On Thu, 22 Feb 2001, Simon Nield wrote:

> do remember that PID control is only a bodge really anyway - it saves you having to analyse the
> system you are trying to control and model its transfer function 'classically' - but it's a bodge
> (aka "engineering approximation") that is very often perfectly sufficient for the "real world" job
> (as opposed to the first year control systems course where they will have you think that you need to
> analyse every system individually and then design a custom controller... still at least it provides
> a bit of background to the discovery later on that in fact 'almost everyone' just buys PID
> controller units and has a the manufacturer tune it for them)

  I don't know who is the teacher at your control systems course but it
has damn right ! You can't find an universal solution just using an
excellent ( and epensive too ) controller. Just a simple example in
heating process: if you must keep at desired value the temperature inside
a small oven ( let say 1000C at 2KW ) and you must use an on-off command
for power supply, you may have the best controller in the world, if you
don't correlate the oven resistor value with your temperature
requirements and mains value ! With other words if you're not able to
dimension well the resistor you'll need an expensive proportional
controller to achieve the same +/- 2 C fluctuation like using a cheap
relay and a simple comparator. ( or a 12C509 to be in topics...)

dictionar:
C = Celcius degree

Regards, Vasile

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