To overcome the problem, ABB and LES Engineering proposed that the old stationary resting blocks for the decks be replaced with active blocks capable of movement. With this new system, ABB Millmate Pressductor load cells
housed in each of the new blocks continuously measure the load across the bridge decks. The load cells, capable of measuring up to 600 tons, relay load data for each deck via a 4-20 mA signal to a programmable logic control (PLC) system.
If the PLC senses imbalanced weight from the load cells, it activates hydraulic cylinders to adjust the position of the resting blocks. This action maintains equal loads across the deck bearings, which will help to both increase their life and that of the overall bridge structure.
The sensing principle of the ABB’s Millmate Pressductor load sensor depends on how the permeability of a magnetic material changes under mechanical stress. The sensor is a membrane machined in the load cell. Primary and secondary windings in the sensor cross at right angles. Alternating current passes through the primary winding creating a magnetic field surrounding it. With no load on the sensor, this magnetic field doesn’t induce a current in the secondary winding.
When, however, the sensor experiences a mechanical force in the direction of measurement the propagation of the magnetic field changes. Now the primary winding does affect the secondary winding, inducing an alternating voltage in it. A control unit converts this alternating voltage into a DC voltage proportional to the applied force. If the measurement force changes direction, the sensor signal also changes polarity.
ABB’s Millmate Pressductor load cells have been widely used in many applications since first developed in 1954. Common applications include monitoring of hot and cold rolling in the steel industry, as well as measurement of tension and flatness.