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    Default good link for steppers+drives

    Quote Originally Posted by jimluu View Post
    I read some where that larger stepper motors lose resolution. I'm not sure what this means, but the post suggest 600oz/in as the largest recommended stepper size. I'm thinking about getting some 900oz steppers from Kelinginc, but I'm not sure about this lost resolution issue. Can someone clarify? thanks.

    here is a link where you can get some 1200oz nema-34 steppers that are water/dust proof for $149 each

    http://www.kelinginc.net/NEMA34Motor.html



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    Quote Originally Posted by haylspa View Post
    hmm not that I am all that great at this stuff but from 90% of the post on this the only answer I can come up with is if I want speed I have to go servo but if I want accuracy and procession + consistency stepper is the only real way to go!
    Servo = accuracy, precision, consistency, and speed
    Stepper = cheap

    Which do you need?

    All modern, commercial grade, metalworking CNC equipement uses servos. It's like the old adage about IBM and job retention. "Nobody ever lost their job because they specified or recommended IBM hardware." Plenty lost their jobs because they didn't.


    Fred Smith - IMService
    http://www.imsrv.com
    Inexpensive servo solutions for desktop machines



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    just gonna throw in my 2c. i have a 4x6 table i built to run both a plasma and a router, and im running a 4-axis 15 amp stepper board from dtllc.com. they sell controllers, breakouts, motors, ect. i buy all my drive parts from sdp-si.com. they have a huge selection of standard and metric. I have had some problems with the steppers and the plasma, I have an old 100amp 3 phase miller, and it makes a TON of HF noise. I have to trigger it manually when I have to use it. I also cut at 180ipm in anything under 3/16". I know that sounds high but thats the cleanest cut. Maybe on a less powerful machine it needs to be a little slower. I found on my system if I drop the plasma head to touch the sheet that Im cutting, that the arc start HF is a very short pulse, rather then a bunch of long pulses, and helped a LOT. Also, make sure you run everything through a good ground, ground your table, controller, and workpiece. I was running my setup through an extension cord with the ground pin cut off and almost fried my controller. A good 14/3 cord and a bunch of good grounds made a big difference. Just my 2c.



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    Default Servo or Stepper for CNC

    I am thinking about buying a viynl cutter, for some smaal biz work and maybe a little at home income. Any idea on eqmt? and what is the deal with a stepper are Servo motor? And what is the deal with the machine that offers vinyl cutting and engraving?
    Thanks for any help



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    My first post, so its probably wrong in some way.
    I am interested in running 600W. brush DC motors with Geckodrive controllers. Will this kind of system allow my motors to position themselves accurately? By this I mean more accurately than the 40 or so Commutator slots which each motor has. I keep reading about encoder line counts. I need my motors to work accurately at around 1000 line counts.



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    Wink

    Servo = accuracy, precision, consistency, and speed
    Stepper = cheap
    Is it too much to ask for both? What about a stepper servo...

    http://www.evarobotics.com/_product_...evelopment_Kit



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    Without reading every letter of every post on this thread... I would like to confirm and correct some statements here...

    A "Servo" really means "closed loop", regardless of the actualy motor type.

    In reality, what most people mean here when they say servo, is actually an "AC Brushless motor with encoder feedback". It should be noted that a DC Stepper motor can actually be a servo motor (you just have to add an encoder and now it's a servo)!

    What most people have to realize is that you'll typically find three types of motors on machines here:
    1. Open loop DC stepper motor
    2. Closed loop DC stepper motor
    3. Closed loop AC motor (i.e. "Servo")

    Q. How do you choose between open and closed loop?
    A. You have to look at the application's required accuracy and/or repeatiblity. In other words, what type of tolerances do you want to hold on your machine. If you're looking to do wood, MDF, foam (or similar) cutting, then an open loop system might be OK. If you're looking to do metal cutting (with assembly) and you need to hold tight tolerances (to reduce stackup), then you will likely need a closed loop system.

    Q. How do you choose between a DC stepper motor and AC brushless "servo"
    A. If we ignore the cost aspect (even though it's a big factor), then it simply comes down to motor performance. Once you have your mechanical system designed, you can calculate the inertia of your system and the required torque (at specific rpm). You can then use this information to look at a motor manufacturer's speed-torque curve and find a motor that meets your needs.

