1.8 or 0.9 degree stepper?

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mawyatt
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Post by mawyatt »

Gene,

Your note. "Doubling the number of teeth/cogs, in their otherwise nearly identical motors, doesn't result in any increase in accuracy. It does result in a ~15% drop in torque though."


However doubling the number of teeth would surely improve accuracy when micro stepping. Given that a 1.8 degree motor would required twice as many micro steps to get to the same position as a 0.9 degree motor. Microsteps can not be more precise than the teeth positions they are micro-stepping from (ref point), so will always add to the uncertainty of the final position, thus decreasing step accuracy. A 1.8 degree motor will be "straddling" a wider angle, twice as much in the 0.9 degree motor, so an intrinsic microstep error from the driver/coils will create more of an error with the 1.8 degree motor.

Since a 1.8 degree motor will require a 1/2 step to get to where a 0.9 degree motor can position, the 15% reduction in holding torque (85% total) for the 0.9 degree motor you mention will actually be higher than the effective holding torque of the 1.8 degree motor under a 1/2 step which will experience a torque reduction due to the 1/2 step (71% total).


Here's a good reference article.

The lure of microstepping a two-phase stepper motor is compelling. For instance, microstepping a 1.8° hybrid stepper motor with 256 microsteps per full step would yield 51,200 steps/revolution.

Sounds great. But there's a catch. The real compromise is that as you increase the number of microsteps per full step, the incremental torque per microstep drops off drastically. Resolution increases but accuracy actually suffers.

Few if any stepper motors have a pure sinusoidal torque vs. shaft position curve and all have higher-order harmonics that distort the curve and affect accuracy. And even though microstepping drives have come a long way, they still only approximate a true sine wave. Significant too is that any load torque will result in a magnetic backlash, displacing the rotor from the intended position until sufficient torque is generated.

https://www.machinedesign.com/archive/m ... ping-myths

It seems that a 0.9 degree stepper will be a better choice where overall accuracy and resolution are concerned, since at a minimum it will not require any micro steps to achieve a 0.9 degree step angle whereas the 1.8 degree motor will require a 1/2 microstep. Also the finer teeth motors usually result in smoother motor operation since they aren't "jumping" from step to step as far. I've noticed this with the motors I have, less setup ringing with the 0.9 degree steppers.

So the less micro-steps one requires to achieve their desired resolution the better, and this implies using a finer step motor where applicable. Of course sometimes other things come into play, like what you have on hand, or cost, or just general availability (NEMA 17 0.9 degree motor are plentiful, NEMA 11s not so).


Best & Happy Holidays,
Research is like a treasure hunt, you don't know where to look or what you'll find!
~Mike

Adalbert
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Post by Adalbert »

Hello Mike,
Since a 1.8 degree motor will require a 1/2 step to get to where a 0.9 degree motor can position, the 15% reduction in holding torque (85% total) for the 0.9 degree motor you mention will actually be higher than the effective holding torque of the 1.8 degree motor under a 1/2 step which will experience a torque reduction due to the 1/2 step (71% total).
And therefore I cannot find any 0.9 degree NEMA 17 60mm long with high torque.
(NEMA 17 0.9 degree motor are plentiful, NEMA 11s not so).
BR, ADi

lonepal
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Post by lonepal »

Dear Mike;

Thanks for the good explanation.

I also find 0.9 stepper motors more reliable for microstepping.
Regards.
Omer

ray_parkhurst
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Post by ray_parkhurst »

Adalbert wrote:And therefore I cannot find any 0.9 degree NEMA 17 60mm long with high torque.
ADi...is there some important mechanical aspect of your system which requires a particular length of motor? IMO that is one of the least important specifications of a motor, but makes sense if you have a very tight spot which you are trying to fit within.

Adalbert
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Post by Adalbert »

Hello Ray,
The length is not so important but the torque.
NEMA 17 with 60mm length has the highest torque.

e.g.
https://en.nanotec.com/products/250-st4 ... r-nema-17/
ST4118D1804-A 17 42 mm 80 Ncm 1.8 °/step 1.8 A 60 mm
ST4118D1804-B 17 42 mm 80 Ncm 1.8 °/step 1.8 A 60 mm
ST4118D3004-A 17 42 mm 80 Ncm 1.8 °/step 3 A 60 mm
ST4118D3004-B 17 42 mm 80 Ncm 1.8 °/step 3 A 60 mm

BR, ADi

ray_parkhurst
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Post by ray_parkhurst »

lonepal wrote:Dear Mike;

Thanks for the good explanation.

