Strain gauge auto-zeroing without using rotational angle

Abstract

A power meter has a strain gauge attached to a crank of a bicycle vehicle to measure a bending force applied to the crank. The power meter also includes an accelerometer positioned on the crank to sense Y-axis acceleration. The power meter also includes a controller having a processor and a memory storing machine readable instructions that, when executed by the processor, cause the controller to: read a bending force value from the strain gauge; reading an accelerometer value from the Y-axis accelerometer; calculating a correction factor based on the accelerometer value and a maximum error force value; and subtracting the correction factor from the bending force value to determine an auto-zeroing bending force value corrected for gravity effects on the crank.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. patent application No. 63 / 433,513, filed on December 19, 2022, entitled “Automatic Zeroing of Strain Gauges Without the Use of Rotation Angle,” the entire contents of which are incorporated herein by reference. Background Art

[0003] A power meter attached to a bicycle crank uses a strain gauge to measure the force applied to the crank and, from this, determine the power generated by the rider. However, as the crank rotates 360 degrees, the mass of the pedals and crankarm exerts a torque on the strain gauge, introducing a force not caused by the rider. When the crank is in the 3 o'clock position (pointing toward the front of the bike), its mass increases the torque measured by the strain gauge; when the crank is in the 9 o'clock position (pointing toward the back of the bike), its mass decreases the torque measured by the strain gauge. The resulting force offset can be as much as ±1.5 Nm, which equates to about 15 watts of power at a typical cycling cadence.

[0004] One known solution for compensating for this force uses the accelerometer of the power meter to detect the X and Y accelerations affecting the crank, determine the angle (e.g., orientation) of the crank based on the X and Y accelerations, and then apply a correction factor to the strain gauge measurements based on this angle. Summary of the Invention

[0005] Certain aspects of the present embodiments include the recognition that conventional power meters calculate crank angle to apply correction for the effects of gravity on the strain gauge's power measurements. The present embodiments improve upon conventional power calculations by automatically zeroing the strain gauge output to counteract the effects of gravity, without requiring crank angle calculation. Furthermore, because the angular error introduced by calculating crank angle is avoided, the accuracy of the power measurement derived from the strain gauge is improved.

[0006] In certain embodiments, the technology described herein relates to a power meter for a pedal vehicle, comprising: a strain gauge for sensing a bending force applied to a crank of the pedal vehicle; a Y-axis accelerometer for sensing a Y-axis acceleration relative to the crank; and a controller having a processor and a memory storing machine-readable instructions that, when executed by the processor, cause the controller to: read a bending force value from the strain gauge; read an accelerometer value from the Y-axis accelerometer; calculate a correction factor based on the accelerometer value and a maximum error force value; and determine an auto-zeroed bending force value corrected for the effects of gravity on the crank by subtracting the correction factor from the bending force value.

[0007] In certain embodiments, the technology described herein relates to a strain gauge auto-zeroing method for determining a correction factor for a bending axis of a crank of a pedal vehicle, the correction factor correcting for error forces caused by the mass of the crank and pedals, the strain gauge auto-zeroing method comprising: capturing a bending force value indicating a force applied to the crank from a strain gauge; capturing an accelerometer value from a Y-axis accelerometer; calculating the correction factor based on the accelerometer value and a maximum error force value; and determining an auto-zeroed bending force value by subtracting the correction factor from the bending force value. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a schematic diagram showing an example crank with a power meter mounted thereon in an embodiment, the power meter including an accelerometer assembly having an X-axis accelerometer (parallel to the crank) and a Y-axis accelerometer (perpendicular to the crank).

[0009] Figure 2 In further example details are shown in the embodiment Figure 1 Block diagram of a power meter.

[0010] Figure 3 is a graph showing an example output of a Y-axis accelerometer as the crank rotates about the crank bearing, in an embodiment.

[0011] Figure 4 is a flow chart illustrating one example method for determining a correction factor for a crankshaft bending axis of a bicycle to correct for error forces caused by the mass of the crank and pedals during crank rotation, in an embodiment. DETAILED DESCRIPTION

[0012] US Patent No. 10,060,738 and US Patent No. 11,033,217 (each of which is incorporated herein by reference in its entirety) disclose power meters and cadence meters for bicycles.

