Development of inexpensive (<$50) open source drag-tilt meters.
Available instruments are $600-2500 USD per unit (6-axis IMU, SD card reader USB-B data transfer)
- 9-axis IMU for sensor fusion (magnetometer, accelerometer, gyroscope) & temperature (±0.05 °C)
- TFT screen with real-time sensor updates
- Onboard SD card
- Bluetooth connectivity (data transfer, serial port)
- USB-C
- Low power mode (3.7V 1100mAh Li-ion battery)
- Open-source code base
- Kalman filtering to update the state estimate for tilt angles and angular velocities to improve measurement accuracy under noise and disturbances
- Switch units on/off using hall effect sensor (US5881LUA)
- Include gyroscope into self-calibrating in zero flow environments
- Variable height measurements? e.g. 30cm, 60cm, 100cm
- Inductive charging
- LoRaWAN surface connectivity to create sensornetworks & surface solar charging
- 4G connectivity
#Hardware*:
- Development board: Arduino Feather ESP32 S3 TFT
- 9-axis IMU: (LIS3MDL/LSM6DS3TRC).
- RTC / SD card: (Adalogger Featherwing)
| File | Image |
|---|---|
| ESP32_TFT_NE_bearing.ino |  |
| ESP32_TFT_pitch_roll_yaw.ino |  |
| LIS3MDL magnetometer calibration |
Contributions of a Gyroscope in a Drag-Tilt Current Meter
- Calibration: In still water conditions, the gyroscope can help define the zero point by calibrating the sensor to account for any inherent drift or bias. This ensures that the tilt measurements are accurate and start from a known reference point.
- Dynamic Tilt Correction: The gyroscope can detect and compensate for sudden movements or vibrations that may temporarily affect the tilt angle. This helps in filtering out noise and ensuring that the tilt measurements accurately reflect the steady-state current flow.
- Enhanced Sensor Fusion: The gyroscope works in conjunction with the accelerometer to provide a more comprehensive understanding of the instrument’s orientation. While the accelerometer measures static tilt due to gravity, the gyroscope measures dynamic tilt caused by rotational movements.
- Compensating for Accelerometer Limitations: Accelerometers can be affected by linear accelerations and vibrations, which can introduce errors in tilt measurements. The gyroscope helps to correct these errors by providing rotational data that can be used to differentiate between tilt and linear motion.
- A gyroscope measures angular velocity around three axes (x, y, and z).
- In still water conditions, ideally, the gyroscope should read near-zero values because there should be no rotation or angular movement of the sensor.
- Using the gyroscope to define the zero point in still water conditions is an effective way to calibrate the sensor for accurate angular velocity measurements to ensure precise and reliable motion tracking and orientation determination.
- Defining the Zero Point:
- Calibration: In still water, you can calibrate the gyroscope to determine its zero point. This involves taking a series of readings over a period of time and averaging them to find the baseline offset for each axis.
- Baseline Offset: The baseline offset (zero point) represents the small bias or drift in the gyroscope readings when there is no actual rotation. This offset needs to be subtracted from future readings to get accurate angular velocity measurements.
- Steps to Define the Zero Point:
- Data Collection: Place the IMU in still water and collect gyroscope data for a certain period.
- Average Calculation: Calculate the average reading for each of the three axes (x, y, and z). This average represents the bias or drift in the gyroscope.
- Offset Storage: Store these average values as the zero-point offsets.
- Calibration Application: Subtract these offsets from future gyroscope readings to correct for the bias and obtain accurate angular velocity measurements.
• 0.625 Hz: The sensor outputs data 0.625 times per second.
• 1.25 Hz: The sensor outputs data 1.25 times per second.
• 2.5 Hz: The sensor outputs data 2.5 times per second.
• 5 Hz: The sensor outputs data 5 times per second.
• 10 Hz: The sensor outputs data 10 times per second.
• 20 Hz: The sensor outputs data 20 times per second.
• 40 Hz: The sensor outputs data 40 times per second.
• 80 Hz: The sensor outputs data 80 times per second.
• 155 Hz: The sensor outputs data 155 times per second.
• 300 Hz: The sensor outputs data 300 times per second.
• 560 Hz: The sensor outputs data 560 times per second.
• 1000 Hz: The sensor outputs data 1000 times per second.
the maximum magnetic field strength that the sensor can measure:
• ±4 gauss: The sensor can measure magnetic field strengths from -4 gauss to +4 gauss.
• ±8 gauss: The sensor can measure magnetic field strengths from -8 gauss to +8 gauss.
• ±12 gauss: The sensor can measure magnetic field strengths from -12 gauss to +12 gauss.
• ±16 gauss: The sensor can measure magnetic field strengths from -16 gauss to +16 gauss.
where Gauss (G) is a unit of magnetic flux density or magnetic induction in the centimeter-gram-second (CGS) system of units. (1 gauss is defined as one maxwell per square centimeter). Earth’s magnetic field is approximately 0.25 to 0.65 gauss (25 to 65 microtesla).
