Passive distribution boxes prove to be best option for machines

Schuster Maschinenbau GmbH is headquartered in Denklingen, Germany, about 60km from Munich in Bavaria. The company was founded 1979 and currently has 65 employees. As well as developing and manufacturing vertical turning machines, the company also produces impact cutting systems, automation systems and custom machines. Schuster Maschinenbau delivers about 35 machines and systems a year, the majority of them - about 30 machines - are vertical turning machines. The company distinguishes three different types of turning machines: flexible turning cells (F series), pick-up lathes (P series) and vertical turning cells with integrated fast handling systems (V series).

Schuster Maschinenbau is a mid-sized company with strong customer orientation: many machines are designed according to customer requirements. These customers are often mid-sized companies in the automotive supply industry.

Herbert Assner is the manager of the electrical department at Schuster Maschinenbau. One of his tasks is installing and connecting the I/O points of the machines and systems in a very simple, reliable and cost-effective manner into the cabinet, and connecting them to the control system.

Three different concepts are established in the market:

• conventional parallel wiring
• passive distribution technology
• fieldbus technology

An important criterion for selecting the right installation concept is the relationship between the I/O points and their distance to the machine. With a very small number of I/O points, conventional one-to-one (point-to-point) wiring may be the most cost-effective approach. However, this requires a lot of installation time, even when the number of I/O points is small. This makes the use of passive distribution boxes a more cost-effective option. Pluggable or moulded homerun cables for distribution boxes are quickly installed and they prevent wiring errors in the field. LED displays show the status of operating voltage and signals. The time required for installation, commissioning and service is significantly shorter, because the I/O level is designed in a pluggable way, which results in lower total costs. In case of extensive installations with a high I/O density that also require comprehensive diagnostic options, an active fieldbus system may be the right option.

The most cost-effective option

Schuster Maschinenbau also uses fieldbus technology in the form of Murrelektronik's Cube67 modular fieldbus system. This is used in impact cutting machines because it is the most cost-effective option. Herbert Assner and his colleagues have worked out, however, that passive distribution is the best approach for many other machines that Schuster produces. The reason is that about 50 I/O points is the typical number for a Schuster machine, and they are located in a relatively compact space. Passive distribution boxes gather the I/O signals directly in the field into the cabinet and connect them to the control system.

Murrelektronik's new distribution concept with Exact12 series passive distribution boxes enables Schuster Maschinenbau to engineer installations in its machines with overall pluggable connection from the control cabinet into the field. If the I/O points are further away from the cabinet, they are connected with a distributor cap with pre-wired or moulded cables. Distribution boxes for rear mounting are used for I/O signals that are located near the cabinet.

Safety connections

For safety applications, Herbert Assner and his colleagues use Exact12 safety distribution boxes. These distribution boxes can handle inputs form up to eight standard sensors and eight actuators for safe shut-off. The relevant ports are marked by yellow rings (360 degree marking). These yellow rings make it easy for the workers in the production and the machine operators to clearly recognise the safety ports. Exact12 series safety distribution boxes are approved according to EN ISO 13849-2. Up to performance level d can be achieved. EN ISO 13849-2 refers to fault exclusion in case of short circuits between adjacent conductors, terminals and multipole connections. Herbert Assner explains: "Most of all in the field of hydraulics - for example, with main drain valves - we use a two-channel switch off for safety reasons. By implementing the well-engineered distribution concept based on Exact12, Schuster GmbH can completely do without terminal boxes that take a lot of space in the machine and require time-consuming, error-prone and expensive installation."

The system also offers other benefits: 11- and 12-pole power strips have been pre-wired to the homerun cables of the distribution boxes to simplify installation. Herbert Assner says: "In the cabinet, they are transferred with a transfer module with spring clamp terminals. This goes very fast, because it is no longer necessary to connect up to 23 terminal points to terminal blocks by hand." When installing the distribution boxes it is not necessary to connect the ferrule ends. And employing the system with pre-wired terminals, Schuster Maschinenbau can be certain that the components used are already 100 per cent tested. This is a huge relief, especially for safety applications. Herbert Assner explains: "This way, one possible error source is already eliminated, when the machine is commissioned."

A service adapter offers an additional option to visually check all I/O signals. If all connections are error-free, the test LED on this control component illuminates and directly shows the signal states. Herbert Assner continues: "This installation concept creates a clear separation between control cabinet and machine peripherals. With this system of transfer adapters in the cabinet we also have a simple separation for transport that facilitates on-site commissioning. The additional connectors that otherwise are used to separate the machine are also no longer required. With this distribution system, we have found the optimum solution."

Clever torque wrench

Herbert Assner appreciates installations based on M12 connectors because they have proved to be very durable and they are easy to handle by the operators. He says: "And the torque wrench is also an additional help, especially with fully populated distribution boxes we can be sure that really all connections are made with the correct torque - and therefore IP67 sealed. Tightly connected, no wobbling."

If space is scarce, Schuster Maschinenbau also uses M8 distribution boxes. This is often the case in robotic applications or in the field of handling and assembly. Herbert Assner says: "Murrelektronik's M8 distribution boxes feature different mounting holes that allow horizontal and vertical mounting, this is another advantage for us," and he is very pleased with this additional flexibility.

Talking about flexibility, when integrating hydraulic power units that are often separately delivered to the end customers, Schuster Maschinenbau also relies on a distribution system from Murrelektronik. Herbert Assner explains: "Here we prefer the cap models of the Exact12 series; this solution helps us to create a simple and quick transport separation."

Go to www.murrelektronik.co.uk for more information about Exact12 passive distribution boxes.