User Applied Force to Assistive Jogger s Interface During Gait Senior Capstone: Assistive Jogger MEE 443 Mechanical Engineering Laboratories University of Maine April 13, 2010 By: Joey Passarelli Alexander Foster Thomas Ciampa Andrew Jacques
Table of Contents Introduction...3 Objectives..4 Apparatus, Equipment, and Instruments 5 Theory 8 Procedure...9 Results...9 Conclusions 12 References..13 Appendix 14
Introduction: Increasingly, engineering principles are making an impact in the design and construction of rehabilitation devices and exercise equipment. The benefits of getting adequate exercise, jogging in particular, are well known and documented, but there is a wide range of people who cannot exploit those benefits due to various limitations. The inspiration of the Assistive Jogger came from two professors in the Department of Disability Studies, Dr. Liz DePoy and Dr. Steve Gilson. Dr. DePoy and Dr. Gilson requested our group to design and fabricate a prototype jogger that provides the user with the ability maintain stability during gait. However, the design had to be aesthetically pleasing to avoid any stigmatization and to encourage the user to use the device. The design developed, shown in Figure 1, was developed based on the principle of building a sturdy structure for the user to off-load some of their weight through a user-interface, while maintaining the maneuverability, aesthetic design, and relatively light weight design. The user interface was designed for the user to place their arms on, shown in Figure 2. The user interface on the jogger serves to provide stability, but also (to a certain extent) alleviate the impact force of the user foot during gait (running). Theoretically, this inherent ability of the user should help minimize stress that results on the joints from running. This principle makes the jogger potentially useful for a wide range of people from rehabilitation after an injury to those who have a difficult time running due to the propagated stress from the repeated impact (elderly, overweight, over trained distance runners, etc.). Therefore, further study beyond the scope of this proposal, of the jogger will be needed to quantify the benefits of the jogger's design on the joint (inherently and over an extended period of time), as well as the particular advantages applicable to targeted demographics. Any further studying or redesign of the jogger for different application will need to know the manner the user applies the load to the jogger and any relationship the load has with different parameters (speed and steering angle). The testing completed will provide a general relationship and interpretation of the load applied to the jogger by the user that may be used in future redesigns and studying of the Assistive Jogger.
Figure 1: Assistive Jogger prototype being used in Healthy 5k Race by Dr. DePoy Arrow Bar Pads, User s Arms Rest Figure 2: Top view of user-interface on the Assistive Jogger.
Objectives: The objective of the experiment was to determine the following information for future use in either studying further implications of the jogger or redesigning the jogger for specific uses and demographics. Determine the peak force applied by the user through the interface during gait. Provide a general relationship of speed (slow-walking and fast-jogging) and steering angle (none-straight line, small-large turning radius, and large-small turning radius). Provide insight into the specifics of how the load is applied (dynamic or static), and explain the wheel wobble. o Determine if the force curve is constant, and if not, find the average change in amplitude of each cycle in the force for that particular recording. Determine the center of force that the user applies to the interface and its possible implications on form or redesign. Determine the area of the user s force is over on the interface during operation of the jogger. Apparatus, Equipment and Instruments Experiment Setup The figure below, Figure 3, demonstrates a schematic of the experimental setup of the jogger.
Figure 3: Experimental setup with the F-Scan System pressure sensors and wireless hardware (by Tekscan) on user interface and user on the left, and on the right the wireless hub connected to the software and power source. The force applied by the user to the support interface of the jogger, Figure 2, will be measured by Tekscan tactile pressure sensor system for different testing parameters. Preceding the experiment, the F-Scan pressure sensors, shown in Figure 3, will undergo a equilibration process that allows the system to compensate for variation in the output of individual sensing elements by applying an uniform pressure to the active area of the sensor, with the device shown in Figure 4, and associating a gain to each sensel so its digital output is equal to the average digital output of all loaded sensels.
Compressed Air Supply Applies Pressure to pressure sensor using soft membrane (bladder). Compressed Air Regulator Figure 4: Equilibration device used for F-Scan sensors in conjunction with software to ensure the accuracy of raw output by sensors. Equipment The only equipment used in the experiment was the Assistive Jogger and the changes made to the padding of the user interface. The support interface will replace the arm pads from arrow bars typically used on road bikes with neoprene rubber and polyethylene foam, as shown in Figure 5. The layering of neoprene rubber and polyethylene foam will provide a surface consistent with the designed applications of the in-shoe sensors.
