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Basic CNC

Discussion in 'CNC IN THE HOME SHOP' started by Bill Gruby, Oct 7, 2012.

  1. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    I have gone over this board a several times now looking for basic info. Most times I have found that it is assumed that you already know what happens up to a certain point or you would not be asking. This is not true in a lot of cases. Also there are some who will not ask for various reasons.

    So here is my plan -- Ask a basic question and get the answer in this thread. Simple --- not really. I would bet there are dome here that do not know how a stepped motor works. I didn't till this morning? I didn't even know that there is more than one type.

    This is stickied to the top so it does not get lost. You neophytes just post whenever you feel the need to ask about something.

    One thing I woud ask of those answering, please do not assume anything. And remember you startes an sero knowledge too.

    "Billy G" :thinking:
     
  2. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    What is a Stepper Motor and why is it needed instead of a normal DC Motor?

    "Billy G" :))
     
  3. Mid Day Machining

    Mid Day Machining United States Active User Active Member

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    Great topic Bill. I'm sure there will be lots of questions. I'll chime in where ever I can help.
     
  4. joe_m

    joe_m Active User Active Member

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    I'm still trying to figure out what I need to do for CNC, but I think I can answer the stepper motor question.
    A regular motor just goes round and round (or back and forth if it's a windshield wiper motor.) A stepper motor shaft goes round in steps or increments. It takes a pulse of power and the shaft rotates a set amount. That amount doesn't vary - one pulse of power and it moves one step. The step might be 1/10 of a full rotation, 1/4, 1/3, whatever - but it will be the exact same each time. And since CNC relies on the computer moving the x/y/z axis of your mil or lathe in very precise measurements you need a stepper motor.
    For example: Your computer program says "move the X axis .01 to the left before making this cut" and if your stepper motor is geared up to the x axis in such a way that each pulse of power makes it rotate just enough to move .005 then the computer will tell the motor to take two steps.
    With a normal motor you get on and off and you just can't control the on/off precisely enough to get the controlled movement you need.

    That's my understanding of stepper motors.
    Joe
     
  5. xalky

    xalky United States Global Moderator Staff Member Director

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    That's a very good explanation. A servo motor does basically the same way except that it has feedback that tells the controller that it did indeed move 2 steps, at least that's how i understand the difference between a stepper and a servo.
     
  6. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    Let's try not to add to any confusion. The original post was Stepper versus Normal DC. Once this is answered we can move to Servo Motors. We need to go sloooow here. No harm done yet folks, just staying on top of things.

    DC motor move in a continuous rotation. Stepper moves in steps caused by electric impulses. Simple as that???

    "Billy G" :thumbsup:
     
  7. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    I have nothing to add here joe is right about how it moves
    the size of the motor has to be large enough to make sure the step is made or everything goes out of position. the computer tells the motor to move the required amount of steps. but in most hobby machines has no way of knowing if it did move that exact amount.
    the dc motor you set a speed and direction and "you" have to stop it when it reaches the point you wanted
    are we going the direction you wanted bill?
    steve
     
  8. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    In less that three hours this thread is off the original question. Yes Servo motors are used -- Reason, greater accuracy. If this thread is going to work we have to stay on track. DC Motors (as in tread mill) vs Stepper Motors. That is the original deal. One step (pun intended) at a time please.

    "Billy G"
     
  9. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    if you just want to turn a motor at a set speed: dc motor
    if you want to turn at a exact speed and number of turns: stepper motor
    steve
     
  10. 7HC

    7HC Active User Active Member

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    So to sum up so far in regard to 'regular' DC motors vs stepper motors, a regular motor will spin continuously when current is applied, while a stepper motor requires a more specialized input, usually from a computer, to tell it to move a step at a time like a ratchet.
    The computer tells it how many steps and how quickly to go from step to step. If it moves from step to step really quickly it can appear to be moving like a regular motor, but that's just an illusion.

    So far so good?


