What torque-speed curve should I use for my stepper motor selection?

[Zalajbeg]

Hello everybody,

I have a project in which I need to continuously rotate some plates attached to rods across a flow and measure the effect of these plates on the flow in downstream. The rotation must be in specific angular velocities.

Let's assume I expect the maximum moment applied on the plates will be 20 N.cm. My questions are:

1) Should I compare this torque with holding torque in technical data sheets with some safety factors in case I cannot find a torque-speed curve? Is the holding torque the maximum torque on the torque-speed curve?
2) Let's say at a given angular velocity the torque-speed curve shows 40 N.cm and the air at this given velocity applies a moment of 20 N.cm in the opposite direction of the motor's rotation. Will the motor be able to move as I defined since 40 N.cm is greater than 20 N.cm?
3) Again I have the same scenario as #2 but this time the moment is applied in the same direction as motor's rotation. Will the motor be able to move as I defined if the motor needs to resist the moment not to rotate too fast?

Thanks in advance

 

[berkeman]

I need to continuously rotate some plates attached to rods across a flow and measure the effect of these plates on the flow in downstream. The rotation must be in specific angular velocities.

How continuously/smoothly? Are you planning on using regular stepper motors (how many steps per rotation?) or micro-stepper motors? Are you considering using any gearing-down between the motor and the moving plate mechanism?

 

[Zalajbeg]

How continuously/smoothly? Are you planning on using regular stepper motors (how many steps per rotation?) or micro-stepper motors? Are you considering using any gearing-down between the motor and the moving plate mechanism?

I want the rotation to be smooth. I may need the steps with some scenarios like "Rotate between angle -x and +x, one step from -x to +x in t seconds and one step from +x to -x again in t seconds, repeat it again and again (for few minutes)".

As I don't know much about the stepper motors I just checked some motors, NEMA-17 motors looked suitable to me but if you can direct me to some resources I am happy to check which one fits more.

Edit: It seems last part of my reply did not go into the post.

I am planning to attach the plate directly to the output shaft with a rod but without any gears.

 

[berkeman]

As I don't know much about the stepper motors I just checked some motors, NEMA-17 motors looked suitable to me but if you can direct me to some resources I am happy to check which one fits more.

I think NEMA-17 refers to the frame size of the stepper motor, rather than to any particular specs. It looks like this stepper motor family is available with up to 4096 steps per revolution (about 11 steps per degree), which is pretty good:

https://www.cuidevices.com/product/resource/nema17-amt112s.pdf

Here is an article about microstep motors, although it is pretty critical about the drop in torque compared to standard stepper motors:

https://www.faulhaber.com/en/suppor...r-tutorial-microstepping-myths-and-realities/

Be sure to figure out a reliable way to couple the stepper motor shaft to the drive shaft of your plates that will be subject to the lateral forces in the flow. Stepper motor shaft bearings are not generally designed for large lateral or thrust forces.

 

[Dr.D]

If you want continuous rotation, why would you use a stepper motor?

 

[Zalajbeg]

I think NEMA-17 refers to the frame size of the stepper motor, rather than to any particular specs. It looks like this stepper motor family is available with up to 4096 steps per revolution (about 11 steps per degree), which is pretty good:

https://www.cuidevices.com/product/resource/nema17-amt112s.pdf

Here is an article about microstep motors, although it is pretty critical about the drop in torque compared to standard stepper motors:

https://www.faulhaber.com/en/suppor...r-tutorial-microstepping-myths-and-realities/

Be sure to figure out a reliable way to couple the stepper motor shaft to the drive shaft of your plates that will be subject to the lateral forces in the flow. Stepper motor shaft bearings are not generally designed for large lateral or thrust forces.

Thanks a lot for your reply. I have been moving to another apartment therefore I did not have proper internet connection. Sorry for my late response.

According to what I read I think I will be OK to go with stepper motor rather than micro stepper one.

To be honest I did not think about the lateral forces. I was only concerned about the rotational forces and as both half of the plate would get air flow I was thinking more or less they would be cancelling each other. Now I have noticed. Thanks for directing me. I will consider my options to reduce the load on the bearings.

My only question for now concerning me is what happens a torque is applied in the same direction as the motor's rotation. If that torque is lower than what the stepper motor can provide but high enough to rotate it faster than I desired will the motor counter this torque to maintain its defined stepping?

