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TerryT
03-31-2008, 12:22 PM
... Is not what I was hoping to hear from the tech. The machine would carve just fine until about 30 to 50 percent mark then it would shift the pattern a half inch to the left (keyboard side) and would carve clear off of the wood. They had me do a few checks but still aren't sure. They are sending a new Y motor to try. It happened three times in a row with two different patterns and I reflashed the card between attempts two and three, no noticeable binding or loose belt. Still no go.

Any other ideas out there.

hotpop
03-31-2008, 02:28 PM
Terry,

I had that problem a couple of weeks ago. The problem was dirt on the "Y" axis rails causing the gear belt to skip a tooth. Once a tooth is skipped the whole pattern shifts about 1/4"

I cleaned the rails with 4X steel wool. Be sure to turn off the machine off when moving the truck assembly.

luckettg
04-01-2008, 11:07 AM
This is a very typical problem for stepper motors, and sort of a mixed blessing. It is better that they slip like this when hung than keep going and causing serious damage...IMHO.
Greg Luckett

Jeff_Birt
04-01-2008, 11:10 AM
The machine does not use stepper motors. All axis are servos.

luckettg
04-01-2008, 11:22 AM
Steppers are a kind of servo, and the motors are steppers on the axis drives. I spent a career designing and building control systems for machines using servos. If the motor is slipping, it is a stepper type. If the slippage is due to it being a brushless DC type servo (these are the two types mostly used), then the timing belt is jumping, or a sprocket is slipping. Actually if the latter, the error distances will not be a function of the belt cogs.

From digging around inside my CW machine, it appears to derive from printer technology and components. In other words, stepper motors.

Greg Luckett

Jeff_Birt
04-01-2008, 11:33 AM
Steppers are a kind of servo, and the motors are steppers on the axis drives.

We are delving OT here, but Stepper and Servos are completely different animals and are driven completely differently. Steppers have multiple field coils and a PM armature. The coils are energized in certain arrangements to move the armature. Do to this arrangement the armature moves in 'steps' (usually about 200 steps/revolution, modern stepper drivers can partially energize the coils to provide partial steps between the real steps. Stepper 'slip' or miss steps due to not being able to provide enough torque for the situation. When it misses a step the controller is not aware of that fact and the pulsing signals it is sending to the coils are now out of time with the actual motor, causing it to actually miss a few more steps until it gets back in time. Stepper motors are generally run open-loop, meaning there are no encoder to provide feedback to the operator.

Servos are any other type of motor with an encoder feedback. The motors on the CW are brush type permanent magnet. A servo motor is always closed loop as its encoder provides feedback to the control.

luckettg
04-01-2008, 11:56 AM
Jeff, I run steppers with encoders for closed loop control all the time and for many years. Steppers are so notorious for slipping that they should never be used without an encoder feedback for position error control. When I have more time I will look at the CW axis motors again to see about the encoders, if any, but it sure looks like open loop stepper control. Servos can be AC brushless, stepper, Vector, or plain old DC/AC motors. The term servo is commonly misused to mean only brushless DC technology, which is what i think you are doing. May I suggest you independantly confirm what I am saying since you do not believe me?
Greg Luckett

Jeff_Birt
04-01-2008, 12:14 PM
No, what I am saying is that stepper motors (open or closed loop) are NOT servo motors (adding an encoder to a stepper provides a supervisory type of feedback, did it make the step or not, but it's positioning is still determined by the inherit number of step the stepper has, normally 200). The fundamental construction of stepper motors is completely different than any other type. Brushless motors by default have some sort of feedback for the electronic commutation, but this does not make them servos either. The encoders on servos provide feedback as to the angular position of the servo and/or the position of the device being driven. With the proper feedback/controller you can make a servo out of brush type motors, brushless motors, even hydraulic and pneumatic motors.

My real point to replying in the first place was to clarify that the CW's motors are all servos.

jackssib1
04-01-2008, 12:39 PM
i just use and hope it don't stop on me,,
thx, jack

Digitalwoodshop
04-01-2008, 01:51 PM
I have a background in this stuff too and I TOO was FOOLED.... I thought for sure they were STEPPERS until I talked to LHR Engineer Chris Raul and he informed me that they are SERVOS... With Encoders for position feedback. Every Servo X, Y, and Z has a Encoder on the back of the shaft for Closed Loop Mode.

