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@@ -22,11 +22,93 @@ We had also planned our work timelines for this week. Our priority is to finaliz
Finished writing all sections related to the Data Acquisition system. Topics finished included writing the subsystem overview on pressure data acquisition, where I discuss the layout for placing FSRs across our shoe insole, the circuitry for amplifying the voltage response of these FSRs, and the requirements and verifications for ensuring that this subsytem performs as desired. The high-level requirement may need some revision, but will wait for Prof. Schuh's feedback before changing anything.
Originally, I was opting for a simple voltage-divider circuit, in which the change in voltage caused by the change in resistance of the FSRs will be fed through a generic op-amp. However, I had found that the response was largely binary. When the FSR was fully pressed, Vout has a maximum of 4V. When not pressed at all, response is at 0 V. There is an exponentially decaying relationship between the FSR resistance and Vout, which will yield innacuracies when doing voltage-pressure conversions.
I instead opted for an adjustable buffer circuit, in which the circuit gain can be adjusted by a ratio of Rin/Rout. Rin refers to the resistance of the op-amp input, and Rout is the resistance of the op-amp output. Rin should not be modified, as the FSR resistance is already a changing component and having a potentiometer connected from the op-amp input to ground will only increase design complexity. As such, we can tune the value of Rout to adjust the output gain more effectively.
This can be done by connecting a resistor between the inverted op-amp input and the output, and one between the inverted op-amp input and ground. I'll refer to these two resistors as R2 and R1. Instead of using a potentiometer, I can adjust the ratio between R2 and R1 such that I meet the following requirements:
Looked over implementing the amplification circuit on the PCB before the design review. Ritvik was finishing up implementing the power distribution subsystem onto the PCB - however, we had kept running into issues of certain parts going out of stock, which required constant revision of our PCB design.
We had just received the parts ordered before Spring break. The first priority was to establish the feasibility of using Bluetooth Low Energy (BLE) for use in transmitting pressure data from the PCB to a phone on an Android app. If this can be established, there will be no need for us to work on the microSD subsystem, which locally stores our data onto the PCB. This will also greatly increase the usefulness of our project, as the user can remotely view their foot pressure as they are walking.
As Ritvik had opted for using a different type of battery (LiPo cell instead of Nickel coin batteries), a few changes had to be made for the power subsystem. As such, the new requirements are that the input voltage recieved by the Data Acquisition subsystem is 3.3V instead of 5V. After playing around with the sim, I had found that the new R2/R1 ratio will be 33k/47k ohms. This gives us a maximum Vout of ~2V, while keeping Vout ~0V when the FSR is not pressed.
- This procedure was repeated until the maximum output voltage (2.005 V) was reached, which was when the container + water weighed 254.5 grams. This was the value in which the FSR entered saturation, and each increase in pressure applied to the FSR would result in no change in output characteristics.
After verifying the FSR characteristics, it was time to move the testing into the shoe insole. The FSR terminals on the breadboard were replaced with jumper cables which connected to the FSR placed underneath the shoe insole. The output terminal was connected to an oscilloscope. Only one FSR was tested to minimize loss in case the FSR was damaged by our testing. The FSR was placed in the heel region, since we expect to find the highest foot pressure in that area.
After pressing the heel multiple times, and attempting to recreate a walking motion, we were able to get good responses on the oscilloscope, indicating that the FSR exhibits the same functionality when placed within a shoe. In particular, we were able to see exactly when the foot both touched and left the ground in a walking motion, as the oscilloscope registered a gradual increase in pressure when my shoe hit the ground, followed by a corresponding decrease as my foot left the ground. This was particularly important, as we can map the change in pressure in a normal walking stride.
Now that the FSRs work as intended, it was time to begin work on building the shoe insole. I labeled the FSRs # 1-6, which were placed in the exact same layout as outlined in our design docs. We soldered wires of different colors (each color corresponds to the sensor) to each terminal of the sensor, and heat-shrank the interface between the FSR and the wire connection to incease durability. Afterwards,