IT201800005292A1 - STRESS / DEFORMATION DETECTOR FOR A COMPONENT OF A BICYCLE TRANSMISSION - Google Patents

Description

STRESS / DEFORMATION DETECTOR FOR A COMPONENT

OF A BICYCLE TRANSMISSION

The invention relates to a stress / deformation detector for a component of a bicycle transmission.

The general knowledge in the field of stress detection in a bar, as typically used in the context of torque measurement applied to a crank arm of a bicycle crank, typically involves the use of two strain gauges, one positioned on one side with respect to the plane crank arm with respect to the useful pedaling force component and the other positioned on the opposite side with respect to the neutral plane.

In the present description and in the attached claims, as well as in general in the mechanical sector, the term "neutral axis" is intended to indicate the geometric location of the points in which the stresses normal to the cross section of the solid - in this case the pedal crank - are zero. By "neutral plane" we mean the geometric location of the points belonging to the neutral axis of each cross section, which in practice can also deviate from a geometric plane.

Considering the crank arm with the pedal axis in the forward position (in the direction of travel) with respect to the axis of rotation and therefore in the "downstroke" (the most effective part of the pedaling cycle), the strain gauge which is in the top position it is subject to and detects an expansion or elongation, while the strain gauge which results in the lower position detects a contraction or compression when the cyclist applies a force on the pedal.

The general knowledge in the field of strain measurement by strain gauge provides that in such a configuration a Wheatstone bridge in half-bridge configuration is used, in which two completion resistors are provided in one branch, while the two are arranged in the other branch. strain gauges, top and bottom, such as active resistors or unknown resistances.

In the case of other components of the crankset, such as spokes of the right crank arm (more generally on the chain / drive belt side) or the bottom bracket pin or the freewheel body of a monolithic sprocket set associated with the rear wheel hub, it is similarly known to use two strain gauges (or two pairs of strain gauges) positioned in two different positions, such that when one is subject to and detects an expansion or elongation, the other detects a contraction or compression.

Furthermore, it is generally known to provide, in each measurement position, a further strain gauge whose tracks are oriented parallel to the tracks of the active strain gauge, the role of this parallel strain gauge being to increase the accuracy of the measurement reading. ; or an additional strain gauge whose tracks are oriented at 90 ° with respect to the tracks of the active strain gauge, the role of this orthogonal strain gauge being to compensate for the resistance variations in the active strain gauge caused by temperature variations and / or to increase accuracy of the measurement reading, detecting the lengthening / shortening due to the Poisson effect.

The general knowledge in the field of strain gauge stress detection provides that when a total of four strain gauges are provided, a Wheatstone full bridge configuration is used, in which each resistor is represented by one of the aforementioned strain gauges.

Stress / deformation detection systems for bicycle components that follow the general knowledge described above are described for example in US2016 / 0031523A1, US8006574B2, US9488668B2, US9459167B2, US2016 / 0003696A1, US9097598B, US2015 / 0355042A1, EP1978342A2.

US7647837B2 and EP1407239B1 describe temperature-compensated strain gauge detection systems, without however any reference to the application to bicycles. In both cases, a temperature sensor is arranged at each of four strain gauges connected in a Wheatstone bridge in full bridge configuration. The four temperature sensors are connected in a second Wheatstone bridge in full bridge configuration, which is connected in series or in parallel with the strain gauge bridge, so as to directly compensate for the apparent strain of the strain gauges due to temperature variations. Alternatively, the compensation can be made through calculations on the outputs of the two bridges.

The Applicant has perceived that in the case of a bicycle transmission component, in particular of a pedal crank, in conditions of high speed running, the air flow that strikes the pedal crank can cool the side exposed to the opposite wind even by some degree centigrade with respect to the unexposed side. As a result, two strain gauges positioned at even a small distance from each other can be in use at a different operating temperature.

This is particularly true in the case of cranks or other components made of composite material comprising structural fiber embedded in a matrix of polymeric material, due to the poor thermal conductivity.

Furthermore, in the case of composite material, the thermal expansion is strongly dependent on the local characteristics of the material, such as actual orientation of the fiber, effective density of the fiber, effective density of the polymeric material, etc., as well as of course on the local geometric characteristics such as wall thickness. and similar.

Therefore, the apparent strain component (due to temperature) detected locally by the two strain gauges positioned in two different measurement positions or regions of the bicycle component - for example on the two sides of the neutral plane with respect to a bending strain to be measured - is different ( giving rise to an overall non-zero apparent deformation) and, consequently, the variation in resistance in the two strain gauges is not equal and opposite, giving rise to a false output of the Wheatstone bridge in the aforementioned conventional configurations.

The technical problem underlying the invention is that of providing a stress / deformation detector in a component of a bicycle transmission, in particular in a crank arm, which is as accurate as possible and as least sensitive as possible to changes in temperature. , so as to be particularly suitable for a component made of composite material.

In one aspect, the invention relates to a stress / strain detector applied to a monolithic component of a bicycle transmission, comprising a first strain gauge positioned in a first region of the monolithic component and a second strain gauge positioned in a second region of the monolithic component, characterized by comprising a first Wheatstone bridge for reading the output of the first strain gauge and a second Wheatstone bridge for reading the output of the second strain gauge, in which each of the first and second Wheatstone bridges is in quarter configuration bridge, or in which the first Wheatstone bridge is in a half-bridge configuration comprising, in addition to the first strain gauge, a third strain gauge and the second Wheatstone bridge is in a half-bridge configuration comprising, in addition to the second strain gauge, a fourth strain gauge.

In the present description and in the attached claims, a "monolithic" component is intended to indicate a structure that is also complex, but made up of parts joined together and not detachable without damaging at least one of them.

Preferably, the first region and the second region are selected in such a way that when the first strain gauge is mainly stressed in tension, the second strain gauge is stressed mainly in compression and vice versa.

Preferably, the first and second regions of the monolithic component are close enough to consider the strain gauges subject to substantially equal and opposite deformations, but sufficiently distant to be found at different temperatures.

Preferably, the first and second Wheatstone bridges share the neutral branch comprising two completion resistors of known value and substantially insensitive to deformation and temperature.

Preferably the third strain gauge is oriented (that is to say, has its detection direction) at 90 ° with respect to the first strain gauge in Poisson configuration and / or the fourth strain gauge is oriented at 90 ° with respect to the second strain gauge in Poisson configuration.

Alternatively, the third strain gauge oriented parallel to the first strain gauge and / or the fourth strain gauge is oriented parallel to the second strain gauge, so as to increase the detection sensitivity of the same stress.

More preferably, in both cases the third strain gauge is positioned in the first region of the monolithic component and the fourth strain gauge is positioned in the second region of the monolithic component.

Mixed configurations are conceivable in which two strain gauges in Poisson configuration are provided in one of the first and second regions of the monolithic component and two parallel oriented strain gauges are provided in the other between the first and second regions of the monolithic component.

Preferably, the stress / strain detector further has a first temperature sensor thermally coupled to at least one of the first strain gauge and the second strain gauge, a reading circuit of the first temperature sensor being provided and processing means configured to perform a temperature compensation of the reading of said at least one of the first strain gauge and the second strain gauge on the basis of the reading provided by the reading circuit.

In the present description and in the attached claims, the expression "thermally coupled" is intended to indicate that the two elements are in positions whose mutual distance is such that in use they have a temperature difference not exceeding a tenth of a degree centigrade .

Advantageously, in this case the numerical ratio between temperature sensors and strain gauges is less than 1: 1. This represents an inventive measure in itself, which can also be used independently of the provision of one of the aforementioned alternative configurations of the two Wheatstone bridges.

Alternatively, the stress / strain detector further has a first temperature sensor positioned at the first region of the monolithic component, thermally coupled to the first strain gauge and a second temperature sensor positioned at the second region of the monolithic component, thermally coupled to the second strain gauge, being provided a reading circuit of the first temperature sensor and a reading circuit of the second temperature sensor and processing means configured to perform a temperature compensation of the reading of the first strain gauge and / or of the second strain gauge on the basis of the reading supplied by the first and second reading circuit.

Preferably, said processing means are configured to perform a temperature compensation of the reading of the first strain gauge on the basis of the reading of the output of the first temperature sensor and a temperature compensation of the reading of the second strain gauge on the basis of the reading of the output of the second. temperature sensor.

The bicycle transmission component can be selected from the group consisting of a pedal crank, a race of a pedal crank on the transmission side, a bottom bracket pin, a freewheel body of a monolithic sprocket set, preferably being a pedal crank.

