Measuring VCM and Spindle Motor Current

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fzabkar
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Measuring VCM and Spindle Motor Current

Postby fzabkar » Thu Apr 25, 2013 11:08 pm

Measuring VCM and Spindle Motor Current

This tutorial is intended to assist in the identification of test points for the measurement of voice coil and spindle motor current. By directly measuring the current, one can determine whether the motor controller IC is driving the respective motors, and whether there is anything abnormal in this area of the circuit. The current is determined by measuring the voltage drop across a current sense resistor and then applying Ohm's Law.

For reference purposes I have used the following datasheet for a typical SMOOTH motor controller:

L7250, SMOOTH, spindle motor + VCM controller, ST Microelectronics:
http://wandrew.regruppa.ru/PCInfo/TechDoc/L7250(Smooth).pdf
http://www.datasheetcatalog.org/datashe ... Xyuswx.pdf


Spindle Motor Current Sensing

The typical spindle motor is a 3-phase brushless DC type. Today's designs have 3 terminals, while earlier ones had an additional common terminal. The motor controller drives the motor by energising each pair of windings in sequence, while sensing the RPM via the back-EMF of the third winding.

The attached block diagram illustrates the current flow through the motor.

L7250_spindle_motor_block_diagram.jpg


There are 3 pairs of MOSFETS, one pair per phase. Two MOSFETs are turned on at any one time, while the remainder are switched off. One MOSFET switches the high side (the +12V supply) while the second MOSFET completes the circuit to ground.

The ground leg passes through a current sensing resistor. This resistor needs to be able to carry 1 or 2 amps of current, so it will be noticeably larger than its neighbours. Often there will be a parallel combination of 2 or more resistors to share the wattage. The equivalent resistance of the array will usually be less than 1 ohm. The motor controller senses the current by monitoring the voltage across this resistor.

For example, in the attached Fujitsu MPE3064AT circuit, there is an 0.30 ohm spindle current sense resistor.

Applying Ohm' Law ...

V = I x R = I x 0.3

... so a current of 1A would produce a sense voltage of 0.30 Volts.

MPE3064AT_motor_cont.JPG


The motor current sense resistor can be distinguished from the VCM current sense resistor in two ways.

- one end of the motor resistor will be grounded whereas the VCM resistor will be floating

- the VCM resistor will connect to one of the VCM pins at the preamp connector

The following tutorial has more examples of current sense test points:
http://www.users.on.net/~fzabkar/HDD/Tu ... 0411N.html

The spindle motor in the above tutorial has 4 terminals -- 3 phases plus a common. The phase-to-phase resistance is 3.8 ohms, and phase-to-common is 1.9 ohms. If any resistance measurement is dissimilar to the others, then this would suggest a problem with one of the windings.

A simple continuity test identifies the motor test points adjacent to the HA13627 motor controller IC.

    Pin #64 = common
    Pin #63 = phase A
    Pin #61 = phase B
    Pin #59 = phase C

Code: Select all

                 Common (Pin #64)
                   o
                   |
                   |
             +-----+-----+
             |     |     |
             |     |     |
             C|    C|    C|
   1.9 ohms  C|    C|    C|        Spindle Motor windings
             C|    C|    C|
             |     |     |
             |     |     |
             o     o     o
             A     B     C

       Pin #63  Pin #61  Pin #59


VCM Current Sensing

L7250_VCM_block_diagram.jpg


The voice coil is driven by 4 MOSFETs in an H-bridge configuration. The attached block diagram illustrates the current flow through the VCM. One out of each pair of MOSFETS is switched on while the other is switched off. The top MOSFET in each pair switches the high side (+12V) while the bottom MOSFET switches the grounded side. The diagram shows current flowing from top to bottom. If the "ON" MOSFETs were now to be switched off, and the "OFF" MOSFETs switched on, then current would flow in the upward direction. This is how the VCM is able to seek in both directions.

In the VCM case, the sense resistor (Rs) is directly in line with the voice coil itself (Rm + Lm). As before, the voltage drop across this resistor reflects the VCM current.


A Note About Resistor Markings ...

The "R" in a marking code of "Rxy" represents a decimal point. Therefore a marking of R33, for example, indicates a resistance of 0.33 ohm.

A Note About Resistor Arrays ...

When n resistors of equal value, R, are connected in parallel, then the combined resistance of the array is R/n, as measured between points X and Y in the diagram..

Code: Select all

          n x R

           ___
       |--|_R_|--|
       |   ___   |
       |--|_R_|--|
 X  O--|         |--O  Y
       |   ...   |
       |   ___   |
       |--|_R_|--|


         |-----|
 X  O----| R/n |----O  Y
         |-----|


ST313021A_resistors.JPG
ST313021A_resistors.JPG (16.86 KiB) Viewed 2246 times

fzabkar
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Samsung SP0411N (PANGO REV06) test points

Postby fzabkar » Thu Apr 25, 2013 11:28 pm

Samsung SP0411N (PANGO REV06) test points

Tutorial - how to locate the basic test points of an unfamiliar HDD PCB:
http://www.users.on.net/~fzabkar/HDD/Tu ... 0411N.html
Attachments
SP0411N_HA13627_3V3_Vcc2.jpg
SP0411N_Motor.JPG
SP0411N_Motor.JPG (44.48 KiB) Viewed 2245 times

hpwlab
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Re: Measuring VCM and Spindle Motor Current

Postby hpwlab » Mon Jul 08, 2013 8:02 pm

this is very goood
thank you

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Re: Measuring VCM and Spindle Motor Current

Postby Spildit » Tue Jul 09, 2013 2:08 am

hpwlab wrote:this is very goood
thank you


I fully agree !
1Q9xrDTzTddUXeJAFRn37aqh1Yr6buDCdw - (Bitcoin Donations)

LarrySabo
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Re: Measuring VCM and Spindle Motor Current

Postby LarrySabo » Tue Jul 09, 2013 3:07 am

Love it! Thanks, fzabkar! And very well explained, too.

Larry


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