Converting accelerometer values???

Technical Technical Discussion
1

Hello,

I am currently working on some autonomous code and I need some help in understanding some information about accelerometers… To start off I have a question in regards to the central voltage and gain… What do these numbers mean and how do i adjust these numbers in order to get a proper reading (apparently 1G) which can later be converted to Velocity and Distance?

Thank you

Technic-R-C

2

Are you using LabVIEW or C++? The specific answers are different for each environment, though the concepts are the same.

The “central voltage” is the voltage the accelerometer outputs when there is no acceleration. It translates to “zero G”.

The “gain” is the voltage the accelerometer output changes by for a given measured acceleration.

The accelerometers we received in the 2009 Kit of Parts have a central voltage of 1.5 volts, with a gain of 300 millivolts per G. This information is in the 2009 Sensor Manual which you will find under Section 8 - The Robot on the 2009 FIRST Robotics Competition Manual and Related Documents](http://www.usfirst.org/community/frc/content.aspx?id=452) page.

3

Great!! Thank you for your responses,

Now to go on and convert this accelerometer value into a change in distance and velocity value… Does anybody have an idea of where i should begin in my setup of this code? (btw I am using LABVIEW to program our robot) or maybe if i’m lucky, is somebody willing to share a bit of their code dealing with the conversion of accelerometer numbers to velocity and distance values :smiley:

Thank you

Technic-R-C

4

I’m using Windriver, not LabView. The basic idea is to integrate the accelerometer output to get velocity, and integrate velocity to get distance.


#define X_ACC_ZERO 1.5
#define Y_ACC_ZERO 1.5

  AnalogChannel *m_accelerometerX;
  AnalogChannel *m_accelerometerY;
  m_accelerometerX = new AnalogChannel(2);
  m_accelerometerY = new AnalogChannel(3);

// initialize:
  XVelocity = 0.0;
  YVelocity = 0.0;
  XDistance = 0.0;
  YDistance = 0.0;

// periodic:
		XVelocity += m_accelerometerX->GetVoltage() - X_ACC_ZERO;
		YVelocity += m_accelerometerY->GetVoltage() - Y_ACC_ZERO;
		XDistance += XVelocity;
		YDistance += YVelocity;

You’ll probably have to add code to manage drift, shocks due to collisions, etc., which will cause the velocity and distance variables to diverge from the actual robot velocity and distance traveled. Depending on what you need the numbers for, you might need to convert the units e.g. to actual velocity in m/s. Note that 1G is 9.81 m/(s*s) so that’s a start.