Page 82 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.32 THE WORK

FIGURE 2.29 Grade Laser Beacon and targets. (Courtesy of Laser Alignment Inc.)

Automatic Grade Control. The basic application of the rotating laser beam is the direct, auto-
matic control of cutting depth (or filling height) of grading machines.
The machine to be controlled must have hydraulic lift for its cutting edge or parts. A vertical
mast is erected on a frame resting on the edge, and is kept vertical by a pendulum-controlled valve
and hydraulic minicylinders. It has a telescopic adjustment for height.
The mast carries a receiver with a number of cells sensitive to the laser light, which are wired
to a solenoid valve that controls the hoist for the cutting edge. This valve opens almost instanta-
neously when activated by the receiver cells.
Center cells are neutral or on grade indicators, and do not activate the valve. Those just above
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or just below center will open the valve for 50 milliseconds or ⁄ 20 second. The top and bottom ones
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open it for 200 milliseconds or ⁄ 5 second.
There may be additional cells to activate a control that will swivel the mast to keep the receiver
facing the laser.
The mast height is adjusted so that the center of the receiver is in the plane of the laser beam
when the cutting edge is exactly on grade, as seen in Fig. 2.30.
The machine is then driven and steered in the normal manner. If the blade drops slightly below
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grade, the hoist valve will be opened for ⁄ 20 second to raise it. This action will be repeated 5 times
per second until it is on grade.
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If the drop is enough to activate the top cells, they will open the valve for ⁄ 5 second. If the
demand for lift is repeated when the beam returns, the valve will stay open continuously until cor-
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rection shifts the work to the ⁄ 20 -second adjustment, or to neutral.