    Q. "Inertia Matching?" what's that?
    A. This is the inertia of the load compared to the inertia of the motor's rotor. The ideal inertial match is 1:1; however, this seldom happens so there's a mismatch. Most motor manufacturers will publish a maximum allowable mismatch for their motor. Regardless of whatever a catalog may say is "possible" use the following rules:
    < 6:1 for best dynamic performance
    6:1 - 10: 1 for ok dynamic performance
    up to max for it'll work

    Ref:
    http://www.sdp-si.com/D795/79501277.pdf
    http://www.motion-designs.com/images...s_Dec_2008.pdf
    http://www.kollmorgen.com/website/co...ay_06_AHTD.pdf
    Getting torque-to-inertia right | Machine Design
    Direct Drives for Inertia Matching - 2010-11-09 17:04:16 | Design News



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    Quote Originally Posted by schbrownie View Post
    In reality, what most people mean here when they say servo, is actually an "AC Brushless motor with encoder feedback". It should be noted that a DC Stepper motor can actually be a servo motor (you just have to add an encoder and now it's a servo)!
    Although AC servo's are now the preferred motor, there are still many people here that are using DC servo's.
    For true closed loop you need a PID control loop which is not usually implemented when an encoder is fitted to a stepper.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    And aren't stepper motors just an AC motor?



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    The common low voltage type are DC steppers, DC pulse driven.
    There are practically identical AC stepper motors wound for 120vac that are used on 50/60hz direct either split phase or two phase, these have the advantage of running synchronously with the AC supply, 72 rpm on 60hz, 60rpm on 50hz, high torque, 'Instant' stop start.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    Step motors are actually "high pole-count AC permanent magnet synchronous motors". There; I'll bet you never knew the lowly and much maligned step motor had such a lofty title and nobly exalted pedigree.

    Seriously, a step motor is classified as a PMSM (permanent magnet synchronous motor) and it is in the same class category as the unfortunately-named BLDC (brushless DC) motors. Both are poly-phase synchronous AC motors. If cared to, you could run a BLDC motor as a 12-step per revolution step motor; it is indistinguishable from a 200 step per revolution step motor except for the 3-phase excitation and the low step-count.

    Mariss

    Last edited by Mariss Freimanis; 08-23-2012 at 03:23 AM.


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    I guess if one wishes to get pedantic about the motor definitions, the brushed motor fed from DC would be defined an AC motor as the commutator reverse polarity in the windings just as polarity in a BLDC does, but electronically.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    Correct. In fact all motors are AC motors.

    Mariss



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    Quote Originally Posted by Mariss Freimanis View Post
    Servos vs. steppers. Nice list, I wrote and I still like it.:-)

    Let me add a little to it that may be otherwise practical:

    If you are designing a machine and you get to motors, the first thing you should do is calculate the power you need. Never buy a motor (stepper or sevro) first and then figure out if it will fit what you need. That is the sign of an amateur or hack.

    Motors are motors. They couple power to your mechanism and power is what makes things happen. The choice of a motor comes after you know what's needed.

    Power is velocity times force or torque times RPM. It doesn't matter if the motors are steppers, servos or a gerbil in a spinning squirrel cage at the start.

    To seperate what motor need (neglect the gerbil), is the power your mechanism needs.

    Rule #1: If you need 100 Watts or less, use a step motor. If you need 200 Watts or more, you must use a servo. In between, either will do.

    So, how do you figure the power you need?

    Method 1: You have a plasma table, wood router or some other low work-load mechanism. You have a clear idea of how many IPM you want but your'e not sure of what force you want at that speed.

    Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. Multiply it by the IPM you want. Say that's 1,000 IPM. Divide the result by the magic number "531". The answer is 75.3 Watts so use a step motor.

    Eq: Watts = IPM * Lbs / 531

    Method 2: You have a Bridgeport CNC conversion you are doing. The machine has a 5 TPI screw and you need a work feedrate of 120 IPM. 120 IPM on a 5TPI screw 5 * 120 or 600 RPM.

    How about force? Not a clue? Use your machinist's experience on a manual machine. The handcrank is about 6" inches in diameter. How much force would you place on the handcranck before you figure you're not doing something right? I hear about 10 Lbs.

    !0 Lbs is 160 oz, 160 oz on the end of a 3" moment-arm (6" diameter, remember?) is 480 in-oz (3 times 160) ot torque on the leadscrew.

    The equation for rotary power is: Watts = in-oz * RPM / 1351

    For this example, Watts = 480 in-oz * 600 RPM / 1351 or 213 Watts.

    213 Watts is servo territory. You have to use a servo motor to get that, about a NEMA-34 one.

    OK. Long post, late night. If anyone cares, let me know. Proper application of servo motors is an entirely different topic, it's involved but not particularly difficult. Servos are not steppers and they are not interchanchable. Let me know if I should continue.

    Mariss
    Hi Mariss, forgive my ignorance but where do we get the magic number 531 or the number 1351 when calculating rotary power above?



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    MECHUP,

    It took about 5 minutes of quality-time with my calculator.

    I started with the basic definition of a horsepower which I memorized in high school as 1HP = 550 lbs lifted one foot in 1 second and 1HP = 746 Watts.


