Automatic strain gauge zeroing without rotation angle
The power meter system uses a Y-axis accelerometer to automatically correct strain gauge readings for gravitational torque without determining crank angle, enhancing measurement accuracy.
Patent Information
- Application Number
- JP2025536055
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-12-19
- Filing Date
- 2023-12-13
- Publication Date
- 2026-01-09
AI Technical Summary
Conventional power meters for bicycles struggle with inaccuracies in strain gauge measurements due to the torque exerted by the crank's mass, which varies with its rotation, leading to significant errors in power calculation.
A power meter system that uses a Y-axis accelerometer to detect gravitational forces on the crank, calculating a correction factor without determining the crank's angle, and automatically adjusts strain gauge readings to compensate for these errors.
Improves the accuracy of power measurements by eliminating the need to calculate crank angle, providing precise power calculations without angle-dependent corrections.
Smart Images
Figure 2026500934000001_ABST
Abstract
Description
[Technical Field]
[0001] This application claims priority to U.S. Patent Application No. 63 / 433,513, entitled "Automatic Zeroing of Strain Gauges Without Using Rotation Angle," filed December 19, 2022, which is incorporated by reference in its entirety. [Background technology]
[0002] A power meter attached to a bicycle crank uses strain gauges to measure the force applied to the crank, and from there, determine the power a cyclist is producing. However, as the crank rotates 360 degrees, the mass of the pedals and crank exerts a torque on the strain gauge, resulting in a force not caused by the cyclist. When the crank is in the 3 o'clock position (pointing toward the front of the bike), its mass adds to the torque measured by the strain gauge; when the crank is in the 9 o'clock position (pointing toward the rear of the bike), its mass subtracts from the torque measured by the strain gauge. This resulting force offset can be as much as ±1.5 Nm, which corresponds to the equivalent of 15 W of power at a typical riding cadence.
[0003] A known solution to compensate for this power involves using an accelerometer in the power meter to detect the X and Y accelerations affecting the crank, determining the angle (e.g., orientation) of the crank based on the X and Y accelerations, and applying a correction factor to the strain gauge measurements based on that angle. Summary of the Invention
[0004] One aspect of the present embodiment involves the recognition that conventional power meters calculate crank angle to compensate for the effects of gravity on strain gauge power measurements. The present embodiment improves on conventional power calculations by automatically zeroing the strain gauge output to cancel out the effects of gravity without calculating crank angle. Furthermore, the accuracy of strain gauge-derived power measurements is improved because the angle error that occurs when calculating crank angle is avoided.
[0005] In one embodiment, the technology described herein relates to a power meter for a pedal-operated vehicle, the power meter including strain gauges that detect bending forces applied to a crank of the pedal-operated vehicle, a Y-axis accelerometer that detects Y-axis acceleration relative to the crank, and a controller having a processor and memory storing machine-readable instructions that, when executed by the processor, cause the controller to read bending force values from the strain gauges, read accelerometer values from the Y-axis accelerometer, calculate a correction factor based on the accelerometer values and the maximum error force value, and subtract the correction factor from the bending force value to determine an auto-zero bending force value that corrects for the effects of gravity on the crank.
[0006] In one embodiment, 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-powered vehicle to correct for error forces caused by the mass of the crank and pedals, the method including obtaining bending force values indicative of forces applied to the crank from strain gauges, obtaining accelerometer values from a Y-axis accelerometer, calculating a correction factor based on the accelerometer values and the maximum error force value, and subtracting the correction factor from the bending force value to determine an auto-zeroed bending force value. [Brief explanation of the drawings]
[0007] [Figure 1]FIG. 1 is a schematic diagram illustrating an example of a crank with a power meter attached that includes an accelerometer package with an X-axis accelerometer (parallel to the crank) and a Y-axis accelerometer (orthogonal to the crank), according to an embodiment.
[0008] [Figure 2] 2 is a block diagram illustrating a more detailed example of the power meter of FIG. 1, according to an embodiment.
[0009] [Figure 3] FIG. 3 is a graph illustrating an example of the output from a Y-axis accelerometer as the crank rotates about the crank bearing, in accordance with an embodiment.
[0010] [Figure 4] FIG. 4 is a flow chart illustrating an example method for determining a bending axis correction factor for a pedal-powered vehicle crank to compensate for error forces caused by the mass of the crank and pedals during crank rotation, according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION
[0011] US Patent Nos. 10,060,738 and 11,033,217 disclose power and cadence meters for bicycles and are each incorporated by reference herein in their entirety.
