Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: rigger61 on October 05, 2007, 03:06:35 AM
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Hello all;
I have a Taig cnc mill , and have been running the demo of Mach 3 for a while. I use a Xylotec board and have Oreintal motor steppers. The motors are 200 steps per rev, and on the Taig 20tpi leadscrew. I have the board set for full step and when I do a calibrate test Mach tills me 32,000 pulses for 1" of movement. Is this right ? It seems way to fast. I have the motors tuned for 14 inch per min and mid settig for accel. I f I run them faster they stall and hum. I can grab the coupling on the stepper and stall it with very light force. I have had the motors for years in a box , cuold they be bad ? I got some specs years ago and they were 180 in/oz motors. Is this to weak for mill ? I love to make stuff on the cnc , but its not realiable, it stalls to much. I have noticed it runs better withstock code from the Mach software ( wizards ) But is actting up with Mastercam G-code. It always stalls at start of program. If I bypass some start settings and get to the meat of the code it works better. I have a 1ghz amd desktop with 512 meg ram, running Windows 2000.
Thanks Mitch .........................
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Hi Mitch, What stepper drivers are you using?
Regards
Les
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Hi Mitch, orry, didn't see the mention of the Xylotec board. You should be running the motors at anything up to 20 times the voltage on thier labels (6v will run really well on about 75 volts without any problems) and the amps at spec . Don't run them above the recommended amps or you will get smoke! They will run quite hot at specified amps but don't seem to have any problems.
Regards
Les
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Hello all;
I have a Taig cnc mill , and have been running the demo of Mach 3 for a while. I use a Xylotec board and have Oreintal motor steppers. The motors are 200 steps per rev, and on the Taig 20tpi leadscrew. I have the board set for full step and when I do a calibrate test Mach tills me 32,000 pulses for 1" of movement. Is this right ? It seems way to fast.
200 steps per rev * 20 threads or revs per inch = 4000 pulses per inch at full step.
I have the motors tuned for 14 inch per min
14 * 4000 / 60 pulses per sec = 933.33 pulses/sec. call it around 1KHz which is around 5 revs per sec - which isn't hugely fast. The faster you want to drive them the more the overdrive ratio of the drive voltage you need. what ratio are you using (supply voltage to motor voltage)?
and mid settig for accel.
whatever that means... until you get your speeds worked out and whether your motors can cut it power wise - turn you accel down pretty low - once you have motors that can run at the speeds you want then you can figure out what accel your motors/rig can handle.
If I run them faster they stall and hum. I can grab the coupling on the stepper and stall it with very light force.
Two (of many) possibilities are a) too low drive voltage ratio b) resonance.
I have had the motors for years in a box , cuold they be bad ?
motors are neither like wine nor milk, they don't get better or worse with age :-)
I got some specs years ago and they were 180 in/oz motors. Is this to weak for mill ?
Well that depends. Of more interest to you at the moment is: Their Amps/phase, Voltage, inductance, PS voltage, whether they're resonating etc. as above.
If I bypass some start settings and get to the meat of the code it works better.
What settings are you bipassing?
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Hi,
My name is Anthony, I am new to the forum and new to the CNC world. I acquired a taig 2027 CNC micro mill to serve as a learning tool for me to gradually lead into the CNC and machining trade. After buying the Mach 3 software and the necessary adapter board to finally get this older mps 2000 controller running and communicating with the computer, I was finally able to get my machine running. After a few test programs, I noticed that my stepper motors were stalling while in jog mode. I tried motor tuning, loosening the gibs on all axes, and adjusting the motor couplers but nothing seemed to work. After doing some research, I realized that resonance is a possible issue, so I decided to completely take the motors off the machine to see if this was the case. When I removed the motors and tried to jog them, they instantly stalled. It seems to me that there is absolutely no torque given out by the motors and all they do is hum. I know for a fact that the motors are not bad because I ran a few programs before and everything seemed to work out fine.
