[FlyingElectron]

Anyone know if there is a formula to roughly estimate how much torque a stepper is providing at a given RPM?

I have a machine that is running steppers that I would like to convert to servos, but I'm not sure how much servo torque I need.

The steppers are from keling KL23H286-20-8B being run in unipolar mode 6.8mh @ 2A from a 36V chopper SLA7062M if it matters. Datasheet for the stepper is http://www.kelinginc.net/KL23H286-20-08B.pdf

The motors are currently running at about 575 rpm before the machine stalls out. The machine has 10mm pitch ball screws. The machine moves as 225 inches per minute currently.

I'm thinking if I can figure out an approximation of the torque being put out, then I would know how much torque is needed to move the machine. And I think that that torque is independent of speed? So then I could figure out what I would need to do to get the machine to move at about 4x the max speed it is running right now.

My understanding is that it doesn't take more torque to move the machine faster, but it's the stepper motors that have decreasing torque as the RPMs increase.

Thanks in advance to anyone that might have any info for me!

[irving2008]

Running those steppers in bipolar parallel mode from a 50v driver will increase the torque at speed by a factor of 2 or 3. Currently they stall because you simply dont have enough drive volts to push the current through them at the higher speed and unipolar mode is the lowest torque mode because only one of the two phase coils is in use at a time. They simply cant generate enough torque to accelerate the load. If you turn the acceleration down you may find you can get it to go faster, but this is only useful for rapids, for cutting you need the acceleration to be as high as you can get it because you want to be able to accurately follow the cutting line.

The key factors in the torque requirement are: torque to overcome friction and/or cutting forces, torque to accelerate the load, torque to accelerate the leadscrew. In most situations the torque required to accelerate the leadscrew is the biggest component. You can reduce the torque requirement by reducing acceleration.

I have written a tutorial on calculating motor sizes here

Before considering servos, which will be very very expensive you would do well to review whats possible with a properly sized stepper matched to a good driver... your current setup is a long way off optimal.


The other issue you might need to consider is the critical speed of your lead screws - 4 x current speed is 2300rpm - depending on the diameter of the lead screw and how it is supported you that speed may not be possible without mechanical changes. Whats diameter/length/support arrangement have you got now?

[Crevice Reamer]

Anyone know if there is a formula to roughly estimate how much torque a stepper is providing at a given RPM?

I have a machine that is running steppers that I would like to convert to servos, but I'm not sure how much servo torque I need.

The steppers are from keling KL23H286-20-8B being run in unipolar mode 6.8mh @ 2A from a 36V chopper SLA7062M if it matters. Datasheet for the stepper is http://www.kelinginc.net/KL23H286-20-08B.pdf

The motors are currently running at about 575 rpm before the machine stalls out. The machine has 10mm pitch ball screws. The machine moves as 225 inches per minute currently.

I'm thinking if I can figure out an approximation of the torque being put out, then I would know how much torque is needed to move the machine. And I think that that torque is independent of speed? So then I could figure out what I would need to do to get the machine to move at about 4x the max speed it is running right now.

My understanding is that it doesn't take more torque to move the machine faster, but it's the stepper motors that have decreasing torque as the RPMs increase.

Thanks in advance to anyone that might have any info for me!

Hi FE.

Bigger is not always better. Many make this same mistake, and give steppers a bad rap. Those 425s are 84V motors. You are starving them for Voltage and running them in the weakest possible method.

Wiring them Bipolar Parallel and connecting them to a larger Voltage PSU would help, but if you REALLY want fast, get this setup:

http://crevicereamer.com/Page__57.html

CR.

[multiplex]

CR, when you say those motors are rated at 84v, where do you see that on the specs?

edit - reading the above link answered my questions. Now to do some calculations on my motors.

I'm at the same point where i'm considering servos. But the one thing I want to get from servos is more repeatability. this seems to be an issue with the steppers when i do a bunch of little moves across a piece and then try to return to the starting point. Or if i'm running multiple pieces off - my start point tends to drift

[irving2008]

CR, when you say those motors are rated at 84v, where do you see that on the specs?

He's not, what CR means is that they should optimally be run from a driver using 84v, based on V= 32 * sqrt(L). L = 6.8mH -> V = 83.5v

Anything lower than this will not get maximum possible torque at speed as it cant build up the magnetic field fast enough.