The full-scale acceleration range indicates the maximum and minimum values that the accelerometer can measure:
• ±2 g: The accelerometer can measure acceleration from -2 g to +2 g (±19.62 m/s²)
• ±4 g: The accelerometer can measure acceleration from -4 g to +4 g (±39.24 m/s²)
• ±8 g: The accelerometer can measure acceleration from -8 g to +8 g (±58.86 m/s²)
• ±16 g: The accelerometer can measure acceleration from -16 g to +16 g (±78.48 m/s²)
g (gravity) is the unit of acceleration due to gravity, approximately equal to 9.81 m/s². Therefore, in the ±2 g range, the accelerometer can measure accelerations up to about ±19.62 m/s².
The angular rate range indicates the maximum and minimum values that the gyroscope can measure:
• ±125 dps: The gyroscope can measure angular velocities from -125 degrees per second to +125 degrees per second (dps)
• ±250 dps: The gyroscope can measure angular velocities from -250 degrees per second to +250 degrees per second (dps)
• ±500 dps: The gyroscope can measure angular velocities from -500 degrees per second to +500 degrees per second (dps)
• ±1000 dps: The gyroscope can measure angular velocities from -1000 degrees per second to +1000 degrees per second (dps)
• ±2000 dps: The gyroscope can measure angular velocities from -2000 degrees per second to +2000 degrees per second (dps)
The data rate determines how frequently the sensor takes measurements and provides data. Here’s an explanation of the different data rates:
• LSM6DS_RATE_SHUTDOWN (0 Hz): The accelerometer is turned off and does not take any measurements.
• LSM6DS_RATE_12_5_HZ (12.5 Hz): The accelerometer takes 12.5 measurements per second.
• LSM6DS_RATE_26_HZ (26 Hz): The accelerometer takes 26 measurements per second.
• LSM6DS_RATE_52_HZ (52 Hz): The accelerometer takes 52 measurements per second.
• LSM6DS_RATE_104_HZ (104 Hz): The accelerometer takes 104 measurements per second.
• LSM6DS_RATE_208_HZ (208 Hz): The accelerometer takes 208 measurements per second.
• LSM6DS_RATE_416_HZ (416 Hz): The accelerometer takes 416 measurements per second.
• LSM6DS_RATE_833_HZ (833 Hz): The accelerometer takes 833 measurements per second.
• LSM6DS_RATE_1_66K_HZ (1.66 KHz): The accelerometer takes 1660 measurements per second.
• LSM6DS_RATE_3_33K_HZ (3.33 KHz): The accelerometer takes 3330 measurements per second.
• LSM6DS_RATE_6_66K_HZ (6.66 KHz): The accelerometer takes 6660 measurements per second.
• LSM6DS_RATE_SHUTDOWN (0 Hz): The gyroscope is turned off and does not take any measurements.
• LSM6DS_RATE_12_5_HZ (12.5 Hz): The gyroscope takes 12.5 measurements per second.
• LSM6DS_RATE_26_HZ (26 Hz): The gyroscope takes 26 measurements per second.
• LSM6DS_RATE_52_HZ (52 Hz): The gyroscope takes 52 measurements per second.
• LSM6DS_RATE_104_HZ (104 Hz): The gyroscope takes 104 measurements per second.
• LSM6DS_RATE_208_HZ (208 Hz): The gyroscope takes 208 measurements per second.
• LSM6DS_RATE_416_HZ (416 Hz): The gyroscope takes 416 measurements per second.
• LSM6DS_RATE_833_HZ (833 Hz): The gyroscope takes 833 measurements per second.
• LSM6DS_RATE_1_66K_HZ (1.66 KHz): The gyroscope takes 1660 measurements per second.
• LSM6DS_RATE_3_33K_HZ (3.33 KHz): The gyroscope takes 3330 measurements per second.
• LSM6DS_RATE_6_66K_HZ (6.66 KHz): The gyroscope takes 6660 measurements per second.
| Specification | FXOS8700CQ | LSM6DS3TR-C + LIS3MDL |
|---|---|---|
| Type | 6-axis (Accelerometer + Magnetometer) | 9-axis (Accelerometer + Gyroscope + Magnetometer) |
| Accelerometer Range | ±2g, ±4g, ±8g | ±2g, ±4g, ±8g, ±16g |
| Gyroscope Range | ±2g, ±4g, ±8g | ±2g, ±4g, ±8g, ±16g |
| Magnetometer Range | ±4, ±8, ±12 gauss | ±4, ±8, ±12, ±16 gauss |
| Magnetometer Sensitivity | 4096, 2048, 2014 (LSB/g) | 6482, 3421, 2281, 1711 (LSB/g) |
| Power Consumption (low power mode) | 8 µA (accel), 40 µA (magnetometer) | 9 µA (accel), 40 µA (magnetometer) |
| Output Data Rate (Accel/Magnetometer) | 800 Hz / 400 Hz | 1600 Hz / 1000 Hz |