Grade Control System
The grade control system provides a way to lay a smooth mat at a specified elevation or to match a longitudinal joint. An electric sensor senses any vertical movement of the screed in relation to some outside reference; a stringline, a previously laid mat, a curb, etc. The tow point is then raised or lowered to adjust the screed back to the desired elevation. Grade control can be used on both sides of the machine, on one side of the machine with the other side controlled by the slope control system, or on one side with the other side riding free on the asphalt.
The grade control sensor is supported by an adjustable mount on the screed end gate. The machine and sensor are initially adjusted so that the screed is at the desired elevation and the sensor is nulled (no electric signal) with the sensor ski riding on the outside reference. If the screed then rises or falls in relation to the reference, the sensor shaft will be rotated, sending an electric signal to the servo flow control valve. The signal causes the appropriate port in the valve to open and send hydraulic oil to the tow point cylinder to bring the screed back to the correct elevation. When the screed reaches the correct elevation, the signal from the sensor stops, the valve port closes and the hydraulic oil stops flowing to the tow point cylinder.
A sensor skate and/or ski are used when the reference being followed is an adjacent mat, curb, gutter, etc. A traveling stringline is available to provide a reference from an existing irregular surface. The length and design of the traveling stringline even out the highs and lows in the grade so that the mat will be more level and will be thick enough on the highs.
Slope Control System
An electric slope sensor is the heart of the slope control system. The slope sensor, mounted on a beam that spans the screed, senses any deviation of the screed from the cross slope set in the system. When deviation from the set cross slope is sensed, the slope sensor sends an electric signal to the servo flow control valve. The signal causes the appropriate port in the valve to open and send hydraulic oil to the tow point cylinder to bring the screed back to the correct cross slope. When the screed reaches the correct cross slope, the signal from the sensor stops, the valve port closes and the hydraulic oil stops flowing to the tow point cylinder.
Slope control is normally used in conjunction with grade control on one side of the machine, but the slope control can be used to maintain a given cross slope with the screed tow point pinned on one side. Slope control can never be used when grade control is being used on both sides of the machine.