Re: [CAD_CAM_EDM_DRO] LARGE Gantry table: Geckos on large servos.

On Wed, 2004-03-10 at 22:45, Jon Elson wrote:

> >If the motor had 80V across it all the time, it would be moving all the
> >time.
> >
> >
> But, it reverses polarity 25,000 times a second. The net current
> averages out
> to zero.

This only happens if the motor is oscillating, which depends upon where
the gain is set, among other things. I've had a scope on the motors and
at rest there generally isn't any/much power going to them.

> >
> Yes, seems reasonable, but MOST PWM full-bridge driver schemes use
> the 50% duty cycle scheme.

Doesn't PWM mean that the Pulse Width is variable... I'm sure it does
run at 50% some times, but it generally runs where it needs to according
to the loop feedback.

> >It may also cause high currents to flow just to
> >the windings if the capacitance to ground is low, or if there are EMI
> >filters in the motor. It might be necessary to put an inductor in series
> >with the motor.
> >
> >
> >
> >I'm not following this argument.
> >
> >
> If the motor's inductance is too small, the triangle wave current
> becomes large.
> The solution is to increase the inductance with a series inductor.

V = Ldi/dt. If L is small, di/dt is large and the currents get large ?

> >
> >
> >>Finally, if the Gecko failed when stopping, the problem is due to the
> >>back EMF of the spinning motor PLUS the extra voltage created by
> >>currents in the motor inductance produces a DC supply voltage GREATER
> >>than the DC supply normally produces. In other words, during deceleration
> >>of large motors and heavy inertial loads, the bus voltage can rise to
> >>dangerous
> >>
> >>
> >
> >I've had this argument with other people. Here is my understanding:
> >
> >a) the terminal voltage of a DC <servo> motor is equal to the IR "drop"
> >of the armature, plus the KV product of the motor's speed.
> ><big snip>
> >
> >Therefore the terminal voltage during deceleration can never really be
> >greater than the supply voltage.
> >
> >
> This is true in a completely linear system. But, this is NOT a linear
> system,
> it is a switched-mode system, and inductance becomes VERY important.
>
> So, the motor can be thought of as the ideal DC motor, as you described
> above,
> PLUS an inductor in series with it! Now, if you are extracting energy from
> the spinning motor, you are applying the reverse polarity from the power
> supply across the motor for short periods, and allowing the current to
> build to a large value. Then, you reverse the polarity for a while as part
> of the PWM switching cycle, so that the motor back EMF and the power
> supply are of the same polarity. But, there's also that inductance with the
> large current in it. As the polarity reverses, the like polarity of the
> two sources (DC supply through transistors and the back EMF) would
> act to reduce the large current in the inductor. AND, when you reduce
> the current flowing in an inductor, you get a large voltage which acts
> to raise the motor terminal voltage appreciably, as it forces a large
> current back into the power supply.

Hmmmmmm..... the Gecko manual doesn't say anything about this...

>
> >Think of the decelerating motor as a generator being turned at a
> >constant speed and the deceleration being a bigger and bigger load on
> >the motor. As the load increases, the terminal voltage drops.
> >
> >
> But, if you were to short the motor for an instant, and then when the
> current
> became very large, break the circuit, you'd get a big spark! This is
> exactly
> what happens in the simple PWM servo system every PWM cycle.

The inductive voltage spike will be handled by either a flywheel diode
external to the FETs or a flywheel diode built into the FETs.

> These systems are always on, one polarity or the other.

Maybe at a very low duty cycle, but not otherwise at idle, unless the
error feedback is large.

> The PWM refers
> to the duty cycle of the waveform. There are other designs that provide
> for the source transistors to be off for part of the cycle, but the
> Gecko 320/340
> is not of that type.

> Jon
>
>
>
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--
Kim Lux <lux@...>