New Machine Build- How slow can a stepper go

[bwprice100]

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Hi

This is my first post here and still finding my way around the site.

I am currently building a 4 axis milling machine using a converted Taylor hobson engraving machine.

Using Mach3
breakout boards from here
http://medw.co.uk/wiki/index.php?pag...Breakout+Board
Stepper motors and driver from here
http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors

Now on my horizontal X and Y axis everything seems fine, motors tuned and turning fine with no missed steps. However on the Z axis I have a much bigger motor 650Ncm (http://www.arceurotrade.co.uk/Catalo...Stepper-Motors) and the 4.2A driver (http://www.arceurotrade.co.uk/Catalo...tepper-Drivers)

Now I have mounted the motor directly on to the exisiting axis spindle drive and although the the required torque is well below the specification of the motor I am having problems I am still seem to be loosing steps. The motor has to turn very slowly or the missed steps get worse. I have improved things by increasing the steps per rev on the motor driver (via dip switches) but stiil not right. I can easily turn the spindle by hand so the motor shouldn't be working that hard

Now the questions is how slow can these motors turn?
Is there an optimal speed range they work best?
Should I look at driving indirectly via a reduction mechanism (gears or pulleys)?

Thanks Brian

[Kipper]

Steppers can turn extremely slowly....a single step if you like.

I find most steppers get flaky at higher speeds.... Is a setting incorrect or something binding?

[bwprice100]

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thanks for the quick reply Keith.

It makes it a bit easier if the motor is ok turning slowly, thanks. I am not happy with the mechanical setting but there does not appear to be any binding. With the power off the motor it is relatively easy to turn and hence move the axis with the existing hand wheel, however I am going to strip it mechanically and check it over. At present there is an intermediate shaft that moves the lead screw via a pair of helical gears and I do suspect excessive end float, which moves the gears out of mesh and could cause binding.
I could drive the lead screw directly via toothed pulley to up the ration so the motor could turn faster but this means extra work that may not be needed.

I have also been increasing the steps per rev on the motor driver via dip switches, it was at 1600 step/rev and is now on 10000 and tomorrow I will try increasing it again but I am not sure why this should make a difference?

Cheers Brian

[amishx64]

Those DIP switches you are changing control microstepping - they tell the driver to turn the motor a fraction of a step per computer pulse vs a whole step. This is good for things that need vibration dampening or more precision. This is most likely not what you want to be doing.

Increasing the voltage to your driver increases max potential torque. What's your voltage?

[bwprice100]

Originally Posted by amishx64 Those DIP switches you are changing control microstepping - they tell the driver to turn the motor a fraction of a step per computer pulse vs a whole step. This is good for things that need vibration dampening or more precision. This is most likely not what you want to be doing. Increasing the voltage to your driver increases max potential torque. What's your voltage?

Hi

The voltage output from the motor driver is 5V and cannot be changed There is a higher current setting available, presently on 8.08Amps but when I up it to the highest setting 9.04Amp the over current circuit cuts in and stops the motor for some reason.

With regard to the steps per rev it was as an experiment and has definitely improved things but not sure why.

Cheers Brian

[Kipper]

Originally Posted by bwprice100 Hi The voltage output from the motor driver is 5V and cannot be changed There is a higher current setting available, presently on 8.08Amps but when I up it to the highest setting 9.04Amp the over current circuit cuts in and stops the motor for some reason. With regard to the steps per rev it was as an experiment and has definitely improved things but not sure why. Cheers Brian

It's a 50V driver with 4.2A max output....5V and 8.08A is an odd figure?

Personally I never go past 1/8th stepping as the loss in torque makes the resolution less appealing.....full steps with reduction are more my thoughts....full steps are stronger/more positive.

[bwprice100]

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Hi Keith

It is the 9A driver I am having problems with. Sorry the input voltage tot he driver is 5V the spec does not say what the output voltage is; I will measure it tomorrow.

In this case what does the 1/8th equate to, is it 8 x 258 micro steps on the driver = 2064 (steps/rev)?

My initial setting was 1600 steps/rev but the motor did seem very notchy when rotating and it has improved by changing to 10000 steps per rev.

Cheers Brian

[dilbert0815]

Originally Posted by bwprice100 It is the 9A driver I am having problems with. Sorry the input voltage tot he driver is 5V the spec does not say what the output voltage is; I will measure it tomorrow.

What do you mean by input voltage ?
If you mean the driver power supply rail be informed that 5V sounds awkwardly low. I would be acually surprised if those chopper driver circuits inside would even work correct at that low power supply level.

Output voltage is always determined by current and motor coil resistance as it's a constant current configuration.
Typically the voltage accross the coil is much lower than the supply rail. Mostly at a magnitude of 1:10 or more.

As Keith already suggested, your figures seems not to make much sense and it may help when you could post a little more details on your power configuration and detailed setup of the driver.
I think there is eigther a misunderstanding of your data or you may have missed some critical things about how to set up the stepper drivers.

If you realy run your driver with just 5V supply then this is the cause of all your trouble.
For serious work you would need much more supply voltage for the driver, most likely something between 20 and 50V. Below 20V is only possible for very slow and simple robotics.

[vger]

Brian,

The motor drivers on the AET site you provided links to in the first post utilize 18 to 50 volts for the 4.2 amp model, and 24 to 80 volts for the 7.8 amp model.

I know the rating on the motor is 5 volts for the 650Ncm motor, however... the driver handles limiting the "average" power through the motor windings by chopping the power at a 20 Khz rate. Basicly, the full supply voltage is switched on to a coil in the motor while a current sensor monitors the actual current flowing through the coil. Once the current reaches the limit (set with dip switches SW1, SW2, and SW3) the power to the coil is switched off. Then 0.00005 seconds later the power is switched back on again and the process repeats, 20,000 times a second.

You must supply the motor driver with at least it's minimum voltage rating to work properly. Looks like you will be using some fairly hefty power supplies.

Steve

[bwprice100]

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Hi Chaps

Some updates and clarification. Some of this is very new to me.

The motor driver is the 9A version from Arc Euro Trade and is no longer listed on their site. It is being supplied with 72V via a hefty transformer.

The motor is again from AET and is their 650Ncm one. The measured torque to move this axis is max. 200Ncm

After much testing the axis is actually gaining steps and seems worse when the driver is first powered up and at slower feed rates.

It has been suggested to me that there is a known problem with these 9A drivers and the Chinese stepper driver rated for 80V but goes non-linear at anything over 24V. Apparently now 'discontinued' by the manufacturer.
AUT don't list it anymore.

Cheers Brian

[vger]

Brian,

I assume the 72 volts from the transformer has a rectifier and filter capacitors for DC output. Is the 72 volts the measured DC voltage? If the transformer's output is 72 volts AC, then after rectification and filtering the coltage could be as high as 100 volts DC. A bit more information on your setup would be helpfull.

Steve

[bwprice100]

Originally Posted by vger Brian, I assume the 72 volts from the transformer has a rectifier and filter capacitors for DC output. Is the 72 volts the measured DC voltage? If the transformer's output is 72 volts AC, then after rectification and filtering the coltage could be as high as 100 volts DC. A bit more information on your setup would be helpfull. Steve

Hi Steve

It's the measured DC volts. I forget what the AC voltage to the rectifier is. There is also a nice big capacitor but no regulator as the voltage requirement can vary from this.

cheers Brian