WO2020120226A1

WO2020120226A1 - Sensor for sensing force normal to a surface

Info

Publication number: WO2020120226A1
Application number: PCT/EP2019/083482
Authority: WO
Inventors: Philippe Garrec
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2018-12-13
Filing date: 2019-12-03
Publication date: 2020-06-18
Also published as: FR3090101B1; FR3090101A1

Abstract

Disclosed is a normal force sensor (50) comprising: - a plate (51); - a measuring cell (52); - a first segment (53) comprising a first bearing point (O) bearing on the plate (51) and a second bearing point (A) bearing on the measuring cell; - a second segment (54) bearing on the plate (51) and on the first segment (53); - a third segment (55) bearing on the first segment (53) and on the second segment (54). Also disclosed are a bearing element comprising such a sensor and an exoskeleton comprising such a bearing element.

Description

Force sensor normal to a surface

FIELD OF THE INVENTION

The invention relates to the field of force sensors and more particularly force sensors.

BACKGROUND OF THE INVENTION

A normal force sensor generally comprises a measurement cell comprising an element sensitive to a specific force or distributed over a determined surface, generally of size less than one square centimeter. For the measurement of a force force normal applied on a larger surface such as a segment of exoskeleton foot on which rests a shoe sole, it is common to use a device comprising a plurality of distributed sensors under the sole and add the measurements. This type of device is expensive and has a high sensitivity to the position of the fulcrum on the sole. Thus, in walking situations during which the point of support of the sole on the foot segment moves, the accuracy of the measurement varies and an assistance command which would use the pressure measurement on the sole would be imprecise. or even unsuitable.

OBJECT OF THE INVENTION

ün object of the invention is to improve the accuracy of a normal force sensor.

SUMMARY OF THE INVENTION

For this purpose, a normal force sensor is provided comprising a plate, a measuring cell comprising an element sensitive to a point force, a first segment comprising a first support point bearing on the plate and a second support point supported by the sensitive element of the measurement cell, a second segment comprising a third support point bearing on the plate and a fourth support point bearing on the first segment, a third segment comprising a fifth support point bearing on the first segment and a sixth support point in press on the second segment. The orthogonal projection of the fifth fulcrum on a straight line connecting the first fulcrum and the third fulcrum is separated from the first fulcrum by a first distance. The orthogonal projection of the fourth support point on a straight line connecting the first support point and the third support point is separated from the first support point by a second distance. The orthogonal projection of the sixth fulcrum on a straight line connecting the first fulcrum and the third fulcrum is separated from the third fulcrum by a third distance. The orthogonal projection of the fourth support point on a straight line connecting the first support point and the third support point is separated from the third support point by a fourth distance. According to the invention, the first connection point as well as the second, third, fourth, fifth and sixth support points being arranged so that the first ratio of the first distance to the second distance is more or less ten percent to the second ratio of the third distance over the fourth distance.

A force, or component, of normal force applied to the third segment produced by a uniformly distributed pressure, a non-uniformly distributed pressure, a point force or a case of complex load in which these different types of force are combined, causes a signal emitted from the measurement cell representative of the resultant normally applied to the third segment and of which the value is weakly correlated with the point of application of the force.

The accuracy of the measurement is further improved when the first ratio equals plus or minus one percent to the second ratio.

A total decorrelation of the signal from the measurement cell and the point of application of the force is obtained when the first ratio is strictly equal to the second ratio.

Advantageously, the first point of connection with the plate is an articulation or an embedding.

According to a particular embodiment, the third segment is connected to the plate.

The forces to which the measuring cell is subjected can be reduced when the sensor comprises means for preloading the third segment.

The realization of the sensor is particularly economical when the precharging means comprise a screw. The preloading means can also include an elastic element.

The invention also relates to a support element comprising a normal force sensor as described above and an exoskeleton comprising such a support element.

Other characteristics and advantages of the invention will emerge from the following description of particular non-limiting embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to the appended figures, among which:

[Fig. 1] Figure 1 is a schematic perspective view of an exoskeleton according to the invention; [Fig. 2] Figure 2 is a schematic detail view of a support element according to the invention;

[Fig. 3] Figure 3 is a schematic sectional view of a sensor according to the invention;

[Fig. 4] Figure 4 is a schematic top view of the sensor of the invention.

DETAILED DESCRIPTION OF THE INVENTION With reference to FIGS. 1 and 2, an exoskeleton, generally designated 1, is carried by an operator 100 shown here in dotted lines. The exoskeleton 1 comprises a pelvis element 10 situated substantially at the height of the pelvis of the operator 100 and connected to the latter by a belt 2. A payload can also be supported by the pelvis element 10.

