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During R&D, 2M discovered that a non-PLC system would struggle to properly diagnose and supervise the processes carried out by the machine in a safe manner. Therefore, we determined that a PLC control system would need to be incorporated into the machine to take charge of all the functionality.
This write-up is about a project which involves Foam Formation and alleviating Electromagnetic Frequency Interference whilst building a Foam-Slitting machine from scratch.
For another Project write-up in the Industry of Foam Formation, here’s Foam Forming with Canon Viking – 2M Automation , whilst Boundaries of Sound & Space – 2M Automation also touches on the issue of EMF interference in another 2M Project.
This product was a step beyond off-the-shelf products because it is the first machine of its kind to be Programmable Logic Controller (PLC) based with a bespoke design. Machines made prior to this development were very clunky and were known to comprise of many unknowns as access and visibility to various components was very limited.
The original specification was for a non-PLC based system to control:
- 4 blades
- 3 conveyors, and
- 4 grinders
Foam Slitting Machine
From Scratch
This machine was to be located at the end of a production line where the foam, after passing through extrusion, enters the machine in a three layer format so that it may be slit. The product then exits the machine for operators to handle.
The four blades inside the gated machine slit the foam which creates the need for extra safety features to ensure the safety of the operators from both the blade and the grinders.
The machine included an infeed conveyor which fed the product and a mid-feed conveyor which fed the material to the blades. The outfeed conveyor carries the product to the operator. There are also two grinders per blade which run continuously whilst the blades are running to ensure that the blades are being sharpened.
Due to the high risks involved, the machine is fully gated with four solenoid activated gates plus further gates to ensure no unauthorised access is granted to operators whilst the machine is running. There is also functionality to allow for manual sharpening of the grinders.
By the end of the project the machine had:
- a much improved throughput,
- flexibility to adjust the speed of the machine due to the design synchronicity, and
- the ability to perform high-risk operations whilst keeping the operators safe.
Safety Integrity Level (SIL)
One particular aspect that created much uncertainty was that around operator access to the various parts of the machine which are high-risk.
The system was originally designed to only allow access to the machine, i.e. opening the Safety Guard Switches, once all the drives including the Blades Drives were disabled and at zero speed to eliminate any hazard and risk to the Operators.
However, the client wanted to change this protocol which meant the safety gates would open whilst the blades were coasting to a stop so operators could get hold of the product inside the machine in time to avoid excessive product waste and possible jams which cause major machine downtime when this occurred in existing machines.
To achieve the above, 2M had to precisely measure the stoppage time for all the blades at different speeds and for different scenarios to ensure there was enough time for a controlled stop of the blades upon entry of the operators, therefore minimizing risk to the operator and at the same time avoiding the machine jamming whilst still maintaining the Safety Integrity Level (SIL) required by the design.
Electromagnetic Interference
A further uncertainty arose due to the fact that the machine was extremely large and therefore not easy to transport from 2M where it was being built to the customer’s site where it was to be installed. To overcome this, the machine was designed to be built in three parts.
A system of plugs and sockets as well as routing for the cables around the machine then had to be designed and constructed to provide required connectivity and also allow for disassembly for transit. Due to the tight space available for the electrical panels, this proved particularly challenging.
A further uncertainty faced was that when the machine was moved to site it experienced Electromagnetic Interference (EMF) with the functionality of the drives. Before the machine was moved over to the end user it was tested in the workshop and all functionalities worked correctly. However, once it was moved over to the client base, functionality issues began to be reported whilst the machine was in production. It was reported that a number of the 11 drives in the machine were cutting off i.e. stopping and then restarting in the middle of the operations with the issue gradually getting worse over time and eventually resulting in issues with all of the drives.
To try and overcome this technological uncertainty, we carried out further diagnostics and site visits and determined from these investigations that the safety enabled signals to the drives were being affected by EMF interference that was present within the factory. 2M first tried to find a solution for this by hard-wiring the safety enabled signals and found that, whilst this would remove the problems completely, it also meant that the drives would remain enabled on E-Stop activiation which would pose significant risk to the operators.
Therefore, temporary safe operational and maintenance provisions were made and agreed with the client whilst further investigations were carried out to find the root cause and remedy for the interference.
After many attempts, weeks of offline software development, and many tests, 2M finally arrived at the required solution to the interference by discovering that the Output Signal Switching Device (OSSD) had to be disabled on all the Safety Output Cards and the hardware links (the above hardwired solution) had to be removed, which left the machine in a safe operational mode and free from any EMF interference.
