"Andy Hall" wrote in message
...
On Mon, 9 Feb 2004 16:59:17 -0000, "IMM" wrote:
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
It constantly moves up and down in an on-off manner.
That's a good technical explanation....:-)
Good enough for you.
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 separte. 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.
It could supply data to other control processes, but it is a separate
control process.
Of course.
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.
And, the modulating 3-way valve has finer accuracy to maintain a setpoint
temperature than switching on-off a boiler.
Yes, except that I was referring to the heatbank.
The heat bank supplies the constant hot water for modulation purposes.
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.
See above.
Glad to see that you clarified it.
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?
The cycle times are not a part of the Integral aspect of the controller
AFAICS.
I am referring to the specific method of operating the control valve -
the signalling between the controller and the valve. The CM67
provides a pulse with a total width of anything from 5 to 20 minutes
A pulse? A signal going on and off continuously?
depending on the what is configured on the setup. In order to
control the controlled device (boiler, motorised valve etc) it will be
on for between 1 minute and 100% of that time while operating in the
proportional band. On the fastest cycle time, that will equate to
a range between 60 and 300 seconds of on time per 300 second cycle.
What I have specifically asked you to provide is the make and model of
a valve which will operate in a modulated fashion given that type of
control signal.
Please reply to that specific point.
The control signal is 230V.
I would be surprised if such a thing exists. I know that there are
voltage controlled modulating valves ( I have some radiator valves
that can be controlled by 0-10v DC); and I could believe that there
are some that would modulate based on a pulse width with a much
shorter cycle time ( a second or less)
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.
There are 230v electric modulating valves available. Look in the CM67
pdf,
it has a diagram of one.
I just have. Presumably you are referring to the sheet I posted -
http://tinyurl.com/2yc23
It doesn't say anything about it being modulating.
It does. A reversible motor. 230v on one terminal and it moves one way,
same the other way. Stop the singal and it stays where it is.
There is a thermal actuator (diagram c) controlling a boiler. This
type of device is a thermally operated relay which controls the boiler
on and off, not modulating.
There is an electric actuator (diagram h) which is an electric
actuator. This is a motorised device to operate some kind of
mechanical entity such as a damper. It even shows the limit switches
which stop travel. This isn't a modulated device either.
It appears to be.
Did you mean something different? Please provide the URL to the CM67
sheet that shows it if I have the wrong one.
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.
What I am on about is a "modulted" 3-port mixing valve, with an electric
actuator. I have a few here.
I know. Please provide the make and model of them. I am simply
asking you to provide details of a *modulated* device which will work
with a CM67.
Any maker makes them. They are common.
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.
It depends on how far you want to take it. A CM67 on a themal store will
learn how long it takes the boiler to heat it up, and bring it on at the
right time.
True.
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.
AFAIK, the cm67 when controlling the setpoint temperature, only applies
PI
control modes. The cycle times appear to be an underlying control layer
which only allows the boiler and PI control on it at certain time
periods.
The cycle time is about choosing a control signal suitable for the
device being controlled e.g. boiler, zone valve, electric heater
AFAIK the CM67 can operate a 230v modulated valve. If that is not the case
(specs vague on this from wjat I have seen), then I think the Landis &
Staefa version can. If so, I would go for this controller not the CM67. I
tend to prefer Landis & Staefa controls anyhow. They are much more
reliable, better made, better user intefaces and generally cheaper too.
snip crap about snakes, which should all be banned.
If Brazil is short of snakes then you should not be having one.
They didn't come from Brazil, they were captive bred in the UK.
They should be sent back to the wild then.