New
servo drive development continues to ride in the wake of Moore's Law and the computer industry. Continuing increases in processor performance,
memory capacity and speed are spawning new, higher performance drives that are
priced at basically the same level of the products they are replacing. The role of drives in many systems is being
transformed by widespread adoption of Ethernet, which is minimizing the role of
dedicated controllers, and a move to more decentralized, soft real-time control.
"Like computers, drive performance
and capability have improved significantly while the prices have drifted lower,"
says Scott Evans, marketing director for Kollmorgen. "The drives themselves are becoming much smarter,
yet simpler, and they are better able to control a wider array of machines than
in the past. The precision of the motion
is much greater due to the ability to keep error small and respond very quickly
to ever higher resolution feedback devices."
Because drives create higher motor performance and are better
equipped to handle high-speed communications over Ethernet, designers are
increasingly comfortable relying exclusively on the drives for critical motion
tasks.
"If a drive responds to commands
more quickly and with less error in the first place, then time required to
correct error is reduced, thereby dramatically increasing machine performance,"
Evans says. He says Kollmorgen's
new AKD drive series, for example, offers a torque loop (640 nanoseconds) that
is an order of magnitude faster than other products on the market.
Evans says better overall performance,
combined with Ethernet communication, is contributing to two interesting trends
in motion control. Centralized control is
becoming less and less dependent on dedicated hardware, and decentralized
control is gaining traction in the marketplace.
Centralized control gives the controls engineer a single
point of contact with the machine and enables them to do all the programming in
one place, in one language.
What
Evans sees as a trend with drives getting smarter and more capable is that end customers
who still prefer centralized control are moving to "soft" real-time control.
"We are seeing a trend away from dedicated hardware and
centralized controllers toward soft machine controls on a PC," says Evans. "It could be a single board computer, workstation,
a thin client or a dedicated industrial computer. Because the drives are getting smarter, engineers
trust the drives to close not only the torque loop, but also the velocity loop
and perhaps the position loop."
This creates a de-centralized feel to centralized control
where historically the front-end controls would close both the velocity loop
and the position loop of every single axis in the machine. But being able to send target positions every
few microseconds to drives over Ethernet, and having the drives communicate
status plus additional data, requires reliable and very high-speed network
communications. Legacy buses such as CAN,
Profibus, DeviceNet and Modbus are simply not fast enough, running at a maximum
rate of 1 or 2 Mbit/sec. Quality
motion applications might require tens of thousands of target positions per
second for even the simplest motion.
Ethernet capabilities within the newest drives increase
communication bandwidth exponentially utilizing robust protocols and topologies
that the computer is already using. Evans
says the Kollmorgen Automation Suite embraces Ethernet and uses the EtherCat protocol
to communicate seamlessly to the AKD drives.
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