Design of Small CNC Grinding Machine System based on Motion Control Card

Xin He No. 1310 Dagu South Road, Hexi District, Tianjin University of education and technology, Tianjin

corresponding author: 514459221@qq.com

Keywords: numerical control system, CNC grinding machine, on-line detection, motion controller

Abstract: In this paper, the general structure of grinding machine system is introduced, and the hardware platform of CNC machine tool system of computer host +PMAC motion controller is introduced in detail. A distance measuring instrument based on induction current principle is used as the detecting device of work-piece, and a new double loop structure tester is designed according to the design requirement. The new detection technology to obtain information through the coil impedance principle of current detection, information obtained through the digital analog conversion by the host computer, and the signal processing and analysis of machining error compensation, improve the machining precision grinding.

1. Introduction

Numerical control technology is the supporting technology in the field of manufacturing processing. It uses digital instruction code form to execute automatic processing of control objects according to the set processing parameters. At present, due to the constant change of market demand, the resource allocation of global manufacturing industry is developing in the direction of "labor force + information + knowledge" under the impetus of science and technology. Since its successful introduction to China, the powerful motion controller (PMAC motion controller) has played an important role in all fields of industrial manufacture.[1] In Chinese, CNC lathe system (CNC) after decades of development, now has formed a certain scale, the company's production of CNC system is the mainstream product of open CNC system, but it is closed by the products, led to the latest CNC technology research cannot be applied to the existing NC system upgrade and improvement.[2] For those schools capable of developing CNC systems independently, developing CNC grinding systems is both a way to save money and a good way to exercise and improve the R & D capability of CNC grinding machine systems [3].

This paper developed a small CNC grinding system, which not only can use the CNC teaching, and in order to ensure the safety of teaching, the use of small sliding mechanical equipment, electrical parts are used in electrical power equipment is very low, also joined the online detection system in the system, with monitoring and fault in the processing of alarm function, so this system has very strong security features, has important significance to carry out teaching work [7].

The motion control of new technologies and new products continue to emerge, has formed a

2018 3rd International Conference on Computer Science and Information Engineering (ICCSIE 2018)

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variety of typical motion control system, the application of the control system consists of PC host computer, motion control card, the driver is widely used now[4]. The system designed in this paper is this kind of structure [5]. The CNC grinding system designed in this paper is mainly used for teaching, on the one hand to meet the needs of teaching, on the other hand the need to ensure the safety in the process of teaching, so this paper mainly CNC grinding machine grinding.

2. Design of NC system grinder

The grinding machine is the working part of the whole system, and has important influence on the realization and performance of the system. The grinding machine mainly consists of three parts: instruction, control and execution. According to the teaching characteristics of grinding system designed in this paper, the design of system safety is the top priority, so the use is very safe and reliable mechanical and electrical properties of the device: the instruction part using the personal computer (PC) as the host computer; the control part for the PMAC motion controller and servo system of the corresponding implementation; part of the three degree of freedom platform system [6]. The PMAC motion control card is inserted into the PC PCI slot to complete the connection instruction part and control part of the PMAC motion controller driver to drive the servo motor with three degrees of freedom movement of the sliding table on the axis of the operation task by servo control [8].

2.1 The system structure is shown in Figure

2.1.1 PMAC motion controller

PMAC motion controller is a motion controller based on DSP processor, which has powerful control function. The PMAC motion controller can operate 8 moving axes simultaneously. The 8 moving axes can move at the same time or move separately, and have high maneuverability. The numerical control system adopting PMAC motion controller has the advantages of bus, open and modularization compared with the traditional CNC structure system. It is a new trend of NC system development.

The PMAC motion controller has the following functions. 1) Perform motion control program 2) Execute the PLC program 3) Servo system update 4) NC resource management 5) Communication with main control equipment

2.1.2 servo-system

A small motor that converts electrical impulses directly into angular displacement or linear displacement. The servo motor is usually not operating separately, need to use special drive servo motor for driving operation, this paper mainly adopts the MSDA series Panasonic servo system, the output power of the system can be in 1 to 500 watts, can adapt to a variety of power frequency, wide application. In order to realize the position control of NC system, this paper adopts "pulse + direction" double end output mode to control[9].

2.1.3 Connection mode

In order to ensure the safety and accuracy of data transmission, the input and output signals of PMAC motion controller is connected by switching and external equipment terminal board, to

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prevent mutual interference between the control circuit and the external voltage circuit of low voltage, internal terminal boards are equipped with optocoupler circuit, the low voltage and high voltage electrical isolation, so the terminal need +24V DC voltage supplied by an external power supply.

