The VÖGELE innovation helps steer an accurate course without physical stringline.
  
     
     
The system for non-contacting grade and slope control and navigation.  
VÖGELE NAVITRONIC:
High-Precision Paving Cuts Cost		  
  By Prof. Dr.-Ing. Henning Jürgen Meyer, Technical University of Berlin  
     
  Nowadays, grade and slope control and navigation systems in road construction machinery are indispensable tools for road builders, as their use enhances substantially the quality of the road being built. Modern computer technology and surveying technology meanwhile allow us to do without physical stringlines and their time-consuming installation.
This paper gives an overview of the geodetic side conditions as well as conditions related to machinery and applications technology, which influence the use of non-contacting systems such as VÖGELE NAVITRONIC.		  
     
     
Requirements for Use of Non-Contacting Grade
and Slope Control and Navigation Systems
In order to be able to do without time-consuming set-up of physical stringlines in road construction, several technological requirements need to be fulfilled, in other words the following needs to be available:
  
-  Non-contacting system for position measurement, adequate for measuring position
of moving objects
- Computer system adequate for mobile construction machinery
- Radio link for transmission and exchange of data between system components
Thanks to the fast progress made in computer technology and the use of modern technologies in the fields of communications engineering, surveying and automation, all these preconditions are fulfilled for road construction machinery. In view of the high cost saving potential and the high potential for rationalization, providers of machine control systems have developed solutions for grade and slope control as well as navigation to be used in the construction sector.
Especially as far as earthworking machinery is concerned like graders, 3D systems for grade and slope control have been applied successfully in recent times. For laying machinery such as asphalt and slipform pavers, however, accurate control is harder to implement due to the higher demands made on accuracy. But meanwhile for these machinery, too, systems for non-contacting control are available performing very well in the field. For road pavers, VÖGELE NAVITRONIC is the only system which allows not only non-contacting grade and slope control but also non-contacting navigation. 		 
Geodetic Side Conditions
Collecting data referring to position of one or several points on the machine is a major precondition for the use of non-contacting control systems. Two methods are currently applied with are:
1.  Satellite-based measurement of position by GPS
2. Laser-based measurement of position by total station
Satellite-Based Position Measurement
GPS stands for “Global Positioning System”,
a system as it is also used nowadays for navigation control in cars, for instance. The background for this system are satellites orbiting the earth. Between 4 to 8 satellites can be observed from the earth simultaneously at any time at an angle of at least 15°.
A GPS receiver collects signals emitted by the satellites. Based on these signals distances can be derived. This information allows to calculate the position of the receiver with pinpoint accuracy.
For machine control, the accuracy attained by simple GPS is not sufficient (Figure 1). Application in this field called for development of differential GPS with Real-Time Kinematics (RTK). The method requires a second receiver as reference station, the postion of which is accurately known. With the help of the reference station correcting values can be calculated for the measured pseudo-distances
  Figure 1: Demands Made on Roads in Terms of Evenness.
 
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and transmitted by radio link to the mobile station. This approach allows to attain a much better accuracy. Even higher precision can be achieved by applying the RTK method (Real Time Kinematics), which provides for an accuracy of 1cm in the horizontal and 2cm in the vertical. This range is absolutely sufficient for earth-moving and construction machinery such as graders, dozers and excavators.

  
Total Stations / Tachymeters for Position Measurement
As to road pavers, however, the accuracy achieved by GPS is still not high enough for grade and slope control of their screeds. Asphalt pavers and also slipform pavers require an accuracy within the range of millimetres due to the fact that the demands in terms of accuracy made on the road to be built also lie within this range (Figure 1). For these applications only those systems are currently appropriate which use total stations with automatic target tracking capabilities for position measurement. Total stations are equipped for optoelectronic measurement of distances and come with a combination of ptoelectronic and microelectronic components for measurement of angles. Another feature of a total station is its capability of automatically tracking a moving target. This capability is a must for use with road pavers, as they move during operation.  
Side Condtions Related to Machinery
(Figure 2) shows in a general form the flow of information for navigation and grade and slope control, including influencing parameters. It starts with measurement of position by a total station and measurement of additional machine parameters such as angles and distances. Based on this information and on the machine’s geometry, it is possible to describe the machine’s position in space with mathematical accuracy. A comparison is then made between the actual model and the specified model calculated from design data for the route.
Based on this comparison signals are calculated for control circuits of grade and slope and navigation, to actuate valves for the correcting elements on the machine. Correcting elements on asphalt pavers normally are the hydrauic rams for the screed’s tow points and the steering system. Selected sensitivity, control response etc. are the crucial parameters for the behaviour of the machine to match the respective situation. 		     
Figure 2: Signal Flow and Influencing Parameters
for Grade and Slope Control.
Zoom in  
As far as valves are concerned, different types can be installed. In the case of on/off valves, fine control of the oil flow to operate the hydraulic rams is brought about by opening/closing the valves for a moment. Characteristic of this type of valves is a discontinuous oil flow. In the case of proportional valves, the control current for operation is proportional with the required oil flow, in other words much more sensitive control is possible. In road pavers, on/off valves are normally sufficient for grade and slope control, as the response of the „floating screed“ compensates the impact released when switching the valves on and off. It is quite evident that non-contacting navigation and grade and slope control makes use to a large extent of the components of conventional machine control. This control, however, needs to be extended by interfaces for the non-contacting system. Today’s microcontrollers, as they are installed in VÖGELE pavers, have interfaces that can be used for information exchange. All that needs to be done is adapting the data transfer protocol. Also possible is direct communication between the external non-contacting systems and the on-board units for navigation and grade and slope control, as is the case with VÖGELE NAVITRONIC, which uses VÖGELE NIVELTRONIC to control paver hydraulics. This solution also suggests itself for non-contacting grade and slope control alone, when a conventional „grade sensor“ is simulated by means of a position measurement system and an on-board computer. 		 
Side Condtions Related to Applications Technology
There is a variety of side conditions related to applications technology. At the time being, non-contacting systems exist in the market but only for grade and slope control. They are widespread above all for graders and dozers. The term of 3D system is often used in this context. These systems, however, do not include automated steering, as for the above kind of machinery this feature can be dispensed with. For laying machines such as road pavers, however, it cannot. Pavers need to precisely follow a given direction and shape of the route, which calls for semi-automated or fully automated steering. At present, only VÖGELE NAVITRONIC is capable of fulfilling these requirements in practical operation.  
Computer Integrated Construction (CIC)
A central role among the side conditions related to applications technology plays the integration of computers. It is quite justified to see an analogy with the production sector and Computer Integrated Manufacturing (CIM). The main purpose pursued by CIM is to create continuous, uninterrupted and transparent information flows for control and management of production operations on the one hand, and upstream and downstream company departments on the other. For the time being, as far as the construction sector is concerned, a continuous information flow does not yet exist. The situation is much like in production, where computers are integrated to a large extent into design and engineering, whereas other areas often are only poorly supported. There is hope, however, that this will change within the next few years.

