Stepping motor torque ripple...

[Klaus Vestergaard Kragelund]

On 27/12/2021 00.44, jlarkin@highlandsniptechnology.com wrote:

On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klauskvik@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus

 

[server]

On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

On 27/12/2021 00.44, jlarkin@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klauskvik@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus

Constant-voltage PWM will result in torque rolloff at high step rates,
and eventually stalling. More voltage with current feedback helps
stuff current into the winding inductance.

Some sort of pre-emphasis without feedback should work too.



--

I yam what I yam - Popeye

 

[Lasse Langwadt Christensen]

mandag den 31. januar 2022 kl. 02.33.22 UTC+1 skrev jla...@highlandsniptechnology.com:

On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

On 27/12/2021 00.44, jla...@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klau...@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus


Constant-voltage PWM will result in torque rolloff at high step rates,
and eventually stalling. More voltage with current feedback helps
stuff current into the winding inductance.

Some sort of pre-emphasis without feedback should work too.

page 38, https://www.st.com/resource/en/datasheet/powerstep01.pdf

 

[Klaus Vestergaard Kragelund]

On 31/01/2022 02.33, jlarkin@highlandsniptechnology.com wrote:

On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

On 27/12/2021 00.44, jlarkin@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klauskvik@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus



Constant-voltage PWM will result in torque rolloff at high step rates,
and eventually stalling. More voltage with current feedback helps
stuff current into the winding inductance.

Yes, that\'s why I wrote that my application has: \"the speed of the motor
is significantly below the max specified speed\"

Some sort of pre-emphasis without feedback should work too.

In some cases the voltage PWM is better than current mode controlled.
For example, around the zero crossing where current feedback has distortion

 

[Klaus Vestergaard Kragelund]

On 31/01/2022 02.46, Lasse Langwadt Christensen wrote:

mandag den 31. januar 2022 kl. 02.33.22 UTC+1 skrev jla...@highlandsniptechnology.com:
On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

On 27/12/2021 00.44, jla...@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klau...@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus


Constant-voltage PWM will result in torque rolloff at high step rates,
and eventually stalling. More voltage with current feedback helps
stuff current into the winding inductance.

Some sort of pre-emphasis without feedback should work too.

page 38, https://www.st.com/resource/en/datasheet/powerstep01.pdf

Nice detailed datasheet, to learn from

 

[server]

On Mon, 31 Jan 2022 11:53:59 +0100, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

On 31/01/2022 02.33, jlarkin@highlandsniptechnology.com wrote:
On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

On 27/12/2021 00.44, jlarkin@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klauskvik@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus



Constant-voltage PWM will result in torque rolloff at high step rates,
and eventually stalling. More voltage with current feedback helps
stuff current into the winding inductance.


Yes, that\'s why I wrote that my application has: \"the speed of the motor
is significantly below the max specified speed\"


Some sort of pre-emphasis without feedback should work too.


In some cases the voltage PWM is better than current mode controlled.
For example, around the zero crossing where current feedback has distortion

Not to change the subject (never!) I\'ve been experimenting with the
jitter implications of PWM, namely how precise in time can a PWM
system be? Better than the PWM period? How much better?

It\'s an interesting perversion of the Shannon/Nyquist sampling
theorem. Maybe I\'ll post some sims.



--

I yam what I yam - Popeye

 

[whit3rd]

On Monday, January 31, 2022 at 7:18:38 AM UTC-8, jla...@highlandsniptechnology.com wrote:

... I\'ve been experimenting with the
jitter implications of PWM, namely how precise in time can a PWM
system be? Better than the PWM period? How much better?

PWM is waveform generation; you can synchronize a feature of that waveform with
good precision, and the PWM period only determines the (at steady-state)
repetition interval, not the features within that interval. Useful examples of PWM
usually alter the features according to some criterion other than timing, i.e.
output voltage under load.

What specific time precision is under consideration? Old IBM game controllers got
pinged at a crystal-clock-synchronized interval, and gave 8-bit resolution.

 

[Klaus Vestergaard Kragelund]

On 31/01/2022 16.18, jlarkin@highlandsniptechnology.com wrote:

On Mon, 31 Jan 2022 11:53:59 +0100, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

On 31/01/2022 02.33, jlarkin@highlandsniptechnology.com wrote:
On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

On 27/12/2021 00.44, jlarkin@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klauskvik@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus



Constant-voltage PWM will result in torque rolloff at high step rates,
and eventually stalling. More voltage with current feedback helps
stuff current into the winding inductance.


