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Stepper Motors, Counts, Steps, Pauses, Cycles

W

W. Watson

Jan 1, 1970
0
I'm looking at the manual of a stepper motor device that turns a focus knob
on a telescope. It talks about micro step pause, step size, cycles and
counts. The software program allows one to specify the # of pauses from 1 to
20, and steps (step size, microsteps) from 1 to 255.

The description is quite baffling. How do these two numbers figure into one
rotation of the stepper motor shaft? Apparently, they produce a rotation
measured in microsteps, which relates to the number of milliseconds in a
microstep. There seem to be too many terms here, cycles, steps, microsteps,
pauses, stepsize. Apparently, it is a small shaft that is geared to the
larger focuser shaft. I quote the manual:

"MicrostepPause. The RoboFocus processor pauses between its microstep moves
for a chosen number of milliseconds. Thus, the longer the microstep pause,
the slower the stepper will move. If the pause is set too small, the pulses
may be too fast for the stepper to follow and movement will cease or be
erratic. A stepper period of about 4 is a good choice for most motors. You
may want a longer period (slower stepping) if you want to perform
diagnostics on the system.

StepSize (Microsteps/Step). The stepping motor goes through four microsteps
before it completes one full electronic/magnetic cycle. One full cycle is
small fraction of rotation of the output shaft (depends on the gearing used
in the particular motor). The RoboFocus controller counts microsteps
internally, but you can set how many microsteps equals one control program
count. This configuration item allows you to set the number of microsteps
(1-255) that will be made when you tell the RoboFocus to move one step
(count). By changing the microsteps per count setting, you can change the
“fineness” of the focus positioning.


Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
"Summertime,
And the livin' is easy
Fish are jumpin'
And the cotton is high ..." -- G. Gershwin

Web Page: <home.earthlink.net/~mtnviews>
 
J

John O'Flaherty

Jan 1, 1970
0
W. Watson said:
I'm looking at the manual of a stepper motor device that turns a focus knob
on a telescope. It talks about micro step pause, step size, cycles and
counts. The software program allows one to specify the # of pauses from 1 to
20, and steps (step size, microsteps) from 1 to 255.

The description is quite baffling. How do these two numbers figure into one
rotation of the stepper motor shaft? Apparently, they produce a rotation
measured in microsteps, which relates to the number of milliseconds in a
microstep. There seem to be too many terms here, cycles, steps, microsteps,
pauses, stepsize. Apparently, it is a small shaft that is geared to the
larger focuser shaft. I quote the manual:

"MicrostepPause. The RoboFocus processor pauses between its microstep moves
for a chosen number of milliseconds. Thus, the longer the microstep pause,
the slower the stepper will move. If the pause is set too small, the pulses
may be too fast for the stepper to follow and movement will cease or be
erratic. A stepper period of about 4 is a good choice for most motors. You
may want a longer period (slower stepping) if you want to perform
diagnostics on the system.

StepSize (Microsteps/Step). The stepping motor goes through four microsteps
before it completes one full electronic/magnetic cycle. One full cycle is
small fraction of rotation of the output shaft (depends on the gearing used
in the particular motor). The RoboFocus controller counts microsteps
internally, but you can set how many microsteps equals one control program
count. This configuration item allows you to set the number of microsteps
(1-255) that will be made when you tell the RoboFocus to move one step
(count). By changing the microsteps per count setting, you can change the
"fineness" of the focus positioning.

As I read it, they aren't saying how many "microsteps" add up to a
full revolution of the motor. The four microsteps are what it takes to
move the stepping motor one step, which they are calling a 'cycle'; the
number of steps per revolution of the motor depends on its design. In
any case, that would still be a fraction of one revolution of the
output shaft, if there is gearing. (The 'microsteps' are intermediate
conditions of the windings of the stepping motor needed to move it from
one notch to the next.)
They are using an abstraction to program the thing, called a
'control program count'. You can control how much movement a program
count corresponds to, over a range of 1 to 255 by setting the number of
microsteps per count. As it said, that controls the fineness of
positioning. The 'pause' thing just controls the speed of the focus
adjustment, from very slow to faster and faster until the stepper motor
can't follow it any more.
They're giving the adjustment range but no absolute information.
 
C

Chris

Jan 1, 1970
0
W. Watson said:
I'm looking at the manual of a stepper motor device that turns a focus knob
on a telescope. It talks about micro step pause, step size, cycles and
counts. The software program allows one to specify the # of pauses from 1to
20, and steps (step size, microsteps) from 1 to 255.

