Electronic PID Controller Design for Gear Lubrication Oil Temperature Control

Author: Gerard Ratoka LekhemaEmail: info@khems.co.za Website: http://www.khems.co.za

Table of Contents Abstract..................................................................................................................... 2 1. Introduction........................................................................................................... 3 2. Mechanical Controller Specifications .................................................................... 4 3. Electronic Controller Instrumentation .................................................................... 5 3.1 Temperature Transducer ...................................................................................... 5 3.2 PLC..................................................................................................................... 6 3.3 Electro-pneumatic Actuator ................................................................................. 6 4. PID Controller Design ........................................................................................... 6 4.1 Electronic Controller Analysis ............................................................................. 6 4.2 PID Controller Analysis....................................................................................... 7 4.3 PID Software Design ........................................................................................... 8 5. System Implementation Considerations ................................................................. 9 5.1 Hardware Implementation ................................................................................... 9 5.2 Software Implementation................................................................................... 10 5.2.1 PLC System Control Program......................................................................... 10 5.2.2 PID Tuning..................................................................................................... 11 5.2.2.1 Proportional Gain Tuning ............................................................................ 11 5.2.2.2 Integral Gain Tuning.................................................................................... 11 5.2.2.3 Derivative Gain Tuning ............................................................................... 12 5.2.3 HMI Graphics Design..................................................................................... 12 6. Project Management ............................................................................................ 12 6.1 Phase 1 Testing.................................................................................................. 12 6.2 Standard Work Instructions ............................................................................... 13 7. Conclusion .......................................................................................................... 13 8. References:.......................................................................................................... 13 Appendix A: Equipment to Use ............................................................................... 14 Appendix B: ROM Variables of the F2614_PID1 function block............................. 15 Appendix C: Variables List ..................................................................................... 16 Appendix D: The General Control Program Illustration........................................... 17 Appendix E: General Illustration of the Designed System........................................ 19

Abstract Discussed in this paper is the analysis and design of an electronic temperature PID controller. This controller is basically an alternative system to replace the existing mechanical PID controller used to regulate the temperature of the gear lubrication oil. The specifications of the current mechanical controller system are first determined. This is followed by the design of the electronic controller based on the determined current system specifications. The PID control algorithm is implemented in Toshiba V3 PLC. The designed system is theoretically evaluated using transient response, steady-state response and system stability. The electronic PID controller tuning principles are also stated and finally the project implementation plan is explained in details.

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1. Introduction The proficient control of machinery used in the plant is one of the essential aspects necessary for consistent operation of plant processes. Different types of control systems classified in terms of control algorithms include linear control systems, sequential or logic systems, fuzzy-logic, artificial neural networks, etc. The general illustration of a closed-loop linear control system is depicted in figure 1 below. One of the advantages of this system configuration is the minimized system susceptibility to plant disturbances such as vibrations, thermal drifts, etc [1].

Figure 1: Illustration of a closed-loop linear system The characteristics which are used to evaluate the performance of the closed-loop control system are transient response, steady-state response and the overall system stability [1]. If these performance measures are not fulfilled, the closed-loop system offers the flexibility of incorporating a compensator in order to attain the desired response. Proportional-plus-Integral-plus-Derivative (PID) controller is the most commonly used industrial compensator mainly because of its functional simplicity and robust performance over a wide range of operating conditions. The PID controller can be implemented by either utilizing hardware components or a software algorithm. Most of the processes carried out in the plant are automatically controlled from Digital Signal Processors such as a Programmable Logic Controller (PLC). The recent PLC systems support PID control functionality. Implementing the PID algorithm for the closed-loop system in the PLC software has several prominent advantages such as improved overall system reliability. This is because the performance of the entire system can be continuously monitored from HumanMachine Interface (HMI) linked to the PLC. Furthermore, adjustments to the PID compensator parameters can be conveniently made in PLC software without any hardware changes. Also, the PID controller stability is maintained since the PLC software is not affected by external plant disturbances. The temperature PID controller for gear lubricant is not implemented in PLC and hence does not exhibit the above mentioned features. The system utilizes a complex pneumatic mechanical PID controller which may be difficult to repair or expensive to replace in case of failure. It is therefore worthwhile to replace the mechanical PID controller with a simple electronic transducer controlled and compensated for in PLC.