As far as non-contacting navigation and grade and slope control are concerned, it suggests itself to use CAD systems for generating data. For VÖGELE NAVITRONIC it is sufficient to get data in a rather simple format. These can be easily generated from those data supplied by usual route design systems.
  

Non-Contacting Grade and Slope Control Systems
for Road Pavers
Figure 3 shows the components of non-contacting grade and slope control systems in a general form. The systems currently available are much alike in their basic designs. Major differences, however, exist in the measurement techniques applied, in the media for data transfer between total station and the machine’s on-board computer on the one hand and the machine’s
on-board computer and machine control on the other. Finally they also differ in algorithms applied for calculation and control.
A total station is used for position measurement of a prism attached to a mast in an exposed place. In the majority of cases this mast is installed within the rear third of the screed arm which, as we know from conventional grade sensors, is the best location for tow point control. The information about position is transmitted via radio link to the machine’s on-board computer. The task of this computer is to calculate a machine model based on the position determined for the point on the machine and on additional measurements. The model created this way maps the screed in a 3D coordinate system. For complete description, measurement of additional geometrical parameters is required. Based on information about its longitudinal grade and transverse slope the screed can, in the mathematical machine model, be modeled as a plane. An advantage as regards slope measurement is that it can easily be performed, as no specific reference is required. 		   
  Figure 3: Basic Concept of Systems for Non-Contacting Grade and Slope Control.
     
 
         
       
 
  NAVITRONIC with ergonomic display for control and monitoring.  
   
  Based on the route’s design data, the specified screed model is generated for comparison with the actual model. The result of this comparison is the error, taken as a basis to determine signals emitted to the valves for the correcting elements. Correcting elements normally are the paver’s hydraulic rams for tow point control.   Total station with automated target tracking capabilities
for position measurement of the paver.		  
     
VÖGELE NAVITRONIC – Non-Contacting Grade and Slope Control and Navigation System for Road Pavers  
The road to be constructed by laying machines is to be built in exactly the specified place. So far, a physical stringline served as a reference. When working with a non-contacting system, position measurement will have to do the job. As far as non-contacting navigation control is concerned, the approaches described below are imaginable. They have to some extent already been tested for suitability in practice:  
-  Semi-automated navigation by tracking a target on the machine. This solution tracks a target on the machine, detects any deviation from the specified direction and displays the error to the operator. He makes a correction.
- Fully automated navigation by tracking a target on the machine. This solution also tracks a target on the machine, detects any deviation from the specified direction and finally corrects the error via a control system.
- Indirect fully automated navigation by coarse steering of the tractor unit and fine control of the screed’s extending units.
   
The VÖGELE non-contacting NAVITRONIC system uses indirect fully automated navigation, as other solutions do not supply satisfactory results. This is above all due to the fact that, as a result of the flexible connection between tractor unit and screed and the difficulty to precisely steer a tracked machine, steering stability cannot be achieved. After some time the machine starts veering from its course, which results in poor positioning of the road pavement.
Indirect automated navigation (Figure 4), similar to all other methods, tracks a point on the machine and, based on this information, calculates a signal for steering the tractor unit. In this case, tractor steering can be rather imprecise, as accurate lateral control of the screed is brought about by high-precision displacement of the screed’s extending units. For picking up positions of the screed’s extending units, an additional system is needed measuring changes of the screed’s extending units in relation to the basic screed. For this purpose VÖGELE NAVITRONIC uses non-contacting displacement sensors of sturdy design. 		      
    Figure 4: Basic Concept of Systems for Non-Contacting
Grade and Slope Control and Indirect Navigation.		  
Past Experience in the Field
Non-contacting grade and slope control systems for road pavers have been tried for several years in practical operation. In contrast to the earthmoving sector, where demands in terms of accuracy are not so high, the accuracy achieved so far for pavers seems to have reached the limits of what automated control is able to perform. A problem that existed for mere non-contacting grade and slope control was the need to set up a physical reference for steering. This problem can be regarded as solved by systems for indirect automated navigation such as VÖGELE NAVITRONIC. A number of units have been in operation for several years.

A vital precondition for use of such systems is the technical integration but also the organizational one. Experience showed that new challenges emerged for the construction team made up of paving crew and people with surveying skills, such as:

-  Acceptance and readiness to deal with new technologies
- Profound knowledge of paving and surveying
- Good team work of all people involved
In modern road construction companies, however, this should be no problem at all.