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Mon 19:59

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Q: How to obtain torque constant for stepper motor?

AmirmkrI want to measure the torque constant of a stepper motor but seems I am doing it wrong. I have an MST342C02 stepper motor (200 step/rev and 4 phases) with SMD42C2 Driver. I have a converter that gives 48VDC 12.5A from 110VAC. I have Teensy 4.1 as the microcontroller and use Accelstepper library. …

stepper-motor torque teensy

tlfong01

tlfong01

6980

Your question is a bit confusing: (1) “motor is stationary the current is 0.4 A” – When you apply power with no load (free running), the not load current is usually small, perhaps 400mA. (2) But if load is more than maximum, or when you force it to “stall” (in your word “stationary”, than the stall current is very big, perhaps a couple of Amperes. (3) PWM255 means high number of steps per seconds, meaning high speed, but not necessary high torque. (4) To measure things, I would suggest to set full stepping mode (ie, no microstepping).

(5) Some useful references: (a) NEMA34 Step Motors MST340, 341 and 342 Product Data – JVL jvl.dk/files/pdf-1/datasheets/ld0042gb.pdf (b) SMD42C2 Driver Datasheet – Motion Control 247motioncontrol.com/media/1230/smd-42.pdf (c) Microstepping myths – MachineDesign 2003oct09 machinedesign.com/archive/article/21812154/microstepping-myt‌​hs

Amirmkr

Amirmkr

11

(1) When the motor is stationary it has holding torque that can hold 9 Nm. (2) Stall happens when more than 9Nm is applied. (3) if you look at the driver manual we have a moving current pin that connects to the microcontroller. The manual says it gets 0-2.5 VDC using the PWM, but using the 3.3VDC digital pins on Teensy I got to have all values from 0-255. This moving current has nothing to do with the speed but the torque. The speed is another pin on the driver named stepclock. By setting 10 for the driver resolution, and knowing 200 step/rev for the motor, 2000 ministep/s equals 1 rev/s.

tlfong01

tlfong01

Question: You said “if you look at the driver manual we have a moving current pin that connects to the microcontroller.” Are you referring to the driver manual of my (5) (b) Above? I cannot find the “moving current” pin!

Amirmkr

Amirmkr

That’s “move current” pin 12 on the driver, sorry I said moving current. (5)b is the motor datasheet. For the driver manual use the following link: jvl.dk/files/_2011clean/pdf/lb043gb.pdf

tlfong01

tlfong01

6980

Oh my goodness, so there is a driver user manual beside the driver datasheet, and this is the root cause of confusion. Updated reference list v0.1: (i) SMD42C2 Driver Datasheet (Manual) – Motion Control 247motioncontrol.com/media/1230/smd-42.pdf (ii) SMD41 and SMD42 Step Motor Ministep (MicroStepping) Drivers User Manual – JVL Industri Elektronik A/S 1998-2005 jvl.dk/files/_2011clean/pdf/lb043gb.pdf, /to continue, …

(iii) NEMA34 Step Motors MST340, 341 and 342 Product Data – JVL jvl.dk/files/pdf-1/datasheets/ld0042gb.pdf (iv) Microstepping myths – MachineDesign 2003oct09 machinedesign.com/archive/article/21812154/microstepping-myt‌​hs / to continue, ….

You might like to let me know if you have a scope to display the waveforms of the stepper motor coil current. A cope is useful to explore the nature of the coil current, especially for microstepping mode operation. If you don’t have much experience in experimenting with microstepping, chopper drives, you may need to read some basic tutorials. You might also like to skim my chats with other forum news, to how to test microstepping/chopper mode operation currents and torques etc. / to continue, …

tlfong01’s chats on microstepping motor current and torques: (a) chat.stackexchange.com/rooms/134387/… (b) chat.stackexchange.com/rooms/133950/… (c) chat.stackexchange.com/rooms/133829/… (d) chat.stackexchange.com/rooms/133206/… (e) chat.stackexchange.com/rooms/134572/…

Amirmkr

Amirmkr

11

Mon 19:59

I checked the chats, I don’t want to make it that complicated. I have access to a range of speeds from 0 to 600 RPM and I can quantify the torque using PWM, my main problem is to find the value of torque using current by having the torque constant.

tlfong01

tlfong01

6980

Ah yes, the chat records are only my suggested solutions workarounds for a coupe of EE SE questions/problems. Your problem seems simple but not very clear. Perhaps we can make it more precise. (1) What do you mean by “torque constant“? (2) Are you assuming that torque is directly/linearly proportional to coil current? For my two questions above you might like to give answers with maths equations and charts. In your answer, you might need to specify the test condition, ie, (a) are you using microstepping, such as full, 1//2, 1/4 etc? , (b) Are you using AccelStepper?

Amirmkr

Amirmkr

11

(1) I assume that T=k*i since it is a DC motor (k: torque constant) and (2) torque is a linear function of the coil current at a certain speed. In the main problem statement, I mentioned that I use AccelStepper library. Also, earlier I mentioned that I use resolution or ministeps/fullsteps is 10. With the knowledge of 200 fullsteps/rev for the motor, when I set stepper.setMaxSpeed(2000), it goes by the speed of 1 rev/s. Thank you for helping.

tlfong01

tlfong01

6980

Your assumption of t = k * I is a bit confusing, because it oversimplified, and the real world is more complicated. I would suggest to use JohsonElectric’s motor docs as a reference: (6.1) DC Motors: Classification in JE Motor Range – JohnsonElectric johnsonelectric.com/en/resources-for-engineers/dc-motors/… (6.2) DC Motors: Overview – JohnsonElectric johnsonelectric.com/en/resources-for-engineers/dc-motors/…. / to continue, …

