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@@ -22,11 +22,93 @@ We had also planned our work timelines for this week. Our priority is to finaliz
# 2/16/2022 - Sensor Circuitry and Design
We had officially elected to develop our own sensor, as the current sensors available on the market do not fit our specific needs.
Our sensor design is as follows:
Through our investigations today, we have determined that there are two types of sensors most ideal for our design: Force-sensing resistors (FSRs), or developing a pressure-sensing matrix array.
# 2/17/2022 - Deciding Between Sensor Options
Currently, we are deciding between developing a pressure-sensing matrix array as the shoe insole or purchasing FSRs from online. The Pros and Cons are weighted below:
PRESSURE-SENSING MATRIX : PROS
- Maximum coverage of foot
- Sensor construction isn't complex
- High theoretical range of sensitivity, based on online demonstrations
PRESSURE-SENSING MATRIX : CONS
- Make the sensor ourselves
- More room for error (as we're making the sensor)
- Increased design complexity, since each copper line requires its own amplification circuitry
- Testing to establish range of linearity
Based on this info, it will make more sense to implement FSRs into an existing shoe insole. The design/testing complexity seems too great to pursue the pressure-sensing matrix route, and we cannot proceed with other subsystems until we know that the pressure sensors work.
# 2/18/2022 - Design Doc Signup
Confirmed date with Hanyin Shao regarding design doc check.
# 2/20/2022 - Design Doc Progress
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.
# 2/21/2022 - LTspice Sims for Amplification Circuit
Made changes to the Design Doc after the results of the LTspice sims.
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.
Based on this response, I need to find a way in which the voltage gain can be adjusted and we can acheive a linear regime between FSR resistance and Vout.
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:
- Maximum output gain is 3.3V
- Well-defined linear response is found
- Output is close to 0 V when FSR is not pressed (ie. Rfsr = infinite resistance)
All three of these objectives were acheived when I found the ratio or R2/R1 to be 1k/1.3k. The circuit was updated in the design doc.
# 2/22/2022 - Design Doc Check
After listening to Prof. Schuh's feedback, we'll need to change our high-level requirements such that the units are intuitive and make more sense. Instead of using weight, it would be mroe accurate to use pressure since the weight is being applied over a defined sensing area of the FSR.
# 2/23/2022 - TA Meeting
Went over the Design Doc with Hanyin to gather her feedback, as well as review any missing info to add before the submission deadline tomorrow. No changes were needed.
# 2/24/2022 - FSR Research
Was looking for ideal FSR candidates for use within our project. As of now, most FSRs part of the Arduino Project toolset had too little of a sensing range (ie. they were only meant to detect touch instead of sensing weight/pressure).
# 2/27/2022 - Finalizing PCB Design
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.
# 3/1/2022 - Editing PCB Design
Based on the feedback of the TA, our PCB looks to be in good shape. However, there are certain areas of the board in which the trace density was too high, and so some parts may need to be rerouted such that they are not too close to the 5V rail. This will decrease the chances of the board shorting.
# 3/22/2022 - Parts have arrived!
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.
# 3/24/2022 - Editing LTspice sim
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.
# 3/27/2022 - FSR breadboard testing
Now that the FSRs have shipped, I can begin testing them to establish their parameters and test the LTspice circuit on the breadboard. My objectives are the following:
- Observe similar behavior between LTspice sims and real-life results
- Gather measurements of the FSR response when the sensor is fully pressed vs. not pressed at all.
I had designed the breadboard circuit to use a different op-amp than what was listed in the circuit (LM340 instead of LT1014). After designing the circuit, I had set the power supply to 3.3V and had connected an oscilloscope to measure the FSR response, and had gotten the following:
FSR depressed --> 0.127 V
FSR fully pressed --> 2.005 V
This was almost identical to the response seen in the LTspice sims for the amplification circuit. This procedure was repeated for the five other FSRs, each receiving similar values within ~1% error difference between the first FSR values mentioned on the top.
# 3/28/2022 - FSR Linearity Testing
Now that the FSR low and high range have been tested, it was time to establish a range of weights in which a linear relationship can be established between the weight applied to the FSR and the corresponding voltage response. I had used the following procedure:
- An empty container was weighed (4.5 grams) and placed on the FSR, which elicited no response. This served as our first data point - and since there was no change in reponse, it was assumed that we had not yet reached the minimum activation force of the FSR.
- Water was then poured in 15 mL increments into the container. The container was weighed after pouring the water, and then placed onto the FSR. The minimum activation weight was found to be 18.5 grams
- 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 plotting the data obtained from this experiment into an Excel spreadsheet, a clear linear range was established between weight and output voltage. This was exciting to see, as we can directly correspond the output voltage to the weight applied to the FSRs with little error.
# 3/29/2022 - FSR testing with Shoe
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.
# 4/6/2022 - Shoe insole construction
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,
# 2/17/2022 - Subsystem Design
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