[0012] FIG. 1 is a schematic side view of an example crank 100 to which is attached a power meter 102 including at least one strain gauge 103 and an accelerometer package 104 having at least a Y-axis accelerometer 108, where 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. The crank 100 is used to power a pedal-driven vehicle, such as a bicycle, where the crank 100 rotates about crank bearings 101, as indicated by arrow 112, to drive the vehicle forward. Pedals (not shown for simplicity) are attached in openings 114. The accelerometer package 104 may include an X-axis accelerometer 106 and a Z-axis accelerometer (not shown), where the X-axis is parallel to the length 116 of the crank 100 and the Z-axis accelerometer is perpendicular to both the X-axis accelerometer 106 and the Y-axis accelerometer 108. The accelerometer package 104 cannot distinguish between acceleration and gravity and continuously detects the Earth's gravity.
[0013] FIG. 2 is a block diagram illustrating a more detailed example of the power meter 102 of FIG. 1. The power meter 102 includes a battery 202 (optionally rechargeable), a controller 204 (e.g., a microprocessor, microcontroller, ASIC, etc.), a strain gauge 103, an accelerometer package 104, and a wireless interface 210. The power meter 102 may include other sensors and components without departing from the scope of this specification. The controller 204 includes a processor 205 and a memory 207 that stores machine-readable instructions executable by the processor 205 to implement the functions described herein. The controller 204 may include at least one analog-to-digital converter that converts analog signals to digital signals for storage in the memory 207 and for processing by the processor 205. Controller 204 also includes a power algorithm 206 stored in memory 207 which, when executed by processor 205, causes controller 204 to determine power 209 input by a user into crank 100 based on input obtained from strain gauges 103 and input obtained from at least Y-axis accelerometer 108. Power algorithm 206 further includes an auto-zeroing algorithm 208 that compensates for the effect of gravity on crank 100 as detected by strain gauges 103 without determining the angle of crank 100. The following pseudocode provides an example of auto-zeroing algorithm 208:
[0014] bend = readStrain().bend accelY = readAccelerometer().y accelY / = ACCELEROMETER_COUNTS_PER_G bend -= accelY * PEDALPLUSCRANK_AT_3OCLOCK
[0015] Thus, correcting for flexion makes power calculations using flexion more accurate than when no correction is applied, and makes the correction calculation more efficient compared to prior art corrections that require crank angle. In one example of operation, power algorithm 206 determines power 209 at regular intervals (e.g., 1 / 26 seconds) by repeatedly taking readings from both strain gauge 103 and Y-axis accelerometer 108 with crank 100 in different positions as it rotates about crank bearing 101. Advantageously, auto-zero algorithm 208 calculates auto-zero flex force value 220 without having to determine the angle of crank 100, thereby simplifying functionality within controller 204.
[0016] The controller 204 controls the wireless interface 210 to communicate with one or more of the smartphone 240, the bike computer 250, and the other computer 260. The power algorithm 206 may also determine cadence as part of determining the power and work performed by the user. Cadence, also referred to as pedaling rate, is a measure of the number of revolutions per minute of the crank 100. Cadence is a measure of angular velocity, which is proportional to, but not identical with, wheel speed. The cadence and the auto-zeroed bending force value 220 are used, at least in part, to calculate the power the user exerts on the crank 100.
[0017] Figure 3 is a graph 300 illustrating an example of the output 302 from the Y-axis accelerometer 108 as the crank 100 rotates about the crank bearing 101. Figures 1, 2, and 3 are best viewed in conjunction with the following discussion.
[0018] As the crank 100 rotates, the accelerometer package 104, and therefore the X-axis accelerometer 106 and the Y-axis accelerometer 108, rotate with the crank 100. Thus, the orientation of the X-axis accelerometer 106 and the Y-axis accelerometer 108 changes relative to the Earth's gravity 110. Therefore, as the crank 100 rotates, the X-axis accelerometer 106 and the Y-axis accelerometer 108 simultaneously sense the Earth's gravity 110 and, because they are oriented 90 degrees relative to each other, output accelerometer values in the form of cosine and sine waves, respectively. As shown in graph 300, the output 302 (e.g., Y-axis acceleration) from the Y-axis accelerometer 108 is zero at the 12 o'clock position 304, 1 G at the 3 o'clock position 306, 0 G at the 6 o'clock position 308, -1 G at the 9 o'clock position 310, and returns to 0 G at the 12 o'clock position 312. Although not shown in the figure, the accelerometer values from the X-axis accelerometer 106 are 1 G at 12 o'clock and -1 G at 6 o'clock. As the crank 100 rotates around the crank bearing 101, the accelerometer values, when plotted (X, Y), form a circle.
