Q. What kind of information typically does a servo send out over a
digital bus network? There seems
to be different schools of thought
about how to use different bus
schemes, or even when an analog
interface is more appropriate.
A. Anywhere from 25 to 40% of
the servo applications we see
are still analog. That said, most
analog connections today tend
to be found between inexpensive
controllers made by one manufacturer and servodrives made
by another. The two may support
digital bus schemes, but perhaps
not the same digital bus schemes.
So the easiest connection between the two is a ±10-V analog
command signal to the drive and
a digital pulse-position signal from
the drive to the controller.
But machine controllers have
no access to detailed servodrive
information that could help diagnose problems unless the two
communicate over some kind of
digital-bus network. For example,
consider the case of a malfunctioning motor-position feedback
device. The drive would fault and
generate a general trouble signal
to the machine controller in the
form of a digital output.
A controller that just gets a digital signal from the drive would only
know that a fault happened somewhere. There are no specifics that
could help solve the problem and
quickly get back online. It is usually necessary to open the control
cabinet, read the fault code on the
drive display, then find the manual and locate details on the fault
code, or call the drive vendor.
In contrast, servodrives on
such digital-bus networks as SynqNet, CANopen, Profibus, DeviceNet, EtherCat or Sercos let
the controller query over the
network for fault information in
the event of a problem. The controller can then display specific
details about the difficulty to the
machine operator — as well as
recommend a course of action via
software in the controller.
Additionally, information available from the drive through the
digital-bus network can warn the
controller of potential future problems. For example, a rising demand
for motor current often indicates
excessive machine wear that could
eventually damage the machine and
reduce product quality. Through
a digital network the operator can
be prompted to schedule maintenance or take other action to keep
the machine performing properly.
Many digital-bus networks also
let the controller store the drive's
application parameters and then
download them to the servodrive. This is particularly helpful
in the case of a drive swap out. It
lets the controller send application parameters to the new drive
quickly and easily.
It is also sometimes convenient to let the controller change
the drive parameters on the fly.
An example is tuning gain in a system that must cut through a work
piece slowly and accurately, but
travel as fast as possible between
the end of one cut and the beginning of the next. Such systems
may be optimized by setting one
PI gain when cutting, a different
one when traversing.
Carroll Wontrop is a senior system
engineer with Danaher Motion
Corp. Got a question about
motion control or mechatronics?
Ask Carroll via e-mail at
contactus@danahermotion.com