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 RELAY coil NO contact NC contact

Relay Picture is downloaded from

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 RELAY SYMBOL WITH 8 CONTACTS coil contacts NO NC

Relay 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 12 V R1 30 mA 5 A R11 R12 ~ 220V

Ladder diagram

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

Basic logic AND LOGIC L + - s1s2 Lamp L will light if switch s1 and s2 are on. In logic on usually symbolized as 1 and off as 0. s1s2L Mathematically written as L = S1 AND S2

Basic logic OR LOGIC L + - s1 s2 Lamp L will light if switch s1 OR s2 are on. s1s2L Mathematically written as L = S1 OR S2

Basic logic NOT LOGIC + - s1 Lamp L will light if not R1 is on R1L L R11 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: 1.The hand brake (HB) is released and the doors (D1-D4) are locked or 2.The safety system is disable by switching on override switch (OS) for maintenance purpose Mathematically the above logic is written as GB = (HB AND D1 AND D2 AND D3 AND D4) OR OS HBD1 GB D2 D3D4 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 startstop 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 R1 R11 motor S1 S2 startstop O M A 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 R11 S1 S2 startstop O M A LS OLOT R1 motor S0 Permissive circuits

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 R1 R11 A B R2 R21 LA R12 R22 R13 R23 LB

Push Button In Quiz Show program R1 R11 A B R2 R21 LB R12 R22 R13 R23 LC LA R12 R3 R11 C R31 R21 R31 R33

Push Button In Quiz Show program R1 R11 A B R2 R21 LB R23 R13 R23 LA R13 R12 R22 Reset 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

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.