Industrial Control and Data Aquisition Technology - Assignment Example

INDUSTRIАL СОNTRОL АND DАTА АСQUISITIОN ТЕСHNОLОGY Name: Institution: Course: Tutor: \ INDUSTRIАL СОNTRОL АND DАTА АСQUISITIОN ТЕСHNОLОGY Mechanical engineers need to find control solutions for: Entire factory Car management system Standalone machine Childs toy Car production line Temperature controller (Floor heating) Wrapping machine The first step in any control system intimately knows what you are trying to control and the questions that need answering are: Speed requirements Environment (Electrical, dust, humidity etc.) Type of control (Electrical, hydraulic, pneumatic) Broadly speaking control systems consist of two parts: 1. Logic control 2. Power control The Logic behind control systems can be implemented in numerous ways: Relay based logic Programmable Logic Controllers (PLCs) Relay based Logic: Used in early control systems For small control solution Input relay works through the following ways On the input side: a coil. On output side: a set (or sets) of contacts. A low current input applied to the input coil can switch a higher capacity load connected to output contacts. Examples - Fuel pump relay in car - High beam relay This is OK for moderate output current requirements. Relays used for control logic have multiple auxiliary contacts. Normally open contacts are a set of contacts that are open when coil is not energised. Normally closed contacts are a set of contacts that are closed when coil is not energised. By using the auxiliary contacts of relays we can construct control logic circuits. Pro`s and Cons of relay logic Pro’s: Electrical personnel understand it. Consequently they can fault find easily Cons Size Mechanical things break down (This is the big problem with all things mechanical) Costly FBD programming Understand FBD programs and the associated Inputs/outputs and function blocks as engaged with in classes and as documented in the FBD handouts. Understand the steps in simulating FBD programs. Be able to write and analyse simple FBD programs. Programmable logic controllers A PLC is a rugged industrial computer that can perform logic control functions. PLCs can be programmed in a ladder logic which is a language that closely resembles relay logic circuits. Similarities include: Power rail on left and neutral on right Output coils positioned on RH rail NC or NO contacts off any coil can be used in a control program Implements timers, counters and other functions A Programmable Logic Controller (PLC) is: Industrial grade computer Rugged (Specifically for control environments) Highly reliable It can be programmed to perform control functions Has functionality to assist fault finding Resistant to electrical noise Resistant to vibration and dirty(dusty) environments Benefits over hardwired (relay) systems 1. Reliability - Logic in software is far more reliable than logic in mechanical (relay) devices. - Software written today will work exactly the same in 1, 2 or 10 years time. - Solid state electronics is inherently reliable 2. Flexibility Software changes are simple to implement in comparison to rewiring relay based systems 3. Speed 4. PLCs operate at quite high speeds compared to relays 5. Security Passwords can be put in place to stop tampering 6. Low cost Small PLCs can be purchased for the cost of small relay based systems 7. Duplication Once a system is working, duplication is a matter of downloading a program to the PLC 8. Communications PLCs have many communications options IP networking, Modbus, cellular network & others A PLC consist of: Computing unit (microcomputer) Input hardware Output hardware Communication (Not on small relay replacement systems) Power supply Because PLCs are designed to operate in harsh electrical environments they need to be highly immune to electrical noise generated from motors, solenoids, contactors etc. PLCs use opto-couplers between the input/output backplane and the computing unit. Hence noise in the I/O modules is isolated from the processing section of the PLC. Types of I/O hardware Two categories: Integrated I/O connections Modular I/O connections Integrated I/O is the domain of small cheap systems. All inputs will be the same voltage. Eg 24vDC, or 240VAC. Outputs will be either, Triac (for AC), Transistor (for DC) or relay (can mix & match, but at the expense of speed). Examples are our Zelio and Allen Bradley controllers. Modular I/O hardware Larger PLCs have an I/O backplane that allows different types of input or output modules to be used as needed. - 24vDC 8 input module - 110VDC module - TTL module - High speed output module - high speed counter module - servo control module - communications module others depending on PLC brand. The PLC scan When in run mode a PLC continuously completes what is called a scan. Scan consists of: Reading the state of input devices (The state of inputs is only updated when the PLC is sure it knows the input state. ie. The PLC needs a steady input) PLC executes the control program. In doing so it uses the stored state of inputs as determined on the last scan in conjunction with logic states stored in the PLC to determine the current state of outputs. The PLC then updates the status