Gavin Stairs on sun 13 sep 98
At 09:29 AM 12/09/98 EDT, Dana Groemminger wrote:
....
> I'm considering a kiln with computerized controller but have not had any
>experience with them. The thing I am most concerned about is maintenance
>and durability of the thing. I know if I have a sitter/timer I can tear it
>apart and find out what's wrong, but with a computerized controller I'm
>afraid repairs may be costly. ...
Hi Dana,
I'm in the process of designing one right now, actually. There are also a
couple of others on the list who have gone through this as well.
The likelihood of failure in a well designed electronic controller is
probably somewhat less than that of a mechanical controller. Mechanical
parts have a greater tendency to wear out and need attention. However,
there are things that can go wrong with electronic controls, and there are
things that can be done to prevent the problems and to ease the pain when
they do go wrong, anyway.
The elements of the controller can be broken down thusly:
1 Power entry circuits
These are thecables, fuses, filters, power line switches. This stuff is
pretty much common between the two, but the electronic type should have
more surge protection and fusing for the electronics portions of the
device. This is because the achilles heel of the electronics devices is
overvoltage spikes coming from the power system. These generally don't do
anything much to electromechanical components (although there have been
exceptions), but they can destroy solid state devices, and do, on occasion.
2 Power pass switches
These come in two flavors: electromechanical (relays) and solid state
electronic.
Relays have contact points and mechanical parts which are subject to
failure. This is especially so if the relay contacts are opened while
current is flowing (that is, if this is not a safety or line disconnect
device used in conjunction with a separate duty-cycle switch). So, any
electromechanical relay which is used as the duty-cycle switch should be
easily replaceable. Socketed or with clip leads.
Electronic switches may degrade from insufficient capacity, overheating,
unprotected line surges and other accidents. Some types may be more
susceptible to this kind of failure than others, but the key is good design
and sufficient protection circuitry. All power switch elements should be
easily replaceable.
3 Connections
Power connections can be susceptible to heat and corrosion failures.
These need to be carefully designed, robustly made, and laid out so as to
avoid overheating. This is the same with all type of controllers. The
electronic type has additional wiring going to the electronics elements,
while the electromechanical type has more wiring going to the sitter and
timer.
Control inputs, especially thermocouples, are also subject to similar
degradation, and must also be carefully designed. This is a particular
concern in the electronics type, since this is the sensory input that the
controller uses to determine the firing control. Therefore it is of
primary importance that these connections be robust and reliable, and that
thermocouples be easily replaceable.
4 Front panel
The operator controls and displays are subject to abuse (surely not from
me?), and may experience failure, especially the keypads or switches and
controls. These should all be easily replaceable. They should also be
heavy duty units for harsh environment service, which means they can be
expensive.
5 The electronics main-board components.
These are the least likely components to fail, provided the other things
have been properly taken care of. To further decrease the likelihood of
failure, most of the components on this board should be soldered in place.
Accordingly, if there is a failure in this main board, the quickest fix is
to replace it all at once. Look for a warranty on this replacement, or at
least a quick replacement policy: you know, same day shipment of replacement.
6 Safety and preventative design
There are safety issues. Clayart recently had a couple of notes about
failure of electronic controllers leading to meltdowns. This can be
prevented, and should be. There is also the issue of fail-closed switches.
Generally, when a solid-state switch fails, it will do so safely, that is
in an off or open condition. One type of failure in mechanical switches in
the fail-closed type, usually from welded contacts. Both types can
fail-closed, and the user should be aware of this. A second level
disconnect is well advised. This would be a timer in the electromechanical
case, and a separate safety relay in the electronic case.
All of this means that the electronic controller need not be more prone
to failure or more difficult to repair than the electro-mechanical
controller. Indeed, a properly designed, maintained and operated
electronic controller has the potential to be more reliable and easier to
maintain/repair than the electro-mechanical devices it replaces.
Nevertheless, caveat emptor: look for the features that are important, and
buy accordingly. Look for easy replacement of the failure prone elements
(see above), ask for a replacement parts price list, ask about the
replacement policy, get an extended warranty if it is offered and gives
protection (i.e., rapid correction) in the case of in-service failures.
And have a look around inside. You want to be able to open the case
easily, and have easy access to all parts. You want to see carefully laid
out wiring with proper feed-through bushings and strain-reliefs, and robust
connections, all heat sensitive components protected from kiln heat and
provision of disconnect and replacement of failure prone components. If
you don't understand what the fix is for a particular failure, ask about
it. Read the manual.
All of this aside, if you are concerned about cost of repairs, you are
probably not considering the controller from the attitude of what it can do
for you, rather than what it will do to you. If you are satisfied with the
standard electromechanical controls (infinite switches, sitters and
timers), then you will probably save money by continuing with them. What
the electronic controls offer is primarily more uniform firing under all
conditions. If this is important to you, then there is no substitute. Any
of the controls now being offered will give you this, to varying degrees,
and at varying costs. None of them should be left to do the whole job
themselves, and especially, none of them, of any type, shhould be left to
fire unattended.
Hope this is of some use.
Gavin
===============================================
Gavin Stairs
Stairs Small Systems
921 College Street, # 1-A
Toronto, Ontario, Canada M6H 1A1
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