Position monitoring in free fall with SIKO encoder

Manufacture of the Franki rod using a percussion process. Photo: © Stump-Franki Spezialtiefbau GmbH

Heavy machinery in custom construction requires precise and reliable components to perform functions safely. SIKO WV58MR absolute encoders are part of the Stump-Franki Spezialtiefbau safety system for postmen.

by Michaela Wassenberg

The special construction company Stump-Franki supplies the load-bearing subsoil for heavy constructions such as football stadiums, motorway bridges, shopping centers or wind turbines. For this purpose, the company manufactures its own piling equipment, which is used to manufacture foundation piles. Through these piles, the load of the structure is transferred to the deeper layers. “We have a mechanical engineering department where we develop, design, assemble, commission and then use the right equipment for us,” explains Stefan Hemmerle, head of mechanical engineering at Stump-Franki.

Especially for postal drivers, in-house production was the best option. Until the 1960s mail drivers could still be bought in Belgium, then Franki Deutschland started to produce their own machines such as Franki rams and from the year 2000, Atlas machines. “It is a clear competitive advantage for us to be able to build these machines based on our ideas, so that we can also use our processes for the benefit of our customers,” says Hemmerle.

The WV58MR is an absolute encoder with redundant position detection, developed for demanding measuring tasks, especially in mobile machines. Photo: © SIKO GmbH

SWEATER AND FRANKI BASE

The pile driver is used as shown in Figure 1. The Franki pile driver has four winches installed at the top, including three SIKO encoders for position monitoring. In the front area, the so-called leader, a frame to which the pile driver or guide pipe is attached, is open at the top and bottom. A plug is created in the pile driver pipe (1) by filling it with concrete and gravel, and the hammer, a solid block of steel, is used to compact the plug with very short strokes (2). The club is a free fall system. When the cap is firmly embedded in the pipe, the drop height is increased thanks to the impact of the hammer and the pile pipe is driven to its final depth in the bearing subsoil. Then the cap is compressed by the hammer (3). The so-called Franki base is formed, which functions as a pin. After the foundation is created, the reinforcing cage is installed and plastic concrete (4) is added to form a continuous pile. After the concrete has hardened and the pile head (5) is covered, the pile is ready to be loaded.

Franki hammers in action: With its enormous weight, the free-fall hammer drops down the guide tube to compact the concrete and gravel in the stable base of the foundation pile. The SIKO WV58MR encoder controls, among other things, the positions of the rope pulleys.

Photo: © Stump-Franki Spezialtiefbau GmbH

MONITORING THE FOUR WINCHES

The four games have different tasks during the piling process. A cable winch is used to pull the guide pipe, another to control the hammer, a third is connected to a bucket for filling with concrete and gravel, and finally there is an auxiliary winch to lift the rebar cage. Three of the four winches are each monitored by a SIKO encoder, so that it is clear at all times where the pile driver, auxiliary hoist and bucket are located.

The auxiliary rope is a technical safety feature to ensure that the pile driver shuts off before a hook pulls through a pulley. This encoder therefore has an end point beyond which the game must not continue to rotate. The play for pulling the pipes is the most sensitive area of ​​the installation; despite the heavy blow of the pile driver, the position must be known at all times. The encoders are integrated into an automatic control system that permanently interrogates the positions and therefore also controls the course of the rope.

ENCODER WITH SECURITY LEVEL PLd “What matters most is the accuracy of position detection and redundancy as a safety feature for the encoder,” explains Hemmerle. “This is ensured by a multiturn encoder, which can absolutely register up to 4,096 revolutions of the game; moreover, we have also deployed a transmission ratio to obtain a huge number of signals, a factor that allows us to permanently detect the position ”. Since individual component failures can have serious consequences in such a heavy equipment application, the safety function of the encoder, with safety class PLd, is critical. The system has a fully redundant design with two separate sensor circuits, ensuring safe control. Furthermore, it is an absolute encoder that accurately detects its position even in a de-energized state and does not requiretarget search

. In this case, the integration into the control system takes place via a CANopen safety interface.

“Absolute robustness is also important for our application,” adds Hemmerle. “When the base is compressed, the pipe undergoes a pulling force of 240 t on one side, at the same time the hammer falls into the pipe with a force of 6.5 t. Pretty much the whole car jumps! The encoders must be able to handle this load, and they can.” The magnetic measuring principle of rotary encoders, which is ideal for use in harsh environments, also meets this requirement.

Stefan Hemmerle, head of mechanical engineering at Stump-Franki Spezialtiefbau GmbH.

Photo: © Stump-Franki Spezialtiefbau GmbH

FOUNDATIONS AND HEAVY LOADS

Driving down foundation piles is necessary in case of poor soil quality and particularly heavy constructions. “The advantage of the Franki rod solution is that it is a fully shifting rod system. This means that we have no excavated soil, and we do not have to dispose of anything,’ explains Hemmerle. “With the compacted base, we can adapt to any terrain. If it is not possible to dig very deep, we can simply make a bigger base and compress it more, so that the structure gets the stability it needs”.

In the case of wind turbines, another parameter comes into play: the height of the wind turbine. For secure anchoring in the ground, slanted rods are required here, which can also be driven into the ground with a Franki post driver and free-fall hammer. From 24 to 36 piles are driven into the surrounding ground; two bars have an outward slant, one is slanted inward. The poles are then placed alternately, as they must withstand both tension and compression depending on the direction of the wind.

MODULAR SYSTEM FOR MOBILE MACHINES

SIKO encoders meet the expectations for safety, accuracy and robustness that Stump-Franki requires for its applications. The WV58MR is part of SIKO’s “PURE.MOBILE modular sensor system”, developed exclusively for the high demands of mobile machines and commercial vehicles.

All sensors with this mark have the following characteristics: They are available in single-channel or safe versions up to performance level d (PLd); they have protection classes up to IP6K9K; withstand an extended temperature range: -40 to +85 ° C (optional: +105 ° C); they have a high resistance to shocks and vibrations; provides expandable and configurable functions such as tilt, DIP switches, digital I/O thanks to the optional PURE.MOBILE cards. “Nothing must be changed about the robustness of the codes!”: Is Hemmerle’s appeal to the commercial partner SIKO. “These properties will continue to be indispensable for our custom construction applications.” SIKO sensor and system specialists have the right answer with their measurement solutions for mobile machines.

© TECNeLab

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