Data acquisition

Table of contents
3.Introduction
4.Specification requirements
5.Solutions suggestions
6.The magnetic fields sensors
7.The inclination measurement system
8.The gyroscope
9.The data acquisition system
10.Communication system
11.The power supply
12.Realisation of the PCB
13.The embedded system
14.Static Library Util.a
15.ViewPort
16.Xcompass
17.Sensors controller commands
18.Test
19.Future improvements
20.Conclusion
21.References

This feature was one of the first parts we designed. It makes easier the dimensioning of the others components.

The microcontroller ATMEGA32 is sufficient for our application. It includes height 10-bits ADC that is not sufficient to measure the angular rate, the magnetic field or the acceleration but enough to control the temperature or the voltage. Therefore we decided to use serial ADC 12-bits (MAX186AEPA). After a calculation of the sensors resolution, it shows that 12 bits is enough. With a 14-bit application, we will not obtain a better signal than the actual ADC; we just collect a more great quantity of noise.

On the Max186, 1LSB = 4,096/2¹²  = 1mV

On the Atmega32, 1LSB = 4mV

The input signal on the ADC is comprised just between 0 and 4,096V. Consequently, all the signals have to be centred on 2,048 V.

The eight input channels of the ADC are divided as follow:

            -2 channels for the accelerometer (one for each axis)

            -3 channels for the magneto-sensors (one for each axis)

            -1 channel for the gyro.

We have still two channels free. They will be used to supervise the reference voltage and the inner temperature of the sensor.

Note: A problem with the microcontroller is that it is not design to operate outdoors (it doesn’t work when the temperature below 0°C).