PLC PROGRAMMING

Machine control design is a unique area of engineering that requires the knowledge of certain specific and unique diagramming techniques called ladder diagramming. Although there are similarities between control diagrams and electronic diagrams, many of the component symbols and layout formats are different. This chapter provides a study of the fundamentals of developing, drawing and understanding ladder diagrams. We will begin with a description of some of the fundamental components used in ladder diagrams. The basic symbols will then be used in a study of Boolean logic as applied to relay diagrams. More complicated circuits will then be discussed.

Control Transformers

For safety reasons, machine controls are low voltage components. Because the switches, lights and other components must be touched by operators and maintenance personnel, it is contrary to electrical code in the United States to apply a voltage higher than  120VAC to the terminals of any operator controls. For example, assume a maintenance person is changing a burned-out indicator lamp on a control panel and the lamp is powered by 480VAC. If the person were to touch any part of the metal bulb base while it is in contact with the socket, the shock could be lethal. However, if the bulb is powered by 120VAC or less, the resulting shock would likely be much less severe.

Fuses

Control circuits are always fuse protected. This prevents damage to the control transformer in the event of a short in the control circuitry. The electrical symbol for a fuse is shown in Figure 1-2. The fuse used in control circuits is generally a slo-blow fuse (i.e. it is generally immune to current transients which occur when power is switched on) and must be rated at a current that is less than or equal to the rated secondary current of the control transformer, and it must be connected in series with the transformer secondary. Most control transformers can be purchased with a fuse block (fuse holder) for the secondary fuse mounted on the transformer

Switches 

There are two fundamental uses for switches. First, switches are used for operator input to send instructions to the control circuit. Second, switches may be installed on the moving parts of a machine to provide automatic feedback to the control system. There are many different types of switches, too many to cover in this text. However, with a basic understanding of switches, it is easy to understand most of the different types.

Pushbutton 

The most common switch is the pushbutton. It is also the one that needs the least description because it is widely used in automotive and electronic equipment applications. There are two types of pushbutton, the momentary and maintained. The momentary pushbutton switch is activated when the button is pressed, and deactivated when the button is released. The deactivation is done using an internal spring. The maintained pushbutton activates when pressed, but remains activated when it is released. Then to deactivate it, it must be pressed a second time. For this reason, this type of switch is sometimes called a push-push switch. The on/off switches on most desktop computers and laboratory oscilloscopes are maintained pushbuttons.

A programmable controller is a powerful machine, but it can only do what it is told to do. It receives all of its directions from the control program, the set of instructions or solution algorithms created by the programmer. Therefore, the success of a PLC control program depends on how organized the user is. There are many ways to approach a problem; but if the application is approached in a systematic manner, the probability of mistakes is less. The techniques used to implement the control program vary according to the programmer. Nevertheless, the programmer should follow certain guidelines. Table1 lists programming guidelines for new applications and modernizations. New applications are new systems, while modernizations are upgraded existing control systems that have functioned previously without a PLC (i.e., through electromechanical control or individual, analog, loop controllers)

ELEMENTS TO LEAVE HARDWIRED 

During the assignment of inputs and outputs, the user should decide which devices will not be wired to the controller. These elements will remain part of the electromechanical control logic. These elements usually include devices that are not frequently switched off after start, such as compressors and hydraulic pumps. Components like emergency stops and master start push buttons should also remain hardwired, principally for safety purposes. This way, if the controller is faulty and an emergency occurs, the user can shut down the system without PLC intervention

A user should begin the problem-solving process by defining the control task, that is, determining what needs to be done. This information provides the foundation for the control program. To help minimize errors, the control task should be defined by those who are familiar with the operation of the machine or process. Proper definition of the task is directly related to the success of the control program. Control task definition occurs at many levels. All of the departments involved must work together to determine what inputs are required, so that everyone understands the purpose and scope of the project. For example, if a project involves the automation of a manufacturing plant in which materials will be retrieved from the warehouse and sent to the automatic packaging area, personnel from both the warehouse and packaging areas must collaborate with the engineering group during the system definition. Management should also be involved if the project requires data reporting. If the control task is currently done manually or through relay logic, the user should review the steps of the manual procedure to determine what improvements, if any, can be made. Although relay logic can be directly implemented in a PLC, the procedure should be redesigned, when possible, to meet current project needs and to capitalize on the capabilities of programmable controllers.

CLICK HERE TO LEARN MORE ABOUT PLC

PLC

CLICK HERE TO LEARN MORE ABOUT PROGRAMMING METHODS

PLC PROGRAMMING METHODS