I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

i

"Ignoramus8975" wrote in message
...
I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

i

you're looking at the motor brushes.

You don't use a tach with EMC. Encoders go to control. Conrol sends -10v to
+10v to amps set in torque mode. Only connections on your amps are DC+ and
DC- in, Power+ and power- to servo, your analog reference signal and analog
ground. Also strongly recommend servo fault output and inhibit input if your
control will do these.

Karl

On 2010-06-08, Karl Townsend wrote:

"Ignoramus8975" wrote in message
...
I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

you're looking at the motor brushes.

OK... They look kind of small...

You don't use a tach with EMC. Encoders go to control. Conrol sends
-10v to +10v to amps set in torque mode. Only connections on your
amps are DC+ and DC- in

That's "power in", got it

, Power+ and power- to servo, your analog

That's "power out", got it

reference signal and analog ground

Is that the +/- 10v control signal?

Also strongly recommend servo
fault output and inhibit input if your control will do these.

OK, I am not there yet to understand this properly. I did not know
that servos had a fault output.

By inhibit input, do you mean things like limit switches, estop etc?

So, the amplifier's tach input simply remains completely unconnected,
right?

i

"Ignoramus8975" wrote in message
...
On 2010-06-08, Karl Townsend wrote:

"Ignoramus8975" wrote in message
...
I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

you're looking at the motor brushes.

OK... They look kind of small...

You don't use a tach with EMC. Encoders go to control. Conrol sends
-10v to +10v to amps set in torque mode. Only connections on your
amps are DC+ and DC- in

That's "power in", got it

, Power+ and power- to servo, your analog

That's "power out", got it

reference signal and analog ground

Is that the +/- 10v control signal?

yep


Also strongly recommend servo
fault output and inhibit input if your control will do these.

OK, I am not there yet to understand this properly. I did not know
that servos had a fault output.

Yes your servo will fault on a number of out-of-bounds conditions. If the
control doesn't know, it keeps trying to move.

By inhibit input, do you mean things like limit switches, estop etc?

No, the control sees the limits. I use the inhibit for machine stop, tool
changes, etc. any time the operator is in the machine like with a door open,
on and on. Some use it for E-stop. My training as a corporate engineer says
this is a no-no. Estop physically kills all power to any device that moves
in my equipment. Estop should kill a broken or misprogrammed machine. I had
to have idiot proof equipment and they had some really ingenious idiots
where I worked.

So, the amplifier's tach input simply remains completely unconnected,
right?

Yep, the tach isn't used at all.

Iggy,
All tachometers are unregulated voltage dc generators that provide a linear output where the output voltage rises as a function of
speed. A simple test can identify this functionality. They are typically used in analog systems and are used to back off the
output gain of an op amp, based on velocity or to apply dynamic braking. The cost of these devices directly relate to their
linearity. They have been replaced by digital electronics because of cost. Very cool technology.
Steve

"Ignoramus8975" wrote in message ...
I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

i

On 2010-06-08, Ignoramus8975 wrote:
I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

It looks it to me.

Some motors have a smaller diameter tach on the free end.

Others simply have a second set of brushes at right angles to
the ones which drive the motor. It looks as though you have a lot of
life left in those brushes.

Hmm ... that red dot at about 4:00 on the brush mounting plate
looks as though it may be close to a similar mark on the inside of the
case (not visible form this angle) to assure reinstallation in the
proper orientation, given those slots for the mounting screws.

The real test is to drive the motor by another source, and look
for DC voltage on the extra pins. (Look at the connector for heavier
and lighter gauge wires if there is only the one connector, exclusive of
the encoder.

And the tach feedback pair may be shielded, unlike the power
*to* the motor.

If these are SEM motors, then they almost certainly have tach
outputs. IIRC, mine had a label inside the cover of the wiring box
which the connector was mounted to. There were more pins than were
needed.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On 2010-06-08, Karl Townsend wrote:

"Ignoramus8975" wrote in message
...
I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

[ ... ]

you're looking at the motor brushes.

