Vanderbilt University

Department of Biomedical Engineering

Senior Design Project

Instrumented Wheel For Wheelchair Propulsion Assessment

Date Reported: April 22, 2008

Group #2

Jacob Connelly

Andrew Cramer

John Labiak

Advisor: W Mark Richter, PhD.

Abstract

The development of upper extremity (UE) pain and/or injury is a prevalent health concern amongst manual wheelchair users. The UEs serve as the principle means for mobility for this population, therefore, any impeding factors, such as pain or injury, will lead to a decreased quality of life. The development of UE pain and injury may be a result of improper propulsion biomechanics or poor wheelchair seating configurations. In order to quantitatively assess a manual wheelchair user’s propulsion technique for training or seating purposes, there is a need for an affordable instrumented assessment tool. The goal of the project was to develop an instrumented wheelchair wheel utilizing strain gauges that have the capability of quantitatively measuring resultant force during propulsion. Strain gauges were placed on the top and bottom of each of the pushrim tabs that couple the wheel to the pushrim. The strain gauges were then wired into a Wheatstone bridge circuit which was then connected to an instrumentation amplifier circuit. The output voltages were then sent via a data acquisition unit with Bluetooth capabilities to a local computer to be recorded, processed, and analyzed in LabView. A standard curve was created relating resultant force to output voltage. The prototype wheel developed demonstrates the ability to assess wheelchair propulsion by measuring strain created by resultant force. Small changes in voltage created by flexion in the pushrim can be sufficiently amplified in order to gain the appropriate sensitivity to clearly track the resultant force applied during propulsion.

Introduction

The development of upper extremity (UE) pain and/or injury is a prevalent health concern amongst manual propulsion wheelchair users. Sie et al. found that 64 percent of individuals within a study of 239 paraplegic patients experienced UE pain [1]. Similarly, Dalyan et al. and Gellman et al. reported the prevalence of UE pain within manual wheelchair user populations to be 59 and 68 percent, respectively [2-3]. The UEs serve as the principle means for mobility, therefore, any impeding factors, such as pain or injury, can lead to a decreased quality of life. The development of UE pain and injury may be a result of improper propulsion biomechanics or unsuitable seating conditions. In order to quantitatively study, classify, or train a manual wheelchair user's (MWU) propulsion techniques, there is a necessity for an instrumented assessment tool.

Current commercially available assessment tools, such as the SmartWheel developed by Three Rivers [6], are exceedingly expensive and measure several biomechanical forces and parameters, many of which are unnecessary for clinical applications. An instrumented wheel designed to asses solely resultant propulsion force and push cadence would severely decrease cost by providing only the necessary clinical data.

A successfully designed instrumented wheel would provide a much needed tool in seating and mobility clinics. This assessment tool would provide important information on the propulsion habits of wheelchair users. Improper propulsion could be quickly and accurately characterized, then corrected through clinical training programs that promote ideal propulsion techniques set forth by the Consortium of Spinal Cord Medicine [4]. Furthermore, modified pushrim systems could be prescribed and fitted for special case MWUs with non-correctable propulsion conditions. Pain and injury in MWUs could also be avoided by improving customized seating conditions. Typically seating is determined solely by the user's biometrics; however, it is important to also consider the user's propulsive style and needs. Richter et al. showed that propulsion characteristics are directly altered by changing wheelchair seating configurations [5]. By analyzing resultant force characteristics it is possible to optimize the user's position in relation to the pushrim to improve propulsive technique and lessen the likelihood of developing UE pain and injury. Essentially, the instrumented wheel will serve as a means to quantitatively compare various wheelchairs and seating configurations to optimize the degree of comfort and propulsive capabilities of MWUs. With comparative propulsion data, the instrumented wheel will also serve as justification for the selection of particular wheelchair equipment for insurance reimbursement purposes.

Methodology

Design of the Instrumented Wheel

Figure 1. The thinness of the tab design allows for some degree of pushrim flexibility.

