Force measurement sensor
A force measurement sensor includes a peripheral portion, a central portion, and a first frame connecting an upper section of the peripheral portion to an upper portion of the central portion. The central portion includes a seat intended to receive the outer ring of a bearing in order to mount a shaft in rotation about an axis Z. The frame includes a core enclosed by two arms which are substantially parallel to one another, such that the upper section of the peripheral portion, the upper portion of the central portion, and the two arms form a parallelogram.
1. A force sensor comprising:
a peripheral portion, a central portion, and a first frame linking a top section of the peripheral portion to a top portion of the central portion,
said central portion comprising a seat configured to receive an outer ring of a bearing in order to mount a shaft rotating about an axis (Z),
wherein the frame comprises a web plate bracketed by two arms that are substantially parallel to one another such that said top section of the peripheral portion, said top portion of the central portion, and the two arms form a parallelogram, and
wherein a bellows links the peripheral portion to the central portion over the entire circumference apart from the zones occupied by the frame.
2. The force sensor as claimed in claim 1 , wherein the thickness of the web plate, on the axis (Z), is less than 50% of the thickness of the arms.
3. The force sensor as claimed in claim 1 , wherein the web plate is in contact with the two aims, with the top section, and with the top portion.
4. The force sensor as claimed in claim 1 , wherein the web plate, the two aims, the top section, and the top portion form part of one and a same piece produced in a single material.
5. The force sensor as claimed in claim 1 , wherein the web plate is planar and is oriented on a radial plane, at right angles to the axis (Z).
6. The force sensor as claimed in claim 1 , wherein, on the axis (Z), the web plate occupies a central position with respect to the arms.
7. The force sensor according to claim 1 , wherein at least one strain gauge is glued onto one of the faces of the web plate.
8. The force sensor according to claim 1 , wherein four strain gauges are glued onto the web plate.
9. The force sensor as claimed in claim 1 , wherein a second frame, which is substantially identical to the first frame, links the peripheral portion to the central portion, and the first frame and the second frame are diametrically opposed.
10. The force sensor as claimed in claim 1 , wherein the bellows comprises an inner wall in contact with the central portion, an outer wall in contact with the peripheral portion and a central wall linking the inner wall to the central wall.
11. The force sensor as claimed in claim 10 , wherein the inner wall and the outer wall are substantially planar and oriented on a radial plane, at right angles to the axis (Z), and the central wall is substantially cylindrical.
12. The force sensor as claimed in claim 10 , wherein a second frame, which is substantially identical to the first frame, links the peripheral portion to the central portion, and the first frame and the second frame are diametrically opposed along an axis (Y′).
13. A crankset assembly to be mounted on a bicycle, said assembly comprising:
at least one right-hand bearing for mounting a crankset spindle; and
the force sensor as claimed in claim 12 , placed around the outer ring of said bearing,
wherein the axis (Y′) forms an angle (ϕ) of between 10° and 30° with a vertical axis (Y) and the first frame is placed behind the vertical axis (Y).
14. The assembly as claimed in claim 13 , wherein the angle (ϕ) is between 20° and 25°.
15. The assembly as claimed in claim 14 , wherein four strain gauges are placed on the first frame.
16. A method for controlling an electric assist motor for a bicycle, comprising:
equipping said bicycle with the crankset assembly as claimed in claim 13 , wherein a signal (S) generated by the force sensor corresponds to the component, on an axis (X′, X′), that is at right angles to (Y′) and to (Z), of the action Rright being exerted on the right-hand bearing, the component being denoted: RRx′ Total.
17. The method as claimed in claim 16 , wherein the part of the component RRx′ Total, which is generated by the cyclist, RRx′ Cyclist, is equal to:
RRX′Cyclist=RRX′Total− K 1· K 2· I
in which, I is the current consumed by the assist motor; K1 is the constant of torque of the assist motor and K2 is a factor dependent on the number of teeth of a chain ring meshing with the transmission chain.
18. The method as claimed in claim 17 , wherein, on each pedal revolution, the minimum value of the component RRx′ min is stored in memory; at each instant, the minimum value is subtracted from the portion of the component generated by the cyclist RRx′ Cyclist; and an averaging operation is performed over a multiple of half-revolutions of the crankset:
Δ
R
Rx
′
Cyclist
_
(
β
)
=
2
n
∑
i
=
1
i
=
n
/
2
Δ
R
Rx
′
Cyclist
(
β
-
2
π
i
n
)
in which n is the number of sampling values.
19. The force sensor as claimed in claim 1 , wherein the thickness of walls of the bellows is between 0.5 mm and 0.8 mm.