You don't use a tach with EMC.

That depends on the servo amplifiers. If you are using the
Gecko drivers for DC servos, then yes, the tach is not used. But the
Gecko is made to make the servo act like a stepper motor.

However -- other servo amps may well accept tach feedback.

Encoders go to control. Conrol sends -10v to
+10v to amps set in torque mode. Only connections on your amps are DC+ and
DC- in, Power+ and power- to servo, your analog reference signal and analog
ground. Also strongly recommend servo fault output and inhibit input if your
control will do these.

Which servo amps are you describing? It is not true of *all*
possible implementations of EMC. Many DC servo amps accept tach
feedback and automatically adjust the power to the servo motor so the
speed indicated by the tach feedback matches the speed commanded by the
control. I have similar SEM servo motors which do provide tach
feedback, and the AeroTech 4020 (+/-40V +/-20A) amplifiers which I have
utilize that tach feedback. You can command speeds which are slower
than the second hand on a clock with that -- even without the encoder
feedback. Just a servo amp, and a power supply which can output little
as a stable 0.001V or less.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On 2010-06-08, Ignoramus8975 wrote:
On 2010-06-08, Karl Townsend wrote:

"Ignoramus8975" wrote in message
...
I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

you're looking at the motor brushes.

OK... They look kind of small...

They are rather small.

You don't use a tach with EMC. Encoders go to control. Conrol sends
-10v to +10v to amps set in torque mode. Only connections on your
amps are DC+ and DC- in

That's "power in", got it

, Power+ and power- to servo, your analog

That's "power out", got it

reference signal and analog ground

Is that the +/- 10v control signal?

Plus there *may* be provisions for using the tach feedback. It
will be a function of the servo amp in use -- not EMC itself.

Also strongly recommend servo
fault output and inhibit input if your control will do these.

OK, I am not there yet to understand this properly. I did not know
that servos had a fault output.

By inhibit input, do you mean things like limit switches, estop etc?

Yes -- to totally shut down power output to the motors from the
servo amp -- so while you are fixing whatever went wrong, you don't
suddenly get a hand trapped between a (non-rotating_ milling cutter and
the workpiece or something else rigidly mounted.

So, the amplifier's tach input simply remains completely unconnected,
right?

Look up the data for your servo amps. Don't take the "EMC does
not use tach feedback" as true. It is not universally true. It is true
when using the Gecko drives, which are emulating a stepper motor, but
not so with many servo amps.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

Which servo amps are you describing? It is not true of *all*
possible implementations of EMC. Many DC servo amps accept tach
feedback and automatically adjust the power to the servo motor so the
speed indicated by the tach feedback matches the speed commanded by the

Don't make life hard for iggy. It's his first one. Or that is, he's a virgin
or at least he used to be.VBG

Iggy has some brush type analog servo amps made by AMC. They work extremely
well with only +/- 10 volt from the control. Simple to install and reliable.

Karl

On 2010-06-08, Karl Townsend wrote:

Which servo amps are you describing? It is not true of *all*
possible implementations of EMC. Many DC servo amps accept tach
feedback and automatically adjust the power to the servo motor so the
speed indicated by the tach feedback matches the speed commanded by the

Don't make life hard for iggy. It's his first one. Or that is, he's a virgin
or at least he used to be.VBG

Iggy has some brush type analog servo amps made by AMC. They work extremely
well with only +/- 10 volt from the control. Simple to install and reliable.

I am reading the drive datasheet right now.

The drive does accept the tachometer signal.

Also, I do think that the object that I showed on the picture, is
actually a tachometer. The wires are too small for carrying motor
power.

I received Jon's PPMC today and I will try to hook it up to the PC
(but not to the mill yet).

i

On Tue, 08 Jun 2010 20:50:12 -0500, Ignoramus8975
wrote:

On 2010-06-08, Karl Townsend wrote:

Which servo amps are you describing? It is not true of *all*
possible implementations of EMC. Many DC servo amps accept tach
feedback and automatically adjust the power to the servo motor so the
speed indicated by the tach feedback matches the speed commanded by the

Don't make life hard for iggy. It's his first one. Or that is, he's a virgin
or at least he used to be.VBG

Iggy has some brush type analog servo amps made by AMC. They work extremely
well with only +/- 10 volt from the control. Simple to install and reliable.

