Cost-effective machine safety
3rd Quarter 2018, Electrical switching & drive systems & components
IMA Klessmann of Lübbecke, Germany is an international manufacturer of trend-setting manufacturing machines for the woodworking and craft furniture industries. In 2017 the company modernised a complex, multi-track transport system for wooden workpieces for one of France’s largest kitchen cabinetry manufacturers, Fournier of Thônes. In the process, a reliable monitoring system that prevents unauthorised entry was implemented in an extremely simple, flexible and cost-effective way using analog sensors and TwinSAFE SC safety technology from Beckhoff.
In the plant area concerned, board-shaped workpieces for kitchen furniture are removed from a sorting warehouse and stacked on pallets in two picking stations. The finished stacks are subsequently transported out of the order-picking areas to the downstream machines. Following destacking, these machines then receive the necessary parts in precisely the right order to assemble a kitchen cabinet. The two picking stations each have six gates to discharge the workpiece stacks.
According to Michael Gube, the software developer at IMA who was responsible for the startup of this project, it must never be possible for a human to enter the risk area. There is a high safety risk involved on account of the high dynamics of the transport portals located in this area and the large masses that are moved. The conventional method to control access to such plant areas is to use safety light barriers and muting functions. However, such measures alone were deemed insufficient in this case. One of the requirements, therefore, was to guarantee personal and process safety through a second safety device. If anyone attempts to gain unauthorised access to the picking area, they must pass through two devices. As soon as they pass the first, the portal switches to the Safely Limited Speed (SLS) mode. As the person approaches the second device, the machine is stopped from the safe speed.
The first safety device consists of three standard transit time sensors. There is always a safety risk when there is either no material stacked in the area of these sensors or when the material stack is not moving in this area. The entry risk during this phase is reliably avoided in the following way: As soon as a board stack moves underneath the transit time sensor area and is subsequently stopped, the transit time sensors measure the current stack height once (latch). If the stack moves completely out of the area, the stack height is given the value 0. The values of the three sensors determined at a standstill are transmitted to the safety controller and continuously compared to the actual values of the transit time sensors. Now if someone attempts to gain entry when no stack is present or by climbing over a stationary stack, at least one of the three actual values deviates from the latched position. This immediately causes the portals to switch to Safely Limited Speed (SLS) mode.
Once a person has overcome the first safety device, he or she must additionally overcome the second set of devices, safety light barriers placed immediately in front of the picking area. If they detect entry, then the axes which are already moving at a safely limited speed are finally brought to a standstill.
For Gube, the prerequisite for an efficient safety solution was the analog signal processing capability of the EL6910 TwinSAFE Logic terminal. “The core is the EL6910 TwinSAFE Logic terminal with its extended safety functionality. In addition to the safety function blocks from the EL6900, it offers certified safety function blocks to process analog signals, as well as more complex functions such as counters, limit value and comparison. In addition, the EL6910 supports the TwinSAFE SC technology, which makes it possible to securely transmit data from standard EtherCAT I/Os via the TwinSAFE SC extension to the EL6910. As a result, analog signals can now be analysed, checked for plausibility and evaluated within the logic.”
The fine scalability of PC-based control technology from Beckhoff resulted in one of the biggest advantages in the installation of the new safety solution, as Gube explains: “The entire production facility is controlled by TwinCAT 2 software. However, the TwinCAT 3 software generation is required to connect the analog sensors directly via the EL6910 TwinSAFE Logic terminal. The modular Beckhoff control technology is scalable to suit the application demands and it allowed this by simply and cost-effectively realising new safety functions via a subsystem that consists of the CX9020 Embedded PC with TwinCAT 3 as well as the TwinSAFE and TwinSAFE SC terminals.”
For more information contact Michelle Murphy, Beckhoff Automation, +27 11 795 2898, email@example.com, www.beckhoff.co.za