Cartesian Robots Are a Simpler, Lower-Cost Alternative to SCARA and Six-Axis Robots
Of the various robots used for automated assembly, the Cartesian robot is the least complex. Also known as a linear 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.
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 5,000 millimeters or more.
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. But, 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 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. The entire system needs to be considered.
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. 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.
Bosch Rexroth’s EasyHandling system is a complete platform for designing, building and commissioning Cartesian robots. Engineers can take advantage of open, user-friendly programming environments as well as precise and reliable linear components to create accessible, easy-to-use Cartesian robots. With a wide range of load and speed capabilities, the system is scalable for use in everything from small laboratories to large, aircraft assembly operations. When equipped with Rexroth drives and controls, it is programmable under IEC61131-3 but also via Rexroth’s Open Core Interface for programming in platforms as simple as Excel all the way up to high-level languages such as C++.
Bosch’s Standard Linear Modules are available in lengths up to 12 meters. Equipped with integrated zero-clearance ball rail systems or cam roller guides, they can be powered by belt drives or ballscrew drives. A compact aluminum frame provides high inherent stiffness.
Available in five sizes up to a length of 5,400 millimeters, the Model MKK Standard Module features a ball rail system and precision ballscrew assembly protected by a sealing strip. Positional repeatability is ±0.005 millimeter.
Available in five sizes up to a length of 12,000 millimeters, the Model MKR Standard Module features a ball rail system and a toothed belt drive. The backlash-free system enables large masses to be moved at high speeds. Positional repeatability is ±0.05 millimeter. A variant, the Model MKR-145, has two ball rail systems and a closed-type aluminum profile frame with particularly high inherent stiffness for high torque load capacity and high speeds. Another variant, the Model MKR—Food and Packaging, was designed for easy cleaning.
Available in two sizes up to a length of 10,000 millimeters, the Model MLR Standard Module features a cam roller guide and toothed belt drive. The backlash-free cam roller guide is especially suited to very high speeds—up to 10 meters per second.
Model CKK and CKR Compact Modules are distinguished by their high power density and compact dimensions. Their width-to-height ratio is around 2-to-1. They are available as complete systems, including motor, controller and control system. They come in five sizes based on a compact precision aluminum profile with two integrated pre-tensioned ball rail systems.
Precision Modules have an extremely compact and rigid precision steel frame with reference edge and integrated guide tracks. Driven by a backlash-free, precision ballscrew, they have a fixed bearing cross tie made of aluminum with pre-tensioned ball bearings and screw journal.
Available in three sizes in lengths of up to 5,500 millimeters, Model OBB Omega Modules are particularly suited to applications where the frame extends into the working area. They are equipped with extremely compact, precision aluminum profiles and ball rail systems. Center holes are provided on the carriage and the end plates. They are driven by a toothed belt for high dynamics and high traversing speeds (up to 5 meters per second).
Model VKK Feed Modules are equipped with a backlash-free ballscrew drive (tolerance grade 7) and a compact aluminum frame with two zero-clearance ball rails. They are ideal for use as the Z axis in a Cartesian robot system.
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