[0013] Figure 1FIG1 is a schematic side view of an example crank 100 mounted with a power meter 102, which includes at least one strain gauge 103 and an accelerometer assembly 104 having at least a Y-axis accelerometer 108, wherein the Y-axis is perpendicular to the length 116 of the crank 100 and parallel to the plane of rotation of the crank 100 (indicated by arrow 112). Crank 100 is used to power a pedal vehicle (e.g., a bicycle), wherein crank 100 rotates about crank bearing 101 (as indicated by arrow 112) to propel the vehicle forward. Pedals (not shown for clarity) are attached to holes 114. Accelerometer assembly 104 may also include an X-axis accelerometer 106, whose X-axis is parallel to the length 116 of the crank 100, and a Z-axis accelerometer (not shown), which is perpendicular to both the X-axis accelerometer 106 and the Y-axis accelerometer 108. The accelerometer kit 104 cannot distinguish between acceleration and gravity and continuously detects the earth's gravity.

[0014] Figure 2 is a block diagram showing in further example detail Figure 1 The power meter 102 includes a battery 202 (optionally rechargeable), a controller 204 (e.g., a microprocessor, microcontroller, application specific integrated circuit, etc.), a strain gauge 103, an accelerometer assembly 104, and a wireless interface 210. The power meter 102 may include other sensors and components without departing from the scope of this disclosure. The controller 204 includes a processor 205 and a memory 207 storing machine-readable instructions that are executed by the processor 205 to implement the functions described herein. The controller 204 may also include at least one analog-to-digital converter for digitizing analog signals for storage in the memory 207 and processing by the processor 205. The controller 204 also includes a power algorithm 206 stored in the memory 207 that, when executed by the processor 205, causes the controller 204 to determine the power 209 input by the user to the crank 100 based on the input captured from the strain gauge 103 and the input captured from at least the Y-axis accelerometer 108. The power algorithm 206 also includes an auto-zero algorithm 208 that corrects for the effects of gravity on the crank 100 as sensed by the strain gauge 103 without determining the angle of the crank 100. The following pseudo code provides an example of the auto-zero algorithm 208:

[0015] bend = readStrain().bend

[0016] accelY=readAccelerometer().y

[0017] accelY / =ACCELEROMETER_COUNTS_PER_G

[0018] bend-=accelY*PEDALPLUSCRANK_AT_3OCLOCK

[0019] Thus, the bend is corrected so that power calculations using the bend are more accurate than when no correction is applied, and the correction calculations are more efficient than prior art corrections that require crank angle. In one example of operation, the power algorithm 206 determines power 209 at intervals (e.g., 1 / 26 of a second) to repeatedly capture readings from both the strain gauge 103 and the Y-axis accelerometer 108 as the crank 100 rotates to different positions about the crank bearing 101. Advantageously, the auto-zero algorithm 208 calculates the auto-zero bend force value 220 without having to determine the angle of the crank 100, thereby simplifying the functionality within the controller 204.

[0020] The controller 204 controls the wireless interface 210 to communicate with one or more of the smartphone 240, the bicycle computer 250, and the other computer 260. The power algorithm 206 can also determine cadence as part of determining the power and work performed by the user, where cadence (also known as pedaling rate) is a measure of the number of revolutions per minute of the crank 100. Cadence is a measure of angular velocity that is proportional to, but not identical to, wheel speed. The cadence and the auto-zero bending force value 220 are then used, at least in part, to calculate the power applied by the user to the crank 100.

[0021] Figure 3 is a graph 300 illustrating an example output 302 of the Y-axis accelerometer 108 as the crank 100 rotates about the crank bearing 101 . Figure 1 、 Figure 2 and Figure 3 Best viewed in conjunction with the following description.

[0022] As crank 100 rotates, accelerometer assembly 104 rotates with crank 100, and thus X-axis accelerometer 106 and Y-axis accelerometer 108 also rotate with crank 100. Consequently, the orientation of X-axis accelerometer 106 and Y-axis accelerometer 108 relative to Earth's gravity 110 changes. Thus, as crank 100 rotates, X-axis accelerometer 106 and Y-axis accelerometer 108 simultaneously sense Earth's gravity 110 and output accelerometer values ​​in the form of cosine and sine waves, respectively, because they are oriented 90 degrees relative to each other. As shown in graph 300, output 302 of Y-axis accelerometer 108 (e.g., Y-axis acceleration) is zero at 12 o'clock position 304, 1 G at 3 o'clock position 306, 0 G at 6 o'clock position 308, -1 G at 9 o'clock position 310, and returns to 0 G at 12 o'clock position 312. Although not shown, the accelerometer values ​​from the X-axis accelerometer 106 are 1 G at 12 o'clock and -1 G at 6 o'clock. Plotting the (X, Y) accelerometer values ​​as the crank 100 rotates about the crank bearing 101 forms a circle.