In order to quantifiably assess manual wheelchair propulsion, a standard 24'' wheelchair wheel [a] and pushrim were instrumented with strain gauges. The pushrim is coupled to the wheel by three connection tabs. The pushrim tabs, which were designed in SolidWorks [b], are shown in Figure 1. The pushrim with the tabs connected is shown in Figure 2. Standard pushrims have six tabs that connect the wheel to the pushrim, providing a highly rigid and robust system with an extremely high safety factor. By reducing the number of pushrim tabs to three, the overall rigidity of the pushrim-wheel system is decreased allowing for a small degree of flexibility. Uniaxial strain gauges were attached to the top and bottom of each pushrim tab [c]. Application of force to the pushrim results in a change in the electrical resistance of the strain gauges. In addition, the increased flexibility of the pushrim from reduction to only three pushrim tabs allows for an adequate change in the resistance of the strain gauges that can be accurately measured.

The strain gauges were wired into a Wheatstone bridge circuit, Figure 3. When force is applied to the pushrim, the resistance of the strain gauges change corresponding to being in tension or compression. These changes in resistance are reflected in changes in the two voltage outputs from the Wheatstone bridge. However, the changes in the differential output voltage from the Wheatstone bridge were small, approximately 10mV, and required amplification. An instrumentation amplifier, seen in Figure 4, was implemented with a gain of 400. Calculations are shown below. Each of the three tabs utilize one Wheatstone bridge and one instrumentation amplifier which generate an output voltage due to the force applied to that tab. A printed circuit board (PCB) containing the Wheatstone bridges and instrumentation amplifiers for each of the three tabs was designed [d,e]. The wheel was then wired with 0.015'' diameter wire allowing the strain gauges to be connected to the PCB. The wire was run through the interior of the wheel, protecting it from environmental exposure and damage. The power supply for the wheel is four AAA batteries providing 6V, however a 5V-5V dc-dc converter was used in the PCB to provide a constant voltage through the circuit. The three output voltages from the three instrumentation amplifiers were then sent to an 8-pin data acquisition unit (DAQ) [f]. The PCB, DAQ, and power supply were mounted on a black plastic board. The plastic board was securely mounted to the spokes of the wheel.

The data was sent from the DAQ to LabView [g] via a USB compatible Bluetooth transceiver [h]. Within LabView the data was recorded, processed through low-pass filters, and analyzed. A standard curve relating voltage to applied resultant force was created by applying known forces (5-25lb) to the pushrim with a spring scale and recording the changes in the output voltages.

Determination of Resultant Force

In order to determine the overall applied resultant force, each tab was analyzed individually. When a force is applied to the pushrim, each tab will be strained leading to one strain gauge being in compression (C) while the other is in tension (T). The resulting strain will cause the resistance of each strain gauge to change; tension causes resistance to increase and compression causes resistance to decrease. As the resistance of the strain gauges change, there is a corresponding change in the output voltage of the Wheatstone bridge (). This change in output voltage is governed by the following equation:

(1)

where (which is the same as the nominal value of the strain gauges) and . The output voltage from the Wheatstone bridge in then amplified using the instrumentation amplifier shown in Figure 4 according to the following equation:

(2)

where is the voltage output of the instrumentation amplifier. The gain associated with the instrumentation amplifier is equal to:

(3)

where, (desired output voltage) and (approximated change in voltage from the Wheatstone bridge), thus:

(4)

The value of the resistor is equal to:

(5)

A standard curve relating output voltage and resultant force was created by applying the known forces of 5, 10, 15, 20, and 25 pounds to the pushrim using a spring scale. The forces were applied in close proximity to each of the three tabs. Each time a force was applied to the pushrim, the magnitude of the change in output voltage for each of the three tabs would change in relation to the location of the applied force. From the three output voltages corresponding to each tab a single voltage corresponding to the resultant force was determined using the following equation:

(6)

For each applied force (5-25 lbs) we determined . We then used these values to create a Voltage vs. Force Standard Curve.

