One control system for safety and process functions
The competency of the plant and machine builder, plus the skill and specialist knowledge of the purchaser are in demand when the relevant machinery is bought new, but where retrofit measures are concerned, it is the design engineers that are faced with a wide range of challenges.
Retrofits and refurbishments in the press area generally involve “significant intervention” into existing, but outdated safety concepts. On the one hand the design engineer has to take into account the requirements of the applicable directives and standards, on the other he is expected to look for innovative automation solutions and to take advantage of any potential savings. The press shop at Ford in Saarlouis, Germany has demonstrated that it’s possible for a balance to be found.
The task there was to upgrade two hydraulic die-spotting presses with a pressing force of 250/50 tons. Built in 1967, the two presses operate in the tool room within the press shop.
The main task of the presses, also known as try-out presses, is to test new press tools in order to optimize them where necessary. After operating for almost 40 years, the mechanics were still completely intact, but the hydraulics and electrics were in need of renovation.
Programmable safety system also performs process control functions
In the search for potential savings, the current market trend — to integrate standard and safety-related functions — presented an economical solution for upgrading the presses. A modular Pilz PSS programmable safety system was already available to monitor the safety functions. The safety system was developed in accordance with IEC 61508 and meets the very highest safety requirements — Category 4 of EN 954-1 and SIL 3 of IEC 62061. A few months previously, all the PSS-range programmable safety systems had been fitted with a more powerful CPU (Central Processing Unit). This process improved its performance by a factor of five, while also quadrupling the program memory. Time-critical processes present no problem in view of the short reaction time. This enhanced performance also creates additional capacities for the PSS standard section, providing greater potential for process control functions.
An opportunity that Hans Josef Richner from the Press & Automation Engineering group at Ford in Cologne was quick to recognize, “We very quickly realized that we have a lot more options with a programmable safety system of this performance class. While the application area of the PSS was previously restricted to safety-related sensor and actuator technology, with its enhance performance the programmable safety system can also now undertake all the complex standard control functions.
The cost saving is immediately apparent because we no longer need the standard PLC in addition to the PSS, nor do we need the standard fieldbus system.”
Software blocks simplify press upgrades
When modernizing the presses it was necessary to take into account not just the legal requirements but also the specifications of the end customer, Ford.
“Even when modernizing presses, or in fact particularly when modernizing presses, one is subjected to considerable cost pressures. This forced us to make way for new solutions”, says Ralf Bratz, managing Director of Bratz Engineering GmbH in Wuppertal, who won the contract to upgrade the presses. “The PSS with the new generation of CPU enabled us to develop additional potential savings. Quite apart from the reduced hardware costs, there is now only one programming tool to use”, Ralf Bratz added.
The old press controllers, including the hydraulic pumps, were renovated on both the diespotting presses. The guard rail for the 250 ton press was also renovated; the slide is now secured using a Sitema clamping device.
The safety section of the PSS monitors various functions such as controller ON/OFF, the ESTOP pushbutton, the start-up disabler and guard monitoring, plus control of the two-hand operation. Also monitored are the press valve controls DOWN/UP, the guard rail, the guard control and the “slide locked” indicator field. The cost of the press upgrades was kept in check by using special software blocks available for the PSS. These software blocks have already been approved by BG and TŰV and help to simplify commissioning and CE certification.
They are incorporated into the PSS WIN-PRO programming tool via simple drag-and-drop.
The standard section of the PSS controls and monitors hydraulic pumps and hydraulic functions, as well as controlling the position of the stroke setting and slide. This is achieved with one absolute encoder each for switching from rapid traverse to creep speed. Even the issue of “preventive maintenance” was taken into account by performing a test on the guard rail (slide lock) via a key switch and two-hand button. The PSS standard section also records and outputs process variables (e.g. slide path). An operator terminal from the PXT range is used for operator guidance; this also enables detailed fault diagnostics to be displayed in the form of plain text error messages. Process variables can be input and output via 25 function keys and a four-line text display.
Apart from the two presses at the press shop, the first of two overhead cranes was also modernized with a PSS 3000 to take care of both the safety-related and standard control functions.
The overhead crane was built in 1967, with a crane track width of 33.5 meters and an ultimate load that can be switched from 13 to 33 tons. Both overhead cranes transport blanks to the presses.
As with the die-spotting presses they were in urgent need of modernization or else they would sooner or later need to be replaced by a new system. Here too, electronics have now replaced relay-based technology.
“The good experience we had when retrofitting the presses meant that the decision to renovate the overhead crane without a separate standard PLC was an easy one”, said Reiner Freichel, master electrician on the press shop maintenance team at Ford Saarlouis.
“As a control system for both safety and control, the PSS performs every function”, he concluded.
Whether cranes, presses, blank loaders, stackers, belt conveyors, etc. — the purchase of almost any plant, machinery or production equipment in press shops involves a very high investment, which takes decades to pay for itself. So a retrofit/refurbishment based on modern safety technology should always be a worthwhile venture.
Without doubt the greatest challenge lies in resolving the safety-related requirements in compliance with the relevant standards. As proven in many cases under real conditions, the performance of modern programmable safety systems gives you the option to cover process control functions at the same time, thereby saving costs.