A lower left member 20 and a lower right member 30 of the exoskeleton 1 are respectively articulated on two ends 11 and 12 of the pelvis element 10 and extend substantially parallel to the lower members 120 and 130 of the operator 100. The left lower limb 20 of the exoskeleton 1 comprises a thigh segment 21 articulated to a leg segment 22 itself articulated to a support element 23 on which the operator's left foot 123 rests. Symmetrically, the right lower limb 30 of the exoskeleton 1 comprises a thigh segment 31 articulated to a leg segment 32 itself articulated to a support element 33 on which the operator's right foot 133 rests. All the articulations of the exoskeleton 1 are motorized and controlled by a control unit 3.

The support element 33 of the right lower limb 30 is identical to the support element 23 of the left lower limb 20.

Referring to Figures 2 to 4, the support element 23 is a sole 40 of a shoe 41 (shown in dotted lines) and comprises a pressure sensor 50 comprising a plate 51 on which is mounted a piezoresistive gauge 52 connected to the control unit 3. A first segment 53 extends parallel to the plate 51 and is screwed into a first support point O on the plate 51. The first segment 53 also includes a second point A bearing on the gauge 52. A second segment 54 parallel to the first segment 53 is screwed into a third support point B with the plate 51 and includes a fourth point C bearing on the first segment 53. A third segment 55 parallel to the second segment 54 and suspended between a first screw 56 and a second screw 57 screwed into the plate 51 comprises a fifth point D bearing on the first segment 53 and a sixth point E bearing on the second segment 54. A first stack 58.1 of Belleville washers 59.1 stretches re the head 56.1 of the first screw 56 and the upper surface of the third segment 55. A second stack 58.2 of Belleville washers 59.2 extends between the head 57.1 of the second screw 57 and the upper surface of the third segment 55.

The first segment 53, the second segment 54 and third segment 55 are here made of stainless steel.

As shown in Figure 3 and 4:

- the orthogonal projection D 'of the fifth point D on a straight line (OB) connecting the first support point O and the third support point B is separated from the first support point by a first distance doo';

- the orthogonal projection C 'of the fourth support point C on a straight line (OB) connecting the first support point 0 and the third support point B is separated from the first support point 0 by a second distance doc ";

- the orthogonal projection E 'of the sixth support point E on a straight line (OB) connecting the first support point 0 and the third support point B is separated from the third support point B by a third distance from the * , ·

- the orthogonal projection E 'of the sixth support point E on a straight line (OB) connecting the first support point 0 and the third support point B is separated from the third support point B by a third distance das ” ;

- the orthogonal projection C 'of the fourth support point C on a straight line (OB) connecting the first support point 0 and the third support point B is separated from the third support point B by a fourth distance dac' .

As visible in FIGS. 3 and 4, the first support point O, the second support point A, the third support point B, the fourth support point C, the fifth support point D and the sixth fulcrum E are arranged so that the first ratio Ri of the first distance don 'over the second distance doc' is equal to the second ratio Rz of the third distance <W over the fourth distance dac '.

The relation can be written in the following form:

Where k is a proportionality coefficient, here equal to one.

Thus for a distance doa equal to twenty-seven centimeters and which corresponds to a foot in size forty-two, the following distances are obtained:

When the values of the first, second, third and fourth distances doo ', doc *, cW, dsc' respect the relation 4 with a coefficient of proportionality equal to 1, the signal returned by the gauge 52 is invariant whatever the point d application of the same force Z on the third segment 55. Thus, in the case of the application of a uniformly distributed pressure, a non-uniformly distributed pressure, a point force or a case of complex load in which these different types of force are combined _/ the pressure sensor 50 delivers a signal 59 representative of the resultant normally applied to the third segment 55 and the value of which is decorrelated from the distribution of the load Z. The control unit 3 processes the signal 59 and converts it into a force value applied to the gauge 52. The control unit 3 then compensates for the preload applied to the third segment 55 by the first stack 58.1 of Belleville washers 59.1 and the second stack 58.2 of Belleville washers 59.2. On the basis of the processed signal 60 thus obtained from the support element 23 of the left lower limb 20 (and possibly of an additional signal coming from the support element 33 of the right lower limb 30), the unit of command 3 determines a set of commands for the motorizations of the joints of exoskeleton 1. The sensor 50 provides acceptable accuracy when the arrangement of the first support point O, the second support point A, the third support point B, the fourth support point C, the fifth support point D and of the sixth point of support E is such that the proportionality coefficient k is between 0.9 and 1.1, that is to say that the first ratio Ri is equal to plus or minus ten percent to the second ratio R2.

