Three-tank System Exercise Advanced Process Control Methods and Process Control Project Work Laboratory of Process Control and Automation 1
Table of Contents 1 Introduction ------------------------------------------------------------------------------ 3 2 Three-tank System ---------------------------------------------------------------------- 3 2.1 Specifications ----------------------------------------------------------------------- 5 2.1.1 Three-tank: -------------------------------------------------------------------- 5 2.1.2 Pumps: ------------------------------------------------------------------------- 6 2.1.3 Valves: ------------------------------------------------------------------------- 6 2.1.4 Sensors: ------------------------------------------------------------------------ 6 3 Control System of the three-tank system --------------------------------------------- 7 3.1 Control Panel GUI --------------------------------------------------------------- 9 4 Preliminary Exercise------------------------------------------------------------------- 10 5 Laboratory Exercise ------------------------------------------------------------------- 11 6 Appendix -------------------------------------------------------------------------------- 13 6.1.1 DAQ card --------------------------------------------------------------------- 13 6.1.2 I/O Terminal ------------------------------------------------------------------ 14 6.1.3 Power Supply ---------------------------------------------------------------- 15 6.1.4 Inverters for the Pump Control --------------------------------------------- 16 6.1.5 Valve Interface Module ----------------------------------------------------- 17 6.1.6 PC ----------------------------------------------------------------------------- 18 2
1 Introduction The three-tank system along with its state-of-the-art automation system is a simple, robust and an efficient platform to achieve the necessary hands-on experience on various stages of control system design. The stages range from process modeling, understanding the process dynamics and designing an efficient control strategy to achieve better control on the process of interest. The advanced control algorithms such as Model Predictive Control (MPC) with significant impact on industrial control engineering can be well understood when handled practically. This practical knowledge coupled with the theoretical knowledge, gives an edge for the student to understand the underlying concepts in control systems. In the preliminary exercise, one is required to model and simulate the three-tank system, linearized it around an operating point and to design the decouplers to efficiently handle the MIMO three-tank system. The open loop characteristic of the three-tank system is to be studied. The closed loop control involving both PID and MPC controllers is to be implemented to achieve an efficient control on the levels of the three-tank system. In the laboratory exercise, one acquires a real experience on the three-tank process by calibrating the level and inlet flows. The closed loop control (both PID & MPC) is to be achieved on the real time three-tank process which is supported by an automation system backed up by a data acquisition card. A detailed description of the three-tank process along with the specifications of its hardware is provided in Section 2. It is followed by brief explanation of the automation system of the three-tank process in Section 3 along with its graphical user interface. The exercises are provided at the end in Section 4 and Section 5. 2 Three-tank System The core process of this laboratory exercise is the three-tank system, which consists of three identical cylindrical tanks as depicted by the scheme shown in Figure 1. Two diaphragm pumps feed liquid to the tank1 and the tank2 from the main reservoir. The 3
tanks are interconnected by the cylindrical pipes of the cross section S n with the interconnecting valves and a nominal outflow from the tank2 to the main reservoir through a pipe of the cross section Sn with an outflow valve. Figure 1: Schematic diagram of the three-tank system Apart from the three valves mentioned above, each tank has a leak valve at the bottom of each tank. The liquid entering the reservoir from the tanks is pumped back to the tanks via the pumps, denoting a closed system. The levels in the tanks are measured by three capacitive pressure sensors placed at the foot of each tank. The whole experimental setup of the three-tank system consists of level sensors, actuators (pumps and valves) and is connected to a PC via a data acquisition card. The pumps and valves are controlled from a PC based on the requirements of the experiments. The pumps are switched off when a liquid level of the tank1 and the tank2 exceeds the specified upper limit, whereas the valves can be opened or closed to isolate each tank (SISO) as in case of interconnecting valves or to drain each tank in the case of the leak valves. The three-tank system facilitates the introduction of sensor and actuator faults to test various scenarios encountered in a real world. Leaks (disturbances) can also be introduced to the process through the leak valves. The three-tank system is a multivariable system with two inlet flows and three level outputs. However, only the levels in the tank1 and the tank2 are considered as outputs in most cases, due to the controllability issues. The three-tank system being a 4
