Inside the Leica TCA2003 from Hugh Baertlein
In 1902, a 25-year-old surveyor named Heinrich Wild was employed by the Swiss Federal Topographic Survey in Bern, Switzerland. While working on a triangulation network, he experienced something that would have a significant impact on his later life. He wrote: “On September 1, 1902, while surveying in the mountains of the lower Valais, I was setting up a theodolite on the summit of Dent du Midi (Elev. 10,686') and had spent two or three hours adjusting and preparing it. When I was finally ready to work, a thunderstorm with snowfall forced me to stop work on the summit for several days.” Unfortunately, almost everyone can relate to that experience.
After several visits to the patent office and years of experimenting, Heinrich Wild joined Carl Zeiss in Jena, Germany, and developed the following seven commandments for work on a new theodolite:
After leaving Zeiss in 1920, Heinrich Wild founded Heinrich Wild, Precision Engineering and Optics in Heerbrugg, Switzerland. The first two Heerbrugg theodolites, called the Wild Th1 (later called the Wild T2), were checked out on December 21, 1923. Much has changed since then.
The inner workings of a modern total station are remarkably complex. Custom alloys have replaced steel, and electronic angle detector systems have replaced optical micrometers. Weight, size and price have decreased, while functionality and performance have increased. With the addition of an electronic distance meter capable of measuring 8500' to a single prism and three high-performance microprocessors, Heinrich Wild would be impressed. This article discusses the inner workings of the Leica TCA2003.
Sometimes Change is Good
Thirteen years ago, as an inertial guidance system engineer using Wild T3s, I evaluated the first Wild sub-second electronic theodolite, the T2000S. The T3 and the T2000S shared a telescope with crystal clear, bright optics, 0.5" angle accuracy (the T3 was actually 0.3"), the finest construction materials and methods, dual-pickoff angle reading system and that “love-it-or-leave-it” green paint. The similarities stopped there. The nickel, brass and glass had been replaced by the integrated circuit. I found that the dynamic absolute encoder system could resolve angles to about 0.1 arc-seconds using a spinning piece of glass about the same diameter as a golf ball (4 centimeters). The displayed result was the average of 512 independent measurements and only took a second to compute—much faster than I could read a T3 optical micrometer.
Some Things Never Change
Gone are the days of the Feinmechanik lapping T2 standards to ensure a perfect tilting axis. Until the advent of on-board compensation for axis errors, theodolite standards were slowly and meticulously “lapped” (scraped and ground by hand) until the trunnion axis was absolutely perpendicular to the standing axis. With present day high-accuracy theodolites and total stations, the standards are engineered to be just as stable as the T2, but the mechanical adjustment is “close enough.” After manufacturing, the standards are tested to verify that they meet manufacturing tolerances. Then an electronic adjustment is performed, saving hours of manufacturing while producing an even more accurate instrument. As an added advantage, the tilting axis calibration can be checked and adjusted in the field.
I was introduced to sub-second alignment techniques at Rockwell International by Ned Cherry, a guru in the aerospace alignment field. Once, while working with a highly sensitive gyroscope component, I noticed the display reading the hundredths of an arc-second was fluctuating as we walked around, despite the fact that we were working on a 3-foot-thick concrete slab that measured 20 x 25 feet. Ned remarked, “Below 0.1 arc second everything turns to Jello.” To this end, the TCA2003 has been constructed using proprietary materials and engineered using the latest finite element analysis techniques. Every Leica instrument is tested and calibrated for -4ºF to + 122ºF to ensure that accuracy specifications are met.
Waterproof or weather proof? The TCA2003 is built to the same weatherproof standards that Wild has used for many years. A rough translation follows: In a 3-inch-per-hour downpour with a 30-mile-per-hour wind, the equipment is designed to operate for 15 minutes. That is usually 14 minutes and 50 seconds longer than it takes for the crew to get to the truck. This standard has proved adequate over the years. Ironically, many moisture-related equipment failures are not caused by rain, but by putting a wet instrument in a case and then sealing the case.
