Re: steppers or servo?

Hi,
If I can jump in here with my thoughts about the stepper vs.
servodrive debate.

I have designed both step motor drives and servodrives my entire
proffesional life, so I have no particular axe to grind regarding the
virtues of one over the other.

Before getting starting I have one myth I've noticed I would like to
dispel. It seems some belive step motors mysteriously "lose steps"
and thus are suspect for any serious application.

It is true if a step motor is overloaded it will miss steps. At that
point it will not just miss a single step, it wiil miss hundreds or
thousands at a time, making it obvois to the operator that something
is amiss. It simly means more is being asked of the motor than it can
deliver.

Uder the same conditions a servodrive would do no better; rather it
will "fault out".

As long as either are operated within their limits, both will deliver
what they are expeced to.

As I see it the advantages and disadvantages of both are:

SET MOTOR ADVANTAGES:

Plug and play. Easty to setup and use.
High torque at low speed
Open-loop operation
Fail safe; if anything breaks, the motor stops
No wear-out mechanism excetp fo bearings, rugged, tolerates abuse.
Standardized motors and drives
Easy to calculate expected performance
Inexpensive

STEP MOTOR DISAVANTAGES:

Prone to resonance, particularly if not microstepped
High pole-count motor; low efficiency at high speeds
Gets hot. Requires high current independent of load
Low stiffness. Equal to ordinal step reselution
No feedback of motor position
Low torque to inertia ratio; limited in acceletration.
Audibly noisy at moderate and high speeds

SERVOMOTOR ADVANTAGES:

Gauranteed motor position. Encoder feedback
High torque to inertia ratio; quick response, fast acceleration
No resonances
Efficient. Twice the power of a step motor for size.
No current at no load.
High stiffness if used with a PID drive.
High accuracy; entirely encoder dependent.
Audibly quiet at moderate and high speeds.

SERVOMOTOR DISADVANTAGES:

Complex; much more difficult to setup, requires servo "tuning"
Not failsafe; requires many safety circuits to prevent run-away
Wear-out mechanism; 1,000 to 5,000 hours brush life
Requires high peak current vs. average current power supply
Expensive

There's probably a lot more than can be added to each list. What it
comes down to is a Ford versus Ferrari thing. Both will get you to
work in the morning, but if you need the speed and ability to take
curves, you'll chose the latter.

Peronally, I would go with a well designed microstep drive system if
what I needed was to meet a low to moderate performance application.
It is dependable but not exciting. If it were a high performance
application, I would bite the bullet and go with a servodrive; it
just can't be beat on performance.

Mariss