Module 5 - Industrial Instrumentation and Automation

H330 Automotive Engineering MODULE 6 INTRODUCTION TO SEQUENCE CONTROL, PLCS - WORKING, SPECIFICATIONS OF PLC ONBOARD/INLINE/REMOTE IO’S, COMPARISON OF PLC & PC, RELAY LADDER LOGIC- PLC PROGRAMMINGREALIZATION OF AND, OR LOGIC, CONCEPT OF LATCHING, INTRODUCTION TO TIMER/COUNTERS, EXERCISES BASED ON TIMERS, COUNTERS. BASIC CONCEPTS OF SCADA, DCS AND CNC Programmable Logic Controllers (PLCs) Introduction - Any computer having input and output interfaces can be used to control external devices. - However most of the computers are not industrially hardened. - Input / Output devices of general-purpose microcomputers are not engineered to handle line-voltages and currents above transistor-transistor logic (TTL) levels. - Also they are not designed to with-stand the temperature, humidity, and vibration on shop floors. - These drawbacks of a general purpose computer have been rectified by developing a Programmable Logic Controller (PLC) with built-in isolation into their inputs and outputs. - “The programmable logic controller is defined as a digital electronic device that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic words to control machines and processes.” - PLCs are generally used for incorporating automation in open loop systems where processes are to be performed in a sequential manner. - PLCs are used for automation of assembly lines in industries. - They are generally designed for multiple input multiple output (MIMO) systems. 2 - In PLCs, instructions are saved in non-volatile memory. - PLCs were first developed in the automobile manufacturing industry to provide flexible, ruggedized and easily programmable controllers to replace hard-wired relays, timers and sequencers. - Since then, they have been widely adopted as high-reliability automation controllers suitable for harsh environments. - A PLC is an example of a "hard" real-time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result - Some of the advantages of PLCs are: Cost effective Flexibility and ability to use similar system for other processes Programming interface is easier in comparison to other processers Resistant to impact and vibration Resistant towards electrical and mechanical noise Ability to work at high temperatures 3 Structure and Functioning - Figure shows the basic elements of a PLC. - It is basically a microprocessor based control system. - Microprocessor communicates with the outside world with input/output devices via a circuitry. - This circuitry protects the microprocessor and other elements of PLC from the high voltages and currents coming to the PLC. - Microprocessor does its basic functions of taking decisions according to the instructions written in the programs which are stored in the memory. - PLC scans a set of sensor inputs rapidly and repeatedly. - Then it evaluates their logic relationships to defined outputs according to a logic program. 4 - At last it sets the outputs according to the programmed logic. Main Parts of PLC - The PLC mainly consists of: A CPU Memory areas Appropriate circuits to receive input/output data. - We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations. - INPUT RELAYS-(contacts): These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors. - INTERNAL UTILITY RELAYS-(contacts): These do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays. There are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are on only once during power-on and are typically used for initializing data that was stored. 5 - COUNTERS: These again do not physically exist. They are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down. - TIMERS: These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s. - OUTPUT RELAYS-(coils): These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen. - DATA STORAGE: Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed. 6 I/O section - It consist of input modules & output modules - I/O section can be of 2 types 1. Fixed I/O - Here the PLC has fixed number of input & output modules attached to it permanently. - This type comes as a package & addition or removal of modules not possible. - This configuration is common in small PLCs & are low in cost. 2. Modular I/O - Here the number of input & output modules are not fixed & can add or remove according to our requirement (maximum number subjected to the PLC specification) - This arrangement gives more flexibility 7 Input module - It forms the interface through which input field devices are connected to the controller Output module - It forms the interface by which output field devices are connected to the controller - PLC employs optical isolation between input/output field devices & controller Programming Device - A PC is the most commonly used programming device - The software allows the user to create, edit, compile, document and load the program to PLCs - The PC communicates with PLC through a data communication link 8 PLC OPERATION - A PLC works by continually scanning a program. - We can think of this scan cycle as consisting of 3 important steps. Step 1 - CHECK INPUT STATUS: - First the PLC takes a look at each input to determine if it is on or off. - In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step. Step 2-EXECUTE PROGRAM: - Next the PLC executes program one instruction at a time. - Maybe program said that if the first input was on then it should turn on the first output. - Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. - It will store the execution results for use later during the next step. 9 Step 3-UPDATE OUTPUT STATUS: - Finally the PLC updates the status of the outputs. - It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. - Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true. - After the third step the PLC goes back to step one and repeats the steps continuously. - One scan time is defined as the time it takes to execute the 3 steps listed above. PLC RESPONSE The total response time of the PLC is a fact we have to consider when purchasing a PLC. PLC takes a certain amount of time to react to changes. 