The use of PLC technology and sensors has increased the range of use for cranes. In Böcker Maschinenwerke (Germany) systems, WDGA absolute encoders with Endra multiturn technology from Wachendorff (Germany) ensure boom stability.
|Böcker cranes can be converted into work platforms in just a few steps. Particularly high safety standards apply to these (Photo: Wachendorff)|
TRUCK-MOUNTED CRANES HAVE BECOME AN INDISPENSABLE TOOL, especially in the roofing trade. Their compact dimensions allow them to be simply driven to the building site and put into position. Although they appear small on the truck, once the support system and telescopic mast are extended, they become an impressive and powerful piece of transport machinery. The operating range of the boom used to be considerably restricted, but now PLC controls and sensor technology ensure that the boom can be pushed to its limits. This is also necessary as parked cars, trees or slopes often mean that the crane cannot be ideally positioned and not all of the outriggers can be completely extended. “The crane has to be set up in the space available on site,” explains Dirk Seiger, who is responsible for electronic and control technology at Böcker. Sometimes every centimeter counts.
Sensors for increased safety
Infinitely adjustable swiveling and telescopic outriggers make positioning easier. However, all of these variables are a great challenge for the control technology and safety system. Of course a machine should not tip over and fall onto a house or even onto people. Measurement values from different sensors are therefore continuously recorded and the current overturning torque and allowable, non-critical operating range calculated at all times. Thanks to the variable support system, the range is not necessarily circular. It can also be pear-shaped or elliptical, depending on how far each outrigger is extended. Before the system reaches critical limits, it locks automatically and will not allow any further movement or any additional load to be lifted. To do this, values such as the incline and rotation angle of the boom are recorded - each one twice and separately from each other. “All safety mechanisms have redundant monitoring,” explains Dirk Seiger. “Many of our cranes can be converted for use as work platforms in just a few steps. Even stricter safety rules apply then, as people are being carried.”
|WDGA absolute encoders record the rotation of the boom on its own axis. For safety reasons, the entire safety system is designed with redundant sensors and controls – this is why there are two encoders side by side, each working completely independently (Photo: Wachendorff)|
The protocol used here is CAN, which is also used in automotive technology. “This allows priorities to be defined particularly easily, and the system is extremely stable,” says Dirk Seiger. The company uses Wachendorff absolute multiturn encoders to determine the boom’s angle of rotation at all times. The encoders use the communication profile according to CiA 301 and the CiA 406 V3.2 class C2 device profile for encoder. They offer up to 127 nodes and a bit-rate of 10 kbit/s to 1 Mbit/s with automatic bit rate detection. The standard settings as well as any customization in the software can be changed via LSS (CiA 305) and the SDO protocol, e.g. PDOs, Scaling, Heartbeat, Node-ID, Baud rate, etc. When a synchronization telegram (SYNC) is received from another bus node in synchronous mode, PDOs are transmitted independently. In asynchronous mode, a PDO message is triggered by an internal event. (e.g. change of measured valued, internal timer, etc.).
As changes in position must also be recorded even if the boom is moved by hand, Böcker now almost exclusively uses absolute encoders with the Endra multiturn technology. Instead of a gear mechanism, they calculate the rotary movement completely electronically via a magnetic field that is built up and discharged during the rotation. The revolutions are determined by means of an energy wire: a permanent magnet accumulates so much energy in the wire that for a defined position the information “Revolution” and “Direction of Rotation” is transmitted to the evaluation electronics. An external energy feed, for example by means of a battery, is not required for this. As a result this patented system is able to work fully autonomously. The benefit of this is that there are no wearing parts or a battery that must be replaced. “In the extreme temperatures that our cranes are subjected to, battery life suffers quite considerably,” says Dirk Seiger, speaking from experience. Endra technology makes these concerns a thing of the past. The encoders are even unaffected by wind and weather: the precision turned encoder housing is tightly sealed and also cemented into place, achieving protection class IP67 or IP69K.
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