Taking Your Pick
One of the main selling points for robots is their flexibility. Unlike conventional automation systems, robots can be programmed to perform any number of tasks-either in sequence as part of a single production cycle, or when assembling different styles or model components.
However, robots can be limited by their end effectors. Once a robot has been equipped with, say, a set of grippers, it becomes severely restricted in what it can do. Imagine having a pair of needle-nosed pliers duct-taped to your hand. Not so easy to drink a beer anymore, is it?
It was with this hurdle in mind that manufacturers began experimenting with automated tool changers back in the 1980s. In the beginning, there were problems. Tool changers could be unreliable and were apt to drop their end effectors if and when they lost power. "Back then tool changers falling off were common," says Bob Little, product manager for ATI Industrial Automation (Apex, NC), adding that he knows of one case of a hurtling end effector barely missing an operator.
Today, however, tool changers are a mature technology, with failsafe mechanisms assuring that any and all welders or grippers stay where they belong. The challenge now is recognizing when tool changers make sense for an assembly application and making sure to purchase the right one.
Changes in Attitudes
When most people think of robotic tool changers, they think of those cases in which a single robot uses multiple end effectors as part of multitask production cycle in a given cell. A single robot, for example, will employ a gripper to pick up and position a part, after which it exchanges the gripper for a welder. When the welding is complete, the robot drops off the tool and reattaches the gripper, so it can remove the piece from its fixture and pick up another.
In fact, this application represents only a fraction of the applications for which assemblers employ tool changers. Far more common are applications in which a robot uses a single effector for a given product run, only changing out end effectors when the product changes.
Stamping lines in particular lend themselves to this arrangement, because a different gripper must be used with each component. Given the cost of presses and dies, manufacturers want to keep them running as much as possible.
"In some cases lines are changing every 2 to 3 hours," Little says, citing door panel manufacture as an area in which tool changers can both cut costs and increase throughput. "They often have short runs of different parts. With a tool changer, within minutes the press line can be ready to start stamping out a new line of parts."
And this use is not just limited to the big boys. According to Charlie Smart, engineering manager for Robotic Interface Systems Technologies (RISTEC, San Diego, CA), this capability also increases a robot's value in the context of smaller assemblies. His company's tool changers are often used for changing out suction grippers employed in electronics or medical device manufacturing.
The same is true for Robohand (a division of DE-STA-CO Industries, Monroe, CT). According to Keith Crawford, general manager for Robohand's advanced development center, many of his company's tool changers are used for disc drive manufacture and surface mount assembly on printed circuit boards, applications in which each board style requires a unique gripper.
"A toolchanger is really a magnificent tool with product life cycles dropping like they are," Crawford says, noting the high mix of products assembled by manufacturers today.
In fact, this increasing need for flexibility has resulted in workcells in which robots don't just perform multiple functions, but multiple functions on different parts. A single robot, for example, will not only be equipped with both a gripper and welder, it will be equipped with multiple grippers so it can it can handle different assemblies for, say, different models of cars.
Finally, there are those applications in which tool changers help reduce the down time resulting from routine maintenance. This use is especially common in welding applications, in which multiple guns will be on hand so production can continue with one gun while operators are cleaning or repairing another. In this case, the tool changer justifies its expense because the welders can be swapped out in a matter of seconds, as opposed to minutes, or even hours.
Make That Change
In terms of design, all tool changers are pretty much the same. A robot, or master, side is attached to the robotic arm. The piece then mates, or attaches, to a tool-side piece, which is attached to the end effector.
The locking mechanism generally relies on a pneumatically driven piston that forces a set of steel balls into a groove in the tool-side piece, both locking it in place and clamping the two pieces together. Various failsafe mechanisms rely on springs or basic piston and groove geometry to ensure the robot- and tool-side pieces do not come apart in the event of lost air pressure.
Of course, when selecting a tool changer, it's vital to select a piece of equipment that can handle the loads to which it will be subjected. In particular, this means calculating the required moment capacity so a tool changer can resist the twisting forces at work without excessive gapping, or wobbling, between the tool and robot sides of the device. At the low end of the scale, this can mean a payload rating of less than 20 pounds and a moment capacity of 150 in.-lb. At the top end there are grippers with payload ratings of over 1,000 pounds with maximum moments as high as 15,000 in.-lb.
When reviewing these capacities, it's important to look closely at how different suppliers rate their equipment in terms of rated payloads and locking strength. According to Little, manufacturers will rate their equipment according to different standards. As a result, two tool changers with the same rating may, in fact, offer markedly different performance. "Look carefully at rated payload. There is no industry standard that is being followed," he warns. "Find the factor of safety."
Beyond that, the question is one of supply of utilities. The real beauty of today's tool changers is their ability to quickly connect and disconnect the air, electricity or I/O required by the end effectors to which they are attached. In the past, these capacities would be part and parcel of the actual tool changer. Today, though, suppliers are taking a modular approach, in which a manufacturer buys a tool changer equipped with a basic number of pneumatic and electrical ports and then customizes that changer with add-on modules for specific applications.
Robohand, for example, offers a DeviceNet bus module, a 200-amp weld power module, a 19-pin I/O module and an optional self-sealing fluid or pneumatic module with its QC series. ATI recently unveiled what it calls the InstaTool to prevent the problem of 3- to 8-second delays when a DeviceNet bus is connected during tool change.
Finally, for those who don't feel the need for fully automated tool changers, there are perfectly serviceably manual versions that offer cost savings, with equivalent performance once an effector is attached.
SAS Automation Ltd. (Xenia, OH), for example, offers the SWM quick-changing mounting chuck in a number of sizes, complete with quick-disconnect pneumatic connection. According to SAS general manager Richard Petz, when using the system, end effectors can be safely and reliably changed in just a few minutes time.
Similarly, Schunk Inc. (Morrisville, NC) not only manufactures the SWS line of automatic modular tool changers-which includes true behemoths for heavy handling and welding tasks-it also offers the HWS line of manual tool changers. These changers employ an integrated, curved lever, which turns a half-round pin that locks or releases the tool-side plate. According to product manger Jesse Hayes, the changers offer a 40 percent savings over automated tool changers.