Basically, I have been at this now for the last couple weeks and I am not sure exactly what I should do. I would really appreciate if someone could steer me in the right direction as to what my next move should be. If my issue is truelly a resonance problem, I would appreciate if someone could tell me where I need to measure the current and voltage from so that it is set correctly. Any suggestions would be greatly appreciated as my frustration with this machine has reached its highest point. I hope to see some chips flying soon! Thanks.
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Hi,
it sounds to me like your drivers have lost the plot.
Testing a stepper driver is not straight forward, you really need an oscilloscope ideally with a high bandwidth current clamp. Do you have acess to
that sort of gear?
Can you tell me a little more about the steppers and the drivers, a few pics would help.
In absence of some good test gear you might be advised to beg/steal/borrow or buy one good driver like a Gecko and hook up your stepper. If the stepper
performs it rather tells you the original drivers are faulty or just plain crap.
Craig
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Hello Craig,
I appreciate your suggestions about the stepper drivers. I will attach some pictures of the stepper motors and drivers below. I'm sorry if the pictures arnt exactly clear, but i hope they can give you some idea of what I am running. Also, I do have access to an oscilloscope and someone who knows how to use it. If you think that my issue can be troubleshooted through testing the drivers, It would be helpful if you could provide some information on how I would go about testing the drivers. To be honest with you, I didn't even know what the drivers were until I did a quick Google search, so any details on how I would test the drivers would be greatly appreciated. Thanks again!
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something I've seen with my setup of a new cnc build, the stepper motors don't like 1/8 excitation. 1/4 or 1/16 work well. I'm trying to find out exactly why.
When the motors stall using 1/8, they turn during the acceleration phase, then stop and hum, and run again during the deceleration ramp.
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Try reducing the velocity and the especially the acceleration to a very low setting and then see if it works. I would suggest a velocity of 5 ipm and an accel of 3.
Mike
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motors are neither like wine nor milk, they don't get better or worse with age :-)
Sadly, very much NOT true.
The old round stepper motors relied on ferrite magnets for their power. After 5 -10 years the ferrites lose their magnetisation and the stepper motor loses its torque. I have a large box of old round stepper motors of all sizes ...
But the square ones shown by OP should be OK. Me, I would guess that the voltage to the drivers is way too low. Well, that, or the drivers are cactus.
On the other hand, if that PCB in the front under a smaller one at the back, is the driver, I would have a lot of doubts about it. It looks like a really crude antique from 30+ years ago. IF I am right, it is bound to be fairly gutless anyhow. Yeah, I have some like that too.
Cheers
Roger
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The old round stepper motors relied on ferrite magnets for their power. After 5 -10 years the ferrites lose their magnetisation and the stepper motor loses its torque. I have a large box of old round stepper motors of all sizes ...
I hope not. ;)
All my steppers are round case and most, if not all, are now more than 10 years old. :'(
Perhaps there are Steppers and there are Steppers (mine are Swiss made, Sonceboz).
Tweakie.
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Hi Tweakie
Do they have ferrite magnets?
Could it be worth while buying a new square one and comparing the drive?
Cheers
Roger
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Do they have ferrite magnets?
Absolutely no idea.
I will contact Sonceboz and see what they have to say about the lifespan vs. torque of their motors - I think they only make round case types.
Tweakie.
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a first step would be to meassure the voltage,
for Motor supply.
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Hi,
another fairly simple test is to install an ammeter in one wire of the stepper.
The stepper driver will be using a sort of pulse width modulation technique to regulate the current. If the full voltage of the DC supply were hooked to the stepper
the current would be so great that the motor would be destroyed in short order. For the purposes of this test a good-old rugged analogue meter is best.
Note also that the current in any given wire will reverse direction with each step, so your meter will try to read backwards, provided the current is not too
high relative to the rating of the meter it will probably withstand being driven backwards.
Another couple of points should be made so that you can be sure that your measurement is accurate.
The first is that with microstepping, and just about all drives for CNC use it, the current builds up to its peak through each intervening microstep to its maximum
that occurs on the motors natural fullstep. I would recommend therefore to switch microstepping off for this measurement, that will means that any current
you measure will be the maximum the drive is capable of or configured for.
