Author: Seyedreza Fattahzadeh
Product Group : eBook
Format: PDF file
Number of pages:130
There is a quotation which was originally stated by Confucius, I would like present it here as a very practical statement on how the human mind approaches the learning process
I hear, and I forget
I see, and I remember
I do, and I understand
Confucius (551–479 BCE)
All this means is the best way to understand anything is to get your hands on it and actually experiment with it. In computer programming this means writing many small programs and using a simulator software to test programs' functionality.
Well, by writing small programs to learn how to use PLC instructions is for the time you are taking an introductory course in collage to learn how to start programming a PLC to do BIG THINGS in future, BUT when you are applying for a job as a PLC programmer, the employer will NOT ASK you to write a PLC program just to "START", and "STOP" a 3 phase electro motor, on the contrary , he will give you a Project scope and will expect you to read it, understand it , and if you do not have any good question to ask, TO DO IT.
And by definition we all know that a Project Scope or Functional Specification, defines in detail how a system is to operate when the project is finished!
I have come up with a solution for those that do have the beginning knowledge on how to work with PLC instructions but now they need to learn how to take a project Scope and turn it into a working PLC program.
This book assumes you have a little background with PLCs- perhaps you have worked with other PLCs from other manufactures or you have helped to install and wire PLCs. Perhaps you are a mechanical or Process Engineer and you need to learn how to program with Siemens STEP7 or LOGO! Soft Comfort software.
The whole idea is to generate a ladder logic program to control 7 mechanical limit switches to detect the exact location of elevator car, 10 pushbuttons to detect which keypad number is pushed the cabin to be directed to that particular floor, 4 limit switches to control the car sliding door when it is closed, and 4 limit switches to turn alarm when any of the floor door is open in absence of car parked at that floor . Hence control ladder logic program has to control total of 25 mechanical limit switches in harmony! + 24 output port terminals to control two DC motors to lift car & car door, 9 LED indicators to display which pushbutton key is depressed, 3 outputs to display which floor # is the car parked at, 2 flicking LEDs to show direction of the CAR, 2 LED indicators to control car lighting and alarm, and finally 1 trigger signal output for the elevator home made announcing system when the car is reached to the destination . Total of 21 outputs!. All these input / outputs must be controlled by the PLC (S7-300 ) in order the system to function right. I assume he who can come up with the solution for this exercise, can do any other project which is even more complex that this EASILY!. Writing a ladder logic program for this project require you to know what you are doing other wise, it would not work right. You have to make use of basic function blocks such as, flags, latches, timers, counters.. Also the text comes with full explanation of each step of operation and full schematics of circuits and PLC and I/O modules wiring diagrams.
In PLC programming business, you have to have complete self confidence on yourself that "you can do it" when applying for a job and you can only emit that self confidence when you believe in yourself and how can you achieve that? By defending your programming knowledge not just by talking, BUT by proving it. You must show your future employer that you can handle the job of writing at least a fairly complex program in few hours. That knowledge can not be achieved unless you had practiced doing a successful project already. You can go out and order a hardware such as my well designed elevator from some company located in CHINA for some thousands dollars and develop a program for that OR you can purchase my item and just use the simulator which comes with both those software and write your program, simulate it and then compare it with mine and LEARN! And then in your resume to write maybe " I am looking for an entry level job in programming PLCs and the most complex program I did write and it worked was "Programming a 4 floor elevator with SIMENS SIMATIC S7-300 & LOGO!....." and watch your employer's face when he is reading your RESUME…
Goal of this project is to design and implement a control program to operate a 4 floor elevator system.
Each floor is to have its own illuminated pushbutton to call the elevator to a demanded floor. The elevator door is to be emulated with the use of a 3 V DC motor that opens, and stays open, and reverses after few seconds at each floor after being called. Also a 12 V DC motor is used to hoist the elevator and mechanical limit switches were used to indicate when the elevator is at the floor in position to open. The final aspect of this experiment is to intelligently organize the ladder logic to make the troubleshooting easier. Hence the purpose behind of developing a manual like this is an attempt to make learning Programmable Logic Controllers fun! Making connection with a familiar control system such as a full functional 4 floor elevator, brings out the how and why latches, timers, limit switches, sensors and relays are used. The programs in this manual were written for SIEMENS SIMATIC S7-300 and LOGO! & their S7-300 / Logo! Soft Comfort software, however the ideas and program layouts are universal to any PLC.
Beside the development of the two full control programs for two different models of Siemens PLCs, I also had to design few other circuits to needed for the hardware structure of the Elevator. These extra hardware designs were:
1x Cabin display PCBs used to display stop floor numbers + 6 pushbuttons
4 x floor display PCBs used to display each floor number + 1 key pushbutton
Hoist motor and cabin sliding door drivers
Design of a voltage level shift circuit to interface TTL logic gates to 24 V DC coming from PLC
Interfacing a home made elevator floor announcing circuit to PLC
Full schematics of all hardware used is included in the book including the mechanical wiring of the PLCs and their related modules.
Control FBD / LDA software developed for LOGO! has consisted over 100 blocks whit full explanation of the instructions used to do different tasks in whole program. Full explanation of the solution program in about 20 pages of the text.
Control LDA / STL software developed for S7-300 has consisted over 140 networks with full explanation of the instruction used to do different tasks in whole program. Full explanation of the solution program in about 60 pages of the text.
Topics covered in the book are:
Presentation of the 4 floor Elevator with LOGO! software Comfort V. 7.0, Binary-coded decimal, Cabin display and Keypad hardware , Cabin sliding door dc motor controller schematic, Cabin lift dc motor controller , Project description, List of Equipments ,Main power supply circuit , Control hardware circuit, List of input / output and flags ,LOGO! Software Program.
Presentation of the 4 floor elevator with STEP7 V5.2 software & S7-300 CPU, voltage divider circuit, installing an elevator floor announcing , explanation of the control ladder logic program.
The schematic of all other hardware used in this project is given: keyboard pad, DC motor controller .etc. if you wish, you can even build your own elevator hardware similar to ours or even tailor it to the one with more features you like. You can learn to master your programming knowledge just by simulating yours and checking out its performance and comparing it against my solution. This way you can improve your programming knowledge! And if you stuck at any step, you can check out our solution against mine and use the explanation to understand the reasoning behind!
Nowadays, HMI, 3 phase motors, and AC motor inverters have lot's of application in PLC automation industry. In chapter 3, I thought maybe it is not a bad idea to add these three elements to my 4Floor Elevator project. So I changed hoist motor which was a small 12 V DC motor to a 3 phase @ 90 Watts with gear box. Since I needed to change direction of the motor to CW or CCW, then I added an AC inverter to my system to let me 1- To get 3 phase motor to operate from 1 phase (220 V AC to 3 phase power supply) 2- To change the motor direction to either CW or CCW under altered ladder logic control software.
igure 3.1 displays how two control inputs of AC inverter that connected to output terminals of the PLC. Comparing figure 3.1 with figure 3, you see how previous hoist motor is replaced with 3 phase motor.To watch the performance of the project tested in our lab, click on the following link :