Chief Delphi - Accelerometer class - isn't reading zero at rest?

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oddjob

02-02-2009 12:44

Accelerometer class - isn't reading zero at rest?

Do we have a bad KoP board or the code is wrong or missing some setup?

Code:

...



// Declare variables for the 3 axis accelerometer (2 channels wired)

Accelerometer *m_accelerometerX;

Accelerometer *m_accelerometerY;



...



// create accelerometers

m_accelerometerX = new Accelerometer(2);

m_accelerometerY = new Accelerometer(3);



...



//m_accelerometerX->SetSensitivity(0.0);

//m_accelerometerY->SetSensitivity(0.0);

//m_accelerometerX->SetZero(0.0);

//m_accelerometerY->SetZero(0.0);



...



accX = m_accelerometerX->GetAcceleration();

accY = m_accelerometerY->GetAcceleration();

...

The accX and accY values are about 1.5 each when the board is at rest. Do we need to use SetZero? I tried putting in 1.5 for the value and then accX went to an entirely different non zero value, -2.5 or something like that. Can I just use AnalogChannel and do the scaling in the code?

EricVanWyk

02-02-2009 12:47

Re: Accelerometer class - isn't reading zero at rest?

You certainly can.

Before you do, be sure you are telling the class that your center point is 1.5V and your sensitivity is 300mV/g. I *think* the class assumes a different model.

kiettyyyy

02-02-2009 14:09

Re: Accelerometer class - isn't reading zero at rest?

The classes aren't really documented.

How do you set the center point to 1.5V? How do you set the sensitivity to 300mV/g?

From what I can tell, the accelerometer class contains the function calls of SetZero() and SetSensitivity().

How do you use these?

Jared Russell

02-02-2009 17:11

Re: Accelerometer class - isn't reading zero at rest?

If you read the WPILib source code, it gives you some hints.

Code:

/**



 * Set the accelerometer sensitivity.



 * 



 * This sets the sensitivity of the accelerometer used for calculating the acceleration.



 * The sensitivity varys by accelerometer model. There are constants defined for various models.



 * 



 * @param sensitivity The sensitivity of accelerometer in Volts per G.



 */



void Accelerometer::SetSensitivity(float sensitivity)



{



        m_voltsPerG = sensitivity;



}



...



/**



 * Set the voltage that corresponds to 0 G.



 * 



 * The zero G voltage varys by accelerometer model. There are constants defined for various models.



 * 



 * @param zero The zero G voltage.



 */



void Accelerometer::SetZero(float zero)



{



        m_zeroGVoltage = zero;



}

So for the KoP accelerometer (zero G = 1.5V, sensitivity = 300mV/G), call:

Code:

myAccelerometer->SetZero(1.5);

myAccelerometer->SetSensitivity(0.3);

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