KRAKsat nanosatellite documentation
  • KRAKsat nanosatellite documentation
  • Introduction
    • KRAKsat mission
    • KRAKsat team
    • Partners
    • Glossary
  • SR-NANO-BUS platform
    • SR-NANO-BUS platform
  • Payload
    • Ferrofluid Reaction Wheel
    • Payload electronics
  • Software
    • Satellite state machine
    • Communication with OBC
    • Sensors
  • Communication
    • Communication specification
    • Infrastructure & Software
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  • IMU
  • Calibration
  • Temperature sensors

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  1. Software

Sensors

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Last updated 5 years ago

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IMU

Reading of the 3D digital angular rate sensor, the 3D digital magnetic sensor and temperature sensor is performed by I2C bus. Data received from sensor are in raw value, and to get the actual measurement, conversion is required.

Calibration

Magnetic sensor calibration

Magnetic sensor calibration should be performed in a magnetic-neutral environment, for example in a park. IGRF-12 model is used to get a vector magnitude of the reference magnetic field ∣br(i)⃗∣|\vec{b_r(i)}|∣br​(i)​∣ in a given location. Raw data from the sensor is gathered while the sensor is being rotated in every axis. Then the data is processed to calculate matrix A and vector o. To achieve calibration parameters, minimalization of the goal function ∑i=1n[∣br(i)⃗∣−∣b⃗^(i)∣]2 \sum_{i=1}^{n} [|\vec{b_r(i)}| - |\hat{\vec{b}}(i)|]^2 ∑i=1n​[∣br​(i)​∣−∣b^(i)∣]2 is performed. The figure bellow shows calibration results, where red points are raw data and green points are calibrated measurements.

Angular rate sensor calibration

Angular rate sensor calibration is performed on a rotating platform. The speed of the platform is constant, known and can be changed. First, the platform has no angular velocity. Raw measurements are collected and based on them, linear translation o⃗\vec{o}o is calculated. Then the platform is accelerated to given various constant rotation velocities and raw measurements are collected once again. Based on them, the scale factor KKK is calculated.

Temperature sensors

There are three temperature sensors in the payload. The first is built into the microcontroller and provides information about its temperature. The other is on the IMU circuit board and measures the temperature on the gyroscope and magnetometer chip. The third is near F.R.W. and monitors the temperature of the electromagnets.

Sensor

Interface

Microcontroller temperature sensor

Analog to digital converter

IMU temperature sensor

Inter-Integrated Circuit (I2C)

F.R.W. temperature sensor

Serial Peripheral Interface (SPI)

All the sensors require simple conversion of raw value with a linear equation. Coefficients of that equation are obtained by performing linear regression on reference measurements.

x - raw sensor valuet - real sensor valuetk,t0 - temperature regression coefficentst(x)=tk⋅x+t0x \text{ - raw sensor value}\\ t \text{ - real sensor value}\\ t_k, t_0 \text{ - temperature regression coefficents}\\ t(x) =t_k \cdot x + t_0x - raw sensor valuet - real sensor valuetk​,t0​ - temperature regression coefficentst(x)=tk​⋅x+t0​

Calibration was performed using a climate chamber. The temperature range for the regression is from -20°C to 50°C.

Based on the measured temperatures, linear regression coefficients were determined for all sensors.

Regresion results

tk

t0

ucTemp

0,93216

7,88081

tempIMU

0,05858

30,73653

tempFRW

0,98121

-2,00320

The determined coefficients were introduced into the temperature determination software.

Magnetic sensor calibration results
Microcontroller (ucTemp) and F.R.W (tempFRW) sensor calibration results
IMU (tempIMU) sensor calibration results