I am reading the drive datasheet right now.

The drive does accept the tachometer signal.

Also, I do think that the object that I showed on the picture, is
actually a tachometer. The wires are too small for carrying motor
power.

I received Jon's PPMC today and I will try to hook it up to the PC
(but not to the mill yet).

i

One should note that many servos/encoders have a number of different
outputs. Some you may need, some you dont need.

Counts AB/BA/Z
Home position
Secondary Home
Velocity
Tach (different than velocity)

And others

Here is a typical wiring diagram of the servos that I use on OmniTurn
CNC lathes
http://www.glentek.com/glentek/dcconnt.aspx



btw..have you seen this?

http://www.homecnc.info/servo%20box.pdf

Gunner

One could not be a successful Leftwinger without realizing that,
in contrast to the popular conception supported by newspapers
and mothers of Leftwingers, a goodly number of Leftwingers are
not only narrow-minded and dull, but also just stupid.
Gunner Asch

I am reading the drive datasheet right now.

The drive does accept the tachometer signal.

Also, I do think that the object that I showed on the picture, is
actually a tachometer. The wires are too small for carrying motor
power.

I received Jon's PPMC today and I will try to hook it up to the PC
(but not to the mill yet).

You won't need a tach input on the drive.

A servo is just a dc motor. Put the leads on your battery charger. It
should turn slowly.

Hope you're having fun and learning.

Karl

Ignoramus8975 wrote:
I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9
Yes, the extreme thinness of the commutator indicates it must be a tach.
I haven't run into many 4-pole tachs, but if it is smaller than the
main motor, it has to be the tach.

Jon

Karl Townsend wrote:
"Ignoramus8975" wrote in message
...
I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

i

you're looking at the motor brushes.

You don't use a tach with EMC. Encoders go to control. Conrol sends -10v to
+10v to amps set in torque mode. Only connections on your amps are DC+ and
DC- in, Power+ and power- to servo, your analog reference signal and analog
ground. Also strongly recommend servo fault output and inhibit input if your
control will do these.
If you are using a velocity servo amp, then you WILL want to use the
tach with your servo amps. If you use the tachs with a servo amp in
velocity mode, you will get much better response. EMC will be perfectly
happy with this arrangement. I am using velocity servo amps with tachs
on my Bridgeport, with EMC.

Jon

Ignoramus8975 wrote:

I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

i


I don't think so. If it was a tach, I believe you would only see two brushes. That is
based on the limited number of servo motors I've had a part. I could be wrong.

Wes
--
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller

On 2010-06-09, Wes wrote:
Ignoramus8975 wrote:

I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

i


I don't think so. If it was a tach, I believe you would only see two brushes. That is
based on the limited number of servo motors I've had a part. I could be wrong.

I will look a little closer. I thought that the wires were too think
to carry the main motor current. I will try to find out for sure
tonight.

i

On 2010-06-08, Karl Townsend wrote:

Which servo amps are you describing? It is not true of *all*
possible implementations of EMC. Many DC servo amps accept tach
feedback and automatically adjust the power to the servo motor so the
speed indicated by the tach feedback matches the speed commanded by the

Don't make life hard for iggy. It's his first one. Or that is, he's a virgin
or at least he used to be.VBG

I'm not trying to make things hard for him -- I'm trying to make
sure that ne knows the possible options for *his* motors and amps. He
will learn quickly enough -- he is good at that.

Iggy has some brush type analog servo amps made by AMC. They work extremely
well with only +/- 10 volt from the control. Simple to install and reliable.

Again -- which amps? I note that at least some of the AMC amps
use tach feedback.

