Emerson 54e pH/ORP Proportional Mode Gain, Proportional Gain Plus Integral Reset

Page 57

MODEL 54e pH/ORP

SECTION 6.0

 

THEORY OF OPERATION

6.9PID CONTROL (continued)

Proportional Mode (Gain)

The simplest control is proportional. Proportional may also be referred to as sensitivity or gain. Although these terms may refer to a different version of proportional, the control function is still fundamentally the same - the error from set point is multiplied by this factor to pro- duce the output.

The Model 54e pH/ORP's proportional mode is referred to as proportional "band" which is config- urable from 0 to 299%. For good control of a specific process, the proportional band must be properly adjusted. The proportional band is the percent of the analog output span (the difference between the 4 (or 0) mA and 20 mA settings) through which the measured variable must move to change the output from mini- mum to maximum. The larger the proportional band, the less the controller reacts to changes in the measured variable. As the proportional band is made smaller, the reaction of the controller increases. At 0 proportional band, the proportional- only controller behaves like an on/off controller (an alarm set at 20 mA).

Most processes require that the measured variable be held at the set point. The proportional mode alone will not automatically do this. Proportional alone will only stabilize the measured variable at some offset to the actual control point. To control at an exact setpoint, proportional plus integral mode is used.

Proportional (Gain) Plus Integral (Reset)

For the automatic elimination of deviation, I (Integral mode), also referred to as Reset, is used. The propor- tional function is modified by the addition of automatic reset. With the reset mode, the controller continues to change its output until the deviation between measure- ment and set point is eliminated.

The action of the reset mode depends on the propor- tional band. The rate at which it changes the controller output is based on the proportional band size and the reset adjustment. The reset time is the time required for the reset mode to repeat the proportional action once. It is expressed as seconds per repeat, adjustable from 0-2999 seconds.

The reset mode repeats the proportional action as long as an offset from the set point exists. Reset action is cumulative. The longer the offset exists, the more the output signal is increased.

The controller configured with reset continues to change until there is no offset. If the offset persists, the reset action eventually drives the controller output to its 100% limit - a condition known as "reset windup". To prevent reset windup, a controller with reset mode should never be used to control a measured variable influenced by uncorrectable conditions. Once the con- troller is "wound up", the deviation must be eliminated or redirected before the controller can unwind and resume control of the measured variable. The integral time can be cleared and the "windup" condition quickly eliminated by manually overriding the Model 54e pH/ORP's analog output using the simulate tests feature (detailed in Section 5.4).

Control Loop Adjustment and Tuning

There are several methods for tuning PID loops includ- ing: Ziegler-Nichols frequency response, open loop step response, closed loop step response, and trial and error. Described in this section is a form of the open loop response method called the process reaction curve method. The reaction times and control charac- teristics of installed equipment and real processes are difficult to predict. The Process Reaction Curve Method of tuning works well because it is based on the response of the installed system. This procedure, out- lined in the following paragraphs, can be used as a starting point for the P and I settings. Experience has shown that PID controllers will do a fair job of control- ling most processes with many combinations of rea- sonable control mode settings.

