Re: [CAD_CAM_EDM_DRO] Stepper or servo?

mariss92705 wrote:

> Hi,
>
> By special request and with apologies to David Letterman, here's
> some "Top 10" lists I came up with for choosing between steppers and
> brush DC servos.

Great work, I'm sure a lot of beople will benefit from this. I think we should
put it in the CAD_CAM FAQ pages, if that is allright with you.

> Top Ten DC Servo Disadvantages:
>
> 1) Requires "tuning" to stabilize feedback loop.
> 2) Motor "runs away" when something breaks. Safety circuits required.
> 3) Complex. Requires encoder.
> 4) Brush wearout limits life to 2,000 hrs. Service is then required.

2000 operating (rotating at full speed) life on a home shop CNC will last
MANY years.

> 5) Peak torque is limited to a 1% duty cycle.
> 6) Motor can be damaged by sustained overload.

However, a proper CNC control should detect this and e-stop.

> 7) Bewildering choice of motors, encoders, servodrives.
> 8) Power supply current 10 times average to use peak torque. See (5).

Well, peak accelerating torque will usually be applied near zero speed, requiring
current, but very little voltage. So, the PWM scheme will prevent the DC power
supply from having to supply the actual motor current. Peak decelerating torque
should be largely developed by the motor's own EMF, again not requiring the
power supply to deliver the full motor current. (This may require series inductors
between motor and servo amp.)

> 9) Motor develops peak power at higher speeds. Gearing often required.

To use the smallest motors, reduction would be needed. Many systems use
oversize motors directly driving the leadscrew to avoid belt drives. For home
systems, I think there is nothing wrong with belt drive, and there are mechanical
advantages (pun not intended) to using a belt in many cases, such as making the
drive assembly more compact.

> 10) Poor motor cooling. Ventilated motors are easily contaminated.

Well, a motor that dissipates heat only when being demanded to deliver power
seems better than a motor that runs hot while standing still! Obviously, a stepper
motor has a similar problem. Yes, DC brush motors can overheat the armature
when required to deliver torque at standstill for a long time, but a properly
configured system should not suffer from that.

Jon