Get the model number, and check against the data he

http://www.a-m-c.com/searchtool/

Of course -- it could be using intervals between pulses of power to
check generated voltage using the same brushes as drive and tach. But
this means that there must be dead periods during which there is no
drive. More common with PWM than with a DC servo amp in my experience.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

I inspected the motor more closely. There is another set of larger
brushes that clearly are for actually running the motor. These tiny
brushes are likely to be a tachometer.

i

On 2010-06-09, Gunner Asch wrote:
On Tue, 08 Jun 2010 20:50:12 -0500, Ignoramus8975
wrote:

On 2010-06-08, Karl Townsend wrote:

[ ... ]

Iggy has some brush type analog servo amps made by AMC. They work extremely
well with only +/- 10 volt from the control. Simple to install and reliable.

I am reading the drive datasheet right now.

The drive does accept the tachometer signal.

Also, I do think that the object that I showed on the picture, is
actually a tachometer. The wires are too small for carrying motor
power.

I received Jon's PPMC today and I will try to hook it up to the PC
(but not to the mill yet).

[ ... ]

One should note that many servos/encoders have a number of different
outputs. Some you may need, some you dont need.

Indeed.

Counts AB/BA/Z
Home position
Secondary Home
Velocity
Tach (different than velocity)

And others

Here is a typical wiring diagram of the servos that I use on OmniTurn
CNC lathes
http://www.glentek.com/glentek/dcconnt.aspx

Useful information.

btw..have you seen this?

http://www.homecnc.info/servo%20box.pdf

Hmm ... that looks like it is using the Gecko servo drivers --
the ones which make the servos look/act like steppers. Note the
presence of the step and direction inputs -- no tach input, but encoder
inputs. You feed it a step signal, and it goes full speed (or perhaps a
speed which is a function of how many steps behind it happens to be)
until the internal step count matches the number of step pulses
received. Which means that you lose the ability to have a very slow
speed on one axis without steps -- so a lathe would turn out a stepped
Morse taper instead of the proper smooth one. How bad that would be is
a function of the resolution of the encoder, of course. I know that it
is terrible with my Compact-5/CNC lathe, which has a minimum step (set
by the computer) of either 0.001" or 0.01 mm *radius*, not diameter.
This turns out a very visible step in diameter when attempting to make a
Morse #2 taper.

So -- this is throwing away one of the benefits of servo motors
for the convenience of interfacing to a version of EMC set up for
steppers, instead of one with an interface board for producing velocity
commands to servo amplifiers. The interface board should be able to put
out an analog voltage between +10V and -10V to command the desired speed
from the amp/motor combination

I also see the possibility of resonance with the step rate and
the mass of the machinery being controlled. With a true servo amp. you
can add damping to control this.

Also -- with only 36V, you are throwing away quite a bit of the
speed that the servo motors are capable of.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On 2010-06-09, Karl Townsend wrote:


I am reading the drive datasheet right now.

The drive does accept the tachometer signal.

Also, I do think that the object that I showed on the picture, is
actually a tachometer. The wires are too small for carrying motor
power.

I received Jon's PPMC today and I will try to hook it up to the PC
(but not to the mill yet).

You won't need a tach input on the drive.

A servo is just a dc motor. Put the leads on your battery charger. It
should turn slowly.

Ignoring AC servo motors, of course.

A DC servo is A DC motor with a special design low-inertia rotor
(usually the windings in air, bound with epoxy or something similar)
with all the iron stationary to allow maximum acceleration.

Many of them have current limits which if exceeded will at least
partially demagnetize the permanent magnets making the field in the
motors. (Only the newest ones with rare-earth magnets, or those with a
DC field winding.)

The benefit of the tach feedback is the ability to run at a very
stable and very slow speed -- so you can move one axis very slowly while
the other moves at a proper cutting speed to generate a very shallow
angle ramp.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On 2010-06-10, Ignoramus967 wrote:
I inspected the motor more closely. There is another set of larger
brushes that clearly are for actually running the motor. These tiny
brushes are likely to be a tachometer.

Great! And you have a servo amp which accepts the tachometer
wires.

