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".
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