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Hard drive clock.

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dr peppers

New Member
I want to build a hard drive clock synced to the msf time signal from anthorn, but to see how it looks I thought I'd build a clone of alan parekhs hard drive clock off the net.
Unless I've missed it I didnt see what speed drive alan used, anyone know, my drive is a 7200rpm, I suspect alans is a 5400 or whatever the lower speed one is.
 

be80be

Well-Known Member
I have used a 7200 rpm I made my own controller so I could slow it down worked out ok
Fun to play with.
 

dr peppers

New Member
A while back I had a go at a hard drive motor controller, but couldnt get it to work, I just provided 3 phases and attempted to start slow and speed up, but I could only get a few rpm out of it.

How did your controller work, did you use hall effect position sensors, or armatire feedback or something?, I'd be interested to know, one thig about your own controller is that the opto slotted switch isnt needed.

Sounds like alans design does run at 5200 revs.
 
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be80be

Well-Known Member
I didn't get into all that I just use PWM and a pot to lower it down to speed that was clocked with a shaft rpm meter and then glued the pot
 

dr peppers

New Member
That explains things.
I'd like to build a hdd motor controller with a pic micro, I'm sure its been done.
I read an article that explains how to use a couple of tranys to provide armature feedback for a brushed motor, I was wondering if this idea would work on a brushless motor, I'd need to work out a way of switching all outputs off to the motor, then 'listen' and wait for a pulse back on one of the phases to get position information, but I couldnt quite think of a way.
I think you were right pwm it.
 

be80be

Well-Known Member
From what I have read you can set the speed real close by how you shift the legs so I went with pwm and set the speed with a rpm meter I wish I still had the rpm meter lost it back a few years ago.
 

Gobbledok

Active Member
If you are driving a brushless motor in open-loop and only get a few RPM, you are doing it wrong. Perhaps you have the polarity reversed on the coils or something (in comparison to the direction you are commutating the coils). I got over 10K RPM from a standard HDD motor in open-loop (no feedback).

Microchip has a nice App Note discussing the control of BLDC motors. In sensored and sensorless modes as well as open-loop and closed-loop. It details the method discussed by dr peppers.


BTW, for a 3 phase BLDC motor, each revolution requires 6 commutations.
 
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dr peppers

New Member
From the app note I am doing it wrong, my circuit used only 3 switching transistors, theres no devices switching to ground, I had the star point connected to ground and the trannys switched to +v.
Might try that again with 6 trannys.

I wondered why the star point is brought out on hdd motors, looking at that article its probably bemf sensing.

I got a little bogged down with some of the app note, but from what I've seen I'm gonna have more success changing the drive circuit as mentioned.
 

be80be

Well-Known Member
If you are driving a brushless motor in open-loop and only get a few RPM, you are doing it wrong. Perhaps you have the polarity reversed on the coils or something (in comparison to the direction you are commutating the coils). I got over 10K RPM from a standard HDD motor in open-loop (no feedback).
Where that come from Mine ran up to 7200 rpm and down to a stall
 

Gobbledok

Active Member
Where that come from Mine ran up to 7200 rpm and down to a stall
They are electronically kept at 7200RPM in a hard drive. It doesn't mean that they can't go faster.

~12K RPM was the fastest I could get it to go before it "went out of sync" with the controller. With a closed-loop controller I'm sure it could go plenty faster.
 

dr peppers

New Member
In my original project I hoped that keeping the load low, ie starting the drive slow and building up to speed slowly would work, I spose this might with a full bridge driver instead of the half bridge I used.
I'm also gonna take a look at the tda5140 chip thats designed to drive bldc motors and see if it simplifies things.
Would be kinda usefull circuit, I have a box full of bldc motors from vcrs, cr roms, hard drives and various bits, earlier ones have hall feedback, later ones do not.
 
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