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The Control Difference

When many people think about a robot, the arm is the thing that comes to mind.

When many people think about a robot, the arm is the thing that comes to mind. Of course, while the arm is certainly important, what makes the arm truly useful is the control that runs the mechanism. Just how true this is can be discerned from a new line of robots thatMotoman Inc. (West Carrollton, OH) is launching, as the XRC ARM (for Advanced Robot Motion) control (combined with a new drive system and encoders), explains company president Craig Jennings, has allowed the development of a new arm architecture. The arm—which is designated "UP" is simpler than that which it will replace, and is more readily accessed for maintenance and repair.

 

arm configuration 1
arm configuration 2
arm configuration 3
A new control results in a new arm configuration for the Motoman UP series robots.

The most obvious difference is the elimination of the parallel link arrangement, or yoke, that is characteristic of the company's SK line of robots that are applicable to a wide range of robotic applications (arc welding, assembly, sealing, cutting, material handling, finishing, spot welding, etc.). Having bearings on two sides of the arm provides a lot of stiffness and rigidity, Jennings says. But that's something that's being achieved purely by mechanical means for the SK.

"That design," Jennings notes, "gives way to vibration suppression in the servo control loop on the UP." The RISC-type processor, operating at more than 100 MIPS, busily figures out where the arm is in space through inputs from sensors. Not only does this mean that there is the suppression of vibration, but it also results in far better linear and circular precision than can be achieved with Motoman's just-previous MRC robot control.

Jennings points out that the MRC controller wasn't in the least bit sluggardly. In fact, he notes that the MRC can control up to 18 axes, which means that there is the capability of running two robots with one controller, which is, he says, "the only way to have two robots work in close proximity" (i.e., in some setups, where there are two robots and two controls, one of the controls gets input from the other control so that there is synchronization, but a difficulty can arise from the fact that there is a slight mis-sequencing as the second control needs to wait for input from the first). According to Jennings, Motoman is the only robot vendor that provides this two robot/one control capability. And that's with the MRC control.

With the XRC control, the capability has gone up. It provides the means to control 27 axes. Or, three robots can work in coordination, under the control of one XRC.

Because the processor operates so quickly, the processes that people need to perform can be done more quickly and accurately. For example, in terms of arc welding, cycle time can be cut because the arm will rapidly move the torch to the start point. In terms of sealing, there's good part quality obtained because there is improved path precision during high-speed operation. (How much speed? Well, in the case of the UP-130, an arm with a 130-kg, or 286.6-lb, capacity, the S (turning), L (lower arm) and U (upper arm) axes can move at 130o/sec; the R (wrist roll) axis moves at up to 215o/sec; the B (wrist pitch/yaw) at 180o/sec; and the T(wrist twist) axis at 300o/sec, which are benchmarked-best numbers as compared with other comparably sized robots.)

The XRC can be integrated to a network via Ethernet, DeviceNET, Interbus-S, Profibus-DP, and others.

The first two robots in the Motoman UP series are the aforementioned UP130 and the UP6 (which has a 6-kg payload capacity). Both are six-degree of freedom units. The UP6 has a reach of 1,373 mm (54.06 in.); the UP130 has a reach of 2,646 mm (104.17 in.). One useful result of the elimination of the yoke is that the arms can reach behind themselves, facilitating, for example, toolchanging.

There are other UP variants on the way, such as the UP165 and UP200. Motoman has also applied the XRC ARM control to the previous generation of manipulators.

Jennings admits that Honda was instrumental in driving the development of both the UP series of arms and the XRC control. Although Motoman has long been recognized for its arc welding and waterjet cutting installations, it is clearly getting into the spotwelding arena in a big way with its bigger UP series robots.—GSV