Specialized Total Stations for Construction Layout, Machine Control and More

Written by Paul F. Hahn

Created on Sunday, 07 March 2010 15:07

In the past decade, manufacturers have made tremendous strides in developing optical total stations specifically designed for the construction industry. In the 1990’s the construction industry was slow to adopt total station technology.  Manufacturers attempted to increase the adoption rate by offering lower priced, de-featured versions of the models they traditionally sold to surveyors.  However, as their understanding of the industry’s needs increased; new model were introduced that better fit the contractor’s needs for layout, as- built’s and machine control applications.  These instruments are highly specialized and very versatile, serving many purposes on the job site.

This group of instruments share common characteristics. They are motorized, robotic total stations capable of one person or unmanned operation.  They contain reflectorless Electronic Distance Meter (EDM) technology and may also contain imaging capabilities as well.  They are able to be used for machine control as well as standard surveying tasks and the new capability to perform surface scans.

When used for machine control applications these specialized instruments differ from standard surveying instruments is several significant ways. The standard surveying total station is designed to measure to static (non-moving) objects.  Therefore, a slight time delay in the obtaining the angle and distance components of a measurement have no impact on the measured position.  If the prism is held by a steady hand and properly plumbed the resultant position will be correct. This type of measurement mode is often referred to as “static mode”. However, measurements to moving objects present a completely different set of requirements.  These measurements are referred to as being in the “dynamic operations mode”.  Depending on the vector and velocity of the moving target, positioning errors will occur if the angles and distances are not captured simultaneously. To overcome this inherent error, manufacturers needed to develop total stations wherein the angle and distance information was synchronized or with very low latency.  As the target (machine) was also moving (dynamic), these instruments needed to have a very fast measurement update rate, so position information could be relayed to the machine in real-time.  They must also possess very fast radios to relay the position data to the machines they are controlling or that latency will result in positioning errors as well.  When all of these challenges were overcome the resultant instrument was capable of traditional surveying work and use as a machine control sensor. Total stations are capable of higher positioning accuracy than unaided GNSS (GPS) technology, especially in the vertical (elevation) component.  This makes them a very good for fine and finish grade applications.  Therefore, the first systems developed were made for motor graders and pavers. In addition, total station-based excavator systems have been recently introduced.  There are several excellent products in this group, including Trimble’s SPS Universal Total Station, Leica’s PowerTracker and Topcon’s LPS-900.  All are fast-tracking robotic total stations with synchronized angle and distance measurements and high update rates.

To give you an idea of how advanced these units are; I will provide some specifications.  The angular accuracy of these units is as high as +/- one arc second. Distance measurements are accurate to +/- 1mm in static mode and +/- 5mm in tracking mode.  Position update rates are very high at 20 times per second. Combining these capabilities, one manufacturer states they can track machines at up to 35 MPH.

The total stations instruments themselves are not the only modifications made to make the systems construction site- applicable.  The software on-board these instruments (or contained within their field controllers) has been specialized as well. The user interface and application software has been tailored to the needs of the job site.  These are not the same application programs that are sold into the surveying market.  Topcon, Leica and Trimble have all created contractor-friendly, construction application-specific programs to support the workflow of construction projects.  They have names like “Site Foreman”, “Layout Manager”, “Site Controller” and “Layout Master”.  These were not named or developed with the land surveyor in mind. To improve the user interface; they all utilize color graphics and construction-centric terminology.  

So now we have available very productive, accurate and powerful robotic total stations for construction layout, topo verification, as-built’s, earthwork quantity determination, and more, with reflectorless and scanning capabilities. And, oh, by-the-way- they are also machine control sensors.

The first total station used for machine control, that I was aware of, was the Spectra Precision (Geodimeter) Advanced Tracking Sensor (ATS). This was developed and introduced to the market in the mid 1990’s. The first use was for tracking motor graders, as part of Spectra Precision’s Blade Pro 3D system. Soon, third party integrators began including the sensor as part of their solution. I believe the first was Somero Enterprises, who used the ATS for their automated screed system.  Since then, many motor grader and paving solution providers have integrated these advanced sensors.  Leica, alone is providing sensors for several paving machine makers, including Gomaco, Miller Formless, Power Curbers and Wirtgen. Somero still offers a new version the ATS (the Trimble SPS Universal Total Station) as part of its 3-D Profiler system.  Another early use was for guidance of tunnel boring machines .There are many motor graders systems available that also utilized these specialized total stations

Just as GNSS systems do today, these total stations provide real-time positioning information.  That data is compared to design positions and appropriate actions takes place via the machine control system. However, total stations do not require line-of-sight to the satellites.  They can replace GNSS technology when machines are working under obstructions or even underground. Manufacturers have made it easy to switch GPS and total station sensors for added flexibility. Total stations are, of course, constrained by the required line-of-sight between the total station and the target on the machine and range can also become an issue.  Some manufacturers have overcome this constraint by making it possible for multiple total stations to be “strung together” to allow for uninterrupted machine control operation over greater distances. Also keep in mind that vertical accuracies are higher with a total station than with un-aided GNSS systems.  It is possible to augment or enhance GNSS positions with laser-based instruments but line-of-sight to the sky must be available and surface scanning capability would not be available.

These new instruments (and their controlling software) fill many needs on the job site and compliment GNSS technology in a wide range of applications. Some of the newer models even include spatial imaging capabilities. Add to this the many machine control applications and you will conclude that a total station is no longer “just a total station”. Your investigation into these units may surprise you as to just how versatile and productive they are.  And that can impact your bottom line.

Resources:
Leica:  http://www.leica-geosystems.us/en/Machine-Control_4677.htm

Topcon:  http://www.topconpositioning.com/applications/construction/

Trimble:   http://trimble.com/sitepositioning.shtml

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