Stepper Motor Torque

Please register (& reply to this post after you've registered so we know who's who).

Too late tonight to give you a proper answer - if no-one else has done the biz by the morning I'll try to help.

Do you have a minimum acceptable acceleration?

Can you easily remove the motor from the stage?

What kind of drive is it (ball screw/belt & pulley etc.)?

Did you make the stage? If not, who was the manufacturer & do you have a part number?

G'night.

Hi, Sorry about that. I have registered now. We do not have acceleration requirements, just minimum speed. The motor can not be easily removed from the stage. We did not assemble the stage and it is an old Nikon stage, but there is no part number to be found. :/. The motor turns a threaded rod and which in turn moves the stage. Hope that helps.

This is a bit of a tricky one, because if (A) you ignore friction and screw efficiency, (B) the load is being moved horizontally and (C) you can take as long as you like to accelerate it, then the force required to move it (and therefore the torque required from the motor) can be as small as you like (depending on how long you're prepared to wait for it to accelerate). Sorry that's not very helpful, but it's a fact (a consequence of Newton's laws of motion).

On to more realistic things:

For a screw drive, the torque is related to the linear force as follows:

T = ( F x P ) ÷ (2 x pi x ε),

Where

• T is the motor torque (Nm, or Newton-meters)
• F is the linear force (Newtons)
• P is the screw pitch (meters)
• pi = 3.142
• ε is the screw efficiency (about 0.3 for an ordinary micrometer type screw)

If you can make some kind of estimate of the force F (which will be dominated by friction for low acceleration rates and horizontal movement), you can feed it into the equation to find T.

Note that for small motors it's more usual to use mNm (milli-Newton-meters) or Ncm (Newton-centimeters). To get these units, enter the screw pitch in mm or cm respectively.

To get a "feel" for how much force one Newton is, it's about the force a big-ish apple exerts when held in your hand.

Hope some of this waffle helps a bit. Ideally, you'd disconnect the motor & measure the torque needed to turn the shaft directly; failing that it's a case of making some rough guesstimates.

Get back to me if you need anything more specific.

[And welcome to CR4].

John, Thanks a lot for your help. A few follow up questions: How do I determine the screw pitch? For the force estimate do you have any idea roughly how much friction I would be encountering? I really appreciate your help.

Sorry I haven't replied - I've been busy. I don't want to go off half cocked - this needs a bit of thought and I don't want to give you a load of duff information. I'll get back to you as soon as I get enough time.

Easy one - the screw pitch is just how far the stage moves per turn of the screw. You can either turn the screw & measure the distance moved, or just measure the distance between turns of the thread.

Cheers, John.

Or, provided you know the type and size of the thread, look it up in an online table. Thats what I do.....

Like here for example:-

http://www.engineershandbook.com/Tables/threadlimits.htm

I have found that provided you are not looking for really fast response, a small motor, via a relatively fine thread (I use 8mm metric threaded rods in my CNC machines for example that "climb" 1.25mm per revolution and 1.8 amp stepper motors) can move very heavy weights very accurately......

Also, one can add a toothed belt and two different sized pulleys as "gearing" to reduce the speed even more if required. T-15 type is a good place to start with.....

As tiny stepper motors can often be turned far faster, this can be quite useful indeed to increase torque!!!

Best of luck.

Thanks a lot for the help and the suggestions. I have calculated the pitch. Now I just need to calculate the linear force. Any advice for that and how to take friction into account? Thanks.

I'm still thinking - but tied up with payin' work at present.

You absolutely sure you can't take the motor off so you can measure the torque directly? It would be way more accurate than any guesses.

BTW, does the motor have any identification marks, or do you have any details about it?

John

John, Don't worry, take your time.

I may be able to take the motor off but preferably I would not have to. Preliminarily I am just looking for an estimate.

Luckily the motor does have identification and I know the manufacturer and model number.

Can you post the motor details, please? It'll give us something to work form (or to).

BTW - I have a very useful .pdf regarding motor selection, which describes how to calculate torque required, gear ratios, moment of inertial, angular & linear acceleration etc. for all sorts of drive types (leadscrew, belt & pulley, gears etc.).

I'll send it to you if you're interested - but I can't do it within CR4; you'd have to pm me your 'net e-mail address so I can attach the .pdf in reply.

If you're not interested, ignore this comment (apart from the first line!).

