Re: [CAD_CAM_EDM_DRO] Re: steppers or servo?

Mariss,

Thanks for the information.

Have you ever looked at the new 3 phase stepper motors.

Bob Campbell

----- Original Message -----
From: Mariss Freimanis <geckohall@...>
To: <CAD_CAM_EDM_DRO@egroups.com>
Sent: Monday, July 10, 2000 12:50 AM
Subject: [CAD_CAM_EDM_DRO] 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
>
>
>
>
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