Re: [CAD_CAM_EDM_DRO] How do you compare steppers to servos

Glenn wrote:

> Suppose a system is advertised using nema23 steppers of some size, say 260 oz-in.
>
> You want to build the same system but instead use servos.
>
> How do you compare the compare power of the the steppers to the servos
>
>

Ohhhh! This will start another culture war!

Well, steppers are rated by HOLDING torque, ie. the torque they can
resist at standstill. The running torque, ie. the torque they can
deliver at some speed, is less. Above 100 RPM or so (varies by motor
and driver) the torque drops off as speed increases. But, you can try
to get a torque vs. speed graph for the specific motor and driver
combination, at some specific DC supply voltage. Then, figure out what
motor speed will be required at the fastest cutting feedrate, and see
what torque the motor will deliver at that speed. Then, figure back
through the drive belts and leadscrew to figure the delivered linear
force on the table.

Servos have a generally flat speed vs. torque rating, and generally can
run MUCH faster than steppers, so in many cases a belt reduction is used
to allow the motors to spin at a more optimum speed. (Power out is
speed X torque).
Where it gets a LOT more complicated is that servo motors have a
continuous and a peak torque rating. They can deliver the continuous
torque all day, and the peak torque maybe for a few seconds out of every
minute, but the peak rating may be 4 X higher than the continuous. But,
most lower-cost systems don't have an RMS integrator to protect the
motor against using the peak rating too much of the time. If you do,
the motor just burns up.

Brushless servo motors have a big advantage over brush type, as the
coils are on the stator and have a good heat conducting path to the
outside. Brush motors have the coils on the rotor, and removing the
heat is a problem.
But, brushless drives are more complicated, hence cost more.

The bottom line is if the continuous rating of the servo motors equal
the holding torque rating of the stepper, you can't possibly go wrong.
But, this will result in larger motors than you really need. In most
cases, if the peak rating of the servo is equal to the stepper's holding
torque, you should be OK, but this is a VERY rough approximation.

If the stepper is run at a very high speed, such as driving a 20 TPI
leadscrew on a Taig or Sherline, then it actually develops very little
torque under those conditions. (60 IPM on a Sherline, for instance,
requires 1200 RPM. Drivers with a low supply voltage won't have much
torque left at that speed, if they can even get there.) With a servo,
you might use 2:1 or even 4:1 belt drive to run a servo at a better
speed, and get even more torque.)

Jon