X-Y-Z: The Modular Robot
Of the various robots used for automated assembly, the Cartesian robot is the least complex. Also known as a rectilinear, rectangular or gantry robot, a Cartesian robot can only move its end effector in straight lines along the X, Y and Z axes. Some Cartesians have an additional axis of motion, in which the end effector rotates about the Z axis or parallel to it.
Each axis is a separate linear actuator, which can be driven by a ballscrew or belt. Depending on length, velocity, payload and accuracy requirements, a linear guide may be necessary to support one of the axes.
Because a Cartesian robot distributes loads evenly across a rigid frame, it can accurately and repeatably position large payloads at high speeds. "Cartesians can come screaming into a spot and stop on a dime," says John C. Clark, national sales manager for Epson America Inc. (Carson, CA).
With their modular construction, Cartesian robots are easily scaled to meet various travel and payload needs. Individual axes can be quickly repaired or replaced, and the entire system can be disassembled for use in other motion control applications.
The maximum reach of a SCARA robot is typically 1,000 millimeters. Cartesians are available with travel lengths of 3,000 millimeters or more.
However, the silver lining of modularity does not come without a dark cloud. Cartesian robots are harder to move than other robots, and some engineers may not want to assemble, align and coordinate the different axes. Managing the control cables for each axis can be tricky, though new high-speed communication technologies are minimizing this issue. Finally, engineers should beware of making conclusions about the robot’s payload, accuracy or repeatability based on the specifications of one component.
In the past, choosing between Cartesian and SCARA robots was a matter of trade-offs. Cartesians were more accurate and less expensive than SCARAs. SCARAs were faster and took up less space than Cartesians. Today, technological advancements have made the two technologies comparable in both price and performance. As a result, distinctions between the two are much more subtle.
One issue to consider is the robot’s work envelope, especially where work space is expensive, such as clean rooms. A SCARA robot’s work envelope is revolute, while a Cartesian’s is rectangular. If you put a revolute work envelope inside a square work area, you can’t use all the available space.
"If everything fits in a rectilinear envelope, a Cartesian is good," adds Clark. "If you need to pick up parts at the three o’clock position and bring them over to the 12 o’clock position, a SCARA is better."
Cartesian robots can be equipped with a variety of end effectors, including screwdrivers, routers, dispensing valves, soldering heads and grippers.
When specifying a Cartesian, engineers should provide the weight of the payload—the end effector and any parts or materials it will carry—as well as how far and how fast the payload will travel, says Kevin D. Gingerich, manager for communications and e-business at Bosch Rexroth Corp. This will determine the length and width of the frame and the size of the motors. And, as with any motion control system, accuracy and repeatability requirements are critical.
Engineers should also specify how and where the robot will be used. If the actuators will be located where maintenance will be difficult, they can be furnished with permanently lubricated bearings. If the robot will be turned off and on frequently, the actuators can be equipped with absolute encoders so the robot won’t have to return to a home position before resuming operation.