John,

The motor is a Phytron ZSS-32 motor. What specifications are you looking for?

Yes I would absolutely be interested in the .pdf. I'll pm you my email.

Thanks.

OK, got the data sheet for the motor. Is there a gearbox between the motor and the leadscrew? What is the leadscrew pitch? What kind of motor drive do you have (type, amps/phase etc. or manufacturer + type number)?

John

PS pdf in the post. You should have it by now. J.

Hey John

Sorry for taking so long but I've been travelling for the past week or so.

I got the pdf, thanks.

I don't think there's a gearbox and the leadscrew pitch is 2mm. We actually don't have a motor drive yet, we are still looking into what would be the best option. We are considering this: http://www.phidgets.com/products.php?category=13&product_id=1063

Thanks,
Kinshuk

I've been away for a while as well.

Quick look over the motor spec and your proposed drive looks like it should work fine.

What's the full part number on the motor? Should be something like ZSS 32.200.1,2 (indicating 200 steps/rev, 1.2A/phase) - there may also be some stuff after that.

You'd probably need a higher voltage power supply than the 12V one recommended on the Phidgets site, but I need a bit more info on the motor.

Back-of-envelope calculation comes out with a linear force of about 35N (≈3.5kgf) assuming 30% efficiency.

The part number is: ZSS 32-200-0.6E.

Do you know how I can determine how large a power supply I need?

Your motor is 0.6 Amps/phase - the Phytron datasheet only shows torque/speed curves for the 1.2 A/phase version, but usually (as you'd expect) you need higher voltage to get the equivalent power (torque) at the lower current. The water is also muddied by the fact that once you start trying to drive at more than a few tens of steps per second, you need to provide a high-ish voltage to get the current high enough between one step and the next (as the coils are inductors, the current will rise exponentially when a winding is turned on - the rise time will depend on the applied voltage).

If you're going to use the Phidgets drive, I'd suggest going for 24V at, say, 1.5A per motor (you can use one power supply for two motors). 24V is a very common voltage for control systems, so there's a wide range of supplies to choose from.

So you are saying we can get a dual output power supply to power both drives? That would be good.

Can you recommend any manufacturers or places I can get one?

I have been looking around a bit at dual output DC power supplies and all the ones I have seen have different voltage and current values for the two outputs. Is that standard and would that be a problem?

Don't need a dual output supply - just wire both drives to the same output. As long as the PSU can deliver the maximum combined current requirement of the drives, it'll work fine. To be on the safe side (the drives are rated at 2A each, tho' you won't actually need that as the motor is only 0.6A) - I'd go for 24V @ 4A. That's 100W, which is a common rating for power supplies.

This would do nicely, but you can probably find something cheaper if you shop around.

I'd recommend getting a modular one that's all sealed up (like the one I linked), as it's very difficult to electrocute yourself or anyone else with that type .

Update: I have found this power supply
http://www.alliedelec.com/search/productdetail.aspx?SKU=2180313#tab=Specs

Would that work? And how can I make sure that I will be able to connect it to the phidgets drive? Meaning, the output and input interfaces on the two match up.

That one won't do, as it has -24V and +24V.

This one would be OK, tho', if you connect both drives to the single output:

http://www.alliedelec.com/search/productdetail.aspx?SKU=2180288

You'll need a power supply connector for each drive - contact Phidgets and ask what size the connector is - it doesn't seem to be stated in the drive datasheet.

When you've settled on all the bits you need, I'll do you a connection diagram.

If anyone is interested in a DIY power supply that is "almost" a switched mode version with an efficiency of around 85%, where you can buy the empty PCBs from the supplier of Elektor PCBs. Send me your private email address via the CR4 email system and I will send the infos via email. DKWarner has already taken "delivery!" and appears to be most happy....

I built one last year, its very very clean output wise and can be set up for any voltage from 12 to 40 volts (with a change of chip to 60 volts) at a max of 10 amps. the required voltage is set up using 3 resistors and a small poti....really simple.

Its quite small and with the addition of a small fan will work for years at the full output if need be. Due to the efficiency, basically you only take from the mains what is required, not more as many simple PSs do....heat is kept to a minimum.....

It was originally designed for a HiFi system, so it is really clean output wise. Its fully safe against short circuits and over temperature.

