Handbook - Eurotherm Ltda

T820 HANDBOOK5.2.3 PID page (Cont.)• Integral Time, TiIn Proportional only control, an error between SP and PV must exist for power to be delivered. Integral time, Ti,is used to achieve zero steady state control error.The Ti term slowly shifts the output level as a result of an error between SP and measured PV. If the measured PVis below SP the Integral time action gradually increases the output in an attempt to correct the error. If it is aboveSP the Ti action gradually decreases the output or increases the cooling power to correct the error. The diagrambelow shows the result of introducing Ti action.TemperatureProportionalControl OnlySPProportional &Integral ControlTimeFigure 5-2-3bIntegral Time (Ti) configurationThe units for the Ti term are measured in time (1 to 99999 seconds). The longer the Ti constant, the more slowlythe output is shifted, resulting in poor response. If the Ti is set too small, it will cause the process to overshootand even oscillate. The Ti action can be disabled by setting PID.Tin Off.Temporarily disabling the Ti term can be useful when a control loop is expected to open, i.e. it may be necessaryto turn heaters off for a short period or switch into manual at low power. In this case it may be an advantage towire it to a digital input that activates when the heaters are turned off. When the heaters are switched on again theTi term is already at its previous value minimising overshoot.In a PID control (3-term control), the Ti term of the PID calculation can be frozen at the current value ifMain.IntHold is set Yes. It will hold the Ti term at its current value but will not integrate any disturbances in theplant/system. Essentially, this is equivalent to switching to PD control with a manual reset value, Ti term value,preconfigured.When the control loop is configured to use PID control, changes between manual and automatic can cause abruptchanges to the output value. By configuring Integral Balance, PID.IntBal, abrupt changes, bumps, can beprevented, and the output power gradually changed in accordance with the demand from the PID algorithm or byan user via a User Screen.NoteOutput bumps can damage valves and destabilise the process.Chapter 5Page 5 - 6HA029131Issue 3 Feb 08