The Basics: Stepper Motors

[Will_D]

Delighted with the chat on the original basics thread. Just whats needed!

So as not to dilute that, this is a sub-thread about stepper motors!

So Santy came early ["Typical Male Problem" says Mrs Claus]

I got these from:

http://www.ebay.ie/itm/3-Axis-CNC-Kit-425oz-in-Nema-23-Stepper-Motor-Driver-CNC-Mill-Router-Lathe-/121787136378?hash=item1c5b14397a

Sorry I missed the bit about using low torque steppers.

So the motors I understand, 2 coils, 4 wires. But what about the stepper driver controllers?

These are M542T units and have 8 dip switches!

Sw1..3 are current limiters [1.5, 2.0, 2.4, 2.8 Amps] This limits the max current to a coil (understood I think)

Sw 4 is the Mode Table [Full or half mode] (not underswtood!!)

Sw 5..8 are for the step angle (understood i.e a pulse will move the rotor by this amount) BUT what should I select (and assume I must tell the cnc controller)

[Will_D]

Now the controller I ordered is another story!

I still fancied the one that started this whole thought process for me:

The 4 axis cnc 1508 (available on banggood but no longer on e-pbay!!)

Anyways ordered this one, got a confirmed the delivery from the seller (Mmmm?) then my ebay summary showed that th e seller was 'de-listed' but if I had paid then no worries! Huh!! So I now have to wait for the China slow boat to arrive, if it does great!, If not I have to order another [like the DDCSV!]

Alarm!!

Take away Indian just arrived so will check and edit this later!

[Joules]

That was Custer's fatal mistake too.

Not a bad set of parts Will, you are still in the low power stepper region (NEMA23)

identify your coils (Black, Green and Red, Blue)  Your driver is labelled A- A+  B- B+  they are your stepper pairs  i.e Black to A- Green to A+ etc  swap over one pair AA or BB to reverse motor direction, but don't connect AB BA  the music is awful.   

Full current, half current usually refers to the holding power when a stepper is powered but at rest between moves.  If you can get away with half current the motors will run alot cooler, but if any axis can be  back driven, you would need full current at rest.  It kind of contradicts the bit they say about auto current control at idle, especially if you don't use the enable line.

If you are just making the motors spin add some tape flags to watch the shafts spinning and start out at 2 microsteps 400steps/rev and try the different options.  In the real world you don't really want to be going beyond 8-16 microsteps, the torque falls off a cliff as you increase the microsteps, so unless you are wildly over spec'd  wouldn't recommend it.

When you get a controller we can advise as it will depend what they ask for.

 

[Will_D]

Many thanks Joules for the info. Again that's exactly whats needed for the "Basics"

I reckoned that 3 Nm should be enough for my Sieg SX2P (at least for X and Y)

Z may be another matter though

Received PM from the seller that he had shipped and gave me a tracking number that seems to work!

PS: Take away Indian was gorgeous!

[Will_D]

So my controller arrived (rhis the one with th emicro-Sd card (the SMC4-4-16A16B))

Hooked power to controller and the controller works as expected.

Connected up one stepper (X-axis) and nowt! didlly squat! No motion.

Connections are:

PUL +ve, DIR +ve
The negatives go to the controller.

Hmm time to read and think and get the meter out.

Eventually found the parameter thats sets the "Step Low Active" bit. Default is 1 so it aint goint to pull down the Pulse line. Set it to zero and now it works as expecetd.

Lesson learned!!

Now its a question of setting the params so that 1 mm of travel in real like equals 1 mm og travel in the controller.

Due to the weird correspondance with E-pray about the seller etc and the fact that it didn't work, and also following Joules' thread of fitting the MPG pendant to the DDCSV1 controller - guess whats on order??

I obviously have a severe case of 

[tom osselton]

A 6 pack of Guinness? 

[Will_D]

My next question:

I have ordered another (dual shaft) stepper for the Y axis.

