PID CONTROLLERS OF INDUSTRY SYSTEM SIMATIC

Transcription

1 PID CONTROLLERS OF INDUSTRY SYSTEM SIMATIC ONDROVIČOVÁ MAGDALÉNA, BAKOŠOVÁ MONIKA, VANEKOVÁ KATARÍNA Faculty of Chemical and Food Technology Slovak Technical University in Bratislava Institute of Information Engineering, Automation and Mathematics Department of Information Engineering and Process Control Radlinského 9, Bratislava, Slovak Republic Abstract: In this paper results of control of reboiler temperature by industry control system Simatic are presented. In the Simatic library occur several controllers and two of them we have chosen to temperature control: FB41 and FB58. The main aim of this paper is compare results of control by two blocks of PID controllers. Keywords: industry system Simatic, control of temperature, controller 1 SIMATIC S The structure of the user s program is given by organization block OB, witch represent main program that is cyclic working. Organization block OB consists from function blocks (FB). They have the memory and enable store variables in block by declaration table. For most often used routines are used Functions (FC). To store user data are used data blocks (DB). To right work of the FB, FC and DB blocks they have be called by organization block OB. Before blocks programming it is necessary to made project, to configure network and define connected and used input and output modules. Visualisation of the project is realised in v Graphics Designer. 2 PID CONTROLLERS IN STEP 7 A library of program Step 7 provides a package of function blocks (FB) that contains of the PID controller blocks. To control of processes user may choose suitable PID controller and to set its parameters. In this paper were chosen two controllers to control of temperature of the reboiler to compare their effect to process control. Function block FB41 is continuous PID controller and block FB58 is PID controller especially to temperature control used. 2.1 Function Block FB41 FB41 CONT_C is used on Simatic S7 to control processes with continuous input and output variables. During parameter assignment, we can activate or deactivate sub functions of the PID controller to adapt the controller to the process. The inputs and outputs of Function Block FB 41 are shown on the Fig. 1. The input parameters are: MAN_ON - if this input is set TRUE, the control loop is interrupted and a manual value is set as manipulated value. P_SEL the PID actions can be activated or deactivated individually in the PID algorithm. The P action is on when the input proportional action on is set TRUE. I_SEL - the PID actions can be activated or deactivated individually in the PID algorithm. The I action is on when the input proportional action on is set TRUE. D_SEL - the PID actions can be activated or deactivated individually in the PID algorithm. The D action is on when the input proportional action on is set TRUE. SP_INT The internal setpoint input is used to specify a setpoint. PV_IN an initialisation value of process variable MAN manual value input is used to set manual value using the operator interface functions GAIN proportional gain R093a - 1

2 TI the reset time input determines the time response of the integrator TD - the derivative time input determines the time response of the derivator unit LMN_HLM manipulated value high limit LMN_LLM - manipulated value low limit Output parameters of controller LMN - manipulated value output LMN_P proportional component output of the manipulated variable LMN_I integral component output of the manipulated variable LMN_D derivative component output of the manipulated variable PV process variable ER error signal Figure 1 - Inputs and outputs of FB41 The block scheme of the PID controller is shown in the Fig. 2. R093a - 2

3 Figure 2 - Block scheme of PID controller FB41 From the block scheme in Fig. 2 we can determine simply form of the transfer function of the controller 1 G R ( s) = Zr(1 + + Td. s) Ti. s To verify outputs of the Simatic controller we used Matlab and Simulink. In Simulink we created scheme of the PID controller (Fig. 3) with as inputs and outputs as block FB41. This scheme enables to us compare outputs of the Simulink and Simatic controllers. Figure 3 - Simulation scheme of PID controller FB41 in Simulink Parameters LMN_P, LMN_I, LMN_D in Fig. 3 have the same meaning as parameters of the Simatic PID controller block FB41 (Fig. 1, Fig. 2). Simulink block Saturation enable to set high and low limits for output of controller. Constant is initialisation value of the process value. 2.2 Function Block FB58 Function Block FB58 is used to control temperature processes with continuous or pulsed control signals. We can set parameters to enable or disable subfunctions of the PID controller and adapt it to the process. These settings can be made simply with the parameter assignment tool. FB58 can be used either purely for heating or purely for cooling. If we use the block for cooling, GAIN must be assigned a negative value. The PID algorithm operates as a position algorithm. The proportional P, integral (INT), and derivative (DIF) actions are connected in parallel and can be activated or R093a - 3