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The main objective of this document is to provide comprehensive details of the analysis and design of an electronic PID controller to replace the existing mechanical PID controller for the gear lubricant temperature control. The analysis and design process is partitioned into the following categories: The current mechanical controller system specifications determination Instrumentation of the electronic PID controller system Design of the PID controller software to be implemented in PLC. Implementation details of the designed system

Each of the individual aspects mentioned above will be elaborated and analyzed in details. Also, emphasis will be made on the details of the overall project management plan. 2. Mechanical Controller Specifications The gear lubrication oil lubricates the gear boxes interconnecting the Induction Motors to the mill stand and the winders. It is essential to maintain the gear lubricant at low temperature, because the lubricant has higher viscosity at low temperatures, which is necessary for preventing excess friction in the gear system. The general layout of the gear lubrication system is shown in figure 2 below.

Figure 2: General Illustration of the gear lubrication system The lubricant system specifications are determined as follows: Tank Capacity: 15 000 Liters Pump: 490 KPa, 500 Liters/Min Oil operating temperature range: 30 0C 50 0C Cooling Water Pressure: 0.35 to 0.59 MPa Cooling Water Temperature: 35 0C To control the lubricant temperature, the water volume flow rate circulating through the heat exchanger is regulated by a pneumatic valve.

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The pneumatic actuator of the valve is a single direct acting diaphragm which utilizes air to close and springs to open [2]. The valve has a fully open fail-safe position. The valve receives control signals from the pneumatic mechanical PID controller. The comprehensive illustration of the current control system is portrayed in figure 3.

Figure 3: The illustration of the Mechanical PID Controller System The current control system exhibits reliable and efficient temperature control characteristics, so the performance characteristics evaluation of the regulator, the pneumatic positioner and the pneumatic valve were not taken into design consideration. The mechanical PID controller Input-Output specifications are determined as follows: Input/Reference Temperature Range: 0 100 0C Air Pressure Supply: 140 KPa Output Pressure Range: 20 100 KPa 3. Electronic Controller Instrumentation The input-output characteristics of the current mechanical controller are applied as basic references for determining the electronic controller components specifications. The electronic controller is comprised of a temperature transducer, the main controller (PLC) and electro-pneumatic actuator. 3.1 Temperature Transducer Different types of temperature transducers exist and the most prevalent types used in industry are thermocouples and Resistance Temperature Detector (RTD). The thermocouple exhibits a number of drawbacks as compared to the RTD. To operate a thermocouple, a constant reference temperature is required as well as a temperature signal transmitter [3]. The RTD has a fast response and is not affected by thermal drifts because it is not self powered. The resistance of the RTD is linearly proportional to changes in temperature over a specified range. A PT100 is selected as the temperature transducer for the electronic controller.

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The PT100 is a platinum based RTD which has linear and repeatable resistance/temperature characteristics over a temperature range of 200 to 800 0C [4]. At 0 0C, PT100 has resistance of 100 . 3.2 PLC Toshiba V3 PLC is main controller for processes at the concerned mill. Signals from PT100 require conditioning components such as a wheat-stone bridge and a low pass filter. However, the Toshiba PLC has a RT318 Analogue-to-Digital converter (ADC) Input Module which is designed to process input signals from the PT100 sensor. The RT318 is an 8 channel, three-wire RTD input Module with low thermal drift (100 PPM/0C) and high resolution (12 bits). The input temperature range is 50 to 270 0C and the converted data range is 800 to 4000. The PLC also has a DA374, 4-channel Current Analogue Output Module suitable for controlling the electro-pneumatic actuator. The output current range of DA374 module is 4 20 mA or 0 20 mA. 3.3 Electro-pneumatic Actuator The electro-pneumatic actuator is used to convert the control electrical signals from PLC into corresponding pressurized air signals for controlling the pneumatic valve. The actuator requires a supply of pressurized air in order to perform this operation. A vibration immune, weather-proof and high resolution WatsonSmith current-topressure converter is selected as an actuator for the electronic controller. The actuator has an input current (I) range of 4 20 mA, output pressurize air range (P) of 20 100 KPa and supply air pressure of 130 400 KPa [5]. The input-output relationship of the converter is given in equation 1.POUT = 5 KPa mA Equation 1