    M
     
  11. jumps4

    jumps4 Global Moderator Staff Member Active Member

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  12. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    Steve;

    The original objective was this:

    1 -- What is a Stepper Motor

    2 -- What is the differance between a Stepper Motor and the normal DC Motor.

    3 -- Why is the Stepper Motor necessary

    Thats it. It is to bring everyone to the same level slowly. There are many out there that would like to know these things in the order they are used. I for one only know enough to be dangerous. This thread is to start at square one and build from there. Assume nothing.

    Nothing is off topic yet, just out of order. All motors will be touched on in the next question that I have. Let's get this one out of the way first. I know it will be tough not to jump right in with what is known by some. I just want to move slow so no one is left behind.

    Hope that explains where I am comming from with this thread. If for some reason it starts to wander, all of you feel free to pull it back. I can't be here 100% of the time. This is your thread, all of you, slow it down if you start to lag behind.

    "Billy G"
     
    Last edited: Oct 7, 2012
  13. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    Can and will do. I started a file on my computer for just that reason. Thank you Steve. I see a long road ahead for this thread, with a great destination.

    "Billy G" :))
     
  14. DMS

    DMS United States Active User Active Member

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    I'll take a shot at the 3 questions here

    1 -- What is a Stepper Motor

    A stepper motor is a electro-mechanical device used for positioning. Specifically it uses electrical signals to change the physical configuration of the device. The most common is to rotate a shaft (rotary stepper), but linear steppers are also available which convert electrical impulses into linear motion. Rotary steppers rated as having a certain number of steps per revolution (the number of pulses, or steps, that cause the shaft to rotate 360 degrees). The most common are 200 steps per revolution (1.8 degrees per step).

    Things to keep in mind about stepper motors.

    * They have high torque at low RPM.
    * They have low torque at high RPM
    * When power is removed they have only a small amount of holding torque
    * They are relatively heavy for the amount of power they put out.
    * You can't just hook a battery up to them and make them "go", they require some sort of controller.
    * There are several ways to make a stepper motor, but this doesn't matter in practice (IE, doesn't matter how they work, as long as they have enough torque)
    * There are several ways to wind the coils of a stepper motor, and this DOES matter in practice because there are different types of controllers, and the controller has to match the motor you are using. The 2 winding types are "bipolar" and "unipolar". You will commonly see these words listed on both motors and controllers. Bipolar motors have 4 leads, and require a bipolar controller. Unipolar motors have 6 wires, and are best driven by a unipolar controller; they can be used with a bipolar controller, though it is not optimal (usually you need to run them at reduced current or they overheat). Some motors have 8 wires, and these can be used as either unipolar or bipolar depending on how they are wired.

    2 -- What is the differance between a Stepper Motor and the normal DC Motor.

    By "normal DC Motor", I will assume this refers to a brushed, permanent magnet DC motor, the kind you find in kids toys, model cars, and cordless drills. As stated above, stepper motors move in a controlled fashion based on electrical input.

    Would it surprise you to learn that DC motors ALSO move in a controlled fashion based on electrical input? They do, but the behavior is different. With stepper motors, 1 pulse == 1 step == a known amount of rotation of a shaft. With DC motors, a fixed voltage == fixed RPM, and a fixed current = a fixed Torque. If you reverse the voltage, the motor reverses direction (think, drill forward, drill reverse).

    While trying to stay on topic, and not go too far down this path, if we put a sensor on the output of the DC motor (this is called an "encoder") that tells us the exact position of the motor shaft, we can adjust the voltage (hence speed) and current (torque) on the motor to zero us in on the position that we want. This is a servo-system, and the dc motor in this system is called a servomotor.

    So let's see if I have answered the question here. Stepper motors take pulse inputs, and require a controller to make them "go". We talked about how we can make a DC motor go at the right speed, but we don't know _where_ it is unless we add an encoder. Something has to look at the output of the encoder, and adjust the input of the motor to get it in the right spot. We can't do it by hand, which means, you got it, a controller.

