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A SYSTEM FOR THE DESIGN AND ANALYSIS OF SEAT SUPPORT SURFACES

TE Karg , DM Brienza , CE Brubaker , J Wang  and CT Lin*  School of Health and Rehabilitation Sciences, University of Pittsburgh * Viscoustech Incorporated, Pittsburgh, PA

ABSTRACT

Despite research that now spans decades, the ability to measure the efficacy of seating to minimize the risk for pressure ulcers is still modest at best. There are some promising and even intriguing theoretical constructs, but none have been subjected to rigorous experimental examination. Vacuum consolidation, foam-in-place and other direct shape measurement techniques are commonly used to provide custom cushion or body support surface shapes for specialized seating. However, there is still an urgent need for knowledge that will allow for the systematic design of support surfaces. A system has been developed to quantify the complex relationships among support surface shape, tissue thickness changes and interface pressures. This system allows for the dynamic formulation of an interface contour on the basis of programmable criteria. It was designed to facilitates the study of the relationships between support surface shape and interface pressure, and support surface shape and soft tissue distortion. The design, instrumentation, and results of system performance tests are presented here. The systemÕs surface optimization technique, as evaluated by subject studies, is presented in a subsequent paper. The performance tests showed that the system has the ability and precision to control surface shape while measuring interface pressure distribution.

INTRODUCTION

Although it is popular among clinicians as a criterion for good seating, the measurement of interface pressure has considerable limitations [1]. The most appealing theory is that optimum seating will be achieved when the soft tissues are subjected to the least distortion. However, the problem is quite complex and meaningful measurements of tissue distortion, tissue thickness, shear stress, and tensile and compressive forces have been difficult to obtain. It is worth noting that significant stresses occur within the subcutaneous tissues due to distortion of the tissue from seated loading. The amount of distortion is related to the relative composition of the soft tissues and the extent to which they are distended in different directions from a nominal unloaded condition. In theory, the optimum shape is the one that would minimize the internal tissue stresses and external pressure on the tissue.

Computer aided design (CAD) and computer aided manufacturing (CAM) technology has aided in investigations designed to study the complex relationships between external pressure and soft tissue stress and strain. CAD/CAM technology for lower extremity prostheses and orthotics has received significant attention by researchers and developers. In contrast, much less has been accomplished with respect to application of computer aided shape sensing and automated fabrication technology in customized seating.

Quantitative measurements of the seat contour have been investigated. A passive seating system has been developed that uses polyurethane foams of various stiffness and thickness to quantify contours resulting from the human buttocks and cushion interface [2]. This effort included an investigation of the relationships between interface pressure distribution and contour shapes on typical wheelchair cushions for SCI subjects and able-bodied subjects. The application of measured contour data and CAM techniques has been developed for clinical use of the custom contoured seat cushions for SCI, CP, and elderly persons [3]. Clinical studies have indicated that custom contoured cushions provide better pressure distribution, comfort and stability [4]. Despite these advances, there is still an urgent need for knowledge that will allow for the systematic design of support surfaces. Current practice is often based on a trial and error process whereby support surface shapes are iteratively altered until satisfactory results are obtained.

The computer-aided seating system (CASS) allows for the dynamic formulation of an interface contour on the basis of programmable criteria. Our purpose in developing the closed-loop, dynamically controlled shape and pressure sensing system is to quantify the complex relationships among support surface shape, tissue thickness changes and interface pressures. Change in tissue thickness are inferred from the force-deflection characteristics of the tissue as measured by external pressure per unit change in deflection, i.e., stiffness. In other words, measurement of stiffness is used as an indictor of tissue thickness.

The primary effect of non-uniformly applied force or pressure on the skin is strain in the cutaneous and subcutaneous tissues. The buttocks soft tissue is generally not compressed as a result of support surface reaction forces. If the tissues were contained or pressure was applied hydrostatically, the soft tissues of the buttocks could withstand relatively high pressures without significant risk of tissue damage. Only when pressure is applied non-uniformly are tissues strained and consequently put at risk of tissue damage. While sitting, the soft tissue of the buttocks is not contained, therefore, support surface reaction forces result in such internal strain.

At present, an algorithm based on interface pressure and tissue stiffness is used to drive the system to an "ptimized" shape. The broader aims of our research are to quantify the relationship of shape, interface conditions (axial) and tissue distortion (primarily based on stiffness) relative to unloaded shape, lesion level and other relevant factors for stratified populations. The capabilities of the CASS allow for these and other potential investigations.

INSTRUMENTATION

The current CASS represents a second generation design for this system. The development, assembly and testing of the first generation CASS has been previously reported [5,6]. As compared to the first system, this CASS has the ability to adjust surface shape four times more rapidly, measure interface pressure directly at the support surface versus indirectly using force measurements as was done in the first design, and uses updated computing and interface equipment.

The system consists of an instrumented seat support surface for measuring interface pressure and controlling support surface shape, an interface unit for processing the pressure transducer signals and controlling the array of drive motors, and an 80486 DOS compatible personal computer for high level control of the system. The adjustable seat support surface forms a 3-dimensional support surface through selective adjustment to the heights of support elements arranged in an 11 by 12 array. Pressure sensors are fixed in the swiveling heads on top of each support element (see Figure 1).

[INSERT] Figure 1. Support element with sensor

The top of the support element rotates freely so that the pressure transducers are oriented in a direction normal to direction of net force. The pressure sensitivity and resolution of the sensor are 0.17 kPa. The 43 cm by 47 cm array of support elements is deployed in a frame consisting of mounting plates for the stepper motor driven linear actuators and bearing plates for the support elements. The vertical position of each support element can be adjusted through its stepper motor driven, lead screw assembly. The range of vertical adjustment is approximately 15 cm.

