Monitoring the force of fingertip manipulation without disturbing the natural sense of touch is crucial for digitizing the skills of experienced craftsmen. However, conventional force sensors need to be put between the skin and the objects, which affects the natural sense of the skin. Here, we proposed a fingertip force sensing method based on changes of blood volume and designed a wearable photoelectric fingertip force sensing system (PFFS) for digitalization of traditional Chinese medicine (TCM) pulse diagnosis. The PFFS does not interfere with the fingertips' tactile sense while detecting fingertip force. This PFFS detects the change of blood volume in fingertip by photoelectric plethysmography and can obtain the change of output current under different fingertip forces. We also studied the effect of various factors on PFFS output signals, including emission lights of different wavelengths, ambient temperature, and the user's heartbeat artifact. We further established the relationship between the change of blood volume and fingertip force by combining experimental and theoretical methods. Moreover, we demonstrated the feasibility of the PFFS to detect fingertip forces under commonly used conditions in TCM pulse diagnosis without sensory interference. This PFFS also shows promise for perceiving the viscosity of objects and recognizing gestures in human-computer interaction. This work paves the way for the digitalization of fingertip forces during TCM pulse diagnosis and other fingertip forces under natural conditions.
Keywords: TCM pulse diagnosis; fingertip force sensing; photoelectric plethysmography; wearable sensor; without sensory interference.