Control loop Tunning


#1

In a control loop, to select the most appropriate control elements are gain and stability. Here in control element is a valve.

The gain of any device is its output divided by its input. The characteristic range and gain of control valves are interrelated. The gain of a linear valve is constant. This gain (Gv) is the maximum flow divided by the valve stroke in percentage.

The gains of a linear controller (Gv = plain proportional) and a linear transmitter (if it is a temperature transmitter, its gain is Gs = 100%/°F) are both constant.

Therefore, if the process gain (Gp = °F/GPM) is also constant, a linear valve is needed to maintain the total loop gain at 0.5 (Gv = 0.5/ GvGcGs = constant, meaning linear).

Case: Non-linear transmitter

If the transmitter is nonlinear, such as in the case of a d/p cell (sensor gain increases with flow), one can correct for that nonlinearity by using a nonlinear valve the gain of which drops with flow increases (quick opening).

If the transmitter is nonlinear, as in the case of a d/p cell (sensor gain increases with the flow), the gain from which drops with flow increases (quick opening) can be corrected for this nonlinearity by using a nonlinear valve.

The efficiency of heat transfer (process gain Gp) decreases as the amount of heat to be transferred increases in the case of heat transfer over a fixed area.

The valve gain (Gv) must increase with load in order to compensate for this nonlinearity (drop in process gain= Gp). For all heat transfer temperature control applications, an equal percentage valve should be selected.

Case: heat transfer

In case of heat transfer over a fixed area, the efficiency of heat transfer (process gain Gp) drops as the amount of heat to be transferred rises. The valve gain (Gv) must increase with load in order to compensate for this nonlinearity (drop in process gain= Gp).

For all heat transfer temperature control applications, an equal percentage valve should be selected.

Case: Flow control

In the case of flow control, one effective way of keeping the valve gain (Gv) perfectly constant is to replace the control valve with a linear cascade slave flow control loop.

The limitation (in addition to its higher cost) of this cascade configuration is that if the process controlled is faster than the flow loop, cycling will occur. This is because the slave in any cascade system must be faster than its master.

The only way to overcome this cycling is to slow the master down (detune) by reducing his gain (increasing his proportional band), which in turn degrades his quality of control. This approach should therefore only be taken into account in slow or secondary loops for temperature control.