11:59 AM
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I 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. ...

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

(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.

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!

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

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, ...

11:59 AM
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.

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?

(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.

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/…

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)?

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