Propose of electromagnetic actuator for high efficiency EDM

Abstract

In this paper, a kind of electromagnetic actuator is developed which can solve the problem of in time controlling inter-electrode gap in traditional EDM. An incoming current in the coil of an electromagnetic actuator generates Lorentz forces that cause the electrode to move axially. When the electromagnetic actuator is working, electrode is actuated positioned quickly, and electrode is produced a displacement of plus or minus 1 mm by control. The experiment shows that the response time of the electromagnetic actuator can meet the machining requirements. The discharge probability is increased and the machining efficiency is improved.

Keywords

Electron Discharge Machining (EDM)high efficiency electromagnetic actuate PID control

1. Introduction

Electrical Discharge Machining (EDM) is used between tool electrode and workpiece generate the pulse spark discharge to removal in metal materials, in order to achieve the required shape, size and surface morphology of a processing method [1]. Due to EDM is based on the high temperature melting evaporation to removal material, belong to the non-contact processing, no macro cutting force in the machining process [2], as a result, compared to the traditional machining method, there is no special requirement for the strength and hardness of the workpiece material, can be processed any conductive materials [3]. It is widely used in machining, abrasive tools manufacturing aerospace, and other fields [4]. However, during EDM, a large amount of machining debris will be generated between the electrode and the workpiece, which will eventually accumulate between the electrode and the workpiece, impeding the processing and reducing the discharge probability.

Traditional motor and ball screw has a slow response, and it is difficult to control the inter-electrode gap in time, which is also a major cause of low efficiency of EDM. When the inter-electrode gap is too large, the discharge channel cannot be formed. When the inter-electrode gap is too small, the inter-electrode discharge gap cannot deionize effectively during the pulse repose, leading to the inter-electrode short circuit, processing cannot be carried out. Therefore, maintaining stable inter-electrode voltage plays a key role in improving EDM efficiency.

A lot of research work is divided into three points to improve ED performance and improve machining efficiency. 1. Improvement of processing medium. 2. Improve chip removal ability. 3. Ensure proper interelectrode gap. Rongyuan Xue et al. [5] used oil-in-water emulsion as the working medium to process titanium alloy and found that the processing efficiency was about twice than kerosene ;Xiaoyou Zhang, Akio Kifuji sing radial magnetic bearing, thrust magnetic bearing and magnetic coupler designed a six-degree-of-freedom magnetic actuator that can be directly connected with ordinary EDM [6].

In this paper, the electromagnetic actuator is designed to solve the problem that electrode axial location is not timely when EDM is abnormal. When the EDM machining, the electromagnetic actuator actuates the electrode to locate rapidly in the axial direction and control the inter-electrode gap within the effective discharge range.

2. Principle and structure of electromagnetic actuator

In order to improve the machining speed, the purpose of this paper is to develop an electromagnetic actuator for EDM, which actuates the electrode movement when EDM machine tool is working. In the axial direction, a displacement of 1 mm above and below can be generated, so that the distance between the electrode and the workpiece can be timely controlled to make it in the optimal discharge gap as far as possible. Thus, the discharge probability and the machining efficiency can be improved.

Fig. 1.

The schematic of control system.

Figure 1 is the overall control of electromagnetic actuator in the process of processing. The electromagnetic actuator adopts displacement double closed-loop control. The inter-electrode voltage is detected through a specific detection circuit connected between the electrode of the EDM machine and the workpiece. The inter-electrode voltage is converted into a voltage signal and fed back to the control system. The control system calculates the difference between the voltage between the inter-electrode voltage and the target voltage, and converts the difference into a displacement signal, which is converted into a voltage control signal and input into the voltage controlled current source after calculation by the controller. The voltage controlled current source converts the control signal into the control current and inputs it into the coil of the electromagnetic actuator to complete the movement of the electromagnetic actuator. After the displacement sensor in the electromagnetic actuator detects the displacement signal, the displacement signal is fed back to the control system to complete the closed-loop control of the displacement. After double closed loop control, the inter-electrode voltage can be controlled at the target voltage to maximize the discharge probability.

In order to adjust the inter-electrode gap quickly, it is necessary to combine the traditional discharge motor with wide-band and high-precision local actuator. The localization response of electrode is improved by using electromagnetic actuator as local actuator. In this paper, we describe in detail a high-speed, axially controlled local actuator that uses thrust to control the direction of movement of the electrodes in order to quickly maintain a suitable distance from the workpiece. The actuator is designed and manufactured preliminarily, and the high-speed response performance of the actuator is evaluated for the control system.

