Stepper Options - Page 2


Page 2 of 4 FirstFirst 1234 LastLast
Results 13 to 24 of 41

Thread: Stepper Options

  1. #13
    Registered
    Join Date
    Jun 2009
    Location
    si
    Posts
    11
    Downloads
    0
    Uploads
    0

    Default Re: Stepper Options

    Comment to one previous post: If stepper loose step you will see it , motor will start stall. You can not see failing of micro stepping, however if position error increases to step motor will stall.

    Micro stepping helps to improve vibration reduction, its purpose is not to improve resolution. Leadshine drivers works fine. Almost like servos. But the price is too high. Cheap and reliable servos can be found for same money.

    Check this page: CNC | Galil

    They are very good.

    kr



  2. #14
    Community Moderator ger21's Avatar
    Join Date
    Mar 2003
    Location
    United States
    Posts
    30566
    Downloads
    0
    Uploads
    0

    Default Re: Stepper Options

    Each microstep on my machine is 0.0001" theoretical resolution
    You can be off several microsteps without losing steps. They will eventually catch up. Consider your actual resolution to be 1/2 step. You aren't losing steps until you lose a full step or more. And the chances of losing a single step are incredibly slim. If you are losing one step, then you'll lose a lot more than one.

    If you don't have encoders then saying that you have "never" lost a step is only a guess. Can you see 0.0001" or even 0.003"
    If you can't tell if you're losing steps, then you're probably not, and don't need encoders.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  3. #15
    Gold Member
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    3055
    Downloads
    4
    Uploads
    0

    Default Re: Stepper Options

    The funny thing about steppers is that once you have lost ONE step, you are very likely to lose the rest as well (as Gerry said). So it is not a case of having to detect 0.0001"; rather it is a case of having the machine stall.
    In addition, you can usually hear when a stepper loses just a few steps.

    A dull tool in aluminum can heat and then clog by welding the aluminum cuttings to the tool. That tool no longer cuts and you wind up trying to push it through the stock. Are you saying that a tool that is no longer cutting because of this can't cause lost steps??
    That is called Built Up Edge (BUE). It is not always due to a dull cutter; it can be due to the aluminium alloy (some are chewing gum) or the surface of the cutter. (Mist lubication is the cure there.)
    Regardless, when it happens you do not lose a few steps: you usually lose the whole cutter with a big bang. And probably the job too.

    Cheers
    Roger



  4. #16
    Community Moderator ger21's Avatar
    Join Date
    Mar 2003
    Location
    United States
    Posts
    30566
    Downloads
    0
    Uploads
    0

    Default Re: Stepper Options

    A dull tool in aluminum can heat and then clog by welding the aluminum cuttings to the tool. That tool no longer cuts and you wind up trying to push it through the stock. Are you saying that a tool that is no longer cutting because of this can't cause lost steps??
    Either the tool breaks, or the machine stalls.
    You need encoders to tell you that?

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  5. #17
    Gold Member
    Join Date
    Feb 2006
    Location
    USA
    Posts
    6953
    Downloads
    0
    Uploads
    0

    Default Re: Stepper Options

    Quote Originally Posted by 109jb View Post
    If you don't have encoders then saying that you have "never" lost a step is only a guess. Can you see 0.0001" or even 0.003". Each microstep on my machine is 0.0001" theoretical resolution. I plan to set my encoder up to fault the machine if commanded position is off by more than 0.005" and if you say you have a way to detect that without an encoder then then please tell how????
    There are so many things wrong with that argument, I wouldn't even know where to begin. It is clear you do not really understand how stepper motors work, nor how using encoders to detect "lost steps" is actually implemented. You will NEVER be able to detect one of your theoretical 0.0001" "lost steps", even it if were possible to "lose" a 0.0001" step (which it is most definitely not). And if you did somehow manage to build a system that could detect such a small loss, it would be constantly false-tripping due to following error alone. In reality, you'd be lucky to reliably detect even a 0.003" static position loss. And I can assure you a 0.003" loss is easy to detect visually.

    I suggest you spend a lot of time reading and researching how stepper motors really respond, in the real world, where things like friction, stiction, inertia, and other physical factors interfere with the "theoretical". Learn how a real stepper motor responds to micro-stepping, and you'll realize even talking about 0.0001" is ridiculous. Factor in the real-world characteristics of the machines being talked about here, and it becomes downright ludicrous. Learn that micro-stepping has virtually nothing to do with increasing resolution, but is, rather, used primarily to reduce resonance at low-to-moderate speeds, and that many drivers automatically switch to full-step mode at higher step rates.

    And, if I follow your argument, I am wrong to suggest that I can tell if my open-loop machine is losing steps, because I can't possibly detect a 0.0001" position error, but you are going to allow a 0.005" position error on your machine before you trip a fault? Do you see a logical contradiction there? How is a 0.005" position loss acceptable, when a 0.0001" loss is not? How do you suppose you are likely to ever achieve a 0.005" position loss without stalling the motor? The fact is, with steppers, once you've lost a single step while moving at anything other than extremely low speed, the motor is almost guaranteed to stall. THAT is easily detected without encoders.

