Hardly any systems today are developed for a single product. Instead, the aim is usually to produce as many variants as possible in as short a cycle time as possible and also provide ample scope for future design and product range changes. Assembly systems have to be designed with the necessary flexibility. Pneumatic handling quickly reaches its limits in the face of such demands, especially when the systems also have to operate as quietly as possible and specifications now call for maximum noise levels close to 60 dB.

Nevertheless, pneumatic networks still dominate in assembly automation - they currently supply around 70% of all actuators. The remaining 30 % are electrically driven actuators, which are usually connected to networks with 400 V or more. In addition, 24 V technology could become the third major pillar in the future. There are a number of advantages in favor of the latter:

• Minimized engineering, installation, programming, commissioning and maintenance effort;
• Reduced need for expertise: mechanics are also authorized and able to commission 24 V systems;
• Elimination of tubing and significantly reduced wiring, enabling tidy system designs;
• Elimination of hydraulic shock absorbers, reducing the risk of unforeseen system failures;
• Reduced noise emissions;
• Reduced system vibrations.

Despite the sometimes enormous advances in pneumatic networks and components - such as valve terminals with intelligent pneumatics - micro valves that can be integrated into actuators or moderate cost prices, these will not fully compensate for the strengths of 24 V technology in the long term, meaning that a redistribution of the market is foreseeable.

A specialist for assembly applications: the 24-volt EGP small parts gripper.

© Schunk

With this in mind, Schunk has also put together a corresponding modular system consisting of grippers, gripper swivel units and linear modules. One component of the solution is the so-called ELP linear axis. Instead of setting throttles as with pneumatic modules or loading new travel sets as with electrically controlled modules, the user only has to define the end position mechanically and regulate the retraction or extension speed on two rotary coding switches with the linear direct-driven ELP - the auto-learn function takes care of everything else. Two to five strokes are all it takes to complete the programming.

During the teach-in process, the maximum possible speed is calculated for the current payload. The travel profile itself is designed as a ramp, i.e. the unit accelerates and brakes automatically depending on the total stroke. Shocks and vibrations as well as uncontrolled travel at the maximum speed of the module are therefore ruled out. If the part weight changes during the process, the axis automatically adjusts its movement profile within a few strokes without the need for operator intervention. As it is controlled via digital I/Os, the ELP series is compatible with all control systems and can easily replace pneumatic modules. In combination with small parts grippers (EGP series) and gripper swivel modules (EGS series), which are also based on 24 V technology, this results in a modular system that can be used to implement complete handling solutions based on 24 V technology.

Highly dynamic and yet gentle

The example of the ELP also shows how big the advantage over the pneumatic world is: even if users have to invest around twice as much as with pneumatic axes, the wear-free module pays off in the medium and long term, because: With a total output of 40 million cycles and a maintenance interval of 5 million cycles, the costs for wear parts, working time and production downtime alone for pneumatic axes add up to a value that far exceeds the total costs of ELP axes. In concrete terms, an assembly system with 17 linear modules, for example, results in savings of around EUR 12,000 over a service life of 40 million cycles - not including the savings on ongoing operating costs such as compressed air! Amortization is already achieved after 20 million cycles.

There are also design advantages: In practice, throttles on pneumatic modules are often opened wide enough in order to save cycle time and get the last reserves out of systems. As a result, shock absorber wear and therefore costs increase rapidly. Larger companies are now planning fixed maintenance intervals in which all shock absorbers are replaced and readjusted regardless of their individual condition. This is an attempt to prevent unplanned system failures.

And yet there are still unpleasant side effects in some places: Time and again, the massive vibrations in the periphery of pneumatic linear modules also affect other components, such as camera systems. This is precisely where the ELP comes in: It enables extremely dynamic movements and yet moves into the end position harmoniously and therefore gently on the periphery. As the compact unit does not require hydraulic shock absorbers, commissioning and maintenance costs are reduced to almost zero. Defective dampers also prevent damage to the system and long downtimes. In addition, system construction can be organized in a much leaner way, as valve islands, hoses, maintenance units and pressure boosters are no longer required. Last but not least, energy chains shrink to half their previous size, as only two 24 V circuits need to be set up for the sensors/logic and actuators. A pleasant side effect: the electric axis is around 20 dB quieter than its pneumatic counterpart.

The ELP linear axles are now also available with an electrically activated holding brake.

© Schunk

To simplify the setup of 24 V systems, the linear modules are optionally available with an electrically activated holding brake. In the event of a power failure, the brake reliably ensures the position of the linear module and prevents vertically arranged axes from falling unintentionally into the working area. Similar to lowering locks on pneumatic modules, this prevents damage to parts, system components or even people. The STO (Safe Torque Off) safety function can be implemented with the aid of the holding brake. The linear module can also be stopped at full speed.

Admittedly: The selection of 24 V handling components is still quite limited. Nevertheless, plant manufacturers and users are now showing great interest in the rapid expansion of corresponding product ranges and are already using available modules such as the ELP linear axis or the EGP parallel gripper specifically as actuators or as assembly systems in small plants and robot cells without a compressed air supply. The topic will rapidly gain in importance at the latest when the companies active in handling technology make the corresponding components available in the most important sizes, from feed technology to robots and safety technology.
Ideally, all components should be easy to program from a central control system. In addition, autonomous functions can minimize the commissioning effort. The best example of this is the ELP with its auto-learn function: as soon as the system is wired, the mechanic can put the handling system into operation even before the system is programmed.

24 volt gripper with IO-Link

The electric gripper swivel unit EGS is available in sizes 25 and 40 with gripping forces of 15 N and 140 N respectively and strokes of 3/6 mm for component weights of up to 0.55 kg.

© Schunk

The aforementioned EPG small parts gripper in a 24 V version will also be available with IO-Link in the future. In this version, it meets the 'IO-Link Class B standard', which is suitable for increased power requirements, and can be connected directly to an 'IO-Link Class B master'. Its fingers can be freely positioned for each cycle. As the gripping force can be individually adapted to the respective workpiece in four stages via software settings, handling scenarios with deformation-sensitive parts can also be implemented.

Within the permissible finger length, both the gripping force and the gripping speed remain virtually constant over the entire stroke. The position of the gripper fingers can be detected over the entire stroke using the integrated measuring system, meaning that no external sensors are required to query the position and intermediate positions or varying workpiece sizes can be queried at any time. An integrated diagnostics tool permanently monitors the voltage, current, temperature and status of the gripper and automatically transmits errors to the higher-level control system. If required, system operators can also store maintenance intervals for the system in the tool.

As the complete electronics of the gripper are installed inside it to save space, it does not take up any space in the control cabinet. Brushless and therefore maintenance-free servomotors and a backlash-free preloaded cross roller guide ensure high efficiency and constant gripping forces over the entire finger length. In the first step, the small parts gripper with IO-Link is available in size 40 with a finger stroke of 6 mm and maximum gripping forces of 140 N. The repeat accuracy is 0.02 mm. It is suitable for handling parts weighing up to 0.7 kg. The maximum finger length is 50 mm. In addition to the version with IO-Link, the 24 V gripper is still available with control via digital I/Os.

Author:
Jakob Khoury is Head of Product Marketing at Schunk in Lauffen / Neckar.