The Importance of Vacuum Flow in Robotic Material Handling Applications
by Andy Lovell
, Application Engineer
PIAB USA, Inc. Posted 05/03/2006
Industrial manufacturing is undergoing a revolution with fresh opportunities and challenges. Average profit margins have declined, putting increased pressure on manufacturers to come up with innovative cost saving alternatives to increase productivity.
From a global perspective, demand for cost efficient engineering is at a premium, as with the trend toward automation and ‘‘lean’‘ manufacturing. These challenges have severely impacted the importance of proper design as it relates to all manufacturing processes, and companies that will remain profitable throughout this revolution are those that recognize the importance of constantly evaluating new technologies to facilitate greater efficiencies.
Improved Productivity through Optimized Vacuum System Design
One of the ways many companies can improve material handling productivity is through the optimization of vacuum system design. Vacuum pressure, created by any pressure lower than atmospheric pressure, is the method of choice for keeping the majority of industrial products moving.
Although much is written and understood about the importance of vacuum pressure, the performance of robotic material-handling systems also depend on vacuum flow. In fact, ensuring adequate vacuum flow is as critical as providing sufficient vacuum pressure. This is because although the holding force of a suction cup depends on the vacuum pressure, it is the level of vacuum flow at the cup that determines how quickly and securely the grip is established.
System Architecture and Vacuum Flow
A suction cup adheres to a surface when the pressure between the suction cup and the surface of the object is lower than the surrounding pressure (atmospheric pressure). To create the low pressure, the suction cup is connected to a vacuum source. The rate of flow from the vacuum source to the cup determines how quickly the desired vacuum pressure is reached, and after any leakage, how quickly this level of pressure can be restored.
As will be illustrated in the latter portion of this article, the architecture of the vacuum system can significantly affect the level of vacuum flow that reaches the cup, especially at the beginning of each cycle. This initial flow is critical to material handling operations; high vacuum flow must occur at the point in time when the cup first makes contact with the part and starts the gripping action.
Importance of a Quick and Secure Grip
Initial flow is defined as the rate of vacuum flow during the time interval from the beginning of the cycle, prior to applying vacuum, until the point when the vacuum pressure reaches the level necessary to achieve a stable grip that has the required holding force. Typically, this interval is the time when the vacuum pressure in the cup rises from 0 to 3’‘ Hg. In the case of bellows-type cups, the initial flow must bring the pressure to about 6’‘ Hg in order to collapse the cup and establish a stable grip.
The higher the vacuum flow at the cup, the faster a stable grip will be established. The faster gripping action reduces cycle time for robotic material handling applications. The robot can go into motion more quickly, minimizing ‘‘dwell time’‘ while the load is secured. Elimination of such waiting time is a prime example of the reduction of unproductive muda (waste) at the core of the Lean Manufacturing management philosophy originally promoted by Taiichi Ohno, the father of the Toyota Production System.
Quick grabbing action is especially important for high-speed material-handling applications, where the cup would otherwise miss picks or drop parts.
High vacuum flow is useful not only for establishing the original grip, but also for recovering from any leakage, compensating for the micro leaks that exist with porous and rough surfaces.
High initial flow and quick gripping action also extend the useful life of the suction cups. In applications where there is horizontal motion of the cups against the part’s surface, the cups will ‘‘drag’‘ across the surface, causing friction and cup wear. Quick gripping action significantly reduces such wear. In addition, when wear does occur, higher vacuum flow can compensate for the increased leakage, and thereby extend the useful life of the worn cup.
Centralized Systems Reduce Initial Vacuum Flow
The overall architecture of the vacuum system significantly affects the strength of the initial flow at the suction cups.
Diminished and delayed initial flow at the suction cups is especially challenging for ‘‘centralized’‘ vacuum system architectures, consisting of a single vacuum source.
Historically, this was the only possible architecture – dedicating a remotely-situated mechanical vacuum pump to a single material handling machine (with multiple suction cups), or dedicating one pump for multiple machines or even for an entire factory. Sometimes the vacuum pump is located quite far from the points-of-use, due to space constraints, maintenance issues, or the desire to isolate the noise and heat generated.