MIMO system, the presence of strong interactions between the levels of each tank are justified by the fact that the level in a particular tank is always effected by the height of the liquid column in the adjacent tank/tanks. The three-tank system is modelled by the well known mass balance equations based on the Bernoulli s law of liquids. The unmeasured flow-rates between the tanks q 13, q 32 and q 20 are determined using the Torricelli-rule. The square root terms present in the model equations induce non-linearity into the process. The liquid levels in the tank1 and the tank2 are independently controlled by manipulating the inlet volumetric flows q 1 and q 2 through the inlet pumps. The level in the middle tank is uncontrollable as is not directly manipulated by inlet flows. However, better controllability can be achieved only when the number of the control inputs is not exceeded by the number of the outputs tracking the reference points. The three-tank system is an open loop stable system and an equilibrium point h > > is achieved finally. The whole system of the three-tank system is 1 h3 h2 considered as a flat hybrid system with four possible state locations, where a differential model can be attained in each location. The conditions corresponding to the above locations are h 1 h3 or h 1 < h3 and h 2 h3 or h 2 < h3. The three-tank system being a nonlinear system is linearized around an operating point by the Taylor expansion to further implement the intended control strategies. 2.1 Specifications The dimensions of the three-tank system and hardware specifications of the threetank process equipment involving pumps, valves and level sensors are outlined as followed (AMIRA, 1996). 2.1.1 Three-tank: Cross Sectional Area of the tank: 0.0154 m 2 Cross Sectional Area of the Connecting Pipes: 0.5 cm 2 Maximum Liquid Level in the tank: 630 mm Tank capacity: 9 litres 5
Main tank capacity: 55 litres 2.1.2 Pumps: Type: DC-motor with a three chamber diaphragm pump Voltage rating: 12VDC Current rating: 1.4A 4.5A(max) Flow rate: 7 litres/min 2.1.3 Valves: Type: Electrical control valve Voltage rating: +24V Current rating: 1A Potentiometer output: 5 kohm Working angle: 90 degrees Operating time: approx 10sec Output signal valve position (0-100%): -10V - +10V The valves are also equipped with additional features such as electronic torque limitation with additional limit switch contacts and a visual position indicator. All the control valves are primarily controlled through an adapter box with a converter module for each valve. The adapter box converts the digital signal to power signal (24VDC/1A) to operate the motor of the control valve, generates digital signals for the fully open and fully close positions of the valves and maps the potentiometer output to the voltage range of -10V to +10V. The converter module consists of a switch for manual control of the valve along with the four LEDs indicating the position of the respective valve. 2.1.4 Sensors: Type: Capacitive Pressure sensors Supply Voltage range: 12VDC- 30VDC 6
Maximum Current: 23A Output signal nominal: 4-20mA Output Current range - Liquid level(0-630mm): 4-14mA 3 Control System of the three-tank system The automation system of the three-tank system is designed around a data acquisition card. Data Acquisition as shown in Figure 2 is the process which starts with the gathering of the real world physical parameters from the measurement sources to a PC as measurable electrical signals such as voltage/current and digitizing the acquired signals for storage, analysis and representation on a PC. Figure 2: PC based Data Acquisition (National Instruments) The NI data acquisition card acts as an interface between the three-tank process and the PC, where various control algorithms such as PID and MPC are designed and implemented in Matlab. The data acquisition cards are configured and accessed through the data acquisition toolbox in Matlab. The acquired parameters such as the levels and pump flows, which are digitalized into electrical signals by the process of data acquisition, are to be calibrated every time by the user to represent the real world parameters of level in mm and flow in ml/sec. Finally a graphical interface is designed to interact with the three-tank system and to implement already developed control algorithms. The architecture of the three-tank automation system along with the vital equipment is shown in Figure 3 and is followed by the description of the graphical user interface. 7
power Architecture: NEW 3-TANK SYSTEM PS1 PS2 inve rter 1 Inve rter 2 NI Matlab NI -DAQmx PXI-1033 with NI-6221 SCC-68 Phoenix 1 Phoenix 2 3Tank Valve box Figure 3: Architecture of the automation system for the three-tank process The focal point of the architecture depicted in Figure 3 is the NI PXI-6221 M series multi-function DAQ card connected to the PXI bus of the PC. The card is selected based on the process input/output requirements. The process information (I/O) as listed in Figure 4 is routed to the DAQ card through NI SCC-68, acting as a junction box to collect all information at one place and to transfer it to the PC. The other hardware is chosen to power or to support the other vital equipment. Input Output AI(mA) Level Sensors 3 Valve Position 6 DI(logical 0/1) Valve fully open 6 Valve fully close - 6 AO(VDC) Pump control 2 DO(logical 0/1) Valve open 6 Valve close - 6 Figure 4: Three-tank process I/O information 8
