Conveyor technology with retrofitted safety technology

The conveyor technology for palletising and feeding the high-bay warehouse was now somewhat outdated. The operator, K+S, therefore planned to modernise and supplement the existing safety equipment.

The control system itself was already a Siemens S7 416, the periphery was connected decentrally by M and S components in remote control cabinets and field distributors via Profibus DP to the CPU S7-416-2DP.

The challenges

Some of the conveyor sections were to be secured by additional fencing. This entailed the installation of additional safety doors with safety contacts and safety light barriers.

According to the applicable Machinery Directive, the connections for this must be of two-channel design. Since a mixed operation of single-channel and dual-channel safety elements was out of the question, all existing safety elements such as doors, safety light barriers and emergency stop buttons had to be upgraded to dual-channel.

Since the entire conveyor system was now protected with safety technology, a way had to be found to ensure that the system operators still had easy and safe access for manual interventions without the entire system always shutting down.

It became necessary to segment the existing system into safety areas that could be separately switched off and acknowledged again. This required a massive rewiring of the existing emergency stop circuit with PNOZ relays.

The drives were switched on and off using standard motor contactors. In the event of an emergency stop, the PNOZ switched off not only the control voltage for the motor contactors but also the main contactor, so a second channel was already available here. However, if this technology had been used further, it would have meant retrofitting a separate main contactor for each new safety area to be created in order to achieve a two-channel shutdown.

For all work in the field and in the control cabinets, only a repair break of three weeks was available. During this time, the existing control cabinets would have had to be massively rewired. At the same time, the safety components would have had to be installed in the field and also rewired to the central control cabinets. Then the re-commissioning of the existing and the re-commissioning of the modified electrical engineering, control software and Scada system would have had to be carried out.

The solution concept

Together with the client, we defined the available time during the repair break as the biggest resource bottleneck. Therefore, all work had to be aligned in such a way that this resource was used the least. We decided on the following conversion plan for the three-week repair break; all work was to be carried out during one shift in the period from 07:00 to 16:00:

  • Week Mon-Thu: All mechanical and electrotechnical work, Fri/Sat standby.
  • Week Mon-Thu: Hardware and signal check with commissioning of existing and new technology, Fri/Sat reserve
  • Week Mon-Thu: test runs, Fri/Sa reserve
  • After that, the three-shift regular operation has to run regularly again.

In order to implement this demanding schedule, we had to rethink. Together with the customer, we developed the following concept:

Switch cabinets

For the switch cabinets, we followed a 0-fault policy. Therefore, the control cabinets are not rewired during the repair break. Instead, they are rebuilt and simply replaced. This massively reduces the time needed during the repair break and eliminates the risk of troubleshooting due to possible wiring errors during the repair break. In order to get fault-free control cabinets on the system, the control cabinets are pre-tested and commissioned by the PLC programmers at the control cabinet builder’s premises together with the final control and safety software.

No conventional motor contactors are used, but Siemens motor starters with F-functionality that are mounted on safety-capable Siemens ET200SP and connected to the CPU via Profinet/Profisafe.

The existing Siemens CPUs S7 416-2DP are replaced by S7 416-3F PN/DP.

The I/O assignment of the new ET200SP will, as far as possible, be adapted to that of the existing ET200S in order to avoid unnecessary address adjustments in the control programme.

A new Profinet network is drawn from the central CPU cabinets to the associated decentralised control cabinets and field distributors.

Field distributor

The Profibus-DP connections of the existing ET200S are replaced by safety-capable Profinet connections.

The field distributors are retrofitted with Safety F-DI input modules.

The emergency stop buttons, safety door contacts, safety light barriers and acknowledgement buttons are connected in two channels to the F-DI modules of the field distributors.


  • Preparatory work
  • Laying the new Profinet cables during running production as far as possible.
  • Testing and commissioning of the new control cabinets at the control cabinet manufacturer, christening of the F-modules
  • Pre-testing the extended control software as far as possible in the office.

Ensure that all required material is available, all safety elements as well as cables, assemblies for retrofitting the field distributors, etc.

Week 1

  • Switch cabinets: The wiring of the existing switch cabinets was removed in such a way that the wires remained on the old terminal strips for the time being. Then the old cabinets were removed, the new cabinets installed and the outgoing cables reconnected. The only new cables were two new Profinet cables for a safe ring.
  • Field
  • Fitting of fences and safety light barriers
  • Replacing old emergency stop buttons with new two-channel ones
  • Laying the missing Profinet cables
  • Two-channel wiring of the existing and new safety elements
  • Retrofitting the field distributors with PN connections and F-DI modules

Week 2

  • Reserve Fr/Sa was not needed
  • Signal check and hardware commissioning of the field components
  • Elimination of isolated terminal faults
  • Adjustment of the safety light barriers
  • Function test of all safety elements in a 4-eyes-principle with supplier and customer, with checklist and documentation of all tests
  • Reserve Fr was needed


  • Test runs partly with manually placed pallets
  • Test runs partly with regular production
  • Readjustment and improvement of the settings of the safety light barriers
  • Complete final function test of all safety elements in a 4-eyes-principle with supplier and customer, with checklist and documentation of all tests.
  • Reserve Fr/Sa was not required
  • Start-up support
  • Production was immediately restarted in three shifts.
  • Production was accompanied in two shifts
  • Optimisation of various parameters in the control programme, for example for the automatic muting of the SI-LS
  • Extension of the emergency stop system by integrating further adjacent machines
  • Instruction of the operating personnel
  • Instruction of the maintenance staff in the new technology


The project for the expansion of the safety technology ultimately also became a modernisation project for the complete control cabinets.

On the one hand, these were unplanned costs. On the other hand, the more than 20-year-old electrical engineering was modernised at the same time. This task would have been due in the next few years anyway. This is an investment in the future of the company.

The operator now has a conveyor system with state-of-the-art hardware, software and safety technology.

All safety elements are dual-channel, including those to neighbouring machines. The safety technology was programmed in Safety S7.

Thanks to the clever segmentation of the system into safety areas, the system operators can still work without obstructions.

The Scada system ProLeiT Plant IT was also modernised and is now state of the art again.

A+K’s scope of services

  • Advice on modernisation, electrical engineering and safety technology
  • Support in the design of the electrical plans
  • Design of the safety areas and the emergency stop area
  • Programming of the safety technology Siemens Safety for S7 400
  • Upgrading of the existing S7 400 control programme and operator panels
  • Upgrading the SCADA system ProLeiT Plant IT from V7 to V9
  • Preliminary IBS of the control cabinets at the control cabinet manufacturer
  • Commissioning of the new safety technology, control programmes and Plant IT V9
  • Start-up support with instruction of the operator’s employees

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