Design methods and precautions for PLC ladder diagrams in equipment renovation

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Design methods and precautions for PLC ladder diagrams in equipment renovation Abstract: This article introduces the methods and steps for designing ladder diagrams based on the original relay circuit diagrams when using PLCs to renovate old equipment. It also provides solutions and precautions for the problems that arise in practical applications. 　　Keywords: Transformation ； PLC ； Design method ； Design methods and points to note for PLC-based ladder charts Abstract: This paper presents the design method for ladder-type charts based on original relay circuits when old equipment is upgraded with PLCs, and identifies the problems that arise in practical applications, along with solutions and key points that need attention. Keywords: upgrading ； PLC ； design method ； Ladder-type chart 0　Preface　PLC is a new generation of industrial control devices developed based on microcomputer technology, and it is now widely used in various fields. It is a general-purpose automatic control device that combines the advantages of relay control with the comprehensive functionality, flexibility, and versatility of computers, using computer programming software logic to replace relay wiring logic. 　　Currently, replacing relay control with PLC control is an inevitable trend. Some relay-controlled systems or devices, after years of operation, have proven through practice to be successfully designed. Since PLCs use a ladder diagram language that is very similar to relay circuit diagrams, if one wishes to switch to PLC control, it is possible to design a PLC control system with the same functions by making appropriate conversions, or in other words, by appropriately \"translating\" the original relay circuit diagrams. 1 Basic Method When designing a PLC control system, the PLC can be considered as a control box within a relay control system. The external wiring diagram of the PLC shows the external connections of this control box, while the ladder diagram represents its internal “wiring diagram.” The input and output relays in the ladder diagram serve as the “interface relays” that allow communication between this control box and the outside world. In this way, the PLC control system can be analyzed using the methods employed for analyzing relay circuit diagrams. The methods and steps for converting a relay circuit diagram into an equivalent PLC wiring diagram or ladder diagram are as follows: (1) Understand and become familiar with the technological process of the equipment being controlled as well as its mechanical operations, and analyze and grasp the working principle of the control system based on the relay circuit diagram ； 　　(2) Arrange the sensing elements and control elements (such as limit switches, buttons) properly and connect them to the input ports of the PLC ； 　　(3) Connect the components under control (such as contactor coils, relay coils) to the PLC’s output ports ； 　　(4) Replace the time relay with the timer/counter built into the PLC, and replace the intermediate relay with the auxiliary relay built into the PLC ； 　　(5) Draw the ladder diagram based on the above correspondence. 2　Common problems and solutions　　　When designing the external wiring diagram and ladder diagram of a PLC based on the relay circuit diagram, direct translation may pose problems due to the differences between the operating mode of the PLC and that of the relay control system, which can lead to errors in the PLC’s operation. Examples are provided below. 　　 　　The forward and reverse control circuit for the spindle motor of a certain machine tool is shown in Figure 1 as an electrical schematic diagram controlled by relays and contactors; when controlled by a PLC, its I/O wiring diagram is shown in Figure 2. The ladder diagram is shown in Figure 3. Before installation and commissioning, the PLC program was tuned and the control functions worked properly. However, during trial operation, it sometimes functioned correctly while at other times an inter-phase short circuit occurred when switching between forward and reverse rotation. The reason for this is that, in the ladder diagram, the PLC operates in a serial manner; therefore, when switching between forward and reverse rotation, the contactors may not be able to respond fast enough to the PLC’s control signals, resulting in a overlapping operating time for KM1 and KM2. If the release and engagement times of these two contactors are set appropriately, normal operation is possible; otherwise, a power short circuit occurs. Furthermore, due to the large variability in the release time and engagement time of the contactor, the above phenomena tend to occur. The solution to this problem is to add a delayed switch in the ladder diagram, as shown in Figure 4. 3 Precautions When designing the external wiring diagram and ladder diagram of a PLC based on the relay circuit diagram, the following points should be taken into account: (1) The grammar rules of the ladder diagram must be followed. In a relay circuit diagram, contacts can be placed to the left or right of the coil, but in a ladder diagram, the coil must be placed on the right side of the ladder diagram ； In relay circuit diagrams, contacts can appear on horizontal lines as well as on vertical lines, but in ladder diagrams, contacts can only appear on horizontal lines. Therefore, it is necessary to analyze the original circuit diagram carefully and make adjustments to some of its connections, as shown in Figure 5b. The I/O assignment for the circuit shown in Figure 5b is as shown in Figure 5c; the intermediate relay is replaced by 1001, and the time relay is replaced by TIM00. The resulting ladder diagram is shown in Figure 5d. 　　(2) Setting up intermediate units: In a relay circuit, when multiple coils are controlled by a series-parallel circuit of certain contacts, such as SB2 in Figure 5a, to simplify the circuit, an auxiliary relay that controls this circuit is installed in the ladder diagram, such as 1000 in Figure 5d; it functions similarly to an intermediate relay in a relay circuit. 　　(3) I/O points should be used reasonably in system design; the number of I/O points should be minimized as much as possible. Increasing the number of I/O points raises the cost of the PLC, so reducing the number of I/O points is a way to lower hardware costs. In a relay circuit diagram, if several input contacts are always presented as a single unit through series or parallel connection, they can be treated as just one input signal for the PLC, occupying only one input point on the PLC. If a relay’s normally closed contact is used as a protective element in the relay circuit diagram and appears only once, then it can be placed on the output circuit outside the PLC, such as the normally closed contact FR of the thermal relay in Figure 2, thereby reducing the number of input points required by the PLC. 　　 　　(4) Design of external interlocks: To prevent a short circuit in the three-phase power supply caused by both contactors that control forward and reverse rotation operating at the same time, in addition to implementing corresponding software interlocks in the ladder diagram, such as the normally closed contacts 0500 and 0501 shown in Figure 3, hardware interlocks must also be installed outside the PLC, such as the normally closed contacts KM1 and KM2 shown in Figure 2. 　　(5) Rated voltage of external loads: The relay output modules and thyristor output modules of PLCs can generally only drive loads rated at AC220 V. If the coils of the AC contactors in the original relay system are rated for AC380 V, those coils should be replaced with ones rated for AC220 V. 4 Conclusion In the modification of electrical control circuits for actual machine tools, when the control circuit is very complex, it can be broken down into simpler components; partial conversions are carried out first before everything is integrated together. When a large number of intermediate relays, time relays, and counters in the relay circuit are replaced by internal components of the PLC, and the complex wiring in those relay circuits is handled using the PLC’s ladder logic, the advantages of using a PLC to replace relay-based control become evident. This post was last edited by 771207 on 2008-1-6 21:26]