The power supplies (PS1 & PS2) provides DC power to both the pumps and the three level sensors of the three-tank system. The purpose of the inverters is to control the input voltage to the pumps according to the requirements, thus controlling the amount of liquid pumped by the pumps. The phoenix contacts provide an interface to collect the information of the valves from the valve box. A detailed description of this equipment is provided as Appendix for further reading. 3.1 Control Panel GUI A simple and straight forward graphical user interface (GUI) was created in Matlab by the graphical user interface development environment (GUIDE). It allows the user to control the vital equipment like pumps and valves of the three-tank system. It also provides access to all the necessary Simulink files developed to calibrate the level and the pump flows and to further analyse the functionality of the three-tank system in the various real time scenarios like the PID control and the MPC control. The ControlPanel, a GUI for the three-tank system along with its Applications menu is shown in Figure 5. Figure 5: Three-tank Control Panel GUI with the Applications menu 9
The ControlPanel GUI of the three-tank system is useful in the preparation tasks of real time execution of the three-tank such as isolating, connecting the tanks and opening/closing the leak valves as per the requirements of various operations. Its presence is eminent to gain manual control of the vital equipment of the three-tank in unwanted situations during the operation of the three-tank process. The pump control pane allows the user to run the pump at full throttle and to stop it when necessary. The valve control pane allows the user to open or close all the six three-tank valves. A modal window opens each time the valve open/close button is pressed. It warns the user of the wait time of approx 10secs, allowing the valve to complete its operation. The modal window restricts the user s access to all the other Matlab windows when it is open. The Application menu at the top left corner of the GUI allows the user to open the respective Simulink window by a mouse click. The EXIT option in the menu closes all the windows corresponding to the three-tank including the ControlPanel. 4 Preliminary Exercise This part of the exercise has to be completed and accepted before the actual laboratory exercise. 1) Obtain the mathematical model of the three-tank system based on mechanistical modeling. Also obtain the state-space model. 2) Linearize the above developed model around an operating point. 3) Design the decouplers for the three-tank system. 4) Simulate the above (both 1, 2 and 3) models in Simulink. Observe and comment on the open loop dynamics of the system. 5) Implement closed loop PID control on the Simulink model of the three-tank system, to control the levels in tank1 and tank2. Tune the PID parameters to obtain a satisfactory control. 6) Implement MPC control on the threetank model to control the levels in tank1 and tank2. Tune the MPC controller to obtain satisfactory control. 10
5 Laboratory Exercise 1) Primary Tasks: a) Power up the Junction box, valve-box and the PXI-1033 chassis. b) Switch on the PC. Note: Please ensure that the PXI chassis is on before switching on the PC. c) Open Matlab and change the current directory to Threetank. Note: Do not open any other applications/windows other than Matlab, as it may the real-time deterministic control of the three-tank process d) Open ControlPanel GUI. e) Ensure that the tanks are empty and all the valves are open. 2) Level Calibration: a) Please ensure that all the tanks are empty before performing this task. b) Open the Level Offset model from the Applications menu and Run it c) Obtain the level offsets. 3) Flow Calibration: a) Ensure that the respective tank is empty and isolated. b) Open Applications Flow Calibration Flow1 c) Configure the level scaling block based on the level sensor specifications and respective offsets obtained in Stage 1. d) Start the real-time operation and wait for the completion of the task. e) Note the slope and intercept of the flow calibration curve. f) Repeat Steps a e for Flow2. g) Empty tanks and open all valves after the completion of the task. 4) Open Loop Assessment a) Ensure that the tanks are empty, leak valves closed and connecting valves open. b) Open Applications Model Assessment. c) Configure the level scaling and flow calibration blocks based on Stage 2 & 3. d) Run the model. e) Empty tanks after the completion of the task. 11
5) PID Control: a) Ensure that the tanks are empty, leak valves closed and connecting valves open. b) Open Applications PID Control. c) Configure the level scaling and flow calibration blocks based on Stage 2 & 3. d) Tune the PID controller to achieve tight setpoint regulation. e) Empty tanks after the completion of the task. 6) MPC Control: a) Ensure that the tanks are empty, leak valves closed and connecting valves open. b) Open Applications MPC Control. c) Configure the level scaling and flow calibration blocks based on Stage 2 & 3. d) Use the MPC controller designed in your preliminary exercise. e) Tune the MPC controller to achieve tight setpoint regulation. Note: that the changes in the MPC controller are to be made on the model and then imported to real time execution but not directly on the real time execution. f) Empty tanks after the completion of the task. 7) Shut Down : a) Ensure that all the tasks are empty and all valves open. b) Exit ControlPanel GUI and Matlab. c) Shutdown the PC. d) Power off all the equipment. NOTE1: The three-tank process has a in-build safety procedure which activates when the level in any tank reaches an alarming level during the experiments. NOTE2: Control the pumps through GUI in times of any unwanted situations during the experiments. 12
