Monitoring of stacked workpieces
July 2018, IS & Ex
IMA Klessmann GmbH 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 (TwinSAFE Single Channel) 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 according to job lists. The finished stacks are subsequently transported out of the order-picking areas via appropriate conveying equipment to the downstream machines. Following destacking, these machines then receive the necessary parts in precisely the right order to assemble a kitchen cabinet as efficiently as possible. The two picking stations, which are among the safety risk areas due to their operating principle, 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, the requirement for this kind of application is that 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. For structural reasons the safety light barriers could only be installed immediately before the risk area. Unauthorised entry would be reliably detected by the light barriers, but there would not be sufficient time to stop hazardous movements quickly enough, even if the maximum possible braking ramps were activated. Other measures, for example the use of safe service brakes, would place an extreme load on the mechanical system and in the long term once again represent a safety risk while endangering the process safety.
Two-stage safety concept provides solution
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 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.
Analog value processing saves considerable costs
For Gube, the prerequisite for an efficient safety solution was the analog signal processing capability of the EL6910 TwinSAFE Logic terminal: “Previously there was a safety deficit on this machine, even though the roller conveyors were manufactured to be inaccessible. However, access was still possible in individual cases, for example if only a base plate normally used underneath a stack was transported. The safety light barriers used for protection were too close to the moving portal, which meant it couldn’t be stopped fast enough in case of imminent danger. The initial solutions considered, such as safety doors or the use of radar scanners, would only have been possible with considerable mechanical rework and cost expense. The alternative with TwinSAFE SC and transit time sensors proved considerably simpler and more flexible for us, while being much more cost-effective.”
Safety function blocks for analog sensor signals
According to Gube, the safety functions based on the analog signals from the transit time sensors can be implemented very conveniently in TwinCAT 3 software with the appropriate safety function blocks, which are above all extremely scalable. The complexity of the system is also not a problem. It consists of two machines with identical hardware and software, each of which makes use of a Beckhoff CX9020 Embedded PC, an EL6910 TwinSAFE Logic terminal and six EL3124-0090 TwinSAFE SC analog input terminals (one for each roller conveyor). Bettina Keller, application/support from Beckhoff, adds: “In addition, each machine uses four EL1904 TwinSAFE digital input terminals for the safety acknowledgement and dual-channel muting inputs and one EL2904 TwinSAFE digital output terminal to control the safety contactors. All necessary functions such as the maximum permitted duration of a muting procedure can be configured conveniently with TwinSAFE function blocks in TwinCAT.” This is also confirmed by Gube, “The most diverse safety functions can be realised simply and quickly with the safety function blocks. A particular advantage of this is that it applies universally, even to the more complex analog input signals.”
Keller explains the building blocks for such a streamlined safety implementation: “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, among other things. These also include more complex functions such as counters, limit value and comparison. In addition, the EL6910 supports the TwinSAFE SC technology, and only this technology makes it possible to securely transmit data from standard EtherCAT I/Os via their TwinSAFE SC extension to the EL6910. As a result, analog signals can now be analysed, checked for plausibility and evaluated within the logic, although for safety reasons at least one of the data sources must be a TwinSAFE SC component.”
Demand-based solution is scalable,yet integrated
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.”
This solution has proven to be extremely flexible in a further regard for Gube. He says: “According to the applicable safety regulations, the hazardous area must be monitored over its entire width in 250 mm intervals. Therefore, we use three transit time sensors on each of the 700 mm-wide roller conveyors. If it should prove necessary in the future to use wider roller conveyors due to larger workpieces, we only need to increase the number of sensors accordingly. The adaptation of the safety functionality can then be configured with little effort via TwinCAT software, especially since safety engineering under TwinCAT 3 is very convenient and efficient.”
For more information contact Michelle Murphy, Beckhoff Automation, +27 (0)11 795 2898, firstname.lastname@example.org, www.beckhoff.co.za