The belt slipping a Cog is MOST LIKELY the cause of this error. Proper tightening will help.

Could be some impacted sawdust in the cogs too causing the jump.

Good Luck,

AL

luckettg
04-01-2008, 02:00 PM
No, what I am saying is that stepper motors (open or closed loop) are NOT servo motors (adding an encoder to a stepper provides a supervisory type of feedback, did it make the step or not, but it's positioning is still determined by the inherit number of step the stepper has, normally 200). The fundamental construction of stepper motors is completely different than any other type. Brushless motors by default have some sort of feedback for the electronic commutation, but this does not make them servos either. The encoders on servos provide feedback as to the angular position of the servo and/or the position of the device being driven. With the proper feedback/controller you can make a servo out of brush type motors, brushless motors, even hydraulic and pneumatic motors.

My real point to replying in the first place was to clarify that the CW's motors are all servos.
Ok, but know that we do disagree about this, which is ok. :cool: My point is that if they are open loop steppers, then it accounts for the positioning problem caused by the track being dirty. That this happened without the machine knowing it means that the motors are being controlled in an open loop fashion.
Does this sound right to you, Jeff?:)

Incidently, check out the Compumotor web site. If I think about it when I have time later I will get the link. Lots of good servo/stepper information and very germaine to a CNC machine.
:)

Jeff_Birt
04-01-2008, 02:20 PM
My point is that if they are open loop steppers, then it accounts for the positioning problem caused by the track being dirty.

That's true. It is also true in this case for servos as the slippage is past the encoder do it will never see it.

As I think about it more, a closed loop stepper drive 'could' is similar to a servo in the fact that it can sense lost steps and try to catch up (would be similar to the servos following error). I am not aware of any stepper drives that use a typical servo loop (PID or similar algorithm) to really control a stepper motor in a 'servo' fashion. Variable reluctance motors/drive would be the closest that I know of.

I think at this point it is getting rather academic and I think you, Al, and I are the only ones who really care. ;)

ncamp
04-01-2008, 04:07 PM
I think at this point it is getting rather academic and I think you, Al, and I are the only ones who really care. ;)


You three are not the only ones who care:rolleyes:

There is a big difference in smaller hobby style servos and steppers. I play around with robotics at times and have built drivers for both. From what I have seen, when you get up into the larger industrial units with encoders, etc, the operational differences between the two are not that noticeable.

The technology is different, but the results are the same. Of course a driver board and motor on one of the larger units will make the carvewright look cheap.

luckettg
04-01-2008, 04:33 PM
Amen to that. I spend that for a couple of motors, encoders, and the control & driver boards, then spend time installing and writing code to run them. This is why I think we are dealing with printer level technology, which is not unsophisticated either.

Eagle Hollow
04-02-2008, 06:42 AM
That's all easy for you guys to say. What kind of motor runs my hand saw?
Jerry
1.126

luckettg
04-02-2008, 07:02 AM
That's all easy for you guys to say. What kind of motor runs my hand saw?
Jerry
1.126

Probably universal. Tell me the brand and age and someone here may have been one of its designers. Most hand tool motors are universal, but the technology has been moving to brushless DC lately.

Eagle Hollow
04-02-2008, 07:14 AM
Probably universal. Tell me the brand and age and someone here may have been one of its designers. Most hand tool motors are universal, but the technology has been moving to brushless DC lately.

That must be why my shoulder gets a kink in it...no brushes in the 8 teeth per inch hand saw.

Please forgive my B.S. You can't see my tongue in cheek...snicker!!

Dhaffner
04-02-2008, 12:56 PM
So is it "No hill for a stepper" or "You got servoed"?
Heh. I've been watching this thread and gearing up to make that joke for a couple days...and I expect feedback unless this thread has gone to pot. :-P

luckettg
04-02-2008, 01:12 PM
So is it "No hill for a stepper" or "You got servoed"?
Heh. I've been watching this thread and gearing up to make that joke for a couple days...and I expect feedback unless this thread has gone to pot. :-P


:roll: Sigh

ncamp
04-02-2008, 06:59 PM
That's all easy for you guys to say. What kind of motor runs my hand saw?
Jerry
1.126

Well, it would be universal. It will drive your saw, hammer, screwdriver, and much more.:D