Monday, November 23, 2009

Measuring Systems Part 1 - Positioning

In traditional Surveying & Mapping, points are determined by measuring an angle and distance to an object from an instrument over a known point (X, Y, Z or elevation).  We have all seen, at one point or another, a person at a total station locating a rod held by another field crew member - often while we zoom past at highway speeds. 

Now, with Mobile LiDAR, we are Surveying & Mapping at highway speeds.

In order to better understand how accurate measurements are collected from our vehicle, it is important to know the instruments onboard and the function of each.  As individual sensors, they do not provide an adequate solution.  But, when used as part of a system, they provide the foundation for accurate information. 

The purpose of this post is to provide a basic overview of our system.  Additional parts will cover the lasers, cameras and other components and processes which provide a complete solution.  If you have questions, please leave a comment.

Global Positioning System (GPS)
The primary method of location for the vehicle is GPS.  We utilize 2 onboard units to provide the position of the vehicle as well as the heading (having a known baseline, distance and direction, between antennas provides a measure of direction of the vehicle).  The fundamental issue with GPS as a sole (hence I used primary) source of positioning is that we measure a position one time per second.  For those of you breaking out your calculator, that equates to 88 feet per second when traveling at 60 mph.  In that second, our lasers could have measured 400,000 points (covered in part 2, stay tuned).  Therefore, we rely on our second measurement instrument.

Inertial Measurement Unit (IMU)
The IMU measures the attitude (roll, pitch and yaw) of the vehicle, much like an aircraft, at a rate of 200 times per second.  By measuring those changes in direction about the X, Y and Z axis, we are able to calculate the vehicle position at those increments.  Using interpolation, we are able to further refine intermediate positions.

Distance Measurement Instrument (DMI)
The often overlooked member of the measurement family is the DMI.  Barely noticeable and not mounted on the "LiDAR Wing", the DMI has two distinct purposes: determine the distance traveled by measuring the revolutions of the wheel 1,024 times per second and tell the system when the vehicle is stopped.  Since there is drift in GPS and the IMU, the DMI basically determines when the wheel stops revolving.  By measuring the diameter of the wheel and calculating circumference, we also know the distance traveled per partial revolution.

For NASCAR fans:  since we measure the diameter of the wheel at rest, the circumference of the wheel is calibrated while we're driving and GPS provides a good solution.

For non-NASCAR fans: as we drive, the tire will begin to heat and will build pressure thereby increasing tire circumference and impact the distances measured by the DMI.

In a nutshell... one system helps calibrate another system.  In a later post I'll cover what happens when we lose GPS!!! 

Comments are moderated, so if you have a question, comment or suggestion please let me know.

Thanks for following!

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