For the sake of brevity, the expression "from the transmission side" will hereinafter be sometimes simplified by the specific term "from the chain side" and sometimes further simplified by the specific term "right", meaning that a belt transmission and also a left crank arm are always included in the case atypical mounting of the transmission. Similarly, the expression "from the side opposite to the transmission side" will sometimes be simplified to "from the side opposite to the chain side" and sometimes further simplified by the specific term "left", meaning that a belt transmission and also a right crank arm are always included in the case of atypical mounting of the transmission.

In the case of a crank arm, the first strain gauge and the second strain gauge are positioned on opposite sides with respect to a plane including the pedal axis and the rotation axis of the crank.

The invention can also be applied to a symmetrical detection system, comprising two subsystems made at each crank arm, or to a non-symmetrical detection system, comprising a subsystem at a crank arm and the other subsystem at the pivot of the crank arm. bottom bracket or a detection system made in correspondence of a single crank arm.

Preferably the stress / strain detector is a bending moment / bending strain detector.

Typically, the first and second strain gauges are each arranged as far as possible from a neutral axis or plane with respect to a main stress / strain to be detected.

Preferably, the bicycle component is made at least in part of a composite material comprising structural fiber embedded in a matrix of polymeric material.

Preferably, the structural fiber is selected from the group consisting of carbon fibers, glass fibers, boron fibers, synthetic fibers, ceramic fibers and their combinations.

Preferably the synthetic fibers comprise polyoxazole fibers, for example Zylon®, ultra high molecular weight polyethylene fibers, for example Dyneema®, aramid fibers, for example Kevlar fibers and combinations thereof.

The invention also finds advantageous application also in the case of a bicycle component made of metallic material, even if the problem of temperature compensation is less felt.

Preferably at least one of the first strain gauge and the second strain gauge, more preferably both, is oriented according to a fiber direction of the composite material.

Preferably at least one of the first strain gauge and the second strain gauge, more preferably both, is oriented in a plane parallel to the neutral plane with respect to a main stress / strain to be detected, but alternatively it can be oriented in a plane orthogonal to the neutral plane with respect to a main stress / deformation to be detected or in a plane forming any angle with the neutral plane with respect to a main stress / deformation to be detected.

In one aspect, the invention relates to a stress / strain detector applied to a monolithic component of a bicycle transmission, comprising a first strain gauge positioned in a first region of the monolithic component and a second strain gauge positioned in a second region of the monolithic component, characterized by further comprising a first temperature sensor positioned in correspondence with the first region of the monolithic component, thermally coupled to the first strain gauge and a second temperature sensor positioned in correspondence with the second region of the component, thermally coupled to the second strain gauge, a circuit being provided reading of the first temperature sensor and a reading circuit of the second temperature sensor and processing means configured to perform a temperature compensation of the reading of the first strain gauge and / or of the second strain gauge on the basis of the reading for ned by the first and second reading circuit.

Preferably, said processing means are configured to perform a temperature compensation of the reading of the first strain gauge on the basis of the reading of the output of the first temperature sensor and a temperature compensation of the reading of the second strain gauge on the basis of the reading of the output of the second. temperature sensor.

Advantageous characteristics of the stress / deformation detector of the second aspect are those stated with reference to the stress / deformation detector of the first aspect.

Further features and advantages of the invention will be better highlighted by the description of its preferred embodiments, made with reference to the attached drawings, in which:

- FIG. 1 schematically illustrates a bicycle transmission;

- FIG. 2 is a perspective view of a pedal crank and associated pedal, with some reference systems noted;

- FIGS. 3-5 are schematic illustrations of forces acting on a cross section of the crank arm;

- FIG. 6 is a schematic view of a data acquisition system and FIG. 7 is a relative electrical diagram;

- FIG. 8 is a schematic view of another data acquisition system and FIG. 9 is a relative electrical diagram;

- FIGS. 10-12 are schematic views of still other data acquisition systems and FIG. 13 is a relative electrical diagram;

- FIGS. 14-16 are schematic views of still other data acquisition systems and FIG. 17 is a relative wiring diagram; And

- FIG. 18 is a block diagram of a power meter.

FIG. 1 shows a bicycle transmission 10.

A bicycle transmission 10 is a mechanism that converts the motion applied by the cyclist into rotary motion used to move the rear wheel.

A crankset 12 is the component of the transmission 10 of a bicycle which converts the motion applied to the pedals 14, 15 by the cyclist into a rotary motion used to move the drive chain 16 (in other cases, the belt), which in turn moves the rear wheel.

In addition to the crank 12, the transmission 10 also comprises the pedals 14, 15, the aforementioned chain 16 (or belt) and one or more pinions 18 at the hub 20 of the rear wheel.

It should be noted that terminologies slightly different from the one used here are also in use; for example the pedals 14, 15 can be considered part of the crankset.

The crankset 12 generally comprises two cranks 22, 23, each having a pivot end 24, 25 configured for coupling with a pivot 26 of the bottom bracket or axle of the cranks 22, 23 and a free end 28, 29, opposite to the pivot end 24, 25, configured for coupling to pedal 14, 15; as well as at least one toothed crown 30 (three shown by way of example) fixed, integral in rotation, to the pedal crank 22 on the chain side.

Typically, the motion transmission 10 is mounted on the bicycle with the transmission chain 16 (and the crown (s) 30 of the crankset 12 and the pinion (s) 18 at the hub 20 of the rear wheel) on the right side; less than usual it is mounted with transmission chain 16, crown (s) 30 and pinion (s) 18 arranged on the left side of the bicycle.

A component called bottom bracket 32 allows rotation of the pin 26 of the bottom bracket itself with respect to the bicycle frame in at least one direction; in other words, the bottom bracket 32 constitutes the connecting element of the crank 12 to the frame.

The axis of the pivot 26 of the bottom bracket is hereinafter also referred to as the axis of rotation X and is horizontal in the normal running condition of the bicycle, in rectilinear motion on the plane.

In the bottom bracket 32, the pin 26 is supported in rotation around the axis of rotation X through suitable bearings.

For the connection of each pedal 14, 15 to the respective pedal crank 22, 23, suitable rotating connection means are provided which allow the pedal 14, 15 to rotate freely around an axis here called pedal axis Y1, Y2, which in its volta rotates around the axis of rotation X with the crank arm 22, 23.

The connection between the pedal crank 22, 23 and the respective pedal 14, 15 is typically of the pin / hole type or of another type which preferably allows the rotation of the pedal 14, 15 around the axis Y1, Y2 with respect to the pedal crank 22, 23. The pin -pedal 34, 35 can be integral with the free end 28, 29 of the pedal crank 22, 23 and the hole can be made in the pedal 14, 15. Alternatively, the pedal-pin 34, 35 can be integral with the pedal 14, 15 and the hole can be made at the free end 28, 29 of the pedal crank 22, 23. Still alternatively, two holes can be provided, at the free end 28, 29 of the pedal crank 22, 23 and on the pedal 14, 15 adapted to accommodate a bolt or screw.

The connection between the pedal crank 22, 23 and a respective axially external end of the pin 26 of the bottom bracket is of a type which makes them integral in rotation and prevents the axial sliding of the pedal cranks 22, 23 with respect to the pin 26.

A crank 22, 23 can be made in one piece with the pin 26, the other crank 23, 22 being coupled to the other end of the pin 26 after the latter has been inserted into the bottom bracket 32. Alternatively, each pedal crank 22, 23 can be made in one piece with a respective pivot element, the two pivot elements being end-connected to each other. Still alternatively, both the pedal cranks 22, 23 can be coupled to a pin 26 which is not in one piece.

For one or both of the cranks 22, 23, for example, a screw coupling, a forced interlocking coupling or by interference can be provided, in particular by coupling grooved surfaces, a coupling with a pin and square hole, a gluing, a welding.

The pedal crank 22 on the chain side 16 (typically right) comprises means for fixing said toothed crowns 30 intended to mesh, one at a time, with the chain 16. Typically, a plurality of spokes 36 are provided which radiate off (generally indicated in English as spider), at the pivot end 24 of the right pedal crank 22, typically in one piece with the pedal crank 22; to the free ends of the spokes 36, the toothed crown (s) 30 are typically screwed. Alternatively, the toothed crowns 30 can be made in one piece with the right pedal crank 22.

The main body or "arm region" 38, 39 of each pedal crank 22, 23, that is to say a portion thereof extended between the axis of rotation X and the pedal axis Y1, Y2 and therefore neglecting the aforementioned spokes 36, is generically in the shape of a bar (or rectangular parallelepiped) extended orthogonal (and cantilevered) to the axis of rotation X. For the sake of brevity, in the following the expression "pedal crank" will sometimes be used, meaning in particular its main body 38, 39 shaped of bar.

