Page 83 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.33

FIGURE 2.30 Laser-controlled scraper.

If the blade moves above grade, similar action will put the valve in the down position. Lights
keep the operator informed of the actions of the system.
A refinement for fast-moving machines (scraper or dozer) is a proportional current system,
responding to the 10-sweeps-per-second or faster laser. In this, the opening of the valve is pro-
portional to the amount of correction needed.
Repetition of automatic adjustments 5 or 10 times per second results in running a very accu-
rate grade, usually within 0.02 or 0.04 of specification. Accuracy is greatest close in, and declines
within these limits to a distance of 1,000 feet, the longest recommended.
With increasing distance, the beam widens so as to lose its pinpoint accuracy, and problems
with diffraction become more likely. Errors caused by curvature of the earth’s surface are given
in Fig. 2.35.

Applications. This system is well adapted to automatic control of ditchers, landplanes, scrapers,
dozers, tube and cable plows, and graders.
The grader requires an additional cross-slope sensor. The laser controls the lift cylinder for the
leading edge, while the slope regulator keeps the trailing edge in proper relationship.
Automatic control is largely limited to finishing operations, as it is not designed to sense loads,
and may therefore tend to make a machine dig into more material than it can move.
When the automatic control overloads the machine, the operator can override it, and raise the
blade. The operator can then return for additional manually controlled passes, finally returning it
to automatic for trimming.
Excavating Trench. To excavate a trench to a given depth and in a specified alignment, Laser
Alignment Inc. provides a laser instrument called the Depthmaster, depicted in Fig. 2.31. It can
be attached to the dipper stick of a hydraulic excavator and gets direction from a laser beacon,
described previously, set up nearby.
The Depthmaster gives readings for vertical alignment and depth of cut. The vertical align-
ment is noted by the three lights at the bottom of the readout panel. If the middle light shows, the