P11011: Modular Motion Tracking Knee Flexion Unit V1

Knee Flexion Error

Knee Flexion Error

There are two main sources of error in the knee flexion device. The first is the input offset voltage of the buffer op amp. The MCP6002 is listed as having an input offset voltage magnitude of up to 4.5 mV with an input noise of 6.1 uVp-p. The input offset voltage also drifts with temperature at 2 uV/C. If the device was calibrated at room temperature (25C) and then used on a hot day (45C) the change in input offset would be 40 uV, used on a cold day (-20C) corresponds to 90 uV of change. All of the error cause by the buffer op amp corresponds to a maximum error of 1 bit or ~0.3 deg.

The second is the noise caused by the ATmega328 chip. Although the effect of this noise on the bits cannot currently be calculated without numerical data, solutions exist such as halting digital data communication at nearby ports or putting the processor to sleep while sampling with the ADC. This noise is not expected to change the results by more than 5 deg, but if it does, then these solutions can be implemented.

In order to cause a critical misrepresentation of flexion angle (>5 deg), the input would have to change by 16.4 bits or 80 mV. If this is occurring, the buffer can be used as an amplifier to fit the range of movement of the patients (~60 deg) into the full range of the ADC.

Back to Detailed Design Review Documents