More specifically, the main body 38, 39 of the pedal crank 22, 23 extends in a generally radial direction with respect to the axis of rotation X - generally meaning that it can also deviate, in one or more points as well as along its entire extension, from that direction. Each crank 22, 23 can in fact be more or less tapered / flared, when viewed along a direction parallel to the X rotation axis, and / or more or less angled when viewed along a direction orthogonal to the X rotation axis.

In the present description and in the attached claims, the plane of rotation P of the pedal crank 22, 23 is intended to indicate any plane orthogonal to the pedal axis Y1, Y2 and to the axis of rotation X, in particular one of the median planes of the pedal crank 22 , 23.

In this description and in the attached claims, with the rotating plane R of the pedal crank 22, 23 or plane of the axes it is intended to indicate the plane containing the axis of rotation X and the pedal axis Y1, Y2. In particular, the rotating plane R is intended to indicate one of the median planes of the pedal crank 22, 23.

With reference to this schematization of the pedal crank 22, 23 as a bar, in the present description and in the attached claims, with longitudinal direction or direction of length L of the pedal crank 22, 23 it is intended to indicate a direction that joins orthogonally the axis of rotation X to the pedal axis Y1, Y2; the direction of length L lies in particular in the rotating plane R.

In the present description and in the attached claims, the transverse plane T to the pedal crank 22, 23 is intended to indicate any plane orthogonal to the longitudinal direction L. In particular, the transverse plane T is intended to indicate one of the median planes of the pedal crank 22, 23.

In the present description and in the attached claims, the cross section of the pedal crank 22, 23 is intended to indicate a section made through the main body 38, 39 of the pedal crank 22, 23 in a transverse plane T. The cross section of each pedal crank 22, 23 ( in the region of the arm 38 for the right pedal crank 22) is generally rectangular, but can be of any type, even if it typically has at least one axis of symmetry. The shape and dimensions of this cross section can be constant along the entire length of the pedal crank 22, 23 or they can vary. The cross section of each pedal crank 22, 23 can be solid or hollow.

In the present description and in the attached claims, the sense of the width G of the pedal crank 22, 23 is intended to indicate a direction lying in the rotation plane P and orthogonal to the longitudinal direction L of the pedal crank 22, 23; the sense of width G lies in a transverse plane T.

In the present description and in the attached claims, the direction of the thickness S of the pedal crank 22, 23 is intended to indicate a direction parallel to the axis of rotation X; the thickness direction S lies in a transverse plane T and in the plane of rotation R.

For clarity, in FIG. 1 these planes and directions are shown on the right crank arm 22 only.

In the present description and in the attached claims, the proximal face 40, 41 of the pedal crank 22, 23 is intended to indicate the face which, in the assembled condition, faces the frame; the distal face 42, 43 of the pedal crank 22, 23 means the opposite face to the proximal face 40, 41. The pin 26 of the bottom bracket extends from the proximal face 40, 41 and the pedal-pin 34, 35 extends from the face distal 42, 43.

In the present description and in the attached claims, with the upper face 44, 45, respectively the lower face 46, 47 of the pedal crank 22, 23 it is intended to indicate the faces substantially orthogonal to the proximal 40, 41 and distal 42, 43 faces, extended along the longitudinal direction L and the thickness direction S, which are in the upper, respectively lower position, when the crank arm 22, 23 is in the active stroke, i.e. with the free end 28, 29 forward in the direction of travel with respect to the end of pivot 24, 25.

In the present description and in the attached claims, in the case of a hollow pedal crank 22, 23 (at least along its region of the arm or main body 38, 39), with the internal surface of the pedal crank 22, 23 it is intended to indicate the surface facing the cavity; with the external surface of the pedal crank 22, 23 it is meant to indicate the exposed surface.

During pedaling, the force applied by the cyclist on the pedals 14, 15 is transferred by them to the cranks 22, 23.

The left pedal crank 23 transfers this force to the pin 26 of the bottom bracket. The bottom bracket pin 26 transfers that force - apart from frictional losses with the bottom bracket 32 - to the right crank arm 22.

The force directly applied to the right crank 22 or transmitted to it by the left crank 23 as above is transferred to the spokes 36 of the right crank 22 and from these to the toothed crowns 30.

From the toothed crowns 30, the force is transmitted to the transmission chain 16 and from this to the pinion pack 18 which, finally, transfers it to the hub 20 of the rear wheel, by means of the freewheel body of the pinion pack 18, if present.

Therefore, in each of the aforementioned components of the transmission 10, stresses and corresponding deformations are generated, which can be a more or less accurate index of the force delivered by the cyclist, as well as being of interest, for example in the design phase of the various components. .

More specifically, pedaling is a cyclical movement with which the cyclist impresses with each leg on the respective pedal 14, 15 such a force as to activate the crankset 12 in rotation, consequently moving the rear wheel by means of the chain 16 and the sprocket set 18 .

During pedaling, the force (F in FIG. 2) applied to the pedals 14, 15 by the cyclist varies in terms of both intensity and direction as a function of the angular position in which the cranks 22, 23 are located and generates a state of effort and a consequent state of deformation in the components of the crank 12.

In the following it is considered, with reference to FIG. 2, the right pedal crank 22, it being understood that what will be described is also valid for a left pedal crank 23, the modifications being within the reach of those skilled in the art.

To evaluate the stresses and deformations of the pedal crank 22 due to the application of the force F to the respective pedal 14 in a predetermined angular position, the pedal crank 22 can be assimilated to a joint constrained beam at its pivot end 24 (on the left in FIG. 2) and the pedal 14 to an interlocking element at the free end 28 of the pedal crank 22 (on the right in FIG. 2), i.e. as if the pedal 14 could not rotate with respect to the pedal crank 22 and the crankset 12 could not rotate with respect to the bottom bracket 32.

The point of application of the force F can be considered to correspond with the center O of the pedal surface 14 in contact with the cyclist's foot.

Considering a generic reference system UVW, in which the direction U coincides with the longitudinal direction L of the crank arm 22, 23 and the direction W is parallel or coincident with the rotation axis X and the thickness direction S, and taking the positive directions as arbitrarily shown in FIG. 2, the force F can be generically decomposed into the following components:

- a radial or parallel component Fu acting along the longitudinal direction L of the crank arm 22, 23,

- a tangential or perpendicular component Fv, orthogonal to the plane of the axes or rotating plane R,

- a lateral component Fw, orthogonal to the rotation plane P of the crank arm 22 and parallel to the axis of rotation X and to the pedal axis Y1.

As mentioned above, the magnitude and direction of the force F vary during the pedal stroke for various reasons, and at any instant, one or more of the components Fu, Fv, Fw could also be oriented in the opposite direction to that shown.

The tangential or perpendicular component Fv represents the only effective component or component useful for pedaling purposes, that is, the one that actually rotates the crank 22.

It is found that the tangential or perpendicular component Fv is of maximum magnitude when the crank 22 is in an angular position such that the pedal axis Y1 is further forward than the axis of rotation X in the direction of travel; this phase is called the thrust or propulsion phase and takes place alternately for each crank 22, 23. The tangential component Fv generates a bending moment Bw around the axis W, which involves a first bending deformation, again indicated with Bw.

More in detail and in a per se well known manner, the flexural deformation Bw comprises an elongation or dilatation deformation T1 on one side with respect to the plane of the R axes (at the top in FIG.

3) and a compression or contraction deformation C1 on the other side of the crank arm 22, 23 with respect to the plane of the R axes (bottom in FIG. 3).

In any cross section of the pedal crank 22 it is possible to identify a neutral axis N1 for the bending moment Bw.

As already indicated above, in the present description and in the attached claims, as well as in general in the mechanical sector, the "neutral axis" is intended to indicate the geometric location of the points in which the stresses normal to the cross section of the solid are zero - in this case the crank arm 22, 23 - considered.

In general, the position of the neutral axis N1 for the bending moment Bw in the stressed crank 22, 23 depends on the characteristics of the material and the geometry of the section, i.e. its shape, whether it is a solid or hollow section, etc. .

If the crank arm 22, 23 were in a homogeneously distributed material and with a full rectangular cross section (as schematically shown in FIG. 3), the neutral axis N1 for the bending moment Bw would be on the plane of the R axes (horizontally in FIG. 3) ).

The neutral axis N1 therefore defines the "boundary" between the portion of the crank arm 22 subject to elongation deformations T1 and the portion subject to compression deformations C1. The neutral axis N1 can also be seen as that axis around which the cross section of the crank arm 22 subjected to the bending moment Bw "rotates".

