Motor Selection for Mill Motion

One of the most crutial aspects of a CNC machine is the motors and control systems used to position the head of the mill on the work surface. Accuracy of these components will of course directly affect the precision and quality of the work done.

What are the Motor Options?

There are two main options for precision positioning motrs which are used for motion control. The first option is Step motors and the second is Servo motors. The first step is to compare the pros and cons of each option and make an informed decision based on their properties and what best suits my budget and application.

What is a Servo Motor?

A servo motor can be either DC or AC, and is usually comprised of the drive section and the resolver/encoder. A servo motor is much smoother in motion than a comparable stepper, and will have a much higher resolution for position control. The servo family is further divided into AC and DC types. An AC servo had the advantage of being able to handle much higher current surges than a DC, as the DC has brushes, which are the limiting factor in this case. Therefore, for our practical considerations, you can get a lot stronger AC servo motor than you could in DC or stepper configuration. Steppers, on the other hand, have economy as an advantage, and can be incorporated into a design to produce very smooth motion also. The trend for manufacturers of "serious" CNC machinery is to use AC servos. "Entry level" machines may have DC servos, or even steppers.

A resolver/encoder is a glass disc with very fine lines on it and an optical encoder that counts those lines as it rotates with the motor. This information is couple to the controller which tracks the counts, the rate that they go by, and through a host of feedback loops, logic, and controlling the amplifiers, produces the desired motion.

What is a Stepper Motor?

A stepper motor is wound in such a way that the rotation has a certain number of discrete "steps". I only know of stepper motors being DC motors. These steps are where the magnetic fields cause the motor to want to settle in one of these positions. The number of steps per revolution is rather high, around two hundred or so, and varies by model and manufacturer. What this means is that the motor has effectively a resolution (smallest controlled movement) equal to the number of steps for that motor. Everything seems to have exceptions, and that applies to steppers also - there are some called micro step, with a higher resolution, but I don't know much about them. Stepper motors may or may not have position feedback.

Stepper systems are often "open loop" which means that the controller only tells the motors how many steps to move and how fast to move, but does not have any way of knowing where they actually are. This can lead to errors, should a situation arise where the motors are unable to comply with the commanded move. This can be very obvious, where the motion stops and it sounds like you stripped a gear, or subtle, where the motor only misses a "few" steps. The result is the same - the controller thinks you are at X25.5, Y15.5 and in reality you might be at X25.3, Y15.4 . This can lead to a cumulative error, which may in turn lead to crashes, not to mention out of spec parts.

Stepper vs Servo Motor

Taken from (http://www.ahha.com/techdata.htm)

Many people are confused by all the hype surrounding the choice between step or servo type motors. We hope that this factual summary will help answer your questions on the subject. It is not a matter of one being good and the other being bad. It's simply an issue of price and performance. Both kinds of systems can cut the same parts for you.
Ah-ha! uses 1990's technology, and our motors and electronics are as advanced as any servo system. We use pulse width modulated MOSFET chopper drives, just like servos. Our motors are brushless just like modern servos, etc., etc..

What's the difference?
Steppers and brushless servos are virtually identical motors. The only real difference is the number of poles. Typical brushless-servos have 4 poles. Steppers usually have 50. 

Benefits of servos
Servos provide two benefits over steppers: First, servos need encoder-feedback to work at all, and this encoder inherently provides an out-of-position indication when something fails. Some people think that having encoders means the machine will never be out of position. Not so! Complex servo systems have all kinds of potential errors, such as: following, dither, overshoot, etc.. In most cases, the encoder does not tell you when you have a following error, an overshoot, etc.. And the boards can and do blow up, just like any other product. So, the encoder is nice, but it's not a cure-all. Second, servos give a faster rapid-rate, if your ballscrews can take it. 900ipm sounds great, but many screws can't handle more than 200-300ipm regardless of the motor type. 

Benefits of steppers
Simplicity: Stepper systems are easier to build, easier to install, and easier to maintain. Fewer moving parts and fewer connections. This equates to a much lower cost for you. Resolution: Steppers let you achieve very high resolution at very low cost, typically 8,000-16,000 steps/inch. Servos need expensive hi-res encoders to do the same thing. Steppers are also very accurate, with no errors larger than 1 step.
Robustness: Servos are sensitive to changes in mass, like putting a 1,000lb mold block on the table; and to changes in friction, like getting a little dirt on the ways. Steppers are less sensitive to these things. The tuning of a stepper system is built right into the motors themselves. When properly set up, they'll run all year without missing a beat. They're reliable, very affordable, and easy to install. 

Look at the functionality per dollar.
As you ponder this subject, remember that a high-tech Ah-ha! stepper system costs only $2,500 to $6,500. A typical servo control runs between $12,000 and $35,000. $35K? For a 2nd ops machine or a manual conversion? Hmmmm.....might be worth a call to ah-ha!

Choosing the Right Option

Based on the above information, I can make a more informed decision. Both motors can have about the same precision. The main pro of a servo being it's speed while a stepper is more robuse and has discrete steps (which can reduce oscillation). Another key factor is the fact that servos are generally 2x more expensive than a stepper and more complex (and therefore more likely to break). That being said, a Stepper motor is the best cost/benefit option for me at this point.

For my application I need three motors (one for each axis). And each motor should have as relatively high torque as it wil decrease the risk of the motor missing steps. Did some research on various sites and did find the following on eBay...

Dual Shaft Stepper Motor, 200 steps per revolution. 0.25" shaft diameter with a ground flat and can run in either unipolar or bipolar mode.
Holding torque 425 oz-in bipolar 304 oz-in in Unipolar. Detailed specifications can be found at: http://www.kelinginc.net/KL23H286-20-08B.pdf

I bought a set of three (one for each axis) on ebay for a little over $50 each, which seemed like a good price.