Logix5000 Controllers General Instructions - SLAC Confluence

Special Instructions (FBC, DDT, DTR, PID) 517Set the deadbandThe adjustable deadband lets you select an error range above andbelow the setpoint where output does not change as long as the errorremains within this range. This deadband lets you control how closelythe process variable matches the setpoint without changing theoutput. The deadband also helps to minimize wear and tear on yourfinal control device.+ deadbandsetpointerror within deadband range- deadbandtime 41026Zero-crossing is deadband control that lets the instruction use theerror for computational purposes as the process variable crosses intothe deadband until the process variable crosses the setpoint. Once theprocess variable crosses the setpoint (error crosses zero and changessign) and as long as the process variable remains in the deadband, theoutput will not change.The deadband extends above and below the setpoint by the valueyou specify. Enter zero to inhibit the deadband. The deadband has thesame scaled units as the setpoint. You can use the deadband withoutthe zero-crossing feature by selecting the “no zero crossing fordeadband” option on the Configuration tab or set the .NOZC bit in thePID structure.If you are using the deadband, the Control variable must be REAL or itwill be forced to 0 when the error is within the deadbandUse output limitingYou can set an output limit (percentage of output) on the controloutput. When the instruction detects that the output has reached alimit, it sets an alarm bit and prevents the output from exceedingeither the lower or upper limit.Publication 1756-RM003I-EN-P - January 2007