Thread: whats the matter!
Well nothing really just that I have just come to the decision that there is no REAL information available for stepper motors and their use.
I had thought that when converting my X1 mill to cnc I had gleaned all the information that was on offer. In selecting the steppers I went with 1.8Nm from ArcEuro as this seemed to be about right( obviously wrong as Zaxis is to weak). I then decided to fit ball screws to try and get better efficiency but completely forget that the pitch was a lot higher that the original screws and therefore making the motors appear weaker. I am in the process of fiting a 3Ncm motor to the z-axis, in so doing I noticed the torque curves for the motor and was amazed at just how weak it was in parallel against series connection.
In hind sight I should have waited and bought a ready made KX1 and saved the headaches.
I have still to try a head counter weight or spring. So I am left wondering why the only info we normally hacve on steppers is their holding torgue wherease we relay need their stepping torque so as to compare them.
I am surprised that anything works at all perhaps its all down to gearing.
The one thing I have learnt is that you never can have enough tooling.
Last edited by ptjw7uk; 28-11-2009 at 08:08 PM.
It is totally wrong looking for a stepper motor based on the holding torque and also the torque speed curve is useless if you run it at a different voltage to what the curve was done at.
I get people who buy our motors and use a low power 24V driver with them and complain that they are not getting the speed that they need and that is shown in the torque speed curve.
They allways think that the problem is with the motor and not with the driver that they have spent the last 4 weeks making or spent £70 on ebay buying from China.
The selection of stepper motors seems to be done at the wrong stage in most situations, just last week i had a customer tell me he has this 4 axis stepper driver that is rated to 3A and 36 V and he wants to have the biggest motors as possible for his 1.2m by 2m router he is making, he wanted to go for the largest 34 motors we had, but was a bit disipointed when i told him his driver is too small for the motors.
The result of this is to talk to someone who knows stepper motors and systems before spending your money.
Also you will get far more output power if the motor is in parallel over series, but you do need twice the current.
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I totally agree with all you have said but I thought that I was following in well trodden paths in choosing steppers that others had pointed out as being strong enough. The mistake I made was in not doing as they had done in dropping the 2 to 1 gearing by using toothed belts etc nad going with direct drive this coupled with the coarser pitch of the ball screws I think has left the z-axis under powered.
I have also just found references to the fitting of dampers to the steppers to alleviate resonance, not that I know what to look for if I have got it!!
On the web the videos supposedly showing stepper resonance and displaying missing steps for the life of me I couldnt hear any difference.
I started with too little then worked my way up to too much. Fortunately I never fell in to the smaller size motor trap, just didn't seem to be enough leverage on the armature. I never had ride that snake which leads all the way back to square one :whistling:
OTOH, not all wasted, I now have gash motors and drivers left over to cnc my lathe.
PS: Gary, seeing as how you are bound to read this, your ISM6045 encoder, is it 12 volts in 5 volts out?
Torque curves are useless.
Sounds a far reaching statement but lets break it down.
For a start under what testing criteria was the motor under when the curves were produced as regards the driver ? This is never published and different drivers will give different results.
Some of the charts compare torque to PPS [ pulses per second ] and some compare torque to revs. There is no standard to any of this, for a start what voltage was used? Even a different of 10 volts will make all the difference to the output.
Add to that these motors are on a continual improvement path and what ships in one container isn't guaranteed to be in the next container even though they have the same part numbers.
How can you guarantee that they even put the right motor with the right data sheet ?
99% of people at home shop user level has no way of ever checking these statements. I have literally spent hours and hundreds of pounds trying out different motor / driver combinations on various applications and just because it works for me it may not work for the next guy if he's got a slightly different setup / design.
I have seem and had large 34 motors what should have oodles of torque crawling away only to perform brilliantly when the motor has been swapped for a smaller higher speed type 23.
Compare the torque curves on these and it doesn't make any sense, that 34 has 3 times the torque of the 23.
The secret with steppers is voltage, you must have 20 to 25 times the motor voltage to get anything like performance out of them.
Many of the so called high torque motors are high voltage low amps like 5.4 volts at 2.1 amps, this puts that motor expecting 108 to 135 volts, something that's not obtainable with current drives.
Now the smaller motor rated at say 2.7 volts and 4.2 amps needs 54 to 67 volts, obtainable with the larger more expensive drives and workable with the lower voltage drives but not at peak torque.
Factor in the worse axis like the Z with it's weight and you now have a whole load of figures that even a mathematician can't work out.
It's no secret that ARC's X3 CNC kit that later morphed into the Sieg KX3 took over a year to just run all the tests to get the right motor / driver / power supply combination.
There is also a move to used switched mode supplies as these are cheap and easily imported. These are not really suited to CNC motor operation as they do not have the capacitance to hold the voltage stabe and still keep the power required on tap.
A simple power supply rated at 60 volts for something like an X3 requires a capacitor of 100 volts working and a minimum of 22,000 uf to be rated correctly. This component will cost about £30 to £35 on it's own so you can see that switched mode supplies must cut corners to arrive at the price they do.
John S.John S -
Thank you JohnS for stating what I have slowly realised is the truth, its a black art form.
So what are we hsm's to do, we do not have the resources to test and must rely on those who have gone before but we bocome stuck if we change to what we assume to be for the better one part of the system only to find it isnt quite right!
Anyone know where I can get some reasonable priced encoders in the UK(only got one arm left and no leg!)
The first worry with them was the heat, but I'm getting used to that, doesn't seem to worry the motors.
New problem is the noise, set at 800 steps per rev they make it clear I'm not letting them get anywhere near the sine wave they want to do. Working on it
Stepper perfection is a never ending series of upgrades :whistling:
We really need Gary to reply on this, I have only ever had one test unit of these to look at and the one we had was just an 80 volt driver with an in built power module.
Gary's may be different, after all there are many driver manufacturers out there although a lot share the same technology.
The one I had was rated at 240 volts but turned out to be designed for a 220 volt power supply, as our mains here can easily get to 265 volts at peak it didn't last long :nope:John S -
Yes I read that but we are not on 220 volts like a lot of countries, so it it defaults on one axis whilst cutting and trips you wreck the job ?
.John S -
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