Re: [CAD_CAM_EDM_DRO] Servo heat-sensors or paranoia? servo protection questions

A fuse will prevent fires and circuit traces being burned off the board
if a transistor
fails in the drive. But, a fuse is a terrible way to protect against
overloads in a servo
application. The higher-end drives have what they call RMS protection.
They integrate
I^2 over time, as I^2 will be proportional to motor heating. So, these
drives might
allow 20 amps to flow for 2 second, 10 amps to flow for 8 seconds, but
only 5 A
to flow continuously.

A properly selected time-delay fuse would protect the motor fine. The
problem is
that the design of the Gecko drive cannot handle the arcing that would
happen
at the moment the fuse blew. It would have a high probability of
popping transistors
in the drive. That probably applies to Rutex and similar drives, as well.


////The thermistor/resistor idea you came up with sounds pretty good. I worked on an Binks high pressure airless sprayer recently(made all new parts and rebuilt it for a neighbor) Anyways, It had an electronic device that was broken. It looked similar to a Solid state bridge rec. How it worked, it was wired in to the motor circuit and when the pressure in the piping reached a certain amount it would sense a high load on the line and break the circuit. Thereby stopping the motor untill the pressure dropped again. maintaining pressure at the nozzle. I sent him up to RS to get one and I cant remember what it was called. Was this a thermistor? I think it had another name and it was a common control component. It had a coil of sorts and a microprocessor inside, if I remember correctly. They knew exactly what it was. And would it work to protect in the event of crash/stalled servo? The device that you outline below would be a great add on for us large servo on a Gecko users. I'd buy three
of them straight away.

>
>Also, what is actually happening with an encoder when a servo is at idle? There is 5v, ground, A channel and B channel. Is there a constant A or B signal back to the drive or is it simply a fast pulse and the drive waits for another to move, correct, etc.? Seems there could be some type of device that monitors and in the event of lost encoder activity which would unlatch the E-stop circuit.
>
>

At idle, most drives tend to rattle back and forth across some deadband
or region where the DC gain of
the servo loop is unable to cause motor movement, due to friction. This
is usually a very small distance,
on the order of +/- 2 encoder counts or so. A circuit to detect a
failed encoder without causing false
trips could be quite hard to do. Comparing a differential encoder's
outputs to make sure the two
complementary signals are actually at opposite logic states might be a
first step to detect broken
cables, popped fuses and actual failure of the encoder circuitry. But,
that would totally fail to detect
an axis jammed against the travel limits.?

///Two different devices, one to protect against crash/stalled servo. And another to monitor encoder presence to prevent the runaway servo issue in an encoder failure scenario. Are the two main safety concerns that appear to exist with this type of control configuration. If I understand it correctly. Lost steps, are not a concern as this appears to never happen and isn't much of a safety concern rather a quality of operation issue. Could a device be designed that would monitor the 5volts to the encoder and A or B signal back regardless of the counts and some how break an Estop circuit? Breaking the motor circuit between the drive is out because as you mention arcing as would result in switching would possibly damage the drive. This would protect in the event of lost current to and/or signal back from encoder resulting in a runaway. If these two devices were available, they would be a definite neccessity for large servo users. Encoders rely on circuitry, mechanical alignment and
mounting, of which all three have a marginal probabilty of failure. It's not an if,just when.

///



One possible scheme would be to put a very small value current sensing
resistor in series with the
motor wires. Because this resistor is rapidly switched between ground
and the DC power supply
voltage, circuits to monitor the current are a bit tricky. But,
monitoring the TEMPERATURE of
the resistor with a PTC thermistor might be quite simple! These PTC
resistors have a very abrupt
rise in resistance when they are heated above about 50 C. You could
string them in series and
glue them to some .1 Ohm (for low-current motors) down to .01 Ohm power
resistors for big motors.

This would basically implement a true RMS current limiting scheme. The
string of PTC thermistors
could be wired in series with the e_stop switches, if their cold
resistance was low enough. This is
so simple, I'm probably going to put it on some of my machines! I will
see how much it would cost
to add it to my PWM servo drives for the next revision.

Jon



Addresses:
FAQ: http://www.ktmarketing.com/faq.html
FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com

Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@..., timg@...
Moderator: pentam@... indigo_red@... davemucha@... [Moderators]
URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO

OFF Topic POSTS: General Machining
If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
http://www.metalworking.com/news_servers.html

http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.

NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
bill
List Mom
List Owner


Yahoo! Groups Links











---------------------------------
Do you Yahoo!?
Yahoo! Mail - Easier than ever with enhanced search. Learn more.

[Non-text portions of this message have been removed]