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'Industrial Environments and PICs'
2003\02\17@171737 by Chris Loiacono

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I have several products that use PICs in industrial machinery - which have
been doing quite well except for the rare, but unpleasant instances below.
These PIC based designs have replaced other commercially available
controllers and PLC's and the like. I have noticed in a couple of machine
installations what seem to be brown outs and resets. I have had similar
occurrences back in the 'old' days when using PLC's - If the building wiring
is weak, or if an electician a bit careless, someone will turn on a large
machine at one end of the building and my PIC, like the PLC's will lock up.

I can tell customers that they need a small UPS for a PLC in this
environment, but it won't cut it with my own design...the idea was to
improve the process while SAVING $$.

I figure this must be a fairly common thing to deal with. I am a bit
surprised because these boards are phase angle controllers and have been
hardened (via snubbing, gating methods and filtering and decoupling) to
withstand the worst noise from the AC switching at peaks.
The lock-ups happen when the AC line is lower than normal, so I have changed
to a(n) LDO Regulator (Thanks, Olin, for your suggestions).
If these events still occur, I am left to conclude that there must be some
interruption of the line of significant duration.
Where might I look for strategies to deal with such a thing? I need 4.5V
<200mA to keep the whole thing running for the duration. Having no
experience with such things as super caps, I am wondering if this is a good
app for one....

C

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2003\02\17@174024 by Des Bromilow

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Is it worth building a very small UPS into the device? (maybe based on a NiCd pack, or some of those supercaps?)

>>> spam_OUTchrisTakeThisOuTspamMAIL2ASI.COM 18/02/03 8:17:00 am >>>
I have several products that use PICs in industrial machinery - which have
been doing quite well except for the rare, but unpleasant instances below.
These PIC based designs have replaced other commercially available
controllers and PLC's and the like. I have noticed in a couple of machine
installations what seem to be brown outs and resets. I have had similar
occurrences back in the 'old' days when using PLC's - If the building wiring
is weak, or if an electician a bit careless, someone will turn on a large
machine at one end of the building and my PIC, like the PLC's will lock up.

I can tell customers that they need a small UPS for a PLC in this
environment, but it won't cut it with my own design...the idea was to
improve the process while SAVING $$.

I figure this must be a fairly common thing to deal with. I am a bit
surprised because these boards are phase angle controllers and have been
hardened (via snubbing, gating methods and filtering and decoupling) to
withstand the worst noise from the AC switching at peaks.
The lock-ups happen when the AC line is lower than normal, so I have changed
to a(n) LDO Regulator (Thanks, Olin, for your suggestions).
If these events still occur, I am left to conclude that there must be some
interruption of the line of significant duration.
Where might I look for strategies to deal with such a thing? I need 4.5V
<200mA to keep the whole thing running for the duration. Having no
experience with such things as super caps, I am wondering if this is a good
app for one....

C

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2003\02\17@181336 by llile

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You need to use brownout protection at the PIC.  THen, when the PIC
reboots due to a brownout fault, you need to have something in the program
to deal with that situation.  Restart in a controlled fashion.

In software, you can also do a sanity check:  On startup, Set a number of
registers (the more the merrier) to a certain value then check to see if
they read that value back.  If not, the PIC should strangle itself with a
watchdog trimeout, reboot  and then test it's sanity again until it comes
out sane.

For a machine like this you should also consider real external
microprocessor supervisor chips.  THese can be as simple as a reset on the
MCLR line on low voltage, to a two-step affair which warns your PIC that
low voltage is coming, allowing it to save a bunch of stuff and shut down
in a controlled way, then a final low voltage signal that tells it to
shutdown.

Brownout situations can be dangerous, causing the PIC to lock up in an
unknown state, leave a load on too long, or do something unplanned.



-- Lawrence Lile





Chris Loiacono <.....chrisKILLspamspam@spam@MAIL2ASI.COM>
Sent by: pic microcontroller discussion list <PICLISTspamKILLspamMITVMA.MIT.EDU>
02/17/2003 04:17 PM
Please respond to pic microcontroller discussion list


       To:     .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
       cc:
       Subject:        Industrial Environments and PICs


I have several products that use PICs in industrial machinery - which have
been doing quite well except for the rare, but unpleasant instances below.
These PIC based designs have replaced other commercially available
controllers and PLC's and the like. I have noticed in a couple of machine
installations what seem to be brown outs and resets. I have had similar
occurrences back in the 'old' days when using PLC's - If the building
wiring
is weak, or if an electician a bit careless, someone will turn on a large
machine at one end of the building and my PIC, like the PLC's will lock
up.

I can tell customers that they need a small UPS for a PLC in this
environment, but it won't cut it with my own design...the idea was to
improve the process while SAVING $$.