2.2 Executive part

The execution system of the grinder system designed in this paper is designed on the basis of three degree of freedom precision slide table. Three degree of freedom platform mainly comprises a bottom plate, bracket, X axis sliding motion mechanism, Y axis sliding motion mechanism, Z axis sliding motion mechanism of slide with ball screw lead to 2mm, the positioning accuracy is + 0.001mm, in line with the CNC Feed processing precision and measuring instrument for detecting distance.

The main technical parameters of the slipway are presented. A. Outline size: 700×600×1000(mm) B. X axis travel: 250(mm) C. Y axis travel: 250(mm) D. Z axis travel: 250(mm) E. Slider size: 80×80(mm) F. Slide load-bearing: 15(KG)

3. Design of on line inspection system

During machining, the host controls the CNC system and runs independently of the automatic inspection system. Considering the material is mainly used in the grinding process of the iron aluminum conductor material, according to the advantages and disadvantages of testing equipment in front of the analysis, this paper decides to adopt the non-contact measurement, detection system structure, shown in Figure 1.

Figure 1: Structure of inspection system.

In the process of precise machining, the accuracy of parts can be accurately measured online by proper is as close as possible to the end of the grinding wheel without affecting the normal operation of the grinding wheel. In addition, the measuring system is controlled by the machine tool rotating shaft, avoiding the conflict between the processing and testing process and the influence of the external environment, and protecting the measuring instrument. Combination of machining tools and measuring equipment. The CNC system mainly adopts this structure.

The design is to install the measuring instrument directly on the grinding wheel box, and to drive the measuring instrument through the feed of the grinding wheel. The position of the measuring instrument.

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3.1 Operational principle

The detection principle of equipment used in this paper is mainly analysis according to the current changes, the basic principle of current detection: placing a coil at the top of the metal plate, through alternating current coil inscribed with a certain frequency, according to the law of electromagnetic induction electric coil will produce a corresponding magnetic field. The coil near the surface of the metal plate (no contact), or according to the principle of electromagnetic induction, the metal surface will produce eddy current corresponding to the eddy current generated will produce a contrary to the original direction of the magnetic field of magnetic field, the magnetic field will affect the inductance of the coil L. According to the variation of the coil inductance L, the distance between the coil and the measured steel plate can be calculated, and the principle of the eddy current ranging is shown in Figure 2.

Figure 2: Principle diagr.

The voltage variations at the ends of the test coils can be represented by Equation (1).

),,,,,,( rfnixFU σµ= (1)

Among, U : Voltage across the coil µ : Themagnetoconductivity of detected conductor σ : The conductivity of detected conductor x : The distance between a coil and a conductor i : The current intensity of magnetic test coil n : Test coil factor f : The current frequency of magnetic test coil r : Measured size factor In Equation (1), assuming that only one parameter in the formula is changed and other

parameters in the formula remain the same, a monotonic functional relation can be formed between this parameter and the measured voltage. The easiest way to detect the current is the current, and the current detection can be done by measuring the voltage of the coil. As for the measurement of the coil wire winded, so it can be put into the equivalent coil inductance and pure resistance circuit, so the coil equivalent circuit is composed of an inductor and resistor, according to circuit principle, available impedance type circuit expresses by Equation (2).

LjXRZ += (2)

XL = θL, θ is the angular frequency of coil excitation, L is inductance of coil, R is equivalent pure resistance, Z is circuit impedance.

According to the principle of electromagnetic induction, when the coil close to the metal conductor, the metal surface will induce the conductor eddy current, the eddy current induced magnetic field will form the corresponding, which in turn affects the change of magnetic field

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induction coil, the mutual inductance of M coil measuring instrument and the induction magnetic field interaction degree of metal conductor between the measured and the equivalent circuit shown in Figure 3.

Figure 3

As shown in Figure 3, assuming the detection coil for the left measuring circuit, so the detection coil current, resistance, inductance for circuit design; right for the measured conductor eddy current circuit, so the current measured conductor, resistance, inductance. In order to detect the frequency of the current in the coil, the mutual inductance between the coil and the conductor is detected. According to the Kirchhoff voltage theorem, the lower form can be obtained and shown in Equation (3).

0ILjIRMIjUMIjILjIR

2221

121111

=++−=−+

ωωωω

(3) The system of equations is solved by measuring the current at one time, shown in Equation (4).

−+

++

=

••

222

222

22

122

222

22

1

11

j LLR

ML

LR

MR

UI

ωωω

ωω

(4)

according to Ohm's law, The impedance of the coil can be obtained and shown in Equation (5).

(5) Comparison of type 2 and type 5 shows that when the detection coil is placed above the

conductor to be measured when the equivalent resistance R and equivalent inductance respectively, shown in Equation (6) and (7).

