Module for Monitoring the Probe-Skin Contact Force in the Study of Vibration Perception on the Wrist

Abstract

This paper presents a module for monitoring the contact force between a probe for measuring vibration perception on the wrist and the skin. The module was designed for an original measuring stand for the automatic testing of the vibrotactile discrimination thresholds using the psychophysical adaptive method of 1 up-2 down with two or three interval forced choices (2IFC, 3IFC). Measurement methods were implemented in LabVIEW software. The inspiration for the project was the need to check the possibility of building a vibrating interface for transmitting information through vibrations delivered to the wrist via a bracelet. The test procedure on the wrist is not standardized; however, during its development, the recommendations of the Polish Norm-International Organization for Standardization PN-ISO 13091-1, 2006 were adopted. This standard contains methods for measuring vibration sensation thresholds on the fingertips for the assessment of neural dysfunction. The key to the repeatability of measurements seems to be the ability to continuously control the pressure of the measuring probe on the skin. This article compares two solutions for measuring the contact force along with an analysis of their accuracy and the impact of vibrations on the measured values. Moreover, the results of measurements of vibrotactile amplitude and frequency discrimination thresholds obtained on the ventral wrist at five frequencies (25, 32, 63, 125 and 250 Hz) are presented.

Keywords: force measurement; smart city for blind people; strain gauge beam; vibrotactile discrimination.

Figure 1

Example course of a psychometric function with marked the point of subjective equality (PSE) and the difference threshold (DTh) (logistic function was adjusted to the data).

Figure 2

The course of the psychometric function—example results obtained in the pilot tests: (a) for subject that did not achieve 100% correct indications for the stimulus pairs with the largest amplitude difference and (b) for subject that almost unmistakably indicated a more intense stimulus even for pairs of stimuli with the smallest difference in amplitudes.

Figure 3

The illustrative sketch of the measuring stand for testing the differential vibration thresholds on the wrist: (A)—probe, (B)—support of the subject’s forearm, (C)—accelerometer PCB M354C03, (D)—modal shaker TMS 2004E, (E)—the probe-skin contact force monitoring module, (F)—vibration insulation.

Figure 4

Schematic diagram of the Wheatstone bridge circuit.

Figure 5

Signal conditioning diagram.

Figure 6

Low-pass filter and a general diagram of an operational amplifier: (a) based on TL084; (b) based on INA128P.

Figure 7

Connection diagram of strain gauge beam NA27 and HX711 module (exc—strain gauge beam excitation; input A—analog input; GND—ground, DOUT—Serial Data Output; PD_SCK—Power Down and Serial Clock Input; MCU—microcontroller unit, VCC—Voltage Common Collector).

Figure 8

The nature of the signal changes from when the system is powered up: (a) AnAmp; (b) HX711.

Figure 9

Changes in max-min in time intervals: 5 min (blue), 1 min (red), 10 s (black): (a) AnAmp; (b) HX711.

Figure 10

The vibrotactile amplitude discrimination thresholds: (a) median value and (b) tabular summary of median and quartile deviation values.

Figure 11

The vibrotactile amplitude discrimination thresholds shown as Weber fraction Δf/f thresholds: (a) median value and (b) a tabular summary of median and quartile deviation values.

Figure 12

Distribution of forces in the presence of vibrations.