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Stepper Motors vs servo motor


									                          Stepper Motors vs. Servo Motors vs.
      Depending on whose web site you visit, you will be offered a variety of
reasons why their type of electronics and motors are better than the rest. The
purpose of this page is to give you the straight facts, without distorted torque
figures or unrealistic cutting speed claims.
      Stepper motors follow a laundry list of x/y axis coordinates provided by the
controller. The torch proceeds to each point in the file, perhaps a half a
thousandth of an inch apart, both axes working in precise synchronization with
each other. The relatively simple circuitry involved provides great reliability,
good low speed torque, and easy set-up. Positioning errors don't occur, since
stepper motors know in advance where they are going and when to stop.
      Servo motors depend upon "closed-loop" circuitry to supply information
back to the computer so that positioning errors can be continuously corrected.
This feedback is provided by encoders, which in some respects, are like the
bicycle wheels that some road builders and others use to measure distances.
      A new type of servo motor called an "intelligent servo motor" has recently
appeared on the scene. It is useful for some applications, and we use it in our
automatic torch height controls. In fact, we probably purchase more of them
than any of our competitors. The motor has some programmable computer
circuitry incorporated into it, which lets it independently follow simple
instructions. It works well with our torch height controls, due to their low
torque demand. We believe intelligent motors are impractical for driving a
cnc machine, for reasons given below.
      The following paragraphs explain each of these types of motors in more
Stepper motors:
      Stepper motors are relatively inexpensive, and provide the same or
greater accuracy as servo motors. Sufficiently powerful stepper motors for a
given application do not lose steps. Stepper motors are no more likely to lose
steps than a servo encoder is to pass bad information back to the controller.
This hoax is pure hype generated by manufacturers who are either unfamiliar
with stepper motors, or who are distorting facts to market their particular
product. If for some reason a stepper motor does encounter an obstacle it
can't overcome, such as a tipped up plate, it will simply skip steps, hurting
nothing. If a servo motor encounters the same obstacle, it will sit there and
fight itself until it breaks a gear tooth or burns up.
      If a stepper motor goes bad, which can happen to all three types of motors,
replacement cost is under $200.00. No elaborate tuning process is needed to
keep it functioning accurately. Best of all, its excellent low rpm torque lends
itself perfectly to cnc plasma and oxy-fuel shape cutting, without the need for
backlash producing gearboxes. The low speed torque makes it possible to
use a timing belt and pulley reduction, with virtually no backlash.
      Bear in mind that the vast majority of cnc cutting movements with stepper
motors are at less than 300 rpm motor speed, which translates to about 130
inches per minute on Torchmate machines. Cutting speeds and rapid
traverse moves up to about 350 inches per minute are possible, although
cutting would rarely require speeds in excess of 150 inches per minute.
Rapid traverse moves faster than 350 inches per minute are virtually never
needed, as the distance between pieces being cut rarely provides sufficient
time for ramping (acceleration and deceleration). Don't be taken in by claims
of super fast speeds. The more important issue in cnc plasma and oxy-fuel
applications is how slow a move a machine can make. Oxy-fuel cutting of
thick steel can require speeds as slow as 2 or 3 inches per minute.
      The use of dual X axis motors is easier and more precise with stepper
motors than with intelligent motors, which require instructions to be passed
along from motor to motor in kind of a daisy chain, rather than directly to both
motors from the controller. With steppers, the two motor drivers receive
identical simultaneous instructions from the computer, one in reverse of the
other, reducing the chance of mis-communication.
Servo motors:
      Servo motors are somewhat more expensive than steppers -- perhaps
double the price, or more. They are generally just as accurate, if maintained
in a proper state of tune, however they rely on encoders to provide positioning
information back to the computer. Thus the complexity of the system is at
least doubled, with no accuracy advantage, greater initial cost, and more
maintenance issues. The "closed loop" rhetoric that some manufacturers
play up sounds convincing to the uninitiated, but provides no benefit over a
simpler and more reliable stepper system.
    Servo motors are available in larger sizes than stepper motors, and
powerful servos are generally used on heavy machines with gantry carriages
in the 500 to 1,000 lb range. They offer no advantage whatsoever on lighter
machines, such as Torchmate and its competitors.
Intelligent servo motors:
    The argument given by cnc cutting machine manufacturers who use these
motors is that since they perform some of the computing internally, there is no
need for external electronics that might break down. They also claim that
intelligent motors reduce the amount of cabling necessary. This, of course, is
baloney. Since intelligent motors combine mechanical parts with computer
circuitry, they are more likely to break down than external electronics with no
moving parts. Which is more likely to break down, a television/dvd player
combination, or a regular television set? When intelligent motors do break
down, the replacement cost is about $600 a pop. Also, consider the fact that
intelligent motors are directly exposed to cutting debris and plasma dust, unlike
external electronics that can be situated 20 or more feet from the cutting table.
      In this photo, the cabling on the left controls two intelligent motors. Two
cables get connected to each motor. Additional custom cabling connects a
control box to the computer.
The same length of cabling used to control two stepper motors is shown on the
right. Only one cable is connected to each motor. The two cables to the
right of the motors are extension cables, which can be added to increase the
working length. A standard printer cable connects the controller to the
Which set of cables is more complex?
      Small intelligent motors with 50 oz. in. peak torque and 28 oz. in.
continuous torque are used on one new competitor's cnc machine, although
this is disguised by the citing of output torque at the gearbox. Although these
small motors are expensive, larger intelligent motors would be far more costly.
The only way these relatively low power motors can drive a gantry on a cnc
machine is to run them at a very high rpm with a large gear reduction. This is
kind of like driving your stick shift car around in low gear. This high rpm
greatly increases motor wear, and introduces planetary gearbox backlash into
the equation. When a gearbox first turns in one direction, and then the other,
as in cutting a circle, the backlash in the gear train must be taken up before the
direction changes. Unless super-expensive low-backlash planetary gear
boxes are used, as on the large $100,000 plus machines, circles don't end up
in the same place they started, etc.
More on planetary gearboxes:
      The photo below shows the difference between an "intelligent servo
motor" and a full-fledged brushless servo motor from an $18,000 Burny
Phantom cnc control system, as found on machines costing over $100,000.
The difference is obvious. The Burny motor (top), while a servo unit, is far
larger and more powerful, and can easily loaf along at the relatively low speeds
needed for cnc cutting without the wear of high rpm operation. The planetary
gearbox on the Burny motor is a low backlash unit with 3 arc minutes of
backlash, as opposed to cheaper planetary gearboxes with 6, or even 12 arc
minutes of backlash. If you are considering the use of motors with planetary
gearboxes, be absolutely certain to find out the manufacturer and model of
gearbox you are buying, and verify the backlash specifications on that
manufacturer's web site. Don't rely on gearbox specifications provided by the
company selling you the cnc machine.
      While most driver software, including ours, has a backlash compensation
feature, it is never as accurate as the lack of backlash in the first place. In our
view, the only satisfactory type of moderate-cost drive system uses powerful
stepper motors in conjunction with a timing belt/pulley reduction without
backlash. We have tried them all. See bottom photo.

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