222

222

22

1 RLR

MRR

ωω+

+= (6)

+−=

22

222

22

1LR

MLX L ω

ωω (7)

The alternating magnetic field produced by the conductor causes the change of the original impedance of the coil of the measuring instrument, shown in Equation (8).

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222

222

22

RLR

MRZ ω

ω+

= (8)

222

222

22

LLR

MX Z ω

ω+

= (9)

Equation (6) and (7) can be expressed as Equation (10) and (11).

ZRRR += 1 (10)

ZL XXX += 1 (11) Generally, the displacement has the greatest influence on the impedance, and both of them have

some function relationship. Therefore, a general measuring instrument can obtain displacement by measuring the voltage of the coil impedance. But in some cases, the function between the voltage of the coil and the impedance Z becomes non-monotonic, that is, a voltage corresponds to several impedance values, which results in incorrect measurements.

In order to verify the impedance properties of non-ferromagnetic materials, a circuit inspection experiment is designed:

Equipment: impedance tester, single coil, iron, aluminum block. Method: control the distance between the coil and the tested object, each time 0.1mm feed,

record the inductance value L, the resistance value R and the impedance value |Z| (Ω) with the tester respectively, until the measured object is close to the sensor. The impedance diagram is shown in Figure 4

Figure 4: Impedance diagram.

Through the analysis on the day that the displacement detection coil can be expressed by the voltage of the coil impedance value, in order to accurately detect the current according to the principle, to avoid the problem of double value, will need to decompose the measurement of impedance voltage X (μ) obtained the inductive voltage UX and resistance R (Ω) voltage were analyzed. We can avoid the problem of double value impedance. Therefore, this paper uses phase sensitive detection circuit to decompose the detection coil voltage and obtain the detection voltage respectively.

Phase sensitive detection circuit is a kind of detecting circuit that can recognize the phase and frequency of modulation signal. In the detection operation, phase sensitive detection technique is often used to decompose the impedance signal, and two orthogonal components are obtained after decomposition. The two orthogonal components are Ux, Uy, and UR= Ux× Uy. In this way, a certain degree of interference can be filtered out, and it is also intuitive. In this paper, the principle of phase sensitive detection is analyzed. As can be seen from above, the displacement detected by the measuring instrument can be expressed by voltage, so the voltage value of the displacement

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signal is as follows. U0 = A sin(ωt + ∅)

A is voltage amplitude, ω is voltage frequency, ∅ is initial phase angle. After the phase sensitive detection, the signal is filtered through the low-pass filter to filter out

the high-frequency component, so that the output can be output, respectively.

Ux =2ABπ

cosΦ

Uy =2ABπ

sinΦ

As can be seen from above, the multiplication of displacement signals and carriers is the process of orthogonalization of signals. The phase inductance detection technique is used to obtain the inductance UX and resistance components UR of the displacement signal, and then the impedance analysis is carried out to obtain the result.

3.2 working process

The on-line inspection process of the grinding machine system is shown in Figure 5. The lower surface of the work-piece is used as the initial position of the system coordinates of the system, and the Z0 is the coordinate of the upper surface of the work-piece in the machining system. The initial coordinate of the measuring instrument in the system is Z1, and the displacement signal detected at work is D1. Therefore, the coordinate of the work-piece surface in the system is Z0= Z1-d1, so the thickness information of the work-piece is obtained. When the measuring instrument is a little closer to the work-piece to be measured, the coordinate of the measuring instrument will change to Z2, and the thickness information of the work-piece to be measured can be expressed as Z0=Z2-d2. In processing, each processing is completed, the detection system can detect the corresponding data, and transmitted to the host computer to save, and with the set of additional value to compare to determine whether the processing meets the requirements.

Figure 5: Detection process.

4. Conclusion

The CNC system of grinding machine based on PMAC motion controller is designed in this paper. The system is developed on the basis of three degree of freedom sliding table. It has the advantages of moderate size, complete numerical control function and reliable electrical performance. Detection section. The independent detection system uses the impedance relation principle of the induced current and the voltage coil to overcome the shortcoming of the single coil structure, namely the double value problem of the measured value. The method has the advantages of strong independence, strong anti-interference and high sensitivity, and can obtain relative displacement accurately. Set up the experimental platform of CNC grinding machine online detection system, and test system of CNC grinding machine, the results show that the system has

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high precision and stability, can achieve the design purpose, and meet the design requirements.

Acknowledgements

Fund project: Tianjin University of education and technology. Academic research project. The project number: KJ1718.

Fund project: Tianjin University of Technology and Education. Academic research project. The project number: K171006601C Construction of parallel teaching platform of NC machining theory based on Internet + and virtual reality.

References

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