Yes, that\'s why I wrote that my application has: \"the speed of the motor
is significantly below the max specified speed\"


Some sort of pre-emphasis without feedback should work too.


In some cases the voltage PWM is better than current mode controlled.
For example, around the zero crossing where current feedback has distortion

Not to change the subject (never!) I\'ve been experimenting with the
jitter implications of PWM, namely how precise in time can a PWM
system be? Better than the PWM period? How much better?

Can you elaborate on that?

The PWM of a microcontroller is a combination of the clock jitter and
the jitter of the PWM counter/compare capture unit, plus noise on the
supply I guess

 

[John Larkin]

On Tue, 1 Feb 2022 01:09:16 +0100, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

On 31/01/2022 16.18, jlarkin@highlandsniptechnology.com wrote:
On Mon, 31 Jan 2022 11:53:59 +0100, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

On 31/01/2022 02.33, jlarkin@highlandsniptechnology.com wrote:
On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

On 27/12/2021 00.44, jlarkin@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klauskvik@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus



Constant-voltage PWM will result in torque rolloff at high step rates,
and eventually stalling. More voltage with current feedback helps
stuff current into the winding inductance.


Yes, that\'s why I wrote that my application has: \"the speed of the motor
is significantly below the max specified speed\"


Some sort of pre-emphasis without feedback should work too.


In some cases the voltage PWM is better than current mode controlled.
For example, around the zero crossing where current feedback has distortion

Not to change the subject (never!) I\'ve been experimenting with the
jitter implications of PWM, namely how precise in time can a PWM
system be? Better than the PWM period? How much better?


Can you elaborate on that?

The PWM of a microcontroller is a combination of the clock jitter and
the jitter of the PWM counter/compare capture unit, plus noise on the
supply I guess

The abstract problem is to send a DC-coupled logic level over an
AC-coupled telecom-type link. Imagine the link has AGC or logic
repeaters so we can\'t send different amplitudes to code things.

One could lowpass filter the logic input, sample and digitize, send
the sample values in serial packets, deserialize and DAC and filter
and compare at the receiver. Pure Shannon Sampling Theorem with some
ADC quantization. Slow and painful.

FSK would work but would be very slow and maybe jitterey. OOK would be
really ugly over a telecom type link. PWM might work.

I\'m thinking about PWM at a \"carrier\" pulse clock rate of, say, 1 GHz.
30% duty cycle is a 0, 70% is a 1. The logic level to be sent is not
synchronized to the PWM clock.

It will certainly work, but what\'s the jitter like? Can it be less
than the 1 ns PWM pulse period? I\'m thinking it can.

It\'s just a thought experiment, with a bit of Spicing, right now.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[Lasse Langwadt Christensen]

tirsdag den 1. februar 2022 kl. 01.40.26 UTC+1 skrev John Larkin:

On Tue, 1 Feb 2022 01:09:16 +0100, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

On 31/01/2022 16.18, jla...@highlandsniptechnology.com wrote:
On Mon, 31 Jan 2022 11:53:59 +0100, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

On 31/01/2022 02.33, jla...@highlandsniptechnology.com wrote:
On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

On 27/12/2021 00.44, jla...@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klau...@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus



Constant-voltage PWM will result in torque rolloff at high step rates,
and eventually stalling. More voltage with current feedback helps
stuff current into the winding inductance.


Yes, that\'s why I wrote that my application has: \"the speed of the motor
is significantly below the max specified speed\"


Some sort of pre-emphasis without feedback should work too.


In some cases the voltage PWM is better than current mode controlled.
For example, around the zero crossing where current feedback has distortion

Not to change the subject (never!) I\'ve been experimenting with the
jitter implications of PWM, namely how precise in time can a PWM
system be? Better than the PWM period? How much better?


Can you elaborate on that?

The PWM of a microcontroller is a combination of the clock jitter and
the jitter of the PWM counter/compare capture unit, plus noise on the
supply I guess
The abstract problem is to send a DC-coupled logic level over an
AC-coupled telecom-type link. Imagine the link has AGC or logic
repeaters so we can\'t send different amplitudes to code things.

One could lowpass filter the logic input, sample and digitize, send
the sample values in serial packets, deserialize and DAC and filter
and compare at the receiver. Pure Shannon Sampling Theorem with some
ADC quantization. Slow and painful.

