I know I should post this in the CNC NG but I know I'll get a better answer here. So far, my only experience with CNC is a Maxnc kit I bought and built a number of years ago to play with. I make the 3 motors rotate in different ways and say: "Isn't that cute!" I'm designing a machine that will use steppers for basis XY movement and it seems the more I research the more in trouble I'm getting. I have talked to people at Parker and have run their "Motor Sizer" software and my application should be no problem for an open-loop stepper system and I think I have the torque issues handled. So, I have a few basic stupid questions:
Motor Speed: It seems that my 2 axis steppers need to achieve at least
1,500 rpm in order to rotate a ball-screw 5 turns in 1/2 second, allowing for acceleration and deceleration. One guy that makes controller boards says I can't get better than 400 rpm from a stepper. Question: It seems that rated speeds of steppers varies from model to model and often they don't include speed in the specs on their websites...WHY? Is speed dependant on the drive used? Is the drive a limiting factor? Chopper, bi-polar, unipolar, half-step, microstep?
Motor Voltage: I see steppers have a wide range of rated voltages. It seems that a power supply of 5 to 10 times the rated voltage is used. Why voltage from 1.5 to 75 volts? What are the advantages and disadvantages? Can I mix and match different voltage motors with resistors or such?
I know I could just call Parker and get a bunch of stuff delivered for...$5,000!!! and if I knew exactly what I was doing, get $100 worth of stuff on ebay. I guess I'm looking for a more "Middle-of-the-road" approach and would appreciate any step-and-direction (pun) that you may offer. I would love to hire a consultant so as to have someone to blame...or praise. Any takers?
Because there is a strong relationship between drive speed and torque; and often a sharp knee in the curve, too. So it's meaningless to say your can drive a particular motor at 1500 rpm unless you can also say what torque it needs to put out at that speed, because, depending on the torque, you may not be able to drive it that fast. It's pretty common with low-end stepper CNC stuff that you find that you can do certain things at certain speeds, but no faster, due to losing steps when the drive speed reaches a point where the torque is doo low to move the system, so steps are lost. Thus, you may be able to air-cut just fine, but need to slow down to actually cut material (mine is mostly wood).
If you really need speed, you need servos, but may things can be done with steppers if you understand their limits.
Greetings Tom, Contact Gecko for some good answers about stepper size and speed. They have a guy in tech support who loves his job and will talk your ear off. I don't remember his name (I'm lousy remembering names) but he is the guy they always give me to talk to when I ask for tech support. ERS
Register with the group and get ready to spend several weeks reading ;-)
This is by far the most informative group I have found for any CNC related questions. I am just about to start building a table top CNC router/engraver/mill and this site has been invaluable to me. I modified a free set of plans on there and the help is second to none.
Here's the basic advice you need: steppers are no longer economical for anything but low speeds and low torque. If you need speed, or torque, or both, use servos. This is due to the advent of inexpensive integrated servo controllers like the Geckodrive series, and likewise quadrature encoders.
Right, without a very sophisticated controller, which becomes uneconomical.
Because performance depends on the controller.
Pac Sci gives you nice speed/torque charts, assuming you use their hideously priced controllers.
Yep. By the time you pay for the stepper drive and power supply for anything but toy performance, you'll have more than paid for a better- performing, simpler, more reliable, servo system.
Steppers are quite easy to drive slowly and with low torque. You can do it with one chip. That is why they make sense in things like PC scanners or disk drives that need motion but little force.
Driving steppers faster requires a multiplied voltage dropped through a resistor and a sophisticated, trouble-prone controller. It isn't worth it for things like machine tools that need motion with substantial force.
I design and build custom servo CNC drives for about $1000/axis.
"Tom Gardner" wrote in news:8Oz8d.4057$ firstname.lastname@example.org:
Some drives and motor combinations can get 1500rpm from a stepper, but its not used because not much if any torque is produced at that speed that speed. As a measure i's often pointless as any load beyond their very limited torque at those speeds will cause missed steps. It's possible to run a stepper fast enough that without any load it will miss steps because it doesn't produce enough torque to drive itself and the various emf's in play.
As the others have indicated here, stepper torque drops off rapidly with speed. Steppers develop most torque when they're stalled and the least at their highest speed, typically the exact opposite of servos.
I'm confused by the need for 1500rpm. If your need is 5 turns in 1/2 sec that 600rpm. In the very limited experience I have building and running my stepper based machine the acceleration and deceleration is a varience of the step rate up to the maximum step rate representing the top speed, in your case 600rpm. So why 1500?.
Different steppers are rated at different speeds, torques and voltages based upon thier manufacturer and construction. These characteristics are also effected by the driver used. *Very* generally the round motors have less torque and speed than the hybrid square form motors. Similarly a unipolar motor will have less torque for the same size and power as a bipolar series motor. The unipolar will hold its torque at higher speeds where a bipolar series will drop of the knee of the curve at farly low speeds. The same bipolar motor wired parallel instead of series will have the same high torque of a bipolar series but also provide torque at at higher speeds like a unipolar. A bipolar parallel wired motor will require more power from the driver than the same motor wired series. You need different drivers for unipolar and bipolar. You need more power supply for bipolar parallel. Chopper drives control the motor by limiting current to a set limit as determined by the motor manufacturer. The voltages can be anything from
10-30 times the plate voltage because the driver limits the power by chopping the current. Chopper drivers are pretty efficient and little is lost to heat. Non chopper drivers cannot increase the rated voltage without some means of limiting the current or the motor will burn out. Typically a power resistor is used, sized to limit the current to the motor rated. This is inefficient because of the heat lost in the resistor and less power produced from the moor because the time taken to ramp up to max voltage is slower than a typical chopper drive. Over the time the coil is energised the mean power is less with a resistor non chopper drive than with a chopper drive.