It is an 8 wire version and so there are 3 different ways to connect it up to a 4 output controller:

1. Uni-Polar  (Rated @ 4.0 Amp)

2. Bi-Polar Series  (Rated @ 2.83A)

3. Bi_Polar Parallel (Rated @ 5.66A)

The spec says that the Holding torque is 2.83 Nm for Bipolar and only 2.0 Nm in Unipolar

The rated currents are as above

Which should I use? and why?

Thanks in advance

[Joules]

Bipolar series against parallel, how quick do you want the machine to run.  Parallel gives you the torque at the top end speed and can run faster (hotter).  Series gives you the torque low down the speed range but has a lower top speed (my preference).  Unipolar operate 0 - X+ volts where as bipolar work -X -0- X+ volts, swing plus and minus (bipolar) and have more torque as they make better use of the windings.  Unipolar have centre taps on the windings, in effect only using half the winding.   So all you really need to know, is how to wire them bipolar.

[Will_D]

Cheers Joules, yes I have the wiring diagram with the colour codes so no problem there.

Don't need hi-speed its a mill not an engraver.

So its Bi-polar series.

Will

[PK]

Joules was explanation was empirically correct, but perhaps didn't do justice to "Why you always want to wire the motors in parallel".
First off. The rated current of the motor is fairly meaningless on its own. Multiply it by the rated voltage (which is even more meaningless on its own) and you have a figure for motor power that is a little bit useful in comparing motors.

The reason the rated current is meaningless is that the motor will only actually draw that current at its rated voltage at 0 RPM. The instant the motor starts to turn, the motor current (and hence torque) will drop away. How fast it drops away depends on the most important characteristic of the motor, its inductance.

Now stepper drives do a clever thing, they take your motor with its (say) 5V rated voltage, and apply 50V to it. You would think this would instantly blow up the motor, but the inductance of the coils means that the current ramps up (fairly) slowly. When the current reaches the set point of the drive, it turns the voltage off and the current starts to drop, on, off, on  this is what makes that squealing/hissing noise you hear with some drives...  Effectively the drive will increase the applied voltage to the motor as it begins to turn maintaining the set current until the motor is turning at a speed that the drive can't provide enough voltage for, at which point the current t (and hence torque) will drop away.

So you end up with a torque curve that looks like this:


What determines the point at which torque drops off are drive voltage and motor inductance..  This is why the more expensive drives can work up to around 100VDC, and why you always want to wire your bipolar motors in parallel.

PK

[Will_D]

Yet again PK many thanks for theses "nuggets of wisdom" that is what this thread is all about!

[Joules]

There you go Will, I'm just too tight to buy these quality high voltage drivers and power supplies. 

[Will_D]

And yet another question:

Leaving my controller (DDCSV1.1) on for a few hours I note that the motors are warm (say 40C) but not Hot!

Is this beacause they are in hold mode by default?

Note I have set hold current to 1/2 setting

[John Stevenson]

Correct and this is fine.

[PK]

Here is my ritual for tuning stepper motors. Be aware that reading this begins your initiation into my secret sect.
So, if your not too keen on initiation ceremonies then look away now.

• Pick a motor current.
• Set a low acceleration.
• Set a low velocity.

('low ' here depends on your ballscrew pitch, for a 5mm pitch screw 500mm/min and 500mm/s/s would be low on most setups)

• Get the steps per <insert unit here> right. You should be able to calculate this, if your calculations don't match what you measure, then find the problem before proceeding.
• Write some G code that sends the Axis out and back. Put a dial gauge set to zero at one end.
• Crank up the velocity until the axis doesn't come back to the same spot.
• Set the velocity to 75% of that number.
• Crank up the acceleration until the axis doesn't come back to that spot.
• Set the acceleration to that number.

(nb, if your machine will be used for full 3D work, do acceleration first).

• Next, and this is the bit that's relevant to this post, run the axis back and forth for at least 15 minutes for a NEMA23 (or just use it for a big job). If you can hold your hand on the motor then increase the current.
• When you are happy that all the motors max out at about 60degC. Re tune acceleration..