Basic CNC

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"
 
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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" :))
 
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...
 
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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"
 
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:
 
John Hill said:
As I understand DC motors, those with brushes and commutators are really like stepper motors in that they have multiple coils that are energised in turn. In a stepper motor these coils are energised in turn by the controller electronics whereas in a commutator motor it is the commutator that as it turns energises the coils in sequence. As an aside, I am sure that some sort of commutator could be fashioned on a stepper to make it run as a DC motor.

Permanent magnet motors have a static magnetic field and the coil that is currently (excuse the pun) energised will seek a particular position according to the field so energising coils in turn causes the motor to rotate. This is true of commutated DC motors and stepper motors.

AC motors are quite different in that it is the magnetic field that moves and the armature of the motor turns to follow the moving field.
Click to expand...

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.
 
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
 
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.
 
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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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