation sensitive elements comprises a strain gauge.

7. The device of Claim 1 , wherein said one direction and said direction different from said one direction are mutually perpendicular directions.

8. The device of Claim 6, wherein said one direction and said direction different from said one direction are mutually perpendicular directions.

9. The device of Claim 2, wherein each of said deformable beams has a cross section selected from the group consisting of a rectangular cross section, square cross section and round cross section.

10. The device of Claim 1 , wherein each of said flexible members has a longitudinal axis and comprises: a first parallelogram deformable in the direction of said one force; a second parallelogram deformable in said direction different from said one direction, said first parallelogram being at least partially overlapped with said second parallelogram in the direction of said longitudinal axis; said first parallelogram being formed at least by a first through slot passing through said flexible member in said direction different from said one direction, and said second parallelogram is formed at least by a second through slot passing through said flexible member in said one

. direction.

11. The device of Claim 10, wherein each of said flexible members further comprises a first pair of through notches which are wider than said first slot and which are connected to both ends of said first slot and a second pair of through notches which are wider than said second slot and which are connected to both ends of said second slot.

12. The device of Claim 10, wherein in each of said flexible members said deformation sensitive means comprise: a first pair of strain gauges located at one end of said first through slot and on opposite sides of said first parallelogram for measuring said one force; and a second pair of strain gauges located at the end of said second through slot opposite to said one end and on the opposite sides of said second parallelogram for measuring said second force.

13. The device of Claim 12, further comprising an electric circuit, wherein in each of said flexible members said first pair of strain gauges forms a first bridge for measuring said first force, whereas said second pair of strain gauges forms a second bridge for measuring said second force.

14. A method of measuring a first force acting in one direction and a second force acting in a second direction which is not parallel to and is different from said one direction, said method comprising: providing a bi-directional force measurement device having a first plate, a second plate, a first deformable member and a second deformable member sandwiched between said first plate and said second plate in diagonally symmetrical positions with respect to each other so that one end of each of said deformable members is attached to one of said plates and the opposite end to the other of said plates; providing each of said deformable members with first measuring means for measuring said first force and with second measurement means for measuring said second force; applying said first force to a plate selected from the group consisting of said first plate and said second plate; applying said second force to a plate selected from the group consisting of said first plate and said second plate; and measuring said first force and said second force simultaneously in both of said deformable members.

15. The method of Claim 14, wherein said bidirectional force measurement device is a part of a friction measurement system having a loading unit, a first specimen, and a second specimen, said first force being a loading force and said second force being a friction force, said method further comprising: attaching said first plate to said loading unit of said friction system; attaching a first specimen to said second plate; causing a relative movement between said first specimen and said second specimen while applying said loading force to said first plate thus developing said friction force; causing deformations of said deformable members under effect of said loading force and said friction force; and measuring deformations in terms of said loading force and said friction force, respectively.

16. The method of Claim 15, comprising: forming each of said deformable members in the form of a first deformable parallelogram deformable under the effect of said loading force in said one direction and a second deformable parallelogram deformable under the effect of said friction force in said second direction, said first deformable parallelogram and said second deformable parallelogram being at least partially overlapped.

17. A device for measuring a first force acting in one direction and a second force acting in a direction which is different from said one direction and is not parallel thereto, said device comprises: a deformable beam having a first pair of deformable portions for deforming said deformable beam in said one direction and a second pair of deformable portions for deforming said deformable beam in said direction different from said one direction; a deformation sensitive means comprising a first pair of deformation sensitive elements attached to opposite sides on one of said deformable portions of said first pair and a second pair of deformation sensitive elements attached to opposite sides of another of said deformable portions of said second pair; a first pair of deformable portions and said second pair of deformable portions being formed in said deformable beam by a first pair of through holes with a first through slot which interconnects said first pair of through holes, said first pair of through holes and said first through slot passing through said deformable beam in one direction, and by a second pair of through holes with a second through slot which interconnects said second pair of said through holes, said second through holes and said second through slot passing through said deformable beam in a second direction which is different from said first direction and is not parallel thereto; each through hole of said first pair of through holes and of said second pair of through holes having a longitudinal axis; a first pair of said through holes comprising a first through hole located close to one end of said deformable beam and having its respective longitudinal axis in said one direction and a second through hole located close to the end of said deformable beam opposite to said one end and having its respective longitudinal axis in said another direction; a second pair of said through holes comprising a third through hole located close to one end of said deformable beam and having its respective longitudinal axis in said another direction and a fourth through hole located close to the end of said deformable beam opposite to said one end and having its respective longitudinal axis in said another direction; a second through hole and said third through hole being located between said first through hole and said fourth through hole.

