Designing an In-Shoe Pressure and Shear Measuring System Using NI LabVIEW, DAQ, and SCXI

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"We used NI LabVIEW, DAQ, and SCXI to create a multichannel system that integrates every sensor into an array with real-time detection capacity for force distribution to measure plantar pressure."

- Vei Jye Thor, National University of Singapore

The Challenge:
Developing a system to gather data for localized plantar pressure and shear to predict and prevent foot ulcerations.

The Solution:
Using NI LabVIEW, DAQ, and SCXI to simultaneously measure localized plantar pressure on sites at risk for plantar ulceration and reulceration.

Author(s):
Vei Jye Thor - National University of Singapore
Jin Huat Low - National University of Singapore

The foot is the weight-bearing interface between the body and footwear during all daily locomotion activities. The weight of the body during locomotion is mainly transmitted and distributed across the calcaneus in the heel, the hallux, and the sesamoid bones of the first and second metatarsal head (MTH). Hence, the plantar soft tissues beneath these weight-bearing regions undergo large loads and deformation during dynamic activities. Abnormal, excessive plantar pressure and shear are potential risk factors for foot-related problems such as forefoot pain, hallux valgus deformity, and calluses. We designed an in-shoe measurement system to examine the foot pressure distribution of patients with foot-related problems.

Knowledge of the 3D plantar force distributions is essential for an enhanced understanding of foot function, the plantar tissue injury risks associated with normal and pathological foot conditions, industrial footwear design, and biomimetic walking robots. The Kistler force plate, currently available on the market, cannot measure shear force. Therefore, in the future, we plan to design a platform to measure static and dynamic plantar pressure and shear stress distribution.

Methodology

We developed an in-shoe pressure measurement system to address the above-mentioned issues. First, we took out the shoe insole and secured it to the foot with rubber bands. We used white correction fluid to mark four spots on the aforementioned regions on the bottom surface of the insole. Next, we inserted toothpicks into the insole at the four marked spots as guides and used a hammer and punching tool to make the holes. Then we inserted four sensors into the punched holes of the insole.

We cut grooves adjacent to the punched holes to insert a small section of the wire connecting the sensor into the groove, which ensured that the sensor did not tilt. We used tape at the lower surface of the insoles to ensure that the sensors are slightly pushed upward so the top of the sensor is flush with the upper face of the insole surface. We cut round acrylic pieces of 1 mm thickness and placed them on top of the sensor to obtain plantar pressure comparable to literature review (soft, deformable, and dispersed force exerted by the foot). In addition, we used masking tape to secure the sensors and wires to the insole. We cut a hole on the right side of the shoe and threaded all wires through that hole. After connecting the wires to the data acquisition unit, we used a cable zip to neatly secure the wires together.

We calibrated the Futek sensors from the prototype with the deadweight calibration method. We balanced deadweights of 50 g, 100 g, 200 g, and 500 g on the sensors and recorded the displacement of the sensing area from a fixed reference point for each weight, ranging at intervals of 50 g. We obtained calibration equations and graphs when we plotted the force against the displacement on an Excel spreadsheet.

Preliminary Results

We connected the prototype to an NI SCXI-1520 strain gage module and, subsequently, an NI PXI-6250 DAQ module. Using NI-DAQmx driver software and LabVIEW, we created a block diagram for the strain gage measurement to test the prototype (see Figure 1). The graph in Figure 2 shows the results we obtained after the recruited test subject took one step with the prototype.

The displacement of the sensor can reach an amplitude of approximately 23 µ. Using equations obtained from calibration, the system converts the amplitude into a force value and gathers localized pressure values by the sensing area of the sensor. The system compares the pressure values with normal values, and if it obtains abnormally high localized pressure, the subject is at risk for developing foot ulcerations. Therefore, the subject must seek early treatment to prevent further problems, which could ultimately lead to amputation as seen in 84 percent of all foot ulceration complications.

One of the more popular products manufactured by our competitor, the Kistler force plate, uses piezoelectric transducers that cover the whole plantar surface of the foot and simultaneously measure all three ground reaction and force components on the plantar surface of the foot. However, this system cannot measure plantar pressure and shear, especially at sites that are at risk for plantar ulceration and reulceration. This motivates us to create a new prototype to address the issue.

Final Results

The results indicate that our system can measure localized pressure at different weight-bearing regions. Tables 1 and 2 show the percentage of body weight distributed among the different weight bearing regions during static standing and walking, respectively. Three values are extracted and compared against literature review:

  • First, Jacob (2001) has reported that 23.8 percent of body weight is distributed under hallux during the push-off phase. This is higher than the values measured by our prototype (23.8 percent minus 9.7 percent = 14.1 percent). Therefore, our preliminary data is not as accurate as we had hoped.
  • Next, the second metatarsal head is the area with the greatest amount of stress and strain during the terminal stance of walking, which corresponds with our data.
  • Third, research (Jacob, 2001) shows that stress is highest at the heel pad during the heel strike, which also corresponds with our data.

The system we created may be useful in measuring the localized plantar pressure and shear which are the key factors of causing foot ulcers especially in people with diabetes. It is helpful in detecting regions of the foot with a high risk of developing ulcerations and in therapeutic footwear design.

In the future, we plan to design a pressure and shear gait platform consisting of nine custom-made miniature tactile force sensors. Each sensor measures the vertical pressure, anterior-posterior, and medial-lateral shear force distributions on the plantar foot surface.

Seamless Integration

We used NI LabVIEW, DAQ, and SCXI to create a multichannel system that integrates every sensor into an array with real-time detection capacity for force distribution to measure plantar pressure. Compared to Kistler force plate, our in shoe pressure measuring system is relatively easier to record data to suit our objective of measuring localized plantar pressure due to its high sampling rate. The maximum sampling rate of the SCXI DAQ platform is 2.27 kS/s (2,270 Hz), which is more than enough for gait applications with contact stresses of approximately 0.3 to 0.5s HaH.

References

Jacob, H.A.C. (2001) Forces acting in the forefoot during normal gait – an estimate. Clinical Biomechanics 16 (2001) 783-792.

Author Information:
Vei Jye Thor
National University of Singapore
21 Lower Kent Ridge Road
119077
Singapore
Tel: 90117102
vj_thor@hotmail.com

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