Revista de Engenharia Civil 2019, No. 56, 28-33

http://www.civil.uminho.pt/revista

Centre for Territory, Environment and Construction

Automated control systems for civil construction machinery using RTK-GNSS: an implementation review

J.M. Lopesa, J.L.A. Trabancoa†

a Department of Geotechnics and Transportation, School of Civil Engineering, Architecture and Urban Design, University of Campinas, SP, Brazil

† Autor para correspondência: trabanco@fec.unicamp.br

ABSTRACT Cronologia do artigo: The modernization of global navigation satellite systems (GNSS) allowed us to obtain positions in three dimensions with accurate precision. Techniques such as real-time kinematic positioning (RTK) provide precise coordinates in the field. All the progress has contributed to construction works that make use of heavy machinery as a way to integrate these positioning systems and ensure accuracy of performed activities with increased productivity. A brief analysis on the implementation of automated control systems for construction machinery with the use of RTK-GNSS receivers is presented herein, as well as their needs and advantages as to programming and operation. The commonly automated machines (earthworks, compaction and machinery paving) are classified here into three categories based on their peculiarities and ability to integrate the system components.

Recebido a 07 junho 2017 Corrigido a 27 setembro 2018 Aceite a 11 abril 2019 Publicado a 27 maio 2019

Keywords: GNSS Global Positioning System Real-time kinematic Machine Control Construction Machinery

1. Introduction

Until the 1990s, precision control of earthworks, compression and paving was only possible through exhaustive work of surveying teams that were present in the field during the entire execution of activities. These teams used to make use of laser levels equipment and universal total stations (UTS).

Even though it was created in the early 1970s, the Global Navigation Satellite System (GNSS) came to operation only in the 1990s. As a revolution in human life, GNSS was valuable both for those who needed positioning with precise coordinates, such as surveyors, and civilian users who started using low-cost receivers as an alternative to old printed maps. Nowadays it is hard to imagine how construction demarcation of land would be performed without the use of this technology.

The real-time kinematic (RTK) technique was the most significant advance after the emergence of GNSS, as it allowed obtaining precise data on the field without the need for post-processing the data.

The possibility of integrating heavy construction machinery with these systems emerged later on, in order to provide better control of accuracy and precision during operation.

In the present study, we make a brief analysis on the implementation of automated control systems for construction machinery with the use of RTK-GNSS receivers. Although a particular focus was given on earthmoving and paving surface compaction machines, it is worth noting that RTK-GNSS technology can be applied to various other constructions tools.

2. History of automated control machines

Until the advent of GNSS-operated machines such as excavators, bulldozers, scrapers, drills, cement spreaders, compactors and mixers, it was necessary an extensive work of a topography team throughout the execution of activities in order to achieve high accuracy and precision levels, using laser levels, theodolites and total stations. Finally, with the development of automated laser levels, control machines became more sophisticated, independent and automated.

The rotating-beam levels can be mounted on tripods and are equipped with pentaprisms that rotate 360 degrees at high speed and at a right angle, emitting a beam of light through the work area. Receivers installed on the machines receive the laser as a guide and guarantee accuracy higher than 1cm at 100m.

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This technology allows the control of leveling, and vertical and horizontal characteristics of the plan (Roberts et al., 1999).

GNSS determines positions in three dimensions (3D, or latitude, longitude and height) with millimeter accuracy. A few years ago the biggest challenge when using this equipment was to ensure accuracy in the definition of the height coordinate. Roberts et al. (1999) were the pioneers to present GNSS-RTK as a valid system to automate construction machinery blade. They presented the application of RTK-GNSS applied to a bulldozer, and compared the system’s accuracy with a laser level equipment. However, at that time the use of mutual navigation satellite systems was not consolidated, but nowadays it is consolidated the use of more than one constellation satellites simultaneously (mainly GPS (Global Positioning System) + Glonass (Globalnaya Navigatsionnaya Sputnikovaya Sistema)). This practice increases the accuracy of all three coordinates by using more satellites as a reference.

In addition, with the evolution of the technology its application spread to several other areas. Examples of this is the application of GNSS-RTK for monitoring crustal deformations (Milyukov et al., 2010), for surveying in forests (Blum, 2015), for deformation monitoring (Martín et al., 2015), for real-time monitoring earth–rock dam material truck watering (Liu et al., 2013), or associated with multipath reflectometry for determination of snow depth at the site (Tabibi, Geremia-Nievinski, and Dam 2017).

3. GNSS

Global Navigation Satellite Systems are made of a constellation of satellites in orbit around the Earth that emit signals that allow us to determine precise coordinates across the globe. The coordinates are determined based on the distance between the satellites and the receiver. The emergence of GNSS was made possible through the creation of GPS by the Americans and Glonass by the Russians in the 1970s. Currently, the European Union, China and Japan also have their own satellite positioning systems (Galileo, Beidou, QZSS, respectively). India is also expected to have their system complete and in orbit until the end of 2016 (Government of India, 2015).

