Control of an industrial process using PID control blocks in automation controller by Michel Van Dessel 1 1 Abstract Many continuous industrial processes can be controlled using programmable automation controllers for digital implementation of classical PID control. For this type of applications, a PID control block has been developed for use in the programming environment PCWorx. Two types of PID control of tank level are discussed: single point control and cascade control. The single point control uses a single PID controller to regulate tank level using a single measurement for feedback being tank level itself. The PID controller output determines the state of the feed valve. The system has limitations in reacting to variations in output flow, which is to be regarded as a system disturbance. The cascade control uses two PID controllers for a better performance of the level control. The master controller is the level controller using the level setpoint and measurement to determine the setpoint for the slave controller driving the input flow valve. Measurements of input and output flow are taken into account, enabling the system to faster respond to output flow disturbances. Both types of control have been implemented using the PID control block in PCWorx for programming a Phoenix Contact PLC. The physical system, tank level control, was simulated in the process simulation environment ProcesSim running on the host PC. This virtual process environment enables the automation engineer to implement and test a PLC program controlling a process without having the actual installation physically available. Results of the PID controllers implemented within a Phoenix Contact PLC type ILC 150 ETH, linked to the simulated process in ProcesSim are presented. 2 PID control with automation controllers Many continuous industrial processes can be controlled using automation controllers for digital implementation of classical PID control (e.g. level control of tanks, flow control). The majority of installations use separate digital PID controllers for the control loops and PLC s for control of the processing sequence, monitoring of system variables, logging and alarms. For this type of applications, a programmable PID control function block has been developed for use in the programming environment PCWorx for implementation in Phoenix Contact PLCs [1]. The PID control is integrated in the PLC program and separate controllers are not needed, saving costs in hardware and installation space. 1 dr.ir. Michel Van Dessel, Lessius Mechelen Campus De Nayer, St.-Katelijne-Waver, Belgium
2.1 Basic PID control function block The internal block structure of the PID controller, as found in the standard library of PCWorx is shown in Fig.1. Fig. 1: Basic PID control function block The signals for this controller are Sp: setpoint, Pv: measured process value, Xout: controller output. The parameters of the PID controller are Kp: proportional constant, Tr: reset time constant, Td: derivative time constant. Kp is normally a positive amplification constant. It can also be negative, if the gain of the process to be controlled is negative. Since the PID controller is implemented as a function block of a digital PLC, special attention has to be paid to the sampling period T s or Cycle time. All function blocks having a Cycle input must not be executed in the default task of the PLC. The cycle time of the default task changes permanently according to the utilisation of the PLC. Therefore it is necessary to execute the corresponding function blocks in a CYCLE -task. The sampling time T s is set in a way, that it approximately corresponds to maximally one tenth of the equivalent process time constant T ETC : T s < 1/ 10 * T ETC 2.2 Advanced PID control function block The advanced PID control function block is found in the PCWorx Control technology library version 1_08. The function block, internal structure and main parameters are given in Fig 2. The function block realises a PID-type controller with a delayed D portion, including all possible sub variants (PI, PD, P and I) [2]. The transfer functions that are equivalent to continuousaction controllers are given in Table 1.
Fig. 2: Advanced PID control function block Table 1: Continuous-action PID controller transfer functions
3 Process simulation environment ProcesSim Fig. 3: Process simulation environment ProcesSim The process simulation environment ProcesSim has been developed by the Institution for industrial higher education of Mons (I.S.I.Ms, Belgium) in co-operation with Campus De Nayer. The software has been developed for use with the Windows operating system to simulate an industrial process or machine and to test, improve and maintain the PLC program that controls this process based on this simulation. This simulation environment enables the automation engineer to implement and test a PLC program controlling a process without having the actual installation physically available [3]. The consisting elements of the virtual process environment are depicted in Fig. 3. From a library of virtual process elements, the process simulation is built up. The program contains a range of industrial pneumatic and electric components, digital or analogue inputs and outputs, displays, product generators,... that can be used to simulate the industrial process. Dynamic visualisation generates a realistic simulation of both continuous as well as discrete components of the process. A local control program describes the way the simulated process elements interact and behave in time. The ProcesSim process environment runs on the host PC. Communication and exchange of variables between the PLC executing the control program and ProcesSim is performed using Modbus TCP.
