Re: What makes a motor a servo?

Good point; guess I still had my mind wrapped around the "unstallable
stepper" idea I've been working on when I wrote that.

Couple of points:

1) Everyone gets hung up on torque when it's power that matters.
Power is speed times torque, gearing changes the mix of speed to
torque to match the load. Think Watts, not Nm or in-oz.

2) Stepper low-speed running torque is equal to the holding torque
when microstepping. Why? No torque is invested in resonating the
motor. Endless dyno tests prove that out.

3) An open-loop stepper should be derated to 50% of what it's capable
of. But why blame the motor when the fault is the way it's being run?
That same motor run closed-loop (a "50-pole AC servo motor" now) can
do considerably better.

4) You build in a 50% derating for steppers but you do not for
servos. What happens as wear sets in and the load on the servo
increases? A step motor is smart enough to shed a load if it is too
large. You call it stalling like it's a bad thing. A servo is too
dumb. It will keep trying to move it while its operating point moves
into the self-destruct range. Gotta keep the playing field level to
compare fairly.

-----------------

There was a larger point I was trying to make. I don't think I did a
good job because it was missed so here goes again:

Steppers (50-pole AC servos) are ideal for applications that have two
distinct operating modes; high-torque/low-speed and low-torque/high-
speed (cutting feedrates and rapids). The motor's speed-torque curve
naturally matches this type of application.

On the otherhand, servo torque is independent of speed, making its
efficient operating range speed very narrow. This makes correct
gearing very important.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" <cnc4me@g...>
wrote:

> Hi Mariss,
>
> You forgot to divide the stepper torque by 2. As you have said in
> many posts, for a reliable system you can not run a stepper at more
> than 50% of its torque. And this is especially true for a machine
> tool were the cutters get dull and federates may not be optimum.
> Also I think you may have inflated the stepper torque 2x more than
> it really is for a total of 4x error in torque.
>
> The first part of 50% safety margin for torque is an obvious

point.

> The second 2x error I think you made is not so obvious. You are
> using a 600 oz-in stepper motor in your example. Typically

steppers

> are rated for there holding torque not there running torque. A
> difference of about 50%. If you meant a stepper with 600 oz-inches
> of running torque then that would be a 1200 oz-in stepper. Here is
> an example of a NEMA 34 stepper rated approximately 1200 oz-in with
> about 600 oz-in running torque. I picked this motor because it has
> very good performance at Gecko drive level.
>
> http://products.danahermotion.com/danaher/modelDetail.asp?
> User=StepMotors2&PkgID=424077
>
> Ill post a few specs here for this motor and do the conversions

from

> Nm to oz-in so people can follow along and check my logic.

Nm/.0071=

> oz-in.
>
> Motor N32xxxJ-L (P) NEMA 34 stepper parallel winding PacSci Mfg.
>
> Holding torque 1195 oz-in.
> Max RPM at 72volts 2400
> Don't forget. We have to divide all these torques by 2 for a

safety

> margin in stepper applications.
> Max. running torque 5Nm = 704 oz-in /2 = 352 oz-in
> Torque at 250 RPM 4.3Nm = 605 oz-in /2 = 303 oz-in
> Torque at 500 RPM 3.4Nm = 479 oz-in /2 = 240 oz-in
> Torque at 750 RPM 2.2Nm = 310 oz-in /2 = 155 oz-in
> Torque at 1000 RPM 1.8Nm = 254 oz-in /2 = 127 oz-in
> Torque at 1250 RPM 1.4Nm = 197 oz-in /2 = 99 oz-in
> Torque at 1500 RPM 1.3Nm = 183 oz-in /2 = 91 oz-in
>
> Let's review here. We have a 1200 oz-in NEMA 34 stepper with 1/2

of

> the torque of our NEMA 23 DC servo rated 60 oz-in continuous geared
> 10 to 1.
>
> So now let's increase our stepper motor torque again until it can
> compete with the 60 oz-in servo motor. Once again I have selected

a

> stepper motor that will run well on a Gecko drive. I wanted a
> stepper motor with a running torque comparable to the servo motor.
> So I looked at the running torque curves until I found one close to
> 600 oz-in.
>
> http://products.danahermotion.com/danaher/modelDetail.asp?
> User=StepMotors2&PkgID=424084
>
> Motor N33xxxJ-L (P) NEMA 34 stepper parallel winding PacSci Mfg.
>
> Holding torque 1713 oz-in.
> Max RPM at 72volts 2250
> Don't forget. We have to divide all these torques by 2 for safety
> margin in stepper applications.
> Max. running torque 8.8Nm = 1239 oz-in /2 = 620 oz-in
> Torque at 250 RPM 6.4Nm = 900 oz-in /2 = 450 oz-in
> Torque at 500 RPM 3.8Nm = 535 oz-in /2 = 267 oz-in
> Torque at 750 RPM 2.6Nm = 366 oz-in /2 = 183 oz-in
> Torque at 1000 RPM 2Nm = 282 oz-in /2 = 141 oz-in
> Torque at 1250 RPM 1.2Nm = 197 oz-in /2 = 99 oz-in
> Torque at 1500 RPM 1.1Nm = 170 oz-in /2 = 86 oz-in
>
>
>
> The conclusion I see here is that a NEMA 23 DC servo rated 60 oz-

in

> continuous with a 10 to 1 reduction has a continues rating of 600

oz-

> in up to 600 RPM. And a stepper motor that can begin to compete

with

> it is a minimum of a 1713 oz-in NEMA 34 stepper.
>
> In your example below you seem to think 320 IPM of the stepper is a
> big selling point over the 120 IPM of the servo motor. In my

opinion

> all it gives you is bragging rights but not much more. In a

machine

> this size some ware between a Sherline and a Bridgeport I would

think

> 80 to 100 IPM is a good speed for a rapid.
>
> The reason I took the trouble to look all this up and type it out

was

> I wanted to see a fair comparison of steppers and servos for

myself.

> I am glad I did. This example has shown me that steppers are much
> worse than is obvious. This comparison just about finishes pounding
> the nails in the stepper coffin.
>
>
> Wally
> RIP
>
>
>
>
>
>
>
> --- 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