Traditionally,
a complex manufacturing machine has one large, powerful controller that governs
sequencing, motion and I/O. Multiple drives and motors are connected to the
controller via a motion network. The controller itself can be linked to a
human-machine interface, a PC, or both. It might also be connected to a
manufacturing execution system or an enterprise resource planning system.
This
approach is not the easiest system to optimize, but it does have its
advantages, particularly if the application requires synchronized motion
between multiple axes. Such applications include six-axis robots, injection
molding machines, milling machines and water-jet cutting machines.
For
an automated assembly system, however, engineers would prefer to take a modular
approach to machine design. A modular system is more flexible, faster to change
over, and—for medical device assembly—easier to validate and simpler to clean.
“With
a modular system, it’s easier to diagnose an issue,” says Chris Knudsen,
product marketing supervisor at Yaskawa America Inc. “You can isolate a problem
to a specific module instead of a whole line. And, if the module is critical to
your process, you can have a spare ready to go in case of a problem or sudden
bottleneck.”
For
the machine builder, a modular approach means less development time, smaller
component inventories, and the ability to integrate a wider range of machine
functions. Machine builders can also rough out common modules, such as
pick-and-place units, in advance of a project.
A
modular system consists of a series of cells or modules. Each cell has its own
control—a programmable automation controller (PAC). A PAC is a compact controller
that combines the features and capabilities of a PC-based control system with
that of a typical programmable logic controller (PLC). PACs are available from
a number of companies, including Yaskawa, Beckhoff Automation, Opto 22 and
Schneider Electric.
“A
modular system has more of a distributed control system vs. a centralized
control,” explains Knudsen. “PACs have become smaller, more connectable and
less expensive, and you can now afford to just pop one of these controls into
each module, as opposed to one big, fancy controller running the entire system.
So you might have a line with five PACs and one PLC that’s just monitoring the
whole line.”
“With
a modular system, it’s easier to diagnose an issue.”—Chris Knudsen, Yaskawa
America Inc.
High-speed
networks, such as Ethernet/IP, Modbus and OPC, have supplanted point-to-point
wiring in assembly systems and enabled engineers to take a modular approach to
design. “Network speeds are now close to the speed of PC or PLC backplanes,”
says Knudsen. “These open network standards also ensure interoperability.”
On
the software side, standardized, object-oriented programming environments, such
as IEC61131-3, are well-suited for modular machine design. IEC61131-3 has been
adopted by numerous suppliers, including Yaskawa, Siemens, Bosch Rexroth,
Schneider, Beckhoff, Omron, Mitsubishi Electric, and B&R Industrial
Automation.
“Object-oriented
programming means that the code is modularized,” says Knudsen “Think of an
object as a subroutine. You feed it some variables and it spits out an answer,
but you don’t necessarily need to know everything that’s going on inside the
subroutine.”
Is
a modular system faster than centralized control? “There’s a speed advantage
within the module, because you have this controller that’s handling everything
just for that module,” says Knudsen. “There may be a slight speed disadvantage
when synchronizing the motion of each module. In a traditional system, if I
want to synch, say, Axis 9 with Axis 1 and Axis 2, I would be doing it across
the backplane of the controller, which is immune to noise and very fast. That’s
why a milling machine will have a central controller, because it’s got to
synchronize everything very tightly.
“On
the other hand, a traditional system will eventually max out on axis count, I/O
count and scan times.”