of outputs. Consequences of the scan? Because inputs are sampled and stored for the duration of the scan, any changes that occur during the scanning process are not seen by the PLC program. Hence the scan places a restriction on the frequency of input that it can reliably detect. As such standard input and outputs of a PLC have limitations as to the speed with which they can respond. Solution 1 High speed I/O modules In cases where the PLC must respond immediately to a stimulus, high speed modules can be employed. Solution 2 Interrupt processing: Interrupt processing is available on advanced PLCs. An interrupt is a signal from a field device that causes the normal scanning process of the PLC to temporarily halt while the code associated with the interrupting device is run. Interrupt processing is a normal feature of computer systems. Early PLCs did not provide such facilities however some current PLCs do. Interrupts will extend the PLC scan and so interrupts need to be handled with some care. Remote I/O I/O racks need not be connected directly to the PLC processor. An I/O backplane can be located remotely from the PLC through a communications link (normally Ethernet) but controlled from the main PLC controller. Networking technologies Modern control systems employ networking technologies for communication between PLCs or PLCs and networked field devices. User Interfaces also rely heavily on Networking. Technologies include: TCP/IP over Ethernet Modbus Profibus Function Block Diagram (FBD) Features include: Wide selection of functional blocks Technical staff find fault finding easy Control programs can include user defined function blocks that meet specific control needs. eg. VSD and associated overloads, inputs etc. Some function blocks have functionality that would take significant ladder code Mathematical functions are supported. FBD programming interface Inputs: Discrete or Analog (can be conditioned/filtered) Appear on the LHS of display Outputs: Appear on the RHS of the screen Wiring sheet: Central part of screen (function blocks go here) Connections Link function blocks to outputs Sequential Function Chart Programming SFC programs lend themselves to processes that have strictly defined steps that the process follows. For instance, a process to create a product according to a recipe (chook pallets, bread, Pizza). In such cases the process follows the steps of: 1. Weighing out ingredients 2. Mixing ingredients with solutions 3. Roll and cut to size 4. Cook Collecting Plant Data Reasons for collecting factory data: To guide future engineering decisions To keep management happy To keep government bodies happy Particularly important in food industries to: Track source of ingredients Track processes followed (eg Cooking time) Types of control To date we have concentrated attention on discrete control. Example: control of motors, conveyors etc. Within a PLC such devices are controlled by a single bit, and will be either ON (1) or OFF(0). This type of control often involves timers or feedback from plant inputs like photocells, limit switches, stop buttons etc. (PLCs are suited to such control) Continuous control involves parameters that can change continuously with time, for instance: Temperature Pressure Level Flow velocity Angular velocity others PID control PID control is used throughout the control industry to control continuous variables (eg Temperature) PID controllers continuously update a control device (Example pump, valve, heating element etc.) in order to maintain a controlled parameter at its desired value (Set point). To do this PID control uses feedback to determine the difference (error) between the actual control variable and its desired value (set point). PID feedback diagram The controller continuously monitors the variable being controlled (y(t), compares it with the set point r(t) to find the error (e(t)). A calculation uses the current values of the user defines constants P, I & D to determine the level of the controller output which in turn manipulates the plant (Valve, pump, heater etc). The selection of values for P, I & D dictates how well the controller works. Meaning of each term P – Proportional The size of the P term is proportional to the current error. The P constant is simply multiplied by the current error. I – Integral The size of the integral term is determined by past errors. Addition of past errors. D – Differential The size of the differential term is a prediction of future errors. Zero or near zero slope indicates minimal change required. Tuning PID controllers If the values of P,I & D are chosen poorly the control system will oscillate around the setpoint in either a growing or decaying sinusoid. If it oscillation increases with time the system is said to be unstable. A system where the oscillation decreases with time is said to be stable and one where the oscillations remain is said to be marginally stable. Tuning Tuning a PID controller can be quite difficult. Systems often work differently when under load, or when outside influences (high ambient temperature), transients and other factors come into play. Modern PID controllers have self tuning functions to automate the tuning of controllers. Manual Tuning Set