Introduction
The AccuGrade ® System is designed for earthmoving equipment in the construction application. The system can be used with a wide variety of sensors in order to provide conventional control or 3D machine control.
Conventional Systems
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Cross Slope | |
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Sonic with Cross Slope | |
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ATS with Cross Slope | |
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Single GPS with Cross Slope | |
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Dual GPS | |
Conventional systems provide elevation or slope control. Conventional elevation systems continually measure the offset between a reference surface and the cutting edge of the machine. This measured offset is compared against a design offset that has been entered into the system. If the measured offsets and the design offsets differ, the AccuGrade System drives the cutting edge of the machine upward or downward until the offsets match. Elevation sensors, such as a Sonic Tracer, provide elevation control.
Conventional slope control systems use sensors that are mounted on the machine in order to measure the cross slope of the cutting edge of the machine. This measured cross slope is compared against a design cross slope that was entered into the system. If the measured cross slope and design cross slope differ, one side of the blade is raised or lowered until the cross slopes match.
Conventional systems do not provide a horizontal position. So, the following items are not used in a conventional system: design files, background maps and avoidance zones.
Sonic Tracer
The Sonic Tracer uses sound in order to determine distance. Usually, Sonic Tracers are attached to one or both ends of the cutting edge of the machine in order to measure the vertical offset to a reference surface. The reference surface may be one of the following items: a string line, a curb, a gutter and previous pass.
The system detects any variation in the vertical offset, and sends the corrections to the hydraulic valve that controls the cutting edge. By keeping the Sonic Tracer over the reference surface, the operator can grade each new pass so that it is parallel to the reference surface.
Cross Slope
For a slope control system, a combination of angle sensors and a rotation sensor are used. The sensors accurately measure the cross slope of the cutting edge of the machine. Corrections are sent to the hydraulic valves for the machine in order to drive and hold the cutting edge at a cross slope that has been predefined. Systems that are slope only can be used in order to spread material at a slope that has been set. Slope control systems can also be used in conjunction with elevation sensors or 3D sensors.
3D Systems
3D Systems use sensors that can measure horizontal position and vertical position. The resulting X,Y, Z coordinates are compared against a design that was loaded into the system. The system sends corrections to the hydraulic valves for the machine in order to drive the cutting edge of the machine to grade. The system uses either a Advanced Tracking System (ATS) robotic total station or Global Positioning System (GPS) receivers as the 3D sensor.
The idea behind the AccuGrade System is very simple. Computers and design software produce accurate 3D models of project designs. In the past, these models stayed in the office with the engineer. The engineer gave information to the surveyor. The surveyor then put stakes in the ground in order to guide the machine operators.
With the AccuGrade System, the 3D model comes into the field as a design surface. By adding GPS, you can accurately determine the position of the blade on your machine in both horizontal direction and the vertical direction on the design surface. The cut or the fill that is to be applied to the design surface can then be computed on the machine in order to provide guidance for the machine.
When all of the hardware is installed on the machine you need to configure the AccuGrade System. You can configure all system components with the Display. The Display is the onboard computer. The default settings support most of the conditions, but the settings can be changed as required. Items that need to be configured in order to make the AccuGrade System work properly include the following items:
- GPS Receiver Configuration (Site Coordinates)
- Light Bar Configurations
- Radio Network Or Frequency
- Machine Settings
- Units Of Measurement
- Machine Dimension Information
- Pitch Sensor Calibration
- Valve Calibration
- Operator Configuration
At the start of any new project or work area the site calibration, and the design are required. GPS data is needed in order to position the blade. GPS signals are received through the GPS Receivers. At the same time, base station data is received through the data radio and sent to the left hand GPS Receiver. The GPS data from each GPS Receiver on the blade is combined with the slope information in the Display to produce a blade position and blade cross slope. The system uses the design in conjunction with the GPS information in order to compute the cut or the fill at the blade tips in respect to the design. The design can be in the form of a DTM (TIN or Grid), road, level or sloping surface. The Display has a color LCD that has brightness control in order to allow the operator to see the LCD in both day light and at night. Information on the screen is available in the following views:
- Plan
- Cross section
- Profile
- Text
The operator will be able to switch between these views at any time. The Display uses the GPS positions and the machine dimensions in order to compute the position of each blade tip. The Display compares the blade tips to the design elevation to calculate the cut or fill needed to be on-grade. The cut or fill data is also displayed on light bars, which act as another visual indication to the operator. When horizontal alignments are defined, the center light bar provides left and right guidance to the selected alignment.
