Re: What makes a motor a servo?

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