It is certainly possible to identify several trends in the machine tool industry. If we take a look at presses in particular, the transition from mechanical and hydraulic presses to electric presses—servo presses—is well under way.
Two key trends are having a particular impact on the way presses are constructed— the demand for increased productivity and energy efficiency. Fitting concepts and products that are able to satisfy such requirements are underway, such as Sick AG’s Motion Control safe drive monitoring.
This means optimised forming processes, shorter retooling and cycle times, and the resulting optimisation of costs. Particular attention is paid to the safe and optimised interaction between man and machine.
The call to switch over to more energy-efficient drive systems has been on the radar since June 2011 when standard IEC 60034-2 came into force. These statutory requirements have also brought about additional potential for press manufacturers to tap into.
The way in which process energy is generated with servo presses compared to hydraulic presses is becoming much more affordable and means that servo presses are less susceptible to loss. Their efficiency factor is therefore higher and energy efficiency is increased.
Electric drives are easy to disconnect from the energy supply and do not experience problems when they are restarted, as in comparison to hydraulic drives, the process is not subjected to any temperature influences from the drive medium. In addition, power and speed profiles can be run separately. State-of-the-art drive and control technology enables servo presses to meet the increased demands for quality and process efficiency.
The Rise Of Servo Presses
The machine tool industry is one of the five largest branches of mechanical engineering and covers the areas of forming and cutting machine tools. Whereas in the past, safety technology was primarily incorporated into the machine as a step at the end of the design phase, it is now increasingly becoming a central element of control design that helps increase the performance of the machine.
“The state of the art is constantly changing,” explained Mathias Ams, product manager for Sick’s sens: Control—safe control solutions sector. In the past, hydraulic drives and mechanical eccentric presses were the market leaders when it came to presses. Now, however, servo technology with electric drives— driven by the demand for more efficiency, flexibility, and lower energy consumption —is making increasing strides forward.
In contrast to other types of machines, such as CNC processing machines, machines such as servo presses are not fully integrated in safety standards. For CNC machines, the requirements regarding safe actuators have already been incorporated into the standards, thereby providing support to the ideal compromise that is being sought between safety and efficiency. There are of course defined operational statuses which permit the operator to access the machine, but the movement of the actuators does need to be monitored and limited.
For servo presses, we are currently still using the empirical values obtained from the CNC sector. Based on existing principles from standards and experience as well as applicable regulations, including those relating to presses, new and improved machine concepts could however be established here. This would also enable the transition to new technologies to take place.
Not limited to press machines, drive monitoring enables an
operator to reach into a machine at reduced risk without
having to power off.
To allow the advantages of drive and automation technology to be fully exploited in a press, we need to rethink our approach to safety technology. The speeds, forces, and torques that arise can be controlled more precisely, more dynamically, and in a decoupled state with servo drives, allowing ideal force motion profiles to be achieved.
This results in optimised processes for aspects such as material thickness and flow behaviour, especially when working with complex part geometries. Forming processes are optimised, quality is enhanced, and waste is minimised. Moreover, in contrast to hydraulic presses, for instance, the production environment and the workpiece are kept free of oily residue.
With conventional presses, retooling them when switching from one production process to another is often time-consuming, as the switch cams require mechanical adjustment. With servo presses, there is no need to do this and the press can be retooled by making adjustments in the control program.
Thanks to safe speed monitoring, Motion Control technology allows the new control program to be commissioned and tested at a minimised level of risk and at times when the protective device is deactivated. The reduction in tooling times that this achieves leads to an increase in the machine’s efficiency, particularly in the case of smaller production batches.
Shorter Cycle Times
Drive monitors are able to safely monitor
existing motor feedback signals and any
encoders that are present.
Safe monitoring has moved on from the simple safety light curtain or safety switch to an all-encompassing safety concept that includes the drive function in the safety controller. This allows machines to be used efficiently during maintenance, servicing, and production.
Thanks to safety technology concepts, Motion Control allows you to monitor the movement of a machine at any time. In addition, all the signals from the safety sensors and actuators can be combined. Additional components, such as sensors or encoders, can also be supplied for the entire machine.
The drive monitor FX3-MOC0 works together with the Flexi Soft safety controller to monitor presses. This means that all types of drives can be monitored, allowing independence from the drive manufacturer (subject to regional technology and market requirements).
The drive monitor is able to safely monitor existing motor feedback signals and any encoders that are present as well as integrate plausibility signals from the process controller. The information can be used to determine whether there is an actual risk for the machine operator when accessing the hazardous area.
This means that unintended shutdowns are avoided, cycle times are reduced, availability is increased, and productivity is raised. The return stroke can be safely monitored, as no hazardous movement is taking place and the hazardous area can be accessed before the end of the cycle is reached.
The combination of safe drive functions and the benefit provided by all the information that is available from the process enable test functions such as mechanical brake tests to be optimised. “Combining these individual optimisations in the production process allows cycle times to be reduced by 25 percent or more,” said Mr Ams.
The reason why the drive monitor is particularly suited to servo presses is demonstrated by the variety of drive safety functions contained in IEC 61800-5-2. These range from “safe stop 2, SS2”, “safe operating stop, SOS”, and “safely limited speed, SLS”, right up to “safe direction, SDI”.
Using these functions in combination with sensor signals and process signals paves the way for making ideal use of the technological innovations offered by control and drive technology.
For example, when a protective device such as a light curtain is accessed, an SS2 can be triggered. The press is stopped electrically instead of mechanically and the cycle can be continued immediately without the need to reference the system again. SDI is the basic function that enables access at the hazardous point of the press for tools with a return stroke, thereby allowing cycle times to be reduced.
These are just a few examples that demonstrate the wide-ranging potential of the technology. PSDI is an efficient way of operating a press, as cycle control is already initiated when the press is accessed.
Conventional presses use cam switching mechanisms for PSDI control, which are not available in modern servo presses. This is another area where the drive monitor could help, as movement monitoring can be used to derive switching signals for controlling PSDI, without the need for additional sensors.
As the drive monitor is able to monitor wide range of drive types, it is very easy to transfer these concepts from electric to hydraulic drives. This allows a high degree of flexibility to be achieved and therefore also independence when it comes to selecting suitable drive concepts and manufacturers.
APMEN Feature, May/June 2017