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Logic Functions PLC Motor Control

Control devices like push buttons, limit switches, and pressure switches are connected into a circuit to attain a logical function in a predetermined manner. All control circuits are basic logic functions or combinations of logic functions. Logic functions are common to all areas of industry. The subjects that utilize the logic function are electricity, electronics, hydraulics, pneumatics, math, and other routine activities. Logic functions include AND, OR, AND/OR, NOT, NOR, and NAND gates.

Common logic functions have been used to develop manufacturing operations since the first electrical circuits were used. Ladder diagrams are one of the oldest and most common methods of illustrating, and understanding basic circuit operation, and connections outside of hard wiring circuits. Programmable logic controllers (PLC), for instance, uses a computer aided design of logic operations to decrease the design time needed for creating manufacturing operations, and add flexibility by allowing the designer or electrician to use the computer to make moves, adds, or changes on the screen using software instead of rewiring.

Knowing the basic parts of what makes up logic function aids the designer in operational sequences. The first part of the logic function is the input, sometimes called Gates. Inputs are control devices like limit switches, pushbuttons, contacts, and selector switches or proximities sensors, and just about any device that can be used to control the flow of current to a load. Some logic functions have only one input while others may contain hundreds, which are all arranged in some manner to control a final outcome in an operation. Every logic function must have a final outcome, whether it is to turn on a light start an electric motor or move a robotic arm. The final outcome in the logic function is often called the output. Output or loads are components that will utilize energy, loads include items such as lights, electric motors, solenoids, contractor, and motor starter coils. Control devices paired with loads and a little logic arrangement are all the components that make up a manufacturing operation that makes our cars, clothes or packages our food.

AND Logic

AND logic is a circuit that contains two or more control devices and one load as seen in Figures 1 and 2. The control device of an AND circuit is connected in series with the load which means it takes two or more control devices to close in order for the load to be energized. AND circuits are used in the industry a lot to make a safety circuit. For instance, a drill press requires the operator to press both push buttons before the motor comes down, and drills into the material.

PLC Program AND Circuit

Fig.1: PLC Program AND Circuit

Hard-wiring schematic AND Circuit

Fig.2: Hard-wiring schematic AND Circuit

OR Logic

OR logic is a logic circuit that contains two or more control devices for one load, as seen in Figures 3 and 4. The control devices of an OR circuit are connected in parallel with each other, and in series with the load. A load of an OR circuit will come ON when either of the control devices is activated by a person manually or control devices are activated automatically by an event or the combination of both manual or automatic. For example, a doorbell for a business with two entry points, the control devices will be the push buttons at the front and back door while the bell will be the load that is controlled by the two pushbuttons. If either of the pushbuttons is pressed it will energize the solenoid of the bill to notify the people inside there is someone at the door.

PLC OR logic

Fig.3: PLC OR logic

Hard-wire schematic OR Logic

Fig.4: Hard-wire schematic OR Logic

NOT Logic

NOT logic is a logic function that only requires one control device that contains a normally closed contact as seen in Figures 5 and 6. Normally close indicates that the device is in its rested state, or when active state is closed. When it is activated the control device opens. A good example of a NOT logic circuit is the emergency button seen on many control panels in an industrial facility. Everything in the process of that particular control panel works great until there is an emergency, and in order for every process to stop at one time, and when the E stop is pressed it is cutting power to everything in the control panel. There are many instances, AND an NOT logic circuit can be found whether it is at a gas station pump or the circuit for the light in a refrigerator.      

PLC Program NOT Logic Circuit

Fig.5: PLC Program NOT Logic Circuit

Hard wiring NOT logic Circuit

Fig.6: Hard wiring NOT logic Circuit

NAND Logic

Another version of NOT logic is the NAND logic functions. NAND logic consists of two or more normally closed control devices connected in parallel with each other to a load as seen in Figures 7 and 8. The function of a NAND circuit connection is that not one control device can turn off a load. For example a dome light in a four-door car. Each door comes in contact with a normally closed switch wired to the light, the light is off inside the car if all doors are closed if any door should open the dome light comes ON.

PLC NAND Logic Circuit

Fig.7:  PLC NAND Logic Circuit

Hard-wire NAND Logic Circuit

Fig.8: Hard-wire NAND Logic Circuit

NOR Logic

NOR logic is a logic circuit that contains two, or more normally closed control devices connected in series to the load as seen in Figures 9 and 10. The load of the NOR logic circuit is ON when power is applied to the circuit and neither control device has been activated, but once either device is activated the load will be energized: For example, a factory with one multiple moving machines, and conveyor systems. There is a need for an emergency system to be in place for any one at any time to cut power to the flow in case of an emergency. All the E stop buttons are connected in series to a coil that will cut power to the breaker if it is turned off by one of the E stop buttons.

PLC NOR Logic Circuit

Fig.9: PLC NOR Logic Circuit 

Hard-wire NOR Logic Circuit

Fig.10: Hardwire NOR Logic Circuit 

MEMORY (latching) Circuits

Many of the circuit logic discussed are not only used to make logic decisions but needed to create circuits that store or retain the signal inputs to keep loads on after the signal has been removed. Since many of the control devices used in industries are momentary push buttons it is essential to have memory circuits that will keep the loads energize after the normally open push buttons have been pressed. For instance, an alarm system is a great example of a latching action. Once the alarm has been armed it will not go off unless a door is opened, the idea here is to keep the alarm on even after the door has been closed. The only way the alarm can be turned off is to reset the system or remove power. To build a latching circuit it is required to have a NOT/AND combination logic functions. A normally closed is connected to a normally open in series; the load must have a corresponding contact to serve as the memory of the circuit after the normally open push buttons have been activated.             

About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.