# FC 3 - Lead / Lag

The output of a lead/lag function code equals the product of the time function and the input value. Specifications S3 and S4

provide lead (S3) or lag (S4) functions. Function code 3 also serves as a lead/lag filter. Outputs:

 Blk Type Description N R Output Value with lead / lag function applied

Specifications:

 Spec Tune Default Type Range Description S1 N 5 I Note 1 Block Address of Input S2 N 0 I Note 1 Block address of Track switch signal: 0 = track 1 = release S3 Y 0.000 R Full Time constant T1 (lead) sec S4 Y 0.000 R Full Time constant T2 (lag) sec

NOTES:

1.Maximum values are:   9,998 for the BRC-100, IMMFP11/12 and 31,998 for the HAC

3.1 Explanation

Function code 3 causes the output of the function block to lead or lag changes in the input signal <S1>. The following equation describes the operation: The S2 term enables or disables this function. If <S2> is a logic 0, then the output equals the input <S1>. If <S2> is a logic 1, the lead or lag function is implemented.

3.1.1 Lag Function

To select the lag function, leave S3 at its initial value (0) and enter a number for S4. The equation then becomes: S4 is the time constant term. This is the time required for the output of this function to reach 63.2 percent of the input value.  The output will not reach approximately 99 percent of the input value until the end of five time constants. In this application, it will be five times S4 before the output reaches the input value. To calculate the S4 term needed for the output to equal the input in a certain number of seconds (t), use the following equation: S4 = Time constant term for function code 3.

t = Number of seconds for the output to reach about 99 percent of the input value.

5 = Number of time constants required for the output to reach about 99 percent of the input value.

For example, for the output to reach the input level in 30 seconds, the S4 term needed would be: To select only a lead function, leave S4 at its initial value of zero and enter a number for S3.  The equation then becomes: The output is set to the value that the input will be in (S3) seconds if it continues to change at the same rate as it did during the last cycle. The lead function is essentially equal to the derivative function except that the block output eventually equals the input if the input remains constant long enough. The output of a derivative function is zero when the input is not changing.

3.2 Applications

Figures 3-3 and 3-4 illustrate some general input and output signal shapes for a function code 3 used as a lag filter and as a lead filter respectively. The input signals shown in Figures 3-1 and 3-2 are ideal waveforms for electronic circuits. Actual outputs and inputs vary because Symphony function codes are pre-programmed algorithms.  Figures 3-3 and 3-4 are simplified examples of using function code 3 in boiler applications. Figure 3-3 shows function code 3 used as a lag to delay decreases in air flow for a load decrease. Figure 3-4 shows function code 3 used as a lead/lag to compensate for drum level shrink and swell due to changes in steam flow.  