Re: What makes a motor a servo?

Scott,

No problem. I don't have a dog in this fight; I design drives for
both steppers and servos (maybe I have two dogs in after all).

The tools for understanding motors so I can design for them is
mathematics and physics. A nice dyno acts as a reality check and is
used to back up all the theory and math.

I'm very careful about what I write; it would be very embarrassing to
make a statement I cannot back up with empirical, reproducable data.
Though I have "seat of the pants" instincts I'm not a cut-and-try
designer. For me math and theory must be borne out by empirical test
data before I believe something. I have problems with anecdotal
evidence.

Mariss





--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
<polaraligned@o...> wrote:

> With all due respect, either this post is completely off or I
> am loosing my mind. Somebody tell me.
> Mariss, you completely ignore the fact that servos have a peak
> torque rating along with the maximum continuous rating. This
> peak rating is usually 4 to 6 times the continuous rating.
> You have put a 600 oz-in stepper against a 60 oz-in "continuous
> torque" servo. You seem to be using a mill machine as the
> example. The servo will "hands down" KILL the stepper in this
> application. Not even close.
> Let's look a the real world. The BOSS steppers were
> about 1200 oz-in and with Gecko drives they still are barely
> adequate with a 1:1 ratio. Performance is moderate at best.
> Ron, a frequent poster on this list, uses 40 oz-in servos on
> his machine and drives the knee with ease. He has used the
> machine in a commercial setting, drilling hundreds of large
> holes in steel and the servo barely gets warm. A 600 oz-in
> stepper would not even move the knee of a Bridgeport. A 600
> oz-in stepper would not even work for the X and Y axis.
> Whereas, most commercial retrofits use servos in the 29 oz-in
> range for all axis and performance is spectacular. These motors
> are half the torque of the motor you use in the example.
> Not only that, my torque vs. speed graphs for my Baldor motors
> show a fairly flat response, not power peaks over a narrow range
> as you have said. The motors greatest continuous torque is at
> the lowest speed range with the torque tailing off slightly at
> the top of the speed range.
> It seems easy to make numbers come out as one wishes, but the
> fact is that servos dominate the market in this application.
> If manufactures could make a reliable large mill with steppers,
> they would. Even Bridgeport changed over to servos on their
> later machines.
> Most users who have changed over to servos would never go back.
> I choose to use real world examples to make my decision. And,
> as of now, servos it is. If I am off base here, please advise. I
> will consider all arguments for steppers. I do prefer real world
> examples though.
>
> Respectfully,
> Scott
>
>
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
> <mariss92705@y...> wrote:
> > The picture is not quite as rosy as painted in favor of DC brush-

type

> > PM servo motors. Consider this comparison between a NEMA-34

stepper

> > and a NEMA-23 servo motor:
> >
> > 1) Servo: 6,000 RPM rated load speed, 60 in-oz continuous rated
> > torque. The max continuous torque is just that; no to be

exceeded.

> > This means you have 50 in-oz available be it at 1 RPM or 6,000

RPM.

> > Power output is 266 Watts.
> >
> > 2) Stepper: 600 in-oz low-speed torque falling to 50 in-oz at

3,600

> > RPM. Power output is also 133 Watts.
> >
> > A lot of CNC applications have two distinct operating modes, low
> > feedrates when work is being done and rapids which reposition the
> > machine at no work load. Let's assume a 5 TPI screw is involved,

30

> > IPM is the work feedrate, 600 in-oz is the work load and 100 in-

oz is

> > needed for rapids.
> >
> > 1) Step motor: The step motor has 600 in-oz of low-speed torque,

so

> > it connects to the screw 1:1. The motor's torque is constant from

0

> > to 300 RPM (300 RPM = 1351 * 133W / 600 in-oz), past that, it

falls

> > off as the inverse of speed. 300 RPM is 60 IPM on a 5 TPI screw
> > though.
> >
> > The maximum speed for rapids is 320 IPM.
> >
> > 2) Servo motor: The servo motor has 60 in-oz of max continuous
> > torque. It requires a 10:1 reduction gearing to get 600 in-oz on

the

> > screw. The motor is turning 1,500 RPM at 30 IPM.
> >
> > 1,500 RPM is 25% of 6,000 RPM. This means 120 IPM is the maximum

work

> > feedrate. The rapids IPM is only 145 IPM (See note at the end for

the

> > boring math).
> > ---------------------------------
> >
> > So what happened? How come a stepper having only 1/2 the servo's
> > power (133W vs 266W) gets 320 IPM rapids to the servo's 145 IPM?
> >
> > The difference is the motors' different speed-power curves. The
> > stepper has a flat 133W power vs speed curve past 300 RPM while

