A Two-Axis Tracking System with Datalogger

Meinhard Seefeldner Meteorological Institute, Ludwig-Maximilians University, Munich, Germany

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Andreas Oppenrieder Institute and Outpatient Clinic for Occupational and Environmental Medicine, Ludwig-Maximilians University, Munich, Germany

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Dieter Rabus Meteorological Institute, Ludwig-Maximilians University, Munich, Germany

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Joachim Reuder Meteorological Institute, Ludwig-Maximilians University, Munich, Germany

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Mathias Schreier Meteorological Institute, Ludwig-Maximilians University, Munich, Germany

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Peter Hoeppe Institute and Outpatient Clinic for Occupational and Environmental Medicine, Ludwig-Maximilians University, Munich, Germany

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Peter Koepke Meteorological Institute, Ludwig-Maximilians University, Munich, Germany

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Abstract

A versatile two-axis tracking system with datalogger is presented. It is designed with regard to high pointing accuracy, high torque and mechanical load, high accuracy of the data acquisition, extended weather resistance, remote operability, and considerable freedom from maintenance on site. The system can be used for a variety of pointing devices and also for completely different positioning tasks, such as, for example, the operation of samplers. Depending on the version of the gear box, the maximum absolute pointing error of the tracking system is 19–71 arc min, the angular backlash of its axes is 2–35 arc min, and its peak torque is 15–20 Nm. The maximum mechanical load on the power takeoff shaft is 550 N. The mechanical, electronic, and software design is considerably modular. The modules can be used in various combinations or even stand alone, and they can be modified for future applications without modifying the whole system. The electronics is based on a programmable logic controller, which can in addition to the tracking system and datalogger also serve any pointing and further measurement devices. This facilitates an easy setup of a wide range of different measuring stations. They all have the advantage of being based on the single time process of the programmable logic controller. Four examples of the system have shown very satisfying results in up to 4-yr-long 24-h operation from alpine to seaside environments.

Corresponding author address: Meinhard Seefeldner, Meteorologisches Institut, Theresienstr. 37, D-80333 Munich, Germany. Email: meinhard.seefeldner@lrz.uni-muenchen.de

Abstract

A versatile two-axis tracking system with datalogger is presented. It is designed with regard to high pointing accuracy, high torque and mechanical load, high accuracy of the data acquisition, extended weather resistance, remote operability, and considerable freedom from maintenance on site. The system can be used for a variety of pointing devices and also for completely different positioning tasks, such as, for example, the operation of samplers. Depending on the version of the gear box, the maximum absolute pointing error of the tracking system is 19–71 arc min, the angular backlash of its axes is 2–35 arc min, and its peak torque is 15–20 Nm. The maximum mechanical load on the power takeoff shaft is 550 N. The mechanical, electronic, and software design is considerably modular. The modules can be used in various combinations or even stand alone, and they can be modified for future applications without modifying the whole system. The electronics is based on a programmable logic controller, which can in addition to the tracking system and datalogger also serve any pointing and further measurement devices. This facilitates an easy setup of a wide range of different measuring stations. They all have the advantage of being based on the single time process of the programmable logic controller. Four examples of the system have shown very satisfying results in up to 4-yr-long 24-h operation from alpine to seaside environments.

Corresponding author address: Meinhard Seefeldner, Meteorologisches Institut, Theresienstr. 37, D-80333 Munich, Germany. Email: meinhard.seefeldner@lrz.uni-muenchen.de

1. Overview

The two-axis tracking system with datalogger was first developed for ultraviolet radiation measurements under variable receiver orientations within the context of a study of the exposure of human skin with different parts of the body (Oppenrieder et al. 2003). It is, however, also suitable for a wide range of other positioning purposes, such as for a highly accurate and fast pointing of meteorological sensors, or even the operation of samplers, etc. (Schreier 2003; F. Wagner et al. 2003, poster from second Int. Workshop on Mineral Dust, Paris, France: available online at http://www.terre.lisa.univ-paris12.fr/dust2003/posters_liste_F.html). Figure 1 shows the system equipped with a sun radiometer.

Up to now six examples of the system have been built, which all have basic properties such as high quality components for long-running continuous operation, extended weather resistance, high pointing accuracy, high torque and mechanical load, high accuracy of the data acquisition, mobility, considerable freedom from maintenance on site, operation and data transfer via the Internet, resistance against unstable electrical power or power breakdown, and safety precautions in the case of mechanical blocking.