I also find 0.9 stepper motors more reliable for microstepping.
Looking at it from a marketing perspective, 400-step motors fill the need for finer step sizes and/or smoother operation, thus they tend to have lighter magnets / lower holding torque / lower starting torque and consequently better microstepping capability. They are sold with names like "smooth step".

ray_parkhurst
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Post by ray_parkhurst »

Adalbert wrote:Hello Ray,
The length is not so important but the torque.
NEMA 17 with 60mm length has the highest torque.

e.g.
https://en.nanotec.com/products/250-st4 ... r-nema-17/
ST4118D1804-A 17 42 mm 80 Ncm 1.8 °/step 1.8 A 60 mm
ST4118D1804-B 17 42 mm 80 Ncm 1.8 °/step 1.8 A 60 mm
ST4118D3004-A 17 42 mm 80 Ncm 1.8 °/step 3 A 60 mm
ST4118D3004-B 17 42 mm 80 Ncm 1.8 °/step 3 A 60 mm

BR, ADi
With torque being so important for your system, and I assume also a desire for high accuracy, why not go with a NEMA-23 motor? It's pretty easy to make a NEMA-17 to NEMA-23 adapter (can even be made out of wood!), so that should not be a limitation. NEMA-23 has more design flexibility, since the innards are larger and thus more room for small teeth. You could get a NEMA-23 with 0.45-deg step and 99Ncm torque, ie smaller step AND more torque than is available in NEMA-17. Here is the Lin page with this type of motor:

https://www.linengineering.com/products ... dard_motor

Edited to add: and specifically, here is a model with currents and voltages that I believe should be easily driven by mjkzz/WeMacro/StackShot:

https://www.linengineering.com/products ... dard_motor

mawyatt
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Post by mawyatt »

ray_parkhurst wrote:
Adalbert wrote:Hello Ray,
The length is not so important but the torque.
NEMA 17 with 60mm length has the highest torque.

e.g.
https://en.nanotec.com/products/250-st4 ... r-nema-17/
ST4118D1804-A 17 42 mm 80 Ncm 1.8 °/step 1.8 A 60 mm
ST4118D1804-B 17 42 mm 80 Ncm 1.8 °/step 1.8 A 60 mm
ST4118D3004-A 17 42 mm 80 Ncm 1.8 °/step 3 A 60 mm
ST4118D3004-B 17 42 mm 80 Ncm 1.8 °/step 3 A 60 mm

BR, ADi
With torque being so important for your system, and I assume also a desire for high accuracy, why not go with a NEMA-23 motor? It's pretty easy to make a NEMA-17 to NEMA-23 adapter (can even be made out of wood!), so that should not be a limitation. NEMA-23 has more design flexibility, since the innards are larger and thus more room for small teeth. You could get a NEMA-23 with 0.45-deg step and 99Ncm torque, ie smaller step AND more torque than is available in NEMA-17. Here is the Lin page with this type of motor:

https://www.linengineering.com/products ... dard_motor

Edited to add: and specifically, here is a model with currents and voltages that I believe should be easily driven by mjkzz/WeMacro/StackShot:

https://www.linengineering.com/products ... dard_motor
Ray,

The motor you quoted won't work properly near rated current (900ma) since the internal resistance is 11.7 ohms with a 12 volt supply, because no voltage room is left for the L*di/dt voltage. With this high internal resistance the motor dissipates lots of power at rated current also!!

You would need to increase the motor voltage to 20~24 volts, which I wouldn't recommend directly connecting to the controllers since they all establish the internal electronic voltage (probably 5 volts, but could be 3.3 volts) from the main input voltage for a regulator. Likely this regulator (unless it's switch-mode) would overheat because of the excessive voltage drop required. This is the very reason I used a switch-mode regulator in my S&S system, so I could run higher voltages for the motors if desired and not cause any issues with the other electronics. Also you must consider whether the motor drivers in these controllers can sustain a higher motor voltage.

I would check with William (Wemacro), Peter (MJKZZ) or Paul (Cognisys) before connecting a higher supply voltage than the nominal 12 volts, just to be safe.

Best,
Research is like a treasure hunt, you don't know where to look or what you'll find!
~Mike

ray_parkhurst
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Post by ray_parkhurst »

mawyatt wrote:
Ray,

The motor you quoted won't work properly near rated current (900ma) since the internal resistance is 11.7 ohms with a 12 volt supply, because no voltage room is left for the L*di/dt voltage. With this high internal resistance the motor dissipates lots of power at rated current also!!

You would need to increase the motor voltage to 20~24 volts, which I wouldn't recommend directly connecting to the controllers since they all establish the internal electronic voltage (probably 5 volts, but could be 3.3 volts) from the main input voltage for a regulator. Likely this regulator (unless it's switch-mode) would overheat because of the excessive voltage drop required. This is the very reason I used a switch-mode regulator in my S&S system, so I could run higher voltages for the motors if desired and not cause any issues with the other electronics. Also you must consider whether the motor drivers in these controllers can sustain a higher motor voltage.