    LINEAR POWER (Watts = Lbs * IPM / 531)------------------------------

    Since most CNC math prefers IPM (inches per minute) instead of feet per second, rephrase the HP definition as lifting 550 lbs at 720 IPM (12" times 60 seconds). Now, let's get rid of that pesky '550' number; make 1 HP = lifting 1lb at 396,000 IPM (550 times 720).

    OK, but that's for 746 Watts; how much is it per Watt? Take 396,000 IPM and divide it by 746 and you get 530.831 as an answer. I just round it off to 531.

    This means 1 Watt of power can lift (or push with) 1lb at 530 IPM. So just multiply pounds of 'push' times IPM and divide the result by 531 to get your answer in Watts mechanical.


    ROTARY POWER (Watts = in-oz * RPM / 1351)------------------------

    Start with the same HP definition but imagine you have a 1/16" radius pulley on a motor. On this pulley you have wound a string attached to a 1lb weight that you will be lifting. 1 oz-in of torque will do the job (1 oz-in with a 1/16" radius will exert a 1lb force on the string).

    The pulley circumference is 2 * pi * radius or 0.3927". If you turn the shaft at 1RPM, you will be lifting the 1lb weight at a speed of 0.3927 IPM. We have a linear force and a velocity now.

    Since we already calculated 1 Watt = 1lb * 530.831 IPM, all we have to do is divide 530.831 IPM by 0.3927 IPM to get 1351.747 as the answer.

    1 Watt = 1 in-oz * 1 RPM / 1351.747 which means I should have rounded the denominator off as 1352 instead of 1351. Doesn't really matter since the result is off by only 0.06%.

    This took 5 minutes to write which is about as long as it took to generate the equations in the first place using just two memorized definitions for power: 1HP = 746 Watts and 1 HP = 550 ft-lbs / sec.

    I'm glad I paid attention in Mr. Long's physics class my senior year at Northwestern High School 45 years ago; you never know when you might need stuff later on in life. They certainly didn't teach this stuff to EE's in graduate school at Ohio State or UCLA.

    Mariss



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    excellent explanation, that clears up a lot of questions i had. examples like this just can't be beaten. thanks!
    i'm a wiser man now!!!



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    Quote Originally Posted by Mariss Freimanis View Post
    Step motors are actually "high pole-count AC permanent magnet synchronous motors". There; I'll bet you never knew the lowly and much maligned step motor had such a lofty title and nobly exalted pedigree.

    Seriously, a step motor is classified as a PMSM (permanent magnet synchronous motor) and it is in the same class category as the unfortunately-named BLDC (brushless DC) motors. Both are poly-phase synchronous AC motors. If cared to, you could run a BLDC motor as a 12-step per revolution step motor; it is indistinguishable from a 200 step per revolution step motor except for the 3-phase excitation and the low step-count.

    Mariss
    Hi all, new to this fine forum, but 6+ years designing AC Synchronous motors, aka BLDC aka PMSM aka PMAC aka SynRM aka SwithedRM yadda yadda.

    A stepper motor specifically has crenellation like features in the rotor and stator (rotor = spinny bit, stator = not spinny bit). These "teeth" take something that's a very low pole count, ie 4 magnets or "2 pole" and make it so each cycle of an AC or binary (DC switched) signal is only a portion of a pole of motion.

    It's not like a gearbox, torque is limited by other design considerations and you can only make so much torque regardless of motor topology based on how much magnetic material there is and the sizes and other details. A stepper is more like you have two handles that independently drive the same wheel or shaft, but they are ratcheting and only go one click at a time, but each one doesn't go far enough to ratchet again until the other goes. So you are pulling or pushing left-right-left-right to march that wheel around the circle. If you didn't have the goshdarned ratchets, you could just yank that wheel around. Who designed this ratcheting nightmare?!?

    Well, the ratchet analogy, much like steppers, makes small movements by design, for each uncontrolled and imprecise action on your (or the controller's) part. It inherently simplifies the controller.

    You can make sinusoidal or trapezoidal stepper motors and drive them like any brushless synchronous motor, from an inverter (be it servo oriented or not). They have an electrically high pole count while having low actual pole counts.

    The downside of steppers have already been mentioned, primarily the low speeds and power density. They are intentionally low speed, that is literally by design. They were developed for precise-ish control before precise electronics to control them existed.

    The ironic thing is that they are cheaper, when they are actually harder to build and have a lot more wasted material than a brushless motor. In truth at same volume and quality a brushless motor is cheaper to manufacture. Steps? They make magnets and stators fragile, easier to damage. They also introduce more air space where you normally want working material. They reduce the copper space and thus lower current limits and efficiency.

    These details aren't going to choose for you what to buy from existing off-shelf suppliers, but my hope is to help you understand more of what is actually physically different.

