Load Cell Story: Implementation of Load Cell for IpsiHand Orthosis Device
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Neurolutions Ipsihand Orthosis
Neurolutions Ipsihand is an FDA approved stroke therapy system that re-trains a new part of the brain to control your disabled hand
Feature Specification: Original design employed two FSR (Force Sensitive Resistors) in the tip of the Ipsihand orthosis to characterize the forward and reverse finger force (in Newtons). The force is tracked over multiple therapy sessions to provide performance data to clinicians and patients.
Testing Existing FSR (Force Sensitive Resistor) Design
Test jigs and procedures were designed to validate FSR (Force Sensitive Resistor) implementation in the device tip assembly. An Imada Force Gauge and Imada Horizontal Precision Sled were employed to gather several back and forth passes of multiple Orthosis Tip assembly builds. Findings and data analysis from MATLAB showed excessive hysteresis and variance over different assemblies when plotting position vs. uncalibrated raw ADC values. It was found that the individual Orthosis tips had mechanical play at a hinge point and positional inaccuracy where the rubber bumper met the individual FSRs. Due to the shortcomings, the FSR implementation and design was ruled out.
Original Tip - Force measurements exhibit
Proposed Load Cell Design
After researching various form factors, several load cells were purchased and evaluated. The load cells also required an amplifier solution to couple to the ADC on the microprocessor (TMS320F28075). The evaluation setup is shown below. The evaluation process involved setting the op-amp gain to accommodate +/- 200N of travel and to position the offset ADC in the middle of its range.
Getting the software and hardware measurement harness working
Testing and Calibrating Load Cell
A measurement jig and software were developed to begin generating real time device data. The orthosis tip was coupled to a Force gauge mounted on a horizontal sled. The orthosis actuator was placed in lock mode as the force gauge was moved forward and backward. Real time force gauge data and orthosis measured data was gathered using software which timestamped the data.
Test data was captured for various spine positions of the orthosis. A MATLAB script was developed to verify a linear curve fit between the initial gain and adjusted gain. Results below show cases where the initial gain setting was between 5% and 30%. The calibration process brought the outcome within less than 0.5%.
Initial results are 30% off, calibration brings error to within 0.21%
Graphs show initial uncalibrated test run on left. Adjusted gain and calibrated result is on right. Force gauge is desired value to match with Load Cell force value. Slope (m) should ideally be 1.00.
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