Production of the Franki pile by driving. Photo: ©Stump-Franki Spezialtiefbau GmbH.
Heavy machinery in specialist civil engineering requires that precise components perform reliably. The absolute rotary encoder WV58MR from SIKO GmbH, a manufacturer of sensors and positioning systems, is part of the safety concept for the pile drivers from Stump-Franki Spezialtiefbau GmbH. They are attached to three winches to detect the positions of the respective elements attached to them.
Franki manufactures its own pile drivers, which are used to construct foundation piles. The piles are used to divert loads of a structure into deeper layers. “We have our own mechanical engineering department to develop, design, assemble, and commission. We then use the equipment that suits us,” explained Stefan Hemmerle, Head of Mechanical Engineering at Stump-Franki. Especially with the pile drivers, self-construction was the best option: Until the 1960s, the pile drivers could still be purchased in Belgium.
Subsequently, Franki Germany began to manufacture its own machines, like the Franki rams and Atlas devices, starting in 2000. “For us, this is a clear competitive advantage to build these machines according to our ideas, so that we can also use our own processes to the advantage of our customers,” said Hemmerle.
The Franki pile driver has four winches installed at the top — including three SIKO rotary encoders for position monitoring. In the front area is the so-called leader, a frame to which the pile or driving pipe made of steel is attached, which is open at the top and bottom. A plug of concrete and gravel is filled into the driving pipe (1), and with the pile driver, a solid block of steel, the plug is compacted with small blows (2). The pile driver is a free-fall system. Once the plug is firmly wedged in the pipe by the blows, the drop height increases, and the driving pipe is brought to the final depth in the load-bearing subsoil. Then the plug is impacted with the pile driver (3). The so-called “Franki foot” is formed, acting as a dowel. After the foot has been fabricated, the reinforcement cage is adjusted, and plastic concrete is refilled (4) so that a continuous pile is formed. After the concrete has hardened and the pile head (5) has been capped, the pile is ready to receive the load.
Figure 2a
Monitoring of the four winches
The four winches have different tasks during the driving process: A winch is used to pull the driving pipe, a second is used to drive the pile driver, a bucket for filling with concrete and gravel is attached to the third, and finally there is an auxiliary winch by which a reinforcement cage is lifted in place. Three of the four winches are monitored with a rotary encoder from SIKO so that it is clear where the pile driver, the auxiliary lift, and the bucket are located. The auxiliary rope is a safety design to ensure that the pile driver shuts off before a hook passes through a rope pulley. Therefore, this rotary encoder has an end-position point beyond which the winch must not rotate. The pipe pulling winch is the most sensitive area of the system; despite the hard blows into the driving pipe.
The rotary encoders are integrated into an automatic control system, which permanently queries the positions and thus also controls the tracking of the cable.
Rotary encoders with safety performance level PLd
Figure 2b. As an absolute encoder with redundant position detection, the WV58MR has been specially developed for challenging measuring tasks in mobile machines. Photos: ©SIKO GmbH
“What is most important to us is the precision of the position detection and the redundancy as a safety function of the rotary encoder,” said Hemmerle. “As a multi-turn encoder, it can detect up to 4,096 revolutions of the winch in absolute terms. In addition, we have also integrated a gear ratio so that we receive an enormous number of signals that enable us to detect the position permanently.” Because failures of individual components can have serious effects in such an application with heavy equipment, the safety function of the rotary encoder with safety class PLd is decisive. It is entirely redundant with two separate sensor circuits and thus ensures safe handling.
Additionally, it is an absolute encoder, which recognizes its position precisely even in the de-energized state and does not require reference travel.
In this case, the integration into the controller takes place via a CANopen safety interface. “What is also important for our application is the absolute robustness. The rotary encoders must be able to withstand the loads — and they do.” The magnetic measuring principle of the rotary encoders, which is ideally suited for use in harsh environments, also meets this requirement.
Foundation piles in case of poor soil and heavy load
The driving of foundation piles is necessary in the case of poor soil quality and particularly heavy structures. “The advantage of our Franki pile system is that it is a full displacement pile system. As a result, we have no excavated soil and do not have to dispose of anything,” explained Hemmerle. “With the impacted foot, we can adapt to any soil. If it is not possible to drive particularly deep, we can simply impact the foot a little larger and compact it more strongly, thus giving the structure the necessary stability.”
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