This week, I worked on the design presentation slides with my team and compared IMUs for our product. I decided on the ST LSM6DSOX ($5.30 at the time of writing) for its low power requirements, which draws only 0.5mA (~1.7mW @ 3.3V) in high-performance mode (many comparable IMUs draw around 1-3mA). One downside is that it outputs raw acceleration/gyroscopic/temperature data, likely requiring us to process it into real-world values on the compute unit.
Two alternatives I rejected were the Bosch BNO085/086 and the ST LSM6DS3TR-C.
– The Bosch BNO085/086 was designed specifically for use in wearable devices and calculates acceleration and quaternions on-chip, but its relatively high power draw (10-15mA) made it too costly for our device’s limited battery life to utilize. While not directly useful, the temperature data from the ST LSM6DSOX will allow us to optimize the thickness of the wearable’s casing if overheating ever becomes a problem.
– The ST LSM6DS3TR-C is a simpler, cheaper chip that can carry out many of the functions that the ST LSM6DSOX can, and would definitely work as our device’s IMU. However, it draws significantly more power, and its measurements are noisier. As such, I decided that the ST LSM6DSOX for our IMU.
For our prototype, we will be ordering the IMU on a breakout board. While buying them separately would save about $7 on our budget, I am not confident in my surface-mount soldering ability – failure might require us to purchase a second or third chip. The prototype does not have the strict size limitations of the final wearable (for which we will order the full PCB assembled by a manufacturer), as it will be used primarily to collect data. Comfort is secondary with the prototype; as long as it can be strapped to one’s forearm, its size is acceptable.
I am not behind schedule, but will work to ensure I do not fall behind.
By next week, I hope to have the webapp’s history tracker implemented.
In two weeks, I would like to have the device prototype completed so that we can gather motion data for use in the compute station.