18. The device of Claim 17, wherein said deformable beam further comprises a first beam attachment means connected to said one end of said deformable beam and a second attachment means connected to said end opposite to said one end.

19. The device of Claim 17, wherein said first deformation sensitive element and said second deformation sensitive element are strain gauges.

20. The device of Claim 18, wherein said first deformation sensitive element and said second deformation sensitive element are strain gauges.

21. The device of Claim 17, wherein said one direction and said direction different from said one direction are mutually perpendicular directions.

22. The device of Claim 18, wherein said one direction and said direction different from said one direction are mutually perpendicular directions.

23. The device of Claim 17, wherein said beam has a cross section selected from the group consisting of a rectangular cross section, a square cross section and a round cross section.

24. The device of Claim 18, wherein said beam has a cross section selected from the group consisting of a rectangular cross section, a square cross section and a round cross section.

25. The device of Claim 17, further comprising: a deformable beam having a longitudinal axis and comprising a first parallelogram deformable in the direction of a first axis and a second parallelogram deformable in the direction of a second axis which is different from the direction of said first axis and is not parallel thereto, said first parallelogram being at least partially overlapped with said second parallelogram in the direction of said longitudinal axis; and a first pair of deformation sensitive means for sensing deformations of said deformable beam in the direction of said first axis; and a second pair of deformation sensitive means for sensing deformations of said deformable beam in the direction of said second axis.

26. The device of Claim 25, where said beam is a solid body, said first parallelogram is formed at least by a first through slot passing through said beam in the direction of said first axis, and said second parallelogram is formed at least by a second through slot passing through said beam in the direction of said second axis.

27. The device of Claim 26, further comprising a first deformable portion on said one end of said deformable beam and a second deformable portion on said end opposite to said one end, said first deformable portion and said second deformable portion being formed by a first pair of through notches which are wider than said first slot and which are connected to both ends of said first slot, and by a second deformable portion formed by a pair of through notches which are wider than said second slot and which are connected to both ends of said second slot.

28. The device of Claim 27, wherein said deformation sensitive means are strain gauges comprising a first strain gauge attached to said first deformable portion on one side of said deformable beam, a second strain gauge attached to said first deformable portion on the side of said deformable beam opposite to said one side, a third strain gauge attached to said second deformable portion on said one side, and a fourth strain gauge attached to said second deformable portion on the side of said deformable beam opposite to said one side.

29. The device of Claim 28, wherein said first strain gauge and said second strain gauge are electrically connected to a first electric bridge circuit, and said third strain gauge and said fourth strain gauge are electrically connected to a second electric bridge circuit.

30. The device of Claim 25, wherein said deformable beam further comprises a first beam attachment means connected to said one end of said deformable beam and a second attachment means connected to said end opposite to said one end.

31. The device of Claim 27, wherein said deformable beam further comprises a first beam attachment means connected to said one end of said deformable beam and a second attachment means connected to said end opposite to said one end.

32. A method of measuring a force in a system provided with a deformable beam for transmitting a loading force from a loading unit of said friction system to a probe which is in contact with a test material, said deformable beam having a first pair of deformation sensitive elements on one end of said deformable beam and a second pair of deformation sensitive elements on the end of said beam opposite to said one end, said method comprising: forming said deformable beam in the form of a first deformable parallelogram deformable under the effect of said loading force in a first direction and a second deformable parallelogram deformable under the effect of said friction force in a second direction which is different from said first direction and is not parallel thereto, said first deformable parallelogram and said second deformable parallelogram being at least partially overlapped; providing said first deformable parallelogram with a first pair of deformable portions; providing said second deformable parallelogram with a second pair of deformable portions; attaching a first pair of deformation sensing elements to said first pair of deformable portions, respectively; attaching a second pair of deformation sensitive elements to said second pair of deformable portions, respectively; applying to said deformable beam said loading force and developing a friction force thus deforming said deformable beam simultaneously in said first direction and in said second direction; and measuring the deformation of said deformable beam in said first direction and in said second direction for determining said friction force in terms of the deformation in said second direction.