Building semicontinuous minicentrifuge with HD motor.

[Markos]

Hi,

I'm working on building a circuit (with MOSFET) and a software (with
Arduino) to control HD motors to be used in a DIY "semicontinuous"
minicentrifuge.

Has anyone worked with these motor? (spindle motor, bldc motor, used in
Hard Disks)

I would like to exchange information with someone with experience in
controlling these motors.

Thanks,
Markos

[Nathan McCorkle]

I've done some microstepper code using the hardware timers of an
arduino, and I know a friend that did some BLDC code on a similar
processor using back EMF.

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-Nathan

[Markos]

Dear Nathan,

I am using a motor with 4-wire and the resistance measurements were:
http://www.c2o.pro.br/en/automation/download/wiring_HD_motor.png

To control I assembled the circuit:
http://www.c2o.pro.br/en/automation/download/BLDC_controller_4_wires_MOSFET.jpg

And I'm using the code (Arduino):
http://bartvenneker.nl/schemas/Code_arduino_hdd.txt

That I found at:
https://www.youtube.com/watch?v=CMz2DYpos8w

Using a encoder from a mouse and a oscilloscope I measured a speed of
3500 rpm.

I have yet to fix two plates of acrylic (plexiglass) which will increase
the weight and I'll probably lose speed.

If I reduce minStepLength less than 1200 than the motor stops turning.

How can I optimize this code to increase the speed?

Thanks for the attention,
Markos

[Nathan McCorkle]

First off, the digitalwrites in that code are sequential, and I'm
almost certain it would be better if they were happening
simultaneously. If this is so, then this will certainly reduce the
maximum speed.

Check out this to get simultaneous pin switching/setting:
http://www.arduino.cc/en/Reference/PortManipulation

It probably isn't required for your purposes, but microstepping can
increase the smoothness of the rotation, though it is probably more
important in low-speed applications, but if you have the time to do
microstepping it won't hurt to do so (i.e. instead of step-transitions
between high and low, there is a curve of power applied, which can be
emulated using PWM (which because of the inductance on the motor
windings should actually smooth out the PWM spikes)):
http://arduino.cc/en/Tutorial/SecretsOfArduinoPWM

I have some arduino code that sets up the sine lookup-table (sine wave
is pretty smooth, so that's what they use), and anytime there's a
transition on the coils, instead of just turning one off and another
on instantaneously, it performs the transitions using PWM (starting at
0% duty, e.g. full-off, progressing up to 100% duty, e.g. full-on). I
can post this code later tonight.

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-Nathan

[Jonathan Cline]

Have you considered using DC brushless motors used on R/C airplanes?  That's what I would look into.  These motors were originally hard drive motors rewired by hand with additional windings.   They often are bundled with speed controllers.  They will also have great starting torque which could be your #1 design problem for a DIY centrifuge.


## Jonathan Cline
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## Mobile: +1-805-617-0223
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[Markos]

Dear Nathan,

Thanks for your attention.

Markos