Automated picking and sorting of mixed packaging is challenging for intralogistics applications. Knowledge of the fundamentals of vacuum gripping can help overcome those challenges, improve throughput and lower labor in mixed-packaging environments.
Mechanical grippers will always be of use, but in many situations—such as the confined spaces of totes and bins, large and small packaging mixed together, hard-to-grasp packaging such as bags or fragile packaging—vacuum end-of-arm tools can provide the best solution. Vacuum grippers can also rapidly pick mixed packaging lying in various orientations from a moving conveyor.
There is a learning curve for original equipment manufacturers (OEMs) designing a vacuuming gripping system and for end users maintaining one. To be successful at automated vacuum gripping requires a fundamental understanding of the basics of vacuum and a strategy for partnering with suppliers.
Fundamentals of vacuum
Vacuum refers to a pressure level lower than atmospheric pressure. Vacuum is measured in units such as millibar (mbar) or pascal (Pa). Understanding the relationship between pressure, the force exerted by a gas or fluid on a surface, and vacuum is important to vacuum generation.
When planning for a vacuum gripping application, one of the first questions to ask is whether compressed air is available where the system will be operating. Compressed air access enables the use of Venturi style vacuum generators to be part of a potential solution. If no compressed air is available, the application will require an electrically driven pump to maintain a vacuum.
Venturi generators driven by compressed air provide high pressure low flow, while vacuum pumps deliver low pressure high flow. If the application involves highly porous material, such as recycled corrugated cases, then electric vacuum pumps are often the better choice. Porosity is essentially a leak in the system and high flow compensates and maintains the grip. For non-porous packaging such as plastic bags with little to no leakage, a Venturi system would be best, as pressure, not flow, is desired. In mixed packaging applications— such as with plastic, foil, corrugated or paper—pumps may be the most versatile choice.
There are two different types of vacuum pumps typically used in automation.
- Positive displacement pumps: These pumps physically capture and remove air from the system. Examples include rotary vane pumps and screw pumps.
- Momentum transfer pumps: These pumps use high-speed jets or rotating blades to transfer momentum to air, pushing them out of the system. Examples include centrifugal and regenerative blowers.
Vacuum part performance
The choice of generator or pump is application specific and should be arrived at after testing the expected mix of packaging and evaluating for porosity, size, shape, weight and configuration. The vacuum gripping system will be sized for the maximum grip required for the package mix.
Vacuum valves play a crucial role in controlling the flow of gas into or out of a vacuum system. Vacuum valves enable the isolation, regulation and modulation of vacuum levels. Common types of vacuum valves include gate valves, ball valves, butterfly valves and throttle valves. Effective sealing is essential for maintaining a vacuum. Vacuum systems use various sealing mechanisms to prevent air leakage into or out of the system. Examples of sealing mechanisms include O-rings, gaskets, flanges and elastomer seals.
Vacuum gauges are instruments used to measure and monitor the level of vacuum in a system. Vacuum gauges provide feedback on the pressure or vacuum level and provide machine operators or the maintenance team with the information needed to maintain the desired vacuum conditions. Common types of vacuum gauges include mechanical gauges (such as Bourdon gauges) and electrical pressure sensors.
Maintaining pressure
Atmospheric pressure plays a crucial role in vacuum generation. By creating a pressure difference between the inside of a vacuum system and the surrounding atmospheric pressure via a pump or through compressed air systems, a vacuum is generated. The greater the pressure difference, the higher the level of vacuum achieved and consequent grip. The maximum vacuum is produced at sea level and is 1 bar (14.7 pounds).
Detecting and addressing leaks is critical in maintaining a vacuum system’s integrity. Various techniques, such as helium leak testing, pressure decay testing and bubble testing are used to identify and locate leaks in vacuum systems.
The size of the tubes between vacuum generation and the suction cup in a vacuum system can have an impact on the holding force. High porosity packaging means larger diameter tubing will be needed. Under sizing vacuum tubes can have a negative impact on holding force.
Time taken at the beginning of the project to measure porosities will be time well spent. The OEM can check porosity with a vacuum generator, vacuum cup and a vacuum gauge. These three components measure how much leakage the system will experience and show the resultant achievable vacuum pressure. Not all corrugated materials have the same porosity, so it’s important to test a comprehensive range of cases, especially with cases made with recycled material.
The importance of suction cups
The material of the suction cup directly affects its holding power, as different materials have varying levels of friction and adhesion properties. The material of the suction cup should be compatible with the surface of the object being handled. Some materials, for example, may adhere better to certain surfaces, while others may cause damage or leave residue.
A rubber suction cup with a softer durometer and a very thin lip is ideal for providing a seal with flexible plastic or foil. The friction and flexibility of the cup will contact the surface and not allow the material to be sucked into the vacuum hole. But if the system is gripping a corrugated case, a thin, soft lip does not hold as well and will wear out quickly. The goal of suction cup selection is package compatibility, longevity in the actual warehouse environment and grip. Research is underway by the supplier community to create a universal suction cup.
Other considerations
Acceleration and deceleration versus speed needs to be understood during the design stage. At constant velocity, there is no load on the vacuum cups. Load occurs during acceleration and deceleration. Since six-axis robots are typically programmed for speed, it is up to the OEM designing the vacuum gripper to calculate load based on anticipated acceleration and deceleration and calculate the vacuum force and cup configuration required to maintain grip and not lose the package during acceleration and deceleration.
When dealing with corrugated cases, the vacuum gripping system typically relies on a vacuum sensor to ascertain that the case is being gripped. In a mixed package environment, it is a good idea to add a proximity sensor for two levels of sensing. For example, a vacuum sensor with a lower-than-normal reading may indicate a clog in the system or a missed grip. A proximity sensor can confirm whether the package is gripped or not.
Intralogistics systems with mixed packaging can benefit from the addition of vision technology because vacuum levels will not be consistent. Vision will provide a more accurate means of ascertaining whether the package is picked, missed, or separated from the gripper.
Moving ahead with vacuum gripping
Purchasing an in-house test kit of components to provide a gripping toolbox builds hands-on experience. Explain the need to a supplier partner and rely on the partner to recommend a suitable bill of materials. Having the vacuum test kit will be valuable not only for a current project, but for future ones as well. It is not always productive to perform vacuum calculations based solely on anticipated conditions. Testing the vacuum cup on the surface of a package or assorted packaging can give the designer useful information.
Partner with a company that considers gripping to be a core competency—one that offers many types of gripping components, both mechanical and vacuum. These suppliers understand the nooks and crannies of the science and the application of gripping. They have worked through the difficulties and seen hundreds of successes and failures that inform their recommendations.
Cory Knight is the Grip-It! team lead at Festo, focusing on automation projects in the Food and Beverage and intralogistics segments as well as providing expertise for end-of-arm-tooling gripping applications.
