Indexing solutions are continuing to
evolve with the integration of motion control technology and roller pinion ring
gear 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 increased
system integration flexibility.
"Our newest technology allows indexer applications to
approach direct drive motor accuracies but with a cost of ownership that is
significantly lower," says David Hein, vice president of engineering for Nexen Group. "We do this by using a
roller pinion that provides zero backlash technology and high accuracy. A key
to the design is using the roller pinion along with a ring feature that becomes
the main driving mechanism for the indexer."
Along with high accuracy, a main value proposition for the
new system is that, for a given level of inertia, the system can index twice as
fast as alternative technology solutions. Because of the mechanical technology
used, the system can be mounted vertically, which is a significant advantage in
welding applications such as robotic welding of automobile frames.
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Two primary drive options include using a servo-driven precision gearhead or a direct drive motor for indexer control. The direct drive motor configuration eliminates the need for a gearhead. Furthermore, these systems eliminate the weak components of a traditional indexer, such as the belt that can break between the servo motor and cam cylinder in servo-driven systems or the highly stressed cam that limits the acceleration of those systems.
"For us, a robust mechatronic system design is how well the mechanical components fit into the overall value of a fully integrated mechatronic system," says Hein. "The goal of our motion control products is to deliver high mechanical precision, zero backlash and extremely stiff components."
"Those three characteristics are critical in a fully and properly designed mechatronic system because they are the main parameters that affect the servo controlled portions of a control system. The more robust the mechanical 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 improved
mechatronic design. A mechanical indexer is, in and of itself, a complete
control system. Up until 1995, indexer designs used an ac motor that drove a
gearbox connected to a cam cylinder. The cylinder has machined grooves while
cam followers ride through the grooves as the cylinder rotates, creating
rotation of the dial (output) plate.
At intervals in the cylinder, the grooves are machined with
no lead, which causes dwell of the dial plate during which operations are done
to a work piece. The positions that the indexer moves to are locked by the
position of the flats machined into the cylinder. This type of mechanical
indexer is a robust product and has been used for 30-40 years for a variety of
applications.
The drawbacks of the traditional mechanical indexer are its
lack of versatility. Cam systems are operated in an oil bath and, because of
this, they can only be mounted in certain orientations and require more
maintenance. More importantly, if the application wants to index to six
positions versus five positions, a new mechanical cam system or basically a
whole new indexer is required. These systems are not easily modified for flexible
automation control.
"Just like in the world of power transmission components
where traditional solutions have been replaced by motion control
configurations, in the world of indexers the same technological shift is taking
place," says Hein.
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One solution that has improved indexers is using a servo system to drive the cam using a high-tension belt. This is an advantage because the servo motor is programmable and the application can use an infinite number of positions and make changes on the fly. But drawbacks remain. The drive belt can stretch, wear or break, and the cam system limits acceleration and mounting options (due to the oil bath). But even with these drawbacks, it still provides a better solution than the traditional mechanical indexer.
Hein says the highest level of indexer integration uses a direct drive motor where the rotor is connected to the dial plate and the stator is the main frame of the indexer. The entire indexer functions as a huge servo motor and provides a very precise solution, but is also costly. With a size 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 750 indexer, a direct drive motor may hold 8-9 arc sec of accuracy, a servo driven system might achieve 14 arc sec of accuracy, and the accuracy of a standard cam fixed indexing system is in the 20-30 arc sec range. Each level of integration offers a higher level of performance from an accuracy and repeatability perspective.
The Future Path
From a geometric perspective, the world of indexers is moving
to large hollow bore centers. The majority of applications are looking for the
biggest hole in the center, the smallest outer diameter and the mechanism to be
flat like a pancake. This design is the most versatile footprint for system
designers.
"What makes our mechatronic solution most elegant is a
combination of zero backlash and extremely high stiffness technology, matched
with the mechanical advantage of the roller pinion technology. We are able to
move outside the general realm of application parameters that servo motors are
usually required to follow," says Hein.
Stiffness is critical in a control system because if the
motor's input specifications (rotor inertia, acceleration) are not matched
correctly with the output specifications (inertia, stiffness) the movement of
the system will suffer. In a low-stiffness system, the output can overshoot the
desired 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 is
commonly referred to as inertia matching.
"We are also able to go outside the traditional rules of
thumb for inertia matching. Common applications are 3:1 or 10:1 but we are
looking at applications where the inertia ratio can be dramatically higher
because of the positive mechanical contribution of the components to the
design," says Hein.
From a technology perspective, the goal
of the design is to maximize value defined as the sum of performance delivered
divided by the sum of all costs. Hein says the new technologies in the indexing
market are shifting the value equation.