PID - Copy
- 1. OBJECTIVE- PID BASED FLOW CONTROL USING LAB-VIEW ABSTRACT- The primary aim of our project is to replace the PID instrument with virtual PID that has equal controlling capabilities as that of instrument. This designing is possible on software called Lab VIEW developed by National Instruments. We also intend to use a DAQ card for interfacing with the hardware. This DAQ card is product of the same company National Instruments. The hardware is a Multiloop Trainer Kit mounted with a tank whose level has to be controlled using a feedback control loop. The flow of project execution is: The designed PID will be generating the necessary controlling electronic signal. This signal will be acquired by DAQ card. The DAQ card transfers it to the I to P converter which will convert the electrical pulses 4-20mA into pneumatic signal 3-15psig to actuate the control valve. This conversion take place as the control valve acts on pneumatic signals only. The control valve controls the fluid flow to maintain the fluid level in tank. The tank is fitted with a capacitive level sensor and a transmitter. This assembly takes the level readings from tank and transmits it to the DAQ card. These values are called process values and are further processed into the designed PID. This way a closed loop system is formed. KEYWORDS DAQ card (Data Acquisition),Lab VIEW (Laboratory Virtual Instrument Engineering workbench),PID. I. INTRODUCTION In the present era of Industrial Automation, ease of work is one of the major concerns. This design enables the operator to operate the process sophisticatedly with an ease. Instead of giving manual inputs to the PID, this designed PID can adjust the input parameters just by mouse clicks. Firstly what is a control system? A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other device(s) or system(s). There are two common classes of control systems: logic or sequential controls, and feedback or linear controls. There is also fuzzy logic, which attempts to combine some of the design simplicity of logic with the utility of linear control. II. PID THEORY The "P" stands for proportional control, "I" for integral control and "D" for derivative control. This is also what is called a three term controller. The basic function of a controller is to execute an algorithm (electronic controller) based on
- 2. the control engineer's input (tuning constants), the operators desired operating value (set point) and the current plant process value. In most cases, the requirement is for the controller to act so that the process value is as close to the set point as possible. In a basic process control loop, the control engineer utilizes the PID algorithms to achieve this. Proportional action: It simply amplifies the error based upon the gain. P mode generates offset. Integral action: The integral term magnifies the effect of long-term steady-state errors, applying ever-increasing effort until they reduce to zero. In the example of the furnace, working at various temperatures, if the heat being applied does not bring the furnace up to set point, for whatever reason, integral action increasingly moves the proportional band relative to the set point until the PV error is reduced to zero and the set point is achieved. In the furnace example, suppose the temperature is increasing towards a set point at which, say, 50% of the available power will be required for steady-state. At low temperatures, 100% of available power is applied. When the PV is within, say 10° of the SP the heat input begins to be reduced by the proportional controller. (Note that this implies a 20° "proportional band" (PB) from full to no power input, evenly spread around the set point value). At the set point the controller will be applying 50% power as required, but stray stored heat within the heater sub-system and in the walls of the furnace will keep the measured temperature rising beyond what is required. At 10° above SP, we reach the top of the proportional band (PB) and no power is applied, but the temperature may continue to rise even further before beginning to fall back. Eventually as the PV falls back into the PB, heat is applied again, but now the heater and the furnace walls are too cool and the temperature falls too low before its fall is arrested, so that the oscillations continue. Derivative action: The derivative part is concerned with the rate-of-change of the error with time: If the measured variable approaches the set point rapidly, then the actuator is backed off early to allow it to coast to the required level; conversely if the measured value begins to move rapidly away from the set point, extra effort is applied in proportion to that rapidity to try to maintain it. Derivative action makes a control system behave much more intelligently. On control systems like the tuning of the temperature of a furnace, or perhaps the motion-control of a heavy item like a gun or camera on a moving vehicle, the derivative action of a well-tuned PID controller can allow it to reach and maintain a set point better than most skilled human operators could. If derivative action is over-applied, it can lead to oscillations too. An example would be a PV that increased rapidly towards SP, then halted early and seemed to "shy away" from the set point before rising towards it again.