ATS System
With an Advanced Tracking Sensor (ATS) System, a prism and a radio are attached to the machine. The prism is mounted on a mast above the cutting edge of the machine. The ATS robotic total station can be set up over a known point on site. The ATS robotic station can also be set up on an unknown point. The position of an unknown point can be determined by methods such as resection. The position of the point must be determined before you start using the ATS for machine control.
The ATS tracks the machine as it moves. Then, the ATS broadcasts the position of the machine to the AccuGrade System via one of the following radios: TC900C and TC2400. An ATS System is usually combined with a Cross Slope System in order to provide guidance across the entire cutting edge of the machine.
The Global Positioning System (GPS)
With a GPS system, one or two GPS receivers are attached to the machine. With single GPS systems, the receiver is mounted on one of the masts that is located above the cutting edge of the machine. With a dual GPS system, the GPS receivers are mounted on masts that are located above either end of the cutting edge of the machine.
GPS is a system of satellites that orbit the earth twice a day at very high altitudes. The orbit radius is approximately 26600 km (16528 miles). GPS was set up by the government of the United States. GPS provides precise timing and positioning anywhere on earth, 24 hours per day with no charge to users.
In very general terms, a GPS receiver computes a position based on radio signals received from several different satellites. The satellites have highly reliable clocks, so the timing of these satellite signals is known very accurately. The GPS receiver calculates the relative distance to each of the satellites. This calculation is based on the travel time of the signal and the speed of light (speed of the signal). The receiver then uses these distances in order to calculate the receiver's location on earth.
As a broadcast only radio system, GPS supports an unlimited number of users. The broadcast frequencies penetrate clouds, rain, and snow. GPS can also accurately guide operations in fog or dust as well as at night. There are different levels of precision and accuracy available from GPS. These levels depend on the GPS receivers and the methods that you use.
The four basic levels of position accuracy (1 sigma) that are available in real time (that is, instantaneously) from GPS are as follows:
- Autonomous, 10 m (30 ft) to 15 m (50 ft)
- DGPS, 0.3 m (1.0 ft) to 1.0 m (3.0 ft)
- RTK (Float), 0.2 m (0.7 ft) to 1.0 m (3.0 ft)
- RTK (Fixed), .02 m (.07 ft) to .03 m (.10 ft)
Illustration 6 shows the GPS accuracy levels.
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GPS accuracy levels (1) Autonomous (2) DGPS (3) RTK Fixed (4) RTK Float | |
The method that is used for machine grade control in construction is the same as that used by surveyors for stakeout in construction. The method is called Real-Time Kinematic (RTK), GPS.
In order to produce RTK positions two GPS receivers are required. One receiver is known as the GPS base station and is fixed in one place. The other GPS receiver is known as the rover. This receiver can be stationary or mobile.
The base station communicates to the rover ('s) through a wireless data link using a data radio. Illustration 7 shows this setup.
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GPS and the AccuGrade® System (1) GPS satellites (2) GPS base station and data radio (3) Repeater radio (optional) (4) Machine equipped with the AccuGrade System | |
Both GPS receivers make observations (measurements) of the GPS signals at the same time. The base station broadcasts the observed information together with the location and other information across the data radio link to the rover. The rover then combines the data from the base station together with its own data in order to compute a very accurate position relative to the base station.
A single base station can support an unlimited number of rovers, provided the rovers are within about 20 km (12.4 miles) of the base station. Normally, the range of the data radio link is the more limiting factor. You can use radio repeaters in order to extend the radio coverage.
In areas where the 900 MHz band is available, the TC900C data radio is recommended. In areas where the 900 MHz band is not available, one of the following radios is recommended:
- TC450
- TC2400