Need speed? Is there a delta robot in your future?

11/22/2006


Machine control, robotics, machine vision

A delta robot consists of three servo-driven 'legs' holding a flex plate that holds the robot's end effector. A rod extending downward between the legs provides rotational motion. Courtesy Rexroth

You've seen them at shows. You've marveled at their speed and agility. Perhaps you've even thought one might be useful in your application. They are delta robots, and one just might be in your future. One is more likely to be in your future than in your past because the patents on the original design are about to run out. That will open the field for more vendors to add them to their product lines and for developers to come up with more design variations.

As Figure 1 shows, a delta robot looks like a three-legged spider holding a flex plate. Each spider leg extends downward from a baseplate mounted to an overhead support. Independently acting servos move each leg's short upper segment a few degrees up or down from horizontal. Each leg's lower segment then hangs from a joint at the upper segment's distal end. The three lower segments come together again at the bottom to hold a flex plate to which whatever grippers or end effectors the application requires.

Each leg's lower segment consists of two rods that connect at either side of the leg's 'knee' by gimbal joints. They similarly connect to two gimbals at the flex plate. These rods form the sides of a parallelogram that helps constrain the flex plate to remain horizontal independent of its motion.

A fourth moving element extends from a servomotor affixed to the baseplate between the three leg servos to a universal joint at the flex plate. This fourth element makes it possible to rotate the gripper around the vertical axis extending through the flex plate's center.

Clearly, such an arrangement is capable of moving up, down, left and right within a three-dimensional workspace. Unlike other robot designs, motion along any one axis results from combined motions of all the servos. The robot's kinematic model—the formulas that calculate what servo motions are needed to effect any given move from point A to point B—is a complex exercise in solid geometry. Motion-system component manufacturer Rexroth demonstrated the delta robot shown in Figure 1 at its Pack Expo 2006 booth (Oct. 29-Nov. 2 in Chicago, IL), showing ability to run the Delta-style robot kinematic profile within their IndraMotion for Packaging control platform, along with kinematics for another 90 robot types.

Dan Throne, food and packaging industry manager at Rexroth pointed out: 'The delta style robot is currently a patented design, and the patent runs out in Europe this year and runs out in the United States [at the] end of next year.'

The patent situation has limited sources of delta robots to only three companies, the original patent holder Demaurex SA ( Doboy in the U.S.), SIG Pack, and ABB Flexible Automation. This situation, Throne points out, limits the design's marketability. 'If there's more demand than those three manufacturers can produce,' he said, 'then you may have seen more limited application of that style of robot.'

The system being demonstrated at the show consisted of seven motion axes: the four for the delta robot and three others to control conveyor belts carrying containers of mints that the robot was to arrange. The robot's task was to arrange the containers in a pattern on one of the conveyors with the logos all in the right direction. A bar set at an angle slid the tins from the 'output' conveyor to the 'input' conveyor while randomizing their positions and orientations.

A Cognex InSight machine-vision system monitored the tins' positions and orientations on the 'input' conveyor. 'The vision system notes [each tin's] position on the belt and the orientation of its logo,' Throne says, 'and the robot then picks them off the in-feed belt and places them on to the out-feed belt in an array with all the logos in a proper direction.

Video View 1: Delta robot, video courtesy Rexroth
Depending on system and network, these AVI files could take a few minutes to download.

Video View 2: Delta robot, video courtesy Rexroth
Depending on system and network, these AVI files could take a few minutes to download.

Video View 3: Delta robot, video courtesy Rexroth
Depending on system and network, these AVI files could take a few minutes to download.

Three views of a delta robot demonstrate its agility at Pack Expo 2006. The robot's task is to arrange tins of chocolates in an array with labels correctly oriented. A horizontal bar sweeps tins from the output conveyor to the input conveyor while randomizing positions and orientations. The demonstration system requires seven robot-control axes. Note that a major problem for the robot is to match speeds with the conveyor as it picks up and lays down each tin. Courtesy Rexroth

'We wanted to show that we could run the Delta style kinematic and the control but also do dynamic belt tracking or dynamic product tracking on the in-feed and out-feed belts as well as vision system integration. It's important to note that incorporating delta robot kinematics into the programmable automation controller software made it possible to write the robotic program in standard IEC 61131 ladder logic language, rather than a proprietary language.'

Throne lists the delta robot's advantages versus other robot styles as light weight, speed, and flexibility. For applications where the loads are small and need to be moved relatively short distances at high speed, the delta robot shines. Delta robots can be made of lightweight composite materials and, according to Throne, can reach accelerations of up to 10 g (98 m-s-2) as long as the load is less than a kilogram (2.2 lb). The robot's geometry allows it to move rapidly between any points within its three-dimensional range and rotate the load to any orientation around the vertical axis.

These advantages make it ideal for pick-and-place applications in the packaging and electronic assembly industries. Indeed, the first application for delta robots was in a bakery, where 6 robots were used to package 510 bretzels (a kind of oversized pretzel) per minute!

Throne lists limited range of motion





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