Figure 5: Layered padding on user interface to emulate the hardness of a shoe, as the sensors were designed for. Left shows the bare interface, and right shows padded with materials. Material Specifications: Neoprene Rubber Durometer: 80A (Comparable to Shoe Sole) Thickness: 0.125 in Polyethylene Foam (common in-sole material) Texture Type: Closed Cell Foam Firmness: Firm, 25% Deflection Instruments F-scan system (pressure sensor system Tekscan): F-scan Wireless & Datalogger System Versa Tek Cuffs (2) Versa Tek 2-Port Hub F-scan software Wireless Unit Datalogger and Memory Card F-scan sensors (in-shoe) Model: 3005E Accuracy: Dependent (equivalent) to Equilibration/Calibration Device Equilibration/Calibration Device for F-scan Sensors Model: PB100E Accuracy: 1 % FS Range: 100 psi Cyclocomputer Brand/Model: Blackburn/Neuro 4.0
Accuracy: 0.1 km/hr Range: Not given Theory The Tekscan F-Scan sensors are piezoelectric sensors designed to measure the dynamic and static plantar (foot) pressure from within the shoe (between the foot and insole). The F-Scan system records the pressure data in through the Tekscan software with many intrinsic data manipulation and analysis techniques. The specific results desired from the program were: force curve over time average pressure area over the recording peak force over recording center of force from average pressure of recording calibrated contour plot demonstrating average pressure map of recording As mentioned in the Apparatus section, the arm pads were replaced with a 0.125 in thick layer of polyethylene foam on top of a 0.125 in thick layer of neoprene rubber. This was done to be consistent with the design of the sensors and to ensure the quality of the sensor output. For the experiment, the subject was instructed to run at a slow speed (average mile walking speed 3 mph) or fast speed (average mile jogging speed 5 mph) [site the walking site]. For the user to control their speed, the jogger was outfitted with a general cyclocomputer. The cyclocomputer operates based on the Hall Effect using the wheel sensor attached to the front fork to detect the magnet (attached to the front wheel hub) each rotation of the wheel. The data is transmitted via a wired connection to the headseat displaying the current speed. This model of cyclocomputer has a delay and is limited to outputting only the current speed, thus only a general relationship of the individual s force applied to the jogger was attained with the individual s speed. An upgraded model was not selected because of insufficient funds, which is the same reason that the force applied by the user was not correlated with the impact force of the foot (another data acquisition hub would be necessary). Procedure First, the system was configured in the manner shown in Figure 3 as instructed by the user manual for the software of the F-Scan sensors, F-Scan Research 6.40. The sensors were then Equilibrated to compensate for manufacturing error in the sensors to ensure quality readings. The equilibrations were completed in the manner instructed in the Equilibration
section of the user manual for the software, F-Scan Research 6.40. After which, each sensor was calibrated with the Step Calibration because, compared to the other calibration methods available in the program, it reduces trial-to-trial variation by implementing a factor for rapid dynamic changes and compensates for time related changes in sensor output. The calibration is completed based on the user s weight and is completed one foot at a time. The same calibration file is applied to all recordings completed of the user during that period of testing. At this point the system was ready to record data for the experiments. The data was collected at both slow and fast speed and at steering angles. The user ran at a speed of about 3.5 mph and 5 mph in a straight line while the F-Scan system recorded the pressure data for the necessary duration. Then a circle with a radius of 7.5 feet and 15 feet was mapped out with cones and the user repeated the process of running at both fast and slow speeds. The user completed 2 trials for each parameter (speed and steering angle) for a total of 12 trials. Results and Interpretations The average results are summarized in the following table; the accumulated data from the recordings (peak force, 2D contour plot showing pressure map and COF, and force vs. time graph) are included in the Appendix. For the recordings when the user was following a curved path the sensors are presented as inside and outside sensor, where inside represents the sensor closest to the path and outside the sensor furthest from the path. Note: The relationship between the turning angle of the steering wheel (ββ ssssssssssssssss ), wheelbase (LL wwheeeeeeeeeeeeee ), and resulting turning radius (rr tttttttttttttt ) is rr tttttttttttttt = LL wwheeeeeeeeeeeeee cot ββ ssssssssssssssss. Peak Force throughout Recording Straight Steering Angle 13 Turning Radius 15 ft Steering Angle 25 Turning Radius 7.5 ft Walking Speed 3 mph Left Sensor - 36.8 lb Right Sensor 25.5 lb Inside Sensor - 17.4 lb Outside Sensor 9.4 lb Inside Sensor 13.5 lb Outside Sensor 10.2 lb Jogging Speed 5 mph Left Sensor 26.9 lb Right Sensor 15.6 lb Inside Sensor 18.2 lb Outside Sensor 9.6 lb Inside Sensor 13.7 lb Outside Sensor 6.2 lb Average Area from Recording Walking Speed 3 mph Jogging Speed 5 mph Straight Left Sensor 4.28 in 2 Left Sensor 2.44 in 2 Right Sensor 7.26 in 2 Right Sensor 5.10 in 2