Therefore, the deformation in the crank arm 22, 23 associated with the effective component Fv of the force F is more pronounced - and therefore more easily detectable - the farther away from the neutral axis N1.

Strictly speaking, it is also observed that since the point O of application of the force F on the pedal 14 is displaced with respect to the plane of rotation P of the pedal crank 22, the tangential component Fv of the propulsion force F also generates a torque around the axis U which generates a torsional deformation Qu in the crank arm 22, 23.

In particular and as shown in FIG. 4, the torsional deformation Qu comprises tangential or shear deformations TG which, in a transverse section of the pedal crank 22, are maximum at the outer periphery and gradually decreasing moving towards the center O1 of the pedal crank section 22, until eventually canceling themselves out.

The radial components Fu and lateral Fw are ineffective for pedaling purposes and therefore represent “lost” components of the force F, which nevertheless contribute to deforming the crank arm 22, 23.

In particular, the radial component Fu, by virtue of the misalignment of the application point O, and the lateral component Fw generate a second bending moment Bv and a second bending deformation still indicated with Bv, which causes the crank 22, 23 to bend towards the frame (in case of positive component in the reference system shown).

As shown in FIG. 5, the flexural strain Bv comprises an elongation strain T2 and a compression strain C2 on the two opposite sides of the pedal crank 22 with respect to the plane of rotation P.

The radial component Fu also generates in the pedal crank 22 an axial elongation deformation (in the case of a positive component in the reference system shown). This axial elongation deformation is neglected in the following, since it is typically negligible with respect to the aforesaid T1 and T2 elongation and C1 and C2 compression deformations.

In any cross section of the pedal crank 22, 23 it is possible to identify a second neutral axis N2 for the bending moment Bv.

Always if the pedal crank 22, 23 were made of a homogeneously distributed material and with a full rectangular cross section, as shown by way of example in FIG. 5, the neutral axis N2 for the bending moment Bv would be on the plane of rotation P of the crank arm 22, 23 (vertically in FIG. 5).

In the case of a pedal crank 22, 23 with a non-rectangular section and / or with a hollow section and / or in a non-homogeneous material, such as for example a layered composite material, and / or with a variable section along the longitudinal direction L of the pedal crank 22, 23 , the state of deformation in the pedal crank 22, 23 is even more complex than that described. Moreover, what has been said in relation to the regions of the pedal crank 22, 23 in which the deformations are of greater entity and, therefore, more easily detectable, remains valid. Furthermore, the aforementioned neutral axes N1, N2 can generally be identified, even if they are possibly displaced with respect to the plane of the axes R and the plane of rotation P, respectively.

Therefore, the evaluation of the force F (for example for a torque meter or a power meter) and in particular of its only effective component, namely the tangential component Fv, can take place on the basis of the measurement of the aforementioned deformation. flexural Bw. Alternatively or additionally, one can rely on the measurement of the torsional strain Qu.

In some cases it could be useful to measure also the lost radial Fu and lateral Fw components, for example to measure the lost power with a power meter; in this case, these components can be obtained from the measurement of the flexural strain Bv and the axial strain of elongation.

In still other cases it may be useful to measure one or more of the various deformations described above, for example to obtain useful information for the design of the pedal crank 22, 23 and / or other.

The main forces acting on the spokes 36 of the crank 22 from the chain side 16 are also substantially bending moments acting in a plane orthogonal to the rotation axis of the crank 22, 23 (rotation plane), although there may also be an effort linear in the direction of the rotation axis (thickness direction) and / or a bending moment around the latter direction.

The pin 26 of the bottom bracket, as well as the sleeve of the sprocket 18 or freewheel body of the monolithic sprocket 18, are instead subject mainly to torsion acting around the X axis of rotation - as well as to a bending moment due to weight. of the pedal cranks 22, 23 associated with its ends and with the predominantly downward direction of the pedaling force F as regards the bottom bracket pin 26 and with the weight of the sprocket set 18 as regards the freewheel body of the monolithic sprocket set 18 .

One tool used to measure strains on a structure, especially a component, is a strain gauge.

In the present description and in the attached claims, the term "strain gauge" refers to an electrical resistance strain gauge.

A strain gauge comprises a flexible insulating support which supports, typically by gluing, a serpentine-shaped grid, i.e. according to a zig-zag pattern of parallel lines made by means of a metal foil (photo-etched strain gauges) or a thin metal wire (strain gauges with wire).

The strain gauge is suitably attached to the component, typically by means of a suitable adhesive, for example a cyanoacrylate or an epoxy resin.

As is well known, the surface of the component to which the strain gauge is glued must be carefully prepared in such a way that the adhesion of the strain gauge to it is reliable and unpredictable measurement errors are avoided.

When the component is stressed, for example due to the application of a force from the outside such as the pedaling force F or those derived from it, the deformations produced on its surface in contact with the strain gauge are transmitted to the grid; the consequent deformations of the grid cause a variation of its electrical resistance.

The sensitivity of the strain gauge is much greater in the direction parallel to the branches of the serpentine - hereinafter briefly referred to as the "direction of the strain gauge" or "direction of detection" and taken as a reference when talking about the orientation of the strain gauge - than in the direction orthogonal to it: when the electrical conductor forming the coil is stretched, it becomes longer and thinner and its electrical resistance increases, while when it is compressed it shortens and widens, and its electrical resistance decreases.

More specifically, the variation in electrical resistance R, not to be confused here with the rotating plane R, is related to the deformation through a quantity known as the calibration factor or Gauge factor GF: indicating with epsilon the deformation, in this case a percentage variation of length given by deltaLe / Le where Le is the length, we have:

GF = deltaR / R / deltaLe / Le = deltaR / R / epsilon (1)

In order to be able to read the small variations in electrical resistance induced by a deformation of the component under measurement and a consequent deformation of the strain gauge, a reading circuit is typically used whose output is an amplified signal which is a function of these resistance variations, typically a circuit bridge at Wheatstone bridge.

As is known, a Wheatstone bridge comprises two resistive branches connected in parallel to each other and to a reference voltage; each resistive branch comprises two resistors connected in series. The bridge output is the voltage difference between the two connection points of the resistors in series; the proportionality coefficient between the measured bridge output and the known reference voltage correlates the values of the four resistors with each other, values that may be partly known and partly unknown.

Ideally, one would like the electrical resistance of the strain gauge to change only in response to the strain resulting from the applied force. However, variations in temperature have various effects. The structure to which the strain gauge is glued changes in size due to thermal expansion, which is detected as deformation by the strain gauge. The resistance of the strain gauge also changes due to the elongation of its own material, and the resistance of the wires connecting the strain gauge to the Wheatstone bridge reading circuit also changes.

Some commercially available strain gauges are made of a constantane or karma alloy, designed so that the effects of temperature on the resistance of the strain gauge compensate for the change in resistance of the strain gauge due to thermal expansion of the component under test. Since different materials have different degrees of thermal expansion, self-compensation in temperature requires the selection of a particular alloy for each material of the component under measurement; however this is not always achievable and in any case represents a considerable burden and constraint.

In a strain gauge that is not self-compensated, the thermal effects instead cause a deformation, called apparent deformation.

For the detection of torsional stresses and / or torsional deformations, typically four electrical strain gauges are provided, arranged two by two on the same cross section to the axis around which the torque acts and in diametrically opposite positions, two on right-handed helices inclined by 45 °, the other two on left-hand propellers always inclined by 45 ° (or in substantially such positions, in the case of a non-cylindrical component); as a result of torsional deformations, one pair of strain gauges elongates so that the electrical resistance increases, while the other pair is shortened with a consequent decrease in resistance; the resistance variations are measured with a Wheatstone bridge circuit in full bridge configuration, ie whose branches are made up of the four strain gauges; the imbalance or output of the bridge is proportional to the torque to be measured. Theoretically, the thermal effects cancel each other out and are therefore compensated for.

For the detection of bending strains and bending moments, various configurations are known.

In a first case, two strain gauges are applied on opposite faces of the component under measurement, oriented parallel to each other, so that the second strain gauge measures a bending strain equal in modulus and opposite in sign to that measured by the first. The variations in resistance are measured with a Wheatstone bridge circuit in a half-bridge configuration, ie in which the two strain gauges constitute the two resistors in series of a branch of the bridge. The bridge exit is a measure equal to double the bending deformation on each face. Also in this case, the thermal effects theoretically cancel each other out and therefore are compensated. This configuration is also suitable for measuring stress strains and strains.

