Stepper vs Servo

Stepper Motors vs. Servo Motors vs. Intelligent Motors


 

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 Bullnose bit 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.  The motor has some programmable computer circuitry incorporated into it, which lets it independently follow simple instructions.  It works well with Z axis 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 detail. 

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 router and 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.

The use of dual X axis motors (slave axis) 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 miss 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 1,000 lb range.  They offer no advantage whatsoever on shaping machines, such as Revolucion 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 foam dust, unlike external electronics that can be situated 20 or more feet from the cutting surface.

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.