10 11 Relays – a real example: - Consider a situation to simply turn on a bell whenever a switch is closed. - We have 3 real-world parts : A switch, a relay and a bell. Whenever the switch closes we apply a current to a bell causing it to sound. Replacing Relays: - Let’s use a PLC in place of the relay. - The first thing that's necessary is to create what's called a Ladder Diagram. - We have to create one of these because, unfortunately, a PLC doesn’t understand a schematic diagram it only recognizes code. - Most PLCs have software which convert ladder diagrams into code. Ladder Diagram: First Step: Translate all of the items we're using into symbols the PLC understands. Second Step: We must tell the PLC where everything is located. In other words we have to give all the devices an address. EEE MITS Final step: We have to convert the schematic into a logical sequence of events. 12 First Step: - The PLC doesn't understand terms like switch, relay, bell, etc. It prefers input, output, coil, contact, etc. It doesn't care what the actual input or output device actually is. It only cares that its an input or an output. First we replace the battery with a symbol. This symbol is common to all ladder diagrams. We draw what are called bus bars. These simply look like two vertical bars. One on each side of the diagram. Think of the left one as being + voltage and the right one as being ground. Further think of the current (logic) flow as being from left to right. - Next we give the inputs a symbol. In this basic example we have one real world input. (i.e. the switch). We give the input that the switch will be connected to the symbol shown below. This symbol can also be used as the contact of a relay. - Next we give the outputs a symbol. In this example we use one output (i.e. the bell). We give the output that the bell will be physically connected to the symbol shown below. This symbol is used as the coil of a relay. - The AC supply is an external supply so we don't put it in our ladder. The PLC only cares about which output it turns on and not what's physically connected to it. 13 Second Step: - We must tell the PLC where everything is located. In other words we have to give all the devices an address. Where is the switch going to be physically connected to the PLC? How about the bell? We start with a blank road map in the PLCs town and give each item an address. - The PLC manufacturers each do it a different way. For now let's say that our input will be called "0000". The output will be called "500". Final Step: - Convert the schematic into a logical sequence of events. The program we're going to write tells the PLC what to do when certain events take place. In our example we have to tell the PLC what to do when the operator turns on the switch. 14 Comparison of PLC & PC - A PLC is a digital computer used to control electromechanical processes, usually in an industrial environment. It performs both discrete and continuous control functions. After all, computers are used to control things; they can even run a software PLC. - A PLC differs from a typical computer in several important ways: 1. It has physical input/outputs (I/Os); electrical inputs and outputs bring real world information into the system and control real world devices based on that information. A PLC’s, inputs can be thought of as "senses" like vision and touch; outputs can be thought of as arms and legs. 2. It is deterministic and processes information and reacts to it within defined time limits. PLCs operate on a timescale of milliseconds or even microseconds. 3. It is often modular; it can have I/O modules, communication modules or other special purpose modules added to it for expansion. PLCs may also take the form of a computer or a small single module. 4. One of the major difference between a PLC and PC is the way in which programs are developed and executed. A PLC usually follows a scan-based program execution, whereas PC software are usually event-driven. 5. PLCs are programmed using languages like ladder logic or proprietary vendor language. In comparison, PC-based programs use languages such as C/C++/.NET. 15 6. Software and hardware are platform specific; components and programming software usually can’t be used between different manufacturers. There are exceptions however. 7. It is rugged and designed for use in industrial environments to run 24/7 and are designed to resist harsh physical and electrical environments. 8. The initial cost of a PC-based solution may be higher than a PLC one but whenever demanding calculations and complex network loads need to be processed, a PC-based program is more cost-effective. In comparison, PLCs may be initially cheap but as processing power increases, their cost follows an exponential trend. 9. PLCs do not require excess of protection equipment to increase their ruggedness and shielding. In comparison, a PC may require considerable investment on shielding it from harsh environment effects so that maximum uptime can be guaranteed. 