In a second case, two strain gauges placed on the taut face of the component under test, oriented parallel to each other, and two other strain gauges placed on the compressed face of the component under test are connected in a Wheatstone bridge circuit in full bridge configuration, between of them oriented parallel; the result is a measurement equal to four times the bending deformation on each face, which is also compensated in temperature.

In a third case, which is not ideal for measuring bending strains, a first strain gauge and a second strain gauge are placed on the same face as the first, but oriented orthogonal to it and designed to measure the transverse contraction. As is known, in fact, in a sample of material stressed along the longitudinal direction by a state of uniaxial tension, a transverse epsilon_t deformation is generated linked to the longitudinal deformation by the Poisson's ratio of the material.

The variations in resistance are measured with a Wheatstone bridge circuit in a half-bridge configuration, ie in which the two strain gauges constitute the two resistors in series of a branch of the bridge. Also in this case it is possible to increase the measured signal and theoretically the thermal effects cancel each other out and therefore are compensated. This configuration is also suitable for measuring stress strains and strains.

Finally, in a fourth case, four strain gauges can be applied all on the same face, two parallel and two orthogonal to the first two and designed to measure the transverse contraction. By connecting the strain gauges subject to traction on a pair of opposite sides of the bridge in full-bridge configuration and the strain gauges subject to transverse contraction on the remaining pair of opposite sides of the bridge, an amplified measurement is obtained, again compensated in temperature.

Furthermore, according to the general knowledge of the sector, it is possible to use a single strain gauge and three known resistances, in a quarter bridge configuration, but only for a measurement of a stress or strain in tension or bending in cases where the thermal effects and therefore the apparent deformation are completely negligible, or in the case of strain gauges self-compensated in temperature.

In fact, in this case the exit of the bridge is given approximately by the formula:

Vout = Old * GF * epsilon / 4 (2)

where Vecc is the excitation voltage of the bridge and epsilon is the total strain measured, in turn given by the sum of the strain caused by the force under measurement and the apparent strain:

epsilon = epsilon_force epsilon_apparent (3)

From the above, it is clear that the technical knowledge of the strain gauge sector suggests, in the presence of thermal effects, to use a single Wheatstone bridge in full-bridge configuration in which to insert four strain gauges in the case of complex detection systems, or a single bridge of Wheatstone in half-bridge configuration in which to insert two strain gauges in the case of simpler detection systems.

In this way, the only Wheatstone bridge present is used as a single reading circuit for all the strain gauges associated with the component under measurement, in the case of interest represented here by a monolithic bicycle transmission component. Said single bridge outputs a single voltage value, in which the deformations detected by the two or four strain gauges are suitably added and / or subtracted.

As described above, the connection of two or four strain gauges in a single Wheatstone bridge is made in such a way that the components due to the stresses that it is not desired to detect and the thermal effects are subtracted and / or that the components due to the stresses are added. you want to detect, in order to obtain a higher value output.

Moreover, in practice, a Wheatstone bridge is hardly balanced (that is, its output is hardly zero) when no stress is applied to the component under measurement. In fact, the completion resistors (i.e. the two / three resistors of known value and substantially insensitive to deformations and temperatures that are present in addition to the strain gauges in the half-bridge / quarter-bridge configurations), the resistance of the connection wires of the strain gauges to the reading circuit, the stress induced by gluing the strain gauge to the component under measurement, and any other components, generate an initial deviation or offset.

Although it is possible to compensate for this deviation by means of a compensation circuit, typically the compensation of this deviation or offset takes place at the software level, in a processor or strain gauge control unit that directly supplies the value of the deformation along one or more selected directions.

According to the known procedure, once the bridge configuration (the type of connection circuit) and the calibration factor K or gauge factor GF of each strain gauge have been set in the control unit, a calibration or calibration procedure is performed during the which an initial (output) measurement is taken in the absence of force applied to the component under measurement; the value of this output in the absence of force applied to the component is then appropriately entered as a corrective in the equations that provide the load as a function of the output.

As mentioned in the introductory part, in the case of a component of a bicycle transmission 10, in particular of a crank arm 22, 23, the air flow that strikes the component, especially in high-speed running conditions, can cool the exposed side. against the wind even a few degrees centigrade with respect to the unexposed side. Other temperature differences may be due to other factors, and this is particularly true in the case of cranks 22, 23 or other components made of composite material, due to poor thermal conductivity. Furthermore, in the case of composite material, the thermal expansion is strongly dependent on the local characteristics of the material, such as actual orientation of the fiber, effective density of the fiber, effective density of the polymeric material, etc., as well as of course on the local geometric characteristics such as wall thickness. and similar.

Consequently, two strain gauges positioned at even a small distance from each other, even on the same face 40-47 of a crank arm 22, 23, may be in use at a different operating temperature so that, even in the case of the aforementioned configurations of strain gauges and respective Wheatstone bridge where theoretically a temperature compensation takes place, this compensation is not found in practice in a truly effective way.

Therefore, the apparent strain component detected by strain gauges positioned in different regions of the component under measurement, for example on the two sides of the neutral plane with respect to the bending strain to be measured, is different (giving rise to an overall non-zero apparent strain) and, of consequently, the variation of resistance in pairs of strain gauges is not the same and opposite as theoretically it should be, resulting in a false output of the Wheatstone bridge in a conventional half-bridge configuration.

A data acquisition system or stress / deformation detector according to the invention makes it possible to overcome this drawback.

In FIGS. 6-7 illustrates a data acquisition system 100 according to a first embodiment of the invention.

The data acquisition system comprises two strain gauges 102, 104 provided in two different positions or regions a monolithic component, in particular such that when the first strain gauge 102 is substantially subjected to an expansion, the second strain gauge 104 is substantially subjected to a contraction and vice versa.

A first reading circuit 106 is associated with the first strain gauge 102 and a second reading circuit 108 is associated with the second strain gauge 104. The first reading circuit 106 is a first Wheatstone bridge circuit in a quarter bridge configuration, comprising the first extensometer 102, a first completion resistor 110 connected in series to the first extensometer 102 at a point 112 to form a reading branch, a second and a third completion resistor 114, 116 connected together in series at a point 118 to form a neutral branch, in which the reading branch and the neutral branch are connected in parallel with each other and to a reference power supply voltage 120, of known value Vsupply, moreover detected as Vref_strain at the output at terminals 122 The output of bridge 124 is the voltage difference Vout_1 between point 112 and point 118.

The second reading circuit 108 is a second Wheatstone bridge circuit in a quarter bridge configuration, comprising the second strain gauge 104, a fourth completion resistor 126 connected in series at a point 128 to the second strain gauge 104 to form a second reading branch, the second and third completion resistors 114, 116 connected together in series at the point 118 as above to form a neutral branch, in which the second reading branch and the neutral branch are connected in parallel with each other. them and at the supply voltage 120. The output of the bridge 130 is the voltage difference Vout2 between the point 128 and the point 118.

Advantageously, the first bridge circuit 106 and the second bridge circuit 108 share the neutral branch comprising the two completion resistors 114, 116; however, each bridge circuit 106, 108 could comprise two respective completion resistors.

Advantageously, the first quarter-bridge circuit 106 and the second quarter-bridge circuit 108 share the supply voltage 120; however each quarter bridge circuit 106, 108 could comprise a respective supply voltage.

A computing unit (the aforementioned strain gauge control unit), preferably in the form of a microprocessor, combines the output 124, Vout_1 of the first reading circuit 106 with the output 130, Vout_2 of the second reading circuit 108. Advantageously, in the combination the two outputs are not merely subtracted or merely added. The calculation unit, on the other hand, combines information from the two strain gauges 102, 104 by applying the following general formula:

U = A1 * (Vout_1 - Vzero_1) A2 * (Vout_2 - Vzero_2) (4)

It is understood that each parameter Vzero_1, Vzero_2 can represent, for example, a value obtained during a calibration procedure to take into account the deviation or offset from an output Vout_1, Vout_2 null in the absence of load. Parameters A1 and A2 allow the reading of the two strain gauges 102, 104 to be "weighted" in a different way in order to compensate for possible differences in the local characteristics of the material and / or in the geometric characteristics of the pedal crank in correspondence with the two measurement regions where they are affixed.

It is worth pointing out that in simpler cases, one or more of the parameters A1, A2, Vzero_1, Vzero_2, may be absent.

Thanks to the fact that instead of a single Wheatstone bridge having a single output there are two Wheatstone bridges whose two outputs 124, Vout_1 and 130, Vout_2 can be read independently from each other, it is possible to calibrate each output independently, so as to individually take into account the various causes mentioned above, which unbalance the output of the Wheatstone bridge when no stress is applied to the component under test. The data acquisition system 100 can therefore be calibrated very precisely and therefore provide a very precise measurement in normal conditions of use.