The description is quite baffling. How do these two numbers figure into one
rotation of the stepper motor shaft? Apparently, they produce a rotation
measured in microsteps, which relates to the number of milliseconds in a
microstep. There seem to be too many terms here, cycles, steps, microsteps,
pauses, stepsize. Apparently, it is a small shaft that is geared to the
larger focuser shaft. I quote the manual:

"MicrostepPause. The RoboFocus processor pauses between its microstep moves
for a chosen number of milliseconds. Thus, the longer the microstep pause,
the slower the stepper will move. If the pause is set too small, the pulses
may be too fast for the stepper to follow and movement will cease or be
erratic. A stepper period of about 4 is a good choice for most motors. You
may want a longer period (slower stepping) if you want to perform
diagnostics on the system.

StepSize (Microsteps/Step). The stepping motor goes through four microsteps
before it completes one full electronic/magnetic cycle. One full cycle is
small fraction of rotation of the output shaft (depends on the gearing used
in the particular motor). The RoboFocus controller counts microsteps
internally, but you can set how many microsteps equals one control program
count. This configuration item allows you to set the number of microsteps
(1-255) that will be made when you tell the RoboFocus to move one step
(count). By changing the microsteps per count setting, you can change the
"fineness" of the focus positioning.


Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
"Summertime,
And the livin' is easy
Fish are jumpin'
And the cotton is high ..." -- G. Gershwin

Web Page: <home.earthlink.net/~mtnviews>

Hi, Wayne. The manufacturer website doesn't seem to have the manuals
online, so you'll have to make do with educated guesses here.

First of all, anyone who'd like to read a stepper primer should see
Jones on Stepping Motors:

http://www.cs.uiowa.edu/~jones/step/

Professor Jones' beginners' guide is the best place to start.

Most precision stepper motors have 200 steps per revolution (1.8
degrees each). For a unipolar stepper, one of the four coils is each
energized at a time in the proper sequence for full steps (full
electronic/magnetic cycles). If you turn the next coil on before you
turn the last coil off, you've created an intermediate position between
the two steps called a half step. For the precision motor above, that
will mean 400 half-steps (0.9 degrees each). Now let's use pulse width
modulation to create another intermediate stage between coil fully on
and coil fully off where there's only half power going to the coil.
That halfway position will mean there are now 800 discrete positions
per revolution (0.45 degrees each), and each is called a microstep.
Four microsteps per step, 200 steps per revolution, so 800 microsteps
per revolution.

Now it starts to get interesting. As you know, there's a frictional
component to focus knobs, with a mechanical lag between the time the
knob is turned and the torque of the screw is relieved by movement.
This allows you to manually tweak the focus in very precisely. It also
makes it much harder for you to manually turn the focus knob quickly
than slowly -- you need a lot more torque.

The torque available at the stepper shaft is dependent on speed. As a
practical matter, the torque on most stepper motors drops off pretty
dramatically at 1 to 2 rev/sec. This is called midrange resonance. It
would make sense for small systems like this to have the stepper
operating at the fastest speed (possibly about 3/4 to 1-1/2 rps) that
will allow maximum torque. It seems your system is one where your
RoboFocus will turn the screw a little bit at optimum speed (you
program in how many microsteps per bump, that's one count), and then
allows you to program in a wait period of a few milliseconds before the
next little bump. In effect, this setup allows you to program in the
average speed of the focus screw without playing with that optimum
stepping speed.

I would get the feeling that if you program in too large of a bump
and/or too small of a time between bumps, the torque buildup will
happen faster than the lens assembly can easily move, and the higher
torque will lead to loss of steps.

Sorry for the state of your docs -- it seems as if they didn't do too
good of a job of explaining things. You might want to give the
manufacturer some feedback -- possibly they can offer a webpage that
goes into detail on the mechanics and the programming method.
Actually, it doesn't sound that complex for someone who's worked around
steppers and positional control software for years -- it probably makes
perfect sense to the engineer who designed the hardware and the person
who wrote the software. ;-)

Hope this has been of help.

Good luck
Chris
 
W

W. Watson

Jan 1, 1970
0
John said:
As I read it, they aren't saying how many "microsteps" add up to a
full revolution of the motor. The four microsteps are what it takes to
move the stepping motor one step, which they are calling a 'cycle'; the
number of steps per revolution of the motor depends on its design. In
any case, that would still be a fraction of one revolution of the
output shaft, if there is gearing. (The 'microsteps' are intermediate
conditions of the windings of the stepping motor needed to move it from
one notch to the next.)
They are using an abstraction to program the thing, called a
'control program count'. You can control how much movement a program
count corresponds to, over a range of 1 to 255 by setting the number of
microsteps per count. As it said, that controls the fineness of
positioning. The 'pause' thing just controls the speed of the focus
adjustment, from very slow to faster and faster until the stepper motor
can't follow it any more.
They're giving the adjustment range but no absolute information.
Thanks for the response.