    So, for position control (the thing we really need in CNC to move the cutter to point A in a controlled fashion) we can use

    1) Stepper motors + controller
    2) DC Motor + encoder + controller (a servosystem)
    3) Any number of other technologies that let a computer control the position of the machine axes. (see, the how is not as important as the what).

    There are benefits and drawbacks to all systems, including speed, torque, cost, ease of use.

    3 -- Why is the Stepper Motor necessary

    In the answer to #2, my conclusion is that "stepper motors are not necessary". Rather, positional control is necessary, get it however you can. Steppers are a good choice for this. They are certainly the lowest cost, easiest to use solution, and they give good performance.



    Sigh... this post is already TLDNR, and I haven't even touched on closed vs open loop control, which is the main difference between steppers and servos...
     
    Last edited: Oct 7, 2012
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  15. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    You stopped in the right place. I am taking notes while I watch this thread. Closed vs open loop steppers will be covered later. Thanx Matt.

    "Billy G"
     
  16. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    Thank you all, great stuff to this point. Let's move ahead a little to "Open and Closed Loop" motors. I believe we can also add "Servo Motors to this one. It's all yours. Any takers?

    "Billy G" :thinking:
     
  17. DMS

    DMS United States Active User Active Member

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    What SSSFOX was (I think) what is referred to a "Brushless DC Motor" or BLDC motor. They look a bit like a stepper motor internally. They don't have a commutator and brushes (one of their benefits). Instead, they have a controller which serves the same purpose. They are referred to as "electrically commutated".

    And yes, they can also be used to drive a CNC axis, or a spindle for that matter.
     
  18. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    to add a little there are encoders for stepper motors that send pulses to the software to say if a step is missed and correct it or stop if not possible, that is a closed loop stepper motor system and glass scales can also be used instead of encoders. the more accurate of the two is glass scales in this method the software is always correcting for flex and backlash because it knows where the part is and not just if the motor moved
    steve
     
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  19. DMS

    DMS United States Active User Active Member

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    To add to what John and Steve have said...

    A closed loop system lets you not only know where you want to be, but where you actually are. An open loop system only lets you know where you expect to be. This is something to keep in mind when designing a stepper based system. Always get steppers that are large enough, and don't push them past their limits. If you try to get more torque out of a stepper than it can deliver it will "loose step". In other words, you will send a pulse, but the rotor will not move as expected. You think you are at 1.010" in X, but really, you are 1.0095. In practice, if you loose steps, you tend to lose a bunch. Every time you loose a step, the controllers idea of where you are gets further and further from reality, and you end up with bad parts. This should not happen if you don't overload the system.

    If you try to overload a servo system you will still be in a bad place, but instead of finishing the part, the controllers will fault. This will shut the controllers down down, and let you know things are bad. Maybe letting you save the part if things haven't gone too far, or maybe just letting you know to dial down your feeds.
     
    Last edited: Oct 8, 2012
  20. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    mine are micro stepping they are set to 2000 steps per rev or since i have 5tpi screws 10000 steps per inch
    the 200 steps 1.8 degrees is in their most basic operation and way to jumpy of a setting for most cnc machines. when they are set that way there is a lot of vibration that is sent back to the table the motor sitting still will vibrate bouncing back and forth like a tuning fork vibrates.
    are we still in line with the goal bill or jumping too far too fast?
    steve
     
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  21. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    You guys are right on track, but soon you will be in unknown territory for me. That is what I expect to happen. You will have to set the pace yourselves then. I hope there are some new people watching this. The view count is tough to read and get an indication from. Great information. I don't think this approach has been tried anywhere. Keep up the good work.

    "Billy G" :thumbsup:
     
  22. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    We will give this a day or so before moving on. I want the new to CNC guys to be sure they understand so far. Please if you are new to this ask any questions you have. If something was left out that you wish to know ask. We don't know id you are understanding the info if you do not tell us. If you do not want to post the question here PM me and I will put the answer out here to be seen.

    Again thank you to all who have shared their knowledge with us so far.

    "Billy G"
     
  23. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    I got three good questions in a PM I will list them one at a time. As one is answered I will list the next. Here goes.