[0019] Strain gauge 103 may represent one or more strain gauges strategically placed on crank 100 to detect forces applied to crank 100 by a user. However, crank 100 (and associated pedals, not shown) have effective mass that applies error forces 120 to crank 100 (e.g., forces based on gravity due to the mass of crank 100 and its pedals), causing the output of strain gauge 103 to include error forces 120 that are not attributable to forces applied by the user. Traditionally, to compensate for error forces, prior art power meters determined the angle of the crank and applied a correction factor based on the determined angle to compensate for error forces 120. Unlike traditional correction calculations, power meter 102 determines the correction factor using only accelerometer values from Y-axis accelerometer 108 without determining the angle of crank 100 (angle cannot be determined from accelerometer values from a single axis). For example, a 45-degree crank in an elevator ascending at 0.3 g acceleration will experience the same Y-axis reading (e.g., 1.0 g) corresponding to the error force 120 as a stationary 90-degree crank. Similarly, without accelerometer values from the X-axis accelerometer 106, the power meter 102 cannot determine whether the crank 100 is pointing 45 degrees forward or 45 degrees backward. Without the ability to distinguish between the forward and backward positions 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 detected gravitational force, but shear loads do not.
[0020] One aspect of this embodiment involves the recognition that the amount of correction factor required for the bend axis (e.g., zero offset adjustment) is proportional to the accelerometer value detected by the Y-axis accelerometer 108, which corresponds to Earth's gravity 110. This is because, for a stationary bicycle, the Y-axis accelerometer 108 detects how much gravity is pulling sideways on the crank, and this value is the value needed to apply a correction to the bend axis and the axial strain gauge axis. Therefore, assuming the bicycle is stationary, the percentage of 1 G detected by the Y-axis accelerometer 108 is also a percentage of the maximum error force (e.g., error force 120) applied to the crank 100; therefore, the same percentage can be applied to the maximum error force to calculate the correction factor 218 (e.g., BendAdjust value), automatically correcting the bend force value 212 and making the power 209 more accurate without calculating the crank angle.
[0021] Therefore, the correction factor 218 for correcting the error force 120 is determined by the following formula:
[0022]
number
[0023] where "BendAdjust" is the correction factor and "TorqueOfCrankAt3OClock" is the error force 120 when the crank 100 is at the 3 o'clock position (e.g., the maximum error force). In one embodiment, to determine the maximum error force, a calibration procedure is used in which the crank 100 is placed at the 3 o'clock position and values are read from the strain gauges 103. Advantageously, the controller 204 does not need to determine the angle of the crank 100 to make this correction; no angle is used in the calculation, and no look-up table is required to determine the correction factor 218 (e.g., BendAdjust).
[0024] The example calculations in Table 1 are not a look-up table used within the power meter 102, but are provided to illustrate the correction factors calculated for various positions on the crank 100. In this example, the user applied force is 10,000 (e.g., strain gauge 103 reading), which is a value measured at the bending axis of the crank 100, and the maximum error force 120, plus the weight of the crank and pedals, has a maximum error offset value of 1500, which is measured at the bending axis of the crank 100.
[0025] Calculated value example [Table 1]
[0026] As shown in the example calculations in Table 1, the G-force measured by the Y-axis accelerometer 108 can be used to accurately determine the amount of adjustment (e.g., correction factor 218-BendAdjust) to correct the measured bending force value 212 of the error force 120 caused by the weight of the crank 100 (and its pedals) at any position of the crank 100, without determining the angle of the crank 100. Advantageously, the calculations within the power algorithm 206 are simplified, which may result in further power savings.
[0027] 4 is a flow chart illustrating an example method 400 for determining a bending axis correction factor for a pedal-powered vehicle crank to compensate for error forces caused by the mass of the crank and pedals. Method 400 may be implemented in power algorithm 206, such as at least a portion of auto-zeroing algorithm 208 of FIG. 2.
[0028] In block 402, the method 400 obtains a bending force value from the strain gauge, which is indicative of the force applied to the crank. In one example of block 402, the power algorithm 206 reads the strain gauge 103 and determines the bending force value 212.
