Sensors in CNC machine tools - Solutions - Huaqiang

Photocoupler

The sensor represented by the position detecting device plays an important role in ensuring the high precision of the CNC machine. Figure 12‐ shows the outline drawing of the CNC lathe. Figure 12‐ shows the internal structure of the CNC lathe after the casing is removed. composition.

The lateral movement of the carriage is the z-axis, and the z-axis feed servo motor is realized by the z-axis ball screw; the radial movement of the carriage on the carriage is the x-axis, and the servo motor is fed by the x-axis through the x-axis ball Bar to achieve. The servo motor end is equipped with a photoelectric encoder for angular displacement measurement and digital speed measurement. The angular displacement can indirectly reflect the linear displacement of the carriage or the tool holder through the screw pitch.

Taking the z-axis as an example, the servo control block diagram of the axis is shown in Figure 12‐. As the servo motor drives the carriage movement, the photoelectric encoder generates a pulse signal proportional to the linear displacement x, the signal reflects the actual position value of the carriage, and is used as the position feedback signal Pf, and the position command Pc obtained by the numerical control system operation. For comparison, a position deviation signal Pe, Pe = Pc - Pf is generated, and a speed control signal nc is generated by the signal conversion circuit. After comparing with the speed feedback signal nf, the nc is driven by the signal adjustment and power drive, and the carriage is driven by the ball screw nut to continue the linear motion.

When the carriage moves to Pf=Pc, then Pe=Pc-Pf=0, the servo motor stops, and the carriage stops at the position specified by the position command Pc.

It can be seen that the resolution of the photoelectric encoder determines the accuracy of the actual displacement value of the table, thus affecting the accuracy of the position control of the CNC machine. The angular encoders in CNC machine tools mostly use photoelectric encoders. In general, the position measurement is incremental, and the important measurement is absolute.

In addition, the photoelectric encoder coaxially coupled with the servo motor is used to measure the angular displacement θ of the lead screw on the one hand, and can also be used for digital speed measurement on the other hand to generate the speed feedback signal nf.

In the high-precision CNC machine tool, the position detecting device can adopt a linear grating, and its measurement accuracy is higher than that of the photoelectric encoder, but the price is also high. Figure 12‐ shows the installation of the grating on the z-axis of the CNC lathe.

The grating ruler is fixed on the bed body. In the feed drive, the scanning head moves along with the carriage, and generates a pulse signal proportional to the linear displacement x, which directly reflects the actual position value of the carriage. At present, the resolution of the grating for CNC machine tools is up to 1 μm, and the higher precision is up to 0.1 μm.

If the servo system using the photoelectric encoder as the table position detecting device is called a semi-closed loop control system, then the servo system using the grating as the table position detecting device is called a full closed loop control system, as shown in Fig. 12- Shown in the dotted line.

In addition, the spindle encoder connected to the spindle is also a photoelectric encoder for the control of the thread. Its role is to match the spindle speed with the z-axis feed to ensure the consistency of the pitch.