4 deactivated individually. This allows P, PI, PD, and PID controllers to be configured. The controller tuning supports PI and PID controllers. In the Fig. 4 is scheme of block FB58. We can see variables on the input and output of the block. Figure 4 - Scheme of block FB58 From the scheme on the Fig. 4 we can obtain equation of controller 1 LMN _ sum( t) = GAIN * ER(0) 1+ * t + D _ F * e TI t TD / D _ F where LMN_sum manipulated value ER(0) error GAIN controller gain TI integrator time constant TD derivative time constant D_F derivative factor Then the form of the controller transfer function is 1 G R ( s) = Zr(1 + + Td. s) Ti. s In the Fig. 5 we can find form of the function block FB58 that is used in organisation block OB1. There we can see setting of the input and output parameters of the controller. The input parameters of the FB58 may be set directly in OB1 or by data block. (Fig. 6) Figure 5 Inputs and outputs of controller FB58 R093a - 4

5 Figure 6 - Data block of the FB58 Inputs and outputs parameters of the block FB58 are: MAN_ON - on/off controller SP_INT input of setpoint PV_INT measured value MAN manual value if MAN_ON is set TRUE LMN - manipulated value PV process value Internal parameters of FB58: LMN_P output of P part of controller LMN_I - output of I part of controller LMN_D - output of D part of controller GAIN gain TI integrator time constant TD - integrator time constant LMN_HLM - manipulated value high limit LMN_LLM - manipulated value low limit ER error PID_ON activation of PI or PID controller 3 DESIGN OF CONTROLLER PARAMETERS Four electric heaters heat a reboiler. The temperature inside of the reboiler depends on the power consumption of the electric heaters. So, a controlled variable is a temperature and we can describe model of the reboiler in the form () Q t d = k υ() t dt where Q - a power of electric heaters k - constant depended on reboiler and liquid parameters υ - temperature Controller design by Pole-Placement method is really generally well - known. We have controlled system with 1 st order transfer function. We can write characteristic equation of the close R093a - 5

6 loop system with suitable controller structure. Than we can choose poles of the close loop system and obtain constants of controller. By this way was obtained gain of P controller. 4 CONTROL OF THE REBOILER TEMPERATURE Visualisation of the project is realised in Graphics Designer of WINCC. This environment enables monitoring and control of process variables as well as setting of parameters of controller. Design of the operator s window is on the Fig. 7. Figure 7 - Design of control display To verify functionality of both controllers we have set the same P part of the controllers. After to set the desired temperature we could see the control response of the temperature (Fig. 8). We compared control of the reboiler by Simatic controller and control of the reboiler model by simulation in Matlab. Outputs of the P part both controllers (without limits) are shown in Fig. 9. Step by step we have set various desired values of the temperature. Values of setpoint, manipulated and measured variables were archived in second intervals. Results of control of reboiler temperature by controller blocks FB41 and FB58 are shown in Fig.10. Comparing of outputs of P parts of function blocks FB41 an FB 58 are in Fig. 11. R093a - 6

7 Figure 8 - Control response of the temperature control by Simatic Figure 9 - Comparison of P parts of Simatic and Matlab controllers. R093a - 7

8 Figure 10 - Comparison of control responses of two Simatic controllers 5 CONCLUSIONS Obr.11 Outputs of P parts of controllers FB41 and FB58 The main aim of this paper was to control of temperature of reboiler by control system Simatic. Library of Simatic offers several types of controllers. Two of them were chosen and used in control program Step7. We obtained and compared results of control by two blocks of Simatic controllers. Control result were the same. To design parameters of the controller we used enthalpy R093a - 8

9 model of the reboiler ansd generally well - known Pole-Placement method. Finally, we compared control responses obtained in Simatic and Matlab. REFERENCES SIMATIC. Standard Software for S7-300 and S7-400 PID Control V5.1. Source: MÉSZÁROS, A., DANKO, J., MIKLEŠ, J., BAKOŠOVÁ, M Základy automatizácie, STU, Bratislava 1997 BAKOŠOVÁ M., FIKAR M., ČIRKA Ľ., Základy automatizácie, STU, Bratislava 2003 Acknowledgement The authors are pleased to acknowledge the financial support of the Scientific Grant Agency of the Slovak Republic under grants No. 1/1046/04 and 1/3081/06. The authors are pleased to acknowledge the financial support of the Cultural and Educational Grant Agency of the Slovak Republic under the grant No. 3/3121/05. R093a - 9