the

> > servo power peaks over a narrow speed range. Here's a comparison:
> >
> > 0000-RPM 000W 000W *no RPM, zero power for both
> > 0100-RPM 044W 004W
> > 0200-RPM 088W 009W
> > 0300-RPM 133W 013W *stepper reaches full power
> > 0600-RPM 133W 026W
> > 1000-RPM 127W 039W
> > 1600-RPM 118W 059W *stepper torque = 100 in-oz (max rapid)
> > 2000-RPM 112W 089W
> > 3000-RPM 098W 133W
> > 4000-RPM ---W 178W *stepper torque too small to be useful
> > 5000-RPM ---W 222W
> > 6000-RPM ---W 266W *servo reaches full power
> > 6500-RPM ---W 192W *servo power begins to drop
> > 7000-RPM ---W 096W
> > 7250-RPM ---W 048W *servo/10:1 reduction torque = 100 in-oz
> > 7500-RPM ---W 000W *servo no-load speed, zero power
> > -------------------------------
> >
> > A good way of looking at a stepper is to imagine a 133W motor

turning

> > at constant speed connected to an infinitely variable gearbox.

Your

> > step pulse rate determines the reduction ratio.
> >
> > This type of an application favors step motors. The load is

either

> > high-torque low-speed or high-speed low-torque.
> >
> > About servos in general: A servo is any system that uses negative
> > closed-loop feedback. A DC servo motor is just a DC motor without
> > feedback. A step motor becomes a "50-pole AC servo motor" with
> > feedback.
> >
> > Mariss
> > ----------------------
> >
> > Math Note (the boring stuff):
> >
> > Assume the 60 in-oz rated servo motor will have a peak torque of

300

> > in-oz (5:1 ratio).
> >
> > 1) Rated RPM = (1 - Rated torque / Peak torque) * No-Load RPM
> > 2) Torque available = (1 - RPM / No-load RPM) * Peak torque
> > 3) Power (Watts) = in-oz * RPM / 1351
> >
> > A step motor's corner speed is where torque begins to drop. This

is

> > approximately:
> >
> > 1) RPM = 0.191 * V / I * L
> >
> > Where V = power supply voltage, I = phase current and L = motor
> > winding inductance.
> >
> > Motor power increases proportionally with speed up to the corner

RPM

> > of the motor. Past that motor power has a slight negative slope

with

> > speed:
> >
> > 2) Power (Watts) = ((V * Holding torque in-oz) / (7,073 * I *

L)) -

> > Detent torque in-oz * RPM / 1351
> >
> > Detent torque is always a loss in a step motor. It subtracts

power

> > from the motor at a rate proportional to speed. This is accounted

in

> > the "-" term in eq. (2).
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...>

wrote:

> > > Alex Holden wrote:
> > >
> > > >On 6 May 2005, at 8:30 pm, Pete Brown ((YahooGroups)) wrote:
> > > >
> > > >
> > > >>What about steppers with encoders then?
> > > >>Such as:
> > > >>http://www.maxnc.com/page13.html
> > > >>
> > > >>
> > > >
> > > >It has closed loop control so I'd say it's a servo, but I

don't

> > > >understand what they gain by using a stepper instead of a DC

motor-

> > I
> > > >thought the only reason you would use steppers is so you can

use

> > open
> > > >loop control and avoid the complexity of an encoder and loop
> > > >controller. Surely this system combines the worst of both

worlds?

> > > >
> > > >
> > > >
> > > Right. The only advantage is that you can DETECT lost steps.

But,

> > without
> > > exotic control software, like Mariss at Gecko is developing

(the

> > > "unstallable"
> > > stepper drive) it doesn't really solve the problem. The big
> > problem with
> > > steppers is the torque falls off rapidly with increasing speed,

due

> > to the
> > > high inductance and the high pole count (50 on a standard 200
> > step/rev
> > > stepper). Two-pole DC brush motors only reverse the current in

two

> > sets
> > > of coils
> > > each time a commutator segment crosses the brushes. Each coil

on

> > the
> > > armature
> > > is pretty small, so the current can be reversed easily. When a

DC

> > brush
> > > motor
> > > stalls, it gives full rated torque right down to zero speed.

When

> > a
> > > stepper stalls,
> > > as soon as it lags more than 180 degrees (2 full steps) the

torque

> > > violently reverses,
> > > and the motor slams to a stop, and produces no usable torque

until

> > the
> > > step rate
> > > is brought down close to zero. The stepper just "gives up"

while

> > the DC
> > > brush motor
> > > continues to give its maximum torque against the load.
> > >
> > > Another difference is that steppers produce great heat at idle
> > unless an
> > > idle current reduction
> > > scheme is used. They also suffer from a lot of iron heating

when

> > run at
> > > high speed (over
> > > 1500 RPM, say). They have no torque reserve, they just put out
> > whatever
> > > torque is available
> > > at the current speed. Servos can be set up with a continuous
> > current
> > > (or torque) limit, and
> > > a peak limit. They can run all day near the continuous limit,

but

> > still
> > > have a reserve of 4 to 8
> > > times greater torque for sudden accelerations.
> > >
> > > Jon