These systems were created due to military interests. By the year 2000, for example, the GPS system was Selective Availability (SA), which consists in a programmed signal degradation tool for civilians so that only the Army would have access to accurate signals. Nevertheless, such degradation tool was discontinued and in 2007 the US Government announced the creation of a new generation of satellites, known as GPSIII, which lack the SA resource. In addition, the US Government claimed no interest in resuming with the degradation tool.

According to the US Government (2015) the GPS space segment comprises 31 satellites in orbit, divided into legacy and modernized satellites. The first to be launched were the Legacy Satellites (between 1990 and 2004), operating at frequencies L1 C/A (for civilian use) and L1/L2 P(Y) (for military use). Modernized satellites make use of three communication frequencies, as follows: L2C, L5 and L1C (the latter is the 4th generation of signal and will only come into operation with the launch of GPS III satellites).

L2C (Second Civil Signal): Operates at the frequency of 1227 MHz and divides the frequency with two other military signals; L5 (Third Civil Signal): Operates at the frequency of 1176 MHz and is designed to meet the security needs in transport. It is transmitted in an exclusive band for aviation security services; L1C (Civil Fourth Signal): It operates at the same frequency of the L1 signal (1575 MHz), but should not be confused with the L1 C/A sign used by legacy satellites. This frequency was created to allow the system to make use of international collaboration while still ensures the US national security. The L1C was originally created to be a shared band with the Galileo, QZSS and Beidou (US Government, 2015).

GNSS receivers generally reach great accuracy when accompanied of post-processing, that is,

when data obtained by the receiver have errors and ionospheric refraction corrected by computer. However, it was necessary to have even more precise data in the field in real time. The real-time kinematic (RTK) technique has emerged as a great alternative to solve this problem. Given its importance in automation of construction machinery, following focus is given to this technique.

4. GNSS real-time kinematic

The collection of GNSS data using real-time kinematic technique allows the user to obtain accurate 3D

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positions (3 cm or less) without the need for the receiver to be at a static position. This accuracy is possible due to corrections of the coordinates that are sent via UHF radio by the fixed base receiver to the rover, which is the receiver that may be moving (Figure 1). A single base station can support an unlimited number of rovers, and its scope is very wide and can meet an area within 20 km (12 miles), but the radio range is often more limited.

Figure 1 - RTK-GPS.

The fixed base must always be installed at a point with known coordinates. By informing the

receiver the point coordinates, it can calculate the difference (error) between both obtained by the fixed base. This information is transmitted by radio to the rover, which applies the errors already calculated to the coordinates that it is getting at the moment, correcting them in real time (Figure 2).

Figure 2 - RTK-GPS correction.

The RTK technique may also have the bases with a radio antenna replaced by available cellular

network (GSM, 3G, 4G, etc.) by installing a chip directly on the rover. This technology known as NTRIP uses continuously operating reference station (CORS) networks for corrections (Edwards et al, 2010; Mekik et al, 2011). This method allows the work be carried out without the need for a fixed base in place, but limits its operation within the signal coverage areas of telecommunication companies, as well as their signal speed.

5. Automation components and operation

In the construction area, the most common machines equipped with automated machinery control systems are earthworks, compactors and pavers. Each of these has its characteristics and the requirements of GNSS as well as the complexity of the installation of the automation components, communication, calibration and pre-work activities. The installation process of the components must be done as much accurately as possible. Each installation error can relate directly to the loss of precision of the activities to be performed.

The GNSS receiver can be combined with other equipment to control the machine, both level laser equipment and UTS. Angular sensors are essential functioning parts of these systems as they allow the control box to have the inclination information of each part of the machine and consequently how

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the area is being modified by the blade, drum or screed.

Excavators and wheel loaders Earth moving machine, such as excavators and wheel loaders, differs from bulldozers, scrapers

and graders by presenting a more dynamic movement of the bucket, which prevents installation of the GNSS equipment directly on the buckets. This feature makes the use of angle sensors important for full control of machine movements. The angle sensor should be installed in every part of the machine that will perform the movement, e.g. in case of excavator, it should be installed in the cabin, boom, stick and bucket. GNSS can be installed with single or double receivers which should be installed on high masts on the back of the booth, at the farthest point from the bucket, so that sensitivity to simple movements is not noticed. All the information from these sensors is received, compiled, and interpreted by a control box installed in the cabin.

One of the most important stages of calibration of the equipment is measuring the distances between the machine motion axes and sensors when the machine is leveled and in early position, and also at the initial position of the boom and the bucket at maximum reach on the floor measured with the GNSS receiver (Figure 3).

Figure 3 - Excavator at maximum reach. Bulldozers, scrapers and graders

The work done by these machines requires less dynamic movements than those mentioned earlier. Because of this feature, it allows the installation of GNSS receivers directly above the blade, in a single or double form.

