User Guide - Eurotherm Ltda

nanodac RECORDER/CONTROLLER: USER GUIDEB2.2 TYPES OF CONTROL LOOP (Cont.)Derivative should not be used to curb overshoot in situations when the output is saturated at Op High or OpLow for extended periods, such as process start up, since to do so degrades the steady state performanceof the system. Overshoot inhibition is best left to the approach control parameters, High and Low Cutback.If Derivative is set to Off, no derivative action will be applied.Derivative can be calculated on change of PV or change of Error. If configured on error, then changes in thesetpoint will be transmitted to the output. For applications such as furnace temperature control, it is commonpractice to select Derivative on PV to prevent thermal shock caused by a sudden change of output asa result of a change in setpoint.B2.2.3 Motorised valve controlDesigned specifically for driving motorised valves this type of control can operate in ‘Unbounded’ mode(VPU) or ‘Bounded’ mode (VPB). Relay outputs are used to drive the valve motor.VPBPositionVelocity= Mechanical linkageVPUVelocity= Mechanical linkageSPPIDChNManualinputTimeproportioningMotorValvePot.LoadTempsensorSPPIDManual input(Pulses)TimeproportioningMotorValveLoadTempsensorFigure B2.2.3 VPB and VPU comparisonUnbounded valve positioning (VPU) does not require a position feedback potentiometer in order to operatebecause it controls directly the direction and velocity of the movement of the valve in order to minimise theerror between the setpoint (SP) and the process variable (PV). Control is performed by delivering a ‘raise’ or‘lower’ pulse to control the velocity of the valve in response to the control demand signal.Bounded VP (VPB) control uses PID (or any other combination of the three terms) to set a required valve position.A feedback potentiometer linked to the valve provides a signal giving actual valve position. This allowsthe control loop to calculate the difference between required and actual position dynamically, andadjust control output accordingly. Control is performed by delivering a ‘raise’ or ‘lower’ pulse to adjust thevalve position.MANUAL MODEBounded VP controls in manual mode because the inner positional loop is still running against the potentiometerfeedback, so it is operating as a position loop.In boundless mode the algorithm is a velocity mode positioner. When manual is selected then the up anddown arrow produce +100% or –100% velocity respectively for the duration of the key press.In boundless mode it is essential that the motor travel time is set accurately in order to allow the integral timeto calculate correctly. Motor travel time is defined as (valve fully open – valve fully closed). This is not necessarilythe time printed on the motor since, if mechanical stops have been set on the motor, the travel timeof the valve may be different.Every time the valve is driven to its end stops the algorithm is reset to 0% or 100% to compensate for anychanges which may occur due to wear in linkages or other mechanical parts.This technique makes boundless VP look like a positional loop in manual even though it is not. This enablescombinations of heating and cooling e.g. PID heat, VPU cool with manual mode working as expected.MOTORISED VALVE OUTPUT CONNECTIONSThe loop output which has been configured as valve position can be wired to the PV input of one of the pairsof relays 2A2B/3A3B or 4AC/5AC which has been configured as Type = ’Valve Raise’. Only one relay inputneeds to be wired as the other relay of the pair will be automatically set to ‘Valve Lower’. For example, ifLoop 1 Channel 1 output is wired to Relay 2A2B and the ‘Type’ is configured as ‘Valve Raise’ then the Typefor Relay 3A3B will be ‘Valve Lower’.Appendix BPage 302HA030554Issue 7 Nov 12