I figure this must be a fairly common thing to deal with. I am a bit
surprised because these boards are phase angle controllers and have been
hardened (via snubbing, gating methods and filtering and decoupling) to
withstand the worst noise from the AC switching at peaks.
The lock-ups happen when the AC line is lower than normal, so I have
changed
to a(n) LDO Regulator (Thanks, Olin, for your suggestions).
If these events still occur, I am left to conclude that there must be some
interruption of the line of significant duration.
Where might I look for strategies to deal with such a thing? I need 4.5V
<200mA to keep the whole thing running for the duration. Having no
experience with such things as super caps, I am wondering if this is a
good
app for one....

C

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2003\02\18@042311 by Alan B. Pearce

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>The lock-ups happen when the AC line is lower than normal,
>so I have changed to a(n) LDO Regulator (Thanks, Olin,
>for your suggestions). If these events still occur, I am
>left to conclude that there must be some interruption of
>the line of significant duration. Where might I look for
>strategies to deal with such a thing? I need 4.5V <200mA
>to keep the whole thing running for the duration. Having
>no experience with such things as super caps, I am
>wondering if this is a good app for one....

I would approach it this way ...

1. Use a power transformer with a higher output voltage, and
  use larger than normal rectifier capacitors. You may even
  want to use a 2 stage filter so the rectifiers do not have
  the extremely high peak currents that very large capacitors
  will produce through the rectifiers, allowing smaller
  devices to be used, than you may need otherwise.

2. Use a switching regulator to get the 5 volts. This will allow
  a more energy efficient regulation at high input voltages, and
  make best use of the energy stored in the filter capacitors
  during brownout states. If you can use a buck-boost design then
  you can keep your circuit running during very low input voltage
  brownout states.

3. If you really get to have long brownouts such that even this
  does not do what you need, then a suitable battery using a
  diode to isolate from the filter capacitors may be required.
  With this on float charge you will have the "UPS" you need.

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2003\02\18@080347 by Ray Gallant

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----- Original Message -----
From: "Alan B. Pearce" <EraseMEA.B.Pearcespam_OUTspamTakeThisOuTRL.AC.UK>
To: <PICLISTspamspam_OUTMITVMA.MIT.EDU>
Sent: Tuesday, February 18, 2003 5:21 AM
Subject: Re: Industrial Environments and PICs


{Quote hidden}

This has nothing to do with your supply but I have had similar problems
where AC from a different line was interrupted which caused some intense
machinery movement. I used an 8 pin IC to monitor that AC now with the PIC.
It's output is TTL. The result is that I have time initiate an E-Stop when
that AC is interrupted. My opinion is that if you have severe power
interruptions, then your equipment and safety must also suffer, so in that
case, E-stop.  Regards, {slewrate}

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2003\02\18@101842 by Chris Loiacono

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{Quote hidden}

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2003\02\18@102458 by Chris Loiacono

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These are all excellent thoughts & ideas....Yet even another comes to
mind...
All the logic inputs I use are optically isolated, but I use 2 analog inputs
which are not.
The idea of having to isolate these with linear opto's sounds challenging -
doable, but challenging. I am wondering if the slightly odd input impedance
requirements of the PIC ADC's make them more susceptible to noise and if
this is a worthwhile trail to chase...

I need to get a board revision done yesterday, so I think I will take the
power supply off board and look more into a supervisory cicuit addition.
Then I can do a supply redesign, using these suggestions.

I'm really curious about those ADC's. Is anyone using them in an Industrial
app that can shed more light from that angle?


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2003\02\18@134104 by Peter L. Peres

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On Mon, 17 Feb 2003, Chris Loiacono wrote:

*>Where might I look for strategies to deal with such a thing? I need 4.5V
*><200mA to keep the whole thing running for the duration. Having no

Imho try to find a 4.8V 50mAh battery that is mass produced and adapt a
simple slow/trickle charger/switchover for it. The battery should be
changed once every two years or so. Assuming half capacity it should carry
your system for 5 minute outages at least.

Peter

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2003\02\18@141958 by Dwayne Reid

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At 10:23 AM 2/18/03 -0500, Chris Loiacono wrote:

>I'm really curious about those ADC's. Is anyone using them in an Industrial
>app that can shed more light from that angle?

Much of my stuff would be classed as industrial - and most of that uses the
a/d inputs on 16c73b PICs.

However, I deal with slowly changing a/d values (thermocouples embedded
within gas-fueled catalytic heaters) and I am able to use analog RC filters
on the a/d inputs as well as digital filtering within the code.

My major problems have always been high frequency noise (VFD motor speed
controllers) and fast transients (motors and contactors turning off).

VFD noise is pervasive - it gets everywhere.  Keeping it out of the
thermocouple transmitters was a challenge.

But the worst problem is the fast transients that happen when inductive
loads get turned off.  My early generation controllers would spontaneously
reset if a contactor opened.  I tried everything I could think of: bypass
every wire that left the PCB, increase the quantity of bypass caps near
each chip, ground plane on the PCB, layout changes, the works.