4. PID Controller Design It is crucial to separately analyse the performance of the electronic temperature controller before the PID compensation can be designed. This is accomplished by simulating the system frequency response using the transfer function of the system. However, it was not feasible to practically determine the parameters of electronic systems dynamic response, so a theoretical analysis is applied to predict the performance of the uncompensated system. From the predicted system performance, the PID compensation algorithm can then be designed. 4.1 Electronic Controller Analysis The entire electronic temperature control system is a high order system and its performance is evaluated using the transient responses (rise time, percentage overshoot and settling time), steady-state error analysis and system stability. The general performance characteristics of stable and unstable uncompensated systems are illustrated in figure 4 for a step change in reference temperature. The control of temperature is normally a sluggish process due to the lubricant thermal inertia as well as the mechanical inertias of the electro-pneumatic actuator and the pneumatic valve.

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These temperature characteristics lead to undesired rise and settling times, high percentage overshoot and significant steady-state error. Also, the system may not be stable; hence the controlled temperature will start oscillating around the reference value. 4.2 PID Controller Analysis The ultimate aim of the PID compensation algorithm is to achieve insignificant values of rise time, settling time, percentage overshoot, steady-state error and also the precise system stability. The closed-loop electronic controller continuously compares the set value (reference temperature) with the process variable (lubricant temperature) and the difference is the Error signal (E(t)) which is used to in determining the process actuating signal. The PID algorithm is used to manipulate the E(t) such that the desired response is established.

Figure 4: Controlled temperature response to step change in reference value The Proportional term (P) induces compensation on output which is directly proportional to E(t) as shown in equation 2.P = K P E (t ) .Equation 2 Where KP = Proportional gain.

In order to yield desired proportional response, KP is adjusted. Setting KP to high will result in the large percentage overshoot and an unstable system with oscillating controlled variable (temperature). The correct value of KP will yield the desired transient time (fast rise time and short settling time). The integral term (I) operates by summing up the instantaneous values of E(t) over time and multiplying the accumulated value by KI as depicted in equation 3.t=N

I = KI

E (t )dt Equation 3t =0

Where KI = Integral Gain, N = time period of integration

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Even if E(t) is small, the integral term will slowly accumulate the errors and generate a response which will eventually drive the steady-state error to zero. The negative aspect of the application of integral term is the increased system percentage overshoot. The Derivative term (D) calculates the rate at which the E(t) is changing over time and multiplies this rate by KD as shown in equation 4.D = KD d E (t ) .Equation 4 dt Where KD = Derivative gain.

The derivative term is used to increase or decrease the rate at which the reference value is reached and this can be used to reduce percentage overshoot and decrease transient time (rise and settling times). The drawback of the derivative term its noise susceptibly characteristic and this causes the insignificant noise disturbances to result in large output response distortions. The complete PID algorithm is the sum of the equations above and is given in equation 5. t=N d PID = K P E (t ) + K I E (t )dt + K D E (t ) ..Equation 5 dt t =0 4.3 PID Software Design Since the PLC is a digital signal processor (DSP), then the continuous-time PID algorithm from equation 5 has to be converted into discrete-time algorithm before implementation in PLC. The discrete-time PID algorithm (PIDD) is given in equation 6 [6]. The PIDD algorithm shown in equation 6 can be easily programmed in any type of PLC, however, the Toshiba V3 PLC have integrated PID function blocks which simplifies the implementation of the algorithm. The Toshiba PLC is programmed using Ladder programming language.PIDD = K P E (kT ) + K I T k=N K [E (kT ) + E ((k + 1)T )] + TD [E ((k + 1)T ) E (kT )]....Eqn 6 2 k =0 Where T = PLC sampling or scanning period, k = number of samples taken

The selected function block for implementing the algorithm is F2614_PID1 auxiliary PID function block. The Set Value (SV) of the lubricant temperature is 40 0C as determined from the lubricant temperature operating limits (30 50 0C). The temperature feedback data range from the RT318 (PT100 input module) is converted to the corresponding temperature values using F2575_SCL2 which is an RTD signal conversion and scaling function block. The F2614_PID1 and F2575_SCL2 function blocks are portrayed in figure 5. The terminals displayed on the function blocks in figure 5 are all utilized in the PID program. The rest of the idle terminals are omitted. The ROM variable members of F2614_PID1 are explained in appendix B.