FSK would work but would be very slow and maybe jitterey. OOK would be
really ugly over a telecom type link. PWM might work.

I\'m thinking about PWM at a \"carrier\" pulse clock rate of, say, 1 GHz.
30% duty cycle is a 0, 70% is a 1. The logic level to be sent is not
synchronized to the PWM clock.

It will certainly work, but what\'s the jitter like? Can it be less
than the 1 ns PWM pulse period? I\'m thinking it can.

It\'s just a thought experiment, with a bit of Spicing, right now.

will your duty cycle survive the AC coupling on the link?
isn\'t such things normally DC balanced with bit stuffing or similar?

 

[John Larkin]

On Mon, 31 Jan 2022 16:52:18 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

tirsdag den 1. februar 2022 kl. 01.40.26 UTC+1 skrev John Larkin:
On Tue, 1 Feb 2022 01:09:16 +0100, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

On 31/01/2022 16.18, jla...@highlandsniptechnology.com wrote:
On Mon, 31 Jan 2022 11:53:59 +0100, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

On 31/01/2022 02.33, jla...@highlandsniptechnology.com wrote:
On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

On 27/12/2021 00.44, jla...@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund
klau...@hotmail.com> wrote:

Hi


I am working on a stepping motor driver


To reduce cost I will be making a discrete driver stage driven from a microcontroller that has all the control features that would normally be included in a micrstepping driver like the A4988

https://www.allegromicro.com/en/products/motor-drivers/brush-dc-motor-drivers/a4988


For easy calculation let\'s say the power stage is driven from 10V and the stepping motor has an inductance of 10mH and the peak current is 100mA


In fast decay mode the rising and falling didt is thus 1A/ms


For micro stepping the current in the 2 phases are set to place the armature in between a full step. So for a position 50% between two steps the currents in the windings are the same to have a resulting vector at that point


If the on time is 5us, the ripple current is 5mApp. So, the current ripple is 5% of the nominal peak current. For lower excitation, at 10mA peak drive, the ripple is 50%


So the magnetic field will cause quite a bit of torque ripple.


Will this have any impact on the drive of the motor, or is the inertia so big that it is insignificant?

Also, at other positions than 50%, the ripple with not cancel out between the phases and generate even more torque ripple

I don\'t see any mention of this, so it is probably insignificant...

Regards


Klaus

I did a microstepper once, for tuning superconductive microwave
cavities in an accelerator. It used a uP and a pair of integrated
h-bridge drivers.

It was basically a DDS synthesizer, a phase accumulator (representing
angular position) mapping into a sin-cos lookup table, which fetched
the duty cycle values to go into the pair of full h-bridges which
drove the motor coils. If the switching frequency is reasonably high,
there won\'t be noticable ripple torque.

My basically constant-voltage drive caused torque to drop off at high
step rates, which wasn\'t a problem in that application.

Most motors are imperfect in that you won\'t get smooth angular
position as a function of sin-cos coil currents; they basically have
distortion. That can be fudged in the trig lookup table if you really
need to.

So, now I am getting closer to having first prototype ready

I was doing the chopping control, peak current setting of the current in
the coils, a loop in the microcontroller that would control this peak
current with a DAC

Thinking about it, seems like it is all to no good. Just using the PWM
sine/cosine lookup table, with no feedback seems to work fine, provided
the speed of the motor is significantly below the max specified speed.
In that case BEMF has little effect, and really the winding resistance
sets the current according to applied voltage

Doing the open loop control would remove need for precise current
sensing and DACs (all though they could just be PWM outputs). Also
reduces the CPU resources needed for the control of the motor

I will keep the comparator to check for shorted halfbridge, at least for now

Cheers

Klaus



Constant-voltage PWM will result in torque rolloff at high step rates,
and eventually stalling. More voltage with current feedback helps
stuff current into the winding inductance.


Yes, that\'s why I wrote that my application has: \"the speed of the motor
is significantly below the max specified speed\"


Some sort of pre-emphasis without feedback should work too.


In some cases the voltage PWM is better than current mode controlled.
For example, around the zero crossing where current feedback has distortion

Not to change the subject (never!) I\'ve been experimenting with the
jitter implications of PWM, namely how precise in time can a PWM
system be? Better than the PWM period? How much better?


Can you elaborate on that?