Component count is low as the two main (identical) chips do all the hard work and no part is at mains voltage excepting the primary input of the transformer used.....so it is relatively safe to work on/with....

This is the reply from Phidgets:
"Well, in this case you have two options. You can wire it directly to the board via the green terminal block (G and V+).

Or you can wire it up to a 5.5mm Barrel power jack connector."

That is not addressing the problem as I understood it......

Having the terminal block makes it easy - you don't need the barrel connectors.

Just connect the +24V output of the power supply to the V+ of both drives, 0V of the power suppy to G of both drives and you're away.

Would I be able to connect this: http://www.trcelectronics.com/Meanwell/rs-100-24.shtml to both drives?

That one is better in all respects. Yes you can use it for both motors as far as I can tell with no problems......

Agreed

Thanks a lot guys. I'm going to go ahead and order that one.

Keep us updated ... feedback is good .

That one is not suitable for several reasons (some already mentioned by John for example) :-

1) Far too expensive for what it is. I could either find a better one on ebay cheaper, or build a great one for even less money (with enclosure and fan), especially if I already had some parts lying around as many hobby people do......

2)This one has dDual outputs with middle ground, far too easy for someone who does not understand electronics to cause a short circuit. (Some of these type of supplies can be "separated" internally and used as two separate 24 volts supplies, or even joined together as a (in this case voltage wise) 2.8 amp 24 volts supply). I personally would (provided the drivers can take it and the output for the motors can be correctly adjusted), use it as a single 48 volt supply, by ignoring the ground connection. (in a very few cases this will also not work as expected, a look at the schematic is needed too and at just how the PS is mechanically built!)

3) It does not even appear to have either cooling or a closed case for that exorbitant price either!!!

4) Using it "one sided" means that one side will be working (at least sometimes) at full output, while the other will not be loaded at all, possibly dangerous for the long term reliability to my mind, but again, a look at the electrical & mechanical design is needed first.

5) If your stepper drivers are completely separate to each other for ground and supply (and ONLY then), you could in fact supply one with 24 volts from the + and G. The other with 24 volts from the G (as +) and the minus....but be careful.

"If your stepper drivers are completely separate to each other for ground and supply ..."

What about the USB links to the PC?

Good drivers are completely opto isolated, so this should not be an issue.....the grounds from the USB should not be in any way be connected to the driver grounds, which allows seperate drivers (I design them singly, never 3 or even 4 axis on the same PCB)....with seperate grounds...

Electrical noise from stepper motor circuits can otherwise easily get back into the PC on the ground line and cause havoc.....many have experienced such havoc, which can show itself as a minute (or worse) loss or gain of steps....very difficult to fix.....very difficult to even find as well.....

Also, I actually have 2 motors I need to control. The drive I am looking at is only 1 axis so I would need to get 2. Do you know of any cheap multi-axis drives off the top of your head? Or know where I could find some?

Generally, drives come for one motor only - you can get multi-axis controllers (at a cost!), but you usually need 1 drive per motor.

Do you need to control the two motors synchronously (as when you need to use, say, an X-Y drive to generate circular motion)? If not, it would probably work out cheaper to use two drives, and let the PC do the coordination. With USB drives, you should be able to control as many as you like from one PC (OK, up to 255, or whatever the limit is on USB ports!).

Which brings me to another point - I'm assuming that you'll have a PC to control the system - correct me if I'm wrong.

Ok then, it sounds like 2 drives is the way to go. I do have a pc to control it.

Hello ,I am trying to run my 2 phase bipolar stepper motor directly by PIC24F16KA102 pic controller chip.I even tried to use ADC to get digital signal out & supply to my four leads of motor.Can you answer my questions ? I am not getting enough current so how can i raise output current of my pic chip? please reply me on my personal email id. nirmal_stevens@yahoo.in Thanks in advance if anybody is willing to help me out.

I'm not going to reply to your personal e-mail ID, as that would reveal mine - and I don't know who you are.

If you register as a member on CR4 and post your question again, someone (maybe even me ) will almost certainly try to help you. It's easy, quick and free.

[It's a bad idea to post with your e-mail address on an open forum, as it can be easily picked up by software which scans sites like this to collect addresses for spamming. If you become a member, you can get e-mail notifications when people reply to your posts, and you can also use the CR4 private messaging system to talk with other members while still keeping your own e-mail address to yourself.]