Preferably, the parameters Vzero_1, Vzero_2, A1 and A2 are obtained by applying a multiple linear regression to the output of the two reading circuits 106, 108 when the component under measurement, in particular the crank arm 22, 23, is subjected to forces applied according to different orientations , so as to provide, for example, a response to both a moment and a tensile stress in the longitudinal direction L, in a method associated with "factorial experiments" or "Design of Experiments, DOF".

In FIGS. 8-9 illustrates a data acquisition system 200 according to another embodiment of the invention. The data acquisition system still comprises two strain gauges 102, 104 provided in two different positions or regions a monolithic component, in particular such that when the first strain gauge 102 is substantially subjected to an expansion, the second strain gauge 104 is substantially subjected to a contraction and viceversa.

The data acquisition system 200 according to this embodiment differs from the one described above with reference to FIGS. 6-7 in that, in addition to the components of the data acquisition system 100, a first temperature sensor 232 is arranged substantially in the same position on the monolithic component under measurement as the first strain gauge 102 and a second temperature sensor 234 is arranged substantially in the same position as the second strain gauge 104.

Advantageously, the first and second temperature sensors 232, 234 are thermistors. A thermistor (or resistance thermometer) is a resistor whose resistance value varies significantly with temperature.

Each thermistor 232, 234 is preferably selected from the group consisting of a nickel plane temperature sensor, a platinum plane temperature sensor, a Balto plane temperature sensor (70% nickel and 30% iron), and a temperature sensor non-flat of the Platinum RTD Resistance temperature detector type, preferably being a nickel temperature sensor.

A third reading circuit 236 is provided for reading the first temperature sensor 232 and advantageously comprises a reference resistor 238, of known value and substantially invariable with the temperature, connected in series to the temperature sensor 232 at node 240 ( with the interposition of the second temperature sensor 234); the first temperature sensor 232 and the reference resistor 238 are traversed by a reference current Iptc. The output 242 of the third reading circuit is given by the voltage difference Vthermal_1 across the thermistor 232.

The temperature at which the temperature sensor 232 and the first strain gauge 102 are located is proportional to the value of the resistance 232, in turn related to, and more particularly given by the ratio between, the voltage Vthermal_1 across the first temperature sensor 232 and the voltage Vref_thermal across the reference resistor 238, as measured at output 244:

T232 = T102 = T1 = F (Vthermal_1, Vref_thermal) (5)

A fourth reading circuit 246 is provided for reading the second temperature sensor 234 and advantageously comprises the reference resistor 238, of known value and substantially invariable with the temperature, connected in series to the thermistor 234 (and to the thermistor 232); the temperature sensor 234 and the reference resistor 238 are crossed by the reference current Iptc. The output 248 of the fourth reading circuit 246 is given by the voltage difference Vthermal_2 across the thermistor 234.

The temperature at which the temperature sensor 234 and the second strain gauge 104 are located is proportional to the value of the resistance 234, in turn related to, and more particularly given by, the ratio between the voltage Vthermal_2 across the second temperature sensor 234 and the voltage Vref_thermal across the reference resistor 238:

T234 = T104 = T2 = F (Vthermal_2, Vref_thermal) (6)

In the case shown, the reference resistor 238 of the fourth reading circuit 246 is advantageously the same as that of the third reading circuit 236, but this is not strictly necessary. The reference resistor 238 of the fourth reading circuit 246 of the second temperature sensor 234 could be different from the reference resistor 238 of the third reading circuit 236 of the first temperature sensor 232. The two reading circuits 236, 246 could be totally disconnected electrically between them, so the reference current Iptc in the two reading circuits could be different.

A voltage divider made by applying a known voltage to the series connection of the temperature sensor and the reference resistor could be used as a third reading circuit 236 and / or as a fourth reading circuit 246, preferably but not necessarily the same reference voltage. Vsupply of the first and second Wheatstone bridges 106, 108.

A computing unit (or strain gauge control unit), preferably in the form of a microprocessor, combines the output 124, Vout_1 of the first reading circuit 106, the output 130, Vout_2 of the second reading circuit 108, the output 242, Vthermal_1 of the third reading circuit 236 and the output 248, Vthermal_2 of the fourth reading circuit 246.

Advantageously, in the combination of the four outputs, a respective calibration parameter is associated with each output.

The calculation unit therefore combines the information of the two strain gauges 102, 104 by applying the following general formula:

U = A1 * (Vout_1-Vzero_1-B1 * (T1-T0))

A2 * (Vout_2-Vzero_2-B2 * (T2-T0)) (7)

It is understood that each parameter Vzero_1, Vzero_2 can represent, for example, a value obtained during a calibration procedure to take into account the deviation or offset from a null output Vout_1, Vout_2 of the reading circuits of the strain gauges 102, 104 in the absence of load.

Also in this case, parameters A1 and A2 allow the reading of the two strain gauges 102, 104 to be "weighed" in a different way in order to compensate for possible differences in the local characteristics of the material and / or in the geometric characteristics of the pedal crank in correspondence with the two regions of measurement where they are affixed.

Preferably, the parameters Vzero_1, Vzero_2, A1 and A2 are obtained in the manner described above with reference to FIGS. 6-7.

Furthermore, each parameter B1, B2, of suitable units of measurement, can represent, for example, a thermal calibration coefficient, which takes into account the thermal properties of the strain gauges and of the material, in particular the thermal expansion of the material, T0 being a temperature of reference.

It is worth pointing out that in simpler cases, one or more of the parameters A1, A2, B1, B2, Vzero_1, Vzero_2, may be absent.

Thanks to the fact that instead of a single Wheatstone bridge having a single output there are two Wheatstone bridges whose two outputs 124, Vout_1 and 130, Vout_2 can be read independently from each other, it is possible to calibrate each output independently, so as to individually take into account the various causes mentioned above, which unbalance the output of the Wheatstone bridge when no stress is applied to the component under test. Moreover, thanks to the independent reading of the two temperatures at which the two strain gauges 102, 104 are located, it is possible to further refine the calibration of the output of the data acquisition system 200, which can therefore provide a very precise measurement in normal conditions of use. .

In less accurate detection systems, a single temperature sensor and a respective reading circuit could be provided, for example by simplifying the wiring diagram of FIG. 9 through the absence of thermistor 234 and output 248, Vthermal_2. The calculation unit could then combine the information from the two strain gauges and the single temperature sensor by applying the following formula:

U = A1 * (Vout_1-Vzero_1-B1 * (T1-T0))

A2 * (Vout_2-Vzero_2-B2 * (T1-T0)) (8)

In this case, the temperature sensor could be arranged on an electronic board comprising the electronic control unit, therefore distant from the strain gauges.

In FIGS. 10-13 illustrate data acquisition or detection systems 300, 400, 500 according to other embodiments of the invention. Each data acquisition system 300, 400, 500 comprises the basic configuration of FIG. 6 and therefore two strain gauges 102, 104 provided in two different positions or regions a monolithic component, in particular such that when the first strain gauge 102 is substantially subjected to an expansion, the second strain gauge 104 is substantially subjected to a contraction and vice versa.

Each data acquisition system 300, 400, 500 further comprises a third strain gauge 350, 450, 550 positioned in the same detection position as the first strain gauge 102 and a fourth strain gauge 352, 452, 552 positioned in the same detection position as the second strain gauge 104.

The third strain gauge 350 has the detection grid parallel to that of the first strain gauge 102 (FIG. 10), or the third strain gauge 450 has the detection grid perpendicular to that of the first strain gauge 102 (FIG. 11), or the third strain gauge 550 has the detection grid at 45 ° with respect to that of the first strain gauge 102 (FIG. 12).

The fourth strain gauge 352 has the detection grid parallel to that of the second strain gauge 104 (FIG. 10), or the fourth strain gauge 452 has the detection grid perpendicular to that of the second strain gauge 104 (FIG. 11), or the fourth strain gauge 552 has the detection grid at 45 ° with respect to that of the second strain gauge 102 (FIG. 12).

The mutual arrangement of the grids of the first strain gauge 102 and of the third strain gauge 350, 450, 550 does not necessarily have to be the same as the reciprocal arrangement of the grids of the second strain gauge 104 and of the fourth strain gauge 352, 452, 552. In other words, other forms are possible embodiments obtained by combining the arrangement of the first strain gauge 102 and of the third strain gauge 350, 450, 550 of one of FIGS. 10-12 with the arrangement of the second strain gauge 104 and of the fourth strain gauge 352, 452, 552 of one of FIGS. 10-12.