Body of the TCA2003
The trend toward increased use of FRPs (fiber reinforced plastics) is prevalent in all manufacturing, and Leica has joined this trend. While working on a prototype instrument for Boeing’s Commercial Aircraft Division, I worked with the Mechanical Design group in Heerbrugg. I questioned the lead engineer about the strength and durability of FRPs used in total station side cover plates. He promptly showed me two cover plates, one made of FRP and one made of an aluminum alloy. The FRP plate was covered with white marks, and the alloy plate was marked by large dents. Both plates had been subjected to a mechanic’s hammer with equal vigor and the FRP fared much better than the alloy. In addition to the durability, FRPs are lighter than alloy and can be easily molded into very complex shapes.
As advanced as FRPs have become, special alloys are still used at the heart of the instrument. The T2 with its steel standards was rugged, but heavy—a T2 weighs more than a 2" total station with on-board data collector. The reason for the weight reduction is the change from steel—chosen for its thermal expansion properties—to a proprietary aluminum alloy. Additionally, aluminum was chosen for its ability to conduct heat evenly and quickly, thereby reducing temperature gradients within the instrument.
How it Works
To measure with the TCA2003, the user sights the instrument by grabbing the standards and turning it near the prism. No tangents are needed. Aiming is performed with the optical sight, and no focusing is needed. When the user presses ALL and waits 2 to 3 seconds, angles and distance are recorded. When the user presses Shift-F4, the instrument automatically plunges and reverses. When the user presses ALL again, angles and distance are recorded.
The entire process is controlled by a system processor that coordinates the operation of the three microprocessors controlling the angle, distance and Automatic Target Recognition (ATR) subsystems. Upon request for a measurement the ATR is activated to search for the center of the prism. If necessary, the system processor commands the motors to center on the prism. The ATR process returns a DHZ and DV angle correction related to the telescope crosshairs. Once the ATR has located the prism, the EDM and angle readings are started. The angle reading is instantaneous. The ATR angle corrections are added to the HZ and V locations of the crosshairs as determined by the HZ and V encoder detectors. The distance is calculated and all values are displayed and recorded if necessary. The total time required is about 2 seconds, depending on the accuracy required.
The EDM in the TCA2003 is designed for accuracy, range and stability. Every TCA2003 is shipped with a certificate of calibration showing results of tests generated by comparing the EDM to a laser interferometer and measurements on a fixed 500-meter baseline. The secret to the TCA2003’s accuracy lies in its precise and stable quartz oscillator. The oscillator is used as a standard when measuring distances and is guaranteed to have a drift of less than 1 ppm per year. In close range—2 to 120 meters—the TCA2003 has a measurement accuracy of ±0.5mm at 3s. This means that 99.73 percent of the time the distance as displayed by the TCA2003 will be within 0.5 millimeters of the actual distance. Distance meter range is approximately 2500 meters in average conditions. The TCA2003 can also measure distances to reflective tape targets.
For all the complex electronics inside a robotic survey instrument, the motion is still provided by simple servo motors with a reduction gear system. The end result must be lightweight, durable and fast and have sub-second positioning accuracy.
“What happened to the tangent locks” is one of the most common questions asked when using a TCA2003 for the first time. The TCA uses friction couplings between the motors and the axes—the locks are gone. To quickly position the unit, simply turn the instrument. Fine pointing is done with either the ATR or the tangents. Even when turning with the tangents you are turning an encoder that signals a processor to rotate the motors.
Automatic Target Recognition System
Boeing’s Commercial Airplane Manufacturing R&D group has studied how accurately an experienced operator can point a theodolite at a well-illuminated, well-defined target. The accuracy attained is usually between 0.75 and 1 arc-second with a fresh operator having a “good day.” We all know what happens as the day goes on—sighting accuracy decreases. Leica’s goal with ATR was to provide a complete angle reading system that provided 0.5" accuracy independent of operator skill level, target illumination and target focusing. The goals were met with the TCA2003.