- 3. III. ABOUT Lab VIEW Lab VIEW, which stands for Laboratory Virtual Instrumentation Engineering Workbench, is a graphical computing environment for instrumentation, system design, and signal processing. The graphical language is named "G". Originally released for the Apple Macintosh in 1986, Lab VIEW is commonly used for data acquisition, instrument control, and industrial automation on a variety of platforms including Microsoft Windows, various flavors of Linux, and Mac OS X. The code files have the extension “.vi”, which is an abbreviation for “Virtual Instrument”. Lab VIEW offers lots of additional Add-Ons and Toolkits. IV. HOW A PID CONTROLLER WORK The PID controller’s job is to maintain the output at a level so that there is no difference (error) between the process variable (PV) and the set point (SP). Figure 1: PID Working Diagram In the diagram shown above the valve could be controlling the gas going to a heater, the chilling of a cooler, the pressure in a pipe, the flow through a pipe, the level in a tank, or any other process control system. What the PID controller is looking at is the difference (or "error") between the PV and the SP. It looks at the absolute error and the rate of change of error. Absolute error means -- is there a big difference in the PV and SP or a little difference? Rate of change of error means -- is the difference between the PV or SP getting smaller or larger as time goes on. When there is a "process upset", meaning, when the process variable or the set point quickly changes - the PID controller has to quickly change the output to get the process variable back equal to the set point. If you have a walk-in cooler with a PID controller and someone opens the door and walks in, the temperature (process
- 4. variable) could rise very quickly. Therefore the PID controller has to increase the cooling (output) to compensate for this temperature rise. Once the PID controller has the process variable equal to the set point, a good PID controller will not vary the output. You want the output to be very steady (not changing). If the valve is constantly changing, instead of maintaining a constant value, this could case more wear on the control element. So there are these two contradictory goals. Fast response (fast change in output) when there is a "process upset", but slow response (steady output) when the PV is close to the set point. Note that the output often goes past (over shoots) the steady-state output to get the process back to the set point. For example, a cooler may normally have its cooling valve open 34% to maintain zero degrees (after the cooler has been closed up and the temperature settled down). If someone opens the cooler, walks in, walks around to find something, then walks back out, and then closes the cooler door --the PID controller is freaking out because the temperature may have raised 20 degrees! So it may crank the cooling valve open to 50, 75, or even 100 percent -- to hurry up and cool the cooler back down -- before slowly closing the cooling valve back down to 34 percent. The above formula is the classic PID formula. Where "Kp" is Proportional constant, "Ki" is Integral constant and" Kd" is Derivative constant and "e" is error. The level of fluid in vessel is controlled by using feedback flow scheme. V. THE FLOW LOOP HARDWARE AND SOFTWARE REQUIREMENTS Flow loop Elements Flow meter : Rota meter (mechanical). Final Control Element : Control Valve (Pneumatic). Transducer: Orifice Plate (mechanical) Transmitter: DPT(Differential Pressure Transmitter) (Electronic). Figure 2: Physical Flow Loop (hardware)
- 5. Interfacing Electronic Card NI USB DAQ 6009: Data Acquisition Card by National Instrumentation(NI). Figure 3: NI USB DAQ 6009 Software Lab VIEW installed computer. PID and Fuzzy Logic Tool Kit. Figure 4: Process flow Block diagram
- 6. VI. SOFTWARE CONFIGURATION AND IMPLEMENTATION Controls: Kc: the value of the proportional controller gain. Ki: the value of the integral gain. Kd: the value of the derivative gain Kd. SP: desired steady state value. variable in order to consider that steady state occurred. Auto Tuning: starts auto-tuning operation. Stop: the abort push button. Output range: defines the minimum and maximum output range. dt(s): controller sampling rate is determined. VII. BENEFITS OF THE APPLICATION This project replaces the conventional PID instrument with a virtual controller. It saves time of manufacturing of instrument. Since the PID logic can be designed on Lab- VIEW in a very short period, as the manufacturing of instrument requires plenty of skills which cannot be implemented in a short span of time. Practically this application needs no maintenance and very easy to upgrade. Whereas instruments require timely maintenance and their up-gradations is very difficult.