Saabergwerke AG, a coal mining company based in Saarbrucken, Germany, has hired PKF UniCom GmbH, a construction company, to rebuild St. Michael’s foundation and stabilize the building, without altering the church’s historic appearance or interfering with religious services. The PKF design team chose hydrostatic sensors with a 100-millimeter measuring range calibrated for a 50-mm deviation. The PKF engineers examined a variety of sensing systems to measure the position of the liquid level in the hydrostatic sensors and chose the MTS Temposonics sensors for the St. Michael’s restoration. When the Temposonics sensors are equipped with multiple magnets, they can measure more than one position function at a time, enabling plastic injection machine operators to reduce the number of sensors. The combined hydrostatic–Temposonics sensors have the capacity to command these massive industrial springs to extend and correct any sagging that they detect. Interface electronics and software using the DeviceNet protocol link the 82 Temposonics position sensors beneath St. Michael’s Church. Mercedes-Benz engineers installed MTS commercial position sensors in each suspension strut of the CL coupe to measure the position of the springs. MTS designed its commercial position sensors for applications that demand high-volume measurement.
When the parishioners of St. Michael’s Roman Catholic church in Wemmetsweiler, Germany, got that sinking feeling, it was not because their pastor chastised them from the pulpit. The 100-year-old church was actually settling into the ground. Many years of coal mining in the area had destabilized the soil beneath the foundations. Brought in to rectify the situation, a German coal company and its construction firm borrowed sensor technology from the sphere of manufacturing to straighten things out at St. Michael’s.
The soil beneath the church was destabilized by years of coal excavation performed by Saabergwerke AG, a coal mining company based in Saarbrücken, Germany. This caused the church’s foundation to sag, most noticeably at the apse. This is the small structure with a vaulted roof that projects off the main body of the church. The apse had sunk 2.5 feet from its original position.
After the congregation and pastor brought their church’s plight to Saabergwerke’s attention, the coal company agreed to fix the problem. Saabergwerke hired PKF UniCom GmbH, a construction company, to rebuild St. Michael’s foundation and stabilize the building, without altering the church’s historic appearance or interfering with religious services.
PKF originally decided to use a hydrostatic level system to forewarn crews working beneath the church of any major shifting in the earth. Hydrostatic level systems measure the pressure created by the weight of a liquid above a sensitive measuring diaphragm by taking into consideration the liquid’s specific gravity.
However, a traditional hydrostatic level system on its own would be unable to provide the long-term continual measurement necessary to monitor the condition of a building for years on end. PKF decided to create a level measurement system that would take accurate, reliable readings of St. Michael’s for as long as coal would be mined in the Saar region.
“We wanted to capitalize on the positive aspects of displacement measurement in constructing our hydrostatic level system,” said Patrick Faust, a spokesman for PKF. The PKF design team chose hydrostatic sensors with a 100-millimeter measuring range calibrated for a 50-mm deviation, “but we had to figure out how to measure a liquid level and its variation in gauge glass,” noted Faust.
The PKF engineers examined a variety of sensing systems to measure the position of the liquid level in the hydrostatic sensors and chose the MTS Temposonics sensors for the St. Michael’s restoration.
The manufacturer, MTS Sensors Group, is part of the MTS Systems Corp. in Eden Prairie, Minn., a company that specializes in the development and manufacture of testing and automation products. The Temposonics position sensors typically serve industrial positioning applications, including plastic injection molding, steel rolling, and packaging. The sensors use a magnetostrictive technology; that is, they are based on the tendency of some materials to change shape, constrict, or expand in the presence of a magnetic field.
The Temposonics sensors consist of a specially designed magnetostrictive wire called a waveguide, contained in a protective tube. Typically, microprocessor-based electronics are located at one end of the waveguide, along with a sensing head element.
A permanent position magnet moves along the outside of the sensor tube as the position of the point being scrutinized changes. The sensor’s electronics send an interrogation pulse to the waveguide, typically lasting one to three microseconds, so that another magnetic field encompasses the waveguide. This creates a strain pulse at the exact point where the two magnetic fields interact.
The strain pulse travels at the speed of sound along the waveguide until it is detected by the sensor head element at the end of the waveguide. The Temposonics microcomputer measures the time that elapses between the launching of the interrogation pulse and the arrival of the strain pulse to determine the position of the magnet. This method enables the Temposonics sensors to measure position as precisely as 2 microns very quickly, giving industrial processors greater control over their machining operations.
For example, the Temposonic sensors provide position feedback information to the controllers of plastic injection molding machines made by Husky in Bolton, Ontario; Milacron in Cincinnati, and Mannesmann in Chicago, among others. These companies typically use Temposonics sensors to measure the position of the screw that injects plastic resin into the mold base, the clamp that holds the movable mold plate in place to form the part, and the ejector that removes the finished part, in order to optimize the molding cycle.
