On Mon, 9 Feb 2004 01:24:06 -0000, "IMM" wrote:


"Andy Hall" wrote in message
news
On Sun, 8 Feb 2004 23:37:40 -0000, "IMM" wrote:


snip cut and pasted explanation

There are three basic modes of control:

1. Proportional - operates on off-set and never reaches setpoint
2. Integral - time.
4. Derivative - anticipation, measurting rate of change.

PID controllers, which incorporate all three, are generally better.
Certainly more flexible. In short, the PI of the CM67: proportional gets
it
near the setpoint, but never reaches setpoint because it operates on
"offset". A proportional controler can sit there all day and not move
and
the setpoint will not be reached. They need a prod. The Integral mode
of
control operates on time and pulses the offset (the differnce bewteen the
setting the proportional mode has decided to stay on an the setpoint) up
to
setpoint, giving high accuracy.

Be careful not to have the proportional band too tight as "hunting" will
occur, known is short cycling to you.

Hunting is hunting. It is not short cycling but movement around the
set point.

It is a form of short cycling an easier for others to understand.

It's a totally different mechanism


In a CM67, the control aspect that times the delayed start is separet
from
the PI temperature control.

They could conceivably be related. the manufacturer doesn't say.

Controlwise they have to be eparte. The user doesn't know this nor care.

Not true. The piece controlling the optimised start could make very
good use of knowledge of the characteristics of the PI part. Whether
it does or not is a matter for the manufacturer.


The time proportions of the CM67, are a crude way of keeping an on-off
boiler within the proportional band.

That was already covered.

The CM67 can control an electric actuator. A better system would be a
heat
bank, heated by a basic on-off control boiler (these are cheaper and have
simple more reliable basic controls. The CH flow and return pipes would
have a modulated 3-way valve, moved up and down by and electric actuator.
This can stop at any point in the travel. This way only PI control would
be
used and guarantee much more accuracy of setpoint temperature control.
As
many heat bank and thermal stores are recommended to be on 24/7, the
boiler
can be controlled only by the heat bank and the CM67 moves the modulated
valve and switches on the pump when it calculates the start time.

I think you are confusing the issue.

Not at all.

You are because the heatbank is only a source of heat if designed to
be oversized by a reasonable amount so that the boiler behaviour goes
out of the equation.




From the control mathematics
perspective this is PI control exactly the same. All that this
achieves, although it will work is perhaps finer grain control of heat
delivery from the heatbank to the radiators. Of itself, the heatbank
is a red herring in this if the boiler is not part of the main control
loop.

Not so. The heat bank can sit there 24/7 keeping itself to its own setpoint
providing a nice constant heat source. Just a store of heat.

Exactly, but that's all.

The CM67 will
then just switch on the CH pump and modulate the CH actuator. Modulating a
3-way valve gives far more precise temperature control than switching in and
out a boiler.

I don't disagree with that notion but can you suggest a make and model
that can be modulated in a precise "analogue" way (i.e. part opened)
by a CM67 type of controller with a long cycle time?

There are certainly valves that can be modulated with a DC voltage (I
have some) and I could imagine pulse width modulated using a much
shorter cycle time - e.g. in the way that model control servos work.

If you can't suggest one that will work with the CM67 type of
controller, then the argument becomes a bit academic because either a
different type of controller is needed, which is not in the same price
bracket, or you would be talking about a standard motorised zone valve
and proportioned on/off control of it. That would be a bit better
than controlling a boiler on/off, although with a lightweight heat
exchange type, I don't think that the difference would be substantial.



Another way is that the thermal store can be switched on also by another
CM67. The desired time must be earlier than the time of the heating. This
is if you can replace the room temperature sensor with a water strap-on
cylinder sensor of course.

Yes of course, but this is adding extra complication. As you said, a
thermal store ought to be on 24/7 anyway.



I would also be surprised if a valve for this application would be
modulated with a ten minute or even 5 minute cycle time. but rather
that it would have some form of electronic control requiring a much
shorter cycle time -seconds or even sub second.

All those cycle times in minutes would be redundant, as only PI control on
an actuator is required.

The proportional pulse width output from the controller would be used,
but can you suggest a valve that will take this and use it to operate
the valve to arbitrary positions.





snip crap about snakes, which should all be banned

Well.... they were around before you and will probably be afterwards.

Don't care They should not be allowed in the country. If a couple of those
get out and breed in the wild.

Don't be silly. Clearly you know very little about herpetology.

First of all, the particular snakes in question (Brazilian Rainbow Boa
- Epichrates Cenchria Cenchria) range in the wild in the Brazilian
rainforest and their habitat is rapidly disappearing together with
their numbers in the wild. They do, however, breed and thrive
perfectly well in captivity, given the right conditions of temperature
and humidity (28 - 31 degrees and 70-80% humidity). In order for
them to breed, the conditions have to be right and there needs to be a
pattern of temperature drop and rise around the breeding time.
The UK climate simply won't create the conditions for them to breed in
the wild and they would be unlikely to live for very long in the wild
anyway. Considering the massive species loss in the rainforests,
anything that can be done to sustain them in captivity until their
natural habitat issues can be resolved is a positive step.


..andy

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