In simpler embodiments, only the third strain gauge 350, 450, 550 or only the fourth strain gauge 352, 452, 552 could be provided.

In each data acquisition system 300, 400, 500, a fifth reading circuit 354 is associated with the first strain gauge 102 and with the third strain gauge 350, 450, 550 and a sixth reading circuit 356 is associated with the second strain gauge 104 and the fourth strain gauge 352, 452, 552. The fifth reading circuit 354 is a third Wheatstone bridge circuit in half-bridge configuration, comprising the first strain gauge 102, the third strain gauge 350, 450, 550 connected in series to the first strain gauge 102 at a point 358 to form a reading branch, two completion resistors 314, 316 connected together in series at a point 318 to form a neutral branch, in which the reading branch and the neutral branch are connected in parallel with each other. them and at a reference power supply voltage 320, of known value Vsupply, moreover detected as Vref_strain at the output at terminals 322. The output of bridge 324 is the voltage difference Vout _1 between point 318 and point 358.

The sixth reading circuit 356 is a fourth Wheatstone bridge circuit in half-bridge configuration, comprising the second strain gauge 104, the fourth strain gauge 352, 452, 552 connected in series at a point 360 to the second strain gauge 104 to form a second reading branch, two completion resistors 314, 316 connected together in series at a point 318 as above to form a neutral branch, in which the second reading branch and the neutral branch are connected in parallel with each other and at the supply voltage 320. The output 330 of the second bridge is the voltage difference Vout_2 between the point 360 and the point 318.

Advantageously, the first half-bridge circuit 354 and the second half-bridge circuit 356 share the neutral branch comprising the two completion resistors 314, 316; however, each bridge circuit 354, 356 could comprise two respective completion resistors.

Advantageously, the first half-bridge circuit 354 and the second half-bridge circuit 356 share the power supply voltage 320; however each half-bridge circuit 354, 356 could comprise a respective supply voltage.

A computing unit (the strain gauge control unit referred to above), preferably in the form of a microprocessor, conveniently combines the output 324, Vout_1 of the first reading circuit 354 with the output 330, Vout_2 of the second reading circuit 356, again applying the following general formula:

U = A1 * (Vout_1 - Vzero_1) A2 * (Vout_2 - Vzero_2) (9)

The same considerations set out above apply with reference to the embodiment of FIGS. 6-7.

In FIGS. 14-17 illustrates detection systems 600, 700, 800 according to other embodiments of the invention, which include the four strain gauges 102; 104; 350, 450, 550; 352, 452, 552, arranged respectively as in FIGS. 10-12. Therefore each data acquisition system of FIGS. 14-16 also comprises two strain gauges 102, 104 provided in two different positions or regions a monolithic component, in particular such that when the first strain gauge 102 is substantially subject to an expansion, the second strain gauge 104 is substantially subject to a contraction and vice versa .

The data acquisition system 600, 700, 800 according to these embodiments differs from those described above with reference to FIGS. 10-13 in that a first temperature sensor 632 is arranged substantially in the same position as the first strain gauge 102 and the third strain gauge 350, 450, 550 and a second temperature sensor 634 is arranged in substantially the same position as the second strain gauge 104 and the fourth strain gauge 352, 452, 552.

It should be noted that advantageously a single temperature sensor 632, respectively 634 is associated with both strain gauges 102; 350, 450, 550 or 104 respectively; 352, 452, 552 provided in a detection position, so that the numerical ratio between strain gauges and temperature sensors is 2: 1. This saves components and complexity with respect to known systems in which a temperature sensor is associated with each strain gauge.

Advantageously, the first and second temperature sensors 632, 634 are thermistors, more preferably components of the type indicated above.

A seventh reading circuit 636 is provided for reading the first temperature sensor 632.

An eighth reading circuit 646 is provided for reading the second temperature sensor 634.

In this regard, all that has been said in relation to the embodiment of FIGS is valid. 8-9; the reference resistor is indicated with 638, the node with 640, the outputs with 642, 644 and 648, that is to say by increasing the references of FIG. 9.

The calculation unit therefore combines information from the four strain gauges 102; 104; 350, 450, 550; 352, 452, 552 and the two thermistors 632, 634 again applying the following general formula:

U = A1 * (Vout_1-Vzero_1-B1 * (T1-T0)) +

A2 * (Vout_2-Vzero_2-B2 * (T2-T0)) (10)

The detection circuit described above, in any one of its embodiments, can advantageously be used in a power meter.

A power meter is a pedaling power meter. From a mathematical point of view, as already described above, the pedaling power is the useful component of the power delivered by the cyclist, given by the torque applied on one or both pedals 14, 15, multiplied by the angular speed of the respective pedal crank 22 , 23.

The torque is in turn given by the product of the force component Fv in the tangential direction for the arm, which substantially corresponds to the length of the pedal crank 22, 23.

From a practical point of view, the power applied by the cyclist also has lost components Fu, Fw for the purpose of the movement of the bicycle - which however are not necessarily to be considered a measurement error, if the cyclist is interested in knowing his actual effort (l actual power delivered) and not the effectiveness of his pedaling.

Angular velocity is typically provided by a cadence sensor. Alternatively, the angular velocity can be obtained from an accelerometer that detects the pedaling cadence from the alternating trend of the force of gravity with respect to the rotating plane R of the crank arm 22, 23.

Alternatively, the angular speed of the crankset 12 and therefore of the pedal crank (s) 22, 23 can be calculated from the speed of the bicycle, in turn possibly determined by a cadence sensor applied to a wheel, and from the current transmission ratio.

The data relating to the applied torque are collected by the power meter at the crankset 12.

A power meter 900, schematically shown in FIG. 18, generally comprises a detection system or data acquisition system 902 comprising at least one sensor 904, for example at least one strain gauge, and electrical components forming one or more reading circuits 906 of the output of the sensor (s); as well as a 908 power source for powering the 900 acquisition system.

In the present case, the detection system 900 comprises, as sensors 904, the strain gauges and possible temperature sensors, as well as, as reading circuits 906, the respective reading circuits, of one of the embodiments described above in detail with reference to the FIGS. 6-17.

In FIG. 18, data connections are indicated with a double arrow and power connections are indicated with a single arrow.

Typically a power meter also comprises a processing module 910, comprising a front-end electronics for the pre-processing of the reading of the acquisition system, for example one or more analog / digital converters 912, as well as a processor (MCU - Micro Controller Unit) 914, to perform mathematical calculations on these readings, namely the aforementioned strain gauge control unit. Processor 914 typically comprises memory means for instructions and data. It could be part of more complex data processing means.

The power meter 900 further typically comprises an angular velocity meter 916 as described above. In this case, the processor 914 also receives the output of the latter at its input and processes the reading of the data acquisition system 902 also on the basis of this output of the angular velocity meter 916.

Alternatively, if the mathematical processing is particularly simple, it can be performed in an analog way, the 912 analog / digital converter / s being absent. Furthermore, if the sensor (s) 904 and / or the reading circuit (s) 906 are digital, the analog / digital converter (s) 912 may be absent.

The 916 angular velocity meter, the 912 front-end electronics and the 914 processor are powered by the same 908 power source as above, or by one or more distinct sources.

Typically a 900 power meter also includes a 918 transmitter of the measurement made, typically a wireless transmitter with a 920 radio frequency antenna. Transmission typically occurs under the control of the 914 processor. Typically, transmission occurs according to a standard transmission protocol, such as Bluetooth Low Energy® (BLE) and / or ANT + ®.

The output signal of the power meter 900 is typically sent to memory means and / or to a computer, in particular to a cycle-computer housed on the frame or on the handlebar or carried by the cyclist, perhaps in the form of an application on a device personal such as a mobile phone, a smart watch and / or a remote computer.

When the or at least one of the power sources 908 is of the smart and / or rechargeable type, the power meter 900 comprises one or more cells 922 (only one shown) and a circuit 924 in charge of managing the cell (s) 922, comprising in a per se known manner a voltage regulator 926 to output a regulated voltage, a protection circuit 928 to avoid overcurrents to the cell, optionally a Coulomb counter 930 to take into account the residual charge of the cell, as well as a charging circuit 932 which receives a charge voltage from a network or from a source external to the power meter.

In particular, the Coulomb counter 930, if present, counts the charge accumulated during the recharging phase, and the charge consumed on the basis of the information coming from the processing module 910 and can supply the residual charge value to the protection circuit 928 to avoid too low and too high levels of charge.

Finally, when the or at least one of the power sources 908 is of the on-board rechargeable type, the power meter 900 comprises or is associated with a recharging device 934.