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5. System Implementation Considerations The implementation of the designed system will be categorized into hardware and software sections. The system hardware components to be implemented are the temperature transducer and the electro-pneumatic converter. The software implementation covers the system control program implemented in PLC and the HMI graphics design.

Figure 5: F2614_PID1 and F2575_SCL2 function blocks 5.1 Hardware Implementation The hardware components are installed in the Cellar of the concerned mill. The PT100 will be mounted in the Temperature Well where the mercury-filled thermometer is currently installed and the electro-pneumatic converter will be mounted next to the vertical plate where the mechanical controller is currently installed. Both the PT100 and the Electro-pneumatic converter require shielded and twisted connection wires to prevent electromagnetic interference. The Remote Cellar PLC (A_R_Cellar V3000) has a remote I/O processor (R3PU45) mounted on the base unit BU74A. The processor is connected through slot 2 of expansion interface module IF721 to the base unit BU35B. The module number 7 on BU35B is the RT318 module with only one channel occupied. The three-wires from the PT100 will be connected to one of the free channel of RT318 module. The output signals to the electro-pneumatic converter will go out through module number 5 (DA374), which is the digital-to-analogue converter module. The Remote Cellar PLC communicates with the Master/Auxiliary PLC through a Time Critical Control LAN (TC Net). The variables used in the BU35A modules have to be registered with a talker block number 432 so that they can be viewed and manipulated in the Master/Auxiliary PLC through the TC Network.

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5.2 Software Implementation 5.2.1 PLC System Control Program The gear lubrication pumps are controlled from the Auxiliary PLC S3PU55, in task Aux_MS030. Each V3 PLC task takes about 4096 program steps. Since the Aux_MS030 task has only 126 program steps, then the system control program can be implemented in Aux_MS030 task. The program variables are categorized into Local, Global and Network variables. The Local variables are used only within the scope of Aux_MS030 task and their states cannot be accessed in other PLC tasks. The status of the Global variables can be accessed from other Auxiliary PLC tasks. The Network variables are the I/O variables and any other variables which are registered on the talker blocks and can be accessed and manipulated through the network from the Master or Auxiliary PLC. The general system control program flow chart is illustrated in figure 6 below.

Figure 6: The system control program flow chart The suggested Local, Global and Network variables as well as the suggested control program are given in tables of appendix C

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5.2.2 PID Tuning Several different types of PID control loop tuning alternatives have been devised and these include practical and mathematical tuning methods. The practical methods embrace Cohen-Coon reactive curve method, Ziegler-Nichols oscillation method, reactive curve method, etc [7]. The mathematical PID loop design includes RootLocus techniques, frequency response techniques, etc [1]. The practical ZieglerNichols oscillation method is applied to experimentally tune the PID loop of the electronic temperature controller. In order to observe the graphical response of the measured temperature, tags should be created in Online Data Gathering (ODG) software for the following network variables: L_GRLUB_REF and L_RTGRLUB_DSP (From appendix C). The highest tags sampling time should be selected in order to get an accurate real-time graph. The tags should be saved in a renamed template such as Gear Lubricant PID Temp FBK and SV. The heater should first be used to heat the lubrication oil to a desired value before the tuning is commenced and during the tuning process, the heater will be repeatedly turned on and off to generate the heat variations equivalent to the heat generated by friction in the gears. 5.2.2.1 Proportional Gain Tuning Firstly the lubricant is heated to about 50 0C, and the Integral and derivative gains are set to zero. Then the proportional gain (KP) is set to a very small value, which is around 2.0. The Set Value (SV) is placed at 40 0C. The temperature feedback (FBK) is observed on the ODG and if the FBK starts to oscillate, then this means that KP is too high or the proportional output range is set too high. The range can be adjusted by changing the OMX and OMN values from appendix B. Very slow changes in FBK indicate that the KP is too low. The value of KP should then be increased by at least doubling the previous values of KP. After each transient response for changed value of KP, the SV should be varied by a small fraction in order to observe the new response. As KP is increased, it will reach a point where temperature feedback (FBK) starts oscillating and immediately when these oscillations begin, the value of KP is recorded as KC. If no oscillations occur, the output range in appendix B should be increased. The final determined value should be entered as (0.5KC). 5.2.2.2 Integral Gain Tuning The integral gain (KI) tuning should be started with a higher value of gain such as 10 and after each KI adjustment, the SV should be changed by a little value in order to observe the response. If FBK never stabilizes, but oscillates, then the KI is too small and should be increase by small fractions. If the FBK is stable but never reaches the SV, then the KI is too high and should be decreased by small fractions. The KI term is crucial to ensure that pneumatic valves reaches a steady operation and does not oscillate. This will reduce the wear and increase the life-time of the pneumatic valve. The value of KI is therefore perfect when there are no oscillations observed from FBK and the steady-state error is approximately zero.