Note that the easiest thing to get wrong is to wire the tach (or
the motor) wires backwards. If that is done, it will shoot to full
speed on one direction or the other no matter what the command voltage
is.

Just test this with no mechanical connection to the machine to
make sure it is right before you let the amp and motor run the machine.

If you have a lab type power supply which can put out any
voltage between 0V and 10V, you can run at any speed within the motor's
range. If the amp is not the one which came with the motor and machine,
you may need to adjust the feedback gain so your 10V is truly the
maximum RPM you want. The motor's label should tell you both the
maximum speed *it* can take happily, and the volts/1000 RPM from the
tach, so you can use a DC meter on the tach leads at the amp to see what
speed you are really getting.

Anyway -- the lab supply will give you all possible speeds in
*one* direction, and you will have to swap ground and + to the servo amp
inputs to get it to run in the other direction.

Or -- you could set up a pot between +10V and -10V with either a
pair of resistors defining a 0 voltage reference and use the pot to get
all speeds within the range of the motors.

Of course, again while playing with this, you want the toothed
belts off the motors so you don't drive anything hard against a stop.

Set up the travel limit switches to shut down the amp if it hits
one of them, and make sure that always works before you let the motors
actually move the machine. (This applies, of course, whether you have
tach feedback or not. :-)

I would suggest using relays on the limit switches, so each one
will stop the servo motor moving in that direction, but allow moving in
the other direction, but other contacts will also hit the E-stop on the
controller as well, since if you hit a limit switch you want to stop all
other motion as well. For convenience, given how buried some of the
belts are, you might want to provide a pair of low voltages (perhaps +/-
0.5V or so) to feed to the servo amps to back off a limit switch while
the computer is shut down. The Anilam conversion which I used at work
had handwheels, but your (and my) Bridgeports do not. If you can find
them, set up military style pull-to-unlock toggle switches (the kind
which arm weapons on aircraft) for connecting the slow motion voltages
into the servo amps. This way, someone has to *think* before they
interrupt the control signals to the servo amps.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

Ignoramus967 wrote:

On 2010-06-09, Wes wrote:
Ignoramus8975 wrote:

I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

i


I don't think so. If it was a tach, I believe you would only see two brushes. That is
based on the limited number of servo motors I've had a part. I could be wrong.

I will look a little closer. I thought that the wires were too think
to carry the main motor current. I will try to find out for sure
tonight.

i

Did you resolve it?

Wes
--
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller

On 2010-06-11, Wes wrote:
Ignoramus967 wrote:

On 2010-06-09, Wes wrote:
Ignoramus8975 wrote:

I am reading about AMC drives and how they are supposed to be
connected to tachometers for feedback.

And my question is, is this thing with brushes and wires a tachometer?

http://tinyurl.com/25zwke9

i


I don't think so. If it was a tach, I believe you would only see two brushes. That is
based on the limited number of servo motors I've had a part. I could be wrong.

I will look a little closer. I thought that the wires were too think
to carry the main motor current. I will try to find out for sure
tonight.

i

Did you resolve it?


Wes, yes, I am 100% certain that it is a tachometer. 9.5 volts per
1000 RPM.

On 10 Jun 2010 02:01:06 GMT, "DoN. Nichols"
wrote:

On 2010-06-09, Gunner Asch wrote:
On Tue, 08 Jun 2010 20:50:12 -0500, Ignoramus8975
wrote:

On 2010-06-08, Karl Townsend wrote:

[ ... ]

Iggy has some brush type analog servo amps made by AMC. They work extremely
well with only +/- 10 volt from the control. Simple to install and reliable.

I am reading the drive datasheet right now.

The drive does accept the tachometer signal.

Also, I do think that the object that I showed on the picture, is
actually a tachometer. The wires are too small for carrying motor
power.

I received Jon's PPMC today and I will try to hook it up to the PC
(but not to the mill yet).

[ ... ]

One should note that many servos/encoders have a number of different
outputs. Some you may need, some you dont need.