A fundamental challenge with this centralized approach is that the vacuum source is set at a distance from the suction cups, and the intervening network of tubing, valves and manifolds adds considerable volume to this system. This additional volume needs to be evacuated, and then returned to atmospheric pressure, during each cycle. The necessary evacuation of the tubing network causes delays in the development of the initial flow at the suction cup, resulting in poor gripping performance and prolonged cycle time.
Restriction in Tubing and Fittings
These problems are exacerbated by flow restrictions in the tubing and fittings that can seriously reduce flow through pressure drop. Restriction through tubing and fittings is probably the greatest factor in reducing system performance and reliability for centralized vacuum system architectures. To mitigate flow restrictions, the internal diameter of tubing and fittings must be sufficiently sized. This seems like a paradox in that larger tubing, fittings and manifolds add volume to the system, yet it is imperative to have as little pressure drop and flow restriction as possible to maintain the benefits of high flow through the system. The increased volume due to larger fittings and tubing is generally insignificant versus the increased performance due to unrestricted flow.
The issue is not only that the initial flow at the cup is diminished and delayed compared to the flow at the vacuum source, but also that the initial flow can vary from cup to cup, depending on the proximity to the source and the presence of any restrictions. This variability causes erratic performance and troubleshooting problems.
Contrary to popular belief, installing larger vacuum pumps does not solve the problem, but merely increases the energy consumption of the system with little additional benefit at the cups.
Decentralized Vacuum System Design and Multi-Stage Ejectors
An alternative, ‘‘decentralized’‘ architecture became possible in the 1970s, with the advent of relatively small, air-driven vacuum generators that could be placed close to the suction cups. Tubing is used to bring compressed air to each vacuum generator, and this tubing remains constantly under pressure, without the risk of the type of restrictions that vacuum tubing experiences.
In a decentralized architecture, only the small volume between the vacuum generator and the suction cup needs to be evacuated and returned to atmospheric pressure during each cycle. With little or no tubing separating the suction cup from the vacuum source, the problems with line loss and pressure drops are minimized or eliminated.
However, the benefit of eliminating line loss and pressure drops is moot if the vacuum source itself does not produce adequate initial vacuum flow. Unfortunately, single-stage vacuum generators do not produce sufficient initial flow for most material handling applications.
New Technology Increases Flow
With the introduction of multi-stage compressed-air-driven generators, it is now possible to produce the high initial flow required for material handling. In fact, the new coaxial style multi-stage vacuum ejector cartridges, from PIAB Vacuum Products, have been reduced to the size of a pencil and made of light weight composite materials, providing three times more flow than a conventional ejector with the same air consumption. Additionally, these vacuum cartridges begin producing vacuum immediately when the pressure valve is turned on, ensuring a fast and secure grip.
The increased flow, with better gripping action and recovery from leakage, adds to the reliability of the vacuum system and gives a higher safety margin. Because less volume needs to be evacuated, the vacuum pumps can often be downsized without affecting performance, and less operating energy is be expended.
To ensure quick and secure gripping action in robotic material handling applications, system designers need to maximize the initial vacuum flow at the suction cup. Traditional centralized architectures necessitate compromises that delay and diminish initial flow at the cup.
Decentralized architectures theoretically provide a superior solution, because there is less volume to evacuate each cycle and reduced risk of restrictions. This advantage of decentralized design can be realized in practice if the vacuum ejectors are designed to produce sufficiently fast and powerful flow. The latest generation of multi-stage ejectors is compact enough to be attached directly to the suction cups and powerful enough to ensure fast, reliable, and safe performance.
Editor’s note - The article’s author, Andy Lovell, is an application engineer at PIAB USA, Inc. He welcomes questions and comments at (800) 321-7422. Visit www.piab.com to learn about ‘‘Optimized Vacuum Management (OVM),’‘ a series of scheduled courses on vacuum system design offered by the PIAB Vacuum Academy (PVA™).