How to design low/high signal select input to relay

[sujins]

In some process design, I saw the low/high select signal input to relay to select the source of output signal. The signal may come from HIC or from two controllers (i.e.one from temperature controller and another come from pressure controller)to compare the signal at the relay. How can we specify which signal that relay will select to send output to the control valve. In my understanding, when we want valve to put to close position, we will use low signal selector compare signal from controller and HIC(manual control). When we want valve to open, we will use high signal selector compare signal from controller and HIC(manual control). Is it right? or wrong? please help suggest. Thanks in advance.

 

[bmsg]

From what I could understand, the object you are describing is some kind of an selector switch - that allows either one or the other input signal (in this case, analog inputs) to pass from input to output.

Is it a manual switch? Then, you have to do the changeover manually, depending on some criteron you deem fit.

Otherwise, if it is an 'automatic' switch, there will be a 'conditional input' [i.e., if a condition is true, power is fed to the input, and the switch outputs the value at input terminal'A', say - otherwise - the value at terminal'B', say].

Good luck.

 

[Ashereng]

Does this answer your question?

A high selector switch selects the highest of the 2 or more inputs, and passes that along to the next point in the control circuit/scheme.

A low selector switch selects the lowest of the 2 or more inputs, and passes that along to the next point in the control circuit/scheme.

For example:

Two PID controller's output, in values of percent (representing the 4-20 mA signal) go to a high selector. One signal is 100% (meaning 20 mA), and the other is 0% (meaning 4 mA). The high selector will pass the 100% to the next point (usually the output channel) because it is a high seletor. In this example, the next point is the output channel (or address) on the output module. It will receive the 100% signal (it is the high one), and in my example, sends out the corresponding 20 mA signal.

The valve opens or closes depending on how it is configured. A 20 mA signal usually opens a valve, BUT, this is only by convention. You need to look at how the valve, actuator, positioner, solenoid, etc. are set up to determine the final movement of the valve.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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