Indeed.

Counts AB/BA/Z
Home position
Secondary Home
Velocity
Tach (different than velocity)

And others

Here is a typical wiring diagram of the servos that I use on OmniTurn
CNC lathes
http://www.glentek.com/glentek/dcconnt.aspx

Useful information.

btw..have you seen this?

http://www.homecnc.info/servo%20box.pdf

Hmm ... that looks like it is using the Gecko servo drivers --
the ones which make the servos look/act like steppers. Note the
presence of the step and direction inputs -- no tach input, but encoder
inputs. You feed it a step signal, and it goes full speed (or perhaps a
speed which is a function of how many steps behind it happens to be)
until the internal step count matches the number of step pulses
received. Which means that you lose the ability to have a very slow
speed on one axis without steps -- so a lathe would turn out a stepped
Morse taper instead of the proper smooth one. How bad that would be is
a function of the resolution of the encoder, of course. I know that it
is terrible with my Compact-5/CNC lathe, which has a minimum step (set
by the computer) of either 0.001" or 0.01 mm *radius*, not diameter.
This turns out a very visible step in diameter when attempting to make a
Morse #2 taper.

So -- this is throwing away one of the benefits of servo motors
for the convenience of interfacing to a version of EMC set up for
steppers, instead of one with an interface board for producing velocity
commands to servo amplifiers. The interface board should be able to put
out an analog voltage between +10V and -10V to command the desired speed
from the amp/motor combination

I also see the possibility of resonance with the step rate and
the mass of the machinery being controlled. With a true servo amp. you
can add damping to control this.

Also -- with only 36V, you are throwing away quite a bit of the
speed that the servo motors are capable of.

Ayup. However...and one should keep this FIRMLy in mind..that why the
servo may freaking scream at 90 volts...not all readers/controls can
keep up with the CPS of both the encoders and the voltage/current
changes as you start up, run and slow down and stop. Sometimes that can
be a REAL issue.

Lets say you program a z move of .100...at 600 IPM. It has to haul ass
for 1/10 of an inch and then come to a dead stop EXACTLY on the mark.

Big BIG surge on the power supply, a hell of a counting job for the
control and getting everything organized, run and completed properly all
in that 1/10th of an inch can get really ****ing confusing to the
control..and is rough on the power supplies etc etc.

This is the reason many "fast" controls on machine tools only make
straight line moves at high speeds. The OmniTurn maxes at 300 IPM...but
if you give it a call of x5.3981z1.4263...it moves diagnally to that
point..at the feed rate you picked..G94F275 (inches per minute at 275
IMP)

Which is one of the reasons I encourage most end users to make delicate
moves to be in G95 (inches per revolution)...it can be just as fast, but
it depends on the spindle turning and you can slow down the spindle with
the manual override..and slow down the moves so you can not only see
whats going on..but catch "issues" before they cost you an $85 inserted
tool. One guy thought he knew more than I did..and took out (6) six
diamond coated inserts and 3 holders (PCD) in one fast move because he
had not only forgotten a decimal point..but put z and x moves on
different lines..meaning they moved z first..THEN x, rather than
diagnally.

Shrug...he was a bit green when he realized he was letting ego get in
the way of learning. And it cost him big time.

Gunner


Enjoy,
DoN.

One could not be a successful Leftwinger without realizing that,
in contrast to the popular conception supported by newspapers
and mothers of Leftwingers, a goodly number of Leftwingers are
not only narrow-minded and dull, but also just stupid.
Gunner Asch

Ignoramus10537 wrote:

I don't think so. If it was a tach, I believe you would only see two brushes. That is
based on the limited number of servo motors I've had a part. I could be wrong.

I will look a little closer. I thought that the wires were too think
to carry the main motor current. I will try to find out for sure
tonight.

i

Did you resolve it?


Wes, yes, I am 100% certain that it is a tachometer. 9.5 volts per
1000 RPM.

Interesting. The 4 brushes seem like overkill for a tach. I wonder why a two brush
design wasn't good enough. EE's, got an explanation?

