Stepper motor dead zone and microstepping

[Larty]

I'm not sure if this is the right group to post these questions, so I
apologise in advance if I should be posting to a different group.

I've looked at the very informative article by Douglas Jones
(http://www.cs.uiowa.edu/~jones/step/physics.html) on stepping motors,
but I am left a little confused whether the dead zone is important
when microstepping.

In my application, I will be driving a 2 phase stepper motor with 2
sine wave currents that are out of phase by 90 degrees. The sine wave
currents will be generated by a PWM chopper IC and DAC.

Surely with this drive arrangement the torque will be constant with
shaft angle since the vector sum of the two currents and hence
magnetic flux will have a constant magnitude? Thus, as long as this
constant torque is above the torque required to overcome friction
there will be no dead zone?

Cheers,
Larty.

 

[colin]

I'm not sure if this is the right group to post these questions, so I
apologise in advance if I should be posting to a different group.

I've looked at the very informative article by Douglas Jones
(http://www.cs.uiowa.edu/~jones/step/physics.html) on stepping motors,
but I am left a little confused whether the dead zone is important
when microstepping.

In my application, I will be driving a 2 phase stepper motor with 2
sine wave currents that are out of phase by 90 degrees. The sine wave
currents will be generated by a PWM chopper IC and DAC.

Surely with this drive arrangement the torque will be constant with
shaft angle since the vector sum of the two currents and hence
magnetic flux will have a constant magnitude? Thus, as long as this
constant torque is above the torque required to overcome friction
there will be no dead zone?

Cheers,
Larty.

if the motor is constantly moving i asume as your driving it woith constant
frequency theyl be no static friction wich is what is usualy responsible for
dead zone, and especialy so with constant torque if its above the static
friction, however the actual torque is dependant on your load at a given
speed etc, not on your driving circuit.

dead zone is when the torque for the given angle is below that necesary to
overcome static friction. the torque is still there its just already used up
...

Colin =^.^=

 

[John Larkin]

I'm not sure if this is the right group to post these questions, so I
apologise in advance if I should be posting to a different group.

I've looked at the very informative article by Douglas Jones
(http://www.cs.uiowa.edu/~jones/step/physics.html) on stepping motors,
but I am left a little confused whether the dead zone is important
when microstepping.

In my application, I will be driving a 2 phase stepper motor with 2
sine wave currents that are out of phase by 90 degrees. The sine wave
currents will be generated by a PWM chopper IC and DAC.

Surely with this drive arrangement the torque will be constant with
shaft angle since the vector sum of the two currents and hence
magnetic flux will have a constant magnitude? Thus, as long as this
constant torque is above the torque required to overcome friction
there will be no dead zone?

Cheers,
Larty.

Two things will make the positioning imperfect. Static friction is
one, but a bigger error is that most steppers, especially cheap
tin-can types, will give you a desired-vs-theoretical angular position
curve that is nonlinear, with a period of four steps (ie, one full
sin/cos cycle.) Lots of motors are so bad that there will actually be
a flat spot in this curve. This is because the motor was optimized for
full-step operation, and the teeth are basicly square, so the sine-cos
thing doesn't track very well. Some motors are designed to microstep
accurately, but you've gotta check. I've seen drivers that included a
tweak for the most common nonlinearity, a "third harmonic" or
something.

John

 

[Larty]

if the motor is constantly moving i asume as your driving it woith constant
frequency theyl be no static friction wich is what is usualy responsible for
dead zone, and especialy so with constant torque if its above the static
friction, however the actual torque is dependant on your load at a given
speed etc, not on your driving circuit.

dead zone is when the torque for the given angle is below that necesary to
overcome static friction. the torque is still there its just already used up
..

Colin =^.^=

Thanks for the input.
The motor will be moving at variable speeds and needs to be able to
come to a stop at defined positions (i.e. number of microsteps from
some origin) accurately in my application.

Cheers,
Larty

 

[Larty]

Two things will make the positioning imperfect. Static friction is
one, but a bigger error is that most steppers, especially cheap
tin-can types, will give you a desired-vs-theoretical angular position
curve that is nonlinear, with a period of four steps (ie, one full
sin/cos cycle.) Lots of motors are so bad that there will actually be
a flat spot in this curve. This is because the motor was optimized for
full-step operation, and the teeth are basicly square, so the sine-cos
thing doesn't track very well. Some motors are designed to microstep
accurately, but you've gotta check. I've seen drivers that included a
tweak for the most common nonlinearity, a "third harmonic" or
something.

John

Thanks for the input John.
The motor I will be using will be optimised for microstepping.

Cheers,
Larty.

 

[colin]

Thanks for the input.
The motor will be moving at variable speeds and needs to be able to
come to a stop at defined positions (i.e. number of microsteps from
some origin) accurately in my application.

Cheers,
Larty

as your using a stepper motor designed for microstepping i hope you can get
hold of the spec spec sheet for it to see whats the best resolution/acuracy
you can get, of course if your driving much of a load like say a screww type
linear motion transl;ator
then that would introduce a lot of static friction unless u used
recirculating ball type wich becomes expensive ... youl always have some
static friction so youl always have some dead zone its just a querstion of
wether u can get it below what u need for acuracy, of course if the
load/torque is variable this will definatly vary the angle.

maybe you need a good position sensor but this is usualy considered overkill
with a stepper, as then you might as well just use a dc motor as a servo.
however u can possibly determine micro step angle from the inductance of the
windings. it may not be linear you may have to calibrate it say by spining
it with a heavy flywheel atatched.

i dun realy know how practical this advice is its all something ive looked
into as i was interested in making a small but precise cnc out of cheep
junk, but never got the mechanical stuff sorted.

Colin =^.^=