The charging circuit 932 receives the charging current from a charging port 936 comprising two contacts and the charging device 934 comprises a charging port 938 comprising two coupled contacts, in turn powered by an external source.

The external source can be the network, in which case the charging device comprises a plug, or a hardware component, in which case the charging device comprises a suitable connector 940, in particular a USB or miniUSB connector 940 as shown.

An instrument similar to a power meter 900 is a torque or other stress meter, that is, an instrument similar to the one described above, but without the angular velocity meter 916. A processor external to the torque meter can take measurements of power by combining the output of such a torque meter with the output of a speed (angular) meter.

In the event that the power meter 900 (or the torque detector) provides for measurements both relating to the crank arm 22, 23 on the chain side, and relating to the crank arm 23, 22 on the opposite side, or relative to the pin 26 of the bottom bracket or to another component of the transmission 10, the data acquisition system 902, or at least its sensor (s) 904, will be doubled or multiplied, including for example at least one strain gauge associated with the pedal crank 22 on the chain side and at least one strain gauge associated with the other pedal crank 23 or with the pin 26 of the bottom bracket. All electrical / electronic components downstream can be doubled / multiplied or shared; similarly the power source 908 or some of its components can be doubled or shared.

In particular, in the case of doubled 918 transmission devices, one of the two will be able to communicate with the other and only the other will be able to communicate with the outside, or each can communicate directly with the outside.

The various electric / electronic components described above can be physically housed on one or more printed circuit boards (PCB - printed circuit board).

Preferably, the strain gauge (s) are instead mounted directly on or in the transmission component 10 to be measured.

Although the invention has been described in detail with reference to strain gauges, the invention can also be applied to sensors of different types, for example piezoelectric sensors.

The foregoing is a description of various embodiments of inventive aspects, and further modifications can be made without departing from the scope of the present invention. The shape and / or size and / or position and / or orientation of the various components and / or the succession of the various phases can be varied. The functions of an element or module can be performed by two or more components or modules, and vice versa. Components shown directly connected or in contact may have intermediate structures arranged between them. Stages shown directly following each other may have intermediate stages carried out between them. The details shown in one figure and / or described with reference to a figure or an embodiment can be applied in other figures or embodiments. Not all the details shown in a figure or described in the same context must necessarily be present in the same embodiment. Characteristics or aspects that are innovative with respect to the known art, alone or in combination with other characteristics, are to be considered described per se, regardless of what is explicitly described as innovative.

Claims (15)

CLAIMS 1. Stress / strain detector (100, 200, 300, 400, 500, 600, 700, 800) applied to a monolithic component (22, 23, 26, 20, 36) of a bicycle transmission (10), comprising a first strain gauge (102) positioned in a first region of the monolithic component (22, 23, 26, 20, 36) and a second strain gauge (104) positioned in a second region of the monolithic component (22, 23, 26, 20, 36 ), characterized in that it comprises a first Wheatstone bridge (106, 354) for reading the output of the first strain gauge (102) and a second Wheatstone bridge (108, 356) for reading the output of the second strain gauge (104) , in which (i) each of the first and second Wheatstone bridges (106, 108) is in a quarter bridge configuration, or in which (ii) the first Wheatstone bridge (354) is in half-bridge configuration comprising, in addition to the first strain gauge (102), a third strain gauge (350, 450, 550) and the second Wheatstone bridge (356) is in a half bridge comprising, in addition to the second strain gauge (104), a fourth strain gauge (352, 452, 552). Detector (100, 200, 300, 400, 500, 600, 700, 800) according to claim 1, wherein the first region and the second region are selected such that when the first strain gauge (102) is predominantly stressed in traction, the second strain gauge (104) is mainly stressed in compression and vice versa. 3. Detector (100, 200, 300, 400, 500, 600, 700, 800) according to any of claims 1-2, wherein the first and second Wheatstone bridges (106, 108, 354, 356) share the branch neutral comprising two completion resistors (114, 116, 314, 316) of known value and substantially insensitive to deformation and temperature. 4. Detector (400, 700) according to any of claims 1-3 when alternative (ii) applies, wherein the third strain gauge (450) is oriented at 90 ° with respect to the first strain gauge (102) in Poisson configuration and / or the fourth strain gauge (452) is oriented at 90 ° with respect to the second strain gauge (104) in the Poisson configuration. 5. Detector (300, 600) according to any of claims 1-3 when alternative (ii) applies, wherein the third strain gauge (350) is oriented parallel to the first strain gauge (102) and / or the fourth strain gauge (352) it is oriented parallel to the second strain gauge (104). 6. Detector (100, 200, 300, 400, 500, 600, 700, 800) according to any of claims 1-5 when alternative (ii) holds, in which the third strain gauge (350, 450, 550) is positioned in the first region of the monolithic component and the fourth strain gauge (352, 452, 552) is positioned in the second region of the monolithic component. Detector (200, 600, 700, 800) according to any of claims 1-6, further including a first temperature sensor (232, 632) thermally coupled to at least one of the first strain gauge (102) and the second strain gauge (104 ), a circuit (236, 636) being provided for reading the first temperature sensor (232, 632) and processing means (914) configured to perform a temperature compensation of the reading of said at least one of the first extensometer (102) and the second strain gauge (104) on the basis of the reading provided by the reading circuit (236, 636). Detector (200, 600, 700, 800) according to any of claims 1-6, further including a first temperature sensor (232, 632) positioned at the first region of the monolithic component (22, 23, 26, 20, 36), thermally coupled to the first strain gauge (102) and a second temperature sensor (234, 634) positioned in correspondence with the second region of the monolithic component (22, 23, 26, 20, 36), thermally coupled to the second strain gauge (104 ), a circuit (236, 636) for reading the first temperature sensor (232, 632) and a reading circuit (246, 646) for the second temperature sensor (234, 634) and processing means (914) being provided configured to perform a temperature compensation of the reading of the first strain gauge (102) and / or of the second strain gauge (104) on the basis of the reading provided by the first and second reading circuits (236, 636; 246, 646), respectively. Detector (200, 600, 700, 800) according to claim 8, wherein said processing means (914) are configured to perform a temperature compensation of the reading of the first strain gauge (102) on the basis of the reading of the output of the first temperature sensor (232, 632) and a temperature compensation of the reading of the second strain gauge (104) based on the reading of the output of the second temperature sensor (234, 634). Detector (100, 200, 300, 400, 500, 600, 700, 800) according to any of claims 1-9, wherein the bicycle transmission component is selected from the group consisting of a pedal crank (22, 23), a race (36) of a crank arm from the transmission side (23), a pin (26) of the bottom bracket, a freewheel body (20) of a monolithic sprocket set (18), preferably being a crank arm (22, 23 ). Detector (100, 200, 300, 400, 500, 600, 700, 800) according to any of claims 1-10, wherein the bicycle drive component is a crank arm (22, 23) and wherein the first strain gauge (102) and the second strain gauge (104) are positioned on opposite sides with respect to a plane (R) comprising the pedal axis (Y1, Y2) and the rotation axis (X) of the pedal crank (22, 23). Detector (100, 200, 300, 400, 500, 600, 700, 800) according to any of claims 1-11, wherein the first strain gauge (102) and the second strain gauge (104) are each arranged as far as possible from a neutral axis or plane (N1, R) with respect to a principal stress / strain to be detected. 13. Detector (100, 200, 300, 400, 500, 600, 700, 800) according to any of claims 1-12, wherein the bicycle component is made at least partially in a composite material comprising structural fiber embedded in a matrix of polymeric material. A detector according to claim 13, wherein at least one of the first strain gauge (102) and the second strain gauge (104), more preferably both, is oriented in a fiber direction of the composite material. 15. Stress / strain detector (200, 600, 700, 800) applied to a monolithic component (22, 23, 26, 20, 36) of a bicycle transmission (10), comprising a first strain gauge (102) positioned in a first region of the monolithic component (22, 23, 26, 20, 36) and a second strain gauge (104) positioned in a second region of the monolithic component (22, 23, 26, 20, 36), characterized in that it further comprises a first temperature sensor (232, 632) positioned in correspondence with the first region of the monolithic component (22, 23, 26, 20, 36), thermally coupled to the first strain gauge (102) and a second temperature sensor (234, 634) positioned in correspondence with the second region of the component, thermally coupled to the second strain gauge (104), a circuit (236, 636) for reading the first temperature sensor (232, 632) and a reading circuit (246, 646) for the second temperature sensor (234, 634) and means of el aborations (914) configured to perform a temperature compensation of the reading of the first strain gauge (102) and / or of the second strain gauge (104) on the basis of the reading provided by the first and second reading circuits (236, 636; 246, 646).