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conversion between　kgm2 and As/rpm

kyt
Expert 2355 points
Other Parts Discussed in Thread: MOTORWARE

Although it is measurement of moment of inertia, on the SpinTAC screen, the unit is [As/RPM].
Usually, would you teach the conversion type, although it is [kgm2] and he thinks that it expresses?

I have checked thread,but the formula was not found.

http://e2e.ti.com/support/microcontrollers/c2000/f/902/t/317108.aspx

Supposing there is no formula, how should it relate?

  • 0
    Guru 12800 points

    Kyt,

    The SpinTAC unit for inertia takes into account the torque capabilities of the motor.  Hence it does not use an SI unit.  The conversion between A/(krpm/s) and Kgm^2 is availabe in the InstaSPIN-MOTION User's Guide: http://www.ti.com/lit/ug/spruhj1c/spruhj1c.pdf  See section 11.5

  • 0 in reply to Adam D. Reynolds
    Expert 2355 points

    Adam,

    On PC screen, moment of inertia is written as 10^-3 [As/RPM], and is.
    Is following PU/pu/s^2 the same as the above? Is conversion still more nearly required?

    best regards.

    kyt

  • 0 in reply to kyt
    Guru 12800 points

    Kyt,

    There is an additional conversion between As/RPM and PU/pu/s^2.  If you look at the code in MotorWare lab 05c, this conversion is taking place in the updateGlobalVariables_motor function.

  • 0 in reply to Adam D. Reynolds
    Prodigy 30 points

    I have a quetion to inertia calculation as well

    i am using DRV8312EVM Rev D and the Anaheim Automation BLY172S motor that comes with the kit.

    From Anaheim-data the Rotor Inertia is given J = 0.00068 oz-insec^2 = 0.00068*0.00706 =4.8e-6 kg m^2

    where 1 oz-insec^2 = 0.00706 N m Sec^2 = 0.00706 kg m^2.

    From Inertia measurement the result is 0.0235e-3 A/(rpm/s) = 1.41 e-3 A/(rps/s) which is equal to J/Kt

    Identified Inertia J = (J/Kt) * Kt = 1.41e-3 A/(rps/s) * 5.03 Oz-in/A = 1.41e-3*5.03*0.00706 N m sec^2 

    = 5.0e-5 kg m^2  (with 1 Oz-in =  0.00706 N m)

    The identified value is a factor of 10 of the value from motor. Perhaps i'm wrong, but where?

    Best regards

    Biao

     

    2

  • 0 in reply to Biao Liu
    Guru 12800 points

    Biao,

    You are not wrong.  I've also noticed the same thing for the Anaheim Automation motor.  I don't believe the value provided in that datasheet.  The way I've confirmed my calculation is by using an inertia that I've calculated based on mass & radius.

    We have a couple test stands where we can couple the motor to a flat plate with holes in order to mount weights on the plate to vary the inertia.  From this plate we can calculate the value for the inertia, and in our testing the value that we calculate is the same value that is produced.  

    It is unclear to me how Anaheim Automation arrived at the inertia value for that motor.

  • 0 in reply to Biao Liu
    Prodigy 30 points

    This is the reply:

    identified (J/Kt) = 0.0235e-3  A/(rpm/s) = 0.0235e-3*60  A/(rps/s)= 0.02353-3*60/ (2*pi) = 0.2244e-3 A/(rad/s^2)

    (we have to use rad/s/s consenquently instead of rps/s )

    J = (J/Kt)*Kt = 0.2244e-3 * 5.03*0.00706/sqrt(3) = 4.6e-6  kg m^2        

    (Kt is given in phase current by Anaheim, but  (J/Kt) is meaused possibly with line-current by TI)

    this is very close to Anaheim- Rotor Inertia of 4.8e-6 kg m^2.  TI made good work.

     

  • 0 in reply to Biao Liu
    Prodigy 30 points

    it's worth mentioning BLY17 has Delta winding thus the factor sqrt(3).