6 Appendix 6.1.1 DAQ card The I/O requirements led to the selection of the NI PXI-6221 multifunction DAQ device capable of handling two analog outputs (AO), sixteen analog inputs (AI) and twenty four digital input/output (DIO). This low cost multifunction DAQ device provided optimized functionality for a cost sensitive application with high-accuracy and high-speed supporting multiple control loops and provides a reliable platform to verify prototype designs with high level of noise cancellation (NI PXI-6221 brochure, 2008). Figure 6: NI PXI-1033 chassis with the NI PXI-6221 DAQ board 13
The NI PXI-6221 DAQ card is placed in a slot 2 of the NI PXI-1033 chassis as shown in Figure 6. It is designed with an integrated Multisystem Extension Interface (MXI) bus for remote control applications by a host PCI Express card residing in a PC. This chassis contains five peripheral slots for the I/O modules connected through an interoperable backplane with the daisy-chained local bus architecture. The built-in MXI-Express Interface was controlled by the MXI-Express chassis controller and provided a transparent remote link with a sustained throughput of 110 MB/s (NI PXI 6221 manual, 2007). 6.1.2 I/O Terminal The function of an I/O terminal block is to collect all the three-tank process input/output (I/O) information at one place and to feed it to the DAQ card. The NI SCC-68 was selected to fulfill the above mentioned purpose and is an I/O connector block for signal connection to the NI M series DAQ cards. The NI SCC-68 features 68-pin male SCSI connector with 84 (54 AI, AO, DIO; 20 SCC; 8 bus; 2 external power) screw terminals as shown in Figure 7 for easy I/O connection, the general bread board area for signal connection and bus terminals for external power and grounding (NI SCC-68 manual, 2007). 14
Figure 7: NI SCC-68 I/O terminal with the SCSI cable. Internal view (left) and top view (right) The I/O information coming from the field terminates at their respective screw terminals to be carried to the DAQ card thorough a NI proprietary SCSI cable SHC68-68-EPM. This board is wall mountable and designed to provide a simple but expandable desktop measurement system with maximum signal isolation and immunization to shock and vibration. 6.1.3 Power Supply The power supplies of the Mascot make Type 9320 as shown in Figure 8 provided an output DC voltage of 13.2VDC, 5A continuous output current rating and 70W of the maximum output power wattage. They were selected to drive the power requirements of the two pumps with a maximum rating of 12VDC voltage and 4.5A. This AC/DC switch mode power supply is equipped with short circuit proofing, current limiting and a zener diode over voltage protection (Mascot 9320 brochure, 2007). 15
Figure 8: Pump power supplies Although, initially these units are intended to power the two pumps only but later there was power distribution between the two pumps and the three level sensors. 6.1.4 Inverters for the Pump Control The need to control the two pumps through the NI DAQ board AO terminals had demanded the presence of the respective number of the pump controllers to control the volume of fluid pumped by the pumps. The transistor DC chopper controller Type GS 24S of EPH Electronik make as shown in Figure 9 was chosen to control the speed of the diaphragm pumps through a step-less speed adjustment by an internal/external potentiometer or by an external reference voltage of 0-10VDC (fed by AO terminals of NI board) preselected by jumper adjustment to external (EPH Electronik tech specs, 2007). 16
Figure 9: Inverters for the pump control These inverters feature an internal potentiometer acting as an overload protection by providing a continuously variable motor current limitation and a high clocking frequency of 18 khz with a minimal noise output. 6.1.5 Valve Interface Module A Phoenix Contact VARIOFACE FLK-D50 as shown in Figure 10 was preferred to interface the information (both AI & DIO) from the valvebox to the I/O terminal block SCC-68. These universal interface modules are electrically connected, position by position between input and output sides simplifying the system wiring between various levels of the automation system. These D50 modules were compact with three-level pin strip test connection and an electrical rating of 125VAC/DC and 2.5A (Phoenix Contact product brochure, 2007). 17
Figure 10: Valve interface modules 6.1.6 PC The PC is the vital component of this automation system where the final control decisions regarding the three-tank process are taken. The PC is intended to be capable enough of handling the flow of information in and out of the PC and its processing. The PC is made compatible with the NI equipment by inserting an addon card, NI PCI-Express card into one of the PC s expansion slot to enroute the information acquired by the DAQ card directly to the processor through a PXI bus. The other hardware of the system is chosen to be of high capacity, capable of handling the process of data acquisition and its associated software platform to efficiently handle the three-tank system and its state-of-art new automation system. 18