Re: What makes a motor a servo?

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