    What is the differance in wired motors. One has 4 wires, one has 6 wires, one has 8 wires? Feel free to take this question right thru BiPolar. UniPolar.

    "Billy G" :thinking:
     
  24. DMS

    DMS United States Active User Active Member

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    I mentioned this in one of my earlier posts, but it was pretty long, and didn't go into much detail. I guess, lets start with what we mean by unipolar and bipolar wound motors.

    Unipolar motors:

    These are the "6 wire motors". They have 2 windings, each with a center tap (2 ends+1 center tap = 3 wires per winding). The benefit of unipolar motors is that they are simple to control. In practice, the center tap is connected to the (-) side of your power supply. One switch (usually some type of transistor) is connected to each end of each winding (a total of 4 switches). Only one switch per winding is activated at a time, meaning, only half of the winding is in use at any one time. This is kind of wasteful (unipolar motors are less powerful than bipolar motors for a given weight). Keep in mind that the switches are almost always integrated into the controller. They are one of the costliest part of the controller, hence the desire to limit the number.

    Bipolar motors:

    These are the "4 wire motors". They have 2 windings, just like a unipolar motor, but with no center tap. The benefit of the bipolar motors is that they are lighter than unipolar motors for a given amount of power. They also tend to be capable of higher speeds than unipolar motors. The key drawback of bipolar motors is that they require a more complicated controller. Instead of having one wire from the coil always connected to (-), the controller with connect the wires from a given winding between (-) and (+) of your power supply. It alternates which side is positive, which side is negative, and which coil is on or off at any given time. Here is a table to illustrate what is going on

    Coil 1 Coil 2
    Wire A Wire B Wire C Wire D
    0 (+) (-) nc nc
    1 nc nc (+) (-)
    2 (-) (+) nc nc
    3 nc nc (-) (+)

    After sequence 3, the cycle repeats. Going in reverse order reverses the direction of the motor.

    You may be asking yourself how the controller switches the inputs on and off, and reverses the polarity on the coil. It does it using a circuit called an "H-Bridge". It's very common in power electronics. It is made up of 4 switches (transistors), and we need 1 for each coil, which means for a bipolar controller, we need 8 switches (twice as many as we need for a unipolar driver). The truth is that he relative cost of high current transistors has come down considerably in the last 10 years, so the reasons for using unipolar motors have mostly evaporated. If given a choice, I would always go for a bipolar controller/motor.

    Where do the "8 wire" motors fit?:

    8 wire motors are wound with 4 separate coils (2 wires per coil * 4 coils == 8 wires). Because of this you can wire them in either a bipolar or a unipolar configuration. The trick is doing this the right way. You can figure things out with a voltmeter, and and some trial and error, but its better if you can track down the manufacturers datasheet.
     
  25. 7HC

    7HC Active User Active Member

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    If I understand you correctly, unipolar motor is heavier and less powerful than a bipolar motor, and the only disadvantage of a bipolar is that it requires a more complicated controller?

    However, the controllers I've looked at (assuming that a controller and driver are the same thing?), make no distinction as to the configuration of motor that they control.

    What puzzles me me is the eight wire motor that can be wired as either a unipolar or a bipolar.
    What purpose does a motor serve that can be used as the least favorable option as well as the most? :thinking:


    M
     
  26. arvidj

    arvidj United States Active User H-M Supporter-Premium

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    I think the group may have moved on but I wrote this yesterday and did not wasn't to just throw it away ...

    Ok, here is my take on it.

    First, lets talk about how any electric motor works.

    Lets start with just a very simple bare shaft inside a very simple empty housing. Add bearings on the end of the shafts as necessary so the shaft can easily spin.

    Great prototype, except it doesn't do anything because it is just a shaft and a housing. We need to improve on this design. Setting the prototype aside for a moment go back to our childhood days and think about the fun we had with two magnets. We thought it was magic that if we had two magnets and tried to push the two 'N' of the magnets together they would repel one another and if put the 'N' and 'S' ends together it would take some force to pull them apart.