If you want continuous rotation, why would you use a stepper motor?

Maybe my explanation was not clear. I want a continuous movement like this:

* Initial angular position is -x
* Go to the angular position +x in t seconds
* Go back to the angular position -x in t seconds.
* Repeat this again and again

Unfortunately I don't know a way to do this without a stepper motor.

 

[Dr.D]

I want a continuous movement like this:

* Initial angular position is -x
* Go to the angular position +x in t seconds
* Go back to the angular position -x in t seconds.
* Repeat this again and again

What you describe sounds rather like a triangular displacement function. This is physically impossible because of the infinite accelerations required to reverse the direction at each turning point.

 

[berkeman]

Maybe my explanation was not clear. I want a continuous movement like this:

* Initial angular position is -x
* Go to the angular position +x in t seconds
* Go back to the angular position -x in t seconds.
* Repeat this again and again

Unfortunately I don't know a way to do this without a stepper motor.

A servo motor with encoder can also do this, but without holding torque. Do you need holding torque?

https://www.kollmorgen.com/en-us/se...pper-motor-or-servo-motor-which-should-it-be/

 

[Tom.G]

A sketch of the proposed device would be helpful, there may be other approaches to the same end result.

For instance driving the vane(s) with a cardioid shaped cam may be simpler.

 

[Baluncore]

I want a continuous movement like this:
* Initial angular position is -x
* Go to the angular position +x in t seconds
* Go back to the angular position -x in t seconds.
* Repeat this again and again
Unfortunately I don't know a way to do this without a stepper motor.

How accurate does it really need to be?
How many cycles? for how many years?

1. I would consider a reversible geared motor, driving the shaft. Turn point detectors in adjustable position to reverse the motor direction. Speed set by a VFD, or by encoder or tachometer rate from the motor, or by DC voltage.

2. If you use a DC servo motor, then there is no holding torque problem because steps cannot be lost. Feedback of speed and turning point control are determined by the accuracy requirements. The loop position error controls the H-bridge driver.

3. A single geared motor directly driving a cam disk, with a roller following the disk edge, moving a link to the shaft. Cam profile and (synchronous?) motor speed set the repeated pattern.

 

[Zalajbeg]

Thanks all for your replies. My first question was only about the stepper motors, therefore I simplified the problem. But now thanks to your guidance I see the topic in a broader angle. Let me fully explain what I need.

* I want to rotate plate/plates in a air flow and want to measure the effect of the plate on the flow at downstream. I have defined the movement I want in the previous post.

A sketch of the proposed device would be helpful, there may be other approaches to the same end result.

For instance driving the vane(s) with a cardioid shaped cam may be simpler.

* It is a nice suggestion but I want to do this with different parameters multiple times. If I use a cam I think I will be limited to some certain angles. I want to go between angle -x and +x in t seconds and do it backwards in t seconds. By adjusting the motor speed I can change the parameter 't' but if I want to repeat it with different 'x' parameters I believe I will need a separate cam for each x.

I have a picture from a poster. The active grid is something similar I want to do.

http://www.hecekli.nl/docs/h_e_cekli_poster_tailoring_turbulence.pdf

A servo motor with encoder can also do this, but without holding torque. Do you need holding torque?

https://www.kollmorgen.com/en-us/se...pper-motor-or-servo-motor-which-should-it-be/

* Now I think the servo motor seems a better option. In the article you shared I see that the torque is not affected by the speed and I don't really think that I will need holding torque.

How accurate does it really need to be?
How many cycles? for how many years?

1. I would consider a reversible geared motor, driving the shaft. Turn point detectors in adjustable position to reverse the motor direction. Speed set by a VFD, or by encoder or tachometer rate from the motor, or by DC voltage.

2. If you use a DC servo motor, then there is no holding torque problem because steps cannot be lost. Feedback of speed and turning point control are determined by the accuracy requirements. The loop position error controls the H-bridge driver.

3. A single geared motor directly driving a cam disk, with a roller following the disk edge, moving a link to the shaft. Cam profile and (synchronous?) motor speed set the repeated pattern.

* Thanks for summarizing the options. Turn-point detectors seem difficult to apply as I need to find some place to attach them and I don't want to disturb flow with additional equipment. Servo-motor sounds like the best option to me.