Steering Angle 13 Turning Radius 15 ft Steering Angle 25 Turning Radius 7.5 ft Inside Sensor 4.86 in 2 Inside Sensor 6.54 in 2 Outside Sensor 2.52 in 2 Outside Sensor 3.06 in 2 Inside Sensor 4.22 in 2 Inside Sensor 5.12 in 2 Outside Sensor 2.94 in 2 Outside Sensor 2.27 in 2 Average Change of Force for each Loading Cycle from Recording (Change in force from low to high point on cycle) Walking Speed 3 mph Jogging Speed 5 mph Straight Left Sensor 17.8 lb Right Sensor 11.1 lb Left Sensor 12.3 lb Right Sensor 7.3 lb Steering Angle 13 Turning Radius 15 ft Inside Sensor 10.2 lb Outside Sensor 5.7 lb Inside Sensor 8.8 lb Outside Sensor 3.1 lb Steering Angle 25 Turning Radius 7.5 ft Inside Sensor 8.2 lb Outside Sensor 3.4 lb Inside Sensor 7.9 lb Outside Sensor 2.7 From each recording the pressure data was condensed into a single frame of average pressure for each sensel, which was used to compare the pressure area (presented above) and the center of force. For convenience all the recording images are in the Appendix; however an example is presented below of a pressure map for straight profile at a high speed. For convenience only one example of a Force vs. Time curve is presented below for a straight profile at a high speed. The rest of the force curves are presented in the Appendix for each recording.
Figure 7: Demonstration of force curve, where green represents the left sensor and red the right sensor. Results Interpretation: The force curves, Figure 7, showed the nature of the loading on the user interface was not constant, but dynamic. The force alternates with each step, which is similar to the force curve for an individual s feet while running. Therefore, when the left foot finishes its impact and is pushing off the ground there is an increased load in the right arm support on the interface that also coincides with a decrease in the force applied to the left arm support. Through all recordings, except one, the center of force was lower on the left sensor than the right regardless of which sensor was inside or outside, which likely corresponds with the subjects right leg dominance in strength (personally know subject). The lower center of force is related to a generally higher force from the right leg and the general motion of a person s arms during gait. The fact that the load is dynamic and the center of force is lower shows that the jogger maintains a person s natural running pattern while providing stability, which increases it s potential as a piece of exercise equipment.
The data in the tables above showed that an increased speed had an effect of decreasing the force difference (from high to low point on cycle) on the user interface and the force on the user interface for when the individual is moving along a straight path. This again proves the versatility of the jogger design. Users that need more stability and less impact force will be moving more slowly and thus naturally off-load a greater amount of their body weight. The subject tested in this experiment offloaded a 24 % of his bodyweight on one arm (there is still a load on the other arm). The higher the off-load the more the impact force of the foot is reduced and thus the stress on the body from running is reduced. The data in the tables above also showed a strong correlation between the turning radius and the force applied to the user interface. As the turning radius decreases the force applied vertically to the user interface decreases, as expected. This results from the greater horizontal force needed to turn the jogger. The change in the high to low point for each cycle decreases as well, which means the force becomes more consistent as the user turn. The fact the change in the high to low decreases as a person turns serves to increase the stability of the jogger in a time where the jogger is most vulnerable to tipping. Conclusions The force curve, center of force locations, and the relationship of the load to the interface with the speed (while moving straight) demonstrates the versatility of the jogger. These facts mentioned in the previous section show that while the jogger provides the benefits of stability and reduced stresses on the body it still allows the natural movement of the user during gait. However, the jogger still off-loads a significant portion of the person s body weight, which justifies and encourages further study to quantify the effect of the jogger on the body and its capabilities to minimize wear and tear on the body from training. The results also demonstrate the need to consider fatigue in the design from the dynamic load. Possible redesigns to make the jogger lighter for users on hills will need to consider both the manner the load is applied (dynamic) and the relation between the significant decrease in force applied to the jogger s interface with decreased turning radii (the larger vertical load serves to stabilize the jogger). Overall, the tests demonstrate the concept of off-loading the user weight does not necessarily restrict the user natural running pattern. The results warrant further investigation into other applications of the jogger and redesigns for wider demographics than just those with stability problems.
References 1. Tekscan. F-Scan User Manual. South Boston,MA:F-Scan Research 6.40, 2010. 2. Validation of F-Scan pressure sensor system: A technical note. Ed. Zong-Ping Luo. 06 July 1997. Dept. of Veterans Affairs. 15 Feb. 2011. <www.rehab.research.va.gov/jour/98/35/2/pdf/luo.pdf>
Appendix: Data Reductions and Images Deduced from F-Scan Software