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
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
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
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.
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
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.
Is it too much to ask for both? What about a stepper servo...Servo = accuracy, precision, consistency, and speed
Stepper = cheap
http://www.evarobotics.com/_product_...evelopment_Kit
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
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
And aren't stepper motors just an AC motor?
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.
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
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.
Correct. In fact all motors are AC motors.
Mariss
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
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!!!
In fact all motors are AC motors.
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.
A small addendum to my last, steppers are two phase machines, vs a typical 3 phase.
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