[TriLUG] cdrom: does pushing the tray in damage them?

[Aaron S. Joyner]

On Mon, 9 Oct 2006, mike shlitz wrote:

> Hi All,
>
> I was told and/or read somewhere long ago (around the
> time cdroms came out), that one should ALWAYS use the
> button to effect closing and NEVER push the tray to
> close  it. 

So let's do a quick analysis of the mechanics involved*, and see if we 
can come to any logical conclusions.  The tray-moving mechanism of a 
CDROM is simply a small electric motor, with a set of big step down 
gears to allow the relatively fast-moving motor to slowly and evenly 
eject the tray.  There are typically two sensors at both ends of the 
travel, to detect when the drive has fully extended, or fully 
retracted.  When you press the button on a closed drive, the micro 
controller inside the drive senses the closure of the switch contact, 
and sends power to the motor in the appropriate polarity to spin the 
motor in the right direction to open the tray.  It continues to apply 
the same power until it detects a closure of the switch which indicates 
the tray has reached full extension, at which point power is removed 
from the motor.

There are then two options for closing the tray, as discussed.  You can 
push the button, and the process proceeds as above, just in reverse.  
The micro controller notices the switch closure of the front panel 
button, and activates the motor in reverse, until detecting the closure 
of the switch that indicates the tray has been fully retracted.  The 
other option is you push on the front of the tray.  There are a couple 
ways I can think of which might be used to detect this pressure.  The 
simplest would be to observe the fact that pushing on the tray turns the 
big gear, which spins the motor, which will generate a tiny fraction of 
current on the motor's power input.  You could observe this with an 
analog pin on the micro controller, and upon detecting such a current, 
follow the normal "retract the tray" sequence.  Some other options would 
be a pressure sensor mounted to the tray, or mounting the entire tray 
and housing on a slightly mobile platform, such that when you push on 
the tray, you shift the tray and motor housing itself against a switch 
in the back of the unit... but neither of these options probably work in 
the typical CDROM housing, as you need to securely spin the disc at 
thousands of RPM, so your platform needs to be steady.

So, presuming that your drive has implemented the logical "detect the 
push with the motor" approach described above, would it be likely to 
exhibit failures in the way you describe?  Let's analyze what's 
happening in your failure state.  You push the button, or on the tray, 
and the closure sequence begins.  Power is applied to the motor, by the 
micro controller, until it detects the switch closure indicating the 
tray has been fully retracted, and stops applying power to the motor.  
In your case though, I'd be willing to bet that the motor doesn't stop 
gracefully, it strains for a second, then reverses its direction, 
opening the tray.  This is consistent with the closure sensor not being 
tripped, and the micro controller either timing out from knowing it ran 
the motor way to long, or detecting the change in resistance of the 
motor as an obstruction, and opening the tray so as not to continue 
crushing your fingers.  To boot, nothing about how the start of the 
closure cycle was started, should have any effect on how the closure 
cycle ends.  I can try to envision some arcane designs that *might* have 
some correlation of failure between those two states (some really 
delicate current measurement circuit, which measures the change in 
resistance of the motor to detect the tray stoppage, and is some how 
sent into an over-voltage condition by your push on the tray), but 
generally all of those designs are so ridiculous as to never make it 
into production, especially since the fact that pushing on the tray is a 
generally accepted method of closing the drive, manufacturers aren't 
blind to this, and are going to design with it in mind.

So in conclusion, my guess is that there is no likely correlation 
between these two events (pushing in the tray, and failure to detect 
closure of the tray), and that all of the observed correlations are 
coincidence, brought about by the common practice of closing the tray by 
pushing on it, and the fact that all things with moving parts fail 
eventually.

Aaron S. Joyner

* - I am conveniently ignoring the mechanics of what the CDROM's actual 
purpose is, and focusing only on the parts which open and close the 
tray.  Feel free to Google if you want to know about the workings of the 
much cooler parts involving "sharks with frickin' laser beams attached 
to their heads".