Stepper Motor RPM

Yep, you pretty much have it figured out. It is permissible to goose it a little with reduced duty cycle, as in forcing more current, but the manufacturer's rating is for 100% duty cycle.

OK, thanks alot.

Im designing a CNC router for a college project, Ive seen people reduce the RPM of steppers via software, how does that affect the torque, and how does it compare to physically gearing the motor down?

Thanks

A stepper motor is virtually the same kind of construction as a synchronous motor with salient poles, and they can be driven by variable sinusoidal sources, but in that case, the stepper motor runs at whatever frequency you supply.

Torque is directly related to current, and whatever voltage it takes to deliver the rated current is all that's required, but you won't have any overhead to get more power for brief periods if your supply is only 3.6 volts. Typically, stepper motor controllers (amplifiers) control or limit the current, so operating it from a higher voltage supply is common. If you're building your own controller, I'd suggest that you use some sort of current limiting output so you won't bog down your power supply.

Controlling the speed of stepper motors is typical, and the output step rate is usually dictated by either pulse input or analog input (the controller produces pulses proportional to signal voltage input). If the inertial load on the motor is high, it will be necessary to ramp up the speed. Power = torque X angular acceleration, and since torque = Ki (K is torque constant, and i = current), Power = Ki X angular acceleration.

As you can see, the amount of torque you can produce with the motor is simply a function of the current you force through the motor. High inductance stator windings automatically limit the rate to some value, above which the motor will operate but at reduced torque.

Gearing is only used on stepper motors to increase output shaft torque. Keep in mind that the power will be the same, and maximum speed is divided by the gear ratio. You will also have some frictional losses in the gearbox, so the power to the load will be reduced.

Always start with maximum necessary speed and design the system from there. Ratings of the motor will then dictate the maximum rate at which you can reach top speed.

A step motor will go 1 step each time you tell it to. If it has 200 steps/rev, and you give it 200 steps/sec, it'll do 1 rev/sec = 60 rpm.

The torque rating of a step motor is usually given as the holding torque - i.e. speed = 0 rpm. The torque-speed characteristic is pretty flat for low step-rates, and typically starts to fall off above a few hundred steps/second (depending on motor type).

What motor drive are you using?

Thank you very much, thats some really helpful info, im going to need to think about that for while

Im not sure about the driver, The purpose of this CNC is that its to be designed as cheaply as possible, so I budgeted upto £60 for the driver, just somthing like this http://www.google.com/base/a/2990988/D6711868053166535936 for example.

You see I want the stepper to be going at around 10-15rpm, and looking into it, gearing it down is a bit messy.

(oh and that pdf you sent me was really helpful, for a quite a few things i need to know, thanks alot john)

That board looks fine as a motor driver (assuming you're going for a 3-axis system) - you may find a cheaper single-axis one around, if that's all you need. I don't suppose you'll need more than 2 or 3 amps/phase (based on our earlier exchanges).

You need to give the driver board motor step and direction signals for each motor (to tell the motor how far to go, how fast and which way). Have you thought that bit out yet?

I'm not trying to put you off, but you've got to realize that this project is going to take a lot of input - either £££ (for ready-made solutions), or a lot of hours (for design work & DIY).

Thanks John. Yeah, ill be using three motors, You've lost me a bit in that second paragraph though . how do you configure the driver board, is that through the software? (the softwares an area i know virtually nothing about)

I'm starting to realise this is a bigger project than i first thought, but im in no rush so im happy to put the time into it.

The driver board is just a bit of gubbins to push the right amount of current through the motor windings in the right sequence (depends on direction) to make the motor move in response to the input step signals. Without these input step pulses, the motor won't go anywhere.

Actually generating the step signals is the tricky bit. It's hard to do directly from a PC port with most operating systems, as you can't control what other things are going on - programs running in the background and asynchronous interrupts from peripherals tend to muck up the timing of the step pulses.

Andy's suggestion of a system running under Linux may work - I've had no experience with it.

The usual approach is to get a dedicated controller, which you connect to the PC via e.g. a com port, and give instructions about the required acceleration rate & speed, and how far to go. The controller then works out what step pulses need to be output to the motor (via the driver), and when.

BTW - you can't stop/start a step motor at more than a certain step-rate - say 100 to 200 Hz (30- 60rpm for a 200 step motor). Any faster, and the motor will stall on starting (and sit there making a horrible whining noise), and if started, will lose track of position when stopping - it will overrun a bit.

If you google: how to drive step motors you'll find some useful links.

[Edit - just spotted your mention of 10 - 15rpm, so you'll probably get away without needing to accelerate.]

To be useful, you need to run the motors at anything up to 10 x the voltage specified. This means a max of 36 volts. The current needs to be set to NOT exceed 2.3 amps.

The software you use can be set up to accelerate and decelerate allowing higher top speeds to be achieved.

If money is a problem, there are several DOS softwares around that will run from almost any old PC that still sports a printer port. The printer port needs to be of the type that gives both 5 volts and ground out. Be aware that many Laptops (old) with printer ports only achieve 3.6 volts and ground!!! This will cause problems if the driver card is expecting 5 volts......you will have to make an interface to bring the outputs to 5 volts....not a real problem....

A good software that runs under Linux, is EMC2 from Sourceforge, it is completely free.....you can check it out and download from here:-

http://sourceforge.net/forum/forum.php?forum_id=760508

Best of luck......show us what you achieve and post a video on youtube for us please when it runs......

Hi Andy,

I downloaded the linux app link, but have not as yet installed it, so i dont know which port it communicates with, if it does at all. Surly some PCB interface is required. Can you shed any light if you have used it before. Any advice appreciated.

Craig

I have not used it as yet,but it uses LPT:1 as standard, the "first" printer port. If you have a newer PC without a printer port, you can add one as a card, PCI is normal as long as you have PCI sockets on the mainboard.

A very new PC is not a good idea.....

The EMC2 can be had as a Ubuntu version (there are 2 versions of this as well.....but with the same EMC2 version) that will install itself, provided you have at least 500mb or more main memory.....

You need to start reading all of the EMC2 documentation, I have only breezed over it for the momrent as I "accidently" bought a driver card with everything on it that does not support EMC2. So EMC2 will be used only when my second machine is finished, probably in 6 months or more.....

EMC2 is loved by some and not by others, but if money is a problem, start to like it as it is FREE!!!

Is that enough info?

Sorry Andy, pls ignore request, found info at linuxcnc.org. 2 quick to cry and not to look

Craig

By the way, I have a CNC build under CR4 Hobbies, its not that informative as yet, I have a load of pictures that need adding this week, but please join us there as well if you wish to.....

Thanks, will do. Ive had 6 years in tool and die making and 23 in electronics so i'm VERY keen to build my own machine and "join" my two worlds.

Thanks alot andy, im just starting to get my head around the software part, and thats some useful information there!

jwest

If I can help further (I am also a bit new to CNC, building my first machine right now!) either ask in this blog or send me a CR4 email....