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Leica TPS1200+ White Paper


Leica TPS1200+ A Telescope with New Opto-Mechanical Design TPS Program Leica Geosystems – Heerbrugg, Switzerland

1 INTRODUCTION From September 2007, Leica Geosystems AG introduces the TPS1200+ with new hardware (HW) features that keeps it the most innovative high-end Total Station in the market. In this paper, the new features of the telescope will be described. Every part of a total station has to be designed in way to be an integral part of the whole system. No doubt that the telescope design, which embraces the actual telescope optics and other sensors, has a vast impact on the reliability and quality of any total station. In Leica Geosystems AG, the telescope of the high-end total stations embraces the Electronic Distance Measurement (EDM), Automatic Target Recognition (ATR) and Power Search (PS) sensors. Aiming at further improving the communication between the different sensors in a way to make the telescope and thus the Total Station more intelligent, a single baseboard is used to cover all the processing. In addition, a single EDM laser diode emitter is used for both, reflectorless and reflector measurements, making the EDM more robust when measuring in difficult environments as the communication between the different parts is reduced to half. See Figure 1.

2 NEW OPTICAL DESIGN Through the new optical design, a number of advantages can be stated: • Improved optical beam path due to the reduced number of parts in the optics • Improvements in the geometric coupling resulting in an improved stability of the EDM beam.

PS EGL

ATR CMOS chip

EDM receiver diode

EDM emitter & mechanics

Baseboard

ATR IR emitter

Figure 1: The HW design of the new TPS1200+ telescope (Side View)

These advantages will: • Improve the MTF (Modulation Transfer Function) to provide sharper optical picture/impression for optical sighting through the telescope • Allow no misalignments or deviations between the reflector and reflectorless beam • Eliminate the need for user adjustment of the laser beam Where the first and second advantages are clear, the third point is essential for the users where they are not obliged to adjust the alignment of the laser beam with the line of sight as done previously. Once the alignment is accurately performed during assembly, it stays stable throughout the TPS usage.


3 EDM SENSOR As mentioned above, a single laser diode emitter is used for both, reflectorless and reflector EDM measurements. The EDM mechanics (Figure 2) is placed in the lower part of the telescope (Figure 1) and positioned above the baseboard, and the receiver diode is placed in the upper part of the telescope.

Revolver Wheel Laser diode Laser

Graded Filter Wheel

Figure 2: EDM Mechanics (top view)

In order to achieve reflector and reflectorless measurements using one laser diode emitter, a revolver wheel is built into the beam path that has three positions: 1. CAL position: the wheel rotates to first 1 3 position where the beam hits a prism and 2 gets deflected inside the telescope for internal calibration purposes 2. Reflector position: the wheel rotates to the second position where the beam penetrates a negative lens that widens the beam to suit for prism measurement. In this case, the laser beam also passes through a circular filter wheel that intelligently positions itself depending on the strength of the reflected laser signal. 3. Reflector-less position: the wheel rotates to a middle position where the laser beam passes through a hole without being changed. The graded filter wheel (Figure 2) is a disc whose surface changes from being completely open allowing all the laser power to go through to a completely dark film that allows no laser signal to pass.

This way, by taking a proper position, the laser intensity of the outgoing signal can be regulated. The optomechanical concept and a new laser diode were selected to optimise the geometrical characteristics of the laser spot in terms of small size, round shape, optimal light distribution, more visibility, etc. These characteristics improve the quality of the distance measurements and make it more reliable to measure small objects and feature edges. The reflected laser after entering the telescope is directed by appropriate optics to a receiver that analyses the data according to: • Reflector target: Leica Geosystems uses a special phase shift method that makes it the most accurate EDM (1mm+1.5ppm) in the market with the longest range • Reflectorless target: Leica Geosystems unique System Analyser provides an accuracy of 2mm+2ppm to distances of 500m; the maximum range has been extended to measure beyond 1000m with an unmatched accuracy of 4mm+2ppm. The System Analyser is a unique reflectorless distance determination method. The EDM technology currently available in the market can be classified as being based on either a time-of-flight (TOF) or a phase shift measurement. Although the later is chiefly used due its superior accuracy, owing to the low optical power signals on reflectorless objects, measurements over distances of more than 300 m might become difficult to achieve. On the other hand, time-of-flight measurements have an advantage over the phase measurements with regard to the range since measurements over distances of 1000 m to reflectorless targets are achievable; yet their low accuracy is a main disadvantage. The System Analyser combines the advantages of the phase and TOF measurements without having to deal with their disadvantages; this is because it is neither a pure TOF metre nor a pure phase shift metre. This technology will permit accurate reflectorless