Drive Speed is not limited by full step, half step, quarter step, 10 step decimal or what ever. It can be limiting because increasing microsteps required more steps per revolution from the controller. The higher the microsteps the more step /direction signals the controller has to provide at required step rate and the driver be able to support. For example a 5tpi screw and 1.8 degree stepper will require 200 steps per rev and 1000 steps per inch. If the target rapid is 120inches per minute thats 4000 step signals per second the controller has to produce and the driver support. 1/8th microstepping would require 32000 signals per second - for one axis. Some PC based controllers range from 25000 to
50000 steps per second for all axis. Some hobby end drivers typically top out between 10000 and 45000. If you need speed beyond that and with reasonable loads you need servos.
I'd suggest you need to match the 'gearing' of your required speeds and loads involved with the leadscrew, motor torque and range, driver speed and capacity, and the Controller step rates. hth
9 kids? You are truely blessed, even if you can't find the remote.
To nutshell my app: Move a 40 lb AL table on Thompson ball-bushings, move it no more than 1" in 1/2 second. No side force, just a drilling opperation. Wait for a switch on the top shaft that says the drill is clear then make the next of 96 moves. Do it again. Only 1 axis will move at a time. The machine makes a 8 x 12 row wire brush. Accuracy of +/- .020" would be acceptable.
High voltage is needed for fast stepping. The reason is that the stepper windings are inductive and the current is slow to build up.. A high voltage will get that current up quicker. The down side is you need current control so that the maximum current doesn't exceed ratings A poor mans current control is just a resistor.
You've already gotten the gist of my advice -- go to servos (even if you use the Gecko drivers which make a servo look like a stepper to the controller).
However, I will offer a bit more detail in one point, at least.
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The reason for high voltages is that each time you step to a new pole (winding in the motor) you have to increase the current in that winding to produce the magnetic field. However, since the motor winding is an inductive load, it takes a fixed time (for a given voltage) to reach that current.
I'm going to skip over microstepping, because that complicates (and slows) things, and reduces the torque available without position error, thus increasing the chance of missing a step or several.
You can decrease the time required by applying a higher voltage across the motor winding for the first few microseconds or milliseconds of the pulse.
An ideal approach is use a much higher voltage supplied with current limiting (a switching regulator (chopper) is a good low loss way to do this). When you switch on the winding, the very high voltage is applied (with zero current for the first nanosecond or so), which then increases as a function of time and the inductance. As the current reaches the proper winding current, the current limit will reduce the applied voltage, until at a stop, the voltage across the winding will be within the manufacturer's suggested values.
The higher the starting voltage, the quicker the current builds up to the maximum specified for the motor, and thus the faster the magnetic field builds up to strong enough to do what is needed at full torque.
If, instead, you stay with a voltage matching the specification, for a certain (longer) time, there is not enough current to apply full torque to the motor. When you step faster, you eventually get to the point where the current *never* gets up to full value, and thus the torque available is reduced, The faster you step, the lower the torque, and the greater the chance of missing steps -- even with the mass of the table being the only load.
With servos, however, the motor is capable of very high speeds (though it can't be told to move only a certain distance). It also produces a feedback voltage with a tachometer generator section (here I am talking about DC servo motors, though AC ones proably are similar -- I just haven't used them.) The feedback voltage is directly proportional to the speed of the motor, and is compared by the servo amplifier to a voltage commanding a speed. The output of the servo amplifier increases or decreases the current to the motor to keep the speed (as reported by the tach) correct.
In the meanwhile if position is important, you also have an encoder, which produces pulses (similar to those fed to the stepper driver) which are counted and used to display the position. Both step and direction can be derived from the encoder -- which may be one on the motor shaft, or may be a linear encoder (similar to a DRO's encoder) on the actual part of the machine being moved.
Note that the Gecko servo drives (at least those that they had when I last visited their page) skip the tach feedback, and simply compare step pulses fed into it (as though it were a stepper motor) to those generated by the encoder. A circuit generates a voltage proportional to the difference between the two, and this is used to command the motor to move. A counter is both driven up by the input pulses, and down by the encoder pulses, and the total value in the counter is used to command the motor. If the motor shaft is locked, it is possible to overflow the counter, which is the equivalent of missing a batch of steps. I forget how many Gecko is using (if it is even documented in the manuals for the servo drivers), but I was once considering building something like that before Gecko started with theirs, and I was figuring that probably 16 counts would be sufficient for normal conditions.
So -- with servo motors, and the Gecko servo driver, you can probably get the speed you need. With a normal servo controller (tach feedback and encoder feedback), you could do so with ease. Servo driven machine tools can be so fast as to be scary. Even an early Bridgport clone converted by Anilam to servo drive was capable of 200 IPS (Inches Per Second) on each axis -- and the handwheels had spring-loaded handles which folded into the wheel, and a solid coned dish instead of spokes, so it is less likely to grab a finger or whack some other part of your anatomy with the handle.
Tachs on servos used for position control are pretty much a thing of the past. Any system that has a digital motion controller derives all the speed and postion information from the motor encoder (or in some cases a resolver). You may occasionally see a tach on a speed control app where the velocity loop is closed directly thru the amplifier rather than thru the controller. In this case the amp will be running in velocity (voltage) mode rather than torque (current) mode, which is the norm for positioning systems. Some amplifiers are capable of generating a pseudo tach signal from an encoder pulse train, further reducing the market for tachs.
On every motion controller I've used you can set a limit on the position error and take some action when it's exceeded; shut off torque, for example. Otherwise, when the motor shaft is released the motor will try to race ahead to reach its commanded position, which is not necessarily a good thing.