Machine tools

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 SUNFAR Electric V260 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.
Introduction of automatic lathe process
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.
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.
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 V260 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.

                          Figure 2 Spindle conversion positioning system principle
 
4. Debugging steps and parameter settings
    The system is involved in PID calculation. Values of proportional gain P, integral time I and differential time D are set on the following rules: the higher P is, the faster the respond is, but the poorer the system stability becomes, where too large gain may cause speed shock; the smaller I is, the faster the response is and the over large the gain is, the poorer the system stability becomes. These parameters are proportional to the inertia. If the inertia is large, parameters should be set to large values.
    In the parameter group, speed proportional gain 1 for improving response speed, is effective in spindle acceleration and larger than speed proportional gain 2; speed proportional gain 2 is used for PI adjustment in steady-state process to improve system stability and is relatively small. In the acceleration, two groups of PI parameters can be switched in hysteresis, continuously or automatically.

Function Code

Function Name

Settings

F0.0.09

Motor type and control mode

0010Closed-loop vector

F1.0.03

Acceleration time 1

0.50S

F1.0.04

Deceleration time 1

0.30S

F8.0.17

Feedback speed ratio

0.75

F8.1.21

Speed proportional gain 1

0.5

F8.1.25

Speed proportional gain 2

0.4

Fb.2.20

Zero frequency torque holding

3locked to the specific stop angle

Fb.2.21

Position locking gain 1

1

Fb.2.45

Spindle stop angle

0.0

Fb.2.47

Position error

5

3. Possible problems and solutions in debugging:
(1) Shaking and noise may occur in motor operation
    Reason to the phenomenon: too large proportional gain P may cause speed shaking, which reduces stability accuracy or even make the system unstable; otherwise, acceleration is slow and system response speed declines. The effects of carrier frequency are mainly on audio noise and thermal effects: although carrier frequency is small and increases the maximum load of inverters, noises will also increases.
Solution: adjust IPD gain (F8.1.21and F8.1.25) properly; increase carrier frequency.
(2) Low positioning accuracy and long positioning time.
    Reason to the phenomenon: too low position-loop gain decreases system response and positioning accuracy therefore declines when conditions change frequently.
Solution: increase position loop PID gain (Fb.2.21).

III. Analysis of experimental results
    A southern CNC lathe factory adopted the inverter positioning program to achieve the spindle servo drive performance and conducted tests on a 2.2KW automatic lathe. The results are as follows:

Conclusion:
    In practical production, the program is applied in automatic lathes to realize a single clamping, save external buttons and intermediate relays, improve processing efficiency by nearly 20%, improve production efficiency, reduce production costs and take fully advantages of economical CNC automatic lathes; after long-term application, the program enjoys advantages of stable operation, fast responses and high positioning accuracy and is more suitable for economical automatic lathe field than the AC spindle drive system.
  
References:
1. Specifications of V260 Series High Performance Vector Inverters, Shenzhen Sunfar Electric Technologies Co., Ltd.