(6.3) Stepper Motors: Classification in JE Motor Range – ohnsonElectric johnsonelectric.com/en/resources-for-engineers/stepper-motor‌​s/… (6.4) Stepper Motors: Overview – JohnsonElectric johnsonelectric.com/en/resources-for-engineers/stepper-motor‌​s/…

Your classification of your step motor as a DC motor is a bit too vague. When you are using microstepping mode, you are chopping the current and the k = f(i) is a bit complicated. (7.1) Stepper Motors: Driver Electronics – – JohnsonElectric  johnsonelectric.com/en/resources-for-engineers/stepper-motor‌​s/…

Amirmkr

Amirmkr

My last question, let’s look at the graph in the motor datasheet. For example, the motor runs at 300 RPM, the graph shows 7 Nm, is this for the max torque at that speed (setting PWM = 255 for the move current)?

tlfong01

tlfong01

6980

You seem to have mixed up Stepper motor with DC motor. It is true that for DC motor, (a) Torque generation: T = k * I, (b) DC Motor Equation: ω = U/k – R/(k**2) * T (Speed is highest at no load condition and decreases with rising load torque) (8) (8.1) DC Motors: Performance Curve (Load, Current, Speed, Torque, Efficiency, Rated operating point) – Johnson Electric johnsonelectric.com/en/resources-for-engineers/dc-motors/…

OK. Now let us focus at the Stepper Motor and Stepper Motor Driver: I have an MST342C02 stepper motor (200 step/rev and 4 phases) with SMD42C2 Driver

And let us use this picture for reference in our further discussion: jvl_mst_342_spec.jpg imgur.com/gallery/w8YbtFd

18 hours later…

tlfong01

tlfong01

6980

yst 13:58

An update of the setup photo.

user image

tlfong01

tlfong01

6980

yst 14:57

Now let us agree on how to setup, test, measure to torque constant of the JVL MST342 stepper motor.

user image

tlfong01

tlfong01

6980

yst 15:19

user image

user image

Please confirm if I understand your setup/wiring correctly.

@Amirmkr It is not clear what your PWM = 255 means. I guess you are using Adruino/Teensy4 PWM 255 on top of the AccelStep which would make the control signal a bit completed.

I usually test by hand, manually, use PWM signal from a PWM/square wave sign gen (See my chat records for more details) So I would put away the MPU Arduino/Rpi, and use 8kHz (ie, 8000 steps per second) to the step input of driver, and dc 0V/5V direction signal, …

tlfong01

tlfong01

6980

yst 15:59

Confusion between Torque and Torque Constants

You question is on how to find torque constant, but our discussion so far only talks about torque, not torque constant. For a particular DC motor product, its torque constant is a constant, a fixed value, a ratio based on the architecture of the particular motor.

On the other hand, torque (not torque constant) is a variable, depending on a couple of things. In other words it is a (dependent) variable depending on other (independent) variables. The following graph shows the independence.

tlfong01

tlfong01

6980

yst 16:26

user image

5 hours later…

tlfong01

tlfong01

6980

yst 21:36

Now, how to obtain torque constant from the datasheet.

user image

Reference: Stepper Motor Two-Phase – ni
zone.ni.com/reference/en-XX/…

13 hours later…

Amirmkr

Amirmkr

11

10:29

@tlfong01 For the move current we need to have a value of 0-2.5VDC (Section 1.4 Driver Manual). This value is chosen by PWM in a digital pin from Teensy 4.1. This value has nothing to do with Accelstepper. As this library only deals with position, velocity and acceleration.

Also by looking at the manual, we see there is another pin for stand-by current. I use this pin with PWM to quantify the holding torque; Howver, today I measured that by setting it to PWM = 255, the holding torque only goes to 6 Nm not the 9 Nm that is in the motor Datasheet.

@tlfong01 My main problem is to find a way to know the Torque at a defined speed, by measuring the current.

@tlfong01 Regarding this, in the main question page I wrote that we would have 1.41 * 9.5 = 13.4 A. And for the torque constant we have (Holding Torque)/current = 9/13.4 = 0.67.

2 hours later…

tlfong01

tlfong01

6980

12:47

Let me read the manual again.

SMD41 and SMD42 Step Motor Ministep Drivers User Manua – JVL 1998~2005
jvl.dk/files/_2011clean/pdf/…

Now the relevant pictures.

user image

2 hours later…

tlfong01

tlfong01

6980

14:49

@Amirmkr There is slight problem here: Have you used a scope to display the Teensy digital pin with PWM 255 signal, to make sure it is an analogue DC signal?

@Amirmkr Well, you might have mistunderstood the adjective “standby” in the standby current. Are you sure it is related to the “holding” of the holding torque?

@Amirmkr As I pointed out earlier, you might have mixed up the concepts with (1) DC motor and stepper motor, (2) torque and torque constant.

@Amirmkr We need to first make sure that we agree on the definition of torque and torque constant, which I pointed out earlier, are very different things. The below Singapore Uni EE&CE prof’s YT gives a good explanation/clarification:

Torque constant and back-EMF constant – Prof Aaron Danner, EE and CE Dept, National University of Singapore
youtube.com/…

tlfong01

tlfong01

6980

15:35

Talk is cheap, let us calculate:

ChiHai Motor CHW4632-370 permanent magnetic worm reducer motor power off self-locking 6/12/24V (6mm shaft dia) – AliExpress US$7 ~ 11
es.aliexpress.com/item/…

user image

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