Chapter 13 : Computer Numerical Control

Figure 13.1 CNC Machine

      Computer Numerical Control (CNC) is one in which the functions and motions of a machine tool are controlled by means of a prepared program containing coded alphanumeric data. CNC can control the motions of the workpiece or tool, the input parameters such as feed, depth of cut, speed, and the functions such as turning spindle on/off, turning coolant on/off. The applications of CNC include both for machine tool as well as non-machine tool areas. In the machine tool category, CNC is widely used for lathe, drill press, milling machine, grinding unit, laser, sheet-metal press working machine, tube bending machine, etc. Highly automated machine tools such as turning center and machining center which can change the cutting tools automatically under CNC control have been developed. In the non-machine tool category, CNC applications include welding machines (arc and resistance), coordinate measuring machine, electronic assembly, tape laying and filament winding machines for composites, etc. There are three basic components of CNC System, such as : Part program, Machine Control Unit (MCU), and machine tool ( lathe,drill press, milling machine, etc).

       Advantages  and disadvantages of CNC :

1. High accuracy in manufacturing
2. Short production time
3. Greater manufacturing flexibility
4. Simpler Fixturing
5. Contour Machining ( 2 to 5-axis machining)
6. Reduced Human Error

      And the drawbacks include high cost, maintenance, and the requirement of skilled part programmer.

Source :

http://wings.buffalo.edu/academic/department/eng/mae/courses/460-564/Course-Notes/CNC%20notes.pdf

Chapter 12 : Programmable Control Logic

     

Figure 12.1 PLCs

  A Programmable Logic Controller is a digital computer used for automation of electromechanichal processess, such as control of machinery on factory assembly lines, control of amusement rides. It uses programmable memory to store instructions and the specific functions that include ON/OFF control, timing counting, sequencing, arithmatic, and data handling. Programmable logic employs electronic processing units in order to process data. The operation of equipment constructed according to this technique is not defined by a circuit diagram, as for that used in hard wired logic, but by a program loaded into the memory of the processing unit. Programmable logic controllers are the basic components used in electronic automated systems and first appeared in the United states in 1969. Today, numerous models of programmable controller are available, from small PLCs suited to dimple applications and installations with a few inputs/outputs, up to multifunction PLCs capable of controlling several thousands of inputs/outputs and designed to control complex processes. The main difference from other computer is that PLCs are armored for severe conditions (dust, moisture, heat, cold, etc) and have the facility for extensive input/output (I/O) arrangements. Another advantages of PLC are : Cost effective for controlling complex systems. Flexible and can be reapplied to control other systems quickly and easy, and allow more sophisticated control because of the computational abilities. The disadvantages of PLC are : Too much work required in connecting wires and difficulty with changes or replacements.

Figure 12.2 Applications of PLC

Source :

http://me.emu.edu.tr/majid/IENG447/IE%20447/PLC%20ppt.pdf

Chapter 11 : Types of processes in process control system

              The types of processes carried out in modern manufacturing industries can be grouped into three general areas, in terms of the kind of operation that takes place, as :

• Continous process
• Batch production
• Individual products production               

            A continous process is one in which raw materials enter one end of the system and the finished product comes out the other end of the system : the process itself runs continously. Once the process commences, it is continous for a relatively long period of time. The time period may be measured in minutes, days, or even months, depending upon the process. In batch processing, there is no flow of product material from one section of the process to another. Instead, a set amount of each of the inputs to the process is received in a batch, and then some operations is performed on the batch to produce a finished product or  an intermediate product that needs further processing. The process is carried out, the finished product is stored, and another batch of product is produced. Each batch of product may be different. Some processes combine the features of the batch and continous types. In such processes, several product materials are treated and stored in batch operation. Then these stored materials are drawn off as required into a continous process. Many chemically based products are manufactured by using batch processes. Two ingrediets are added together, mixed, heated, a third ingredient is added, processed, and then stored. Each batch made may have differing characteristics by design. 

        The individual product production process is the most common of all processing of all processing systems. With this manufacturing process, a series of operations produces a useful output product. The item being produced may be required to be bent, drilled, welded, and so on, at different steps in the process. The workpiece is normally a discrete part that must be handled on an individual basis. In the modern automated industrial plant, the operator merely sets up the operation and initiates a start, and the operations of the machine are accomplished automatically. These automatic machines and processes were developed to mass-produce products, control very complex operations, or to operate machines accurately for long periods of time. They replaced much human decision, intervention, and observation.

