What are the criteria for selecting controller mode for a given process? Describe the ratio controller with clearly specifying the types of processes for which it is used.

• Selection Criteria for Controller :-

1. Process reaction curve that can be faster sluggish, slow or moderate.

2. No. of process capacities handles com be single process, self regulating, with multiple load.

3. Process time lag may vary from less time to moderate time. The dead time may vary from small time moderate.

4. Load changes from small to moderate in a process control. The speed of load may also change from small to high.

5. Depending upon the selecting criteria. The suitable control action can be takes such as a position, Integral mode, Pl, PID, or PD.

• Ratio Controller :-

1. Ratio controller is a type of feed forward controller. In this controller before the controller controls the system the noise is measured and a ratio is obtained of two input variable and sent further to ratio station. The obtained ratio and Set ratio are compared and effective measure is taken by the controller. Thus it is called as Ratio controller.

2. Ratio controllers are used for mixing of two elements in a process. Consider a process in which the ratio of water and salt is to be maintained. To obtain the concentration the I/P of both the streams i.e. water and salty water a ratio is set.

1. The ratio is generate as per the requirement of the final output. The Same ratio is given as set point to the controller. The objective. is to maintain the flow rates of both inputs to obtain the describe ratio constant. But the flow rate of both I/P is not controlled instead only one input flow rate is controlled. The input stream which is not controlled is called as wild stream.

1. To obtain desired ratio, flow sensor are used at both input and flowrates are determined. By using an Electronic divider ratio is obtained and passed to ratio Controller. The set ratio value and the news obtained ratio value are compared & the error is determined. The error value is used to set the flow rate of controlled stream to obtain the desired salt concentrations in the mixer tank.

2. The ratio required for the salt concentration at the final output is $R = \frac{S}{W}$, where R is the ratio, S is the salt concentrate flowrate, W is the water stream flowrate.

3. The wild flowrate of water 'w' is measured & multiplied to R to get desired value of "S". Thus when the water stream 'W' varies the stream of salt concentrate 'S' will also vary to maintain the ratio 'R'.