Logix5000 Controllers General Instructions - SLAC Confluence

516 Special Instructions (FBC, DDT, DTR, PID)To set a bumpless restart:Do ThisConfigure the 1756 analog output module’s channelwhich receives the control variable from the PIDinstructionDetails:Select the “hold for initialization” check box on the properties page for the specificchannel of the module.Enter the Inhold bit tag and Inhold Value tag in thePID instructionThis tells the analog output module that when the controller returns to Run mode orre-establishes communications with the module, the module should hold the analogoutput at its current value until the value sent from the controller matches (within0.1% of span) the current value used by the output channel. The controller’s outputwill ramp to the currently held output value by making use of the .BIAS term. Thisramping is similar to auto bumpless transfer.The 1756 analog output module returns two values for each channel in its input datastructure. The InHold status bit (.Ch2InHold, for example), when true, indicates thatthe analog output channel is holding its value. The Data readback value (.Ch2Data,for example) shows the current output value in engineering units.Enter the tag of the InHold status bit as the InHold bit parameter of the PIDinstruction. Enter the tag of the Data readback value as the Inhold Value parameter.When he Inhold bit goes true, the PID instruction moves the Inhold Value into theControl variable output and re-initializes to support a bumpless restart at that value.When the analog output module receives this value back from the controller, it turnsoff the InHold status bit, which allows the PID instruction to start controllingnormally.Derivative SmoothingThe derivative calculation is enhanced by a derivative smoothing filter.This first order, low pass, digital filter helps to minimize largederivative term spikes caused by noise in the PV. This smoothingbecomes more aggressive with larger values of derivative gain. Youcan disable derivative smoothing if your process requires very largevalues of derivative gain (K d > 10, for example). To disable derivativesmoothing, select the “No derivative smoothing” option on theConfiguration tab or set the .NDF bit in the PID structure.Publication 1756-RM003I-EN-P - January 2007