measurements to distances beyond 1000 m (Kodak White, 90% reflectivity) being achieved within seconds. Furthermore, dependencies on general atmospheric influences, such as, dust content or smoke, will be reduced. In addition, multi-target recognition is made possible through this method. The existing methods perform inadequately when multi-target detection is needed. In phase metres, the phase of the received signal of the various target objects can be superimposed in an inseparable manner so that no recognizable separation into the components of the individual target objects is possible by the phase difference method. Although multiple target capability in time-of-flight metres is possible in principle, the poor accuracy of distance measurement and complexity of the device are among the major disadvantages of this measuring principle. The unique System Analyser technique offers the following advantages when compared to the existing sub-optimal techniques employed by competition total stations • Permits accurate (in the mm range) RL measurements to objects over large distances (> 1000 m) within few seconds. • Permits identification of multiple targets. • Permits distance measurements independently of general atmospheric influences, such as, dust, smoke, mist, rain or snowfall, etc. • Makes on-board distance calibration available which runs simultaneously with the distance measurement to avoid thermal drifts and interrupts of measurement flow. • Small visible laser spot Some of the key properties of this new measurement principle are summarized as follows: • Only high frequencies of at least 100MHz are generated in order to collect distance information with exclusively high resolution. In this way, every frequency contributes to the final result giving high accuracy. Thus, the

measurement time gets shorter and sub-mm distance resolution is achievable. No time is wasted for ambiguity resolution. • The number of used modulation frequencies depends on the strength of the received signal: At high signal levels 4 frequencies are sufficient to measure with the specified precision At low signal levels up to 10 frequencies are emitted and analysed. • After sampling the received signals, a merit-function is calculated which is comparative to a time-of-flight signal. The time-of-flight signal has its maximum at the distance to be estimated. Further advantages of the new measurement principle are its high sensitivity and its inherent ambiguity resolution.

4 AUTOMATIC TARGET RECOGNITION SENSOR Automatic Target Recognition (ATR) is the sensor that identifies the prism and measures its position on an image sensor to determine its exact angular location. The ATR is available on the robotic total stations to allow automated measurements, precise measurements without manual aiming, one-man surveying, etc. With the TPS1200+, the imaging technology is based on Complementary Metal–Oxide–Semiconductor (CMOS) twodimensional (2-D) array technology instead of the CCD. The advantageous of CMOS camera lies in the clear and sharp images even with the existence of bright background lights and fast image processing. In addition, the new ATR CMOS camera has a pixel size of 6-μm allowing higher resolution images that guarantee superior measuring accuracy. The improvements in the ATR also allow an improved in range. Further to the above advantages of the new design, the LOCK has been improved


to provide a more robust solution even under high dynamics. As in the previous design, an infrared laser beam is transmitted coaxially with the telescope line of sight towards a prism that is in the FOV (Field Of View); the laser is reflected back to print the image on the CMOS chip. In the new design, the ATR emitter is located in the lower part of the telescope and CMOS chip is located on the upper part (Figure 3). As in most image measuring technologies, the pixel information is the base of measurements. CMOS 2-D arrays (like its predecessors CCD technology) consist of horizontal and vertical pixels that are referred to according to a local coordinate system; thus, each pixel has unique coordinates (x,y). This image pixel information is linked to the object (in this case the prism) angular information by the help of the borehole geometry. By simplifying the geometry, Figure 4 shows the relationship. Let’s define the following straight lines: • Line of Sight: the straight line connecting the centre of the objective and the centre of the CMOS chip (dotted line) • Line of Object: the straight line connecting the centre of the object and the centre of the CMOS chip (blue line) • Line of Image: the straight line connecting the centre of the objective and the centre of the image on the CMOS chip (red line)

CMOS camera chip

ATR IR emitter

Figure 3: ATR emitter and receiver

The angle between line of sight and line of object equals to the angle between the line of sight and the line of image. The image pixel position on the CMOS array is linked to the second angle (the red angle) by the knowledge of the focal length and the size of the pixel. The translation to the 2-D geometry (Figure 5) is easily done through measuring the Hz and V pixel positions of the image, thus leading to the Hz and V angle deviations of the prism from the line of sight. These deviations are used to either steer the motorisation of the telescope to the centre of the prism, or measure true angles of the prism after adding the angle of the encoders that refer to the line of sight.

Objective

CMOS array Line of Sight

Prism Focal Length

Figure 4: Simplified image geometry


3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 100

Figure 5: ATR Hz and V deviations

Depending on the measuring mode (STD, FAST or TRK) and to minimise the time for measuring, the system chooses if it wants to steer the motorisation or determine the final angles. If the deviations are less than 50cc (16’’) and the EDM measuring mode is STD, then the system directly determines the final angles; otherwise, the system steers the motorisation until the deviations are within the 50cc. The remaining deviations are used to determine the true Hz and V angles to the prism-centre, even if the crosshairs are not aimed precisely at the centre of the prism. As for the FAST and TRK, the threshold of the deviation is 400cc (130’’). When the prism is not located within the telescope’s FOV, an ATR search starts according to a pre-defined search window. If the prism is not found within this window, the ATR automatically expands the window until the prism is found. This happens in an iterative manner and the search window takes the form of a (growing) vertical rectangle. During assembly, the geometry of the CMOS chip is carefully calibrated so that the measured Hz and V pixels are measured to the sub-pixel level to guarantee arcsec accuracy in angular deviation. Within the 50cc (16’’) deviation, the accuracy of these deviations matches the accuracy of the angular encoders themselves (~ 1’’). When the deviations are up to 400cc (130’’), the typical angular accuracy is around the 2-3 arcsec. Figure 6 shows the accuracies of the ATR angular measurements at different deviation angles from the telescope’s crosshair.