My cycles will be very limited. I want to run a case maybe for 10 minutes and start to a new case. The cam option seems limiting my parameters.

---------------------------------------------

Firstly I was thinking on stepper motor and now the servo motor option seems a better one to me. The problem I have is I don't have sufficient knowledge to chose a proper set.

* At max I expect a pressure of 300 Pa on one of the plates.
* The plate(s) will be about 10 cm x 10 cm and the rod will pass through the center of the plate.
* In reality both half of the plates will be affected by the flow. But with a conservative approach to calculate the torque I assume only one half is under the pressure. Therefore the total force is 300*0.1*0.05=1.5 N and the torque exerted on the rod is 1.5 * 0.025 = 0.0375 N.m = 3.75 N.cm
* The lateral force on the plate is 300*0.1*0.1=3N

Under these assumptions would something like a MG90S servo motor work for me? The datasheet shows a torque value about 2 kgf.cm which is higher than 3.75 N.cm.

https://datasheetspdf.com/pdf/1106582/ETC/MG90S/1

To be honest intuitively they seem very small and it gave me the impression that they may crack. Do you have any suggested servo motors?

In the poster below there are more concrete looking motors. I read the article of the author and it says they used servo motors. Can you recognize what type of servo motors are they?

http://www.hecekli.nl/docs/h_e_cekli_poster_tailoring_turbulence.pdf

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Lastly I still have the same question, if somebody can answer this I will be very grateful.

* Let's assume the motor (servo or stepper) can provide 20 N.cm torque.
* I define a positioning that requires 2 N.cm torque when there is no external force.
* An external force creates a torque of 10 N.cm in the same direction of the rotation.

Now, will it rotate faster than I have defined or will the motor balance the torque created by the external force?

 

[Baluncore]

Now, will it rotate faster than I have defined or will the motor balance the torque created by the external force?

The speed of a stepper motor will be limited by the step rate. If insufficient torque is available, the stepper will fail to advance, or will be pushed ahead, and lose its reference direction.

The speed of a servo motor will be determined by the position sensing feedback loop. A servo controller will always know where the motor is, and where it should be, the sign of the error will be used to drive it in the required direction. If there is insufficient torque the system will stall, then it will pick up again when the torque falls on the return stroke. The servo motor is capable of very high speeds when it is able to recover from a stall.

You have over-parameterized your question.
What is the range of x. What is the range of t?

 

[Zalajbeg]

The speed of a stepper motor will be limited by the step rate. If insufficient torque is available, the stepper will fail to advance, or will be pushed ahead, and lose its reference direction.

The speed of a servo motor will be determined by the position sensing feedback loop. A servo controller will always know where the motor is, and where it should be, the sign of the error will be used to drive it in the required direction. If there is insufficient torque the system will stall, then it will pick up again when the torque falls on the return stroke. The servo motor is capable of very high speeds when it is able to recover from a stall.

You have over-parameterized your question.
What is the range of x. What is the range of t?

Thanks for your answer. Then I understand that a stepper motor will not be pushed ahead and will be able to maintain its step rate even though there is an external torque in the same direction of the rotation if a sufficient torque is available.

For x, my cases will fall between 0 to 90 degrees (i.e. -30 to +30 degrees, -75 to +75 degrees but not -120 to +120 degrees)
For t, my cases will be around 0.5 to 1 seconds (i.e. -90 to +90 degrees in 0.5 seconds)

 

[Baluncore]

For x, my cases will fall between 0 to 90 degrees (i.e. -30 to +30 degrees, -75 to +75 degrees but not -120 to +120 degrees)
For t, my cases will be around 0.5 to 1 seconds (i.e. -90 to +90 degrees in 0.5 seconds)

The simplest solution would be a rigid pendulum rod attached to the control shaft.
Set the period by adjusting the position of the weight.
Set the angle by raising the pendulum to that angle before you let go.
Increase the weight if it is damped too quickly.

An 0.5 to 1 second will need a maximum of one metre to swing, so you might need a shaft extension and support bearing.

 

[Tom.G]

For the servos, all I can get from the Eindhoven University paper and some photometry is the dimensions of the surface facing the camera, and then only about ±5% accuracy: 5.6 x 6.8cm ( between 5.3 x 6.5 to 6.0 to 7.7cm)

Cheers,
Tom