The interface unit, partially located on the ISA bus of the personal computer, off loads the computers main processor by handling several low level control functions. Low level control of the pressure transducer signal scanning and processing is accomplished using a programmable ISA bus, microprocessor-based data acquisition processor. Low level processing of the motor drive signals is accomplished using an ISA bus stepper motor control board with eight simultaneously and independently controlled channels. The computer hardware and software control the motor array using a 96 channel digital I/O board located on the computer bus, the aforementioned stepper motor controller, and a custom built control signal multiplexing circuit.

Signals from the pressure transducers are continuously scanned by the data acquisition processor and stored locally for recall by the controlling software. Thus, the system controls the height of each of 128 stepper motor driven support elements to alter the support surface shape and has the ability to scan the 128 pressure transducers at a rate of 195 complete scans per second. This capability allows for the open-loop control of support surface shape with or without interface pressure measurements, automatic closed-loop control of interface pressure via control of support surface shape, and/or the characterization of soft tissue in contact with the support elements through monitoring changes in interface pressure conditions relative to changes in support surface shape.

The supporting structure, shown in Figure 2, was designed so that the support surface could be used in a flexible, simulated seating environment. The structure includes an adjustable sling backrest, armrests, and a footrest. Seat depth, seat to back angle, armrest lateral position, armrest vertical position, footrest height and footrest angle are all adjustable.

Core software has been developed to control the motor array, communicate with the data acquisition processor to receive pressure measurements, and display support surface shape and pressure. The core software is expandable to allow for the development of control algorithms using support surface shape and interface pressure as parameters.

SYSTEM PERFORMANCE

In order to demonstrate the ability of the instrumentation to perform the intended application, the various subsystems had to be evaluated and their performance parameters defined or verified. The subsystems that needed analysis were the pressure sensing system and the support element positioning mechanism.

[INSERT] Figure 2. CASS Seating System

A device was developed for calibration of the pressure sensors. A plexiglass tube was used with a thin rubber material covering one end. The height of a water column in the tube applied a known pressure through the rubber membrane. The tube was placed on top of the pressure sensor, perpendicular to and covering the entire sensor's surface. Increasing the height of water in the tube, allowing for known pressure values, the voltage output from the sensor was recorded. In order to reduce the effects of friction, a lubricant, such as ultrasound gel, was used between the rubber membrane and pressure sensor. The linearity of the individual sensors was demonstrated and the maximum error was found to be 8%.

The accuracy of the positioning of the support elements is another important and basic parameter of the seating system. Position accuracy and repeatability under different loads, velocity, step rate, step size and the amount of adjustment will directly affect the convergence of the algorithm, the computation of parameters derived from position changes and the determination of the optimum shape.

An evaluation of the accuracy of the elements'position was determined using a mechanical travel dial indicator. Four sensors were arbitrarily chosen from different locations in the seating system to be evaluated. The test was repeated for variations in starting position, load, step size, and step rate during single step and continuous motion.

The measurement error was minimal for the element in the lower or higher position, while being moved up or down, with an error of less than 2%. The variation in load test results showed that the elements have a small positioning error and good repeatability when the load is less than 45 N. The root-mean-square deviation for 0 to 45 N was from 0.02 to 0.03 mm. For variations in step rate from 100 steps/s to 1500 steps/s, the results indicated good repeatability and small errors for either single step or continuous motion. Positioning errors were determined for step sizes of 0.1 mm, 1 mm, 2 mm, and 10 mm. The deviation from the expected position was found to be less than 0.027 mm with a relative error of less than 5% for step sizes of 1 mm and greater. Overall, the relative error measured for position adjustment can be expected to be 5% or less with a standard deviation of 0.03 mm or less.

CONCLUSION

The system has the ability and precision to control surface shape while measuring interface pressure distribution. The system performance checks clearly indicated that the system is capable of making repeatable and precise measurements of pressure and element deflection and is a reliable tool. The CASS system provides a tool for the investigation and quantification of the complex relationship among the mechanical properties such as shape, interface pressure and tissue deformation under different load and stiffness conditions. This ability is of great significance for clinical study and application. While the CASS itself is not likely to be developed into a viable clinical tool for support surface design, the basic information gathered though its use may prove fundamental to successful design of custom contoured support surfaces.

REFERENCES

[1] M.W. Ferguson-Pell. Seat Cushion Selection, J Rehab R&D Clinical Supplement, 2:49-74 1990.

[2] K.C. Chung. Tissue Contour and Interface Pressure on Wheelchair Cushions, Ph.D. Dissertation, University of Virginia, May, 1987.

[3] K.C. Chung, C.A. McLaurin, C.E. Brubaker, D.M. Brienza and B.A . Sposato. A Computer-Aided Shape Sensing System For Custom Seat Contours, RESNA Annual Conference Proceedings, Washington, DC., June 1990.

[4] S.H. Sprigle, T. Faisant, and K.C. Chung. Clinical Evaluation of Custom Contoured Cushions for the Spinal Cord Injured, Arch of Phys. Med and Rehab, 71(9) 655-658, 1990.

[5] D.M. Brienza. Seat Contour Optimization Using Force Feedback, Ph. D. Dissertation, University of Virginia, August 1991.

[6] R.J. Kwiatkowski and R.M. Inigo. A Closed Loop Automated Seating System, J of Rehab R&D, 30(4), 1993.

ACKNOWLEDGEMENTS

This work was supported by a grant from the National Institutes of Health, National Institute of Child Health and Human Development, National Center for Medical Rehabilitation Research, grant number R01-HD30161.

Tricia Karg, MSBME Rehabilitation Technology Program University of Pittsburgh UPARC-915 William Pitt Way Pittsburgh, PA. 15238 412-826-3138 tkarg@pitt.edu A System for the Design and Analysis of Seat Support Surfaces