Magnetometer Testing Board

Presented byJianer Shi

04/25/2011

SECTIONS

Sections

1. Background2. Board Design3. Code

2.1 Microcontroller2.2 Graphic User Interface (GUI)

4. From here on…

BACKGROUNDSection 1/4

Background

• Toque coil is used to produce a magnetometer dipole which controls the satellites attitude.

• In order for the toque coil to behave as expected, an accurate measurement of magnetic field is required.

• Magnetometer is an essential part to determine the earth magnetic field

Background

• Previous testing done by Ryne Beeson in 2009 shows noise of 0.02 Gauss, which is unacceptable for accurate attitude determination and control. Further testing is suggested and a magnetometer testing board is built for easy testing and lower electronic noise.

BOARD DESIGNSection 2/4

Board Summary

PCB Laying Tool: Eagle 5.11.0Microcontroller: PIC18F2221-ICommunication Port: Mini – USB BPower: Mini-USB 5VCrystal Oscillator: 11.0592MhzUART Communication Baud Rate: 57600Communication: Serial-USB (FT232RL)

Design Criteria

• Microcontroller– Support I2C– Support UART

• Board– Small size– Mountable

• User Interface– Able to pool and save Matlab friendly data for later process

• Communication– Error tolerance

Microcontroller

• PIC18F2221-I– Minimum Pin Number to satisfy the requirement– Small size (SOIC Package)– Has 1 UART and 1 I2C– 5V Power Supply from USB– Errata page provided by manufacturer has no

significant problem with UART and I2C– Compare to other Microcontrollers, this is the

minimum one satisfies the design criteria.

Power and Communication

• Mini-Usb Port– Small size– Provide both 5V power and communication

• FT232RL USB-Serial Chip• Provide 3.3V output - eliminate 2 voltage regulators

and has 500mA cutoff current protection• Provide UART to USB link

Crystal Oscillator

• 11.0592Mhz– This choice will resulte in 0% error with a wide

range of baud rate choice.– We choose 57600 baud rate

611.0592 1057600 64(2 1)

PIC

Magnetometer

USBTo

Serial

Mini USB

Connect to ICD2 to

Program

3.3<->5.5 I2C Logic Level

Conversion

MagnetometerZoff+,Zoff-,Yoff+,Yoff-

Ground Polygon

Top Layer

CrystalOscillator11.0592

Mhz

Ground Polygon

Mounting Hole

Bottom Layer

MICROCONTROLLER CODESection 3.1/4

Microcontroller Task Overview

MicrocontrollerHMC6343Magnetometer

FT232RLUSB Serial Computer

I2C

UART

USB Cable (serial)

I2C

• Communication between magnetometer and microcontroller

• Library provided by i2c.h and pic18f2221.h from C18 Compiler

I2C Write

I2C Read

Magnetometer I2C

• Slave address(0x32) for write(0x33) for read

Magnetometer I2C

• Functions(0x40) Accelerometer Data of 3 axis(0x45) Magnetometer Data of 3 axis(0x50) Heading Data of 3 axis(0x55) Tilt Data of 3 axisReturn value: 6 byte, 2 byte per axis (in hex: xxxx)

(0x82) Reset the processor, no return value

Magnetometer Spec

• 500ms power up delay before extract data• 1ms delay for each read

3.3V to 5.5VI2C Logic Level Conversion

• Logic Level conversion is necessary because components operate on different power

Magnetometer 3.3VMicrocontroller 5V

3.3V to 5.5VI2C Logic Level Conversion

Source: <Bi-directional level shifter for I²C-bus and other systems. Philips>

UART

• Library– Usart.h

• 2 Pins– TX for sending– RX for receiving

Serial Protocol

In order to send information that the computer side can decode, all information pulling from the magnetometer is sent through a protocol (in hex)

(eeeeeeee) (00) (function 1 byte xx)(data 2 byte xxxx)(ffffffff)

Serial Protocol

(eeeeeeee) (00) (function 1 byte xx)(data 2 byte xxxx)(ffffffff)

• (eeeeeeee) starting flag• (ffffffff) end of line• (00) There are still new function data to be transferred

Serial Protocol

(eeeeeeee) (00) (function 1 byte xx)(data 2 byte xxxx)(ffffffff)Functions

(41) Accelerometer X(42) Accelerometer Y(43) Accelerometer Z

(51) Heading X(52) Heading Y(53) Heading Z

(46) Magnetometer X(47) Magnetometer Y(48) Magnetometer Z

(56) Tilt X(57) Tilt Y(58) Tilt Z

GRAPHICAL USER INTERFACE(GUI)

Section 3.2/4

Complete GUI

GUI Programing Language

• Python– Pyserial– wxtools

• Qt4– Matplotlib

• Toolchain– Pyqt4

Python

• State Machine to decode protocol– 24 State to decode serial protocal– Error Tolerance imbedded even though the UART

will in theory result in 0% communication error

Qt4

Data Saving Button

• All raw data will be saved in a tabular format, This can be easily imported into a matlab vector.

• Button to control start saving data and stop saving data

• Reason to keep raw data– Decimal data will result Matlab underrun (not

enough precision)

Real Time Plot

• Based on the open source code developed by Eli Bendersky• 3 colors to display the 3 axis data on the same graph simultaneously

FROM HERE ON ...Section 4/4

From here on

• The board will handed over to whoever is going to test the magnetometer

• Since the GUI has been revised many times for easy testing, the tester will find the whole system easy to use and test

THANKS!