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How uniform are the steps at various microstep sizes on a stepping motor? For that matter, what is the accuracy of the steps and microsteps?
The reason that I ask is that similar techniques used in ADCs for example, result in some steps disappearing at the smallest sizes, while others are over large - this is called differential non-linearity.
Thanks,
Mike.
On a typical stepper the linearity of the main 1.8deg steps is about 1.5%. Halfstepping is about the same. Once you get into microstepping below that then its down to the driver and the load. A typical 8bit driver will give a further 1 - 1.5% non-linearity on an unloaded motor but the load will determine the accuracy to a great extent as the magnetic forces act as a spring against the load. Microstepping (other than halfstep) should never be used to improve resolution only smoothness of motion.
Thanks again Irving. As I willl be using ballscrews with a 5mm pitch, I think that halfstepping at 0.0125mm per half step is perhaps a bit lumpy so I am now looking at using a toothed belt drive for the ballscrews.
Mike.
Hi MikeQuote:
Originally Posted by leadinglights
I use a 4:5 tooth belt simply to drop the steppers below the top of the table and stop faffing about with motor alignment.
4:5 effectively reduces the 5mm pitch to a 4mm pitch so 400 steps/rev = 0.01mm. As the machine weighs less than a ton this is hardly achievable because it will bend under the tool pressure, but I still microstep to smooth it out a bit :naughty:
Don't skimp on the pulley size, I use 32:40XL
Robin
I have 16mm x 5mm pitch ballscrews on x,y and z on my machine running at 1/4 step, runs very nice and not noticed any lumpyness. Did try them at 1/8th and 1/16th step just to see how they ran but did not seem to make too much difference.Quote:
Originally Posted by leadinglights
Mine are direct drive but belt driving them will give you more mounting options so definitely worth thinking about.
Just my two-penneth..!
I was probably spoiled by the first CNC that I built - based on a Proxxon MF70, it worked it's little heart out to give almost unmeasurable errors, really smooth curves etc.. I would like to get a step resolution of 0.0025mm but would settle on 0.005mm. I am thinking of 2.5:1 (1000 half-steps per leadscrew revolution) with 16T to 40T Syncroflex 2.5mm pitch pulleys and 6mm wide belt - would that work with a 3Nm stepper motor?
Probably don't have to tell you but remember belt driving at 2.5:1 will increase your resolution but will reduce your max feed rates.
My machine uses 3Nm steppers and I am driving them at 36V 5A, absolute maximum feed I can get without losing steps is 7500mm/min but have reduced it to 5,000mm/min so that I am not pushing it to the limit all the time and this is driving the screws direct without any belt reduction.
So 5,000mm/min divided by 2.5 will only give you 2,000. Does depend on voltage's, driver type etc but I would say that 2,000mm/min may not be enough.
2000mm/min sounds like the speed of sound to me, my Proxxon manages about 160mm/min - going by Youtube videos of similar machines this is average to good for this type of conversion.
Did I read that correctly....
Your maximum feedrate was 160mm/min????
That is seriously slow, are you sure about that?
Bloody hell you could eat a packet of Hob Nobs in that time..............
How big a packet of Hob Nobs?
The Proxxon is limited to about 160mm by 40mm movement in X and Y. As the 1Nm steppers are only driven by 13.5V it could probably be sped up a bit, but a typical job might be a 25mm dia brass gear, or maybe a 80mm by 30mm PCB.
That would be over 6rps on my lead screw handles :eek:Quote:
Originally Posted by leadinglights
I can ramp it up to incredibe speeds but start to get uneasy at around 3rps.
Guess I'm not very brave :whistling:
I have just been playing with the numbers on toothed belt drives and stepper motors but I don't know if the answer I have come up with are right.
Using the formulae given in the Transdev datasheet for Syncroflex toothed belt drives, I get a figure for tooth shear strength when using a 10mm wide belt and pulleys of 16T and 40T, 6 teeth engaged, of just a tad under 200 Newtons at the stepper motor. The Tension Member is not much better at about 300 Newtons.
Although I haven't settled on a stepper motor yet, the most likely candidate, the so-called 3Nm NEMA 23s that are available at the moment, put out about 2Nm at stall - about 160 newtons at the periphery of a 16 tooth pulley.
Is toothed belt really that wimpy, or have I lost a decimal place somewhere?
Hmm, a quick edit here. Looking at the datasheet for Zapps SY60STH88-3008BF, the maximum radial load is given as 75 Newtons. I assume that if you are applying a torque of 2Nm at a 12.5mm radius then you will get something like 160 Newtons of radial force reacting to it. I may have to think a bit more about the motors as well.
Hmmm
2Nm torque, 5:2 reduction, 5mm pitch screw gives a driving force of 0.63 tons.
Probably best if the belt fails before something else does :naughty:
Robin
How to calculate it...
! rev = 5/2.5 = 2mm
2mm/(2*pi)= 0.3183mm = 0.0003183m torque radius
2Nm/0.0003183m = 6283N = 0.63 tons
Hmm, I had better apply myself to Irving's tudorial and motor calcs program. I had avoided that until I get some idea of the masses that I will be throwing around.
(Also, avoid getting finger trapped between table and frame at 0.63 tons)
f=ma
So 6283 Newtons would accelerate a mass of 18.5kg to Mach 1 in approx 1 second.
Robin :heehee:
But I will be using Mach3, does that take 3 seconds?
1.85638543287408 Hob Nobs [approx]
.
Indubitably :beer:Quote:
Originally Posted by leadinglights