[0029] In block 404, the method 400 obtains accelerometer values from the Y-axis accelerometer. In one example of block 404, the power algorithm 206 reads the accelerometer values 214 from the Y-axis accelerometer 108.
[0030] At block 406, the method 400 calculates a correction factor based on the accelerometer value and the maximum error force value. In one example of block 406, the power algorithm 206 invokes the auto-zeroing algorithm 208 to calculate a ratio 216 by dividing the accelerometer value 214 by a value corresponding to 1 G of force detected by the accelerometer, and then multiplying the maximum error force value by the ratio 216 to calculate the correction factor 218.
[0031] In block 408, the method 400 subtracts the correction factor from the flex force value to form an auto-zero flex force value. In one example of block 408, the power algorithm 206 subtracts the correction factor 218 from the flex force value to determine the auto-zero flex force value 220.
[0032] Thus, the power algorithm 206 does not need to determine the angle of the crank 100 to calculate the adjustment, and by using the auto-zero bending force value 220, the quality of the calculated power 209 has been improved.
[0033] Changes can be made in the above-described methods and systems without departing from the scope of this specification. Accordingly, it should be noted that the matter contained in the above description or shown in the accompanying drawings should be construed as illustrative and not limiting. The following claims are intended to cover all general and specific features described herein, as well as all statements of applicability of the methods and systems herein, and may lie therebetween as a matter of language.
[0034] Combination of features The features described above and claimed below can be combined in various ways without departing from the scope of the present specification. The examples listed below are non-limiting examples of some possible combinations.
[0035] (A1) A power meter for a pedal-operated vehicle, comprising: a strain gauge that detects a bending force applied to a crank of the pedal-operated vehicle; a Y-axis accelerometer that detects a Y-axis acceleration relative to the crank; and a controller having a processor and a memory storing machine-readable instructions, which, when executed by the processor, cause the controller to read bending force values from the strain gauges, read accelerometer values from the Y-axis accelerometer, calculate a correction factor based on the accelerometer values and the maximum error force value, and determine an auto-zeroed bending force value that corrects for the effect of gravity on the crank by subtracting the correction factor from the bending force value.
[0036] (A2) In the embodiment of (A1), the maximum error force value corresponds to the error force detected by the strain gauge when the crank is in the 3 o'clock position.
[0037] (A3) In either embodiment (A1) or (A2), the Y-axis acceleration is perpendicular to the length of the crank and parallel to the plane of rotation of the crank.
[0038] (A4) In any of the embodiments (A1) to (A3), the memory further stores machine-readable instructions that, when executed by the processor, cause the controller to calculate a ratio by dividing the accelerometer value by a value corresponding to 1 G, and to calculate a correction factor by multiplying the maximum error force value by the ratio.
[0039] (A5) In any of the embodiments (A1) to (A4), the memory further stores machine-readable instructions that, when executed by the processor, cause the controller to calculate the power input to the crank based on the auto-zero bending force value.
[0040] (A6) In any of the embodiments (A1) to (A5), the memory further stores machine-readable instructions that, when executed by the processor, cause the controller to repeatedly perform readings, calculations, and subtractions at regular intervals to determine an auto-zero bending force value at any position of the crank as the crank rotates to drive the pedal-powered vehicle.
[0041] (A7) In any of the embodiments (A1) to (A6), the auto-zero adjustment bending force value is determined without determining the angle of the crank.
[0042] (B1) A strain gauge auto-zeroing method for determining a correction factor for the bending axis of a crank of a pedal-driven vehicle and correcting for error forces caused by the mass of the crank and pedals, the method comprising: obtaining a bending force value indicating the force applied to the crank from the strain gauge; obtaining an accelerometer value from a Y-axis accelerometer; calculating a correction factor based on the accelerometer value and the maximum error force value; and subtracting the correction factor from the bending force value to determine the auto-zeroing bending force value.
[0043] (B2) In the embodiment of (B1), the steps of obtaining and calculating are repeated at regular intervals as the crank rotates to drive the pedal-powered vehicle.
[0044] (B3) In the embodiment of either (B1) or (B2), the auto-zero bending force is determined without calculating the angle of the crank.
[0045] (B4) In any of the embodiments (B1) to (B3), in the step of acquiring the bending force value, the strain gauge is attached to the crank at a position where bending of the crank is detected.