2.1. Working principle

When different directions of current are passed in the coil, the actuator will generate upward or downward Lorentz force to control the movement in the Z direction. The displacement sensor above the axis is used to monitor the displacement of the actuator in the Z direction in real time, so that the electrode and the workpiece can quickly maintain the most suitable distance. Figure 2 shows the working principle of the proposed electromagnetic actuator for EDM.

Fig. 2.

Working principle of electromagnetic actuator.

2.2. Structure

According to the output force and displacement requirements of the electromagnetic actuator, design the structure size of permanent magnet ring and electromagnetic coil, and consider how to connect with the EDM machine in the structure. Figure 3 shows the structure of the electromagnetic actuator.

It is mainly composed of spindle, housing and coil. Two leaf spring are fixed at both ends of the shaft to constrain the translation of the rotor in the X and Y directions, the rotation around the X and Y axes, and the rotation around the Z axis. According to the research of Huifang Liu et al. [7], the symmetrical distribution of magnets will produce a better magnetic field, so two permanent magnets with opposite magnetic poles fixed on the spindle and coils fixed on the housing constitute a thrust electromagnetic bearing.

Fig. 3.

Electromagnetic actuator structure diagram.

3. Characteristic analysis experiment

In order to enable the thrust magnetic bearing to generate the maximum electromagnetic force to control the motion of the actuator, the optimal size of each part of the actuator is obtained through orthogonal analysis simulation. The simulation results show that the maximum electromagnetic force generated by the optimal size is 33.976 N, which meets the experimental requirements. In order to obtain more clearly the relationship between the output force and the input current of the electromagnetic actuator designed in this paper, an experimental system of the output force of the electromagnetic actuator was built in this paper. Figure 4 shows the experimental device, which includes: 1. Electromagnetic actuate power supply. 2. Computer. 3. dSPACE. 4. Micro platform. 5. Tension sensor. 6. Electromagnetic actuator 7. Tension sensor display.

In order to compare the experiment with the simulation, the experimental system and the simulation system need to be the same. The leaf spring used to fix the actuator was removed from the experimental system, and the moving element of the actuator was kept in no contact with the coil and the housing. The enclosure and coil of the electromagnetic drive, namely the stator part, are connected to the force sensor, which is connected to the fretting platform and connected to the support which is fixed vertically on the test platform, and keeps the end face of the actuator and the platform horizontal. The moving element is fixed vertically on the test platform, so that the stator and the moving element are on the same axis, and the fretting platform is adjusted to make the moving element of the actuator coincide with the stator’s center point, namely, the location designed. The purpose of zeroing the tension sensor after starting is to offset the gravity generated by the stator and start the experimental system. The actuate power supply is controlled by dSPACE to input a certain current to the electromagnetic coil, and the display number of tension sensor is observed and recorded.

Fig. 4.

Output force measurement experiment system.

Based on the above simulation results, the relationship between the output force of the electromagnetic actuator and the input current is drawn, as shown in Fig. 5(a), where the input current refers to the current of each turn wire. In the experiment, convert the control voltage of phase synchronous length into drive current by voltage controlled current source. In the simulation, the current with the same step size can be directly input. It can be seen from the figure that the force obtained by simulation is slightly larger than the electromagnetic force obtained by experiment. The main reason is insufficient magnetization and some magnetic leakage phenomenon. The simulation and experimental results are consistent trend, the relation between the electromagnetic force and the current can be obtained: F = 5.66 i. The experiment of relation between the input current and the displacement of the actuator is also studied Fig. 5(b) is the relationship between the actuator displacement and the input current. When the coil current is greater than 6 A, a displacement of 1 mm can be generated, which meets the requirements of the experiment.

Fig. 5.

The relation between output force ∖ actuator displacement and input current.

4. System dynamics model and simulation

The dynamic model of the actuator is established through EDM magnetic actuator. Figure 6 is the schematic diagram of the dynamic model.

Fig. 6.

Dynamic model of electromagnetic actuator.

The dynamic models of the magnetic actuator is shown as below. $\begin{eqnarray}\displaystyle F+Mg=Ma+2ky+F_{d}+cv+F_{0} & & \displaystyle\end{eqnarray}$ (1) Where, F is the electromagnetic force; M is the mass of the actuator; a is the acceleration of the motion of the actuator; k is the stiffness of the leaf spring; y is the kinematic displacement; F_d is resistance; c is the friction coefficient; F is the discharge power.