I would check with William (Wemacro), Peter (MJKZZ) or Paul (Cognisys) before connecting a higher supply voltage than the nominal 12 volts, just to be safe.

Best,
I did check with mjkzz a while back and the voltage can be increased, but I don't know the limit, and probably would not recommend that anyway.

Note that the "torque" ratings are holding torque, not dynamic drive torque, and having never developed a motor drive system, I am honestly not sure how they relate.

There are other motors in the series which have lower resistance, like the 5704X-10 (9.6 ohms, 0.9A), or if that's still not enough headroom the 5704M-2 (3.1 ohms, 1.8A).

mawyatt
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Post by mawyatt »

ray_parkhurst wrote:
mawyatt wrote:
Ray,

The motor you quoted won't work properly near rated current (900ma) since the internal resistance is 11.7 ohms with a 12 volt supply, because no voltage room is left for the L*di/dt voltage. With this high internal resistance the motor dissipates lots of power at rated current also!!

You would need to increase the motor voltage to 20~24 volts, which I wouldn't recommend directly connecting to the controllers since they all establish the internal electronic voltage (probably 5 volts, but could be 3.3 volts) from the main input voltage for a regulator. Likely this regulator (unless it's switch-mode) would overheat because of the excessive voltage drop required. This is the very reason I used a switch-mode regulator in my S&S system, so I could run higher voltages for the motors if desired and not cause any issues with the other electronics. Also you must consider whether the motor drivers in these controllers can sustain a higher motor voltage.

I would check with William (Wemacro), Peter (MJKZZ) or Paul (Cognisys) before connecting a higher supply voltage than the nominal 12 volts, just to be safe.

Best,
I did check with mjkzz a while back and the voltage can be increased, but I don't know the limit, and probably would not recommend that anyway.

Note that the "torque" ratings are holding torque, not dynamic drive torque, and having never developed a motor drive system, I am honestly not sure how they relate.

There are other motors in the series which have lower resistance, like the 5704X-10 (9.6 ohms, 0.9A), or if that's still not enough headroom the 5704M-2 (3.1 ohms, 1.8A).
Ray,

The L*di/dt is the voltage required to "push" the current thru the motor inductance, this voltage must be added to the current times motor resistance (I*R) voltage and be available from the supply. You can experience the effects of not having enough supply voltage by reducing the supply voltage while the motor is operational, it will chatter, miss steps, hiccup, cough, then stagger and stop depending on the motor/controller/voltage and other things. Certainly not what we want for our precision stacking sessions!

The higher inductance motors require a higher voltage (L*di/dt) to "push" the current, while a lower inductance motor can cause problems with the controller decay modes unless setup properly. I try and maintain at least 6 volts available overhead after the motor IR voltage and driver voltage drops are subtracted.

The experimental S&S System I'm developing uses a motor driver than can sustain 35 volts max, and a 90% efficient DC to DC converter can down convert from 40 volts max to 5 volts (actually 5.3 volts) for the electronics and Raspberry Pi computer, so plenty of flexibility regarding motor supply voltage without having to worry about regulator and electronics supply problems. Don't envision having to operate anywhere near these upper voltages though, but the capability is "built in" if the need ever arrises.

Best,
Research is like a treasure hunt, you don't know where to look or what you'll find!
~Mike

ray_parkhurst
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Post by ray_parkhurst »

mawyatt wrote:The L*di/dt is the voltage required to "push" the current thru the motor inductance, this voltage must be added to the current times motor resistance (I*R) voltage and be available from the supply. You can experience the effects of not having enough supply voltage by reducing the supply voltage while the motor is operational, it will chatter, miss steps, hiccup, cough, then stagger and stop depending on the motor/controller/voltage and other things. Certainly not what we want for our precision stacking sessions!

The higher inductance motors require a higher voltage (L*di/dt) to "push" the current, while a lower inductance motor can cause problems with the controller decay modes unless setup properly. I try and maintain at least 6 volts available overhead after the motor IR voltage and driver voltage drops are subtracted.

The experimental S&S System I'm developing uses a motor driver than can sustain 35 volts max, and a 90% efficient DC to DC converter can down convert from 40 volts max to 5 volts (actually 5.3 volts) for the electronics and Raspberry Pi computer, so plenty of flexibility regarding motor supply voltage without having to worry about regulator and electronics supply problems. Don't envision having to operate anywhere near these upper voltages though, but the capability is "built in" if the need ever arrises.
Your description of required headroom for inductive spikes jibes with my experience with the motors I've used successfully, which typically require 0.7A at 7-10 ohms, so have 5-7V headroom with a 12V supply. So for ADi's high torque purposes (with 0.45-deg resolution) the 5704M-02 may be the better choice without pushing the limits of the supply voltages.