One of the problems was tracked down to a shift register: we used to use
TPIC6B595 (note the "B") to drive relays and 2 of the outputs from one of
those also controlled a 4051 analog multiplexor.  One of my engineers
finally proved to me that the TPIC6B595 was susceptible to transient
electric fields: he mounted one on a ground plane, connected LEDs to the
outputs, fed in a pulse train to set the outputs to a pattern, then
physically shorted all the control lines to the ground plane (smallest
possible loop area).  Operating a piezo sparker anywhere in the vicinity
completely scrambled the pattern shown on the LEDs.  Changing to the
TPIC6595 (note: no "B") completely eliminated that problem.  The reason
this was leading to resets is that one of the inputs on the mux was at a
known verification value: the code checks 1 input on every mux for a known
value every time through an input scan and bails if any verification input
is out of spec.

Going back to an earlier rev of the same board and changing the shift
registers to the non-B version showed an almost complete immunity of fast
transient fields.  However, there was still a very rare spontaneous
reset.  I went through all the board revisions that I had tried and found
that the version that made the most difference had more ground area under
the PIC and connecting the peripherals (muxes and such).  I can't really
call it a ground plane - its only a 2 layer board with traces top and
bottom.  But the ground trace is somewhere around 0.25" wide as opposed to
0.10" wide on the earlier boards.

My advice, for what its worth, is to abuse your controller in every way
that you can imagine.  I don't have access to a commercial ESD gun but get
similar results from an ordinary piezo ignitor and from a high frequency
ignitor used for starting xenon lamps.

Finally, plan for failure.  My oven controller boards use an external
supervisor (Xicor X5043) and have a simple 2 transistor monostable that
monitors the reset line into the PIC.  Each time a reset pulse is detected,
the timer is reset and disables the fuel gas relays.  There are other
hardware disabling techniques as well as multiple software tests that all
have to pass before allowing the fuel gas relays to operate.

Hope this helps!

dwayne

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2003\02\18@150019 by Chris Loiacono

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I like this idea - I only need 1 - 2 seconds, though. A supervisory circuit
would be good since bad data stuck on an output has the potential to burn
down a $1M host machine - or a building full of them

While the old analog controllers were inaccurate and required a high skill
level for field techs, they did simply continue running after a supply
glitch. With PLC's, I always specified a commercial UPS.

There must be a supervisor chip that will manage battery switching also.
somewhere....
I know, I'm mixing concepts and methods here - I can wish though, can't I?

C

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2003\02\18@151054 by Chris Loiacono

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{Quote hidden}

Yikes - this is the next priority on my suspect list. I have OBSERVED
contactor openings to coincide with resets at one time...
How does this get to the PIC? Des the non-AC wiring to the board become a
series of antennae, or do these events pass through the supply?
If all is isolated, I can see the ground as a big antenna - are you saying
that a large ground mass of Cu very near the chip can do this?

C

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2003\02\18@163252 by Dwayne Reid

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At 03:09 PM 2/18/03 -0500, Chris Loiacono wrote:

>Yikes - this is the next priority on my suspect list. I have OBSERVED
>contactor openings to coincide with resets at one time...
>How does this get to the PIC? Des the non-AC wiring to the board become a
>series of antennae, or do these events pass through the supply?
>If all is isolated, I can see the ground as a big antenna - are you saying
>that a large ground mass of Cu very near the chip can do this?

Nope - I was saying the opposite: a large ground mass near and under the
PIC and connecting the analog muxes and SPI peripherals reduced
(eliminated) the problem.

It all comes down to loop area: if the ground line is thin and meanders all
over the board picking up the commons from the peripherals, you will have
problems.  I've known this for years and had tried to make the ground area
as large as possible.  However, spending more time on the layout allowed me
to move things around and make the ground area about triple the size that
it was originally - that seemed to clean up the last of those phantom resets.

dwayne

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2003\02\19@195215 by Peter L. Peres

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On Tue, 18 Feb 2003, Chris Loiacono wrote:

*>contactor openings to coincide with resets at one time...
*>How does this get to the PIC? Des the non-AC wiring to the board become a
*>series of antennae, or do these events pass through the supply?

All of the above plus capacitive coupling.

*>If all is isolated, I can see the ground as a big antenna - are you saying
*>that a large ground mass of Cu very near the chip can do this?

To understand this imho it is recommended to review H. Hertz's original
experiment wrt. em wave propagation. He used a spark gap exciting a simple
closed dipole with a spark gap between the terminals, and another
identical dipole with spark gap as receiver. He got to 10 meters or more
having sparks on the receiver dipole gap and determined the wavelength in
air (using standing waves from a large metal plate - the lab door I
think).  The frequency was ~300 MHz and the voltage at tx about 2kV
(judged by gap spacing). I will stop now.

Peter

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