[0023] Strain gauge 103 may represent one or more strain gauges strategically positioned on crank 100 to sense the force applied to crank 100 by the user. However, crank 100 (and the attached pedals, not shown) have an effective mass that applies an error force 120 (e.g., a gravity-based force based on the mass of crank 100 and its pedals) to crank 100, causing the output of strain gauge 103 to include error force 120 that is not attributable to the power applied by the user. Traditionally, to compensate for this error force, prior art power meters would determine the angle of the crank and apply a correction factor based on the determined angle to compensate for error force 120. Unlike conventional correction calculations, power meter 102 uses only the accelerometer value from Y-axis accelerometer 108 to determine the correction factor, without determining the angle of crank 100 (the angle cannot be determined from a single-axis accelerometer value). For example, a crank at 45 degrees in an elevator accelerating upward at 0.3 g experiences the same Y-axis reading (e.g., 1.0 g) as a stationary crank at 90 degrees, which corresponds to error force 120. Similarly, without the accelerometer value from the X-axis accelerometer 106, the power meter 102 cannot determine whether the crank 100 is tilted 45 degrees forward or 45 degrees backward. Because it cannot distinguish the front-to-back position of the crank 100, the power meter 102 cannot apply correction factors to all possible strain sensing elements. Bending and axial loads increase in proportion to the sensed gravity, but shear loads do not.

[0024] One aspect of this embodiment includes the recognition that the amount of correction factor required on the bending axis (e.g., zero offset adjustment) is proportional to the accelerometer value sensed by the Y-axis accelerometer 108, which corresponds to the Earth's gravity 110. This is because, on a stationary bicycle, the Y-axis accelerometer 108 detects "how much gravity is pulling the cranks sideways," which is the value required to apply corrections to the bending and axial strain gauge axes. Therefore, assuming the bicycle is stationary, the fraction of 1G sensed by the Y-axis accelerometer 108 is also a fraction of the maximum error force applied to the crank 100 (e.g., error force 120), so the same fraction can be applied to the maximum error force to calculate the correction factor 218 (e.g., Bend Adjust value), thereby automatically correcting the bending force value 212 to make the power 209 more accurate without having to calculate the crank angle.

[0025] Therefore, the correction factor 218 used to correct the error force 120 is determined by the following equation:

[0026] Bend Adjust=(Y AccelerationIn Gs)×Torque Of Crank At 3OClock

[0027] Where Bend Adjust is the correction factor and Torque Of Crank At 30Clock is the error force 120 when the crank 100 is at the 3 o'clock position (e.g., the maximum error force). In some embodiments, the maximum error force is determined using a calibration routine in which the crank 100 is positioned at the 3 o'clock position and a value is read from the strain gauge 103. Advantageously, the controller 204 does not need to determine the angle of the crank 100 to perform this correction, the angle is not used in the calculation, and a lookup table is not required to determine the correction factor 218 (e.g., Bend Adjust).

[0028] Table 1, "Example Calculated Values," is not a lookup table used within the power meter 102, but rather is used to display calculated correction factors for various positions of the crank 100. In this example, the user-applied force has a value of 10,000 measured on the bending axis of the crank 100 (e.g., the reading of the strain gauge 103), and the maximum error force 120 applied by the crank plus pedal weight has a maximum error offset value of 1500 on the measured bending axis of the crank 100.

[0029] Table 1 Example calculated values

[0030]

[0031]

[0032] As shown in Table 1, "Example Calculated Values," the G-forces measured by the Y-axis accelerometer 108 can be used to accurately determine an adjustment (e.g., correction factor 218 - BendAdjust) to correct the measured bending force value 212 for the weight-induced error force 120 of the crank 100 (and its pedals) at any given position without having to determine the angle of the crank 100. Advantageously, calculations within the power algorithm 206 are simplified, which can further save power.

[0033] Figure 4 is a flow chart illustrating an example method 400 for determining a correction factor for the bent axis of a crank of a pedal vehicle to correct for error forces caused by the mass of the crank and pedals. The method 400 is implemented in the power algorithm 206 and is, for example, at least partially based on Figure 2 The auto-zero algorithm 208 is implemented.

