Temperature effects and EMC under control
To ensure reliable and precise measurement, the experts at Dr. Linse Ingenieure designed a comparably extravagant structural solution. In the region of the lighting gantry suspension, they welded two metal holders for the strain transducers on the underside of the main girders (compression zone), at each of two points.
The variations in the load on the lighting gantry cause the supporting beam to deform, with the deformation being transferred to the transducers via the metal holders. Type SLB700A strain transducers from HBK are used here. The transducers work with strain gauge full bridges made from stainless steel and feature a very low zero offset and good temperature compensation. To increase the precision of the measurement, the transducers were fitted with an insulated housing. “This made it possible to minimize what in some cases were extreme temperature fluctuations, as well as the associated temperature gradients”, explains Dr Schmiedmayer. “We also attached temperature transducers in the housings, to perform additional temperature compensation.”
The structure is even more complex for the four spindles of the main girders. Here the engineers designed quadripartite nuts that fitted exactly onto the threaded spindles and which can be held in place by a bolted connection. Two of these nuts on each of the threaded spindles act as holders for four strain transducers. In contrast to the situation on the lighting gantries, these are not connected in parallel, they are evaluated individually. This allows any bending of the spindles that occurs with a load and which cannot be compensated by the needle bearings, to be detected.
The engineers also had to get the effects of electromagnetic interference under control. “In the first preliminary tests, we noticed that when the large spotlights right next to the measurement technology were switched on and off, there could be major interference”, remembers Dr Schmiedmayer. Shielded measuring leads then had to be used, that were also run through continuously enclosed metal pipes, to double the shielding effect.
At the same time, care was taken to ensure that none of the measuring leads were longer than 20 m. MX840A modules from the HBK QuantumX data acquisition system are used as the amplifiers. These compact amplifiers each have eight channels and are easily installed on the gantry structure, making it possible to keep measuring leads short. A total of four amplifiers transmit the measurement data with an interposed fiber-optic link to a type CX27 gateway module, acting as a central measurement node.