MTS Sensors - PRODUCT NEWS
Position measurement in the perpetual ice.
The future of German Antarctic research has begun. In February, 2009, the new "Neumayer Station III" polar research base was opened, starting scientific operation in the Antarctic after a construction period of only seven months. Sixteen hydraulic piles with integrated linear-position sensors prevent the station from getting buried in the perpetual ice to ensure scientific research for decades.
With its Neumayer Stations operating since 1981, the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven contributes decisively to long-term meteorological and climate research in the Antarctic. In the past, polar stations were built according to what is termed "tube design", with housing and work containers surrounded by protective steel tubes. Due to annual snow falls of roughly one meter, these stations were destined to being swallowed by the ice over the years. For example, the Neumayer Station built in 1992 is already 12 meters below the surface of the ice. As the snow load causes distortion of the steel tubes, the research base had to stop its operation. It had to be replaced by a new, weatherproof utility building that is suitable to withstand continuous snow; the Neumayer III polar research base. This is the third German research station in the Antarctic.
State-of-the-art technology and aerodynamic design
The extreme conditions prevailing in the Antarctic put high demands on the new polar research station which was built by the Arbeitsgemeinschaft JHK und Kaefer in Bremerhaven on behalf of the Alfred Wegener Institute. The shelf ice as a building foundation is in motion continuously and causes the station to flex at a magnitude equalling several decimeters. Due to a significant snow drift, the building must be designed according to aerodynamic principles. Snow banks forming behind every obstacle would cause buildings implanted directly in the ice to disappear quickly.
For this reason, the architects of the Neumayer III Antarctic station have chosen not to follow the tube design. Its patented state-of-the-art construction principle reflects present technology and its aerodynamic shape underwent testing in the wind tunnel. The station offers living and working space for a team of up to forty scientists throughout the year. Presently, nine researchers of the Alfred Wegener Institute are busy performing meteorological, geophysical and ecological studies that are possible only under the climatic conditions of the South Pole.
Six meters above the ice
A unique feature of the Neumayer III polar research station is its construction "on piles". It prevents snow drifts due to turbulence and the drift snow can pass underneath the building. On a two-story, 82 x 20 m platform, the buildings for research, scientific operation and habitation are installed at a height of six meters above the snow surface. A supporting structure connects the platform with an opening excavated into the snow. The two floors below the covering platform accommodate the garage and the storage rooms. In total, 4473 square meters of usable surface are distributed throughout these four levels of the polar station.
Hydraulic height adjustment
The construction of a platform at considerable height above the ice reduces snow accumulation to a minimum, but cannot eliminate drift completely. Thus, the distance between the platform and the ice sheet decreases over time. For this reason, the second important characteristic of the new station is a hydraulic lifting system. It compensates for the snow accumulation and corrects the height of the platform without structural components remaining in the snow. The complete hydraulic system including sensor technology and control equipment was supplied by Parker L+S, formerly known as Lingk & Sturzebecher GmbH, in Stuhr near Bremen.
Height correction of the 2300-ton research base is performed at the push of a button by a hydraulic system. The elevating device uses sixteen height-adjustable piles, each with two hydraulic cylinders, accommodated at the underground garage level. These bipods are supported by a steel foundation plate on the snow floor of the garage. Before lifting, each steel foundation is pulled up hydraulically. The space underneath is filled with snow and then the load is set onto the pile again. When all foundations are provided with a snow underlay, the elevating device is extended and lifts the station. Finally, the excess space between the steel stands is filled with snow to create a level garage floor. Thus an annual elevation of 80 - 100 cm keeps reestablishing the minimum distance between the platform and the ice sheet over the years.
Integrated elevation measurement
To operate the new polar station reliably under the most adverse conditions, performance requirements are placed on the components used for construction, including the thirty-two hydraulic cylinders used in the lifting system. Installed in pairs at the piles, their maximum displacement is 1.5 m per lifting operation. Each bipod has an elevating capacity of up to 300 tons.
A particular challenge is to ensure the evenness of the elevation movement, in order to minimize the tension in the steel frame. For this reason, magnetostrictive position sensors monitor the cylinder elevation. Each elevating cylinder integrates a rod-shaped, linear position sensor. Using a CANbus protocol, the sensors transmit the position of the lifting movement to the control system, which can correct each cylinder stroke individually to keep the platform horizontal.
