Coupling by RELI-A-FLEX

PRECISION SINGLE-PIECE
FLEXIBLE COUPLINGS IN ROBOTS

By John Ricker, RELI-A-FLEX Group, Baldwin, NY, and Dan Jones, Incremotion Associates, Thousand Oaks, CA

Robots have been used in U.S. industry since the early 1960s, and many process applications such as die casting, spot welding and spray painting have reached various plateaus of maturity. Today, the U.S. factory floor is seeking to use the increasing flexibility of the electrically-driven robot in electronic assembly applications. The accuracy needed in many assembly robots requires the use of precision mechanical components and high performance feedback devices. While many robot manufacturers are using direct-drive motion system technology, a number of tabletop assembly robots are incorporating more conventional single-piece flexible couplings to cost-effectively achieve essential positional accuracies.

Robot Arm Configurations

A robot must be able to reach work pieces and tools. This motion requires a combination of waist, arm, wrist and finger (grip) axes. The more complicated robot with vision capability requires a lightweight but rigid arm with an integrated servo motor, plus a feedback device to significantly reduce unnecessary mechanical arm flexural errors, servo system backlash and compliance errors. Robot arm speeds of 2 meters (80 inches) per second and position accuracies of 0.01mm (0.0004 in.) are typical for high performance vision robots used in semiconductor electronic component assembly and material handling applications.

ROBOT MOTION AXES

Many pick-and-place and fluid dispensing robots require arm, wrist and finger (grip) axes of motion with less demanding, lower cost motion and mechanism components. Freedom of movement is usually limited to two or three directions: in and out, left and right, and up and down. The Cartesian coordinates robot best represents this type of robot, while the cylindrical coordinates robot is also used in these less demanding applications. A miniature single-piece flexible coupling is used to connect the drive motor shaft to the lead screw used in the Cartesian coordinate robot arm as well as the height (z - zeta) axis and the extend/retract (g - gamma) axis in the cylindrical coordinates robot arm. The pedestal (q - theta) axis of the cylindrical coordinates robot arm uses direct rotary motion, usually with a position feedback device (encoder) directly installed on the drive motor rear shaft. A planetary or spur gear train is often used to reduce overall drive motor rotary speed. Many times a second encoder is attached via a single-piece flexible coupling to the q axis drive shaft in order to reduce the effect of q axis error. Any error in the pedestal axis drive is reflected into the other robot arm axes drives. The second encoder position signal is used to correct the pedestal axis drives' gear train compliance and backlash.

The wrist axis drive directly interfaces to the business end of the robot, the end effectors. These are the moving parts that have to grasp, lift and manipulate the work pieces without causing any damage and without letting go. Pick-and-place and limited sequence robots typically require no more than one extra axis drive (two degrees of freedom) beyond the wrist. The gripper (finger) axis is usually a simple pneumatic actuation system based on controlling holding force.

PRECISION SINGLE-PIECE FLEXIBLE COUPLINGS

The bellows single-piece coupling has been the miniature flexible coupling usually selected for use in tabletop robots, because of its overall performance capabilities. However, it suffers from lower peak torque, lower peak shaft speeds and moderate life capabilities as well as higher acquisition cost when compared with other miniature flexible couplings. Yet, the bellows coupling does provide the highest torque stiffness and lowest lateral radial bearing loads reflected from the flexible coupling to the drive motor and axis mechanism shafts. The bellows coupling also provides excellent performance with respect to overall coupling transmission error. Transmission error is defined as error introduced into a robot axis system by non-uniform transfer of motion from one hub of a coupling to the other hub. This parameter quantifies the combined effect of all the motion errors that occur when a coupling undergoes a reversing motion that is normal for position servos and step motor driven motion systems in incremental (start-stop) motion.

THE RELI-A-FLEX ALTERNATIVE

The miniature single piece flexible coupling developed by RELI-A-FLEX of Baldwin, NY, provides an alternate solution to the bellows coupling with higher torque capability, longer life capability, lower unit price and near equivalent performance in other important operating parameters.

In development and life testing over the past three years, the coupling has a unique radial slotted pattern (patent pending) developed to fill a market need for a single piece flexible coupling that could challenge if not exceed the overall performance of bellows coupling (equivalent size) in most performance characteristics.

The family of helical or spiral beam flexible couplings provides a range of performance based on the number of beams (i.e. ­ typically 1, 2, 3, 4, or 6) for these single-piece miniature flexible couplings. The RELI-A-FLEX flexible coupling is a new member of this group. Its improved performance footprint was developed to perform in increasingly demanding technical applications.

The unique slotted radial beam configuration in the coupling offers higher torsional stiffness and enables this coupling to outperform all traditional one-piece flexible couplings. The slotted radial coupling may also be used where bellows couplings have previously been the only viable choice.

This shorter compact flexible coupling differs from other one piece flexible couplings of its size in that it has two sets of body slits. When its hubs are offset from a common axis, the second set of slits introduces an equal and opposite angle allowing it to cater for parallel as well as angular offsets. This configuration is identified as a double Hooke's joint. The principle of the double Hooke's joint is that a driven shaft can be made to revolve at the same speed as the drive motor shaft at all instants. This condition significantly reduces the stress on the coupling since there is no change of the center piece angle.

PERFORMANCE COMPARISON

Several operating parameters are used in a performance comparison study, where the flexible coupling outside diameters (ODs) are closely matched along with axial length.

Peak reversing torque. The RELI-A-FLEX flexible coupling has the highest peak reversing torque capability followed by the typical 6-beam spiral flexible coupling and the bellows coupling.

Coupling speed. The top coupling speed capability belongs to the slotted radial beam flexible coupling, which can operate at speeds up to 20,000 rpm. No competitive single-piece flexible coupling can operate at speeds above 4000 rpm. In fact, most are rated for 2000 rpm maximum operation.

Torsional stiffness. The bellows flexible coupling has the best torsional stiffness -- > 22,000 ounce-inches per radian -- followed by the slotted radial beam coupling at 16,000 ounce-inches per radian. There is a significant decrease in torsional stiffness with the 3-beam spiral flexible coupling torsional stiffness value measured at approximately 1000 ounce-inches per radian.

Lateral radial bearing load. This is seen at the drive motor front bearing or robot axis front bearing. It is caused by the couplings under test experiencing a parallel offset of 0.1mm (4 mils). The bellows coupling, the typical 4-beam spiral and the RELI-A-FLEX flexible coupling exhibit similar radial bearing loads of 0.6 to 0.75 Newtons (2.2 to 2.7 ounces). The typical 6-beam one-piece flexible coupling was measured at 2.7 Newtons (9.8 ounces) radial load. Higher bearing radial loads will degrade bearing life.

Transmission error. As a component in the robot axis motion system, the bellows cou pling and the slotted radial RELI-A-FLEX coupling have the lowest (best) transmission error, the final performance characteristic. They are followed by the 6-beam radial single-piece flexible coupling at approximately three times the radial slotted flexible coupling transmission error (88 arc-seconds), and a single-beam spiral single-piece coupling at approximately five times (180 arc-seconds or 3 arc-minutes) the radial slotted flexible coupling. Test conditions were driving a 2 ounce-inch load with a 4 mil offset.

The RELI-A-FLEX radial slotted beam flexible single-piece coupling leads in three performance categories: peak reversing torque, maximum speed and unit life. The bellows coupling also leads in three performance categories: torque stiffness, transmission error and lateral radial bearing load. The lower unit cost of the slotted radial beam flexible coupling makes it an excellent choice for accurate measuring and responsive precision motion control where servos and steppers are used.

For more information:

Circle 490 - RELI-A-FLEX