First of all, wether you have loop or not is up to you.
If you need it, you can add feedback to any drive.
Getting feedback to monitor actual position of the axis
is basic requirement to make a loop. If you use the
feedback to control the position - you got yourself a loop.
Steppers were traditionally used as open loop positioning
systems but this is not a must.
Typical characteristics of the stepper systems are:
- cheapre than servo
- ok for slow speed
- pretty high torque
- they tend to resonate on cretain speeds (really loudly!)
- if load is much lower than available torque and application
is not mission critical, you can avoid feedback and hope
that counting step pulses you send to stepper will
match actual position. if the torque ration is not big
or you try to accelerate quickly, chances are that
stepper system will "skip" some of the pulses so
it is not good idea to rely on pulse counting as
replacement for feedback.
Another difference is that with the servo system, positioning
loop is closed in the motion controller instead in your
program and because of that it can have much faster
response without chocking the CPU of your PLC.
Basically you would configure servo for different
motion profiles (jog, home, absolute move to a position,
indexed move, batch move, gearing two or more axes,
circular interpolation etc.) and then use PLC program
just to trigger right move at the right moment.
Servos are true kings of the motion control systems:
- very fast
- VERY flexible (besides positioning and speed control
you can usually have torque control and many different
features that are not available in other systems)
- can be very expencive (but the prices are falling)
- come in all shapes and sizes, for example you can have
linear system or motion controller, amplifier, motor
and encoder all built in ONE unit (check the SilverMax
devices for example).
I hope this helps