        Machines were originally mechanically controlled, they were electromechanically controlled, and today they are often controlled by purely electrical or electronic means through programmable logic controllers (PLCs) or computers. The control of machines or processes can be divided into the folllowing categories :

1. Electromechanical control
2. Hardwired electronic control
3. Programmable hardwire electronic control
4. Programmable logic control (PLC)
5. Computer control

Source :

Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.304-305

Chapter 10 : Count Control

                                                                       Figure 10.1 Counters

       Counters are devices that will receive a string of count pulses from a machine operation and perform an output function based on a number of counts predetermined by the user. Most counters, like timers, can have interval and delay operation. Interval operation means that a load will be actuated when the unit is counting. Delay operation means that a load will be actuated at the end of the counting cycle. Solid-state and electromechanical versions are available.

         Counters are generally thought of as devices that tabulate or count "things" such as bottles, cans, boxes, castings, and so on. In many industrial control systems, it is necessary to count something that affects a controlled process. When the count reaches a certain number, a control action is initiated. In a mechanical counter, every time the actuating lever is moved over, the counter adds one number, and the actuating lever returns automatically to its original position. Resetting to zero is done by a pushbutton located on the side of the unit. In an electromechanical counter, the count setpoint can be adjusted by the knob on the front of the unit. A progress pointer, indicating the count progression, advances clockwise, from setpoint to zero. A solid-state counter has high speed pulse operation with 100 percent accuracy and has may programmable features. Counter output action occurs when the count total indicated by the thumbwheel switches is reached.

 (a) Electromechanical Counter

(b) Mechanical Counter

(c) Solid-State Counter

10.2 Types of Counters used

     

The circuit of counters operates as follows :

• The sustained control switch is closed to energize clutch and enable the counter to- receive and register counts
• Instantaneous contacts transfer
• Each time the count switch is momentarily closed, a pulse is applied to the cout motor to register a count by moving the count progress pointer toward the zero point on the dial
• When the progress poiter reaches zero, the unit is counted out and the delay switch operates to turn output A ON and output B OFF
• Additional counts will not be registered until the unit is reset
• Opening the control switch to remove power from the clutch resets the counter

     Most solid-state counters can count up, count down, or be combined to count up and down. An up-counter will count up or increment by 1 each time the counted event occurs. A down-couter will count down or decrement by 1 each time the the counted event occurs. Normally the down-counter is used in conjuntion with the up-counter to form an up/down-counter equipped with separate count-up and count-down inputs.

Source :

Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.291-293

Chapter 9 : Motor Reversing

Figure 9.1 Three-phase reversing starter

                     A three-phase reversing starter consists of two contactors enclosed in the same cabinet. As seen in the power circuit, the contacts (F) of the forward contractor, when closed, connect L1, L2, and L3 to motor terminals T1,T2, AND T3, respectively. The contacts (R) of the reverse contactor, when closed, connect L1 to motor terminal T3 and connect L3 to motor terminal T1, causing the motor run in the opposite direction. Mechanical and electrical interlocks are used to prevent the forward and reverse contactors from being activated at the same time, which would cause a short circuit.

                                                              (a) Mechanical Interlock

(b) Electrical Pushbutton Interlock

                                           (c) Electrical Auxiliary Contact Interlock

                        Figure 9.2 Reversing starter mechanical and electrical interlocks

           With the mechanical interlock, the first coil to close moves a lever to a position that prevents the other coil from closing its contacts when it is energized. Electrical pushbutton interlocks use double-contact ( NO and NC) pushbuttons. When the forward pushbutton is pressed, the NC contacts open the reverse-coil circuit. There is no need to press the STOP button before changing the direction of rotation. If the forward button is pressed while the motor is running in the reverse, direction, the reverse control circuit is deenergized and the forward contactor is energized and held closed. The reversal of a dc motor can be accomplished in two ways :

1. Reversing the direction of the armature current ad leaving the field current the same
2. Reversing  the direction of the field current and leaving the armature current the sameca

        Most DC motors are reversed by sitching the direction of current flow through the armature. The switching action generally takes place in the armature because the armature has a much lower inductance than the field. The lower inductance causes less arcing of the switching contacts when the motor reverse its direction.

Source :

Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.251-253

Chapter 8 : Magnetic Contactor

      The National Electrical Manufactures Association (NEMA) defines a magnetic contactor as a magnetically actuated device for repeatedly establishing or interrupting an electric power circuit. Unlike relays, contactors are designed to make and break electric power circuits without being damaged.