Adalbert
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Post by Adalbert »

Hello Ray,
https://www.linengineering.com/products ... n-features
Yes, it sounds really good, but this motor is very expensive, so I’ll start with the 42BYGHM809:
http://www.wantmotor.com/product/42byghm.html
BR, ADi

mawyatt
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Post by mawyatt »

I've wondered about the 2 phase motor power consumption considering it has 2 coils driven by an approximate sinusoid. Using twice R(I^2) since we have two coils is certainly a safe maximum estimate.

Here's a little quick analysis for a better estimate.

Since each coil is driven by a sinusoid (Sin and Cos) the overall motor power based upon I squared R is

Motor Power = R(I*Sin(x) + I*Cos(x))^2 where x is step angle, I is max motor coil current and R is motor coil resistance.

Motor Power = R(I^2)*{Sin(x)*Sin(x) + Cos(x)*Cos(x) + 2*Sin(x)*Cos(x)}

From Trig, Sin^2 + Cos^2 = 1

Motor Power = R(I^2)*{1 + 2*Sin(x)*Cos(x)}

From Calculus, take derivative and set to 0 to find maximum

d(Motor Power)/dx = 2(Cos(x)*Cos(x) -Sin(x)*Sin(x)) = 0

Cos(x)*Cos(x) = Sin(x)*Sin(x), solve for x

x = pi/4 or 45 degrees, also 135, 225 & 315 degrees.

So the maximum Motor Power = R(I^2)(1 + 2*Sin(pi/4)*Cos(pi/4))

Max Motor Power = 2R(I^2), same as original estimate!!

Note that maximum motor power occurs at 45, 135, 225 and 315 degrees.

Minimum power occurs when Sin(x) or Cos(x) equals zero, or 0, 90, 180, & 270 degrees.

Min Motor Power = R(I^2)

So the average motor power when moving (not stationary) is

Average Moving Motor Power = (Max Motor Power + Min Motor Power)/2

Average Moving Motor Power = 1.5*R(I^2)

This is an interesting result that the unloaded moving motor power is actually lower than the peak stationary motor power at certain angles. Of course this doesn't consider any other motor losses, non-ideal sinusoidal current, or friction, or load dynamics.

I'll try and run some tests when I get some "free" time to see if these motors behave this way, certainly an interesting possibility IMO.

Has anyone experienced this result?

Best & Happy Holidays,
Last edited by mawyatt on Sat Dec 29, 2018 8:01 am, edited 1 time in total.
Research is like a treasure hunt, you don't know where to look or what you'll find!
~Mike

ray_parkhurst
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Post by ray_parkhurst »

mawyatt wrote: ...
This is an interesting result that the unloaded moving motor power is actually lower than the peak stationary motor power at certain angles. Of course this doesn't consider any other motor losses, non-ideal sinusoidal current, or friction, or load dynamics.

I'll try and run some tests when I get some "free" time to see if these motors behave this way, certainly an interesting possibility IMO.

Has anyone experience this result?
I'd look at it more in terms of energy than power. The actual movements don't take very long, so not much energy is dissipated in the windings, but the stationary power is nearly continous, so makes up the bulk of the heat generation.

mawyatt
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Post by mawyatt »

ray_parkhurst wrote:
mawyatt wrote: ...
This is an interesting result that the unloaded moving motor power is actually lower than the peak stationary motor power at certain angles. Of course this doesn't consider any other motor losses, non-ideal sinusoidal current, or friction, or load dynamics.

I'll try and run some tests when I get some "free" time to see if these motors behave this way, certainly an interesting possibility IMO.

Has anyone experience this result?
I'd look at it more in terms of energy than power. The actual movements don't take very long, so not much energy is dissipated in the windings, but the stationary power is nearly continous, so makes up the bulk of the heat generation.
Ray,

Power is the right metric to use since power is energy divided by time and heat (dissipated) is the parameter we are trying to manage. 100 Joules energy dissipated over 1000 seconds wouldn't make much of a temperature change in our motors, however 100 watts certainly would!

Thermal impedance is expressed as temperature change/power, or C/W. A motor has a certain thermal impedance to the ambient Ta, called Rt.

The motor temperature is Ta + Rt*Pm, where Pm is power dissipated in the motor.

So a motor with a Rt of 5 Degrees C /Watt will have a temperature rise of 50 degrees C when dissipating 10 watts, or about 75 degrees with Ta~25C.

Best,
Research is like a treasure hunt, you don't know where to look or what you'll find!
~Mike

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