10. PCs have a higher cost than PLCs when no external peripherals need to be added as PLCs usually support standard industrial equipment. However, when a solution requires a plethora of external controls, a PC-based solution is much better-suited than a PLC-based one. 16 17 RELAY LOGIC • Relays are the most popular components of the PLC hardware • Relays are used as outputs in the ladder diagram • They can be used to control ON/OFF actuation of powered device • A relay can be latching or non latching • A latching relay needs an electrical impulse to close the power circuit. Another impulse is needed to release the latch • Non latching relays hold only while the switching relay is energized and require continuous electrical signal Main Elements of ladder logic 1. Rails - These are vertical lines and provide the sources of energy to relays and logic system 2. Rungs - These are horizontal and contains the branches, inputs and outputs 3. Branches 4. Inputs 5. Outputs 6. Timer 18 7. Counter Realization of logic gates using ladder diagram NOT gate Input, X Output, Y 0 1 1 0 Input 1, X1 Input 2, X2 Output, Y 0 0 0 0 1 1 1 0 1 1 1 1 Input 1, X1 Input 2, X2 Output, Y 0 0 0 0 1 0 1 0 0 1 1 1 OR gate AND gate 19 NOR gate Input 1, X1 Input 2, X2 Output, Y 0 0 1 0 1 0 1 0 0 1 1 0 Input 1, X1 Input 2, X2 Output, Y 0 0 1 0 1 1 1 0 1 1 1 0 Input 1, X1 Input 2, X2 Output, Y 0 0 0 0 1 1 1 0 1 1 1 0 NAND gate XOR gate 20 XNOR gate Input 1, X1 Input 2, X2 Output, Y 0 0 1 0 1 0 1 0 0 1 1 1 Timers - A timer is a device that introduces a time delay in a circuit or system during its ON or OFF condition - In PLC timer, the time delay is introduced by programming Classification of timers PLC Timer ON delay timer Retentive Non retentive OFF delay timer Retentive MITS Non retentive 21 Schematic diagram of a PLC timer block - The contacts on the left side of the timer function block are the timer enable contacts - When they are closed, power passes to the left terminal of the timer, its clock is enabled and it starts timing. - When they are open, power stops flowing through this terminal, and the timer stops functioning - A timer function block has three output contacts. - When the timer is timed out, DONE BIT(DN) is set. - The ENABLE BIT follows the input enable contact status. - If the enable contact is true then output ENABLE BIT(EN) is true. - The timer timing(TT) bit is set when the timer is operating 22 Functions in TIMER Variety of time base is available The most common time bases are 0.01 sec, 0.1 sec and 1 sec Accumulator value(ACC)- This is the time that has elapsed, since the timer was last reset. When enabled, a timer updates this continuously Preset Value(PRF)- This specifies the value that the timer must reach before the controller sets the done bit The programmer determines the preset time. When the accumulator value becomes equal to or greater than the preset value, the timer stops operating and the done bit is set This bit can be used to control an output device 23 Timer on delay - The instruction is used to delay turning an output ON - The TON instruction begins to count time base intervals when the rung condition become true. - As long as the rung condition remains true the time increments its accumulator value, over each scan until reaches the preset value. - The accumulator value is reset when the rung condition becomes false, regardless of whether the timer has timed out Timer off delay - The instruction is used to delay turning an output OFF - The TOFF instruction begins to count time base intervals when the rung condition makes a true to false transition. - As long as the rung condition remains false the timer increments its accumulator vale over each scan until it reaches the preset value. - The controller resets the accumulated value when the rung conditions becomes true regardless of whether the timer has timed out 24 Retentive and non retentive timers - Retentive refers to the device's ability to remember its exact status such that when the circuit is again activated, the timer continues from the previous point. - RTO - Retentive Timer. Counts time base intervals when the instruction is true and retains the accumulated value when the instruction goes false or when power cycle occurs. - The Retentive Timer instruction is a retentive instruction that begins to count time base intervals when rung conditions become true. - Non-retentive timers reset to zero and start from zero each time the timer function block is energized. Counters - Counters are used to count the number of items produced, and the number of operations performed. - PLC counter utilizes a sensor t count operations, which is processed by software execution in the PLC. - Thus the failure rate is reduced and the accuracy level is increased in a PLC counter. - The major difference between the counter and the timer is that timer instructions will continually increment its accumulative value at a rate determined by the time base when the enable contact is on. 25 - Counter must see a complete contact transition from 0 to 1 each time it increments the accumulative value. - This means that the contact must returns to its zero state before it can have a transition for a second time. Classification of counters - There are 3 types of counters 1. Up counter (CTU) - An up counter is shown in figure - It has the inputs CU, R & PV and outputs Q & CV - The quantity to be counted