Design a Universal End-of-Arm Tool for Intralogistics Applications

Vacuum grippers hold boxes in a warehouse. Adding pneumatically based grippers would give this robot greater flexibility to pick and sort mixed-product packaging.

Fast and efficient robotic picking and sorting at distribution centers is difficult to achieve given that mixed-product packaging comes in a myriad of shapes, sizes, weights and materials. End-of-arm tool changeover is a potential solution to handling mixed-product packaging, but one that is complex, challenging and costly to implement and maintain.

The goal for original equipment manufacturers (OEMs) and system integrators is to develop an end-of-arm tool that stays on the robot and is effective and easy to maintain. The optimum tool approaches universality by being able to grip poly bags, bubble wrapped objects, envelopes, cartons and cases. End-of-line packaging OEMs also seek more versatile gripping options for case packers and palletizers.

End-of-arm tooling that combines vacuum and mechanical gripping comes as close as possible to the one tool that does it all and reduces the number of tool changeovers to a few or zero. The optimum universal tool features force, position and speed control as it grips. It must also be lightweight, easy to work on and offer a long service life.

A selection of suction cups. The importance of testing various cup designs on packaging can’t be overstated.

Master the Basics of Vacuum Gripping

It is essential for OEM and system integrator designers to master the basics of vacuum gripping when designing an end-of-arm tool incorporating multiple functions. The designer must consider the two ways of generating vacuum, vacuum pump or venturi effect utilizing a pneumatics system, then apply the method most effective for the application and best aligned with the distribution center’s infrastructure.

Designers must become familiar with the various shapes of suction cups and the application each cup performs. The choice of cup also depends on its material—natural material like rubber or a synthetic, such as neoprene or PVC—as each material affects performance and longevity.

Learn about Bernoulli-effect gripping that uses positive air pressure to hold objects. Bernoulli grippers are excellent for gripping lightweight flat items, bubble wrap, porous materials or perforated sheets and they have minimal impact on the object’s surface. Consider whether adding a Bernoulli gripper alongside suction cups would give the tool a useful extra level of flexibility.

Work with a pneumatics supplier and benefit from their expertise in vacuum applications. Ask the partner whether they can provide vacuum test kits for assessing various suction cup configurations on actual packaging. Designers take advantage of the many workshops and online courses on vacuum that are available today.

Weight Considerations Drive Design for the Mechanical Gripper

The universal end-of-arm tool for mixed-product packaging will incorporate both mechanical gripping and vacuum gripping. Mechanical gripping will be used to apply force to the sides of packaging that is not conducive to vacuum due to its weight, size or configuration. The paddles pressing against the package must be adjustable not only for position but also speed and force. Long-stroke grippers may also be used to justify case layers on a pallet. Sometimes the mechanical gripper jaws are used to stabilize the product in the gripper after vacuum gripping has been applied to the top of the package to secure the package during vertical or lateral acceleration or deceleration.

One might think that servo-electric control would be excellent for powering mechanical gripping because of its positioning capabilities and flexibility. The issue with electrics in end-of-arm tooling is the significant weight deficit of motors and actuators. They are heavy: For every ounce that the end-of-arm tool adds to the system, an ounce is subtracted from the robot’s payload capability. Stepping up to a larger robot to accommodate a heavier payload adds cost and expands footprint. This is not something designers want to do if there are alternatives such as finding a lighter end-of-arm tool.

The alternative to motor-driven motion is pneumatically powered actuation. Pneumatic cylinders are basically a piston with an attached rod, which moves inside of cylindrically bored housing. Pneumatic cylinders not only offer low weight and compactness, but also provide cost, speed, force and maintenance advantages compared to motor driven gripping.

Pneumatic cylinders can be driven by the same system that powers venturi-based vacuum grippers for one system that controls both. If pneumatics is going to power mechanical gripping, the question becomes what type of pneumatics will do the job—traditional, servo or controlled?

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.

Three Choices in Pneumatic Gripping

Traditional pneumatic actuators have two positions—fully extended and fully retracted. Cylinders operate with a single force profile, which means every object is gripped with the same force. This is not ideal for mixed-product-package systems, where fragile and light packaging requires less force and heavy and large packaging requires more.