The signals of gauge 52 are easier to process when the proportionality coefficient k is between 0.99 and 1.01, that is to say that the first ratio Ri is equal to plus or minus one percent to the second ratio R2. Of course, the invention is not limited to the embodiments described but encompasses any variant coming within the scope of the invention as defined by the claims.

In particular,

- although here the sensor comprises a piezoresistive gauge, the invention also applies to other types of measurement cell comprising an element sensitive to a point force such as for example a capacitive sensor comprising a membrane whose position is linked to a force applied to it, an inductive sensor comprising an electrode whose position is sensitive to a force applied to it or a displacement sensor coupled to a spring;

- although here the first segment, the second segment and the third segment are parallel to each other and parallel to the plate, the invention also applies to other configurations of the first, second, third segments such as for example non-rectilinear segments and / or not parallel to each other;

- although here the first segment and the second are screwed onto the plate, the invention applies to other means of establishing a first and / or a second support point embedded with the plate, such as for example a welded, riveted or glued connection point;

- Although here the first segment is screwed onto the plate, the invention applies to other types of support on the plate, such as for example an articulated or pivoting support;

- although here the third segment is suspended between a first screw and a second screw screwed into the plate, the invention also applies to other means of connecting the third segment to the plate as for example using bolts, support spring or sliding columns;

- although itii the third segment is preloaded using a first and a second stack of Belleville washers on a screw, the invention also applies to other means of preloading the third segment as for example of coil springs, embedding of the third segment on the first and second screws or means of preloading between two clamps integral or not with the plate;

- although here the preloading means comprise a first and a second Belleville washer stack, the invention also applies to other types of elastic elements such as, for example, coil springs, an elastomer block, springs with gas;

- although here the first, second and third segments are made of stainless steel, the invention also applies to other types of materials such as for example composite material or steel;

- although here the third segment is connected to the plate the invention also applies to other types of mounting of the third segment in the sensor such as for example a third segment resting exclusively on the fifth and the sixth point of support.

Claims

1. Normal force sensor (50) comprising:

- a plate (51);

- a measurement cell (52) comprising an element sensitive to a point force and which is mounted on the plate (51);

- A first segment (53) comprising a first support point (O) bearing on the plate (51) and a second support point (A) bearing on the sensitive element of the measuring cell (52) ;

- A second segment (54) comprising a third support point (B) bearing on the plate (51) and a fourth support point (C) bearing on the first segment (53);

- A third segment (55) comprising a fifth support point (D) bearing on the first segment (53) and a sixth support point (E) bearing on the second segment (54);

the orthogonal projection (D ') of the fifth support point (D) on a straight line connecting the first support point (O) and the third support point (B) is separated from the first support point (0) by a first distance (don ');

the orthogonal projection (C ') of the fourth support point (C) on a straight line connecting the first support point (O) and the third support point (B) is separated from the first support point (0) by a second distance (doc ');

the orthogonal projection (E ') of the sixth support point (E) on a straight line connecting the first support point (O) and the third support point (B) is separated from the third support point (B) by a third distance (dra ”);

the orthogonal projection (C ') of the fourth support point (C) on a straight line connecting the first support point (0) and the third support point (B) is separated from the third point support (B) by a fourth distance (decO;

the first connection point (O) as well as the second, third, fourth, fifth and sixth support points (A, B, C, D, E) being arranged so that the first ratio (Ri) of the first distance (doo ”) on the second distance (doc ·) is more or less ten percent to the second ratio (Ra) of the third distance (of _E ') on the fourth distance (d _{BB *} ) ·

2. Normal force sensor (50) according to claim 1, wherein the first ratio (Ri) is equal to plus or minus one percent to the second ratio (R2).

3. Normal force sensor (50) according to any one of the preceding claims, in which the first fulcrum (O) with the plate (51) is a hinge.

4. Normal force sensor (50) according to one of claims 1 or 2, wherein the first fulcrum (O) with the plate (51) is a recess.

5. Normal force sensor (50) according to any one of the preceding claims, in which the third segment (55) is connected to the plate (50).

6. Normal force sensor (50) according to any one of the preceding claims, comprising preloading means (56, 57, 58.1, 58.2,59.1, 59.2) of the third segment (55).

7. Normal force sensor (50) according to claim 6, wherein the preloading means (56, 57, 58.1, 58.2, 59.1, 59.2) comprise a screw (56, 57).

8. Normal force sensor (50) according to claim

6 or 7, in which the preloading means (56, 57, 58.1, 58.2, 59.1, 59.2) comprise an elastic element (59.1,

59.2).

9. Support element (10) comprising a normal force sensor (50) according to any one of the preceding claims.

10. Exoskeleton (1) comprising a support element (10) according to claim 9.