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.


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.


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. 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.

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.
Can you suggest a make and model that would have the characteristics
to work with a controller like the CM6x which has a much longer time?



Bandwidth is a bad word to use but is the usual one. Normally it's
used in connection with radio and network engineering to mean
something completely different.

It is not "bandwidth". It is "Proportional Band - width". The "width" is
the range of the band, e.g., 2C. That is the setpoint and 1C either side.

Obviously, and that is explained in the data sheets. However it is
often written as a single word in controller data sheets.



snip crap about snakes, which should all be banned

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

This is a real life application using an industrial type of PID
controller and served as a reasonable explanation of some of the
principles.

I felt that it was a better illustration than simply cutting and
pasting material from a web site


..andy

To email, substitute .nospam with .gl