A preliminary study on introductory teaching of PLC

摘 要：The focus of this paper is the introductory teaching of plc （Programmable Logic Controller） in secondary vocational schools， In order to make the students know PLC intuitively and progressively， the introductory teaching are offered based on the combination of PLC and relay system. This paper guide students to understand the plc control by jog motor control and self-locking control， and the obvious result is obtained.

关键词：PLC， introductory teaching， relay system， secondary vocational schools

Introduction

Programmable logic controllers are microprocessor based control systems which manage a machine or an industrial foundation by means of change of situations and a program edited before. Basically， a PLC is composed of a microprocessor， a power supply and input/output units. Due to the powerful， high reliability， simple programming， strong anti-interference ability， etc.， PLC has been widely used in the field of industrial control. PLC technology has now become a professional core Mechatronics Courses in most China's secondary vocational schools.

The main content of PLC is the instruction system and programming， Students should possess PLC programming and application capabilities after they learn PLC. Teaching PLC in secondary vocational schools are more difficult than in Higher Vocational school and university， because most of the students lack the basic knowledge learned in Middle School. The reason why most of the Vocational school students do not like learn PLC is the lack of enthusiasm and motivation. In order to attract the attention of students in teaching PLC， teachers should stimulate students' enthusiasm by some new teaching method， such as Project-based Approach and Task-based approach. In the introductory teaching of PLC class， teachers should design some simple projects to help students to know PLC intuitively and progressively， so I use the jog motor control and self-locking motor control to introduce PLC technology， and have received obvious results. Next， this paper will detail the process of introductory teaching of PLC.

1.Structure and working principle of Jog motor control

Jog motor control is usually used in industrial production， such as a rapid traverse device of machine can moves rapidly in both horizontal and vertical direction though jog motor control. As we know， Jog motor control is the simplest control circuit of relay systems which mainly contains a run button， an AC contactor and a motor. Figure1 show the jog motor control in relay system

When push the run button， current will passes through the electromagnet， and then a magnetic field is produced which attracts the moving core of the contactor. The moving contact is propelled by the moving core， with the increase of the current， the force developed by the electromagnet will hold the moving and fixed contacts together. And then the electric motor will begin to run. When the contactor coil is de-energized， gravity or a spring returns the electromagnet core to its initial position and opens the contacts， the electric motor will stop running.

2.Jog motor control using PLC （With MTTSUBISHI FX （subscript 2N） series PLC for example）

Actually during the practical process operation， there is no need to use PLC to realize jog control. However， the application of jog control is beneficial to the teaching of PLC because it can help students to comprehend the hardware and software of PLC， for instance， the external wiring and ladder diagram of PLC.

The Figure 2 is a PLC jog control system which shows the external wiring and ladder diagram. Ladder diagram is a graphical language based on ladder programming， it consists of a left and a right power rail， between which there are so called ladder rungs that consist of contacts and coils. Figure 3 depicts some common elements in a ladder program. PLC programs are typically written in a special application on a personal computer， then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. Often， a single PLC can be programmed to replace thousands of relays.

PLC jog control system consist of an input（x0） represented by contact of the start button（"normally open"） and an output（y0） to the coil which connected with electric motor. When start button is pressed， the contact（x0） will be closed， and then a true （logic 1） value will be passed to the coil output（y0）， thus motor will begin to run. When the button is released， the contact will be opened， and then a false （logic 0） value will be passed to the coil output， thus motor will stop running.

3.The difference between PLC and relay system

Most of the traditional motor control using relay control system which consist of buttons， contactors， relays， the limit switches and so on. Although it is cheap， the relay control system， with the low control precision， stability， reliability and high complexity， cannot satisfy the demand for higher quality of modern industry. However， with easier realized and reliability， the PLC control system is an available method to instead of the relay control system.

For example， In order to achieve self-locking motor control， it must be re-wiring in the relay system， however， it just need reprogramming in PLC control system. Compared with relay systems， plc have the following advantages： First， high reliability and anti - interference ability. Second， Small size and low energy consumption. Third， easy maintenance and transformation. Fourth， fully functional applicability and cost-effective. All in all， with easier realized and reliability， the PLC control system is an available method to instead of the relay control system.

4.Conclusion

This paper introduces the introductory teaching of PLC. The hardware （wiring） and software （ladder diagram） of PLC control system are described in detail though a jog motor control and a self-locking control. By comparing PLC and relay system， students can understand PLC more intuitively and progressively. The application of Project-based Approach is of great significance in arousing the students' enthusiasm and initiative and improving their learning abilities.

References

[1] W. Bolton， Programmable Logic Controllers， Fifth Edition， Newnes， 2009

[2] JIANG Xin- Qian， On PLC Project teaching [J]， Journal of Wuhan Institute of Technology， 2007， vol. 6 N0.4

[3] PEI Gui- ling， QI Yan The Exploration of Stratified-Teaching Mode in Project Teaching of PLC in Higher Vocational Colleges [J]， Guangzhou vocational education forum， Vol.13 No.1