[0034] At block 402 , the method 400 captures a bending force value from the strain gauge indicating a force applied to the crank. In one example of block 402 , the power algorithm 206 reads the strain gauge 103 to determine the bending force value 212 .

[0035] At block 404 , the method 400 captures an accelerometer value from the Y-axis accelerometer. In one example of block 404 , the power algorithm 206 reads the accelerometer value 214 from the Y-axis accelerometer 108 .

[0036] At block 406, method 400 calculates a correction factor based on the accelerometer value and the maximum error force value. In one example of block 406, power algorithm 206 calls autozero algorithm 208 to calculate fraction 216 by dividing accelerometer value 214 by a value corresponding to a 1 G force sensed by the accelerometer, and then calculates correction factor 218 by multiplying the maximum error force value by fraction 216.

[0037] At block 408 , the method 400 subtracts the correction factor from the bend force value to form the auto-zero bend force value. In one example of block 408 , the power algorithm 206 subtracts the correction factor 218 from the bend force value to determine the auto-zero bend force value 220 .

[0038] Thus, the power algorithm 206 improves the quality of the calculated power 209 by using the auto-zeroed bending force value 220 without having to determine the angle of the crank 100 to calculate the adjustment.

[0039] Changes may be made to the methods and systems described above without departing from the scope of this disclosure. It should be noted, therefore, that all matter contained in the above description or shown in the accompanying drawings is to be interpreted as illustrative and not restrictive. The following claims are intended to cover all generic and specific features described herein and all statements of the scope of the methods and systems described herein that, from a language perspective, could be said to fall within their scope.

[0040] Feature combination

[0041] The features described above and the features claimed below can be combined in various ways without departing from the scope of this document. The following examples illustrate some possible, non-limiting combinations:

[0042] (A1) A power meter for a pedal vehicle, comprising: a strain gauge for sensing a bending force applied to a crank of the pedal vehicle; a Y-axis accelerometer for sensing a Y-axis acceleration relative to the crank; a controller having a processor and a memory storing machine-readable instructions, the machine-readable instructions, when executed by the processor, causing the controller to: read a bending force value from the strain gauge; read an accelerometer value from the Y-axis accelerometer; calculate a correction factor based on the accelerometer value and a maximum error force value; and determine an auto-zero bending force value corrected for the effects of gravity on the crank by subtracting the correction factor from the bending force value.

[0043] (A2) In the embodiment of (A1), the maximum error force value corresponds to the error force sensed by the strain gauge when the crank is at the 3 o'clock position.

[0044] (A3) In either embodiment of (A1) or (A2), the Y-axis acceleration is perpendicular to the length of the crank and parallel to the rotation plane of the crank.

[0045] (A4) In any of the embodiments of (A1)-(A3), the memory further stores machine-readable instructions that, when executed by the processor, cause the controller to: calculate a fraction by dividing the accelerometer value by a value corresponding to 1 G; and calculate a correction factor by multiplying the maximum error force value by the fraction.

[0046] (A5) In any of the embodiments of (A1)-(A4), the memory further stores machine-readable instructions that, when executed by the processor, cause the controller to: calculate a power input to the crank based on the auto-zero bending force value.

[0047] (A6) In any of the embodiments of (A1)-(A5), the memory further stores machine-readable instructions that, when executed by the processor, cause the controller to: repeat the reading, the calculating, and the subtracting at intervals to determine an auto-zero bending force value at any position of the crank as it rotates to drive the pedal vehicle.

[0048] (A7) In any of the embodiments (A1)-(A6), the automatic zero bending force value is determined without determining the angle of the crank.

[0049] (B1) A strain gauge automatic zeroing method for determining a correction factor for a bending axis of a crank of a pedal vehicle, the correction factor correcting an error force caused by the mass of the crank and pedals, the strain gauge automatic zeroing method comprising: capturing a bending force value indicating a force applied to the crank from a strain gauge; capturing an accelerometer value from a Y-axis accelerometer; calculating a correction factor based on the accelerometer value and a maximum error force value; and determining an automatic zeroing bending force value by subtracting the correction factor from the bending force value.

[0050] (B2) In the embodiment of (B1), the capturing step and the calculating step are repeated at intervals while the crank rotates to drive the pedal vehicle.

[0051] (B3) In either embodiment of (B1) or (B2), the automatic zeroing of the bending force is determined without calculating the crank angle.

