Logic Control

What is Logic control Logic control is a control based on a logic concept, that is the on-off state of variable and/or equipment Logic control is often used to control combinational and/or sequential events such as lift control, automatic production line, engine start-up, etc. Originally used device such as switches, relay, timer, drum, and any other mechanism to enable changes of the on-off state

SWITCHES

Toggle Hand Switches ~ Single pole single throw (SPST)

Toggle Hand Switches ~ Single pole double throw SPDT switches

Toggle Hand Switches DPST DPDT

Hand Switches 3PST Rotary Swtich

Push button Hand Switches Normally open NO Normally close NC

Push-Push Switch This looks like a momentary action push switch but it is a standard on-off switch: push once to switch on, push again to switch off. This is called a latching action.

Microswitch usually SPDT Microswitches are designed to switch fully open or closed in response to small movements. They are available with levers and rollers attached.

Keyswitch A key operated switch. The example shown is SPST.

Reed Switch Usually SPST The contacts of a reed switch are closed by bringing a small magnet near the switch. They are used in security circuits, for example to check that doors are closed. Standard reed switches are SPST (simple on-off) but SPDT (changeover) versions are also available. reed switches have a glass body which is easily broken!

DIP Switch DIP = Dual In-line Parallel This is a set of miniature SPST on-off switches, the example shown has 8 switches. The package is the same size as a standard DIL (Dual In-Line) integrated circuit. This type of switch is used to set up circuits, e.g. setting the code of a remote control.

Multi-pole Switch The picture shows a 6-pole double throw switch, also known as a 6-pole changeover switch. It can be set to have momentary or latching action. Latching action means it behaves as a push-push switch, push once for the first position, push again for the second position etc.

Multi-way Switch Multi-way switches have 3 or more conducting positions. They may have several poles (contact sets). A popular type has a rotary action and it is available with a range of contact arrangements from 1-pole 12-way to 4-pole 3 way. The number of ways (switch positions) may be reduced by adjusting a stop under the fixing nut. For example if you need a 2-pole 5-way switch you can buy the 2-pole 6-way version and adjust the stop.

Process Operated Switches These switches is constructed using one of the above switches. A process variable will initiate a displacement to switch the switch Limit switch Proximity switch Pressure switch Level switch Temperature switch Flow switch etc

SWITCH CAPACITY On a switch usually there is a label informing the voltage and current capacity, e.g.: 250 V 5 A It means that: the maximum current allowed to pass the switch is 5 A. The maximum voltage across its terminal allowed is 250 volt I<5 A ~ V<250 V ~

RELAY

Relay A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches. Relay consist of coil and contact Usually a relay has 1 coil and many contacts both NO and NC NC contact NO contact coil RELAY

Relay Picture is downloaded from www.kpsec.freeuk.com/components/relay.htm

Relay In electrical diagram relay is symbolized as shown A relay can have many contacts both NO and NC The coil of a relay typically passes 30mA for a 12V relay, The contacts can drive 5A or more depending on the size of relay contacts coil NO NO NO NO NC NC NC NC RELAY SYMBOL WITH 8 CONTACTS

Relay 30 mA R11 R12 5 A R1 ~ 220V 12 V Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 220V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical. N

Ladder diagram

Ladder Diagram To make such as previous diagram easier to read a ladder diagram is used + - S R1 R11

Basic logic AND LOGIC + - s1 s2 L 1 s1 s2 Lamp L will light if switch s1 and s2 are on. In logic on usually symbolized as 1 and off as 0. s1 s2 L 1 Mathematically written as L = S1 AND S2

Basic logic + - OR LOGIC s1 s2 L 1 s1 s2 Lamp L will light if switch s1 OR s2 are on. s1 s2 L 1 Mathematically written as L = S1 OR S2

Basic logic + - NOT LOGIC R1 L 1 s1 Lamp L will light if not R1 is on 1 Mathematically written as L = NOT(R1)

Combinational logic Suppose you want to design a safe car with the following criteria: The gear box (GB) will not engage unless: The safety belt (SB) is fastened and the doors (D1-D4) are locked or The safety system is disable by switching on override switch (OS) for maintenance purpose Mathematically the above logic is written as GB = (SB AND D1 AND D2 AND D3 AND D4) OR OS SB D1 D2 D3 D4 GB OS