    Finally, consider this: If simply adding inexpensive encoders to a stepper motor (and decent optical encoders can be bought for a few $ now) has such wonderful, wide-ranging benefits, why is it so very, very rarely done, even in the industrial world, where cost is not nearly as much of an issue as it is here? A great many extremely high-precision machines use open-loop steppers, with virtually 100% reliability.

    Regards,
    Ray L.



  6. #18
    Registered arizonavideo's Avatar
    Join Date
    May 2008
    Location
    USA
    Posts
    1075
    Downloads
    0
    Uploads
    0

    Default Re: Stepper Options

    Ont thing that happens with a stepper that looses a step is when a coil does not "sink a load" or transfer its power to the armature of the stepper, it just dissipates the field all around and disrupts the surrounding magnetic fields.

    Often under heavy load a stepper will just give up and loose 100 steps or stall. So you dont have to guess if you lost a step.

    Again running a stepper at 60% of full load to leave room for peak loads and you will almost never loose steps.

    The only time I have problems with steps is if a tool clogs and melts the metal. You can hear this from next door when this happens and its not much better with my servos.

    After years of milling with both steppers and servos, only a few times would encoder have helped, and only if they would have stopped the mill and not re set like I have the encoders set now.

    If money is not a issue and the extra wires are OK then do encoders. To me it is just one part part to fill with chips and water.

    youtube videos of the G0704 under the name arizonavideo99


  7. #19
    Gold Member
    Join Date
    Jul 2005
    Location
    USA
    Posts
    2391
    Downloads
    0
    Uploads
    0

    Default Re: Stepper Options

    Quote Originally Posted by 109jb View Post
    If you don't have encoders then saying that you have "never" lost a step is only a guess. Can you see 0.0001" or even 0.003". Each microstep on my machine is 0.0001" theoretical resolution. I plan to set my encoder up to fault the machine if commanded position is off by more than 0.005" and if you say you have a way to detect that without an encoder then then please tell how????
    You DO NOT have .0001" resolution. You cannot count micro stepping to increase resolution. Those who claim that are called "Marketing" people. It's there to smooth out the motion at lower RPM and prevent mechanical resonance. It eats power so you only get about 70% of the torque to the load. Most modern drivers disable mircostepping over about 200 RPM because it is basically ineffective due to motor and load inertia and is a waste of power, and limits upper torque. Couple that with the inherent 5% step error and you have a lot less raw "accuracy" than you think. Your linear mechanics have error as well . Something the encoder on the motor cannot compensate for.

    Having an encoder on an axis for absolute positioning so it has to move to a given spot and stop is a useful design. Trying to use it in a PID servo type loop is less so. It's your money and your idea. Its obvious you are a man of conviction and like to plow your own fields. Go for it. If you want high accuracy you go with servos , high encoder counts, precision slides and precision anti-backlash ballscrews. Gantry mass is important to prevent flexing. All costly, but high accuracy has high costs.



    A dull tool in aluminum can heat and then clog by welding the aluminum cuttings to the tool. That tool no longer cuts and you wind up trying to push it through the stock. Are you saying that a tool that is no longer cutting because of this can't cause lost steps??
    Having cut literally hundreds of miles of aluminum with both steppes and servos I can tell you this: As soon as a bit starts to clog , about 1.5 seconds later it either snaps off (if its smaller) or makes a cut that looks like you cut it with a chisel! So there is no making up lost steps it will simply finally stop motion.

    I am not bashing steppers. I think they are great and absolutely fine for the task, but still disagree that encoders are useless on them.




  8. #20
    Gold Member
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1586
    Downloads
    1
    Uploads
    0

    Default Re: Stepper Options

    Quote Originally Posted by ger21 View Post
    Either the tool breaks, or the machine stalls.
    You need encoders to tell you that?
    I have personally had a tool load up, the machine lose steps, the tool break below top of part, and the machine then went merrily on its way. The remaining upper portion of the tool with some of the upper part of the flutes intact then destroyed any possibility of salvaging the part while milling into my soft jaws as well. I definitely could tell that it happened after the fact but was not in the shop to stop it when it did. If I would have had encoders installed the machine would have faulted, and at a minimum saved my soft jaws, and possibly the part I was making.



  9. #21
    Gold Member
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1586
    Downloads
    1
    Uploads
    0

    Default Re: Stepper Options

    You DO NOT have .0001" resolution. You cannot count micro stepping to increase resolution.
    Re-read and understand my post. I said theoretical right in my post because I do understand this.

    Your linear mechanics have error as well . Something the encoder on the motor cannot compensate for.
    That is what screw mapping takes care of and has nothing to do with whether a rotary encoder would detect an error in the commanded rotary position.

