Re: [CAD_CAM_EDM_DRO] Just an idea

Mariss Freimanis wrote:

> Has anyone used a battery in conjuntion with a servomotor power
> supply?
>
> Reason for the question:
>
> Brush DC servomotors have peak torque values that are up to 10 times
> their rated continuous torque, thus peak current draw can be up to 10
> times the continuous draw. This peak current event can last from 10
> millisec to 1 second and the duty cycle must be under 10% to stay
> within the motor's dissipation rating.
>
> Unfortunately the power supply has to be sized to this peak power,
> when on average a supply 1/10 the size would do.
>
> My idea would be to use a small NiCad or NiH battery connected to the
> power supply output bus via a diode. During a high current pulse the
> power supply voltage would sag, the diode would forward bias and pick
> up the load for the duration. A quick-charge IC would handle the
> charge restoration to the NiCad between the pulses.

The danger here is that a blown transistor in the servo amp could
cause some real fireworks, if current isn't limited.

> Even little AA .5 AH NiCad cells would do; a 25 Amp, 1 second load
> would discharge only .007 AH (25Amp / 3600 sec). The internal
> resistance of a Nicad cell is on the order of .02 ohms, so they are
> quite capable of supplying this level of current.

If you think you can get 25 Amps from an AA cell, you'd better talk to
the
manufacturers of these batteries. Yes, in fault conditions, you can
definitely
get those sorts of currents, but it is destructive to the battery. A 75
Volt
system would need 60 cells, so destroying a 60 cell battery every few
months would not be very cost effective.

A few hundred thousand microFarads (or something like .2 F) seems
quite satisfactory for Bridgeport-size machines. Yes, some commercial
servo systems that guarantee .01" max following error at 1000 IPM need
more power, but I find no need for such a system. The safety aspects of

making errors at the console with a 1000 IPM-capable machine gives
me the willies! I have had a number of crashes, and once hit the wrong
button (spindle DOWN instead of spindle up) and brought a boring head
down on my hand. If the machine had any more power, or the feed rate
had
been set any higher, I would have had a broken hand, at the very least!

I am TOTALLY satisfied with the performance of my servo system, and
think any higher performance would just increase the danger when
something
goes wrong (usually operator error).

Jon