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5.2.2.3 Derivative Gain Tuning In order to reduce the percentage overshoot as well as the FBK settling time, the derivative gain KD is utilized. The initial value of KD is determined using equation 7 below. 1 .Equation 7 [8] KD = 6 KI The percentage overshoot is reduced by reducing the value of KD slowly and settling time is reduced by slowly increasing the value of KD. 5.2.3 HMI Graphics Design

The HMI Graphics for the concerned mill are designed using the WindowMaker Development environment of InTouch HMI Applications Development Software. The parameters such as Temperature set value, and the measured temperature feedback values will be displayed on the Gear Lubrication System HMI page. The Function block FB_HMI_DSI can be incorporated to process execute and clear commands for changing the temperature set value on the HMI screen.6. Project Management

Before installation can be carried out, the technical reference manuals of both the electro-pneumatic converter and the PT100 must first be studies and profoundly understood. The list of equipment to use is given in appendix A. The sequence of the design, implementation and commissioning tasks is given in figure 7 below. The critical tasks (task 7, 8 and 9) must be performed while the mill is stopped.

Figure 7: The proposed design, implementation and commissioning tasks schedule6.1 Phase 1 Testing

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The phase 1 testing is a preliminary testing strategy to evaluate the effectiveness of the PID Gains and ROM parameters. The PT100 probe will be heated and cooled with external devices in order to obtain variation in temperature feedback. The actuation current signal to be sent to the electro-pneumatic converter will be considered as the system output. The temperature set value will be varied on the HMI and the current signal response will be viewed on the ODG as the temperature feedback is slowly changed.6.2 Standard Work Instructions

The PLC I/O modules should first be isolated from the power supply before wiring can be carried out on them. The main pressurized air supply should be turned off before pneumatic tubes can be installed. The hazardous work and the plant access permits should first be acquired before any work can be carried out in the cellar of the concerned mill. The additional safety equipments required are Oxygen gas monitor and the oxygen air supply pack.7. Conclusion

The analysis and design of the electronic temperature PID controller has been carried out. The designed controller is exhibited in appendix E. This controller is basically a system to replace the existing mechanical PID controller used for regulating the gear lubricant temperature. The WatsonSmith current-to-pressure converter is used for electronic actuation and the PT100 probe is used for temperature feedback to PLC. The Toshiba PLC is used as the main system controller. To simplify implementation, the readily available control function blocks of Toshiba PLC are used. The PID tuning principles were presented as well the implementation and commissioning details of the entire system. Since the project is a paper work design, the comprehensive system details will be acquired during the practical system implementation.8. References:[1] Nise Norman S, 4th Ed. Control Systems Engineering. John Wiley and Sons, INC, California. [2] CV3000 Series Pressure-Balanced Cage type Control Valve http://www.yamatake.com/products/bi/iap/ss/cv/SS2-ACP110-0100.pdf Last accessed 10 April 2008 [3] Basic Instrumentation Measuring Devices and basic PID Control http://www.pacontrol.com/download/BASIC-INSTRUMENTATION-MEASURING-DEVICESAND-BASIC-PID-CONTROL.pdf Last accessd 13 April 2008 [4] Bentley J P, Principles of Measurement System, 4th Ed. Pearson Education, London, 2005. [5] WatsonSmith Electronic current to pressure (I/P) Type 425

http://keison.co.uk/watson/pdf/watson_425.pdf, Last accessed 13 April 2008

[6] Phillips C L, Parr J M, Riskin E V. Signals, Sytems and Transforms, 3rd Ed Pearson Education International, Upper Saddle River 2003. [7] Classical PID Control http://csd.newcastle.edu.au/book_slds_download/Ch06c.pdf Last accessed 10 April 2008 [8] Temperature Control: Tuning a PID (Three Mode) Controller http://www.omega.com/temperature/z/pdf/z115-117.pdf, Last accessed 02 June 2008