    Having an encoder on an axis for absolute positioning so it has to move to a given spot and stop is a useful design. Trying to use it in a PID servo type loop is less so. It's your money and your idea. Its obvious you are a man of conviction and like to plow your own fields. Go for it. If you want high accuracy you go with servos , high encoder counts, precision slides and precision anti-backlash ballscrews. Gantry mass is important to prevent flexing. All costly, but high accuracy has high costs.
    Nobody said anything about using the encoder for a PID servo type loop. Only for error detection and to throw a fault stopping machine motion if a deviation is detected which is quite simple on LinuxCNC and has been done by others using steppers and rotary encoders

    Having cut literally hundreds of miles of aluminum with both steppes and servos I can tell you this: As soon as a bit starts to clog , about 1.5 seconds later it either snaps off (if its smaller) or makes a cut that looks like you cut it with a chisel! So there is no making up lost steps it will simply finally stop motion.
    Again, you are misinterpreting my post and how I intend to use the encoders. I do not intend to "make up lost steps". Only to detect when the motor shaft isn't where it is supposed to be and stop everything. I am not trying to get it back to position.



  10. #22
    Gold Member
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1586
    Downloads
    1
    Uploads
    0

    Default Re: Stepper Options

    Quote Originally Posted by SCzEngrgGroup View Post
    There are so many things wrong with that argument, I wouldn't even know where to begin. It is clear you do not really understand how stepper motors work, nor how using encoders to detect "lost steps" is actually implemented. You will NEVER be able to detect one of your theoretical 0.0001" "lost steps", even it if were possible to "lose" a 0.0001" step (which it is most definitely not). And if you did somehow manage to build a system that could detect such a small loss, it would be constantly false-tripping due to following error alone. In reality, you'd be lucky to reliably detect even a 0.003" static position loss. And I can assure you a 0.003" loss is easy to detect visually.
    So if you have a pocket that is 2.997" long and it is supposed to be 3.000" long, you can detect that VISUALLY?

    I suggest you spend a lot of time reading and researching how stepper motors really respond, in the real world, where things like friction, stiction, inertia, and other physical factors interfere with the "theoretical". Learn how a real stepper motor responds to micro-stepping, and you'll realize even talking about 0.0001" is ridiculous. Factor in the real-world characteristics of the machines being talked about here, and it becomes downright ludicrous. Learn that micro-stepping has virtually nothing to do with increasing resolution, but is, rather, used primarily to reduce resonance at low-to-moderate speeds, and that many drivers automatically switch to full-step mode at higher step rates.
    Why is it that everyone has to pick this out. I said theoretical exactly because I know it isn't real. It was only to depict how the machine is set up.

    And, if I follow your argument, I am wrong to suggest that I can tell if my open-loop machine is losing steps, because I can't possibly detect a 0.0001" position error, but you are going to allow a 0.005" position error on your machine before you trip a fault? Do you see a logical contradiction there? How is a 0.005" position loss acceptable, when a 0.0001" loss is not? How do you suppose you are likely to ever achieve a 0.005" position loss without stalling the motor? The fact is, with steppers, once you've lost a single step while moving at anything other than extremely low speed, the motor is almost guaranteed to stall. THAT is easily detected without encoders.
    I see no contradiction at all. What is acceptable is dependent on what you are making. A 0.005" detection error for the types of parts I usually make would mean that a fault at 0.005" would likely mean I would be able to change a tool or whatever, re-home, and then save the part. 0.005" would also insure that no further damage is done to machine, fixtures, etc.

    Finally, consider this: If simply adding inexpensive encoders to a stepper motor (and decent optical encoders can be bought for a few $ now) has such wonderful, wide-ranging benefits, why is it so very, very rarely done, even in the industrial world, where cost is not nearly as much of an issue as it is here? A great many extremely high-precision machines use open-loop steppers, with virtually 100% reliability.
    I never said that a machine with steppers without encoders wouldn't work or that it is necessary, I only said that it is "not totally useless"



  11. #23
    Registered
    Join Date
    Dec 2004
    Location
    USA
    Posts
    67
    Downloads
    0
    Uploads
    0

    Default Re: Stepper Options

    Anyone from the open loop stepper (OLS) tribe, who embraces artifacts normally used in servo tribe rituals, should be banished forthwith from the OLS tribe. Seriously though, an encoder can be had for around $24.00 these days, also, purchasing dual shaft stepper motors would be the foundation for a servo setup further down the road if so desired. Even adding another shaft to an existing single shaft stepper motor is possible if one has a suitable lathe and the aptitude. I used to add shafts to tiny BDC motors back in the day.

    Answering the "Why would one want to do this?" question would probably be similar to answering the "Why would anyone want to use servos instead of OLSs?" question.



  12. #24
    Gold Member
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    3055
    Downloads
    4
    Uploads
    0

    Default Re: Stepper Options

    And to think, encoders used to cost hundreds of dollars - once.
    Hum ... capacitive rather than optical? Interesting.

    Cheers
    Roger



Page 2 of 4 FirstFirst 1234 LastLast

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

Stepper Options
Stepper Options