Servo pneumatic systems can position the paddles and adjust the force exerted on the package, something traditional pneumatics cannot do. Servo pneumatic systems consist of a PLC, a proportional control valve and an actuator with linear displacement encoders attached along its length. The PLC selects a defined position, and the proportional valve changes the pressure and the flow rate to cylinder ports 2 and 4 to hunt for that position. Pressure will be equalized at that point, holding the piston in place.

One advantage of positioning is the ability to pre-stage the paddles to be close to the width of the package it is about to grip. This reduces the time it takes to close on the package and improves response time. Defined acceleration, motion profile and deceleration are achieved through proportional control.

Along with position control, servo pneumatics also provides force control. By increasing pressure proportionally, the force on the piston is increased, and by reducing pressure, the force is decreased. In gripping applications, force control allows grasping of soft delicate objects or heavy rugged workpieces within the same set-up.

Bernoulli gripper offers flexibility in the wide range of light, flat materials it can grip, from textiles to bubble wrap.

The third mechanical gripping option is controlled pneumatics, which was developed just a few years ago. Controlled pneumatics is sometimes referred to as digital pneumatics. Digital pneumatics relies on a new type of flow or proportional pressure control valve featuring piezo electric cartridges with integrated sensors and control algorithms—all packaged in a single valve housing which is commonly referred to as a smart valve. Long-life, low-heat and silent piezo operation open a host of capabilities as compared to traditional spool valves, not the least of which is an up to 50% energy savings for the application.

In controlled pneumatics, piezo valves can be integrated into valve hardware in many ways with each arrangement offering a different capability. For example, a single 3/3-way piezo cartridge, with sensor, and a control algorithm can act as a proportional pressure regulator. Or a single valve made up of four 2/2-way cartridges arranged in a bridge circuit and integrated with sensors and control algorithm can control two different channels at the same time.

Controlled pneumatics solutions range from a standalone proportional control valve and proportional pressure regulator up to multi-valve units, including a proportional pressure valve terminal for medium-flow applications and a high-flow-rate valve terminal with integrated controller, sensors and Ethernet-based communications.

The smart pneumatic valve terminal provides positioning capabilities comparable to electric servo motors, but with the advantages of light weight and compact footprint. This is the solution used for a multifunctional end-of-arm tool.

The end-of-arm tool solution consists of the smart valve terminal and long-stroke cylinder or rack-and-pinion actuator, both integrated with analog sensors for location information. With this solution, the designer has the servo-pneumatic capability to pre-stage the position of the paddles for a faster response and change the force depending on the next package to grasp.

In addition, the controlled pneumatics valve terminal can detect leaks down to an individual cylinder, lower energy consumption by reducing pressure once the cylinder is in motion, achieve soft stop on both extend and retract, and preset travel time to ensure that despite variations in pressure, paddles open and close at the specified speed.

The smart valve terminal also controls vacuum in a pneumatic system. This simplifies the installation as one terminal located on the robot or near its base controls both vacuum and mechanical gripping on the multi-functional end-of-arm tool. The terminal can also control conventional pneumatics on the tool, such as ejector pins, giving the designer a unified control solution—one with IIoT capabilities.

One of the additional benefits of using the smart valve terminal is that the flow and pressure controls are digital, not manual, which eliminates the industry-wide issue of “knob turners,” personnel making unauthorized pressure and flow adjustments. Controlled pneumatics tamper proofs the tool.

When designing an end-of-arm tool for mixed-product environments, or packaging machine applications such as case packing or palletizing, envision a solution that includes both vacuum and mechanical gripping. Acquire expertise in the mechanics of vacuum and Bernoulli-effect gripping and create a lab to evaluate various configurations of suction cups with an assortment of materials.

Strive to reduce the weight of the tool while maintaining its durability by utilizing pneumatics. Apply the capabilities of servo pneumatics or controlled pneumatics when automating mixed-product package picking and sorting. Adopt the motto of striving to do more with less by reducing the number of tool changes to a few or zero.