Servo Positioning Program of Inverter in Automatic Lathe

Abstract:
Spindle servo positioning of traditional automatic lathes is mostly realized by dedicated AC spindle servo positioning system. However, the system fails to gain popularity in economical CNC automatic lathes due to its high price. The article introduces a spindle frequency control system based on SIMPHOENIX Electric V360 inverters, which can position the spindle fast and accurately. The programme has advantages of low costs and high stability and has been widely applied in economical CNC automatic lathes.
Keywords: Inverter; Automatic lathe; servo positioning
Introduction
Lathe machining has a wide range. Various workpieces and processes use different tools and require different speeds of execution units in lathes. Therefore, main lathe movement should have the function of speed regulation. Lathe users attach more importance to the performance of speed control in inverter products.
Spindle is an important of lathes and plays a significant role in improving processing efficiency, expanding material range and enhancing processing quality. Inverters are mainly used in spindle speed control system. Inverters with low costs have gained great popularity in current economical automatic lathe industry.
I.Introduction of automatic lathe process
1.Introduction of automatic lathes
Automatic lathe is a kind of knife-type automatic lathes with high performances, high precision and low noise. It is processed automatically with process programs controlled by the cam. Besides, there are some CNC automatic lathes, pneumatic automatic lathes and heart-type automatic lathes. The basic core is that they can be set and adjusted to process the same products in a long time, especially suitable for the processing and manufacturing of precision parts such as copper, aluminum, iron and plastic, small parts in industries such as instruments, clocks and watches, motorcycles, bicycles, glasses, stationery, hardware sanitary wares, electronic components, connectors, computers, mobile phones, electro-mechanics and military.
2.Specifications of automatic lathes
Install workpieces: rotate the spindle to the fixed position. Put the workpiece into the chuck with a manipulator;
Process workpieces: accelerate the spindle to required speed and move processed products with the knife set;
Unload workpieces: after processing, push products out of the chuck and the chuck rotates fast to the fixed position, ready for next workpiece.
II.Site process requirements and debugging program
1. Site requirements:
(1) Rapid start/stop speed—requirements of acceleration and deceleration time: acceleration 0.5S, deceleration 0.3S;
(2) Quick positioning response—positioning time: within 1 second;
(3) High requirements on positioning accuracy—within 2 pulses;
2. Debugging program
Program used for modifying the system: high performance closed-loop vector inverter + inverter card. In the program, backup inverter card of the inverter lets the system work in closed-loop vector control mode for frequency control positioning. Since load inertial is large and deceleration time is short in automatic lathes, brake resistors are required. For specific selection methods, please see Specifications of V360 Series High Performance Vector Inverters.

3. Spindle conversion positioning system principle

The overall control block diagram of the program is shown in Figure 2. The inverter gains power from three-phase grid by means of the uncontrolled rectifier. Spindle motor is controlled in real-time by the three-phase inverter circuit through SVPWM and Field orientation control. Speed signal is fed back to inverter control unit through transmission mechanism. The speed signal is converted to angle signal by the inverter card. Its difference with the set angle is calculated by the PID unit (acceleration PID and stable PID). The next speed instruction on spindle motor is worked out (with torque limiting function). Finally, the system realizes the accurate control of spindle speed and positioning angles.