Abstract. Siemens S7-200 internal PLC position-controlling wizard can realize the positioning control. However, it is a kind of relative positioning control. In a complicated positioning control system, it would easily lose the envelope and thus wrong control occurs. As for Siemens MAP instruction, it adopts absolute positioning control method, which performs significant stability in the application of servo position control. This kind of method has realized its accurate position control between each station in YL-335B automatic simulation production line. The experiments show its superiority.
Key words: servo position control; MAP instructions; S7-200 PLC; automatic production line
1.Introduction
Position control is generally realized through the control of stepping motor or servo motor by the controller. Position control is a type of automatic control in which the input commands are the desired position of a body. In the accurate position control system, it usually adopts servo motor as the execution institution [1].
S7-200 series PLC is a kind of programmable logic controller that is small in size and easy to be programmed and controlled. The position control of the servo motor is usually realized adopting the following three kinds of methods: PTO and the professional positioning control model EM253 as well as the RS232 communication interface [2]. The servo motor is usually control adopting the method of PTO. There are two kinds of realization methods of the internal pulse output.
1)In the first place, the PTO is realized through the instruction directions that are in the program software.
2)In the second place, the pulse output instruction MAP base, the base of MAP is offered by Siemens [3].
An assembly line is a manufacturing process in which parts are added to a product in a sequential manner using optimally planned logistics to create a finished product much faster than with handcrafting-type methods. For example, YL-335B automatic stimulation production line [4]. Assembly lines are designed for a sequential organization of workers, tools or machines, and parts. The motion of workers is minimized to the extent possible. All parts or assemblies are handled either by conveyors or motorized vehicles such as forklifts, or gravity, with no manual trucking. Heavy lifting is done by machines such as overhead cranes or forklifts. Each worker typically performs one simple operation. The accurate position is usually realized by the control of the stepping motor or the servo motor by the PTO [5].
The internal pulse output of S7-200 PLC offers two kinds of numerical output channels (Q0.0 and Q0.1). The assembly line concept was independently redeveloped throughout history and not invented at one time by one person. Its exponentially larger development at the end of the 19th century and beginning of the occurred among various people over decades, as other aspects of technology allowed. The development of tool path control via jigs, fixtures, for machine tools such as the screw-cutting lathe, metal planer and milling machine during the early century provided the prerequisites for the modern assembly line by making interchangeable parts a practical reality. The transition to other methods began as manufacturers took advantage of the opportunities that the aforementioned machining developments presented. Thus, before the modern assembly line took shape, there were prototypical forms in various industries, as outlined below. The PTO pulse numbers of S7-200 is 1-232-1, which is 1to 4,294,967,295. The circle is 10μs(100K)to 65535μs or from 2ms to 65535ms.
2.The Using Method of MAP Instructions
S7-200 PLC offers two positioning control instruction bases, which are MAP SERV Q0.0 and MAP SERV Q0.1. They are respectively applied in Q0.0 and Q0.1. The functions of these two instruction bases are the same. In the followings, the principles and the methods will be introduced taken MAP SERV Q0.0 as the example.
MAP SERV Q0.x instruction base has included nine function blocks. Each function block is shown as the table 2-1:
Table 2-1 the function block and their functions of MAP SERV Q0.x instruction base
Q0_x_Home function block: determine the referential position, which is shown in Fig.2-2.
EN：Allowing terminal; EXECUTE: touching terminal; Position is the referential position value; Start DIR is the starting point; Done is the finishing point; Error.
Q0_x_MoveAbsolute function is shown in Fig.2-3.
EN：Allowing terminal; EXECUTE: touching terminal; Position is the referential position value; Start DIR is the starting point; Done is the finishing point. Done=1 when it is finished.
Q0_x_Stop function block is shown in Fig.2-4.
What draws attention is that it should be combined with Q0_x_Stop function block.
Q0_0_CTRL function block: the definition of parameter and the primitive control.
In the main program, the Q0_0_CTRL function block should be ensured to be used. Velocity_SS is the minimum operation speed while Velocity_Max is the maximum operation speed. accel_dec_time is the minimum accelerating speed and the block is controlled within 0.02 ~ 32.0 seconds. It is generally no more than 0.5 seconds. Fwd_Limit and Rev_Limit are the buttons and the C_Pos is the current possiton.
Q0_x_LoadPos function block is to upload the current position as the reference point again.
EN：Allowing terminal; EXECUTE: touching terminal; Position is the referential position value; Start DIR is the starting point; Done is the finishing point. Error is the wrong information label while C_Pos is the current position.
Considering that the position of MAP instruction base is as a matter of fact the absolute position control. The principle problem is to determine the position of the reference point. Execute Q0_x_MoveAbsolute function block can realize the position control according to the preset requirements.
3.The Position Control of MAP
3.1.The processing control
YL-335B type automatic stimulation production line training platform is composed of by the following five stations: the supply station, the processing station, the assembly station, the sorting station and the transmission station.
The processing control is specifically shown as the followings: the supply station outputs the work pieces to the charging table. The mechanical hand of the transition station transfers the work pieces to the assembly station. After assembly, the work pieces are returned to the processing station. After finish it is sent back to the sorting station and returned back to the reference station. The position control has four places, which are （1）From the starting point to the assembly station;（2）From the assembly station to the processing station;（3）From the processing station to the sorting station;（4）From sorting to the referential point, which are shown as the Fig.3-1.
3.2.Hardware circuit design
In order to realize the four position control shown in Fig.3-1, the corresponding hardware circuit is designed which is shown in Fig. 3-2.
The transmission controller adopts the type of S7-226 DC/DC/DC PLC. The execution adopts Panasonic minas-A5 series servo driver and the supporting servo motor. The code of the servo motor is 2500 and frequency of four times. PLC adopts 24V DC power supply. I0.0 shall connect the reference point. Q0.0 is pulse output. Q0.2 is the direction control. In order to ensure that the reference point is searched accurately, the initial direction is realized through PR0.06=0.
3.3.Software design
The software design mainly includes one main program and two sub-programs [6]. In every scanning circle of the major program, initial Q0_0_CTRL function scale is operated according to minimum operation speed, the maximum operation speed and the starting time of setting the operation control process. The two sub-programs are respectively the searching reference point sub-program and the operation control sub-program.
The sub-program of operation control is a sequential control sub-program which is divided into five positioning controls. They are shown as the followings: from supply station → assembly station; from assembly station → processing station; from processing station → sorting station; from sorting station → near referential station; from near reference point → reference point.
The program flow diagram is shown in Fig.3-3, 3-4 and 3-5.
4.Conclusions
In the paper, S7-226 PLC, Panasonic AC SERVO DRIVER and SERVO MOTOR were adopted. They have realized the accurate position control between each station in a relatively good manner. The realization method is simple and with good stability, which is fully equipped with amazingly good application values.
5.Acknowledgment
Foundation Project: the education science project of the “Eleventh Five-Year Plan” in Guangxi Province, 2010 (Project Number: 2010C251); education reform project of new generation in Guangxi Province, 2010 (Project Number: 2010JGZ084); Science and development project in the Department of Education of Guangxi Province (Project Number: 201106LX807).
6.References
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[4]Lv jing-quan. The assembly and adjustment of the automatic production line（edition two）[m].china railway publishing house, 2009,12.
[5]Luo geng-xing，ling yu-shan.the control of stepping motor based on plc [j]. Mechanical & electrical engineering technology，2007,36（10）：66-67,84.
[6]Liao chang-chu. The basic course of s7-200plc (edition 2)[m].china machine press，2009,2.