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

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

> -----Original Message-----
> From: R Rogers [mailto:rogersmach@...]
> 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
>
>
>
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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.
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