is given as input in CU - The maximum value of count is entered in PV (pre-set value) The current count value is available in CV (count value) When CV = PV, Q become high To reset the counter, a high pulse is given to R 26 2. Down counter (CTD) - A down counter is shown in figure - It has the inputs CD, PV, LD and outputs Q & CV The quantity to be counted is given as input in CD The value from the count down should start is entered in PV (pre-set value) When a high pulse is given to LD (load), the preset value is loaded to counter. The current count value is available in CV (count value) When CV is less than or = 0, Q become high - To reset the counter, a high pulse is given to LD, so that the counter starts to count down again 3. Up down counter (CTUD) - An up down counter is shown in figure - It has the inputs CU, CD, R, LD, PV and outputs QU, QD & CV - This counter can perform both up counting & down counting 27 Latching in PLC - There are often situations where it is necessary to hold an output energized, even when the input ceases. - A simple example of such a situation is a motor, which is started by pressing a push button switch. Though the switch contacts do not remain closed, the motor is required to continue running until a stop push button switch is pressed. - The term latch circuit is used for the circuit used to carry out such an operation. - It is a self-maintaining circuit in that, after being energized, it maintains that state until another input is received. - There are 2 methods by which latching can be done in PLC Method 1 : Latching by using the output - Here the output is used for latching by connecting it across the input as shown in figure - An example of a latch circuit is shown in Figure. When the input ‘A’ contacts close, there is an output. However, when there is an output, another set of contacts associated with the output closes. 28 Method 2 : Latching by using the memory - Here for latching the operation of a switch, it is stored in a memory location - Now, this memory address can be used for latching as shown in figure PLC timer based program examples 1. When the motor control switch is closed, the motor turn on instantly and when the switch is opened, the motor turns off after a delay of 10 sec. Write a PLC ladder program. 2. When a switch is closed, the lamp turn on after a delay of 5 sec and when the switch is opened, the lamp turn off instantly. Write a PLC ladder program. 29 PLC Counter based program examples 1. Write a PLC ladder program to count the number of products finished in a production line in a day. 2. Write a PLC program to count the number of cars present in a car parking lot having a maximum capacity of 10. 30 Basic Concepts of SCADA - Supervisory control and data acquisition (SCADA) is a system of software and hardware elements that allows industrial organizations to: Control industrial processes locally or at remote locations Monitor, gather, and process real-time data Directly interact with devices such as sensors, valves, pumps, motors, and more through human-machine interface (HMI) software Record events into a log file - SCADA systems are crucial for industrial organizations since they help to maintain efficiency, process data for smarter decisions, and communicate system issues to help mitigate downtime. - The basic SCADA architecture begins with programmable logic controllers (PLCs) or remote terminal units (RTUs). - PLCs and RTUs are microcomputers that communicate with an array of objects such as factory machines, HMIs, sensors, and end devices, and then route the information from those objects to computers with SCADA software. 31 The SCADA software processes, distributes, and displays the data, helping operators and other - employees analyze the data and make important decisions. - SCADA systems are used by industrial organizations and companies in the public and private sectors to control and maintain efficiency, distribute data for smarter decisions, and communicate system issues to help mitigate downtime. - SCADA systems work well in many different types of enterprises because they can range from simple configurations to large, complex installations. 32 - SCADA systems are the backbone of many modern industries, including: Energy, Food and beverage, Manufacturing, Oil and gas, Power, Recycling, Transportation, Water and waste water, and many more - Modern SCADA systems allow real-time data from the plant floor to be accessed from anywhere in the world. - This access to real-time information allows governments, businesses, and individuals to make data-driven decisions about how to improve their processes. - Without SCADA software, it would be extremely difficult if not impossible to gather sufficient data for consistently well-informed decisions. - Also, most modern SCADA designer applications have rapid application development (RAD) capabilities that allow users to design applications relatively easily, even if they don't have extensive knowledge of software development. - The introduction of modern IT standards and practices such as SQL and web-based applications into SCADA software has greatly improved the efficiency, security, productivity, and reliability of SCADA systems. - SCADA software that utilizes the power of SQL databases provides huge advantages over antiquated SCADA software. - One big advantage of using SQL databases with a SCADA system is that it makes it easier to integrate into existing MES and ERP systems, allowing data to flow seamlessly