The Lead/Lag (LL) function block provides dynamic compensation for an input value. The block can apply a lead time function, a lag time function, or a combination of the two. A specified gain is applied to the compensated value and the value is high/low-limited based on the block mode.
The lead time and lag time values are specified by the LEAD_TIME and LAG_TIME parameters. You can specify a gain factor to be applied to the compensated value with the GAIN parameter.
The compensated value is high- and low-limited based on the block's mode. In Automatic (Auto) mode, the value of IN after dynamic compensation is multiplied by the GAIN value and written to OUT. However, when the FOLLOW discrete input is active (FOLLOW = True), no dynamic compensation is introduced by the block.
A bias value is added to the compensated input value to allow bumpless transfer on the transition from Manual (Man) or Out of Service (OOS) to Auto mode. The bias value is ramped to zero over the time specified by the balance time parameter (BAL_TIME).
The Lead/Lag function block supports signal status propagation. There are no standard alarms in this function block. Custom alarms are supported.
FOLLOW is the value that allows OUT to track IN × GAIN.
IN is the input value and status.
LAG_TIME specifies the lag time constant.
LEAD_TIME specifies the lead time constant.
OUT is the output value and status.
The following diagram shows the internal components of the Lead/Lag function block:
