Re: SLA7062 Sync Mode (was New Article bla bla)

Hi,
Typically the 7062 runs each of the 2 phases asynchronously to one
another; each phase is "doing its own thing". The combinations of
various PWM frequencies generates noise in the audible range, as each
phase is turning ON at different times. This noise (often referred
to as squeeling, grunting, hissing, spitting - and perhaps other
uncouth sounds) is a combination of the two high pitched PWM
frequencies to make a lower pitch sound.

There is one place this will not happen: when only one phase is ON
(or alternatively when one phase is OFF). In this case (when one
phase is OFF), there is only one PWM frequency (the one ON), and it
is generally above the audible range. At this point having the PWM
be "synchronous" doesn't matter since there is only one ON anyway.

At all other step angles the frequencies can combine to make
noise. There is however one place that you can reduce noise (other
than 0-100), and that is at 70.7. In this position, both phases are
trying to deliver the same amount of power, thus should be just fine
with the *same* PWM ON/OFF times. In this case, rather than letting
the circuit run the PWM so they are "doing their own thing", the
start of the PWM signal will be synchronized for both. They both
turn the switches ON at the same time (and should therfore be turning
them OFF at close to the same time as well). The 7062 PWM circuit is
a fixed-off time circuit, so after a "fixed-time" the circuit will
try to turn the switch ON again. In synchronous mode, this is only
allowed to happen when BOTH are ready to be turned back on. At 70.7
the time when both are ready to turn back ON will be very close to
one another, so minimal (if any) power will be lost.

The problem for steps other than 100/0 or 70.7/70.7 is that if both
are started synchronously (at the same time), one will reach its turn
OFF level sooner (less current to build up). The other phase which
has to build up more current will remain ON longer. Now, the one
that turned OFF sooner would generally wait its fixed-off time, and
turn back ON, but if it now has to wait until the other phase is
ready to turn ON (has reached *its* peek current and elapsed its off-
time), the first phase continues to loose power through decay as it
waits to turn back ON synchronously. This loss of power leads to mis-
position since it is no longer giving the correct amount of current
for that particular step angle.

Thus, too much decay while waiting for the other phase to be ready to
turn ON synchronously --> loss of current --> mis-position.

Jeff



--- In CAD_CAM_EDM_DRO@yahoogroups.com, John Dammeyer <johnd@...>
wrote:

>
> Hi,
>
> I'm also a bit confused by the SLA7062 data sheet. All they say is

that

> noise is reduced when the unit enters sync mode. It doesn't really

say what

> sync mode does.
>
> The way I reduce current in the ELS driver which uses LMD18245 is by
> shifting the reference voltage from 5 volts to something like

1.25V. This

> causes the motor current sensing circuits to start chopping at

0.75A instead

> of 3A (when configured for full drive current).
>
> The side effect of this is that the motor now becomes quite a bit

noisier

> since the chopping happens differently from the full power mode.

The

> LMD18245 is also a 'fixed off time' chopper.
>
> The only other way to reduce current would be to change the current

through

> the individual windings to perhaps reduce one winding to 0 current

and the

> other to some low level that causes less noise. However, when I

tried that,

> (before using the 'reducing the reference voltage' approach), I

found the

> motor always moved so I gave that up.
>
> John Dammeyer
>
>
> > Keeping track of steps
> > is a logic
> > function, and is (or at least should be) unaffected by the
> > PWM operation.
> >
>