51

Page 56 Page 58
Image 57
Page 56 Page 58
Contents Model 54e pH/ORP PH/ORP Hart Analyzer/ControllerElectrical Shock Hazard Essential InstructionsPage Description and Specifications Model 54E PH/ORP Microprocessor AnalyzerSection Title Appendix Title List of TablesTable No Title About This Document Table of Contents List of FiguresSection Description and Specifications General DescriptionDescription of Controls Power Specifications Physical Specifications GeneralRelative Humidity 95%, non condensing Alarms Current OutputsRecommended Sensors Analyzer Specifications @ 25COrdering Information Code OptionsPart NO. Description Mechanical Installation Section InstallationLocating the Controller Unpacking and InspectionPanel Mounting Power Input Wiring Section WiringAnalog Output Wiring Alarm Relay Output Wiring454EPH02 PH Sensor Wiring PH Sensor CompatibilityTions will Result in Controller MALFUNC- Tion 454EPH01 Cable Dressing Final Electrical Check Wiring to Model Introduction Section CalibrationHold Mode Off Adjust temp 25.1 CTemperature Calibration Autocal buffer Buffer 1 -- WaitAutomatic TWO-POINT Calibration Auto buffer cal done Use the arrow keys to select the correct bufferCalibration Notes Pt 04.00pH 25C Calibrate pointPt calibration done Manual TWO-POINT CalibrationSINGLE-POINT pH Calibration Standardize 7.01 pHTemp comp Auto Temperature Compensation OptionsOutput trim 59.16 mV/pHHold Mode Output trimTrim Outputs PH Settings List Section Software ConfigurationRanges Factory Settings User Settings Diagnostics Section Outline of Menu Levels Changing Alarm Setpoints Alarm High 14.00pHSimulate tests Setpoint 07.00 pH4mA 0.00 pH 20mA 2.00 pH Setpoint 7.00 pHSetpoint + 06.90 pH Changing Output Setpoints PID onlyOutput OutputMA 0.00 pH 20 mA 14.00 pH Output 1 12.00 mA MA +00.00 pHTest output Test alarm Alarm setpoints Output setpointsTesting Outputs and Alarms Simulated testsTest alarm 1 Open Test alarm 1 OpenMeasure pH Display right AL2Security Caution Display contrast Timeout OnDisplay Control ModeChanging Output Parameters Output Control ParametersFault 22.00 mA Range 4-20 mAlRange 4-20 mA Output 2 control Output 2 setup Fault 22.00mAHold feature setup Hold Disable featureAlarms Alarm 3 controlInterval timer Setpoint 0.01 pH Delay 0 sec Alarm LowHysteresis 0.01 pH Delay 0 sec Relay Default NoneSetpoint +07.00 pH Relay default NoneTime period 30 sec URV 2.00 pHFeed limit Disable Alarm FaultTimeout 3600 sec Alarm 4 SetupTimer Disable Recovery 600 secOn time 120 sec Interval Timer Examples Diagnostics Diagnostics OffZero offset 60 mV Auto Calibration Setup DiagnosticsAutocal Standard Stabilize pH 0.01 pH Stabilize time 10 secStandard Buffers Auto Calibration SetupSection Theory of Operation Continuous Sensor DiagnosticsInterval Timer Typical set pointsSee .8 for instructions on chang- ing these setpoints Time Proportional Control TPC Mode Code Alarm RelaysAnalog Outputs Normal ModeController Mode Priority Chart Controller Mode PriorityCondition Definitions Action DefinitionsPID Control Code PID Control Mode CombinationsPID Control Measurement and Set Point Feedback ControlProportional Mode Gain Proportional Gain Plus Integral ResetControl Loop Adjustment and Tuning Process Reaction Curve Process Reaction Curve MethodPID Control Section Special Procedures and Features Password ProtectionLevel 1 3 Password Privileges Security Configuring SecurityCustom Curve Lock allSolution Temperature Compensation Temp coeff +.000Sensor iso 07.00 pH Operate iso +07.00 pH Troubleshooting is easy as 1, 2, 3… Section TroubleshootingDiagnostic Messages Description of problem Diagnostic MessagesSymptom Action Quick Troubleshooting GuideDiagnostic Variables Theoretical pH vs. Millivolt Values at 25C 77F Temperature Compensation Circuit Troubleshooting GuidelinesTroubleshooting Procedure Ohms Ambient TemperaturePreamplifier Troubleshooting Procedure Preamp in a junction box FigureController preamp check Figure Junction Box Preamplifier Check No Power Troubleshooting GuideCoated glass electrode Clean with soft cloth and clean water Password protected Part Number Description Replacement PartsNON-WARRANTY Repair Warranty RepairSection Return of Material GeneralQuick Startup ORP Appendix a ORP ConfigurationItem Choices ORP Settings User Settings Table A-1. ORP Settings ListDiagnostics Section Choices ORP Settings User SettingsSimulated tests Output setpointsORP Calibration Standardize 400 mVSensor or Circuit Board only Return of Materials RequestReturn of Material WarrantySpecifications subject to change without notice
Top Page Image Contents