Motor Start Stop (sequential logic) The following ladder diagram is used to switch a motor on and off R1 motor R11 R12 R13 R14 S1 S2 start stop Latching action

Auto start of water pump Suppose that the motor is used to drive water pump and we want that the pump can run or stop automatically depending on the water level In addition we also want to override the automatic control using manual start and sop control LS

Auto start of water pump Off, manual and auto motor control O S1 S2 M R1 start stop A motor R11 LS

Permissive circuits Often it is desired that a piece of equipment is allowed to start if several conditions are met. For example overload switch and over temperature switch must be closed in order the motor can be started Each process condition is called a permissive, and each permissive switch contact is wired in series, so that if any one of them detects an unsafe condition, the circuit will be opened.

Auto start of water pump with protection Suppose we want to protect the motor against over load and over temperature Permissive circuits O S1 S2 M R1 OL OT S0 start stop A R11 motor LS

Interlock circuits Often it is desired that only one piece of equipment is allowed to start if all other equipments are in off condition. For example push button circuit used in Quiz show program where several contestant have to answer a question. The first one who pushes the push button will disable the other’s push button switch This circuit is called interlock since acting one circuit will lock the others to function

Push Button In Quiz Show program LA R12 R22 R13 R23 LB

Push Button In Quiz Show program LB R22 R32 R13 R23 LC LA R12 R3 R14 C R31 R24 R33

Push Button In Quiz Show program LB R23 R14 LA R13 R12 R22 Instead of pushing the PB continually it is desired that just pushing once is enough for the contestant to claim that they are the first team pushing the button The presenter must push the reset button to reset the system back to original state Reset

Interlock Another example of interlock is the forward circuit of motor must prevent the reverse circuit, otherwise the motor will damage Note: Motor contactor (or "starter") coils are typically designated by the letter "M" in ladder logic diagrams.

Time delay relay If the motor is carry a high inertia load it is dangerous to reverse the direction of the motor instantaneously. Time delay relay can be installed to prevent such occurrence to happen

Fail safe design Consider an alarm system as shown. It can be design in 2 ways Both ways work exactly in the same manner The second design however gives fail save design. Murphy’s law is true. If something can go wrong it will.

PLC

Programmable logic controllers Before the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays. Relays are far from obsolete in modern design, but have been replaced in many of their former roles as logic-level control devices, relegated most often to those applications demanding high current and/or high voltage switching. Systems and processes requiring "on/off" control abound in modern commerce and industry, but such control systems are rarely built from either electromechanical relays or discrete logic gates. Instead, digital computers fill the need, which may be programmed to do a variety of logical functions.

Programmable logic controllers In the late 1960's an American company named Bedford Associates released a computing device they called the MODICON. As an acronym, it meant Modular Digital Controller, and later became the name of a company division devoted to the design, manufacture, and sale of these special-purpose control computers. Other engineering firms developed their own versions of this device, and it eventually came to be known in non-proprietary terms as a PLC, or Programmable Logic Controller. The purpose of a PLC was to directly replace electromechanical relays as logic elements with a solid-state digital computer with able to emulate the interconnection of many relays to perform certain logical tasks.

Programmable logic controllers A PLC has many "input" terminals (X), many output terminals (Y). The input-output relation is programmable To make PLCs easy to program, their programming language was designed to resemble ladder logic diagrams. Thus, an industrial electrician or electrical engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC to perform the same control functions. ~220VAC

Programmable logic controllers + A B C S1 S2 Suppose that the PLC is wired as shown and we want that : Lamp A will light if S1 AND S2 is pushed Lamp B will light if S1 OR S2 is pushed Lamp C will light if S1 EXOR S2 is pushed

Programmable logic controllers + S1 S2 A B C X3 X4 Y1 LA=X3 AND X4 LB =X3 OR X4 X3 Y3 X4

Programmable logic controllers + S1 S2 A B C X3 X4 Y5 LC=X3 EXOR X4 X4 X3