[0046] (B5) In any of the embodiments (B1) to (B4), further comprising calculating a power input to the crank based at least in part on the auto-zero bending force value.
[0047] (B6) In any of the embodiments (B1) to (B5), in the step of acquiring the accelerometer value, the Y-axis accelerometer is positioned on the crank so as to detect acceleration perpendicular to the length direction of the crank and parallel to the plane of rotation of the crank.
[0048] (B7) In any of the embodiments (B1) to (B6), the maximum error force value corresponds to the error force detected by the strain gauge when the crank is in the 3 o'clock position and no other forces are being applied to the crank.
[0049] (B8) In any embodiment of (B1) to (B7), the calculating comprises calculating a ratio by dividing the accelerometer value by a value of 1 G, and calculating a correction factor by multiplying the maximum error force value by the ratio.
Claims
1. 1. A power meter for a pedal-operated vehicle, comprising: a strain gauge for detecting a bending force applied to a crank of the pedal-operated vehicle; a Y-axis accelerometer for detecting a Y-axis acceleration relative to the crank; a controller having a processor and a memory storing machine-readable instructions; The instructions, when executed by the processor, cause the controller to: Reading a bending force value from the strain gauge; causing an accelerometer value to be read from the Y-axis accelerometer; calculating a correction factor based on the accelerometer values and the maximum error force value; The power meter subtracts the correction factor from the bending force value to determine an auto-zeroed bending force value that corrects for the effect of gravity on the crank.
2. 2. The power meter of claim 1, wherein the maximum error force value corresponds to the error force detected by the strain gauge when the crank is in the 3 o'clock position.
3. 2. The power meter of claim 1, wherein the Y-axis acceleration is perpendicular to the length of the crank and parallel to the plane of rotation of the crank.
4. The memory further configures the controller to, when executed by the processor: Calculating a ratio by dividing the accelerometer value by the value corresponding to 1 G; calculating a correction factor by multiplying the maximum error force value by the percentage; 10. The power meter of claim 1, further comprising machine-readable instructions stored therein.
5. The memory further configures the controller to, when executed by the processor:
10. The power meter of claim 1, further comprising: storing machine-readable instructions for calculating a power input to the crank based on the auto-zero bending force value.
6. The memory further configures the controller to, when executed by the processor:
6. The power meter of claim 5, storing machine-readable instructions that cause the reading, calculation, and subtraction to be repeated at regular intervals to determine the auto-zero bending force value at any position of the crank as the crank rotates to power the pedal-powered vehicle.
7. 7. The power meter of claim 6, wherein the auto-zero bending force value is determined without determining the angle of the crank.
8. 1. A strain gauge auto-zeroing method for determining a bending axis correction factor for a crank of a pedal-driven vehicle and correcting for error forces caused by the mass of the crank and pedals, comprising: obtaining a bending force value indicating a force applied to the crank from a strain gauge; Get the accelerometer value from the Y-axis accelerometer, calculating the correction factor based on the accelerometer value and the maximum error force value; and subtracting the correction factor from the bending force value to determine an auto-zero bending force value.
9. 9. The strain gauge auto-zeroing method of claim 8, wherein the steps of obtaining and calculating are repeated at regular intervals as the crank rotates to drive the pedal-powered vehicle.
10. 10. The strain gauge auto-zeroing method of claim 9, wherein the auto-zero bending force is determined without calculating the angle of the crank.
11. 9. The strain gauge automatic zero adjustment method according to claim 8, wherein in the step of acquiring the bending force value, the strain gauge is attached to the crank at a position where bending of the crank is detected.
12. 9. The strain gage auto-zeroing method of claim 8, further comprising calculating a power input to the crank based at least in part on the auto-zero bending force value.
13. 9. The strain gauge automatic zero adjustment method of claim 8, wherein in the step of acquiring an accelerometer value, the Y-axis accelerometer is positioned on the crank so as to detect acceleration perpendicular to a length direction of the crank and parallel to a plane of rotation of the crank.
14. 9. The strain gauge auto-zeroing method of claim 8, wherein the maximum error force value corresponds to the error force detected by the strain gauge when the crank is at the 3 o'clock position and no other forces are being applied to the crank.
15. The calculating step comprises: calculating a ratio by dividing the accelerometer value by a value of 1 G; and 9. The strain gauge auto-zeroing method of claim 8, comprising calculating the correction factor by multiplying the maximum error force value by the percentage.