Leica had to decide whether to use an active or passive target. Leica’s ATR system works with a standard EDM prism. Because the EDM prism emits no light or other energy, it is called a passive target. Remote prisms that emit light to aid in targeting the prism are called active targets. Leica chose the passive target system for several reasons, including:
It is important to talk about two terms that will be used in this article that describe the TCA2003 angle system: absolute and static. Since 1984, Leica (then Wild Heerbrugg), has been using absolute encoder systems in its theodolites. Absolute encoders retain their orientation even when the theodolite is switched off or loses power. An absolute encoder system also eliminates having to reset the vertical axis when the unit is switched off. The end result is more productivity and fewer headaches in the field. In 1984, Wild Heerbrugg introduced the T2000, a 0.5" theodolite with absolute, dynamic encoders. In this case, “dynamic” meant the encoder was rotated 360º by a small motor for every angle reading. The reading was the average of 512 separate angle readings over the entire circle, eliminating circle eccentricity and graduation errors. Although extremely accurate, because of its measurement method a dynamic encoder cannot be used to accurately measure a moving target. The T1000, introduced in 1986, used a static encoder that allowed angle readings while moving the theodolite. The TCA2003 uses a static encoder capable of high-speed angle readings while achieving 0.5" angle accuracy for static measurements.
After assembly, the TCA2003 is tested on a Theodolitprüfmaschine (TPM; see “Wild Heerbrugg: Quality and Innovation,” Professional Surveyor, November/December 1997) and both HZ and V angles are compared against master sensors. The TPM calibration data is stored on board the TCA2003 and used for on-the-fly corrections of the encoder systems. To enable high speed tracking and maintain the highest possible accuracy, the TCA2003 can operate using only one of the four angle detectors and apply the correction values from the TPM.
With the proper ATR-based instrument the surveyor will be able to handle new applications and address existing jobs with a different spin. Because of the co-axial target detection system and the use of conventional EDM prisms, daily operation will remain unchanged. What will change is the speed with which data is collected. Topo surveys are automated by putting the TCA in Autorecord mode—the instrument follows the rod person and automatically records a point at specified distances, time intervals, or whenever the rod is held steady for more than a certain time. Take advantage of the measuring speed and have multiple rod people on larger jobs. The TCA2003 can even turn sets of angles while the user prepares for the next traverse point.
Take automation a step further and do some “no-man” surveying. Robotic total stations are already being used in hazardous areas to provide continuous monitoring of structures or processes. Certain sites link measurement systems with civil defense agencies and law enforcement groups. An offspring of “no-man” surveying is machine guidance. TCAs guide road headers, tunneling moles and paving machinery. A surveyor is in charge of installing the robotic survey station at predetermined locations and lets the robotic unit inform machine operators where they are relative to design information. The machine operator reads the machine position from a small display receiving position updates from a base station. The base station can be robotic, GPS or a combination of a number of sensors.
When a new technology is introduced to a market the consumer must perform a cost/benefit analysis to determine the technology’s value. This can be calculated easily by renting the equipment for a month and comparing the time and labor required with new equipment. ATR and robotics add new dimensions to this analysis by overlapping existing markets and adding new markets to the surveyor’s customer base. New applications and systems will require even more skilled personnel closer to the job site and promise an exciting future. What does this mean to the surveyor? The technologies integrated in the TCA2003 will allow expansion into new markets. Here creativity comes into play. Look for problems and apply the solutions.
Hugh Baertlein is the Northeast Region Manager for Leica Geosystems Inc. He holds a B.S. in Mechanical Engineering from the University of Arizona and an M.S. in Systems Engineering from California Polytechnic State University. He spent almost four years as an applications engineer at Leica Heerbrugg before joining Leica Geosystems Inc. as a product specialist in 1994.