P10010: Motion Tracking Sensor


Table of Contents

Step 1. Clarify The Problem

Without storing the data obtained from the limb and back sensors, there would be no way to analyze the movements of a patient over the period of an entire day (thus rendering the project worthless). Therefore, finding various means of storing sensor data is important when searching for and programming the microcontroller unit (MCU) for this project. Portability (size/foot print) and power requirements are also important.

Preliminary Specifications:

Data Acquisition and Processing Concerns:

Step 2. Search Externally

Technologic Systems TS-7800

Technologic Systems TS-7500

Technologic Systems TS-7260

TI MSP-EXP430F5438

Arduino Mega

Rabbit LP-3500

Step 3. Search Internally

The data from the sensors may need to be fed through an amplifier in order for signal levels to be high enough to work with. In order to accomplish this, there are many different op-amp configurations that may be used. If the sensors have differential outputs, it may be beneficial to use an instrumentation amplifier configuration such as the one shown in Figure 1.


Figure 1. Instrumentation Amplifier using 3 LM741 op-amps

Here is a good amplifier IC: INA2126PA Instrumentation Amplifier

Furthermore, the data from the sensors may contain a lot of noise, which needs to be filtered out. Most likely, the noise will be high frequency, so a simple Sallen-Key Low Pass Filter, Figure 2, would provide effective filtering.


Figure 2. Sallen-Key Low-Pass Filter (active, uses 1 LM741 op-amp)

The UAF42 Universal Filter

Finally, the output voltage of the sensors, amplifiers, and filters needs to be compared to the maximum allowable input voltage for the microcontroller I/O pins. While standard voltage dividers would work, they are prone to failure or accidental misconfiguration, which may damage the microcontroller. Instead, using a Schottky diode, Figure 3, to clamp the MCU input signal would work substantially better (faster reaction time) and are much less prone to failure. Note that in Figure 3, "Vs" should be set to a safe maximum input voltage for the MCU pins (minus the diode threshold voltage). When the voltage on the MCU input (anode of diode) exceeds Vs + the diode threshold voltage (~0.7 V, or ~0.3V for schottky, depending on datasheet), the diode turns on and prevents the voltage at the anode from exceeding Vs+Vthresh. This type of circuit doesn't protect for excessively large negative voltages, however.


Figure 3. Schottky Diode for Voltage Clamping

Step 4. Explore Systematically

Step 5. Reflect on the Results and the Process

Generate Product Concepts