through an entire organization. - While SCADA platforms provide a vast number of benefits and reductions of cost and downtime of the system, there are still many security threats that need to be worked out. - The drive of SCADA platforms is to provide users with quick access to PLCs/RTUs and provide simplistic integration of equipment controls to user interfaces. - These systems can be a great tool, but need to be heavily monitored through HMIs. DCS (Distributed Control System) - Distributed Control System is a specially designed control system used to control complex, large and geographically distributed applications in industrial processes. - In this, controllers are distributed throughout the entire plant area. - These distributed controllers are connected to both field devices and operating PCs through high speed communication networks as shown in figure. - Discrete Field devices such as sensors and actuators are directly connected to input and output controller modules through communication bus. - These field devices or smart instruments are capable of communicating with PLC’s or other controllers while interacting with real world parameters like temperature, pressure, etc. - Controllers are distributed geographically in various section of control area and are connected to operating and engineering stations which are used for data monitoring, data logging, alarming and controlling purpose via another high-speed communication bus. 34 - These communication protocols are of different types such as foundation filed bus, HART, Profibus, Modbus, etc. - DCS provides information to multiple displays for user interface. 35 Basic Elements of DCS - Distributed Control System continuously interacts with the processes in process control applications ones it gets instruction from the operator. - It also facilitates to variable set points and opening and closing of valves for manual control by the operator. - Its human machine interface (HMI), face plates and trend display gives the effective monitoring of industrial processes. 1. Engineering PC or controller - This controller is the supervisory controller over all the distributed processing controllers. - Control algorithms and configuration of various devices are executed in this controller. - Network communication between processing and engineering PC can be implemented by simplex or redundant configurations. 2. Distributed controller or Local control unit - It can be placed near to field devices (sensors and actuators) or certain location where these field devices are connected via communication link. - It receives the instructions from the engineering station like set point and other parameters and directly controls field devices. 36 - It can sense and control both analog and digital inputs / outputs by analog and digital I/O modules. - These modules are extendable according to the number of inputs and outputs. - It collects the information from discrete field devices and sends this information to operating and engineering stations. 3. Operating station or HMI - It is used to monitor entire plant parameters graphically and to log the data in plant database systems. - Trend display of various process parameters provides the effective display and easy monitoring. - These operating stations are of different types such as some operating stations (PC’s) used to monitor only parameters, some for only trend display, some for data logging and alarming requirements. - These can also be configured to have control capabilities. 4. Communication media and protocol - Communication media consists of transmission cables to transmit the data such as coaxial cables, copper wires, fiber optic cables and sometimes it might be wireless. - Communication protocols selected depends on the number of devices to be connected to this network. - In DCS, two or more communication protocols are used in between two or more areas such as between field control devices and distributed controllers and other one between distributed controllers and supervisory control stations such as operating and engineering stations. Features of DCS To handle complex processes - In factory automation structure, PLC-Programming Logic Controller is used to control and monitor the process parameters at high speed requirements. - However due to limitation of number of I/O devices, PLC’s cannot handle complex structure. - Hence DCS is preferred for complex control applications with more number of I/O’s with dedicated controllers. - These are used in manufacturing processes where designing of multiple products are in multiple procedures such as batch process control. System redundancy - DCS facilitates system availability when needed by redundant feature at every level. - Resuming of the steady state operation after any outages, whether planned or unplanned is somewhat better compared to other automation control devices. - Redundancy raises the system reliability by maintaining system operation continuously even in some abnormalities while system is in operation. 