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Relative Attitude Determination

Orientation Sensors

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   Inclinometer   


Sensors
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Calibration
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Using the VN-100
   Embedded Library Example

Using the VectorNav VN-100 As a Gyro-Stabilized Inclinometer

A inclinometer is an instrument used for measuring angles of tilt with respect to gravity.  This is also known as a tilt meter, tilt indicator, pitch & roll sensor, level meter, and gradiometer.

Existing Technology

Tilt sensors and inclinometers generate an artificial horizon and measure angular tilt with respect to the horizon.   They are used in a wide variety of industrial systems including platform leveling, aircraft flight controls, antenna tracking, and many other applications that require tilt measurements.  Existing inclinometer sensors include precision bubble levels, pendulums, liquid capacitive, and solid state silicon accelerometers.

Problems with Existing Solutions

For most static applications it is hard to beat a precision bubble level for its precision, reliability, and ease of use.  For dynamic applications however, things become much more complicated.  Anyone who has used a bubble level is familiar with its sensitivity to motion.  Most mechanical sensors exhibit this same sensitivity since they rely on some physical substrate that is influenced by gravity and thus also translational motion.

Advantages of Using the VN-100 as a Inclinometer

The VectorNav VN-100 is a extremely compact solid state MEMS based sensor module which uses sophisticated onboard algorithm to calculate its tilt and heading angles at a rate of 200 times a second.  It uses an accelerometer to sense the combination of linear motion plus gravity.  The accelerometer can be thought of as the digital equivalent of the bubble level.  What makes the VN-100 different from other digital inclinometers is its use of 3-axis gyro to calculate changes in orientation at an extremely fast rate. A gyro measures how fast an object is rotating in 3D space.  The MEMS based 3-axis gyro, unlike the accelerometer and bubble level, isn't confused by movement and acceleration.  The gyro enables a better measurement of tilt in the presence of dynamic motion.

Gyros do suffer from long term drift in their sensor outputs, which if left un-corrected could lead to errors in the orientation output.  The trick to the VectorNav VN-100 is it's ability to predict the long-term drift of the gyro using the measurement of the accelerometer.  In this way both sensors are used for exactly what they do best. The gyro is used to capture high update rate estimates of the tilt angle, and the accelerometer is averaged over a long period of time and used only to keep the gyro in check.

The VectorNav VN-100 gives developers access to a rich feature set of functionality provided by the onboard processor.  Both serial and programmable SPI (serial peripheral interface) port are provided for easy communication access.  The factory calibration provided with each unit and user-settable configuration parameters enables designers without sensor expertise to quickly, easily, and cost-effectively integrate gyro-enhanced performance into any application, vastly improving dynamic performance.

Along with the gyro and accelerometer the VectorNav VN-100 also provides a 3D magnetic sensor allowing the sensor to calculate compass heading angle as well as pitch and roll.  This full 3D angle sensing adds an entirely new dimension to tilt sensing applications.

Gyro-Stabilized Filter Tuning

The VectorNav VN-100 uses a sophisticated sensor fusion algorithm that seamlessly blends the data from the gyro, accelerometer, and magnetometer into the final solution.  What makes the VN-100 stand out from the other sensors on the market is the level of performance customization that is provided to the developer using the easy to use serial interface.  The easy to use software interface allows designers with limited sensor expertise to quickly adjust the sensor's performance to match their particular projects needs.  The designer can tell the sensor how much trust he wants the sensor to place in each sensor when blending the measurements.  For example, if you don't want the sensor to use the magnetic measurements then just send a command to the sensor to tell it not to trust the magnetometer. Or consider another case where you have the sensor mounted on a platform besides an electric motor and when the motor turns on it creates a vibration in the X-direction that enter the accelerometer reading.  How does one deal with this  Program the sensor to not trust the X-axis accelerometer.  Or maybe you want to trust the accelerometer most of the time, but when the electric motor turns on you want to turn off the accelerometer and only trust the gyros.  Again, not a problem.  The amount of trust that you place in the sensors can be updated as often as you like, even 200 times a second if you wish.  With the VectorNav VN-100 you have full high-level control over exactly how you want the sensor to combine its measurements.

The VectorNav VN-100 changes the way you will approach motion sensing problems.  Think of it as the swiss army knife of tilt sensors.  In one small package you have access to 3D accelerations, 3D rotation rates, 3D magnetic measurements, pitch angle, roll angle, and even the magnetic heading angle, without having to perform any complicated math.