Future trends point to mobile, autonomous and energy efficient systems.
Contributed by Harald Kuolt, Functional Coordinator Pre-Development, J. Schmalz GmbH, Glatten, Germany
Automated production is the key for marketable products. Automation technology makes it possible to cut process costs, increase output and productivity, and raise quality—all of which lead to highly efficient production and better, less expensive and readily available products. That is why the market for automation systems and industrial robots is on the rise.
And in any automated production system, handling is a critical function. In general, users are interested in process-safety with suitable accuracies without influencing or damaging the workpiece. Add to that are high uptime for equipment, short cycle times, and lower up-front investments and running costs.
Especially for gripping and clamping of workpieces, vacuum automation plays an essential role across virtually all industry segments for just these reasons. The core part of a vacuum gripping system is vacuum generation, and pneumatically driven vacuum generators have for years been widely used in handling applications.
That may be changing. Recent technological developments have led to new types of automated systems, such as mobile handling platforms and human-robot collaboration. Such applications will make compressed-air powered vacuum systems less desirable or unavailable. Here’s a look at how electrically driven vacuum generators can overcome the challenges and increase the energy efficiency of future handling systems, and also influence new forms of work.
Vacuum handling efficiency
To ensure customer value during a vacuum handling process, first, the workpiece has to be placed quickly and safely at its target position. Therefore, suction cups must be supplied with the required fluid power. This is influenced by short evacuation times, good sealing conditions, minimal leakage, short lengths of tubing and hoses, and low flow losses within the vacuum gripping system. All these factors also play an important role regarding energy efficiency and operating costs.
Energy efficiency in the field of vacuum handling technology is still low, due to the need to convert electrical energy into compressed air, and then compressed air into vacuum. In addition, the infrastructure, workpiece, suction cups and, of course, the process parameters all influence energy consumption of a vacuum gripper.
Efforts to improve the energy efficiency of vacuum handling systems are still relatively unexplored. A handful of research projects have delved into this topic, and pneumatically driven vacuum generators as a key component have been the focus of a number of studies. But due to features like air-saving functions that can influence the handling process, these research results are not suitably meaningful to evaluate entire handling processes under real production conditions.
Trends in automated production
Nowadays, the borders between fully automated processes done by machines and manual handling tasks done by humans have become blurred. Instead, many activities now involve what is called “human-machine interaction.” This means that production personnel and machines are working (temporarily) together in the same physical space.
Robots used for these kinds of applications are typically small and with a lightweight construction. In the field of “autonomous warehousing,” they are even used on mobile platforms to handle goods, for example for stocking shelves or pulling products for shipment.
In the scope of vacuum handling there is one important restriction: mobile applications cannot carry a compressed air line. The same situation holds for stationary, lightweight robots with built-in sensors. It is not possible to mount a compressed air line at the robot, because this would influence the sensors and would interfere with the robot’s functions.
So when operators of handling technology want to use vacuum handling and its benefits in the future, new approaches are needed to generate vacuum without compressed air. An initial technology study carried out by Schmalz in 2013 showed approaches with no external energy supply for handling applications. The core element was a so-called “push-push mechanism” including a piston actuated by robot movement in a vertical direction, when the gripper was placed on the workpiece. To set the workpiece at its target position, the mechanism had to be actuated again. This solution worked well when handling airtight, rigid workpieces without leakage. However, for industrial applications where leakage or other effects influence the vacuum handling process, other designs are necessary.
Before mobile robot platforms came on the market, users believed that electric-driven vacuum generators were only practical in handling solutions if the performance — primarily the vacuum level and flowrate — is comparable to that of pneumatically driven vacuum generators that operate based on the venturi principle.
In general, standard air-powered vacuum generators can be mounted on the robot structure, connecting arms or peripherals, or even near a single suction cup. The closer a suction cup is placed to the vacuum generator, the more the size and weight of a vacuum generator affect the moment of inertia and the total weight of the gripper. An additional benefit of placing the vacuum generator near to the suction cup is that less airflow is required to manage the handling solution. If the suction cup is near the vacuum generator, less inner volume of the hose has to be evacuated compared to when the distance between cup and generator is greater, especially if multiple suction cups are supplied with vacuum.
Standard electrical-driven vacuum generators—for instance, a piston pump or rotary blower coupled to an electric motor—are not suitable for direct use on the gripping system. They can only be placed near the robot due to the weight and performance. However, this only applies if performance is based on the maximum air flow rate at free flow. As mentioned, a correspondingly greater airflow is necessary to achieve short evacuation times and, based on that, short cycle times in the application.
But if the volume to be evacuated can be limited by reducing the inner volume of suction cups, hoses and other components, smaller electrically driven vacuum generators like membrane pumps can achieve comparable—or at least acceptable—evacuation and cycle times. The smallest pneumatically driven vacuum generator based on venturi nozzles generates a suction flow of about 35 lpm at free flow. Small, lightweight membrane pumps, which are generally suitable for a vacuum application near the suction cup, can generate a maximum airflow of about 50% of this value.
Especially if supplying compressed air to a machine is difficult or even impossible using traditional means, there are new applications using electrically driven vacuum generators which were previously not possible. Further, such developments are leading to new discussions concerning energy efficiency and necessary airflow-rates in “standard” applications. Even in conventional cases where compressed air is readily available, engineers can reduce the airflow rate in free flow conditions without a loss of performance if application requirements can be satisfied, much like standard solutions.
Another consideration is that in standard vacuum ejectors based on the venturi principle, compressed air is flowing at sonic speed. The flow conditions in ejector nozzles can generate tremendous noise emissions. When there is no additional silencer mounted on the ejector, this can lead to hearing impairment of workers. That is why regulations typically mandate the use of silencers. Silencers reduce noise, however they also influence performance because a silencer creates a flow resistance.
In applications where humans and robots are working together, a pneumatically driven vacuum ejector with a high noise emission level could be dangerous in the immediate proximity of the worker. Also in such conditions, small electrically driven vacuum generators have benefits in comparison to standard ejectors, because membrane pumps do not have extremely high flow rates and noise emission levels are lower than
those of ejectors.
Pneumatically and electrically driven vacuum generators are routinely used in automated production processes. However, standard electrically driven vacuum generators are considerably larger and heavier than are ejectors with a comparable performance. That is why these kinds of electric-drive vacuum generators cannot mount directly on a gripping system.
Normally, these generators are installed near the robot or handling device. In such cases, vacuum hoses are necessary to supply the suction cups on the gripper system, with vacuum generated next to the robot. This leads to greater volume inside the hoses and other components, which must be evacuated during each cycle of the handling process.
In mobile applications, this kind of vacuum generation is not possible. Thus, other approaches are necessary. Engineers should first determine which electrically driven principle is suitable for a given handling application, so the gripping system ensures a sufficient vacuum level and the highest possible airflow rate.
Basically, the air flow rate at free flow is not particularly important in an application. It is just necessary that short evacuation times and, in the end, short cycle times can be achieved. That’s why smaller electrically driven vacuum generators are suitable if the application performance is comparable or acceptable in comparison with the standard solution.
The overall design matters, too. The closer a suction cup is placed to the vacuum generator, the lower the volume inside hoses, suction cups and other components, which all must be evacuated. This is especially important in mobile applications because gripping systems can be built smaller and with less weight. This is also the focus of future work concepts, like human-machine-interaction, where people and robots work together safely without protection devices.
The need for vacuum handling in cobots, mobile platforms, and similar applications will increase tremendously in the future. Successful installations are just not possible with traditional pneumatic-powered vacuum systems. Therefore, electrically driven vacuum generators are necessary and, fortunately, are now ready for the market.