Re: What makes a motor a servo?

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