The tracking system consists of two identical rotary drive modules for azimuth and elevation. They are mounted on a portable wind-resistant mast roughly 2 m above the ground. The mast can be adjusted vertically with high accuracy. A variety of pointing devices can be mounted on the power takeoff shaft of the elevation drive.

The complete electronics for the tracking system and the datalogger is housed in a weather-resistant aluminum box, which is located outdoors close to the mast. It is linked with an up to 40-m distant control personal computer (PC), which is placed indoors. The control PC can be linked to additional PCs via the Internet.

The mechanics, electronics, and software are subdivided to a great extent into self-contained modules. They can be used in various combinations or even stand alone, and they can be modified for future applications without modifying the whole system. Consequently, the electronics is also based on a programmable logic controller (PLC), which itself is constructed as an upgradeable modular system. Thus, compared to a solution with PC boards the electronics is a self-contained, weather-resistant unit, which can be placed close to the measurement devices and basically can work independently from any PC. In addition to the tracking system and datalogger, the PLC can also serve any pointing and further measurement devices. This facilitates an easy setup of a wide range of different measuring stations. They all have the advantage of being based on the single time process of the PLC.

At the present state of development the system is prepared, but not yet equipped, for example, for an operational angle range ≥360° of the drive modules (presently ≤ 354°), extremely low temperatures, underwater applications, or a real-time pointing on a moving platform. For these upgrades a rotary encoder had to be implemented, a few components had to be exchanged for their low temperature, or high pressure versions, respectively, and a software module had to be added for the compensation of the process time. Here, of course, the achievable performance depends on the ambient conditions.

2. The drive modules

Each drive module consists of a high quality stepper motor with reduction gear box (both supplied by Phytron Elektronik GmbH), a coupling, the bearing of the power takeoff shaft, a lead-through sealing for the shaft in the housing, three optical interrupters for the position management of the shaft, a power supply for the optical interrupters, a drying agent, and a humidity indicator (see Fig. 2).

Housing and power takeoff shaft are made from anodized aluminum. The housing is hermetically sealed. The lead-through sealing of the power takeoff shaft is a combination of a wiper and an oil seal. Sealing and bearings are treated with a low temperature fat. The cylindrical shape of the housing allows a user-defined orientation in the mounting. The stepper motor and reduction gear box are available in fat-lubricated versions for an operating temperature down to −75°C and are suitable for years-long continuous operation (in fat-free versions they are even available for operating temperatures down to 4 K). There is no safety clutch necessary, because the peak torque is preset by the phase current of the stepper motors. The admissible peak torque is defined by the selected version of the gear box. The mechanical angle range of the power takeoff shaft is unlimited as there is no stop. The shutters of the optical interrupters are mounted directly on the power takeoff shaft. Their orientation is adjustable without restriction. The supply voltage for the optical interrupters is internally low-pass filtered and stabilized. A probe for the motor temperature is implemented. The drive modules are supplied by specially designed cables with black silicon sheathing, which remains flexible even at low temperatures.

The optical interrupters provide one reference point and two end points for the position of the power takeoff shaft. Both end points are adjusted with a small overlap. Thus, at power-on, the position of the shaft is nonambiguous: either it is in the operational angle range, where no end point or at most one is active, or it is in the overlap angle range, where both are active. The absolute position of the shaft equals the number of steps starting from the reference point. This absolute position scale must be initialized one time at every switch-on by crossing all three optical interrupters. As there is no additional position measurement, the error of knowledge of position equals the error of acquisition. It is assumed that no steps get lost, for example, due to blocking of the power takeoff shaft, which can be checked at every crossing of the reference point. If no steps are lost, the error of acquisition is composed mainly of the backlash and inaccuracy of the gearbox plus the drift of the reference point.

The drive modules are equipped either with a planetary-type reduction gear box with 35 arc min backlash (PT1), or with 12 arc min (PT2), or with a Harmonic-Drive (HD) reduction gear box. Data of a drive module stand-alone are given in Table 1, and the complete tracking system is given in Table 2. Here the specified maximum absolute pointing error was calculated from the error contributions specified in the tables. At the present development status the lower limit of the temperature range is defined by the version of the wiper.

