Mechatronics for robots.
Mechatronics--the union of mechanical, electrical, and computer engineering in design--is at the heart of efforts to develop high-precision robots. This approach has been crucial in achieving the ever increasing accuracy and repeatability required by today's manufacturing assembly applications. It is employed in the design of the individual components used in a robot's manipulator, actuators, and end effectors, as well as in the conceptualization of the robot as a complete system.
A notable recent example of mechatronic design is the Megamation robot by Megamation Inc. (Lawrenceville, N.J.). It is built around a novel, 2-D linear stepper motor, resulting in 0.0005-in repeatability and 0.0025-in accuracy.
At the Massachusetts Institute of Technology, a mechatronics research approach has led to the development of a robot's wrist equipped with a thumb to dampen vertical oscillation. Similar oscillation-damping benefits have been documented by a research group at the Georgia Institute of Technology, working with a robot wrist braced for added stiffness.
At Grasp Inc. (Troy, N.Y.), the Clamp (closed loop assembly micropositioner) was developed in 1989. Clamp is an end effector (end-of-arm tool) mounted on a robot that was also designed and built by Grasp. The robot's manipulator (arm, links, and joints) executes large-motion commands, and the Clamp end effector (which is in effect a small robot) executes the fine positioning motions needed to bring the robot into position to pick up (dock with) the workpiece and perform the appropriate manufacturing tasks.
With Clamp, the manipulator becomes simply a transfer device to move the end effector from docking site to docking site. Manipulator accuracy is no longer a strict requirement, and the manipulator's components can be configured and sized for larger, gross motions. Instead, fine-positioning accuracy is handled by the end effector. Because the motions of the end effector come from small moment arms, it is easier to keep to tighter tolerances and meet the required accuracies.
Clamp is the successor to a number of designs built for precision assembly tasks. Perhaps the earliest example of a robot developed specifically for assembly applications is the selective-compliance articulated robot arm (Scara), which was developed in Japan in the late 1970s. Scara makes use of the principle that, for many tasks, it is advantageous if the manipulator is very stiff in the direction of part insertion during assembly, and compliant, or easily movable, in the other two directions. The Scara concept is finding increasing use in precision assembly applications. It is most popular among Japanese robot manufacturers. In the United States, a Scara design is marketed by GMF Robotics Corp. (Troy, Mich.).
Along with the selective compliance arm and a related concept, the remote center compliance (RCC) device which was developed at the Charles Stark Draper Laboratory (Cambridge, Mass.), came the development of better motors (actuators) that could more accurately position a robot's manipulator. A direct-drive motor was developed for the Scara robot.
In the late 1970s and early '80s, major directions in robotics research included structurally stiffer robot arm members, rigid bearings, and more powerful controlling computers. Limited results from the industrial and academic research labs became commercially viable. However, research efforts did not focus on performance of the entire robot.
Gains in precision in robot designs were influenced primarily by advances in computer and electrical engineering, along with some help from developments in mechanical engineering. For example, new laser tracking techniques were developed. Although these tracking systems could determine the position of the end effector more accurately than the robot alone, they tended to be too complex and costly to consider using them as an integral part of a robotic system.
In determining which approach to apply to the task of developing an improved robot for precision assembly, Clamp's designers considered several questions.
* What is the primary task of an assembly robot? Robots designed for a specific family, rather than an all-encompassing range, of operations may be the direction of the future. Many experts see this as a way to lower costs that have been a major stumbling block towards wider use of robots.
* Where is the precision needed? Most current industrial assembly robots can keep their precision at any position and orientation in space. Here, too, precision within a limited range may be more cost effective, especially since most precision tasks are performed just above a work surface.
* What are the manufacturing difficulties in keeping the required tolerances? How difficult and demanding will it be to keep tolerances of individual robot components tight so that the total system tolerance will be satisfactory?
* How easy is it to demand that robot actuators move fast at times, and yet slow and precisely at others? This common requirement places harsh demands on component specifications.
Studying these requirements suggested a mechatronics approach. Clamp is constructed from off-the-shelf components--self-aligning bearings and other standard components have proved adequate to meet the required accuracies. In addition, dc servomotors with harmonic drive units have given Clamp very fast cycle times.
Timing belts are used as drive trains and two servomotors are mounted on the robot base to reduce the moving inertia. The fourth joint is driven by a pneumatic cylinder. All moving structures are fabricated from aluminum since robot arm stiffness is not a tight requirement.
Clamp is controlled by an IBM PC/AT computer using a three-axis motion control card. Standard amplifiers are used to boost the outputs to a level needed to drive the actuators. A stepper motor control in the PC is used for the micropositioner actuators. An I/O card in the PC interfaces with the amplifier relays and the z-axis control. I/O signals and serial communication are available to drive an interface that connects to the work cell.
The Clamp end effector is connected to a mounting plate by a compliant coupling, which allows for limited 3-D translation and rotation. The mounting plate is in turn attached to the robot. A set of springs, conical pieces, and conical holes keep the device rigid when not docked.
When the Clamp end effector docks with the worktable, it forces two conical tips on its locating legs into conical and wedge locating receptacles on the worktable's surface, until they are seated. A third locating leg, which is a conical tip with a flattened nose, is forced to rest against a flat surface on the worktable. The three legs fix Clamp in 3-D space.
This docking setup is based on the commonly accepted "321" fixturing principle that is often used to locate a part for accurate machining. Any attempt to use other more constraining methods, such as three conical legs docking with three conical holes, would be difficult to manufacture and align.
Because of the compliant coupling that helps connect the end effector to the manipulator, and tapered surfaces on the locating legs, the Clamp end effector can miss its targeted location by up to 1/4 in and still dock successfully. Once docked, the force exerted by the robot holds the end effector rigidly in place.
The repeatability of the end effector was measured to be 0.0001-in. in the x and y directions, 0.00005-in. in the z direction, and 0.001 deg in the [theta] direction. The accuracy of the end effector is 0.0001 in. in the x and y directions, 0.00025 in. in the z direction, and 0.01 deg in the [theta] direction. The docking operation was successful while operating the robot at maximum speed despite robot positioning errors up to 1/4 in. Once docking is achieved, there is no need to wait for robot settling since the compliant coupling isolates the end effector from robot vibration.
Clamp can dock with a wide variety of workpiece fixture sites, including single sites, multiple pallet sites, and intermittently moving pallets. The locating receptacles can be affixed directly to the pallet. No vision or complex sensor system is required. Pallets need not be completely registered with respect to the work cell.
Degrees of Freedom
As with any mechatronic device, there are some limitations. The first is the number of degrees of freedom of the manipulator within the Clamp end effector. Building a miniature six-jointed Clamp robot that could perform nearly every precision task would be costly because overall system tolerances build up as the number of degrees of freedom increases. Therefore, a practical limit of two or three degrees of freedom exists.
A second limitation is the work cell flexibility trade-off. The Clamp concept is based on a structured work surface environment. Parts to be assembled cannot be randomly placed on the work surface if they are to be properly reached. This limitation is not severe, however, since most robot environments need to be fairly structured to be fast, dependable, and cost effective.
A final limitation is based on performing tasks that would require very large precise motions at a single docking location. This could require an oversized Clamp device. Though the Clamp design can be scaled up to build a larger model, a 6-ft-long Clamp device having accuracy in the range of 0.001 in would be difficult to achieve.
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|Title Annotation:||mechanical, electrical, and computer engineering|
|Date:||Jul 1, 1990|
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