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09. des. 2014

Special Task for MVK Metal

Fieldbus installation concepts using MVK Metal are well-established solutions for highly automated production lines. Sometimes, Murrelektronik's rugged fieldbus module is used in applications where you wouldn't quite expect it. A prime example is the Halder GmbH mobile debarking system. The machine builder, located in southern Germany, retrofitted their systems with MVK Metal, a modern Profibus fieldbus solution.

MVK Metal Close to the Process, with Double Valve Connectors

Cube20S in a Switch Box in the Operator Cabin

Mobile Debarking System Working

Berthold Halder, Director of the Company

Machinery used in the forestry industry has to be extremely rugged. This includes the mobile debarking system that operates in different locations in the wooded area in southern Germany. Each system is operated by two workers sitting in the operator cabin of the MAN truck. They skillfully maneuver long and heavy trunks onto the vehicle where the bark is rapidly removed and the trunks are stacked along the road.

"Our mobile debarking system has been operating since 1996", says Berthold Halder, President, who has a passion for machine building. The system operates for approximately 1,200 hours a year, sometimes more, sometimes less. "The machine has been operating for almost 38,000 hours." It is a reliable and robust machine: forest paths full of mud, heavy rainstorms and constant vibrations will not damage the machine.

Debarking systems are commonly ordered by saw mills that don't have a permanently installed debarking system on their premises. The mobile system is also very useful if the trunks have to dry rapidly as the wood dries much faster when the bark is removed. "In 45 minutes, we can handle about 100 solid cubic meters", says Berthold Halder. This is an impressive pile of wood. A single 500HP motor is used to drive the vehicle and the eleven hydraulic pumps of the debarking system during operation.

The cutting blades that strip the bark of the trunks have hydraulic suspension and are installed on a rotating drum. The rotor disk is driven by a powerful hydraulic motor. Feed rollers transport the trunks through the machines. A bark centrifuge, that can be variably positioned, transports the stripped bark to the side. The speed of all three movable parts can be adjusted. It is also used to regulate the throwing distance. A large number of sensors are installed in the machine to monitor the operating state of the actuators including the inductive proximity switches installed at the outriggers. They make sure the outriggers are retracted when the machine is driving on the street.

The machine has many I/O points. "About two thirds of them are actuators and one third are sensors", explains Halder. When he wired the debarking system for the first time, over two decades ago, he used conventional single wire connections. Alois Öhler, a control technology specialist from R&Ö, remembers: "This was a very tricky job because there were winding cables of up to 20 meters in length." Throughout the years, the installation has been continuously simplified, part of it with the first fieldbus components. When it was time for a big retrofit, Halder knew that this time they wanted a Profibus fieldbus solution for the machines. Öhler adds: "Our many years of experience helped us as we looked for suitable components for our special application."

“The fully potted modules are reliably sealed.”

Together they decided to use Murrelektronik's MVK Metal fieldbus modules. One deciding factor was the rugged housing of the components. Öhler says: "The fully potted MVK Metal modules are reliably sealed." Additional rubber buffers absorb heavy vibrations and even with very bad weather and cold, the official temperature range for an MVK module is -25 up to +55°C, the modules work reliably without failures. "Even when its 20 degrees below zero there aren't any problems", explains Berthold Halder as he smiles a bit. "However, we don't benefit from this advantage. Even though we work with all kinds of weather, we have to quit at 10 degrees below zero because you can’t strip back from the trees at this temperature."

MVK Metal's multi-function ports were a great advantage during the retrofit phase. "This provided flexibility. We could use and configure the individual ports as input or output, according to the specific requirements", explains Alois Öhler. In the end, a complete fieldbus solution was implemented with a relatively small number of modules. If a port is not used, it is sealed with a blind plug. However, this doesn't happen often as most of the ports are used twice, most commonly for double valve connectors that control the hydraulic pumps. The integrated hex thread on the M12 cordsets is another advantage for Halder. They are able to accurately tighten them with the torque wrench and quickly loosen them for repair work.

Another advantage of the MVK Metal fieldbus solution is that the machine operators are able to troubleshoot problems themselves. "If it should happen that a fieldbus module doesn't work, they can take a spare module, set the correct addresses and replace the defective module", explains Berthold Halder. The automatic topology discovery does the rest.  This avoids abandoning an operation or having to do troubleshooting on site – not that either happens very often. Berthold Halder says: "To be honest, so far there was only one failure with a Profibus connection that interrupted the fieldbus system, and this was caused by inappropriate use." Another project, installing a display in the driver's cab to easily use MVK Metal's diagnostic options, has already been initiated.

In the vehicle's operator cabin and in some smaller, decentralized cabins of the mobile debarking system, Murrelektronik's Cube20s system has been installed. "There isn't much room in these cabins. Therefore it is very important to install high density compact components", says Berthold Halder. Like the MVK Modules, these modules are also exposed to vehicle shock and vibrations", says Berthold Halder. However, this doesn't cause a problem. According to Alois Öhler: "The wires are connected to vibration-resistant spring clamp terminals to make a safe connection." He also appreciates that the terminals are easy to reach. Another important point for Halder is that the fieldbus solution is continuously supplied with power. Because, when the motor is started, the generator needs a lot of power. This is always a critical moment as "It can happen that voltage is interrupted for a brief time, in the worst case scenario even for a few seconds", comments Berthold Halder. Here, Murrelektronik's buffer module MB Cap helps to protect the 24 V supply of the control and the fieldbus system.

In the end, Halder is very satisfied with Murrelektronik's products and solutions. "It is important for us that the machines work and we aren't standing in the rain, in the woods, having to look for failures”. Components from Murrelektronik help to ensure this.

Text & Images: Alexander Hornauer