To prevent blocking by ice, the affected surfaces are smooth, without edges, and, in case of several examples of the drive modules, black anodized. The stress, which is necessary to shear ice off anodized aluminum, was measured to be 0.12 N mm−2 at −12°C. This corresponds to an ice breaking torque of 2.6 Nm at the power takeoff shaft, which is well less than the admissible peak torque of the drive modules. The deicing of a surface by the solar radiation is considerably affected by its solar absorptance. One test cube with a black and a second one with an uncolored anodized surface were covered with roughly 1 mm of clear ice at an air temperature of −4.3°C and then exposed to the sun at 453 W m−2 global irradiance. On the black cube the ice was completely molten and drained after 20 min, whereas the ice on the uncolored one did not melt at all.

To obtain a maximum safety against blocking, the drive modules are operated close to the admissible peak torque. A step-frequency dependence of the torque was determined by means of a spring dynamometer, a rope, and a pulley (see Fig. 3). For the assessment of the maximum torque at low-step frequencies and in full-step mode the commonly used formula M = Mhn η cos 45° is applied. Here Mh is the maximum holding torque, n the gear transmission ratio, and η the efficiency of the gear box; the factor cos 45° arises from the enhancement of the load angle by the step angle at the beginning of each new full step. The formula yields M = 8.7 Nm, which is in good accordance with the dependence. Note that the dependence was determined in half-step mode.

The friction moment of the lead-through sealing of the power takeoff shaft was measured to be <0.03 Nm at temperatures >−25°C. A test setup for the sealing was cooled down to −25°C and then abruptly exposed to a 0.1-m water column for 24 h without significant deterioration of the dryness inside.

3. The electronics

The electronics is based on a PLC system (supplied by Jetter AG). From this the basic controller, two stepper motor driver modules and one A/D converter module are used. The basic controller is linked to the control PC by an RS232 link. This link is used for programming the basic controller, for the control of its program, and for the exchange of data. The stepper motor driver modules contain their own processors for the position management, which provides a near-to-real-time access to the motors. The A/D converter-module has four analog inputs with a resolution of 12 bits. One of these is upgraded with a self-made multiplexer with 32 inputs, thus providing 35 analog inputs in all. Three of the 32 inputs, on the other hand, are connected to reference voltage sources with <160 ppm (°C)−1 drift for a real-time three-point calibration. The multiplexer with the reference voltage sources can optionally be shifted from the electronics box to any external measuring device to reduce the number of wires. There are 10 temperature probes provided for the electronics box, the drive modules, and for additional external measurement devices. The power supply is designed for a 230-V line. Except for the stepper motor power stages the total power consumption is approximately 5 W. The supply voltage for the stepper motor power stages is 55 V unregulated, in case of the maximum load of 160 W. The electronics is contained in a nested housing with 80-mm air gap between the inner and outer box. The inner box has a ventilation aperture to the outer box, and the outer box a ventilation pipe with roughly half a meter length at its outside. Thus, condensed water, which collects at the inside of the outer box, cannot drop onto the electronics parts and evaporates through the pipe, whereas snow and dust cannot enter. The cables are conducted through the pipe. A hand-held box for manual operation of the drive modules, which communicates with the PLC, is provided.

4. The software

The software is designed in such a way that operation, diagnosis, and maintenance can widely be managed via the Internet. As much computing load as possible was shifted to the control PC, thus providing a small and clear set of near-to-real-time tasks for the PLC. These tasks are managed by the control PC via the comparatively slow RS232 link.

The position management and the initialization of the position scale are done by the software of the stepper motor driver modules. Before the initialization starts, the software in the basic controller of the PLC checks if the drive modules are in the overlap angle range (both end points are active), and if so, rotates it in the operational angle range. During operation the position scale is checked for a loss of steps at each crossing of the reference point taking into account the backlash of the gear box. If steps are lost, the data that have been recorded since the last proper crossing of the reference point are ignored, and the position scale is reinitialized. If the initialization does not succeed, for example, due to blocking, the system is shut down and an error message is given. A cable can become coiled only if an external torque rotates a drive module more than a full revolution while the PLC is switched off. At the present development status this case cannot be detected. It is very unlikely, so no precautions were taken against it.

For the control PC a set of software modules is provided, such as a module for sun following based on a four-quadrant detector, a module for pointing in astronomical coordinates including the atmospheric refraction, a PID controller for an electric heater, and a graphic user interface. A module for the determination of the zero points of the angle scales of the drive modules, of the error of perpendicularity between the axes and of the error of alignment of the mast is planned (at the present development status the zero points of the angle scales are determined one time from a sun bearing, meanwhile both the errors are neglected). Time and the geographical coordinates are imported from a GPS module. The control PC is equipped with a watchdog that restarts the computer in case of a system crash. In this and the case of a power breakdown the system reboots automatically, initializes the drives, and proceeds with the measurement process.

