Functional Evaluation of Artificial Feet by Load vs. Deflection Recordings
R. L. Daher, M.Sc
P. J. Nelson, M.Sc
J. B. Heath
This study was supported by a National Health Grant #606-7-258, Department of Health and Welfare, Canada.
Functional characteristics of the various available artificial feet may be compared by means of a continuous recording of load vs. deflection (durometer) data in the loading and unloading phases of a controlled test. By this method a hysteresis curve which indicates the foot's ability to recover from deformation is obtained. The Prosthetics/Orthotics Research and Development Unit, Health Sciences Centre, has developed and built electronically controlled equipment which automatically charts complete hysteresis curves and also records data on deformation resulting from normal wear or cyclic testing.
The Testing Apparatus
Fig. 1 shows the durometer tester complete with control panel. The tester consists of a vertical column driven by a variable speed 1/4 h.p. Bodine Direct Current (DC) motor. A force plate is mounted on top of the column and consists of four vertical columns running in linear bearings with a Schaevitz Model FTD-IU500 load cell located in the centre of the base plate. The load cell contains a linear variable differential transformer which accurately converts load into an electrical signal. A ten-turn potentiometer mechanically coupled to the vertical column by means of a gear train converts the position of the force plate to an electrical signal. Load and position signals are recorded on a Hewlett-Packard Model 7035-B X-Y recorder. The recorder is calibrated so that a 25-lb. load results in a pen displacement of 1 inch in the Y direction; a 1/4-in. vertical movement of the column causes a 1-in. pen displacement in the X direction.
Fig. 2 and Fig. 3 show the foot in position for testing. To avoid removal of the foot for the individual heel and sole tests, the attachment plate mechanism is designed to allow rotation and linear shift. For the heel test (Fig. 2 ) the attachment plate is rotated to 15 deg. where it automatically locks into position. Similarly for the sole test (Fig. 3 ), the attachment plate is rotated and locked at 30 deg. The linear shift required for the two tests is made by means of a roller-track assembly as shown in the two figures. To avoid linear movement during a test, the roller assembly is locked into a specified position by a clamp-friction arrangement.
The "touch point" in a test is defined as the instant that load is applied to the foot, i.e., when the force plate just makes contact with the foot. The "touch point" is automatically recorded. Because of the difference in travel from the initial starting position of the force plate to the "touch point" for a heel test, as compared to the "touch point" for the sole test, it was necessary to control manually the starting position of the chart recorder pen. Consequently, two ten-turn potentiometers, with dial indicators, one for the heel testing, one for testing the sole, were mounted on the control panel. The appropriate position control potentiometer is automatically activated by positioning the foot for heel or toe test. The potentiometers are wired in parallel with the potentiometer on the vertical column, from a common power supply. By means of Integrated Circuit (IC) differential comparators, the voltage across the selected potentiometer on the control panel is compared to that on the vertical column. As the column rises from its starting position, the voltage across the potentiometer increases in direct proportion to the distance. When this voltage reaches the preset voltage on the panel potentiometer, the IC differential comparators trigger a pair of reed relays which in turn start the recorder pen moving in proportion to the column travel. Thus by resetting a specified position on the panel potentiometer dial indicator, a number of tests may be repeated at later dates and compared. This permits recording of permanent deformation in a foot after cyclic testing or normal walking, as the pen-start position will always be identical.
To avoid overloading the foot under test, two additional ten-turn potentiometers with dial indicators were installed in the control panel. Each potentiometer allows individual presetting of maximum load during either the heel or sole test. Similar to the position control circuit, the voltage output from the load cell is compared to that on the selected potentiometer by means of IC differential comparators. When the voltages become equal at the maximum specified load, the differential comparator changes the state of a bi-stable magnetic latching relay which then initiates a one-second time-delay relay. The time-delay relay allows the motor to come to a complete stop before reversing its direction. Consequently, a delay of one second results between the loading and unloading phase of the test. Other safety features include automatic power shutoff which limits both the lower and upper ranges of travel of the vertical column or force plate.
A typical chart showing both heel and sole tests is presented in Fig. 4 . The base reference point or zero deflection corresponds to the preselected starting point of the chart recorder pen. As mentioned, this is controlled by the pen-start position selector on the control panel. For the data shown in Fig. 4 , the heel "touch point" is just over 0.50 inches and that for the sole, 0.80 inches. The upper line of the hysteresis curve is recorded during loading; the lower line records unloading. The dotted lines are the load-deflection ratings for both heel and toe as specified by the Veterans Administration Prosthetics Center, New York. As noted, the deformation is less during loading than when unloading. To obtain ratings on the tested feet, the VA specifications are superimposed over the chart recording with the zero load, zero deflection position corresponding to the touch point of either the sole or the heel.
Preliminary test results indicate a definite relationship between the area inside the hysteresis curves and the ability of the SACH foot to recover from deformation. It was also noted that all new SACH feet showed softening within ten sequential cycles on the tester. The equipment described is used in conjunction with an artificial-foot cycle-testing program in operation at the Centre. A complete report on the results of all tests will be made available upon completion of the current program.
Health Sciences Centre, Rehabilitation Centre Winnipeg, Manitoba, Canada