Ok guys (and gals?), I need your assistance. I am writing a report to go along with my design (school requirement). Although I have already picked my specific components, I need to create some data in regards to what a "Customer" would use in choosing a specific design. Therefore, I need to know what Characteristics you would consider when choosing a motor for a CNC conversion. This could be a stepper motor, servo motor, or another option that I may not be aware of.

I am not looking for specific brand/type recommendations. I am simply looking for CHARACTERISTICS!

Examples:

Cost
Reliability
Noise
Accuracy
Repeatability
Speed
Working Lifespan
Maintenance Cost
Power Consumption
etc.

The above list is what I could think of off the top of my head. If you can think of things to add to this list, please do so.

Thanks!

-Caleb105-

By customer, do you mean professional machine shop or Hobiest?
I would say that hobiest's are mainly driven by cost, hence the popular use of Stepper motors.
The vast majority of off the shelf CNC systems suppliers now use either DCBL or more often, AC sinusoidal motors.
AC sinusoidal provide synchronous control, together with BLDC that includes very quiet operation, very reliable operation, PID control, low inertia, and low power consumption.
With the above motors, position is constantly monitored from the controller.
Another advantage with closed loop controls is the ability to use Electronic Gearing and Electronic Cam features by gearing one motor off another motors encoder, instances where this is used is double gantry motors, threading and tapping operations.
Steppers however are traditionally much cheaper than their AC or DCBL counterpart.
Al.

Ok guys (and gals?), I need your assistance. I am writing a report to go along with my design (school requirement). Although I have already picked my specific components, I need to create some data in regards to what a "Customer" would use in choosing a specific design. Therefore, I need to know what Characteristics you would consider when choosing a motor for a CNC conversion. This could be a stepper motor, servo motor, or another option that I may not be aware of.

I am not looking for specific brand/type recommendations. I am simply looking for CHARACTERISTICS!

Examples:

Cost
Reliability
Noise
Accuracy
Repeatability
Speed
Working Lifespan
Maintenance Cost
Power Consumption
etc.

The above list is what I could think of off the top of my head. If you can think of things to add to this list, please do so.

Thanks!

-Caleb105-

Torque
Inductance
Current Draw

Al,
Thanks for the input.

escott,
I can't believe I forgot Torque!...Duh!
I was kinda thinking about including inductance into cost, but I think I should make it a separate item.

Anyone got anything else?

Thanks!

-Caleb105-

+ Sealing of connectors and shaft (against fluid)
+ Flexibility in customizing the output shaft and mounting face to the application.
+ Warranty

Steppers however are traditionally much cheaper than their AC or DCBL counterpart.
Al.

This may be a foolish question, but aren't Stepper motors BLDC motors?

Thanks!

-Caleb105-

Technically yes, they are brushless and stepped by a DC pulse, but the rotor control is different, BLDC are commutated by a feedback pulse on the rotor, also the torque/RPM characteristics are not the same.
The Torque curve is almost flat up to typical rpm's of 10k.
Although the BLDC has three windings the same as a AC sinusoidal, they are referred to as brushless DC because they are virtually the same as a brushed motor only turned inside out, only two windings are powered at any one time, unlike the AC which can have power to all three windings.
One other thing is BLDC require feedback and are tuned by PID loop from the controller.
Al.

Gotcha.

So, if I was to create a list of the general "Types" of motors I could choose from, they would be:

DC Stepper
AC Servo (aka AC Sinusoidal)
DC Servo (aka BLDC)

Is this correct?

Thanks!

-Caleb105-

Actually you would also have DC Brushed as well as DC brushless.
Al.

I would group them as:

Stepper
Brush Servo
Brushless Servo
Induction Motors

A stepper drive applies DC voltage to alternating phases in the stator of the motor, stepping it along. If you apply DC voltage to any 1 winding, you'll either move one step or apply holding torque and position will stay.

A brush servo is applied DC voltage, and its speed is adjusted through PWM modulation which effectively raises or lowers the voltage. If you apply DC voltage, the motor will spin.

A brushless servo also has DC voltage applied, but is applied sinusoidally between phases based on the rotor's position relative to the stator. If you apply DC voltage between any two windings, the motor will move to a pole and stay there.

An AC Motor, is also brushless, but its speed is based on the frequency of the AC voltage supplied by the VFD, and all three phases are applied. If you apply 3 phase AC voltage to an AC motor, it will spin based on the frequency of the voltage.

Hope this makes sense,

Matt

A brushless DC (BLDC) generally does not have a sinusoidal signal it has two out of its three windings energised at one time, although it resembles an AC sinusoidal motor in construction.
An AC sinusoidal can actually be run as a BLDC.
http://users.tinyworld.co.uk/flecc/4-pole-bldc-motor031102.swf
If you use the step mode, you can observe the sequence.
Also a Brushed motor can be run on SCR drives rather than PWM as this was the earlier form of control before PWM became popular.
KB and Baldor still make SCR controllers.
An AC servo motor is classed as a synchronous motor, An AC induction motor although dependent on frequency, is not synchronous.
Al.

To confuse things a bit more steppers can be run closed loop with an encoder as a high pole count bldc moter, with excellent low speed torque characteristics and none of the resonance problems that steppers normally have.
Amplexus

I'm trying to follow all this. ;-)

I'm trying to make a list of EVERY possible option, so that I can explore the positives and negatives of all of them. From what you all have said, my options are then:

Stepper (open loop)
Stepper (closed loop w/ encoder)
Brush DC Servo
Brushless DC Servo
AC Servo (sinusoidal)
Induction Motor

Is this correct?

Thanks!

-Caleb105-

The inclusion of Induction motor would generally be for Spindle usage, they would not be selected for either open or closed loop servo use.
Al.

The inclusion of Induction motor would generally be for Spindle usage, they would not be selected for either open or closed loop servo use.
Al.

Thanks Al!

So, it should look like this:

Stepper (open loop)
Stepper (closed loop w/ encoder)
Brush DC Servo
Brushless DC Servo
AC Servo (sinusoidal)

Last call for any changes!?

-Caleb105-

You may ce focusing too much on motor types and not enough on drives. the electronics make a huge difference in performance especially with steppers, also tuning is very important with servos as is the chouce of encoder.
Amplexus

Amplexus,

Thanks for the input. Yes, I am focusing only on general motor types right now. Once I have those nailed down, the next step will be the appropriate drivers/encoders for each category.

Thanks!

-Caleb105-

how about linear motor on that list?