[0052] (B4) In any of the embodiments of (B1)-(B3), in the step of capturing the bending force value, a strain gauge is attached to the crank at a location where bending of the crank is sensed.

[0053] (B5) In any of the embodiments of (B1)-(B4), further comprising: calculating the power input to the crank based at least in part on the auto-zero bending force value.

[0054] (B6) In any of the embodiments of (B1)-(B5), in the step of capturing accelerometer values, a Y-axis accelerometer is positioned on the crank to sense acceleration perpendicular to the length of the crank and parallel to the rotational plane of the crank.

[0055] (B7) In any of the embodiments of (B1)-(B6), the maximum error force value corresponds to the error force sensed by the strain gauge when the crank is at a 3 o'clock position and no other force is applied to the crank.

[0056] (B8) In any of the embodiments of (B1)-(B7), the calculating includes: calculating a fraction by dividing the accelerometer value by a value of 1 G; and calculating the correction factor by multiplying the maximum error force value by the fraction.

Claims

1. A power meter for a bicycle, comprising: a strain gauge for sensing a bending force applied to a crank of the pedal vehicle; a Y-axis accelerometer for sensing Y-axis acceleration relative to the crank; as well as A controller having a processor and a memory storing machine-readable instructions that, when executed by the processor, cause the controller to: reading a bending force value from the strain gauge; Reading an accelerometer value from the Y-axis accelerometer; calculating a correction factor based on the accelerometer value and the maximum error force value; as well as An auto-zero bending force value corrected for the effects of gravity on the crank is determined by subtracting the correction factor from the bending force value.

2. The power meter according to claim 1, wherein The maximum error force value corresponds to the error force sensed by the strain gauge when the crank is at the 3 o'clock position.

3. The power meter according to claim 1, wherein: The Y-axis acceleration is perpendicular to the length direction of the crank and parallel to the rotation plane of the crank.

4. The power meter of claim 1 , wherein the memory further stores machine-readable instructions that, when executed by the processor, cause the controller to: calculating a score by dividing the accelerometer value by a value corresponding to 1 G; and The correction factor is calculated by multiplying the maximum error force value by the fraction.

5. The power meter of claim 1, the memory further storing machine-readable instructions that, when executed by the processor, cause the controller to calculate power input to the crank based on the auto-zero bending force value.

6. The power meter of claim 5 , the memory further storing machine-readable instructions that, when executed by the processor, cause the controller to: repeat the reading, the calculating, and the subtracting at intervals to determine the auto-zero bending force value at any position of the crank as it rotates to drive the pedal vehicle.

7. The power meter according to claim 6, wherein: The automatic zeroing of the bending force value is determined without determining the angle of the crank.

8. A strain gauge automatic zeroing method for determining a correction factor for a bending axis of a crank of a pedal vehicle, the correction factor correcting for error forces caused by the masses of the crank and pedals, the strain gauge automatic zeroing method comprising: capturing a bending force value indicative of a force applied to the crank from a strain gauge; Capture accelerometer values ​​from the Y-axis accelerometer; calculating the correction factor based on the accelerometer value and a maximum error force value; as well as An auto-zero bend force value is determined by subtracting the correction factor from the bend force value.

9. The strain gauge automatic zeroing method according to claim 8, wherein: The capturing step and the calculating step are repeated at intervals while the crank rotates to drive the pedal vehicle.

10. The strain gauge automatic zeroing method according to claim 9, wherein: The automatic zeroing of the bending force is determined without calculating the angle of the crank.

11. The strain gauge automatic zeroing method according to claim 8, wherein: In the step of capturing the bending force value, the strain gauge is attached to the crank at a location where bending of the crank is sensed.

12. The strain gauge automatic zeroing method according to claim 8, further comprising: A power input to the crank is calculated based at least in part on the auto-zero bending force value.

13. The strain gauge automatic zeroing method according to claim 8, wherein: In the step of capturing the accelerometer values, the Y-axis accelerometer is positioned on the crank to sense acceleration perpendicular to the length of the crank and parallel to the rotational plane of the crank.

14. The strain gauge automatic zeroing method according to claim 8, wherein: The maximum error force value corresponds to the error force sensed by the strain gauge when the crank is at a 3 o'clock position and no other forces are applied to the crank.

15. The strain gauge automatic zeroing method according to claim 8, wherein: The calculating includes calculating a fraction by dividing the accelerometer value by a value of 1 G; and calculating the correction factor by multiplying the maximum error force value by the fraction.

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