Project development and test of an artificial multifunctional foot

Abstract

Keywords: Footwear testing; thermal comfort; lab prototype; sweating; thermal insulation

Introduction

The extremities like feet and hands mainly do human thermal regulation, thus footwear has an essential role in maintaining the thermal balance of the body (Strickland et al., 1997). 31

Specifications, development and assembly of the final prototype The foot manikin was based in a model-shaped foot and built according anthropometric dimensions, after which it was submitted to a 3D scanning digitalization using the 3D Laser ZScanner®, from DeltaCAD. The resulting file was then converted into a *.stl file type for a final SolidWorks® processing stage. After this stage, all seven segments were manufactured in a CNC machine using an aluminium alloy (Fig. 1).

perception and consequently in the individuals’ thermal comfort. Furthermore, it will also bring an opportunity to increase competitiveness and add value to the footwear industry, with a strong implementation and strategic importance in the Portuguese economy.

Development

All the development of the project was divided in several stages, each one of them with its own degree of importance for achieving a final and complete version prototype working. Physiological parameters gathering The tests to evaluate the temperature gradient across the foot were performed with 8 volunteers (4 females and 4 males, aging between 25-42). 8 iButton® sensors were distributed equally over the feet (using 2 sensors in diametric opposite points on the foot’s dorsal and 2 sensors in diametric opposite points adjacent to the foot’s toes). The subjects walked during four consecutive days, each one for a period of 60 minutes, wearing WOOK® footwear. The final values approached to average values of 31,5 ºC for temperature and 70 % for relative humidity across the foot section. A set of tests were also designed using 8 individuals (again 4 females and 4 males, aging between 25-42) in order to evaluate and determinate average feet’s sweating rates. These tests were made in 3 different conditions (during 8h of continuous lab daily work; in periods of 15, 30 and 60 minutes, and during 30 minutes of cycling exercise), each one of these carried out during 4 consecutive days. The average environmental temperature and relative humidity for the tests was, respectively, 23 ºC and 50 %. Based on this values, it was decided to adopt an average sweating rate between 0,2-1,15 g/h as a reference value.

Figure 1 Final FOOT Assembly With The Seven

The sweating system consists of a peristaltic pump with 16 channels and a set of 7 terminals fed by 7 PVC pipes divided into 4 independent zones. The control system represented is capable to monitor and register the measurements of the thermocouples, to define the parameters for the tests and to enable the display of the results of the tests for further analysis. The control unit receives the inputs from the users parameters, introduced in the software, and the resultant outputs are send to the thermal foot manikin in order to simulate the selected thermal gradients and sweating rates. The developed software also enables the user to carry out dry thermal insulation tests or thermal insulation and evaporative resistance tests, with sweating simulation. Before each test the user needs to introduce the footwear’s thickness, height, duration of the test, temperature’s setpoint and sweating rate. As output, the control system gives results for total and instantaneous local and global thermal insulation, water mass absorption and evaporation as well as local and global heating power dissipated. The heating system is represented in the following bloc diagram (Fig. 2):

Laboratory tests based on obtained evidences Using an aluminum bloc representative of a single prototype’s segment, probes were taken in order to try to mimic the results obtained during the tests with human subjects. Following this goal, the bloc was drilled with 6 sweating terminals, fed by pipes connected to a peristaltic pump and heated by 2 heating flexible electrical resistors (total power 15 W), controlled by 2 type-K thermocouples. It was concluded that the heating elements should be fixed to fit the segment’s geometry and that the most important element to assure an uniform temperature was the heating power (with a recommended power of 15 W). Several membranes were also tested to select the one whose behavior better matched the human’s skin properties. The chosen membrane was a bilaminated membrane of 110 g/m2, 100 % Jersey in the inner face and 100 % PU in the outer face, with a permeability of 3,76 mg/cm2. This membrane was selected because of its ability to absorb and spread uniformly the water and to maintain a good heat balance.

The reasons for the adoption of this test protocol were to allow us to compare the performance of the developed testing (foot manikin) prototype with the results of the reference interlaboratory study (Kuklane et al., 2003), which is also based on the standard ISO 15831:2004.