    The interesting part was that the 'force' was invisible and therefore somewhat magic. If we used used a lever or a pry bar to push things apart it was not magic as we could see the lever or pry bar touching the two items and therefore it was obvious what was pushing them apart. The same way if we used a bolt and a nut to pull two things together ... it was obvious what was happening. But these magnets and this thing called magnetism ... able to push thing apart and pull thing together without the need for a physical connection between the two items. It was an invisible magic force with the only minor constraint being that both items had to be magnetic.

    "the only minor constraint being that both items had to be magnetic" ... lets address that immediately. There are two common ways to "be magnetic". One is to be a magnet in the first place ... like the kind you were playing with as a kid. The other way would pass electricity thru a strand of any conductive material. Around this strand of conductive material will be a magnetic field ... i.e. be a magnet. Great, we now have a magnetic we can can control by simply turning the electricity on or off.

    But our solution to the 'we need a magnet' has a minor problem. The magnetic field around a single conductive strand is relatively weak. It can be measured ... has been since the early 1800's ... but it is not strong enough be of much use. But again, we can resolve that issue by taking a long strand of our conducting material, covering it with an insulating material that does not conduct electricity then wind the strand up in to a coil. Note that we need to use an insulated conductive material to keep the conductive material in each wind of our from touching each other. If the conductive material of one winding touches the conductive material of another winding you have what is called a 'short' and very shortly will utter the words "where did that smoke come from".

    Anyway, now that we have a coil we will notice that when we pass electricity thru the conductor the magnetic field is much stronger and something that we may be able to put to use. Of course we can improve on this basic design by winding the coil around a suitable material ... like an iron core ... and being very careful and creative about the exact winding of the core, but at least we have a basic understanding of what id going on.

    So now, back to our prototype shaft and housing. What would happen if we put a magnet ... either the static kind we used as a kid or our second generation controllable "electromagnet" on the shaft and also put another magnet ... again static kind we used as a kid or our second generation controllable "electromagnet" on the housing. If we put the magnets on the shaft and the housing in the proper orientation we would see that the magnets would attract or repel each other and in the process turn the shaft.

    Great, we now have a "magnetic" motor in that the shaft will turn ... with a minor issue to be resolved. The shaft turned part way and then stopped. Not exactly what we had planned but at least it is progress. What has happened is that our design has a magnetic field on the shaft and a magnetic field on the housing that are static ... they do not change. So the two magnetic fields did what they were designed to do ... either were attracted or repelled each other and caused the shaft to turn ... but the shaft quickly reached a point of equilibrium and stopped turning.

    Think about it for a minute. Lets assume the magnet in the housing and the shaft repelled each other. The magnet on the shaft would start turning away from the magnet on the housing but as the shaft rotated it would eventually rotate far enough that the magnet on the shaft would start going back towards the magnet on the housing ... and that can not happen because the magnets are set up to repel one another. So the shaft would turn and stop at a point where it was "happy" because it had turned as far away from the housing magnet as it could but also was "happy" because the magnet was not moving towards the housing magnet.

    Note that we can apply the exact same logic if the magnets attract each other, just the shaft would "be happy" in a different location ... probably 180 degrees from where it "was happy" if they repelled each other.

    At least our shaft was happy even if we aren't happy with our motor. We have to resolve this minor issue.

    It would seem that if we could exert come control on the magnetic field we might be able solve this problem. Given that we have no control over the magnetic field if we use the static "kids" magnets we will throw out that design. No control means no improvement so to the trash with that one. But we are still left with three alternatives ... controlled magnetic field on the housing and static magnets on the shaft; static magnetic field on the housing and controlled magnetic field on the shaft; controlled magnetic field on both the housing and the shaft.