38 Lot of Predefined function blocks: - DCS offers many algorithms, more standard application libraries, pre-tested and pre-defined functions to deal with large complex systems. - This makes programming to control various applications being easy and consuming less time to program and control. More sophisticated HMI: - Similar to the SCADA system, DCS can also monitor and control through HMI’s (Human Machine Interface) which provides sufficient data to the operator to charge over various processes and it acts as heart of the system. - But this type of industrial control system covers large geographical areas whereas DCS covers confined area. - DCS completely takes the entire process plant to control room as a PC window. - Trending, logging and graphical representation of the HMI’s give effective user interface. - Powerful alarming system of DCS helps operators to respond more quickly to the plant conditions 39 Scalable platform: - Structure of DCS can be scalable based on the number I/O’s from small to large server system by adding more number of clients and servers in communication system and also by adding more I/O modules in distributed controllers. System security: - Access to control various processes leads to plant safety. - DCS design offers perfect secured system to handle system functions for better factory automation control. - Security is also provided at different levels such as engineer level, entrepreneur level, operator level, etc. 40 CNC (Computer Numerical Control) - Computer Numerical Control machining is a process used primarily by manufacturers to produce machined parts, products, items. - CNC machines are what accomplish the process. - A CNC machine is a machine that utilizes Computer Numerical Control over machine tools like lathes, routers, grinders or mills. - Computer Numerical Control is different from typical PCs in the type of software used to control the machine, which is specially customized and programmed with G-Code — a specific CNC machine language that allows precise control of features like speed, location, coordination and feed rate. - Specialized software drives the computerized machining. - The software has customized G-code, the language that enables precise control of coordination, feed rate, location and speed, among other factors. - The G-code-infused software sits within a computer that looks like a sophisticated desktop. - One programmer at the computer console can command machine work that would be the equivalent of multiple operators on lathes, grinders, routers, mills and shapers. - The computerized, automated machining method can achieve reaches, holds and other actions human operators and conventional machines typically don’t do efficiently. 41 - Manufacturing finds CNC machining particularly useful because the industry needs large amounts of metal and plastic parts, often in complex shapes. - A variety of CNC machines offer the advantage of having multiple axes that can adjust to difficult angles and help manage hard-to-cut materials. - Basic machines have a cutting implement along X- and Y- axes that can each work independently, yet simultaneously. - Advanced machines may have up to five axes that perform similarly and have the capacity to turn and flip the part. - For example, the Z-axis moves up and down. Why is CNC Machining necessary? • To manufacture complex curved geometries in 2D or 3D was extremely expensive by mechanical means (which usually would require complex jigs to control the cutter motions) • Machining components with high Repeatability and Precision • Unmanned machining operations • To improve production planning and to increase productivity • To survive in global market CNC machines are must to achieve close tolerances 42 Major components related to CNC machine tools - Any CNC machine tool essentially consists of the following parts: Part Program - A series of coded instructions required to produce a part - Controls the movement of the machine tool and on/off control of auxiliary functions such as spindle rotation and coolant. - The coded instructions are composed of letters, numbers and symbols. Program input device - The program input device is the means for part program to be entered into the CNC control. - Three commonly used program input devices are punch tape reader, magnetic tape reader, and computer via RS-232-C communication. 43 Machine Control Unit - The machine control unit (MCU) is the heart of a CNC system. It is used to perform the following functions: • To read the coded instructions. • To decode the coded instructions. • To implement interpolations (linear, circular, and helical) to generate axis motion commands. • To feed the axis motion commands to the amplifier circuits for driving the axis mechanisms. • To receive the feedback signals of position and speed for each drive axis. • To implement auxiliary control functions such as coolant or spindle on/off and tool change. Machine Tool - CNC controls are used to control various types of machine tools. - Regard less of which type of machine tool is controlled, it always has a slide table and a spindle to control position and speed. - The machine table is controlled in the X and Y axes, while the spindle runs along the Z axis 44 Feed Back System - The feedback system is also referred to as the measuring system. - It uses position and speed transducers to continuously monitor the position at which the cutting tool is located at any particular instant. - The MCU uses the difference between reference signals and feedback signals to generate the control signals for correcting position and speed errors. 45 Previous University Questions 1. Compare programmable logic controller with personal computer. 2. 3. 4. 5. 6. What are the key features of DCS? Draw the PLC ladder diagrams to realize two input AND, OR and XOR gates What are the main components of SCADA? Give the significance of timers and counters in PLC. What are the hardware elements of DCS? 46