Position monitoring in free fall

Heavy machinery in special civil engineering requires precise and reliable components in order to be able to carry out the respective functions in a process-safe manner. In the pile-driving machines of Stump-Franki Spezialtiefbau GmbH, the absolute encoders WV58MR from Siko GmbH, manufacturer of sensors and positioning systems, are part of the safety concept. They are mounted on three cable winches to detect the positions of the respective elements attached to them.

The special civil engineering company Stump-Franki makes subsoil load-bearing for heavy structures such as football stadiums, motorway bridges, shopping centres or wind power plants. For this purpose, it manufactures its own pile-driving equipment with which foundation piles are produced. Loads of the structure are transferred to deeper layers via the piles. "We have our own mechanical engineering department to develop, design, assemble, commission and then use the equipment that is right for us," explains Stefan Hemmerle, head of mechanical engineering at Stump-Franki. Especially in the case of pile-driving equipment, building your own was the best option: until the 1960s, pile-driving equipment could still be purchased in Belgium. Afterwards, Franki Germany started to manufacture its own machines such as the Franki pile drivers and, from 2000 onwards, Atlas equipment. "For us, this is a clear competitive advantage to build these machines according to our own ideas, so that we can then also use our own processes for the benefit of our customers," says Stefan Hemmerle.

Ram bear and Franki foot

The Franki pile driver has four winches installed at the top - including three SIKO encoders for position monitoring. In the front area is the so-called leader, a frame to which the steel driving tube is attached, which is open at the top and bottom. A plug of concrete and gravel is filled into the driving pipe, and the pile driver, a massive steel block, is used to compact the plug with very small blows. The ram is a free-fall system. When the plug is firmly wedged in the pipe by the blows, the drop height is increased and the rammed pipe is brought to its final depth in the load-bearing subsoil. Then the plug is rammed out with the rammer. The so-called Franki foot is formed, which acts like a dowel. After the foot has been produced, the reinforcement cage is adjusted and plastic concrete is added so that a continuous pile is formed. After the concrete has hardened and the pile head has been capped, the pile is ready to take the load.

Monitoring the four winches

The four winches have different tasks during the driving process: One winch is used to pull the jacking pipe, a second is used to drive the pile driver, a bucket is attached to the third to fill it with concrete and gravel, and finally there is an auxiliary winch that is used to lift in a reinforcement cage. Three of the four winches are each monitored by a SIKO encoder so that it is clear at all times where the pile-driving pipe, the auxiliary hoist and the bucket are. The auxiliary cable is a safety-related design to ensure that the piling rig shuts down before a hook pulls through a pulley. This encoder therefore has an end position point beyond which the winch must not continue to rotate. The pipe pulling winch is the most sensitive part of the installation; despite the hard blows into the pile-driving pipe, the position must be known at all times.

The encoders are integrated into an automatic control system that permanently queries the positions and thus also controls the tracking of the cable.

Rotary encoder with safety performance level PLd

"What matters most to us is the precision of the position detection and the redundancy as a safety function of the encoder," says Stefan Hemmerle. "As a multi-turn encoder, it can absolutely detect up to 4,096 revolutions of the winch; moreover, we have also integrated a transmission ratio, so that we get an enormous number of signals, which enables us to permanently detect the position." Since in such an application with heavy equipment, failures of individual components can have serious consequences, the safety function of the encoder with safety class PLd is crucial. It has a completely redundant design with two separate sensor circuits, thus ensuring safe handling. In addition, it is an absolute encoder that accurately detects its position even in a de-energised state and does not require a reference run.

In this case, integration into the control system takes place via a CANopen safety interface. "What is also important for our application is the absolute robustness. At the latest when the foot is stamped out, on the one hand the pipe is pulled with 240 tonnes and at the same time the ram falls into the pipe with 6.5 tonnes. Practically the entire machine bounces! The encoders have to be able to withstand this load - and they do." The magnetic measuring principle of the encoders, which is ideally suited for use in harsh environments, also helps.

Foundation piles in poor soil and with heavy loads

Driving foundation piles is necessary when the soil quality is poor and the structures are particularly heavy. "The advantage of our Franki pile system is that it is a full displacement pile system. This means we have no excavated soil and do not have to dispose of anything," explains Stefan Hemmerle. "With the stamped-out foot, we can adapt to any soil. If it's not possible to drive particularly deep, we can simply tamp out the footing a little bigger and compact it more, giving the structure the stability it needs."

With wind turbines, another parameter comes into play: the height of the wind turbine. To anchor it securely in the ground, inclined piles are needed here, which can also be driven into the ground with the Franki pile driver along with the free-fall weight. 24 to 36 piles are anchored in the ground all around; two piles have an outward inclination, one pile inclines inwards. In this way, the piles are arranged alternately, as they have to withstand both tension and compression, depending on the wind direction.

Modular system for mobile machines

Siko's encoders meet the expectations of safety, precision and robustness that Stump-Franki needs for its applications. The WV58MR is part of Siko's "PURE.MOBILE modular sensor system", which was developed exclusively for the high demands of mobile machines and commercial vehicles. All sensors of this designation have the following features:

• Available in single-channel or safe variants up to performance level d (PLd).
• Protection classes up to IP6K9K
• Extended temperature range: -40 to +85 °C (optional: +105 °C)
• High shock and vibration resistance
• Expandable and configurable functions such as tilt, DIP switches, digital I/O thanks to PURE.MOBILE option cards

"Just don't change anything about the robustness of the encoders!" is Stefan Hemmerle's appeal to business partner Siko, "these properties will continue to be indispensable for our applications in special civil engineering." Siko's sensor specialists have the right answer to this with their measuring solutions for mobile machines.

By Michaela Wassenberg, freelance journalist