Output force F on electromagnetic actuator is written as follows. $\begin{eqnarray}\displaystyle F-F_{0}=k_{i}i & & \displaystyle\end{eqnarray}$ (2) Where i is the input current and k_i is the current stiffness.

The state space equation of the electromagnetic actuator is: $\begin{eqnarray}\displaystyle \left\{\begin{array}{@{}l@{}}{\dot{X}}=AX+BU\quad \\ Y=CX+DU\quad \end{array}\right. & & \displaystyle\end{eqnarray}$ (3) $\begin{eqnarray}\displaystyle X=\left[\begin{array}{@{}cc@{}}x & {\dot{x}}\end{array}\right]^{\text{T}};\quad Y=\left[\begin{array}{@{}cc@{}}1 & 0\end{array}\right]\left[\begin{array}{@{}c@{}}x\\ {\dot{x}}\end{array}\right];\quad U=i. & & \displaystyle \nonumber\end{eqnarray}$ Where X is as input of the electromagnetic actuator, Y is as output of the electromagnetic actuator; U is the input variable.

Coefficients of state space equation are obtained as $\begin{eqnarray}A=\left[\begin{array}{@{}cc@{}}0 & 1\\ -175 & 0\end{array}\right]\quad B=\left[\begin{array}{@{}c@{}}0\\ 28.3\end{array}\right]\quad C=\left[\begin{array}{@{}cc@{}}1 & 0\end{array}\right]\quad D=0.\end{eqnarray}$

The dynamic simulation experiment of the electromagnetic actuator is carried out by using MATLAB-Simulink, and the response time of the actuator. Considering that the discharge power generated during EDM will cause interference to the control system. White noise is added to the simulation to simulate the discharge power with a frequency of 10000 Hz and noise power is 0.001, it is going to have a force of 10 N. Figure 7(a) input 1 mm ascending step signal when 0.1 s, and Fig 7(b) input 1 mm descending step signal when 0.1 s. As can be seen from the figure, the response time of 1 mm ascending step is 0.0566 s and that of 1 mm descending step is 0.0585 s.

Fig. 7.

1 mm step response.

The results show that the control system is disturbed by the discharge power, but the control system can still maintain good stability, and the response time can meet the requirements of EDM. In conclusion, The control system can adjust the inter-electrode gap in time and has good controllability which can meet the requirements of EDM.

5. Experimental verification

The figure 8 shows the displacement control experiment which includes: 1. Displacement control includes electromagnetic actuator. 2. Eddy current displacement sensor. 3. Computer. 4. dSPACE. 5. Power amplifier. The displacement signal detected by the displacement sensor is fed back to the controller, and the deviation resulting from the comparison with the given expected displacement is converted into a voltage signal, which is amplified by the power amplifier to generate a drive current input to the electromagnetic drive to complete the desired displacement.

Fig. 8.

The displacement control experiment.

During the experiment, the PID controller parameters are: K_p = 3, K_i = 0.8, K_d = 0. Figure 9 shows the 1 mm ascending step signal input at 1 s and the 1 mm descending step signal input at 1 s.

Fig. 9.

Ascending step signal and descending step signal.

As can be seen from the figure, the response time of 1 mm ascending step is 0.045 s, and that of 1 mm descending step is 0.05 s. The response time can meet the requirement of EDM. The experimental results show that the electromagnetic drive has good visibility and controllability and short response time, which can meet the requirements of EDM. In the process of processing, if the inter-electrode voltage changes so that the machining process can’t proceed normally, the electromagnetic drive will quickly move the electrode to the appropriate position, so that the inter-electrode voltage remain stable. The inter-electrode gap is timely and effectively controlled in the ideal range of distance, so that the processing process can proceed smoothly. The discharge probability is increased to the greatest extent, so as to improve the processing efficiency and ensure the smooth processing.

6. Conclusions

A kind of electromagnetic actuator is developed which can solve the problem of in time controlling inter-electrode gap in traditional EDM, this paper introduces the principle and structure of electromagnetic actuator, in order to verify the correctness of the theoretical model, finite element analysis of electromagnetic actuator, establish the dynamics model of the actuate and has carried on the simulation and experiment, the results of simulation and experiment results showed that the actuate can control electrode displacement of plus or minus 1 mm. Compared with the driving mode of motor and ball screw, the designed actuator has a faster response speed. On this basis, the next step of this paper will verify the high efficiency of the actuator through EDM experiments.

Footnotes

Acknowledgements

This research is supported by Liaoning Revitalization Talents Program (No. XLYC1802077), Scientific research fund project of Liaoning Provincial Department of Education (No. LJGD2019011).

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