Precise measurement of the distortion
Despite the controlled elevation movement, lowering deviations between the bipods are possible due to differences in the ice condition. This would lead to excessive local strain of the steel structure, which must be avoided. Therefore, a water-level system is installed that utilize pipes to communicate any levelling variation. Each bipod is fitted with a vertical tube which is filled with a liquid suitable especially for low temperatures. All tubes are interconnected with a hose system. If the station is horizontal, the liquid level is identical in all tubes. If one point of the station is deeper, the liquid level increases at that location. A magnetostrictive sensor installed in each tube immediately measures any liquid level deviation and, therefore, height variation.
Extreme demands made on the position sensors
As on the hydraulic cylinders, extreme demands are also placed on the installed position sensors. The units must provide reliable and precise operation in an ice-cold environment - over decades, because the station is designed for a life cycle of twenty-five to thirty years. Maintenance or replacement of position sensors during this time would result in considerable expenditure and high costs.
For this reason, the selection was made in favour of Temposonics® R-Series linear-position sensors manufactured by MTS Sensor Technologie in Lüdenscheid. These robust position sensors determine position, movement and levels according to the magnetostrictive measurement method. The magnetostrictive technology offers a decisive advantage for this building in the freezing cold; position measurement is non-contact and free of wear and maintenance.
The rod-shaped R-Series Temposonics® position sensors are installed in the bipods at the polar station. These sensors were designed especially for direct-stroke measurement in a hydraulic cylinder. The sensor measuring pipe - a pressure-resistant stainless steel rod - is inserted into the open plunger. It protects the sensing element in which generates the measuring signal. The sensor head houses the complete electronics package that provides the active signal processing and measurement criteria resulting in utmost accuracy. Double screened interface modules ensure safe operation. A ring-shaped position magnet mounted on the bottom of the piston slides without contact over the sensor pipe and marks the measuring point exactly through the pipe wall.
The design of the Temposonics® sensors which are fitted in the water-level system is identical to the transducers installed in the bipods. Designed as level sensors, the sensing tube submerges into the liquid. The position magnet is integrated into a special plastic float. Partly submerging into the liquid, the float follows its movement up and down the sensor pipe. Thus the float provides continuous and precise measurement for the liquid level in the pipe.
Non-Contact position measurement
To measure the position, the sensor electronics sends a current pulse through the sensing element in the pipe of the position sensor. A partial twist near the position magnet produces a torsional wave which travels towards the ends of the sensing element with a known sonic speed. There it is processed into a standard analog or digital output signal by a special signal converter. The position is determined exactly by measuring the elapsed pulse time, with the magnet position calculated from the time between the start of the current pulse and the arrival of the electrical response signal.
As the measurement principle makes use of magnetomechanical effects, direct contact between the position-determining magnet and the sensing element is not necessary. This is the reason why Temposonics® linear-position sensors provide complete non-contact and wear-free operation. A signal converter responding only to torsional pulses offers the advantage of being insensitve to vibration, because torsional waves cannot be generated by external mechanical effects.
High accuracy at temperatures below -40 °C
Position sensors installed in the elevation mechanism of the polar station feature a measuring length of 1600 mm and a direct CANbus interface. Due to a linearity of = ± 0.01% F.S., they measure the cylinder stroke with a very high accuracy. The resolution is 2 µm enabling minimal differences between the 32 cylinders to be detected and precisely corrected. With inclination measurement, the sensors also measure level differences of only a few micrometers in the pipes.
Due to non-contact position measurement, MTS sensors operate reliably even with arctic temperatures below -40 °C. Changing hydraulic liquid does not affect the sensor's ability to maintain accurate measurement either. As magnetostrictive sensors offer absolute measurement, they are immediately ready for operation again after power failure. A reference run, as required by various other measuring systems, would not be possible at the polar research base.
With the Neumayer III polar station, Alfred Wegener Institute has created the prerequisites for many years of research activities in the Antarctic. Magnetostrictive Temposonics® linear-position sensors in the lifting system and for inclination measurement are an important contribution to keep the research base on top of the ice in the long run. Sophisticated measuring technology and non-contact position measurement permit operation during extremely low temperatures. Unlike other technologies that are prone to wear, maintenance-free MTS sensors offer a remarkably long service life - not only in the Antarctic.
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