This document proposes measuring pinch force using the geomagnetic sensor in a smartphone. A device with embedded magnets and springs is attached to the smartphone. As force is applied, the magnet's distance from the sensor changes, altering the magnetic flux density. Measurements found a strong correlation between force and magnetic flux density. Future work includes testing different smartphone models and collecting user feedback to improve usability.

1. Pinch Force Measurement Using a Geomagnetic Sensor
1 ) K e i o U n i v e r s i t y
2 ) L Y C o r p o r a t i o n
3 ) T o k y o U n i v e r s i t y o f T e c h n o l o g y
S a r i i Y a m a m o t o 1 ) , K a o r i I k e m a t s u 2 ) , K u n i h i r o K a t o 3 ) , Y u t a S u g i u r a 1 )
2024 IEEE/SICE International Symposium on System Integration

2. • The importance of hand muscle strength in medical practice[1]
• Types of hand strengths: Grip, Pinch(Tip, Key)
• Few opportunities to measure hand muscle strength
• Commercial measuring instruments are expensive and bulky, making
widespread use difficult
2
[1] Mathiowetz V, Kashman N, Volland G, Weber K, Dowe M, Rogers S. Grip and pinch strength: normative data for adults. Arch Phys Med Rehabil. 1985 Feb;66(2):69-74. PMID: 3970660.
[2] TOEI LIGHT, https://www.toeilight.co.jp/recommend/20410/, (Accessed: 2022/07/03
[3] TIGHER, R-364-A, https://www.tigermedical.jp/r364a.html, (Accessed: 2022/07/03)
Background
Grip meter[2] Pinch meter[3]

3. • Enable measurement of pinch strength on a daily basis
• Use smartphone sensors
• No external battery or sensor other than the smartphone
• Lower cost than purchasing a dedicated measuring device
• Aim for use in treatment and rehabilitation of hand diseases and
trauma
• Simple rehabilitation system at home
• Possible even in the absence of a physician
• Increase opportunities for pinch strength measurement
Purpose
3

4. Related Work: Grip Strength Measurement by Flexible Objects
[4] Matsumoto, N., Fujita, K., Sugiura, Y.: Estimation of grip strength using monocular camera for home-based hand rehabilitation; In SICE Journal of Control, Measurement, and System Integration, Vol.14,
pp.1-11 (2021).
[5] Jaber, R., Hewson D. J., Duchˆene, J.: Design and validation of the Grip-ball for measurement of hand grip strength; In Medical Engineering Physics, Vol. 34, pp. 1356-1361 (2012).
• Measure the angle of the fingers from an image
of a hand holding a ball[4]
• Limited measurement range
• A pressure sensor built into the ball that
estimates grip strength from the pressure value
at the time of gripping[5]
• Costly to create devices with embedded sensors
Matsumoto et al.[4]
Jaber et al.[5]
4

5. Related Work: Tracking by Magnet
[6] McIntosh, J., Strohmeier, P., Knibbe, J., et al.: Magnetips: Combining Fingertip Tracking and Haptic Feedback for Around-Device Interaction; In Proceedings of the 2019 CHI Conference on Human Factors in
Computing Systems, pp.1-12 (2019).
[7] Miyatake, Y., Liang, R., Chen, B., Siio, I.: FieldSweep: A 2D Tracking System With Embedded Magnets and a Smartphone; In Extended Abstracts of the 2021 CHI Conference on Human Factors in Computing
Systems, Vol.215, pp.1-6 (2021).
Magnetips[6]
• Operate the smartwatch with a magnet
attached to the fingertip
FieldSweep[7]
• 2D tracking using magnets and
smartphones
McIntosh et al.[6]
Miyatake et al.[7]
5

6. Proposed Method
Pinch force measurement using a smartphone with
built-in 3-axis geomagnetic sensor and neodymium magnet
• Apply load to the attachment with magnet attached
• Acquire changes in magnetic flux density
• Create a regression model from changes in load and magnetism
An operating principle
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7. Principle: 3-axis Geomagnetic Sensor
[8]AsachiKASEI, https://www.akm.com/jp/ja/products/tri-axis-magnetic-sensor/what-makes-akm-different/why-use-tri-axis-magnetic-sensors/, (Accessed: 2022/07/03)
• Enable to detect magnetic fields applied from any direction
• Uniaxial and biaxial magnetic fields cannot be detected in some directions
Geomagnetic sensor: [Left]uniaxial [Right]3-axis[8]
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8. Pinch Measurement
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9. Implementation (1/3)
• Built-in magnet in upper aluminum plate
• When the plate is pushed in, the upper plate moves up and down
with the spring in the pillar
• Distance between smartphone and magnet changes
Device configuration
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10. Implementation (2/3)
• Magnetic flux density of the magnet is 240 mT
Parts Width[mm] Length [mm] Height [mm]
Upper plate 40 50 3
Lower plate 40 50 3+55(Pillow length)
Diameter[mm]
Natural
length[mm]
Coherent
length[mm]
Material
Maximum
load[N]
13 50 25.3 SWP-A 43.25
Device Dimensions
Spring Features
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11. Implementation (3/3)
• Smartphone: HUAWEI P30 lite
• Geomagnetic sensor mounted in the upper left corner of the
smartphone
• Device on top of the case
Device attached to smartphone
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12. Measurement
• Investigate the relationship between force and
magnetic flux density
• Apply force from 0N to 200N in 5N increments by
force gauge
• Difference of magnetic flux density between at the
start of the measurement and the maximum during
the measurement
• Magnetic flux densities are absolute values on
the three axes
• 𝑥2 + 𝑦2 + 𝑧2
Force gauge
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13. Result: Regression Model
𝐹 N = −0.0003 × 𝑛2 μT + 0.5091 × 𝑛 μT + 2.1704
• 𝐹: Force
• 𝑛: Magnetic flux density
• The measurement results show a strong correlation between force and
magnetic flux density
Regression Model
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14. Verification under Noise Conditions
(A): with a smartphone in the surroundings
𝐹 N = −0.0004 × 𝑛2 μT + 0.5447 × 𝑛 μT − 1.2694
(B): with a smartphone and laptop in the surroundings
𝐹 N = −0.0004 × 𝑛2 μT + 0.5444 × 𝑛 μT + 2.0227
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y = -0.0004x2 + 0.5447x - 1.2694
R² = 0.999
0
50
100
150
200
250
0 100 200 300 400 500 600
Force(N)
Magnetic Flux Density (μT)
Force(N)
Poly. (Force(N))
y = -0.0004x2 + 0.5444x + 2.0227
R² = 0.9989
0
50
100
150
200
250
0 100 200 300 400 500 600
Force(N)
Magnetic Flux Density (μT)
Force(N)
Poly. (Force(N))
(A): Regression Model (B): Regression Model

15. Constraints and Future Work
Constraints
• Error due to geomagnetic effects
• Calibration before measurement
• Reconstruction of regression model when using different springs and
magnets
Future Work
• Investigate compatibility of different model smartphones
• Conduct user surveys
• Usability improvement
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16. Summary
Background The importance of hand muscle strength in medical practice
Related Work
Grip strength measurement by flexible objects
Tracking by magnet
Proposed Method Pinch force measurement using a geomagnetic sensor in smartphone
Implementation
Embedded neodymium magnets in a device composed of aluminum and springs
Attach the device to smartphone
Results There is a strong correlation between force and magnetic flux density
Future Work
Investigate compatibility of different model smartphones, magnets and springs
User surveys
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