The second issue that it is common for stepper drives to reduce the current delivered to the stepper if the drive is sitting at idle for any length of time. So if
the stepper drives to a point and then sits there, still with its maximum current its going to get warm so if its stationary for more than a second or so the drive will
reduce its current by half to save the stepper from getting to hot. For this measurement you want to disable that current reduction feature so that you will
at all times get maximum current and therefore a realistic appreciation of the motor current.
Now you have to decide does the measured motor current correspond to the manufacturers rated current?
Craig
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Hi,
there are other ways to measure the motor current.
I have a high bandwidth current clamp from DC to over 1MHz, very useful for such tasks but not usually in the realm of hobbyists kit.
You could put a low value resistance, say 0.1Ohm, in the wire and measure the voltage drop with an oscilloscope, should avoid the disadvantage of driving
an analogue meter in reverse. Also with the bandwidth available with an oscilloscope you should be able to see the signature of PWM that generated the current.
Another possibility is to find the shunt resistor on the driver circuit board. All the current to the motor will pass through a low value resistor and the circuit will
measure the voltage drop to control its PWM loop. If you can identify the shunt you could measure the voltage drop with an oscilloscope with the same advatages
as above. Will require that you study the board and trace out the circuit to the extent that you can identify the shunt resistor. That requires some knowledge
and experience, its part of my training and very much a part of my job which is why I mention it, you may have similar skills/experience and could therefore
adopt this very simple test.
Craig
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Hi,
I have read that stepper magnets can be irrevocably demagnetised if the stepper is disassembled. It always makes me wonder how they make them I the first place?
Do they assemble them real quick while the magnets are still hot and before all the goodness drains out of them?
Any permanent magnet can be demagnetized if a sufficient magnetizing force is applied contrary to its field, if memory serves the parameter which measures that threshold
is called coercivity. Ferrite magnets have vey useful coercivity, about 1/2-3/4 Tesla but rare earth magnets are better again 1-1.5 Tesla. You have to try that much
harder to demagnetize a rare earth magnet.
I think the risk comes when you disassemble a stepper the magnet is now dislocated from its soft iron magnetic circuit. It doesn't suddenly lose magnetism it just that
it doesn't have the easy magnetic conditions it is accustomed to. This is in effect a counter field equal to the remenance of its own field. In the case of ferrites
the difference between it remenance and its coercivity is fairly small, about 0.1 Tesla. Any additional demagnetizing flux due to a current in a winding could cause a
ferrite to demagnetize. Rare earth magnets the difference between remanance and coercivity is about 1/4-1/3 Tesla and so unlikely to encounter flux levels
sufficient to demagnetize certainly by accident.
My contention is that unless a ferrite is removed from its soft iron magnetic circuit or was deliberately spun in generator mode well in excess of it ratings that it
will not demagnetize and not therefore have a 'lifespan'. Whether my contention is correct is another matter!
Craig
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I just put an analog ammeter in the supply line to the driver (all drivers). That tells me most of what is going on.
Cheers
Roger
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Hi Craig
Whether my contention is correct is another matter!
Just so.
It does seem to be accepted by many in the game (or trade) that the old round steppers with ferrite cores do lose their torque over time. For sure, I can report that my old (20 - 30 yrs) round steppers have lost most of their torque. They go around still, but I can stop even the big ones with my hand (Gecko 203V driver).
Cheers
Roger
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I wrecked a nice Nema
46 ooops, that's 42 stepper by disassembling it years ago.
Inquired about getting it re-magnetized and was told that they are magnetized while assembled. Some special machine.
The air gap around the rotor is critical. When the rotor went off center and was pulld out, the magnetism dropped buy over 60%. IIRC, the cost to repair was pretty steep. Was also about the time I was going with AC servos .. so I scrapped it. Lesson learned.
I've also read that continuously overloading a stepper (servo) motor will degrade the magnetism.