Tinkering Accelerometer + Gyro AKA IMU (Part 3)

Now that we know how to read the sensors, we need to make sense of it. Lets have a good look at the sensor datasheet.

ADXL335 3-axis Accelerometer
Accelerometer, measures resultant acceleration acting on the sensor.
This ADXL335 outputs an analogue voltage based on acceleration. Its sensitivity of the ADXL335 is 300mV/g, which means that the output will change by .30v per 9.8m/s² .

The ADXL335 has a ratiometric output, meaning 0g is always around half of the supply voltage Vcc, The datasheet stated that the zero Voltage is typically at 1.5V.

Hence,
Accel  = Vaccel - 1.5V

BUT I prefer to get the actual count reading when the accelerometer is resting on horizontal position

and Sensitivity is typically 0.3V/g, the Arduino ADC gives a 10-bit value based on a 3.3v reference, the gain on the raw ADC value can be calculated by:
Hence,

(1g/0.3V) X (3.3V/1024) =0.01627 g/LSB

Hence,

Position of  the accelerometer = Accel *0.011

IDG500 2-axis Gyro
This gyroscope measures angular rate and return a analogue voltage signal proportional to the rate of rotation. The at the 4.5x, IDG500 has a sensitivity of 9.1mV/°/s, i.e. the output changes 9.1.0mV for every °/s of rotation. Since the Arduino ADC gives a 10-bit value based on a 5v reference, the gain on the raw ADC value can be calculated by:

(1°/s/0.0091V) X (3.3/1024) = 0.354 °/s/LSD


deduct the factory set 1.35V from raw analogue input & multiplying result with 0.354°/s/LSD  will return the angular velocity in degrees-per-second.

Gyro rate =(Gyro_reading - 1.35V)*0.354 deg/sec



Since we are doing discrete maths with our controller, we multiply the known interval between gyro reading which is the loop time; we get the change in angular position in degree

Gyro Angular change = Gyro rate * Loop time

Now we have enough data to measure angular position with the gyro with the following line.

Gyro angle= Gyro angle + Gyro Angular change.

What we had just done is to rationalised both the accelerometer & gyro output to angular position with the same unit of measurement.

It is now convenient to write code to compare the sensors on the same page.

Tinkering Accelerometer + Gyroscope AKA IMU (Part 2)

The IMU is a combo board consisting of 2 sensors.

What good is a sensor if we can not read it.

Here I jury jig a bench top test for reading the sensors.

Hardware involved:-
Arduino Duemilanove
5DOF IMU
some hook-up leads
- 3V3 to IMU Vss & ARef pin
-Gnd to Gnd
- accel X axis to Analog input 0
- accel Y axis to Analog input 2
- gyro X axis(4x output) to Analog input 1
- gyro Y axis(4x output) to Analog input 3
- USB cable

We still need to write some codes to get the job done.

Following  is a simple code which I came  up with



// Reading 5DOF IMU


//Analog Pins

int Xaccel_pin = 0;      //connect accel X axis to Analog input 0
int Yaccel_pin = 2; //connect accel X axis to Analog input 2
int Xgyro_pin = 1; //connect gyro X axis(4x output) to Analog input 1
int Ygyro_pin = 3; //connect gyro Y axis(4x output) to Analog input 3

//Digital Pins

int Xaccel_reading;          //X axis Accelerometer reading
int Yaccel_reading;          //y axis Accelerometer reading

int Xgyro_reading;      //X axis Gyro reading
int Ygyro_reading;      //Y axis Gyro reading

float loop_time = 0.05;
int last_loop = 0;
void setup(){

Serial.begin(9600); // start Serial monitor to print values

pinMode(Xaccel_pin,INPUT);
pinMode(Xgyro_pin,INPUT);
pinMode(Yaccel_pin,INPUT);
pinMode(Ygyro_pin,INPUT);
pinMode(13,OUTPUT);
}

void loop() {
  Xaccel_reading=analogRead(Xaccel_pin);
  Xgyro_reading=analogRead(Xgyro_pin);
  Yaccel_reading=analogRead(Yaccel_pin);
  Ygyro_reading=analogRead(Ygyro_pin);

while((millis()-last_loop)
{
    delay(1);
}
last_loop=millis();

Serial.print("X-Accel: ");
Serial.print(Xaccel_reading);
Serial.print("  ");

Serial.print("X-Gyro: ");
Serial.print(Xgyro_reading);
Serial.print("  ");

Serial.print("YAccel: ");
Serial.print(Yaccel_reading);
Serial.print("  ");

Serial.print("Y-Gyro: ");
Serial.print(Ygyro_reading);
Serial.print("  ");

}

Once loaded, you can see the IMU reading via the "Serial Monitor" or <>

Now that I have IMU readings, the next gargantuan task is to figure out what can I do with them...

Maybe in Part 3 I'll have an answer....