Optimized vacuum system design improves productivity

Andy Lovell -- Control Engineering, 11/1/2006

Vacuum pressure (pressure lower than atmospheric pressure) is the method of choice for handling the majority of industrial products. The performance of robotic material-handling systems depends on vacuum flow as well as vacuum pressure because, although the holding force of a suction cup depends on the vacuum pressure, vacuum flow at the cup determines how quickly and securely the grip is established.

Centralized vacuum system architecture uses one centrally located vacuum source, which leads to problems with initial vacuum pressure strength and timing.

Compact air-driven vacuum sources can be placed near grippers they power, thereby improving overall handling-system performance.

A suction cup adheres to a surface when the pressure between the suction cup and the surface of the object is lower than atmospheric pressure. To create this low pressure, the suction cup is connected to a vacuum source. The flow rate 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.

Vacuum system architecture can significantly affect the level of vacuum flow that reaches the cup, especially at the beginning of each grip/ungrip cycle. Initial flow is the rate of vacuum flow during the time interval from the beginning of the cycle until the vacuum pressure reaches the level necessary to achieve necessary holding force. In the case of bellows-type cups, the initial flow must bring the pressure to about 6 in_Hg 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. Faster gripping action reduces cycle time for robotic material handling applications.

High vacuum flow is useful for establishing the original grip and for recovering from any leakage. The vacuum system has to scavenge any air leaking through microscopic leaks in porous or rough mating surfaces.

High initial flow and quick gripping action also extend the useful life of the suction cups. Horizontal motion of cups against the part's surface causes 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 a worn cup.

Overall architecture of the vacuum system significantly affects initial flow at suction cups. Centralized vacuum systems consist of a single vacuum source connected to multiple vacuum cups through long tubes. 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. Historically, this was the only possible architecture.

The network of tubing, valves and manifolds adds considerable volume, which needs to be evacuated then returned to atmospheric pressure during each cycle. In addition, tubing and fittings introduce restrictions that seriously reduce flow.

Tubing and fitting restriction is probably the greatest factor in reducing system performance and reliability for centralized vacuum system architectures. To reduce flow restrictions, the tubing and fitting internal diameters must be increased. While this adds a performance penalty due to increased system volume, it is generally insignificant compared to the performance increase from reduced flow restriction.

In addition to reducing initial flow at the gripper, having different tube lengths causes flow to vary from cup to cup. Such variability causes erratic performance and troubleshooting problems.

Simply installing a larger vacuum pump does nothing to solve the problem. Since the ultimate vacuum at the pump inlet is generally very close to the maximum theoretically possible (32 in_Hg), system performance at the cup depends almost entirely on tube volume and restrictions. Increasing the pump size merely increases energy consumption.

Decentralized vacuum system 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. 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 reduced.

However, the benefit of eliminating line loss and pressure drops is moot if the vacuum source does not produce adequate initial vacuum flow. Unfortunately, single-stage vacuum generators do not produce sufficient initial flow for most material handling applications.

Multi-stage compressed-air-driven generators produce the high initial flow required for material handling. For example, some multi-stage vacuum ejector cartridges have been reduced to the size of a pencil. They 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 improves vacuum system reliability and increases gripping safety margin. Because volume that needs to be evacuated is low, it is possible to reduce vacuum pump size and energy consumption without compromising performance.