5 Apr
2015
5 Apr
'15
9:54 a.m.
On 04/05/2015 02:55 AM, Robert Jarratt wrote:
Do the gate signals not have the wobble? (Wobble could be
the PWM of the speed control logic, so could be normal.)
If so, then the FET may be bad. In fact, the FET may be
entirely OPEN, and the waveform you see is coming from the
other two driven phases.
Kind of strange that one gate signal is a smooth ramp, the
others have some wave on the top.
But, the drive probably has a current sense resistor in the
common source leg of the low-side transistors, so that is
feeding some of the current waveform back to the gates. So,
it may well be the one with the linear ramp on the top
indicates the drain is open.
Jon
-----Original Message-----
From: cctech [mailto:cctech-bounces at classiccmp.org] On Behalf Of Jon Elson
Sent: 04 April 2015 22:18
To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic Posts
Subject: Re: RD54 Stopped Spinning
On 04/04/2015 04:06 PM, Robert Jarratt wrote:
sensors to
So when I fired it up again with the scope, the
disk started to spin.
This was a stroke of luck as I was able to probe what I believe were
the outputs of the hall sensors. All three oscillated, but one of them
showed significant sideways wobble on the scope, which suggests to me
that the signal is not absolutely regular. It does seem to suggest
that one of the sensors is not working well. However, would this be
enough to explain why it stopped dead in its tracks when it was
working before?
Yes, the logic is generally that you decode 6 legal states of the
Hall decide which of the 3 motor terminals to drive
high and low. if the Hall
sensors
give a signal that has all 3 high, or all 3 low,
the decode logic will
fail, generally
leaving all transistors off. If it gets in that
state again, a vigorous
twist of the
drive around the spindle axis might shift the
rotor to a position where it
will
start up again.
few
Changing one of these sensors is probably going
to be beyond me :-(
Thanks Rob
I'd get the data off it quickly and retire the drive. You may only
have a minutes run time before the sensors go more flaky
and the drive shuts off.
But,
now that you know the secret, you ought to be
able to recover data, if
that is
the plan.
Anyway, it seems you have completely diagnosed the problem.
Jon
I checked again this morning, because something was bothering me. I realised
that the pins I was observing with the scope were the D pins on the FETs and
not the hall effect sensor outputs. When I went back to measure the sensor
output properly the outputs looked fine and perfectly stable. The outputs
from the Z8 also looked fine. The outputs of the inverters into the FETs
looked stable, although not square (pics here: http://1drv.ms/1a39cwz here:
http://1drv.ms/1a39kfs and here: http://1drv.ms/1a39pju). It is only the
output of one of the FETs that looks a bit unstable, but that could be a
triggering problem because the signal is a bit irregular (pic here:
http://1drv.ms/1a39ymO), the other two FETs have similarly shaped outputs
but do not wobble in the same way. I wonder if replacing that FET might do
the trick? Or is the inverter the problem as the inputs are perfectly
square?