Application of AC servo spindle drive system in CNC machining center

The spindle drive system is a high-power actuator for CNC machine tools. Its function is to accept the S code speed command of the CNC system and the M code auxiliary function command to drive the spindle for cutting. The CNC machining center has high control requirements for the main shaft. Firstly, it is required to realize wide-range stepless speed change based on the large torque and strong overload capability. Secondly, it is required to realize the fixed angle stop (ie, quasi-stop) in the automatic tool change action. It makes the machining center spindle drive system more difficult than the general variable frequency speed control system or small power AC servo system in circuit design and operation parameter setting. The drive of the main shaft can be controlled by AC frequency conversion or AC servo. The AC variable frequency spindle can be steplessly shifted but cannot be stopped. It needs to be equipped with a spindle position sensor and cooperate with the logic program of CNC system PMC (index control system built-in PLC). The quasi-stop speed control and positioning stop are completed; the AC servo spindle itself has the quasi-stop function, its own axis control PLC signal can be directly connected to the PMC of the CNC system, and the quasi-stop positioning control can be completed with the simple PMC logic program, and The latter's control accuracy is much higher than the former.

Based on the principle analysis of the spindle servo drive system, combined with the application of the Mitsubishi SJ-PF system AC spindle drive system on the H400 and V400 vertical and horizontal machining centers, this paper discusses how to use the high stability of the modern intelligent drive system. High-precision features and convenient parameter adjustment functions make the spindle drive system meet the process and accuracy requirements of the machining center through reasonable capacity selection, circuit connection and related parameter adjustment.

First, the working principle of AC servo spindle drive system

The AC servo spindle drive system consists of a spindle drive unit, a spindle motor and a rotary encoder that detects the spindle speed and position. The closed loop speed control is mainly completed, but the closed loop position control is completed when the spindle is stopped. Since the spindle drive power of the CNC machine tool is large, the spindle motor adopts the squirrel-cage induction motor structure. The rotary encoder can be installed outside the spindle or integrated with the spindle motor. The closed-loop control and vector operation of the spindle drive unit Both are implemented by an internal high-speed signal processor and control system, and its block diagram is shown in Figure 1. In the figure, the CNC system sends a speed command to the spindle drive unit. The drive unit compares the command with the actual speed measured by the rotary encoder. The digitized speed regulator and the flux linkage function generator calculate the current desired torque and hope of the rotor. The flux vector is then compared with the actual torque and flux linkage results, and the torque current component and the excitation current component of the equivalent DC motor (two-phase rotating shaft system) are converted into two phases by the torque and flux linkage controller. The static shaft system finally enters the three-phase static shaft system through 2/3 vector transformation to obtain the desired value of the three-phase stator current of the frequency conversion device. By controlling the SPWM driver and the IGBT frequency conversion main circuit, the load three-phase current follows the desired value, and the completion can be completed. Speed ​​closed loop control of the spindle.

Figure 1 Block diagram of the AC servo spindle drive unit

The Mitsubishi spindle drive unit adopts current-type vector control mode, with high-speed digital signal processor DSP as the core, and large-scale ASIC and high-speed power module as the realization circuit, which makes the drive unit have small size, light weight, high calculation speed and position. It has many advantages such as short steering time and easy high-speed stop. The drive unit can accept digital speed reference, can also accept analog speed reference, with over-current, over-voltage, overload, under-voltage, over-temperature, over-speed and many other protection monitoring and diagnostic functions, with communication function, quasi-stop The parameters and operating parameter settings can be preset by PC or manually, and the design is compact and easy to use. When the spindle drive is in the position of the stop, it does not actually accept the CNC speed command, and only completes the closed loop position control according to the preset quasi-stop position and the quasi-stop speed.

Second, the capacity selection of the spindle drive system

Take the capacity selection of the H400 vertical machining center spindle drive system as an example to illustrate the selection process of the spindle drive system capacity.

1, select conditions

(1) Primary selection of Mitsubishi SJ-PF series AC spindle drive system, the maximum speed of the spindle motor is 6000r/min, and the maximum speed of the spindle constant torque output is 1500r/min.
(2) Cutting parameters (maximum milling force condition), when milling medium carbon steel workpieces with high speed steel end mills, the tool diameter is 125 mm and the maximum milling speed is 40 m/min.

2. Calculate the main drive power of the main shaft

(1) Spindle no-load power. The average diameter of the front and rear support journals of the H400 vertical machining center spindle is 70mm, and the idle power of the spindle can be calculated by the following formula.

Where: k, c is a constant; dm is the average diameter of the front and rear bearing journals; n is the maximum speed of the spindle constant torque output.

Substituting the value gives P0 = 0.92 kW.

(2) Maximum cutting power of the spindle. The cutting load of the spindle when the maximum cutting power of the spindle is the maximum milling force. When milling medium carbon steel workpieces with high speed steel end mills, the maximum milling force can be calculated by:

Set the maximum cutting speed v=40m/min to find the maximum cutting power

(3) The main drive power of the main shaft is

Since the drive chain of the CNC machine tool is short, the total efficiency of the main drive takes a high value of 0.85, and the value of the main drive can be obtained as PE=2.39kW.

As a complete servo drive system, the Mitsubishi spindle drive unit and the spindle motor capacity have been matched. According to the principle that the design power is greater than the main drive power, the SJ-PF series spindle motor model selected for the H400 vertical machining center is SJ-PF5. 5, the drive unit model is MDS-A-SPJA-55, the capacity is 5.5kW, the corresponding output power is 3.7kW, the rated speed is 1500r/min, and both motors have built-in encoder and cooling fan.

Third, the spindle drive system and CNC connection circuit

The machining center adopts the INCO-M40F open architecture CNC system which conforms to the industrial PC standard as the main controller. The CNC system completes the spindle motion control of the machining center through one set of AC servo drive system, and the three sets of servo feed drive system completes X. Y, Z three-direction feed linkage control, and through two PLC expansion I / O board to complete the machine tool complex logic control (including CNC system work mode management, hand wheel management, tool magazine automatic tool change, double table exchange , the workbench indexing stop, etc.), the logic control software is developed using C++ language, and the coordination management of each subsystem is realized by the 80586CPU of the numerical control system. The overall structure of the electrical control system of the machining center with the INCON-M40F CNC system as the core is shown in Figure 2.

Figure 2 Overall structure of the machining center electrical control system