Re: PID control

Dear Paul Montgomery

Dead time is perhaps best described generally and for any process.

Consider a system that has one input and one output - and the input and output are measured. A system has a delay if there is no measurable output even though there is an input that will eventually excite an output

Perhaps A good example would be to consider a system where the input is energy and the output is the temperature of water/steam in say a steam turbine is the output. Using sufficient Energy as an input to the system - the output temperature will rise to 100[deg. C] and then will cease to rise until the vapour becomes superheated (pressure measurement required)

You can then argue that a delay in temperature rise is due to some physical attribue of the system causing a DELAY to the MEASURED variable.

A delay can conceivably happen anywhere in a system.

Hysteresis would be present in the system IF you reversed the above process and the deadtime didnt occur in exactly the same place. i.e.he reverse is not truly true

All the above relates to the PHYSICAL PROBLEM

The DIGITAL DOMAIN of this problem is related to the ACTUAL MEASUREMENT and control of the system - this is of course assuming DCS. As a measurement of anything depends upon what frequency in the measured signal needs to be represented.- the sampling frequency of the conversion from analog to digital is linked to the process itself. The choice of sampling frequency can become complicated for large systems especially when there is an actual physical time delay in the system.

Hope this helps

Kieran

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