200

300

400

Deviation from image centre cc

Figure 6: Angular accuracies determined from pixel positions

5 POWER SEARCH SENSOR Depending on the specific application, the iterative solution of the ATR search might consume some time and the user has to instruct the ATR to retry the search every time the prism is not found. This potential drawback is overcome by the unique solution that Leica Geosystems offers: Power Search (PS) that allows automatic prism detection within a short period of time – maximum 10 sec. In the new design, the PS sensor is located on the upper part of the telescope (see Figure 1). The PS sensor consists of a transmitter and a receiver (Figure 7). When PS is activated, the instrument starts rotating around its standing axis and the transmitter emits a laser swath with dimensions 20 degrees vertically and a half a degree horizontally. If the laser swath hits a prism, it reflects back to the receiver and the rotation is brought to a halt and an ATR search is performed along the vertical axis.

EGL light exit

PS receiver

PS emitter

EGL emitters

Figure 7: PS emitter and receiver (and the EGL)


The laser swath that is reflected back (Figure 8) can be the “image” of either a prism or a foreign reflecting surface; this “image” is a 3-D surface representing the strength and size of the reflected signal.

Station in the market because of its fastest workflow for Search-Lock-Measure. The continuous innovation in Leica Geosystems products continues in providing the leading solutions in the geospatial industry.

Received signal Amplitude Target object: a prism

Signal Amp.

Plausibility volume: upper tolerance limit Plausibility volume: lower tolerance limit

A foreign object: e.g. traffic sign

Image

Distance

Figure 8: An image created by the PS laser swath

The detection, hence, of a prism and its differentiation from foreign reflecting surfaces is assessed through the knowledge of the reflected signal’s strength and scanning duration (i.e., the size) of the received 3-D image - relative to the distance – according to a plausibility test. See Figures 9 for example; in case the reflecting object is a traffic sign, its signal signature and image size will fall outside the tolerance limits that are assigned for Leica’s prisms.

6 CONCLUSION Having our target the customer satisfaction, TPS1200+ aims to offer surveyors and engineers the best high-end total station with unique solutions of unmatched performance. With the new design the key issues are: • Unparalleled reflector and reflectorless EDM with long accurate ranges • Robust and long range accurate ATR, with 1 arcsec accuracy (ISO 17123-3) • Fast and reliable PS up to 400 m These new changes on the TPS1200+ make it the most efficient One-Man-Total-

1m

100m

10m

1000m Dista

A foreign object: e.g. traffic sign

Scanning duration (sec) Plausibility volume: 0.1 upper tolerance limit

Target object: a prism

0.01

1.10-3 Plausibility volume: lower tolerance limit

1.10-4 0

10

20

30

40

1

50

60

70

80

90

100

Distance [m]

Figure 9: Schematic performance of the plausibility test

Previous Publications: Bayoud, Fadi (2006): Leica’s Pinpoint EDM Technology With Modified Signal Processing And Novel Optomechanical Features. TS27.3 Measurements; Proceedings of the XXII FIG Congress: Shaping the Change, Munich 2006 Bayoud, Fadi (2007): Leica Geosystems Total Station Series TPS1200. White Paper Leica Geosystems. www.leicageosystems.com/corporate/en/products/tot al_stations/lgs_4547.htm


Whether you want to survey a parcel of land or a construction site, a facade or indoors to create as-built plans or carry out high-precision measurements of bridge and tunnel constructions – Leica Geosystems’ surveying instruments provide the right solution for all measuring tasks. The System 1200 Series instruments as well as the software are designed to meet the daily challenges of modern surveying. They all have outstanding, easy to read and user-friendly interfaces. Their straightforward menu structures, their clearly outlined scope of functions and high technology perfectly mate GPS and TPS applications in the field. Whether you use the advantages of both technologies combined or each separately – due to the exceptional flexibility of Leica Geosystems instruments, reliable and productive surveying is assured. When it has to be right.

Illustrations, descriptions and technical specifications are not binding and may change. Printed in Switzerland – Copyright Leica Geosystems AG, Heerbrugg, Switzerland, 2007. 762676en – IX.07 – INT

Leica Geosystems AG Switzerland www.leica-geosystems.com

TPS1200 White paper  

A Telescope with New Opto-Mechanical Design

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