Design Method of PLC Frequency Conversion Speed ​​Regulation in Electrical Control System of M-type Grinding Machine

Abstract: M-type grinding machine is a kind of electromechanical equipment with high automation requirements. The control circuit of the traditional relay-controlled centerless grinding machine is transformed by using PLC and variable frequency speed regulation technology, which changes the traditional time control method with time relay as the core, realizes the automatic control of the centerless grinding machine process and achieves good economic benefits.
Keywords: centerless grinding machine; PLC; frequency converter; automatic control

0 Introduction
M-type grinder is the abbreviation of M1050 centerless grinder. It is a machine tool equipment with high automation requirements. It is mainly used for cutting-in grinding of cones with a taper less than 1:20 and various molded rotating parts. With the addition of a through-grinding bracket, it can be used for through-grinding. The control system of the grinder has always adopted the relay logic control method. The speed adjustment of the guide wheel is obtained by controlling the DC motor by the DC conversion device, which is not convenient and has low efficiency.
In order to solve the shortcomings of the above-mentioned grinder control system, it was decided to implement a new design for its electrical control system. The control circuit of the traditional relay-controlled grinder was transformed by PLC and variable frequency speed regulation technology, which changed the relay logic control method with time relay as the core, and realized the automatic control of the grinding process. After the speed of the grinder guide motor is controlled by the inverter, the previous DC motor is replaced by an AC motor, which can greatly improve the transmission efficiency and is simple and convenient.

1 Process requirements and process of M-type grinder control
The M-type grinder is driven by five AC motors, one of which needs stepless speed control, and the motor is controlled by a frequency converter to obtain stepless speed change. The objects controlled by the main control motor are lubrication, cooling, grinding wheel, hydraulic pressure and guide wheel.
According to the cutting grinding process requirements of the M-type grinder, three solenoid valves are used to control four working objects in one operation cycle. The operation flow of one cycle is shown in Figure 1. As can be seen from Figure 1, the four working objects are the guide wheel frame fast and slow forward, the guide wheel frame fast retract, the push rod push, and the push rod return.

2 Electrical control system of M-type grinder
2.1 Design of main circuit of motor of M-type grinder
The wiring schematic diagram of main circuit of variable frequency speed control of M-type grinder is shown in Figure 2.

In the figure, QS is an air switch, hydraulic motor M2 and guide wheel motor M5 can be started and stopped randomly, grinding wheel motor M1 and lubrication motor M3 are in sequential control mode, that is, lubrication motor M3 starts first, and grinding wheel motor M1 starts later. Guide wheel motor M5 is controlled by a frequency converter and has stepless speed regulation.
The frequency converter is set to external control mode 2, and the method is to write the content of Pr. 79 as 2. Stepless speed regulation is obtained by adjusting an external potentiometer on the frequency converter.

2.2 PLC I/O address allocation
The inverter uses the Japanese Mitsubishi FR-E500 series inverter, and the PLC uses the Japanese Mitsubishi FX series FX2N-48MR PLC.
According to a working cycle of the M1050 centerless grinder shown in Figure 1, the actual cutting-in grinding cycle of the grinder is realized by different working combinations of three hydraulic solenoid valves, namely 1CT, 2CT, and 3CT. For example, the slow feed of the guide wheel frame is realized by the simultaneous connection of 2CT and 3CT, and the pushing of the push rod is realized by the connection of 1CT. The cutting-in grinding cycle of the grinder is shown in Table 1.

Note: 0 means working, - means stop.
The conversion between each process is determined by the signal of the photoelectric sensor or the timer. There are 4 photoelectric sensors, namely: the guide wheel frame feed fast and slow sensor 1XK, the guide wheel frame feed to the bottom sensor 2XK, the guide wheel frame back to the bottom sensor 3XK, and the push rod push to the bottom sensor 4XK.
As mentioned above, the I/O address allocation of the PLC is determined as shown in Table 2.

2.3 PLC hardware wiring diagram
According to the contents shown in Figure 2 and Table 2, the PLC hardware wiring diagram is designed as shown in Figure 3.

3 PLC Program Design
3.1 PLC State Diagram
According to the contents shown in Figure 1, Figure 2, Table 1 and Table 2, design the PLC state diagram as shown in Figure 4.

In Figure 4, there are 6 states. The S0 state is the waiting start selection state. When the PLC is powered on, M8000 always opens S0. In this state, the lubrication and cooling motors should be started first, and then the grinding wheel motor can be started. The hydraulic motor and the guide wheel motor are randomly started according to the processing needs. After the guide wheel motor is started, its speed can be steplessly adjusted through the frequency converter. A variable potentiometer R is connected to the frequency converter, as shown in Figure 3. Rotating R can change the speed of the guide wheel motor.
The S10-S14 state is the cut-in grinding cycle of the M1050 centerless grinder. Press the cut-in grinding cycle start button X8, and the centerless grinder enters the S10 state. In the S10 state, Y5 is connected, the solenoid valve 2CT is energized, and the guide wheel frame is fed quickly. The fast forward is in place, the guide wheel frame feed fast and slow sensor 1XK (X10) is closed, the fast forward stops, and the grinder enters the S11 state. In the S11 state, Y5 and Y6 are connected, solenoid valves 2CT and 3CT are energized, and the guide wheel frame rotates quickly and feeds slowly. When the guide wheel frame reaches the end of feeding, the sensor 2XK (x11) is closed, the slow feed stops, and the grinder enters the S12 state. In the S12 state, Y5 and Y6 are connected, solenoid valves 2CT and 3CT are energized, timer T1 is set for 10 seconds, and the grinder enters the light grinding stage. When the light grinding time is up, timer T1 is closed, the light grinding ends, and the grinder enters the S13 state. In the S13 state, all solenoid valves lose power, and the guide wheel frame quickly retracts. When the guide wheel frame retracts to the end, the sensor 3XK (X12) of the guide wheel frame retracts to the end is closed, the fast retraction stops, and the grinder enters the S14 state.
In the S14 state, Y4 is connected, the solenoid valve 1CT is energized, and the push rod pushes the material. When the material is pushed into place, the push rod pushes the material to the bottom, and the sensor 4XK (X13) is closed, the material pushing stops, and the grinder enters the S0 state. The push rod returns to the first step S0 state, waiting for the next new working cycle.
3.2 PLC step ladder diagram
According to the PLC state diagram shown in Figure 4, the PLC step ladder diagram can be designed, and then the PLC program list can be written, which is omitted here.

4 Conclusion
The system combines the technologies of machinery, PLC, frequency converter, etc., and successfully redesigns the original control system at a lower cost, making the M-type grinder meet the requirements of automatic control. In actual use, it has stable performance, convenient operation, high production efficiency, and has achieved good economic benefits.