When the Temposonics sensors are equipped with multiple magnets, they can measure more than one position function at a time, enabling plastic injection machine operators to reduce the number of sensors. For example, Negri & Bossi in Cologno, Monzese, Italy, uses two Temposonics III sensors in place, each equipped with two position magnets, rather than several strain gauges and accessory clamps, to measure position on its V370 and V480 plastic injection molding machines. Negri & Bossi sells an average of 800 injection molding machines per year.
Noncontact Temposonics sensors do not have any moving parts to wear down, making them long-lived, typically lasting more than 20 years. These qualities found the sensors serving a higher calling at St. Michael’s church in Wemmetsweiler.
A Perfect Challenge
MTS Sensors division markets its Temposonics line to manufacturers through its offices in Cary, N.C., and Lüdenscheid, Germany, and relished the challenge of measuring St. Michael’s.
“Although this is not the industrial type of application Temposonics position sensors were designed for, it is perfectly suited to the project,” said MTS product manager Mark Rossi. “Mining will continue in the area for years, causing ongoing surface destabilization beneath St. Michael’s. Also, the hydrostatic system needs to be able to adjust for weight differential when the church is empty and when it is full of people. Temposonics position sensors allow for continual, absolute, and flexible measurement, which PKF determined was important to overall long-term performance.”
PKF installed 82 hydrostatic sensors beneath St. Michael’s. They consist of glass vessels containing fluid, which are sealed to prevent humidity from interfering with the accuracy of their readings. A single Temposonics DeviceNet sensor was inserted in each hydrostatic sensor.
A crucial difference of the Temposonics sensors used at St. Michael’s is that they were equipped with a glass float containing a magnet in place of a standard position magnet. The float moves in relation to the building, as would the position magnet, and the same magnetostrictive technology is used to measure position.
PKF used laser-calibrating technology to mount sensors at the appropriate points on the columns and outer walls of St. Michael’s before the crews began securing the building. The sensors are linked via a controller area network using a DeviceNet interface. This interface enables up to 64 sensors to be linked by a single cable, thereby eliminating the need for multiple wire runs. The interface also provides sensor-based diagnostics and the ability to expand the number of sensors.
The Temposonics sensors beneath St. Michael’s took up to 7,500 readings per second to sense shifts as minute as 2 microns in the church’s position. This information was displayed on a computer.
Once the position sensing system was in place, PKF installed pile foundations to support the structure temporarily. PKF crews then installed steel-and-concrete grating, consisting of longitudinal girders and traverses, to underpin all walls and columns. The grating was covered with a new floor to make the entire building very rigid.
After the floor was in place, PKF laid substantial concrete foundations beneath St. Michael’s. Most importantly, the workers installed 48 hydraulically actuated, spiral spring packages in the concrete foundation. Each spring package supports up to 120 tons and is connected to the same closed loop with the sensor system.
The springs are hydraulically actuated when the sensor system detects movement of the foundation. This way, if the sensors show a shift in the church structure, they will command the appropriate springs to extend to correct it.
If the soil subsides to the point where it exhausts the spring deflection, the spring packages can be retightened and underlined to take up the slack.
The St. Michael’s project was completed in 1998. The MTS measurement system was moved to the church’s newly air-conditioned basement to work in conjunction with the spring packages to keep the church stable. The success of this project has opened other applications for Temposonics position sensors to save other churches suffering imbalance, for example, at the Mountain Chapel of Inllingen, Germany.
Germany’s automobiles, as well as its churches, are using the Temposonics position sensors to keep things level. Commercial position sensors, designed specifically for medical, marine, and automotive applications, are speeding along the autobahns in the Active Body Control suspension system of the Mercedes CL coupe, which made its debut on European roads late last year.
The ABC system uses a combination of sensors and computer-controlled hydraulic cylinders to stabilize the car body within fractions of a second to greatly reduce the roll and pitch that occur when a driver accelerates, corners, or brakes a car. The ABC system reduces the roll angle of the Mercedes CL by 68 percent, thereby improving driver safety and comfort.
The ABC system consists of hydraulic servocylinders that are connected to steel springs in the Mercedes CL’s suspension struts at each of the coupe’s axles. The servocylinders are powered by a high-pressure hydraulic system whose pump is commanded by two microcomputers that are installed in the car. The microcomputers calculate the pressure and duration of hydraulic power that must be sent to the servocylinders to compress the suspension springs to the degree needed to stabilize the car body. These changes in hydraulic pressure are made continuously within milliseconds to ensure a smooth drive.
Mercedes-Benz engineers installed MTS commercial position sensors in each suspension strut of the CL coupe to measure the position of the springs. MTS designed its commercial position sensors for applications that demand high-volume measurement.
The hydraulic cylinders can automatically level the car according to the load on the vehicle. The driver also can adjust the ABC system to provide more freedom of movement for a sportier ride, or to correct the level of the car for greater comfort.