Figure 8.1 Magnetic Contactors

      A contactor has three components. The contacts are the current carrying part of the contactor. This includes power contacts, auxiliary contacts, and contact springs. The electromagnet (or "coil") provides the driving force to close the contacts. The enclosure is a frame housing the contact and the electromagnet. Enclosures are made of insulating materials to protect and insulate the contacts and to provide some measure of protection against personnel touching the contacts. Open-frame contactors may have a further enclosure to protect against dust, oil, explosion hazards and weather.

     The advantages of using magnetic contactors instead of manually operated control equipment include the following :

1. Where large currents or high voltages have to be handled, it is difficult to build a suitable manual apparatus. Furthermore, such an apparatus is large and hard to operate. On other, it is a relatively simple matter to build a magnetic contactor that will handle large current or high voltages and the manual apparatus must control only the coil of the contactor
2. Contactors allow multiple operations to be performed from one operator (one location) and interlocked to prevent false and dangerous operations
3. Where the operation must be repeated many times an hour, a distinct saving i effort will result if contactors are used. The operator simply has to push a button and the contactors will automatically initiate the proper sequence of events

      The principal parts of a magnetic contactor are the electromagnet and the contacts.  . The magnetic circuit consist of soft steel with high permeability and low residual magnetism. The magnetic pull developed by the coil must be sufficient to close the armature from being held in by residual magnetism, a permanent air gap must be provided in the magnetic circuit. This is generally accomplished by placing a shim of nonmagnetic material between the core and the supporting frame, under the core head or at the core face.
Types of magnetic Contactors :

There are four basic types of electromagnetic contactors :

Figure 8.2 Magnetic contactors type

• Clapper Type - It contains a hinged armature that pivots to seal in, thus closing the moveable contacts against the stationary contacts.
• Horizontal Action - The armature and the contacts move horizontally in a straight line.
• Vertical Action - The armature and contacts move in a straight vertical line.
• Bell Crank - A bell crank converts the vertical movement of the armature into a horizontal motion. Longer contact life and reduced contact bounce result from lessened shock on armature pickup.

Source :

http://en.wikipedia.org/wiki/Contactor

http://ecmweb.com/content/basics-contactors

http://www.myodesie.com/index.php/wiki/index/returnEntry/id/2976

Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.216-218

Chapter 7 : Electromechanical Control Relays

Figure 7.1 Electromechanical Relay (EMR)

       Electromechanical Relay ( EMR) is a magnetic switch. It turns a load circuit ON or OFF by energizing an electromagnet, which opens or closes contacts in both electric and electronic circuits. EMRs may be used in the control of fluid power valves and in many machine sequence controls such as drilling, boring, milling, and grinding operations. A relay will usually have only one coil,  but it may have any number of different contacts. Electromechanical relays contain both stationary and moving contacts.The moving contacts are attached to the plunger. Contacts are referred to as normally open (NO) and normally closed (NC).

Figure 7.2 Parts of Electromechanical relay

Basic parts and functions of electromechanical relays include: 

1. Frame: Heavy-duty frame that contains and supports the parts of the relay.
2. Coil: Wire is wound around a metal core. The coil of wire causes an electromagnetic field.
3. Armature: A relays moving part. The armature opens and closes the contacts. An attached spring returns the armature to its original position.
4. Contacts: The conducting part of the switch that makes (closes) or breaks (opens) a circuit

Figure 7.3 Mechanical operation of Electromechanical relay

      When the coil is energized, it produces an electromagnetic field. Action of this field, in turn, causes the plunger to move through the coil, closing the NO contacts and opening the NC contacts. Normally open contacts are open when no current flows through the coil but closed as soon as the coil conducts a current or is energized. Normally closed contacts are closed when the coil is deenergized and open when the coil is energized. Each contact is usually drawn as it would appear with the coil deenergized. Most machine control relays have some provision for changing contacts normally open to a normally closed, or vice versa. It ranges from a simple flip-over contact to removing the contacts and relocating with spring location changes.

        In general, control relays are used as auxiliary devices to switch control circuits and loads such as small motors, solenoids, and pilot light. The EMR can be used to control a high-voltage load circuit with a low-voltage control circuit. This is possible because the coil and contacts of the relay are electrically insulated from each other. From a safety point of view, this circuit provides extra protection for the operator. Another basic application for a relay is to control a high-current load circuit with a low-current control circuit.This is possible because the current that can be handled by the contacts can be much greater that what is required to  operate the coil. 

Sources :

http://www.galco.com/comp/prod/relay.htm

Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.202-204