HDD motor as a servo.

[dr pepper]

Heres an interesting project I did for fun but might find usefull.

A 3 phase hard disk motor servo with a arduino, the servo accepts position commands over serial, 0 - 1440 so 0.25 degree accuracy.
As is the servo counts the position at turn on as zero, it would benefit from a photo interrupter and a run to zero at turn on subroutine.
The motor in the vid is a bit notchy as its running of 5v, I couldnt be bothered using a bench supply as I was on my desk at work, the motor is just going to random positions.

 

[Nigel Goodwin]

Nice, apart from the misleading title

It's an HDD motor as a stepper motor - nothing like a servo as there's no feedback, and thus no positioning mechanism.

 

[kal.a]

It's neither but nice project nonetheless.

 

[dr pepper]

Aw Spoilsport.

Ok then its a open loop servo, there is such a thing in fact they are common.

I was demonstrating you can do interesting things with dead computer bits.

 

[MaxHeadRoom78]

Ok then its a open loop servo, there is such a thing in fact they are common.
.

A stepper motor is an example of a open loop servo!
Max.

 

[dr pepper]

Yes I was just thinking that.
Analogue car instruments, older desktop printers, cd players are all examples of stepper motor servo's (I spose you could say the cd has feedback from the disk).
Mach 4 which is a enthusiast cnc machine control system uses steppers on its axes too.
It could be argued that a bldc motor and an aircore motor are steppers, bldc's are engineered to be less notchy than steppers as they tend to replace brushed motors where smoothness is important.

 

[MaxHeadRoom78]

Actually BLDC have More 'cogging' effect than a stepper due to a lower pole count, typically a maximum of eight pole, they are smoother in a CNC application where PID loop via encoder feedback provides a finer resolution, but when used as open loop velocity control, the cogging can be seen at lower rpm.
Max.

 

[dr pepper]

I think it depends on the motor, I have a box of salvaged ones you cant tell much cogging by hand, but then thats without power applied.
At speed with any kind of rotating mass it would be damped anyway.
My servo program uses a sine wave lookup table and 'microsteps' the motor so cogging is low, if I implemented current control, like use a chopper controller ic then it would be really smooth.

 

[Nigel Goodwin]

cd players are all examples of stepper motor servo's (I spose you could say the cd has feedback from the disk).

But of wrong and right there

CD players don't use stepper motors at all, they use common little brushed DC motors - with the speed of the disk tightly controlled in a servo loop from the information read from the disk (this is why they tend to go excessively fast if the disk can't be read).

 

[dr pepper]

I meant the lens traverse, the older ones I've pulled are steppers.
The disc motor is a bldc yes.
You'd know that better than me Nige.
One of my experiments with a cd stepper:

 

[Nigel Goodwin]

I meant the lens traverse, the older ones I've pulled are steppers.
The disc motor is a bldc yes.

No, not BLDC - both disk and traverse motors are commonly just cheap brushed DC motors, in almost all players, and certainly anything that isn't completely ancient

The very first (incredibly expensive) players used various schemes, including some with linear motors.

However, I notice your video is a CD-ROM, and not a CD Player - but it's likely to be a pretty early one if it has a stepper.

 

[dr pepper]

My bad, assumption made that cd players use the same mech's as rom's, I have dismantled a couple of dvd players that had cd rom mech's in, but I havent pulled a cd player apart for a while now.
The pc I'm using now has a tiny little dc brushed motor on the traverse for the cd rom.
I made a edm drill using that mech above, worked well.

 

[throbscottle]

I've pulled a few PC CD/DVD drives apart - spindle motors are commonly BLDC, though the old ones are brushed. With the CD drives the traverse motor tends to be brushed with lots of gearing, with DVD drives it is a stepper with a long worm screw. Door motor tends to be brushed on both types, though I think I may have seen a BLDC one of those too. Of course there's no such thing as a modern CD/R drive now they're all DVD/R, so age is relative anyway.

I'm wondering whether to dismantle an old floppy drive. What's the betting if I do so I'll need it for something?
Actually did wonder about adding one of them to my work toolkit (company provided me with a crappy USB one). Some archaic stuff being used in modern retail!

 

[dr pepper]

I think they use bldc for torque and speed, cd's and dvd drives whizz round these days.

I have a few 3 1/2" floppy's, take note allthough there is a stepper, later ones are like cd rom steppers there is only one bearing in the motor, the other is the one at the end of the track, once pulled to bits they fall apart.

 

[Nigel Goodwin]

I have a few 3 1/2" floppy's, take note allthough there is a stepper, later ones are like cd rom steppers there is only one bearing in the motor, the other is the one at the end of the track, once pulled to bits they fall apart.

You want a nice OLD 5.25" one, they have really nice steppers in them

 

[throbscottle]

I did once, Nigel. I kept all the parts for a long time even though I wasn't doing any electronics, then had a clear out thinking I was never going to bother and got rid of most of my stash, year or two later got I had an itch I just had to scratch (faulty multimeter) and I've been regretting that clear out ever since

 

[dr pepper]

I used a couple of 5.25's with my beeb computer, airpax 48 step motors.
Hdd's used to have steppers before they went voice coils.