Wes

Wes wrote:


Interesting. The 4 brushes seem like overkill for a tach. I wonder why a two brush
design wasn't good enough. EE's, got an explanation?

Yup, the big problem with DC tachs is keeping the brushes in good
contact with the commutator. My guess is they use an odd number of
slots/windings/commutator segments. With only two brushes, you'd still
have one brush jumping across commutator segments every small amount of
rotation. With 4 brushes, paired with their opposite brush as you do
with a 4-pole DC motor, then there is always one of the pair making
contact with the commutator. So, this should greatly reduce brush noise
and produce a much cleaner tach signal. Some small tachs use a
silver-plated commutator and sliver-wire brushes. These have limited
life, however, like the Maxon motor/tachs.

Jon

Wes wrote:

Ignoramus10537 wrote:

I don't think so. If it was a tach, I believe you would only see two brushes. That is
based on the limited number of servo motors I've had a part. I could be wrong.

I will look a little closer. I thought that the wires were too think
to carry the main motor current. I will try to find out for sure
tonight.

i

Did you resolve it?


Wes, yes, I am 100% certain that it is a tachometer. 9.5 volts per
1000 RPM.

Interesting. The 4 brushes seem like overkill for a tach. I wonder why a two brush
design wasn't good enough. EE's, got an explanation?


Lower contact noise from the commutator. This allows for a higher
degree of accuracy in the control loop.


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.

On 2010-06-12, Wes wrote:
Ignoramus10537 wrote:

I don't think so. If it was a tach, I believe you would only see two brushes. That is
based on the limited number of servo motors I've had a part. I could be wrong.

I will look a little closer. I thought that the wires were too think
to carry the main motor current. I will try to find out for sure
tonight.

i

Did you resolve it?


Wes, yes, I am 100% certain that it is a tachometer. 9.5 volts per
1000 RPM.

Interesting. The 4 brushes seem like overkill for a tach. I wonder why a two brush
design wasn't good enough. EE's, got an explanation?

My guess is a slight overlap of switching time between two sets
of brushes so one switches while the other makes full contact to
minimize glitches in the feedback signal -- which could cause glitches
in the speed of the motor, and mar the finish of the workpiece.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

"DoN. Nichols" wrote in message
...
On 2010-06-14, David Billington wrote:
Michael A. Terrell wrote:
Wes wrote:

[ ... ]

Interesting. The 4 brushes seem like overkill for a tach. I wonder
why a two brush
design wasn't good enough. EE's, got an explanation?



Lower contact noise from the commutator. This allows for a higher
degree of accuracy in the control loop.



Some years back a mate of mine mentioned he was involved in an
instrumentation project on a diesel engine in a bus or truck and they
used a synchronous motor, as found in mains timers etc, for the
tachometer. IIRC the AC generated was rectified but I can't recall what
they did with it after that, maybe a frequency to voltage converter. Do
you have any ideas as to the benefits or disadvantages of this sort of
tachometer. I can see that there would be no contacts so possibly
reduced noise but not sure what effect methods of converting the AC to a
DC voltage proportional to RPM would have in the system. I have put a
synchronous motor onto a scope and spun it to generate a AC waveform but
not tried it beyond that.

The DC tach provides pretty much a constant voltage for a given
speed -- even at a near standstill. I would expect the frequency of the
generated AC to be so low that even rectified and filtered, you would
get variation in the output voltage for a full rotation with a constant
speed.

And with rectified and filtered AC output, you would have much
slower response to variations in actual motor speed, so the ability to
control the speed would suffer.

I believe that the AC servo motors have a pair of Hall effect
sensors (at 90 degree spacing) and a rotating permanent magnet, and
analyze the two signals to determine the actual physical position, and
from rate of change in that determine the speed. A lot more electronics
to do the same thing -- but the advantage is that there are no brushes
to wear out.

Enjoy,
DoN.


typically not an AC generator but a selsyn for this application (did I
spell that right? special variant of a synchro resolver)