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

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "AbbyKatt" <cnc@a...> wrote:

> All fancy stuff aside... :)
>
> Would my original idea of just putting a heat-sensitive resistor on

my

> motor which would trip E-stop when the outer casing exceeded 50 or

75

> degrees centigrade be a worthwile ploy?
>
> ~Abby

It's kinda like putting a heat sensor on the side of your house to
detect an internal fire. There could be conditions where the thermal
lag of the coils having to heatup the case would be greater than the
time to cause internal damage.

The measuring of the temperature rise of a series resistor is the
measurement of power (wattmeter). The thermal lag of the resistor
will cause error's but would work to integrate the instanteous
current measurement.

The difference in the two methods is just in the time it takes to
change the reading. In the case of the resistor you are measuring
total current draw of the motor whereas in the temp sensor on the
motor, you are measuring the heat loss (inefficiency) of the motor.

I think for a worst case scene where you want to shut things down to
keep from frying a motor, the simple case sensor (you will need some
electronics to set a switch point since the reading would be a semi-
linear output) is sufficient, but if your intent is detect a fault
condition (jammed motor, long term overload) than something with a
faster response is in order.

>
>
>
>
> > -----Original Message-----
> > From: R Rogers [mailto:rogersmach@y...]
> > Sent: 12 December 2004 16:47
> > To: CAD_CAM_EDM_DRO@yahoogroups.com
> > Subject: 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@a..., timg@k...
> > Moderator: pentam@c... indigo_red@q...
> > davemucha@j... [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]
> >
> >
> >
> > 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@a..., timg@k...
> > Moderator: pentam@c... indigo_red@q...
> > davemucha@j... [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