Results And Discussion

As observed in Fig. 3, the total thermal insulation (It) of the WS boot measured at 5 ºC and 70 % RH (0,225 m2.°C/W), is within the range of values for the tests performed in similar environmental conditions, i.e., 0,189 m2.°C/W for Babic et al.(2008), 0,335 m2.°C/W for Kuklane (1999) and 0,27 m2.°C/W for Kuklane et al. (2003). It is relevant to take into account the contribution of the differences in temperature and RH conditions, as well as the distinct thermal and sweating systems, whose together have a major contribution to the differences observed in the obtained thermal insulation measures.

Methods

After testing and validating each one of the sub-systems independently, several tests were developed in order to compare the results obtained with the developed working model. The experimental protocol used as a reference for the determination of the thermal insulation was adopted from the one proposed in a standard-project developed by Kalev Kuklane (2007). The general conditions adopted for the tests were the following: • Air temperature: 5ºC ± 0,5 ºC • Relative humidity: 30-70 % ± 5 % • Average superficial temperature of the manikin: 30-34 ºC, ± 0,3 ºC • Three different footwear’s models: WS boot, a warm boot of impregnated leather, with Thinsulate and nylon for insulation; AS boot, a leather boot without insulation layer; VS boot, a leather boot with an insulation layer of nylon • Each model is to be tested in duplicated tests of 90 min each one • The system starts the data acquisition when all the segments reach the same setpoint temperature • In the tests with sweating simulation, the flow rate is 5 g/h

The Three BOOT Models, Tested In TWO

ENVIRONMENTAL CONDITIONS: A COMPARISON IS CARRIED OUT WITH THE INTERLABORATORY TESTS, BABIC TESTS AND KUKLANE TESTS.

Comparing the 3 boot models it is also possible to observe that the measurements are reproducible and that there is an internal coherence between all the values (It AS<It VS<It WS), for the tests conducted at 5º C and 70 % RH. The effects of the variations in environmental temperature and RH conditions during the tests in the thermal insulation values, are visible when comparing, for VS boot, the total insulation measured at 14 ºC, 85 % RH (0,138 m2.°C/W), with the total insulation measured at 5 ºC, 70 % RH (0,181 m2.°C/W). This finding is very important because it confirms the need of a rigorous control of the environmental conditions during the tests, particularly of the temperature, in order to achieve thermal insulation values more accurate and representative.

SUMMARY AND CONCLUSIONS The novel aspects of this prototype, comparing it with other similar models already developed, are its anthropometric shape and dimensions, which make these tests much more realistic and accurate. An analysis of the results enable us to conclude that the designed and developed system is capable of producing reproducible results within the range of expectable values, according to the interlaboratory reference tests and approaching the results obtained with human volunteers. Despite the favorable results, these tests require a more accurate and complete approach, once each partner/entity of the interlaboratory tests have distinct issues related to the acquisition/processing methods, temperature and heating power calculations, so the outcome of thermal insulation’s values may have considerable intrinsic variations between each one of the results. This is very important in order to guarantee a rigorous comparison and validation of obtained data. It is also recommended to repeat the tests increasing the number of replications, in order to eliminate error factors like heating system stabilization and changes in environmental temperature and relative humidity conditions, which, as it has been demonstrated, are very important sources of uncertainty in the thermal insulation measures.

Psikuta, A., Rossi,R., 2010. Opportunities and Constraints of Presently Used Thermal Manikins When Used for Simulation of the Human Body. In: Proceedings of the 8th International Meeting for Manikins and Modelling. Victoria, Canada, 2010. Strickland, P., Reid, G., Burrows, B., 1997. Thermal profiles in footwear design: an in-sole measurement system. In: 4th Annual Conference on Mechatronics and Machine Vision in Practice. Australia, 1997. Taylor, N.A.S., Caldwell, J.N., Mekjavic, I.B., 2006. The sweating foot: local differences in sweat secretion during exercise-induced hyperthermia. Aviation, Space and Environmental Medicine, Vol. 77, nº10. Uedelhoven,W., Kurz, B., 1999. CYBOR sweating concept. In: Proceedings of the Third International Meeting on Thermal Manikin Testing 3IMM. Sweden, 2001. Yuhong, S., Zhihua, J., 2001. Measurement and evaluation of heat-moisture comfort of footwear. In: Proceedings of the Fourth International Meeting on Thermal Manikins. EMPA, Switzerland, 2001.

Acknowledgments

This project was supported by FCT (the Portuguese Foundation for Science and Technology) and CTCP (Footwear Technology Centre of Portugal) because of its potential interest and added value to the footwear industry, which is one of the most profitable business areas in Portugal.

Share and Cite