Results

Figure 5 shows an image of the final design of the instrumented wheel. The wheel was constructed in order to maximize the voltage output for a given applied force. The final design utilized only three tabs of a thin aluminum design. This was intended to maximize the displacement of the pushrim upon the application of a force, increase strain on the pushrim attachments, and in turn increase the change in gauge resistance and voltage output accordingly. Strain gauges were mounted to the top and bottom of each tab using adhesive, and wiring was run from the strain gauges through the wheel well (Figure 6) to the Wheatstone bridge. The electronic circuit constructed included a half-bridge circuit and an instrumentation amplifier for each of the three tabs respectively. The instrumentation amplifier performed according to the anticipated specifications and calculations made in the Methods section. The recorded output voltages, seen in Table 1 below, are approximately 350.0 times higher than the voltage output from the corresponding strain gauges. When compared to the calculated gain of 400, the amplifier yielded an insignificant level of error and achieved the desired result of amplification. The Wheatstone bridge, instrumentation amplifier, DAQ with wireless Bluetooth transceiver, and power supply were mounted to the wheel spokes, as seen in Figure 7.

Trial 1 / Trial 2 / Average
Tab / / / / / / / / /
Force (lb)
5 / 0.1 / 0.3 / 0.05 / 0.45 / 0.05 / 0.4 / 0.2 / 0.65 / 0.550
10 / 0.4 / 0.2 / 0.25 / 0.85 / 0.15 / 0.75 / 0.2 / 1.1 / 0.975
15 / 0.6 / 0.2 / 0.3 / 1.1 / 0.2 / 0.8 / 0.35 / 1.35 / 1.23
20 / 0.85 / 0.35 / 0.3 / 1.5 / 0.3 / 0.9 / 0.4 / 1.6 / 1.55
25 / 1.1 / 0.3 / 0.3 / 1.7 / 0.4 / 1.25 / 0.4 / 2.05 / 1.88

Table 1. Output voltage data was recorded over a range of applied force values and was used to construct the standard curve.

Table 1 shows the voltage data from trial 1 and 2 and their average. Trial 3 produced inaccurate data and as a result the data was omitted. Using the average values for each applied force we created a Voltage vs. Force standard curve, which is shown in Figure 8. The graph shows a strong linear relationship between voltage and force, with a correlation coefficient of 0.9946. For any output voltage the applied force can be determined using the equation for the best-fit line:

(4)

Though our prototype has fulfilled the project goals by producing a measurable change in voltage in response to an applied resultant force and a strong linear correlation based on recorded experimental data, there still remain a few challenges in producing a marketable product. The first of these challenges is the sensitivity within the electronic instrumentation of the wheel. Due to the initially small voltage outputs from the strain gauges and the necessity of high gain amplification, the components within the circuit have caused imprecision within our results. Despite acquiring highly relatable voltage differentials, the raw voltage data is significantly different. That is, the baseline voltage for each of the three tabs – 0.0, 1.4, and 2.0 V for tab 3, 2, and 1, respectively – varies depending on the actual resistivity amongst each nominal resistor in the bridge and gain resistor in the amplifiers. During experimentation we found that the 1.0% tolerance of the 350 Ohm resistors used in the half-bridge significantly affected the baseline voltage output for the respective tab.

As a secondary challenge, the voltage differential decreases as the applied force moves farther away from the tab's point of attachment. This causes a lower and unrepresentative change in voltage compared to the applied force, rendering the standard curve ineffective. However, this problem can be easily fixed by incorporating an angle sensor to supply multiplicative calibration factors in order to augment the decreased voltage differential depending on the point of application (coinciding with wheel angle). Another option is to include software coding that supplies similar calibration factors based on force application (coinciding with percent differences in voltage differential). A simple suggestion for such function is shown in Appendix I.

Economic Analysis

Market Analysis

The instrumented wheel will be primarily target seating and training clinics, rehabilitation centers, and research labs across the U.S. These centers will benefit from use of the instrumented wheel since it gives them a means of assessing the propulsion techniques of manual wheelchairs users which can then be used to properly seat and train users as to the most biomechanically efficient propulsion technique. As shown in Table 2, there are approximately 130-200 centers across the United States that could benefit from use of the instrumented wheel. The SmartWheel, which is priced at around $25,000, is currently in use in over 60 rehab centers across the U.S. [6]. Given that the price of the instrumented wheel will be considerably less, we feel we can expect the instrumented wheel to be put into use by approximately 100-150 centers in the U.S.