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Appendix A: Equipment to Use

1. WatsonSmith Electro-pneumatic converter. Columbus stores Item Number: 80101214 RS-Components Cost Price: R 6,038.11 2. Three Wire PT100 Columbus Stores Item Number: 802012586 RS-Components Cost Price: R 404.09 3. Twisted, Shielded Cables 4. Terminal blocks 5. Junction Box 6. Pneumatic tubes

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Appendix B: ROM Variables of the F2614_PID1 function block

The ROM variables can be determined practically based on the observed behaviour of the process. The cooling water temperature is 35 0C and the gear lubricant temperature set point is 40 0C. It is assumed that the gear lubricant can reach the highest temperature of 60 0C and a minimum value of 30 0C. From the HMI gear lubrication system page, it is observed that heater is started when the temperature goes below 20 0C as measured by temperature switch. The Output up and low values are set with very narrow limits (deviation Gain of 1) in an attempt to minimize output oscillations and system instability. The proportional dead band set value is used to increase or decrease the proportional gain based on the deviation value of the feedback temperature from the set value. Table 1: Variable Members of the ROM variable of the F2614_PID1 function block Variable Data Type Comment Suggested Initial Member Values 0 OMX REAL Output up limit ( C) 20 OMN REAL Output low limit ( 0C) -10 PMX REAL Proportional out up limit ( 0C) 10 0 PMN REAL Proportional out low limit ( C) -5 IMX REAL Integral out up limit ( 0C) 10 IMN REAL Integral out low limit ( 0C) -5 0 DMX REAL Differential out up limit ( C) 5 DMN REAL Differential out low limit ( 0C) 0 DVMX REAL Deviation in up limit ( 0C) 20 DVMN REAL Deviation in low limit ( 0C) -10 SCNT REAL Sampling time sec PLC scan time RTH REAL Control off rate 30 RTL REAL Control running rate 300 ION REAL Integral on start range 30 DB_S REAL Proportional dead band set ( 0C) 5 DDL REAL Differential control delay time 0.1 OFT Double Integer Control off delay time (sec) 10 sec (DINT)

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Appendix C: Variables List Table 1: Module 7 RT318 and Module variables I/O Word I/O Variable No. 1 Batch IN L_RTGRLUB 5 DA374 Input-Output (Network) Data Type INT Comment

1

Batch OUT

L_PRCTRL

INT

Gear Lubricant Temperature Input Electro-pneumatic Pressure Control Output

Table 2: Task Aux_MS030 Global Variables Variable Data Type Comment G_RTGRLUB_R REAL For Temperature Int_to_Real Conversion G_RTGRLUB REAL Scaled Input Temperature G_RTGRLUB_TERR REAL PT100 Temperature Error G_PID_ON_CMD BOOL PID control on command G_PID_RUN_CMD BOOL PID control run command set high if 1 pump is running

Table 3: Task Aux_MS030 Local Variables Variable Data Type Comment PT100_GRLUB_H REAL PT100 temperature upper limit PT100_GRLUB_L REAL PT100 temperature lower limit GRLUB_GP1 REAL PID Proportional gain GRLUB_GP2 REAL PID dead band proportional gain GRLUB_GI REAL PID integral gain GRLUB_GD REAL PID derivative gain GRLUBPID_OUT_W REAL PID output word GRLUB_T_NEG BOOL Negative status of the PID output word GRLUB_X_W REAL Temporary word for PID output scaling

Table 4: Task Aux_MS030 Network Variables Variable Data Type Comment GRLUB_REF REAL PID Temperature Set Value L_RTGRLUB_DSP REAL Display Temperature feedback on the Online Data Gatherer

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Appendix D: The General Control Program Illustration

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Appendix E: General Illustration of the Designed System

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