Basic CNC

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:
 
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"
 
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:
 
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.
 
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 motor.................................

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
 
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:

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.
 
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

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

"Billy G":thinking:
 
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