The PWM of a microcontroller is a combination of the clock jitter and
the jitter of the PWM counter/compare capture unit, plus noise on the
supply I guess
The abstract problem is to send a DC-coupled logic level over an
AC-coupled telecom-type link. Imagine the link has AGC or logic
repeaters so we can\'t send different amplitudes to code things.

One could lowpass filter the logic input, sample and digitize, send
the sample values in serial packets, deserialize and DAC and filter
and compare at the receiver. Pure Shannon Sampling Theorem with some
ADC quantization. Slow and painful.

FSK would work but would be very slow and maybe jitterey. OOK would be
really ugly over a telecom type link. PWM might work.

I\'m thinking about PWM at a \"carrier\" pulse clock rate of, say, 1 GHz.
30% duty cycle is a 0, 70% is a 1. The logic level to be sent is not
synchronized to the PWM clock.

It will certainly work, but what\'s the jitter like? Can it be less
than the 1 ns PWM pulse period? I\'m thinking it can.

It\'s just a thought experiment, with a bit of Spicing, right now.

will your duty cycle survive the AC coupling on the link?
isn\'t such things normally DC balanced with bit stuffing or similar?

Yes, telecom links assume balanced data, NRZ or 8b10b or something.
That makes it hard to send DC!

I\'ve tested the proposed link and it can propagate duty cycles,
numbers like 30% and 70%; maybe 35 and 65 to be conservative.

Just now I can\'t think of a better modulation scheme, at least one
that\'s practical to actually build.

The edges of a PWM pulse can be positioned with arbitrarily small time
resolution, so sub-clock timing is possible.



--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[Anthony William Sloman]

On Tuesday, February 1, 2022 at 2:18:38 AM UTC+11, jla...@highlandsniptechnology.com wrote:

On Mon, 31 Jan 2022 11:53:59 +0100, Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
On 31/01/2022 02.33, jla...@highlandsniptechnology.com wrote:
On Mon, 31 Jan 2022 01:52:20 +0100, Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
On 27/12/2021 00.44, jla...@highlandsniptechnology.com wrote:
On Sun, 26 Dec 2021 23:38:44 +0100, Klaus Kragelund <klau...@hotmail.com> wrote:

<snip>

In some cases the voltage PWM is better than current mode controlled.
For example, around the zero crossing where current feedback has distortion
Not to change the subject (never!) I\'ve been experimenting with the
jitter implications of PWM, namely how precise in time can a PWM
system be? Better than the PWM period? How much better?

It\'s an interesting perversion of the Shannon/Nyquist sampling
theorem. Maybe I\'ll post some sims.

You might think about the use of quadrature coding to send a 10kbit/sec data stream over a telephone line.

Those modems were quite popular when telephone pairs were our only way of getting our home computer hooked up to the internet.

--
Bill Sloman, Sydney

 

[Jan Panteltje]

On a sunny day (Mon, 31 Jan 2022 16:40:14 -0800) it happened John Larkin
<jlarkin@highland_atwork_technology.com> wrote in
<kmugvgldukcs48h7v2sf3ae378nqfvokj2@4ax.com>:

The abstract problem is to send a DC-coupled logic level over an
AC-coupled telecom-type link. Imagine the link has AGC or logic
repeaters so we can\'t send different amplitudes to code things.

In video (that has a DC conponent reference) we used FM modulation
for example to record to tape.
Used it over phone line too for slow scan TV.
That was before all went digital.

 

[piglet]

On 01/02/2022 12:40 am, John Larkin wrote:

I\'m thinking about PWM at a \"carrier\" pulse clock rate of, say, 1 GHz.
30% duty cycle is a 0, 70% is a 1. The logic level to be sent is not
synchronized to the PWM clock.

It will certainly work, but what\'s the jitter like? Can it be less
than the 1 ns PWM pulse period? I\'m thinking it can.

It\'s just a thought experiment, with a bit of Spicing, right now.

Does the link preserve polarity - after an arbitrary number of
inversions 30% and 70% duty cycle may be indistinguisable. How about 50%
as one state and 30/70 as the other?

piglet

 

[whit3rd]

On Monday, January 31, 2022 at 5:24:01 PM UTC-8, John Larkin wrote:

Yes, telecom links assume balanced data, NRZ or 8b10b or something.
That makes it hard to send DC!

Seismometers are too low frequency for direct tape recording, so they\'re always
frequency-converted for field tape-recorder stations. The tape drives have a multiplicity
of tracks, so one of \'em gets a timing reference, the others record data. It\'s done
without a bias frequency, which saves precious battery power.