[irving2008]

CR, when you say those motors are rated at 84v, where do you see that on the specs?

edit - reading the above link answered my questions. Now to do some calculations on my motors.

I'm at the same point where i'm considering servos. But the one thing I want to get from servos is more repeatability. this seems to be an issue with the steppers when i do a bunch of little moves across a piece and then try to return to the starting point. Or if i'm running multiple pieces off - my start point tends to drift

Then you are losing steps, probably because you are trying to move the load faster than your steppers will allow... try bringing down your acceleration bit by bit till the problem goes away. Or you have excessive backlash in the system. A properly setup and optimal stepper drive will hold perfect position.

[ger21]

But the one thing I want to get from servos is more repeatability. this seems to be an issue with the steppers when i do a bunch of little moves across a piece and then try to return to the starting point. Or if I'm running multiple pieces off - my start point tends to drift

That's because of two things.

One, you're trying to go faster than the machine is capable of going. When you exceed the limits of steppers, they lose steps. If you exceed the limits of servos, they'll fault and stop. Whether you use steppers or servos, you must use components properly chosen for the performance you want to use them at, and usually, you'll want at least a 20% safety margin.

Or, two, you could have some electrical noise interfering with your step signals, which could result in incorrect position. Again, with a servo system, the results would be the same.


You can choose a target velocity and acceleration and design either a stepper or servo system to run reliably at those speeds. Choose the wrong components and it won't.

[H.O]

Hi,
Here's a method of measuring the torque of a step motor outlined by Mariss @ Geckodrive on his Yahoo-group in -03.

Hi,

Yes, strange but true, it can be done and accurately as well. All you need is an efficient step motor drive, a calculator, a paper towel, a pair of channel lock pliers and a multimeter (or two).

How to do it:

1) Attach the test motor and drive to your power supply.

2) Set the multimeter to "DC Amps" and put it in series with the power supply "+" to drive lead.

3) Run the motor (unloaded) up to the test speed you want to measure it's torque at.

4) Measure the DC current flowing from your power supply.

5) Measure the power supply voltage while the motor is running unloaded at the test speed.

6) Multiply the results of (5) and (6) and save the answer.

7) Load the test motor with the paper towel folded over several times until it is only 1/2" wide. It makes a great brake-shoe. For bigger motors you will need the channel-lock pliers to apply sufficient pressure. You may also want to wet it a little to keep it from smoking or catching on fire when you do.

8) Slowly load the motor while watching the DC ammeter as you do.
Note the reading the instant the motor stalls. Note the power supply voltage as well at stall if it is unregulated. This may need to be repeated several times until you get the hang of it.

9) Again, multiply the current and voltage readings you got in (8) at the instant of motor stall.

10) Now for the calculator work. Subtract the results of (6) from the results of (9). The difference is the mechanical Watts the motor delivered to the paper towel brake.

Knowing that and the speed of the motor, the following identity gives you the torque at the test speed:

in-oz = Watts * full steps per second / 4506
The above formula was wrong and was in a later message corrected to:
in-oz = Watts * 4506 / full steps per second



I have a carefully maintained and calibrated 500W dynomometer accurate to +/- 0.5% I use to generate speed-torque curves. It is of my own design utilizing a low-inertia DC servomotor as the test motor load. It has been compared against a really nice Varitrol hysterisis type dyno I splurged on a month ago. Both agree within 1%.

The amazing part is the results of the plier-multimeter method (I call it the Delta W method) agree with the dyno results within 2% over a speed range from 500 full steps per second on up.

The Delta W method falls apart at very low speeds because "delta W"
necessarily trends towards zero for near zero speeds. It requires exquisite care in measurement technique at speeds near 100 full steps per second to get meaningful results. Above 500 though it is very accurate in its results.

I'm killing time until I get the final G2002 boards next week. That's why I'm doing this fun stuff.

Mariss

Never tried it myslef but I seem to remember reports about it being accurate. The topic name was: Measure Step Motor Torque With Just a Multimeter if you want to look it up on Geckodrive Yahoo-group.

With that said, take the other guys advice and see what you can get with the current motors.

/Henrik.

[FlyingElectron]

I have written a tutorial on calculating motor sizes here

Before considering servos, which will be very very expensive you would do well to review whats possible with a properly sized stepper matched to a good driver... your current setup is a long way off optimal.