As a bit of background, the focuser moves a range of about 1/2". The idea is
to calibrate the length of travel, so the start and stop positions are known.

So we have, cycle = step = four microsteps. Where are the pauses inserted?
Between microsteps? Is a microstep the same length (time or distance) as a
pause? H

ow does a count get specified? The software dialog shows no entry for count.
If I specify the # of microsteps, then I'm actually specifying the count?
How do I tell the device how many counts I want to move the stepper motor?
It sounds like position and count might be related (see above, first paragraph).

The dialog does show a postion entry. Triggering it causes the focuser to
move to that position. Would a count equal a position?

Is there a diagram somewhere that shows this type of operation?


Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
"Summertime,
And the livin' is easy
Fish are jumpin'
And the cotton is high ..." -- G. Gershwin

Web Page: <home.earthlink.net/~mtnviews>
 
J

John O'Flaherty

Jan 1, 1970
0
W. Watson said:
Thanks for the response.

As a bit of background, the focuser moves a range of about 1/2". The idea is
to calibrate the length of travel, so the start and stop positions are known.

So we have, cycle = step = four microsteps. Where are the pauses inserted?
Between microsteps? Is a microstep the same length (time or distance) as a
pause? H

They mentioned that the pause was between microsteps, which means it's
as evenly distributed as possible. A microstep will map to a distance;
a pause is the time per microstep (ignoring the time to actually
execute the microstep).
ow does a count get specified? The software dialog shows no entry for count.
If I specify the # of microsteps, then I'm actually specifying the count?
How do I tell the device how many counts I want to move the stepper motor?
It sounds like position and count might be related (see above, first paragraph).

The dialog does show a postion entry. Triggering it causes the focuser to
move to that position. Would a count equal a position?

I can only guess. How finely can you specify a position? In what terms
is it specified? Degrees? What are the maximum and minimum values? It
does sound as if the position in this range, in whatever terms it is
specified, might be equivalent to the 'count'.
Is there a diagram somewhere that shows this type of operation?

It may be time to call the manufacturer directly and ask them. That's
if you're looking at the item in a catalog. If you actually have the
thing attached to a telescope, just try things and it should all come
clear :).
 
W

W. Watson

Jan 1, 1970
0
Chris said:
Hi, Wayne. The manufacturer website doesn't seem to have the manuals
online, so you'll have to make do with educated guesses here.

First of all, anyone who'd like to read a stepper primer should see
Jones on Stepping Motors:

http://www.cs.uiowa.edu/~jones/step/

Professor Jones' beginners' guide is the best place to start.

Most precision stepper motors have 200 steps per revolution (1.8
degrees each). For a unipolar stepper, one of the four coils is each
energized at a time in the proper sequence for full steps (full
electronic/magnetic cycles). If you turn the next coil on before you
turn the last coil off, you've created an intermediate position between
the two steps called a half step. For the precision motor above, that
will mean 400 half-steps (0.9 degrees each). Now let's use pulse width
modulation to create another intermediate stage between coil fully on
and coil fully off where there's only half power going to the coil.
That halfway position will mean there are now 800 discrete positions
per revolution (0.45 degrees each), and each is called a microstep.
Four microsteps per step, 200 steps per revolution, so 800 microsteps
per revolution.

Now it starts to get interesting. As you know, there's a frictional
component to focus knobs, with a mechanical lag between the time the
knob is turned and the torque of the screw is relieved by movement.
This allows you to manually tweak the focus in very precisely. It also
makes it much harder for you to manually turn the focus knob quickly
than slowly -- you need a lot more torque.

The torque available at the stepper shaft is dependent on speed. As a
practical matter, the torque on most stepper motors drops off pretty
dramatically at 1 to 2 rev/sec. This is called midrange resonance. It
would make sense for small systems like this to have the stepper
operating at the fastest speed (possibly about 3/4 to 1-1/2 rps) that
will allow maximum torque. It seems your system is one where your
RoboFocus will turn the screw a little bit at optimum speed (you
program in how many microsteps per bump, that's one count), and then
allows you to program in a wait period of a few milliseconds before the
next little bump. In effect, this setup allows you to program in the
average speed of the focus screw without playing with that optimum
stepping speed.

I would get the feeling that if you program in too large of a bump
and/or too small of a time between bumps, the torque buildup will
happen faster than the lens assembly can easily move, and the higher
torque will lead to loss of steps.