    To summarize:

    AC Synchronous Motors
    + With "Steps" (Teeth/crenellations)
    ++ Permanent Magnet
    ++ Switched Reluctance
    + No "Steps" (profiled or smooth surfaces)
    ++ Permanent Magnet Motors
    ++ Synchronous Reluctance Motors
    ++ Switched Reluctance
    ++ Separately Excited
    ++ Multiply-Fed (Field Synchronous)

    But WAIT!! WTH is a DC motor? In short it's a myth. A DC motor is mechanically switched via a brush commutator or other means instead of electrically switched by silicon chips. Take the brush commutator out and you have a brushless AC motor. Any motor can be mechanically commutated, it's just yet another way to eliminate the need for precision electronics. Mechanical inverter rather than electrical inverter.

    My list above isn't even complete, for example any motor can be "stepped", it's just not historically done. Not would a new motor design usually have a justification for stepping the motor.

    Also, fun fact, any motor can be operated open loop! You can run any AC "servo" open loop, often even with a stepper controller. You just need to make sure it has enough time to complete the much larger "step". High pole counts also exist in large motors. For a given size though, think of it this way, going smaller than a width of about 4mm on a magnet is incredibly difficult to assemble or magnetize in place. If you wanted to go smaller you could, but you almost certainly wouldn't. That's where a stepper shines, high electrical pole counts in small sizes. With precision electronic control, it's not necessary, but it still has its place.



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    A small addendum to my last, steppers are two phase machines, vs a typical 3 phase.



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    Default Re: Servo or Stepper for CNC?

    The actual control accuracy of stepping and servo is not much different, because no matter how small the step angle is, the machine is the key to ensuring the accuracy, but it is difficult for the machine to make the accuracy.

    The key difference between them is the speed. Stepper motors are generally used at 3 to 400 revolutions. They can also be used for thousands of revolutions even with an acceleration time of hundreds of milliseconds, depending on the quality of the drive and the inertia of the motor. And inductance and current. But at high speed, its torque is almost lost, and a little reaction force can stop it, so the actual application can only be at 2, 3, and 4 hundred revolutions. Some imported drives also add closed loop speeds like servos. A little higher, but the structure of the stepper motor has determined that its high-speed response is not fast, because its rotor is very heavy. The performance of the actual application servo is relatively stable, and the stepping speed will jitter.

    Servo is definitely better than stepping, but it is also more expensive, about twice as expensive as stepping, and the working environment is also harsher than stepping. In addition, the servo is a constant torque, and the actual servo with the same torque is larger than the stepping torque. The biggest difference between them is speed and price.

    http://cncmakers.com/cnc/controllers/CNC_Controller_System/CNC_Retrofit_Package.html


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    Default Re: Servo or Stepper for CNC?

    Absolutely, the on-shelf product difference are exactly as you described. These differences are largely due to design legacy and not actual limitations, but they exist nonetheless.

    I would say with total certainty, as a motor designer, I'd rather design a servo than a stepper. It's simpler and you usually get paid more . That said, you could almost certainly do a fair bit of refinement on steppers if you cared to, there just isn't enough of a market to justify it. Keeping in mind that while you can get within 5-10% of real on cheap to free magnetic FEA software, you can't get better until you shell out >$10k monthly or large annual leasing. With something that's pennies of margin on the dollar vs high margin 'servos', there is no reason a motor designer would start aggressively optimizing legacy architectures.

    The control world is different, they can be prototyped faster and simulated with good accuracy, tuned, and the availability of chips is astounding (the global shortages of today notwithstanding). So there is a lot more pennies to be had in improving control systems rather than the motor hardware.

    With the higher margins, new materials and manufacturing methods, and new/old architectures being made practical, the simpler but more flexible synchronous 3-phase motor is much preferred by motor designers (and their wallets). There is also a greater control challenge there because it's a floating rather than indexing positioning problem, on 3 phases, with at least 2 common winding interconnect schemes (though it's possible to run independent phases like a 2-phase stepper, using 2-wires per phase). The motor also needs to be a consistent and shape conformal waveform or your controller has to be highly integrated via tuning or design. For example, a generic controller can spin a trapezoidal or a sinusoidal motor fine, but a sinusoidal focused controller will do worse on the trapezoidal motor and better on the sinusoidal. They may have nearly identical hardware, but different firmware or even control strategies. One is a nearly square current device and one is a sinusoidal device! Think that sinusoidal sounds easier than a square-ish current? Well, low resistance and inductance means that you need tight current control with a very high speed PID loop, actively tracking that sinusoid while every on cycle makes the current want to jump significantly and every off makes it want to drop. It's a game of fast reactions and high (current) stakes!

    But again, when deciding on your purchases you need to go by the specs, the actual performance at the shaft will not be so noticeably different between the two types except in extreme cases, for a stepper or a servo with the same ratings.



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