4 Application: single point level control Two types of PID control of tank level are discussed: single point control and cascade control. Both types can be implemented using the PID control block in PCWorx for programming a Phoenix Contact PLC. The single point control uses a single PID controller LC to regulate tank level h using a single variable for feedback being tank level measurement provided by the level transmitter LT. The PID controller output determines the state x of the feed control valve CV, as shown in the process diagram. The input flow mi is proportional to the valve state x through valve constant Vc: mi = Vc. x mi Vc CV x τ LT At Pv h h set Sp LC ρ mu ry Fig. 4: Process diagram for single point level control [4]. The ProcesSim virtual process environment simulates the plant, while the PLC ILC 150 ETH performs the control using the advanced PID control function block. The user interface for the PID controller is a part of the ProcesSim GUI, as seen in Fig. 5. The results for the PID control of the tank level are plotted in Fig. 6, which shows the level ('Process value') and the level controller output. The system starts with a tank level of 50%, that is maintained at a constant value. After 18 seconds the output valve is fully opened, causing a drop in the level. This deviation is only corrected by a dramatic increase in the controller output, causing the input valve to be opened to almost 100%. Then follows an oscillation in the controller output and thus in the input valve state x. At 85 seconds the controller output is switched to manual mode and output 0, closing the input feed valve. Since the output valve is still open, the tank level gradually drops to zero. The system with single point level control has limitations in reacting to variations in output flow mu, which is to be regarded as a system disturbance.
Fig. 5: ProcesSim user interface for single point level control. 120 100 80 Controller output 60 Process value 40 20 0 0 20 40 60 80 100 120 Opening valve Activate manual Fig. 6: Results of single point level control
5 Application: cascade level control The cascade control uses two PID controllers for a better performance of the level control. The process diagram is shown in Fig. 7. The master controller is the level controller LC using the level setpoint and measurement LT to determine the signal b, that contributes to setpoint u for the slave flow controller FC driving the input flow valve CV. Measurements of input flow mi and output flow mu are taken into account (FiT and FuT), enabling the system to react to output flow disturbances. The setpoint for the flow controller is calculated by a computing relay CR: u = a + b - k = m u + master output load factor Vc mi x Pv LT h At CV τ FiT FC u CR k - + + h set Sp LC ρ mu ry FuT a b Fig. 7: Process diagram for cascade level control [4]. The virtual process for cascade level control in ProcesSim is shown in Fig. 8. The results of cascade level control are shown in Fig 9. The test starts with a tank level of 50%. The valve1, without flow measurement is opened first, causing a limited decrease in the level. The master controller reacts to this change and sends a signal b to the computing relay CR in the process diagram, increasing the input u for the flow controller FC. The output variation of the slave controller controls the feed valve to restore tank level to 50%. Valve 2, with output flow measurement is opened at 19 s. In this case the input a of the computing relay asks for an increase in the input for the flow controller, without the master controller having to intervene. The valve 1 is closed at 32 seconds and valve 2 is closed at 39 seconds. Since there is no output flow possible in this situation, the input feed valve has to be closed by the slave controller whose output drops to zero. This is under command of the master controller that measures tank level, controlling it to stabilise at 50%.
Fig. 8: ProcesSim user interface for cascade level control. 80 70 60 50 40 30 Tank level Master out Slave out 20 10 0 0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 Valve 1 open Valve 2 open Valve 1 closed Valve 2 closed Fig. 9: Results of cascade level control.
6 References [1] PCWorx IEC61131-Programming, Phoenix Contact GmbH & Co, Blomberg, Germany. [2] CLC library for the Control System PCWorx, Phoenix Contact GmbH & Co, Blomberg, Germany. [3] ProcesSim version 9.2, I.S.I.M., Mons, Belgium, http://processim.hecfh.be [4] Simulatie van technische systemen met MATLAB/Simulink, T. Van der Veen, Academic Service, ISBN 90 395 0584 5.