For calibration, it should be flush to a point with known elevation, and it is important for the blade to be in its normal working position, i.e., on the floor and in the same height as the track of the machine.

Soil and asphalt compactors and pavers

Compression activities require great accuracy in their results and GNSS applied to such processes can ensure and control the compression quality more broadly.

In these activities, this system can also be used to obtain information about the progress. Meehan et al. (2013) used interpolation methods (Kriging and Inverse Distance Weighting Methods) to create three-dimensional planes of compacted layers using coordinates obtained by GNSS RTK. Given that GNSS data is about point positions, the interpolation was shown to be an excellent alternative for compaction control of large areas through the creation of plans which represent the 3D compacted layers.

Compactors are typically equipped with only one GNSS receiver, which may be installed above the cabin or close to the drum. In this inclination sensor, compression and temperature can be added to the system.

Pavers are machines that move straightforward at low speed and have small range of movement of the screeds. The installation and control of the equipment is very similar to compactors. GNSS receivers can be installed in single or double way directly in the screeds.

6. Automation in construction and its relationship with industry 4.0

When we compare the construction sector with the agricultural sector or the industry sector, it is clear that those have passed in recent years through a process of innovation and adoption of new technologies that have allowed a greater automation of their processes. The civil construction, was initially left behind

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by being composed of dynamic activities and by each activity differs greatly from one work to another. However, society is currently discussing the emergence of Industry 4.0, where physical, cybernetic and biological meet, and their harmonious integration allows for more and more intelligent decisions and actions to be taken (Schwab, 2016). Industry 4.0 promises to affect all layers of society by allowing "things" to be part of a network (also known as the Internet of Things - IoT) and communicating by providing data. A classic example of IoT is the hospital bed that recognizes that the patient is moving abnormally and automatically triggers the help of the nurses.

Such innovation will not exempt civil construction from its benefits. Despite the precise blade control activities for earth moving, compacting and paving still depend totally on the positioning systems mentioned earlier in this paper, several technological solutions have been used in the field in order to increase the quality of construction activities and also to reduce their costs. One of the main highlights is the remote monitoring system, which aims to allow all machines and their parts to be connected to the same network (Autodesk, 2015). In this way, it would be possible to monitor the integrity of the equipment in real time. This would allow managers to predict a possible equipment failure, or need for supply. This type of system acts in the prevention with the two main expenditures with equipment failures: the one of stopped time for the repair of the machine in the field and the waste of productive potential.

In addition to the technologies directed directly to the machines, several others are already being used in the field. An example of this is the use of drones to monitor the progress of constructions as well as the individual activities of their workers (Knight, 2015), single-task construction robots (STCRs) (Bock and Linner, 2016), and 3D printing technologies in the field (Kothman and Faber, 2016).

One of the great challenges for the rise of Fourth Revolution technologies is security (and data privacy). The RTK-GNSS technology discussed here has been widely used in agriculture (Bangert et al., 2013, Jacobs et al., 2017) and associated with the new technologies allows agricultural machines to operate in the field autonomously. However, civil construction has a more dynamic and busy working field, with several workers circulating between the machines. Currently, although the technologies used in agriculture allow the expansion to automation in construction, they are not applied due to safety issues, so until the moment, there are no fully autonomous machines working in civil construction (Dadhich, Bodin and Andersson, 2016). Despite the delay in the use of technology, the industry can still benefit from the advance of Industry 4.0, which has as a priority robotization associated with the concept of collaborative machines. These allow a worker to work in collaboration with a highly productive robot, without their safety being put at risk.

7. Discussion and conclusion

It is consensual that the advent of GNSS technologies has led to major advances in several areas, including construction. The RTK method also allowed obtaining real-time highly precise coordinates.

When applied to the automated control of machines, RTK-GNSS can still achieve good accuracy (+/- 3 cm), but other variables and equipment can directly affect the quality of an automated work. As all systems installed on machines do not operate only with GNSS receivers, any error in system calibration, or even the installation of ancillary components, can interfere with the desired accuracy.

Obtaining good coordinates is a relatively easy task for manufacturers of GNSS receivers. As this system is composed of several components, communication between the different parts is the main challenge to proper functioning. It can be noted that each type of machine requires a different way of calibration, thus requiring that the workers involved in such activities are properly prepared and trained for the procedures, and above all, understand the importance of each procedure.

The adoption of a RTK-GNSS machine control will require excavation, paving and compaction projects to be made first, in order to serve as a guide for the machines.

When the challenges of installation and operation are overcome, machine control systems can result in significant increase in productivity, allowing the contractor to decrease by up to three times the runtime of the activities or the number of working vehicles. Altogether, it demonstrates that although expensive this system can pay for itself with middle-term productivity.

8. Acknowledgements

The authors thank the Coordination for the Improvement of Higher Education Personnel (CAPES/Brazil – Process/Project 01-P-04376-2015) for financial support. The authors are also grateful to Fernando Catrau (Trimble Brazil) for the support and for provide information on Trimble machine operation.

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