September
1997, Issue 86
PC/104
Quarter:
Motion Control with PC/104
START
MOVE
Start
move gets one or more axes moving. Don't use it for
immediate move, but it can be used for all other moves.
If
single-axis moves are required, they may or may not
require a start command depending on the variation of
the specified move. For example, a 25-hex command byte
makes a timed move after a start command is issued,
while a 26-hex command begins the timed move at message
transfer.
Start
move gives the host at least some control over the node,
yielding a higher degree of control flexibility.
STOP
Stop
is the most important motion command. Unlike steppers,
which "like" to stop, servo motors like to
run away.
While
running away doesn't necessarily imply being out of
control, it does mean the motion is not doing what it
was told to do. This failure may be due to mechanical
breakage or electrical noise, among other things.
It's
impractical for a node to always know the actions of
other nodes. The host, on the other hand, should know
what's going on within the system and be able to require
all nodes to stop whatever they're doing.
The
stopping action may be smooth, decelerated, abrupt (full-power
regeneration), or a coast, which happens when motor
current is removed and the motor winds down at its own
rate.
Based
on the system's move and real-time requirements, a node
controller is selected. The PC/104 computer can be an
excellent choice. It's small, cost-effective, and easy
to program. It also packages well and uses minimal power.
Looking
at the prime objective using nodes, system capability
is affected only if the PC/104 system can't support
the required motion, I/O, or language.
Computers
chosen for embedded applications should be industrially
hardened since they will be in environments nonconducive
to its electronic construction (e.g., welding and plasma
cutting).
PC/104
ADVICE
PC/104
is a good little workhorse for many embedded applications,
and it fits in most. It's small and inexpensive, too,
so most customers won't mind maintaining a PC/104 inventory.
When
choosing an operator interface (i.e., keyboard and display),
standardize on it so you won't find out you can't do
what you want. And, practice with it before being tossed
into a situation. This will free you up to focus on
the application and not get sidetracked with the PC/104
form factor.
And
finally, when designing for embedded operations, keep
in mind that embedding the control means more than just
sticking the computer into a machine cabinet. It means
designing the computer into the machine so it looks
like an integral part of the main design objective.
Smaller
may not always be better. But, don't believe that just
because it's small, it can't do the job.
