I am fresher, I have seen many controls engineers in this group. I want to develop my knowledge so please help me

1. what is control constant?
2.How to identify constant and variables?

 

himeshvarshan,

You found controls engineers on a World Wide Web forum called Control.com.

Since you posted your question in the Power Generation section, I'm going to presume you have been exposed to a GE Mark* turbine control system because it uses Control Constants in programming and configuration.

A Control Constant in a GE Mark* turbine control system. There can be hundreds of Control Constants. Usually Control Constants are analog values, such as a temperature or a pressure or a ratio or a pressure/percent speed. Control Constants can be compared against a real pressure or temperature from an input to the control system, and used to generate an alarm (low pressure, for example) or high temperature (again, an example). Control Constants can also be used to determine a control valve position or whether a solenoid should be energized or de-energized or a fan should be running or not running.

MOST, but not all, Control Constants are values which can be changed or adjusted--with knowledge of the proper password, of course. SOME Control Constants are NOT adjustable.

Those real temperature or pressure inputs (which can include position, level, humidity, flow-rate, etc.) are what I would call variables--they can vary when the unit is being started or loaded or shut down, or even when at rest.

Is this what you are asking about?

Because if it's not, read your post to see if you gave anyone a real idea of what you are referring to. While many manufacturers and programmers/configurators of control system employ similar methods for control and protection they aren't always called Control Constants, or even variable.

Controls engineers are not omniscient--contrary to popular belief. The more information you provide related to your queries here on Control.com, the better the responses you will receive. Help us to help you.

Welcome to Control.com.

 

Thank you for reply sir

 

himeshvarshan,

I've been trying to determine how to answer your second question, and I was hoping your response would include some clue as to whether or not you are actually working on a GE Mark* turbine control system or not. But, my hopes were dashed.

Presuming you are working on a GE Mark* turbine control system, well-chosen Control Constant signal names will almost always have the letter 'K' in the third position of the signal name. For example, FPKGNG or TNK14HR1 are both Control Constants--and they have the letter 'K' in the third position of their signal names. But that's a loose standard of sorts, though it is pretty well implemented in a lot of Control Constant signal names.

I hope this helps! But, I'm still guessing due to the lack of information. And, really, I don't like guessing--but I still do it from time to time.

 

I am newly joined, don’t have experience in Markvie sir

 

From a generic controls viewpoint:

If a "Control Constant can be compared against a real pressure or temperature from an input to the control system, and used to generate an alarm (low pressure, for example) or high temperature (again, an example).

or, if a "Control Constant can also be used to determine a control valve position or whether a solenoid should be energized or de-energized or a fan should be running or not running",

then the generic term in the world of controls for control constant would be "setpoint", or as I tell people, what the value the process is supposed to be, or "what you want"

The process variable is the value the process is currently at, or, "what you got".

The concept of process control is to get what you've 'got' to change to what you 'want', and then stay there.

 

From a computer science point of view a constant is a parameter which does not change. However, when I first started working with gas turbine controls many years ago, it was immediately obvious that while some "control constants" were constants from a computer science programming point of view, many "control constants" were not constant at all. Some "control constants" could be adjusted so they were "constant" until they were changed and the "constant" became a value that was different than it had been previously. Other "control constants" were being calculated and changing in real time, all the time.

What helped me wrap my brain around the concept of a "constant" not being constant, was thinking about an exponential decay and Euler's number e. The exponent part of an exponential decay contains a "time constant". Once the "time constant" is defined, the speed of the decay is determined. However, the "time constant" may depend upon a number of variables so it could be a calculated number, and even though the "time constant" can change value in real time it still makes sense to call it a "time constant".

Also, there are many other instances when it makes sense to call a variable which may change a "constant" because once you it set to a value you leave it alone for long periods of time and it remains "constant" until you decide to change it to another "constant" value. As mentioned above, regulator set points can be an instance of this kind of "control constant" that is "constant" until it is changed.

 

In any Control System, there are some adjustable parameters provided for tuning the performance ( like Proportional, Integral and Derivative gains in a PID Controller). They are usually referred as "Control constants"