On 2010-07-09, Ned Simmons wrote:
On Fri, 09 Jul 2010 11:46:37 -0500, Ignoramus30064
wrote:

On 2010-07-09, Pete C. wrote:

Ignoramus30064 wrote:

On 2010-07-09, Karl Townsend wrote:

"Ignoramus30064" wrote in message
...
On 2010-07-09, Karl Townsend wrote:

I could never get it to run at super slow speed in voltage mode, but
could easily do so in tach mode. It was kind of nice to see.

Disclaimer:My control uses a Galil dedicted real time controller so your
results may differ.

Torque mode is the preferred mode for most all applications, certainly
for
what you're doing. Precise smooth control is not an issue at all with
lathe
and mill applications.

Karl, could you explain to me, why do you think so? Wouldn't the
milling table move at very high rates of speed if it encountered no
resistance, in torque mode?

i

http://www.a-m-c.com/content/support/FAQ.html#setup03

read question 3



Karl, that does not really answer my question (about possible high
speed).

i

Axis speed is controlled by the CNC control giving the motion commands,
not by the servo drive. The CNC control doesn't say "move to position X"
and the drive goes full tilt to that position. The CNC control manages
the movement to move at the specified feed or rapid rate, i.e. G01 X10
F25 moves from the current X position to X10 at a 25 inch per minute
rate.

But how does the control tell the drive what speed to use, if the
drive is in torque mode?

I do not quite understand this.

i

When the amp is set to velocity mode, the setting when you're using a
tach, it acts as a voltage to voltage amplifier. In torque mode, which
as Karl said is what the vast majority of motion control apps use, the
amp's output *current* is proportional to the command voltage. If the
controller sees, via the encoder feedback, that the motor is moving
too slowly it increases the command signal, the amp increases current,
motor torque goes up, and speed increases. Repeat until the controller
is satisfied.

I've designed and built 15 or 20 different systems with perhaps 50
servo axes over the last 20 years, and I can think of only one
instance that used tach feedback, and even that was synthesized from
an encoder signal. And in that case there was no traditional
controller.

Re your earlier remark that tach feedback decreases the load on the
controller -- every modern digital motion controller I've worked with
is constantly updating the servo loop unless you specifically tell it
not to. The typical rate is in the range of 10 KHz.


OK. So, I think that I am making a mistake by using tachometer and
instead, I should go to torque mode. It will actually be easier to
use torque mode, anyway. If everyone does it, so will I. It is also
safer and I can remove a whole bunch of cables from the control that I
do not need. Seems to be a win-win all around.

i