    Now that we have agreed we will used a controlled magnetic field, what will we do with it. The obvious 'control it' but how. We saw that the shaft turned until it reached an equilibrium point. What would happen if we then altered the magnetic field in such a way that this equilibrium point changed. Maybe we put in a second coil mounted in a different location and orientation in either the housing or the shaft. We then turn on the original configuration and as soon as the shaft reach its equilibrium we turn off the original configuration and turn on the second set of coils. The shaft would then try to seek a "happy" spot based on the new magnetic field.

    Our first try at this was not quite what we hope. When we mounted the second set of magnets Murphy intervened and the location was such that when the second set of coils were engaged the "happy" spot for this configuration was "behind" the "happy" spot for the first configuration so our shaft would rotate part of a turn clockwise and then part of a turn counter clockwise as it sought out the "happy" spot with each change. The shaft did not go around, just oscillated back and forth between the two position. But at least it is progress in that the shaft no longer just sat there. It was doing something, just the wrong thing.

    We noodle over this minor issue for a little while and decide that if Murphy does not intervene, and we put the magnetic coils in the correct locations, and possibly add more magnetic coils, and then make sure the the timing electricity to of each of the coils is right, we can come up with a scheme where we energize and de-energize the coils in such a way as to keep the "happy" spot just a little bit in front of where the shaft is at any moment in time and therefore the shaft continually turns in the "correct" direction seeking out ... or chasing if you will ... the happy spot that is always just out of reach.

    Note that we have not talked about steppers, servos, AC or DC up to this point. Just "how do electric motors work" ... at least from my perspective.

    Assuming I am not banned from the board for being too long winded, too basic, etc., I can move on from this starting point to AC, DC, steppers and servos as we talk about how we can orchestrate or synchronize the "happy spot just out of reach" that we think will solve our problems. Or you can PM me and tell me to go away ... or publicly tell me to go away. I've been married for 28 years so I am use to harsh criticism.
     
    silverforgestudio likes this.
  27. DMS

    DMS United States Active User Active Member

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    Hopefully I can clarify some things. I found this page from Gecko that is pretty good

    http://www.geckodrive.com/support.html

    Take a look about halfway through at figure 8 where they show a 6 wire (unipolar) motor connected in a bipolar fashion. Yes, this does work, but read the fine print; if you connect the controller across the two outer leads (leave out the center tap), you have to set the controller up at _half_ the rated motor current (otherwise you risk overheating the motor, which is bad mmm'kay). If you use half the coil (one end, and the center tap) you can run at the full current, but the voltage is gonna be less. Basically with any electric motor the rule of thumb is that torque is proportional to current, and speed is proportional to voltage. So you are basically trading off torque and speed. To top it all off, the motor is still not going to put out as much power as a bipolar motor of the same weight.

    As far as why you would want to have an 8 wire motor. With an 8 wire motor, you can do a lot of different things, and tune the motor to your situation. Some people make their controllers, and in that case, unipolar is simpler. Also, in industry, sometimes nothing is more important than price. It's also easier for the manufacturer because they don't have to produce both a unipolar and a bipolar version of the motor.

    Oh, and remember that thing I said about there being a tradeoff between speed and torque? With an 8 wire motor you can re-wire and get more of one or the other, or work around limitations (ie current) in your controller.

    Another thing I noticed is that the Gecko controllers don't say explicitly that they are bipolar... but they are. The drives from Kelinginc are like that too. I guess if you want to be sure, you gotta check the manual and see how to hook motors up to your given controller.
     
  28. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    OK then, we seem to have covered question #1, it's on to #2. What is the differance in sizes aka Nema 17, 23, and 34?
    When would each be used?

    "Billy G" :thinking:
     
  29. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    the more torgue you need the bigger the winding needs to be nema is the standard for interchange if the requirement excedes the frame size 17 you move up to the next size 23 and so on hey you forgot my nema42 4200 oz/in that motor has a 3/4" output shaft and is rated at over 1.5hp thats a heck of a stepper motor .
    steve
     
  30. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    OK, now on to #3 & #4. "What is Micro Stepping?" "Why is it used" Combine these into one answer if needed.

    "Billy G":thinking:
     
    Last edited: Oct 9, 2012

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