5. Practical experiences

Up to now three examples of the tracking system are in 24-h operation under outdoor conditions since roughly 4 yr. They are equipped with hemispherical ultraviolet photometers and scan the sky in a duty cycle of roughly 2-s run to 1-s stop for each position. Among other situations two of them were operated on Zugspitze Mountain in an alpine climate at 2650-m MSL for 19 months, and one of them at the coastline of the North Sea island Sylt for 1 month. Furthermore, one system with harmonic-drive gear boxes is equipped with a sun radiometer, which requires an absolute pointing accuracy of 50 arc min for finding the sun, and a relative one of 3 arc min referred to the sun. It has been in 24-h operation in urban environment for 1 yr.

Until now not a single failure or deterioration of the performance has occurred. All design criteria mentioned above are fulfilled quite satisfactorily. Under freezing-rain conditions, the drive modules could possibly have been blocked if they were not working at this time. An external heater for deicing was not necessary (perhaps it would have made things even worse, because under very cold and windy conditions an insufficient heating presumably would yield an accumulation of ice). Because of a lack of comparison we cannot say if the black anodized surface of some of the drive modules was essential to avoid blocking by ice compared with an uncolored one. The temperature of the motors during summer and winter remained between 0° and 50°C. Six of the gear boxes had to be relubricated. This is within the specified operation time of 10 000 h for a lubrication. The drying agents of the drive modules had to be exchanged every 1 to 2 yr of exposure to outdoor conditions. The black silicone sheathings of the cables of the drive modules show no deterioration by the solar radiation or the permanent folding. Even in case of the sun radiometer, which requires a comparatively high computing power, there are still plenty of resources left in the PLC.

6. Conclusions

The presented two-axis tracking system with datalogger is distinguished in particular by its versatile possibilities of application; by its modular design, which allows comparatively simple adaptations to future requirements; and by several years proof of satisfying performance in harsh environmental conditions. Further information, details, and manufacturing sketches are available for noncommercial use.

Acknowledgments

The development and manufacturing of the first three examples of the system were sponsored by the Bavarian Research Network BayForUV and the Environmental Research Station Schneefernerhaus. More information about the study can be obtained online at http://www.bayforuv.de/englisch/topindex.html.

REFERENCES

  • Oppenrieder, A., Hoeppe P. , Koepke P. , Reuder J. , Seefeldner M. , and Rabus D. , 2003: Measuring UV-radiation on inclined surfaces. Proc. SPIE, 5156 , 339342.

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  • Schreier, M., 2003: Charakterisierung und Inbetriebnahme des Radiometers SSARA (in German). Diploma thesis, Meteorological Institute, University of Munich, 103 pp.

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Fig. 1.
Fig. 1.

The tracking system equipped with a sun radiometer

Citation: Journal of Atmospheric and Oceanic Technology 21, 6; 10.1175/1520-0426(2004)021<0975:ATTSWD>2.0.CO;2

Fig. 2.
Fig. 2.

Cross section of a drive module

Citation: Journal of Atmospheric and Oceanic Technology 21, 6; 10.1175/1520-0426(2004)021<0975:ATTSWD>2.0.CO;2

Fig. 3.
Fig. 3.

Step-frequency dependence of the torque of a drive module

Citation: Journal of Atmospheric and Oceanic Technology 21, 6; 10.1175/1520-0426(2004)021<0975:ATTSWD>2.0.CO;2

Table 1.

Data of a stand-alone drive module, equipped either with one of two different versions of plantary-type reduction gear boxes (PT1 or PT2) or with a harmonic-drive reduction gear box (HD)

Table 1.
Table 2.

Data of the complete tracking system

Table 2.
Save
  • Oppenrieder, A., Hoeppe P. , Koepke P. , Reuder J. , Seefeldner M. , and Rabus D. , 2003: Measuring UV-radiation on inclined surfaces. Proc. SPIE, 5156 , 339342.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schreier, M., 2003: Charakterisierung und Inbetriebnahme des Radiometers SSARA (in German). Diploma thesis, Meteorological Institute, University of Munich, 103 pp.

    • Search Google Scholar
    • Export Citation
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