J. Hartung

Reliability of Pressure Measurement on Car Seats

Often, pressure distributions on seats are referred to as an objective method to quantify seat comfort. However, the reliability of seat pressure measurement is yet unknown. Therefore, it is important to know how reliable these pressure measurements are, before using them effectively as a means to predict seat comfort based on pressure measurements. Experiments were conducted to determine the inter-individual and intra-individual variation in pressure distribution. The obtained variation ranges of the mean pressure in defined body areas are presented.

Development of a 3D Finite Element Model of Thigh and Pelvis

The development times of car seats decrease while the demand for more comfortable seats increases at the same time. To fulfill this trade-off, numerical simulation of the body/seat interaction could be used. Therefore numerical models of the sitting human are needed. For this reason a 3D-FE-model of the thigh and pelvis of a 50 th percentile male has been developed. The surfaces of the thigh and of the bones were gathered by a laser scanner. For the undeformed outer shape a test subject was chosen who has the anthropometry of a 50 th percentile male. The geometry of the bones was scanned from a skeleton of a 50 th percentile male. From the scanned data 3D CAD surfaces were derived. Within the CAD-system the bony structures were positioned inside the outer shape of the thigh and pelvis using computer tomography images. The biomechanic properties of the soft tissue were determined through indentation tests on test subjects. The results of these tests are force-deflection curves of thigh and pelvis. The 3D CAD model was meshed and imported into FE-computing software for simulation. Material properties were gained by simulation of the experiments. To validate the model the test subject sat on a plain wooden plate. During the experiment the pressure distribution between test subject and seat was gathered by a pressure mat. After that the experiments were rebuild within the FE-model and the pressure distribution from the experiment was compared with the results of the simulation.

Predicting Overall Seating Discomfort Based on Body Area Ratings

For car manufacturers, seating comfort is becoming more and more important in distinguishing themselves from their competitors. There is a simultaneous demand for shorter development times and more comfortable seats. Comfort in automobile seats is a multi-dimensional and complex problem. Many current sophisticated measuring tools were consulted, but it is unclear on which factors one should concentrate attention when measuring comfort. The goal of this paper is to find a model in order to predict the overall seating discomfort based on body area ratings. Besides micro climate, the pressure distribution appears to be the most objective measure comprising with the clearest association with the subjective ratings. Therefore an analysis with three different test series was designed, allowing the variation of pressure on the seat surface. In parallel the subjects were asked to judge the local and the overall sensation. During the experiment the pressure distribution between test subject and seat was gathered by a pressure mat. The test series are based on 84 subjects and over 500 measuring results, which include three different settings for seat cushion and backrest.

Sensitivity of Human Pressure Feelings while Sitting

To describe feelings of discomfort while sitting, many experiments have been conducted to find a link between interface pressure on the seat and the feelings of discomfort of test subjects. Most of these experiments found no relation or correlation between discomfort and pressure while others actually found a relation. A question which arose was how sensitive the human body is to pressure differences during sitting. The attempt of this study was to determine the sensitivity to pressure for an area of the thigh. Therefore a test stand was designed allowing the variation of pressure on an area of a thigh during sitting. In parallel, the subjects were asked to judge the pressure sensation. By analyzing the frequencies of answers given by the test subjects, a curve could be derived which describes the sensitivity of pressure sensation. In this paper the test stand, the test procedure, the results and further experiments will be discussed.

Measuring Soft Tissue Compliance of the Human Thigh

The lack of measured data is a problem that often occurs when modeling physical human properties. This problem also occurs when trying to describe the human thigh with a finite element model. In this pilot study, an experimental chair was used to gather the required information. Each cushion of the chair consists of 81 pistons. Each piston is controlled individually by a computer. This experimental chair was used in two experiments. In the first experiment, the chair was used to measure the force length characteristics of deformed human thighs during unsupported sitting. In the second experiment, the measurement of the characteristics of undeformed thighs deformed by one piston was attempted. In this paper the test stand, the test procedures, the results und further experiments will be discussed.

An Experimental Setup to Parameterize and Simulate Upholstered Seats

The increasing number of 3D-CAD-systems integrating computer based man models make it possible to evaluate the design of the man machine interface in an early state. The interaction of human and seat is not simulated in a satisfying way yet due to the extreme complexity of the task. At the Lehrstuhl für Ergonomie, Technische Universität München an experimental setup has been developed to investigate this interaction systematically. With the help of this setup upholstered seats can be parameterized by their spring characteristics and geometry to be simulated as physical representation of the seat with the same setup. Pressure distributions and deflection profiles can be measured with integrated sensors. The simulated seat also provides the possibility to vary the seat characteristics systematically. The experimental setup consists of two cushion units, adjustable in size and angle. Each unit consists of 9 by 9 adjustable positioning units, technically realized as programmable springs. Tests with selected positioning units have shown that the accuracy of positioning, as well as the accuracy of the measured and simulated spring characteristics is sufficient. Existing seat cushions have been simulated. Based on mathematical indices the similarity of measured pressure distributions on the simulated cushions and the corresponding seats were compared. Based on this results it can be stated that the characteristics of real seats can be simulated with sufficient accuracy. So it is possible to use the experimental setup as a tool to gain the necessary data to model the interaction between the human body and upholstered seats. Further research is going to be performed on the modeling according to the physical parameters geometry and spring characteristics of the seat as well as the pressure distribution and deflection profiles caused by the subjects interacting with the seat.