Ki and Kd to zero. Then set Kp to a value that makes the system oscillate. Now reduce Kp to half that value. Now increase Ki so that any droop offset is corrected in sufficient time for the process. (Droop is a steady state error & occurs in the absence of an Integral term) Finally increase Kd to speed convergence after a disturbance. Supervisory Control And Data Acquisition (SCADA) systems SCADA systems are normally used in large control environments to monitor and control the entire site. PLCs, PACs (Programmable Automation Controllers) and other controllers perform control at local level, while the SCADA system oversees the entire installation. SCADA systems are normally implemented using off the shelf SCADA software packages and run on standard or industrial hardened computer hardware. SCADA software SCADA software packages are required to: Gather data from one or more PLCs or other controllers Facilitate the generation of information pages that incorporate graphical plant representations for users in the form of: Motors, conveyors, VSD, valves, piping, vessels, etc. Alarms, trend graphs Allow interaction with operators Interact with database systems for information storage Scope of SCADA PLCs and local HMI (Human machine interfaces) have local influence, controlling a restricted area of an industrial plant, infrastructure system (water, gas, electricity etc) or other controlled process. SCADA overseas all aspects of the controlled environment, and allows operators to instruct localised control system to modify their high level control objectives. SCADA systems work at a level of abstraction above that of the local control systems. ERP systems & Databases The level of abstraction above SCADA systems is database systems. These are used to hold historical data. At the top of the hierarchy are ERP (Enterprise Resource Planning) systems that integrate all aspects of business processes, including: Material procurement, manufacturing Finance, sales Customer relationship management Vulnerability to attack Because SCADA systems are developed from standard computing hardware using industry standard protocols, SCADA systems are vulnerable to cyber attack. The high level overseeing role SCADA systems have could have dire consequences if attacked maliciously. The Queensland Maroochy’s shire water authority had issues with pumps and other control elements not working as expected. The issue was found to be a disgruntled former employee that still had access to the SCADA system. Stuxnet A computer worm named Stuxnet was discovered in June 2010, believed to have been created by the US and Israel to attack Microsoft Windows systems and Siemens PLCs. The worm seems to have been intended to target machines involved in Iran’s Nuclear enrichment infrastructure. The moral – When engineering control system that employ SCADA technology, spend a little time considering security issues Microcontrollers A microcontroller is essentially a single chip computer. Uses of microcontrollers in the industry. Automotive applications Home appliances (Washing machines, Dishwashers) Home entertainment Mobile electronics Toys (Xbox controller, some TV remotes) Many other application because of their flexible nature Input & Output devices Input and output is handled by I/O subsystems. I/O often incorporates: DMA (Direct memory access. ie. bypass the CPU) Interrupts (interrupt the normal sequence of instruction execution to perform an Interrupt service routine) Access I/O via I/O instructions Access I/O via memory locations Setup() function code Code to define pin operation. The function to define if pins are Digital IN/OUT: pinMode(8, OUTPUT); pinMode(7, INPUT); Internal pullup resistors onto Input lines digitalWrite( 7, HIGH); //enable pullup resistor on this input digitalWrite( 6, HIGH); The code to setup the Serial Monitor (Discussed later) is: Serial.begin(9600); The Loop() function After the setup() function has run once the loop() function is called on a continuous basis. (Somewhat like a PLC scan). This is where the control & data acquisition code should be located. For instance: Code to output a high/low level to a digital output. Code to input the current state of an input Reading the current value of an Analog input Etc. PASCO Pasco specializes in Data Acquisition equipment. Their Hardware & software has application in the areas of science & engineering. Pasco market to schools and universities and have developed a large range of off the shelf experiments for this market. Pasco hardware (three tier system) Pasco hardware fits into 1 of 3 categories: 1. Sensors (These include signal conditioning & batteries as necessary) 2. Interface hardware Both Bluetooth & USB interface hardware provide connectivity between sensors and the display hardware 3. Display Hardware Options include: Personal Computer (Windows or Mac), iPad, iPod, iPhone, Android tablet, Pasco Xplorer GLX. Pasco sensor hardware Pasco offer a large (total of 70) range of mix and match sensor that can detect: Temperature, acceleration, PH, Humidity Force, Time duration Linear/rotary motion Load etc. etc (See web site for details) Microcontrollers Understand what was happening in the Atmel Studio 6 simulation. Read More