Indexing solutions are continuing toevolve with the integration of motion control technology and roller pinion ringgear systems replacing the traditional cam mechanism in indexers.With the technological advantage of zero backlash and high system accuracies,this new approach provides a programmable solution that reduces system noise,increases system life and offers optimized mounting options for increasedsystem integration flexibility.

"Our newest technology allows indexer applications toapproach direct drive motor accuracies but with a cost of ownership that issignificantly lower," says David Hein, vice president of engineering for Nexen Group. "We do this by using aroller pinion that provides zero backlash technology and high accuracy. A keyto the design is using the roller pinion along with a ring feature that becomesthe main driving mechanism for the indexer."

Along with high accuracy, a main value proposition for thenew system is that, for a given level of inertia, the system can index twice asfast as alternative technology solutions. Because of the mechanical technologyused, the system can be mounted vertically, which is a significant advantage inwelding applications such as robotic welding of automobile frames.

Two primary drive options include using a servo-drivenprecision gearhead or a direct drive motor for indexer control. The directdrive motor configuration eliminates the need for a gearhead. Furthermore,these systems eliminate the weak components of a traditional indexer, such asthe belt that can break between the servo motor and cam cylinder inservo-driven systems or the highly stressed cam that limits the acceleration ofthose systems.

"For us, a robust mechatronic system design is how well themechanical components fit into the overall value of a fully integratedmechatronic system," says Hein. "The goal of our motion control products is todeliver high mechanical precision, zero backlash and extremely stiffcomponents."

"Those three characteristics are critical in a fully andproperly designed mechatronic system because they are the main parameters thataffect the servo controlled portions of a control system. The more robust themechanical components, the more the integrator can optimize the drive system,"Hein says.

Indexers and the Evolution of Mechatronics

The evolution of indexers is an excellent example of improvedmechatronic design. A mechanical indexer is, in and of itself, a completecontrol system. Up until 1995, indexer designs used an ac motor that drove agearbox connected to a cam cylinder. The cylinder has machined grooves whilecam followers ride through the grooves as the cylinder rotates, creatingrotation of the dial (output) plate.

At intervals in the cylinder, the grooves are machined withno lead, which causes dwell of the dial plate during which operations are doneto a work piece. The positions that the indexer moves to are locked by theposition of the flats machined into the cylinder. This type of mechanicalindexer is a robust product and has been used for 30-40 years for a variety ofapplications.

The drawbacks of the traditional mechanical indexer are itslack of versatility. Cam systems are operated in an oil bath and, because ofthis, they can only be mounted in certain orientations and require moremaintenance. More importantly, if the application wants to index to sixpositions versus five positions, a new mechanical cam system or basically awhole new indexer is required. These systems are not easily modified for flexibleautomation control.

"Just like in the world of power transmission componentswhere traditional solutions have been replaced by motion controlconfigurations, in the world of indexers the same technological shift is takingplace," says Hein.

One solution that has improved indexersis using a servo system to drive the cam using a high-tension belt. This is anadvantage because the servo motor is programmable and the application can usean infinite number of positions and make changes on the fly. But drawbacksremain. The drive belt can stretch, wear or break, and the cam system limitsacceleration and mounting options (due to the oil bath). But even with thesedrawbacks, it still provides a better solution than the traditional mechanicalindexer.

Hein says the highest level of indexer integration uses adirect drive motor where the rotor is connected to the dial plate and thestator is the main frame of the indexer. The entire indexer functions as a hugeservo motor and provides a very precise solution, but is also costly. With asize 750 indexer (750 mm diameter), a current servo driven system may cost$15,000 while a direct drive control solution may cost five times as much.

He says that, with regard to accuracies on a size 750indexer, a direct drive motor may hold 8-9 arc sec of accuracy, a servo drivensystem might achieve 14 arc sec of accuracy, and the accuracy of a standard camfixed indexing system is in the 20-30 arc secrange. Each level of integration offers a higher level of performance from anaccuracy and repeatability perspective.

The Future Path

From a geometric perspective, the world of indexers is movingto large hollow bore centers. The majority of applications are looking for thebiggest hole in the center, the smallest outer diameter and the mechanism to beflat like a pancake. This design is the most versatile footprint for systemdesigners.

"What makes our mechatronic solution most elegant is acombination of zero backlash and extremely high stiffness technology, matchedwith the mechanical advantage of the roller pinion technology. We are able tomove outside the general realm of application parameters that servo motors areusually required to follow," says Hein.

Stiffness is critical in a control system because if themotor's input specifications (rotor inertia, acceleration) are not matchedcorrectly with the output specifications (inertia, stiffness) the movement ofthe system will suffer. In a low-stiffness system, the output can overshoot thedesired location and cause a resonance as the motor attempts to correct it.Think of it as trying to pull a paperweight with a rubber band. This iscommonly referred to as inertia matching.

"We are also able to go outside the traditional rules ofthumb for inertia matching. Common applications are 3:1 or 10:1 but we arelooking at applications where the inertia ratio can be dramatically higherbecause of the positive mechanical contribution of the components to thedesign," says Hein.

From a technology perspective, the goalof the design is to maximize value defined as the sum of performance delivereddivided by the sum of all costs. Hein says the new technologies in the indexingmarket are shifting the value equation.