Sorry for the state of your docs -- it seems as if they didn't do too
good of a job of explaining things. You might want to give the
manufacturer some feedback -- possibly they can offer a webpage that
goes into detail on the mechanics and the programming method.
Actually, it doesn't sound that complex for someone who's worked around
steppers and positional control software for years -- it probably makes
perfect sense to the engineer who designed the hardware and the person
who wrote the software. ;-)

Hope this has been of help.

Good luck
Chris
Thanks for the info. I can see that Jones' site is good, but it's probably a
little too much for me. Yes, you are right on about "perfect sense". I've
considered the mfger's descriptions weak, including how to install the
device. Too much jargon. He forgets to approach the subject from the
reader's point of view. In his install procedure, his first instruction
contains "jack shaft" without any explanation of where or what it is. I
rewrote his first instruction and sent it to him. I could have easily gone
for more. Not a single figure of picture. Haven't heard back. Oh, well.


Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
"Summertime,
And the livin' is easy
Fish are jumpin'
And the cotton is high ..." -- G. Gershwin

Web Page: <home.earthlink.net/~mtnviews>
 
W

W. Watson

Jan 1, 1970
0
John said:
They mentioned that the pause was between microsteps, which means it's
as evenly distributed as possible. A microstep will map to a distance;
a pause is the time per microstep (ignoring the time to actually
execute the microstep).




I can only guess. How finely can you specify a position? In what terms
is it specified? Degrees? What are the maximum and minimum values? It
does sound as if the position in this range, in whatever terms it is
specified, might be equivalent to the 'count'.




It may be time to call the manufacturer directly and ask them. That's
if you're looking at the item in a catalog. If you actually have the
thing attached to a telescope, just try things and it should all come
clear :).
Thanks again. He's, a single individual makes the device, not at all been
responsive to my questions. His product does have a good reputation, and in
wide use. I have talked to him in the past. His install instructions too are
very weak. I called and e-mailed him to ask for a figure to go along with
the text he provided. No response. I sent him a picture I took and asked him
to show where the "jack shaft" (I had no idea what or where or what it was)
and some gear name (again some unfamiliar name) were, and what orientation
he was speaking of with right and left? He actually sent me a picture that
was helpful, but without any further explanation. A friend who was familiar
with such equipment came over and helped me. It took us 4-5 hours to install
it all. He was equally unhappy with the instructions. The user's manual has
been almost as bad. Another friend has come to my rescue and at least has
provided insight to the proper parameter values. Too much mumbo jumbo with
this product.


Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
"Summertime,
And the livin' is easy
Fish are jumpin'
And the cotton is high ..." -- G. Gershwin

Web Page: <home.earthlink.net/~mtnviews>
 
C

Chris

Jan 1, 1970
0
W. Watson said:
Thanks for the info. I can see that Jones' site is good, but it's probably a
little too much for me. Yes, you are right on about "perfect sense". I've
considered the mfger's descriptions weak, including how to install the
device. Too much jargon. He forgets to approach the subject from the
reader's point of view. In his install procedure, his first instruction
contains "jack shaft" without any explanation of where or what it is. I
rewrote his first instruction and sent it to him. I could have easily gone
for more. Not a single figure of picture. Haven't heard back. Oh, well.


Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
"Summertime,
And the livin' is easy
Fish are jumpin'
And the cotton is high ..." -- G. Gershwin

Web Page: <home.earthlink.net/~mtnviews>

Looking over John's posts yesterday, I think he has a better read on
the timing delay being between microsteps rather than between bursts of
microsteps.

But again, as he also said, you should be able to just try it and find
out. If you program it wrong, nothing should break. The stepper
almost certainly doen't provide that much torque.

Good luck with your telescope. I kind of envy you having the time and
the space.

Cheers
Chris
 
W

W. Watson

Jan 1, 1970
0
Chris said:
Looking over John's posts yesterday, I think he has a better read on
the timing delay being between microsteps rather than between bursts of
microsteps.

But again, as he also said, you should be able to just try it and find
out. If you program it wrong, nothing should break. The stepper
almost certainly doen't provide that much torque.

Good luck with your telescope. I kind of envy you having the time and
the space.

Cheers
Chris
It took a long time to get where I'm at. The time finally arrived when I
have the time, and it was then a matter of space. The latter was probably
more easily achieved. :) Amateur astronomy (and professional) astronomy
have advanced quickly in the last 10 years thanks to the availability of CCD
camera and personal computers.


Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
U.S. Deficit Forecast: A. U.S. Treasury--$760B (std. audit
method), White House--$318B (method??), Full Audit--$3.5T!!
(Corp. method) -- Source: U.S. Today (current forecasts)
Guess which one is most often given in the news? WH
Web Page: <home.earthlink.net/~mtnviews>
 
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