The other issue you might need to consider is the critical speed of your lead screws - 4 x current speed is 2300rpm - depending on the diameter of the lead screw and how it is supported you that speed may not be possible without mechanical changes. Whats diameter/length/support arrangement have you got now?

Thank you so much for that tutorial. I've been studying it all morning and have been plugging my current setup numbers into the formulas. It's amazing how must fast acceleration ups the amount of torque required.

The machine I am trying to upgrade is the X axis on a pick and place machine. Since it's a pick and place machine, it's got almost no load on it, it just has to get from position to position as quickly as possible. The X axis has a chinese ballscrew 1 meter in length, 25mm diameter, 10mm pitch and rides on chinese copies of thomson roundrail guide systems. The ball screw is supported by bearings on both ends. I did a worse case scenario of the X axis pushing 50kg just to run some numbers through the formulas.

With my current setup and running at 225 inches per minute (575 RPM) it only takes 11.2 oz in of torque to move and accelerate.

Just as a sanity check, would the torque on 282oz in steppers really drop off so much when run at that speed to below 11.2 oz in? That's a loss of 95% of the torque.

[ger21]

With my current setup and running at 225 inches per minute (575 RPM) it only takes 11.2 oz in of torque to move and accelerate.

Not sure how you figured that out, but 11 oz-in of torque will not accelerate a 1" ballscrew very quickly at all. I think your numbers are way off.

[FlyingElectron]

Not sure how you figured that out, but 11 oz-in of torque will not accelerate a 1" ballscrew very quickly at all. I think your numbers are way off.

Oops, my mistake, I forgot to say that the axis currently has really awful acceleration and takes 0.375 seconds to reach full velocity.

To double check my sanity, if I change the acceleration time to something like 0.01 seconds to reach full acceleration, the calculated required torque jumps to 362 oz-in.

If that still doesn't sound right, then I need to go back and check the math again, maybe i put a formula in the spreadsheet wrong from the tutorial. :-)

[FlyingElectron]

I was inspired by torque measuring post earlier about grabbing the motor while it was running to measure torque.

I ran the axis at full speed and if I gently clamp the ballscrew with my thumb and forefinger and a papertowel the axis stalls. I guess I really am getting very little torque out of the motor at speed.

My target speed requires the stepper motors to turn at 2500RPM. Even with excellent stepper drivers like the gecko drives, is it unreasonable to expect the stepper motors to have any kind of torque at that RPM?

[ger21]

For most applications, steppers are really not usable over 1000 rpm. Maybe 1500 for rapid moves, but to get that high, your acceleration will suffer. Acceleration requires the same torque at the end of acceleration as it does at the start. So, if you accelerate to 1500rpm, you can only accelerate as fast as the available torque at 1500 rpm will allow, which may be only 10% of the holding torque.

[irving2008]

Exactly, which is why the formulae in the tutorial (and the Excel spreadsheet on page 2) assume a linear acceleration from rest to M m/s in N secs... and works out the torque required at M m/s.

I dont know if Mach3 or EMC2 can do so, but some software I wrote for a specific job changes acceleration in stages - so fast initially and then reduces it as the speed increases to best utilise the torque available from the motor...

[ger21]

There's an older development version of Mach, called Quantum, that does S Curve acceleration. It's available for download.

[irving2008]

There's an older development version of Mach, called Quantum, that does S Curve acceleration. It's available for download.

Obviously wasnt sufficiently well utilised with a properly optimised modern stepper to justify getting into the final software cut... 90% of the time a linear acceleration will work just fine!

[ger21]

Or not enough hours in the day for a one man development team.

[FlyingElectron]

Thanks everyone for sharing their knowledge and helping me.

I've decided to go with servos for this machine. Steppers may work but it looks like it would be more closer to the upper limit of what would be possible for steppers then what I feel comfortable with. I think in my situation it's a good match for servos since every move is a rapid and I can benefit from the faster acceleration of the servo and the closed loop positional accuracy of the encoders.

I'll try to post back with the results of the servos after I buy and install them.

[MadMachines]

Is it possible to install a gear ratio that can turn the ball screw faster at lower motor RPM, this would also use the higher torque area of the motor, just a thought? cheers

[Mariss Freimanis]

At higher speeds step motors are Constant Power motors. That means Torque times RPM gives the same result as you change the speed. If you cut your speed in half, you get twice as much torque, just like a 2:1 gearbox but without the gearbox losses.

Mariss