- Detail

Design of high-precision laser direct writing digital servo filter

laser direct writing technology is an ultra precision machining technology widely used in recent years. This technology is a kind of regular high-precision scanning on the surface of the substrate by using the laser beam with variable intensity. During scanning, the lithographic substrate moves with the object platform. Therefore, the quality of the lithographic element depends on the positioning accuracy of the carrier platform and the stability of motion, and the rapidity of the lithographic element depends on the responsiveness of the system

motion controller based on digital servo is the key of ultra precision positioning system. Because digital servo filter is the core of digital 3 technology to introduce you to the motion controller of machine performance servo, the design of digital servo filter will affect the positioning accuracy of the system

digital servo filter means that the closed-loop control and regulation of the system adopt digital technology, and all control and regulation realize software. The whole software of the regulator makes many control ideas and means in the control theory applied. At the same time, it is easy to complete the parameter liberalization and fault self diagnosis functions by using the software, which greatly improves the system control performance, thus overcoming the shortcomings of the analog closed-loop servo system, such as the signal-to-noise separation of weak signals is difficult, the control accuracy is difficult to improve, it is easy to be affected by mechanical friction and temperature, and the position loop control produces zero drift error

1 servo control system structure and analysis

1.1 servo control system structure

the hardware structure of the whole servo control system is shown in Figure 1. The upper device is composed of DSP processor and D/a conversion module, and the servo unit is composed of Yaskawa servo driver. The whole system is a closed-loop servo motor control system. The DSP processor generates the digital pulse signal of trapezoidal motion curve, directly drives the D/a converter to generate analog voltage through the designed digital filter, and drives the servo motor through the servo unit. The position and speed signals in the actual movement are fed back to the optical encoder by the motor, and the digital signals are generated by the optical encoder, and then transmitted to DSP for acquisition and processing

1.2 system analysis

the key of this scheme is to solve the problems of viscous friction between motor shaft and load and external interference to motor and converter. Due to the existence of friction ring and external interference, the dynamic and static performance of the system is greatly affected, mainly manifested in crawling at low speed, large static error or limit cycle oscillation at steady state. In order to meet the requirements of laser direct writing, the system must also have the characteristics of fast response, short positioning time and high steady-state accuracy. If the starting speed is too slow or overshoot, and the stopping time is too long, the system will have strong oscillation and high noise

2 design of digital filter

2.1 design of digital servo filter model

through a large number of experiments, after analyzing the system, it is found that using PID control method for position deviation control can improve accuracy and step response; The steady-state accuracy of the system can be improved by adding the feedforward compensation control method of speed and acceleration; Friction compensation can overcome the influence of friction. Therefore, this scheme is not only a closed-loop control system using position error, but also an open-loop control system using the given position signal

then the system control output U (T) =up (T) +uva (T) +uf (T), where up (T) is the PID controller, UVA (T) is the speed and acceleration compensation controller, and UF (T) is the friction compensation controller

pid servo filter control law is shown in Figure 2

in Figure 2, KP is the proportional gain, Ki is the integral gain, KD is the differential gain, kvff is the speed feedforward gain, Kaff is the acceleration feedforward gain, KF is the viscous friction coefficient, en is the position deviation, VT is the speed at time t, at is the acceleration at time T. the output B static error is mainly used to compensate the control axis for the influence of gravity. The analog quantity corresponding to the output of the filter is limited by the output saturation controller

2.2 parameter adjustment

in the position PID regulator, the proportional gain KP determines the rapidity of the system, and the integral gain ki is used to eliminate the static error of the system. The function of differential gain kd is to increase damping and reduce oscillation. The adjustment process is to adjust KP first, then Ki, and then KD. When setting ki gain for the first time, setting ki to a non-O value will cause a sudden "jump". To avoid this situation, it is necessary to set the integral limit (saturation controller of the integral part) to 0, Ki to the expected value, and then set the integral limit to the expected integral limit. In this way, all previous integral values are cleared, so that the integral can be smoothly calculated from the previous point. Then adjust kvff and Kaff to improve the steady-state accuracy of the system. Finally, adjust KF to overcome the influence of friction

adjust kvff in the start-up phase. Too much Kaff will make the speed too fast and lead to position overshoot. Adjust kvff in the deceleration phase. If Kaff is too small, the positioning time will be too long. According to the optimal control idea, if the system starts according to the maximum acceleration, moves at the maximum speed, and brakes at the maximum deceleration, it can reach the coordination point without overshoot in the shortest time. Therefore, the parameter adjustment should be set according to the three stages of startup, constant speed and deceleration

3 matlab design and simulation

3.1 simulation module design

according to the design principle, the deviation counting module in Figure 1 is equivalent to a PID controller with feedforward compensation, and is designed into the corresponding part in Figure 2, and the D/a converter is equivalent to discrete data through the zero order holder; The Yaskawa servo driver is equivalent to the speed loop and current loop; The output signal is simulated to show that the anchor fatigue testing machine is in operation. 1. Pay attention to whether the parts are loose, and observe the wave. Therefore, the whole servo three loop PID simulation principle is shown in Figure 3

among them, Rin (k) is the position input signal at sampling time K. in order to simulate the actual effect, set the input Rin (k) as an irregular position signal, and the input command is a sinusoidal superposition signal at this time; Drin (k) is the speed input signal at sampling time k; Ddrin (k) is the acceleration input signal at sampling time k, and drin (k+1) = (RIN (k+1) -rin (k))/TS, ddrin (k+1) = (drin (k+1) -drin (k))/ts

3.2 simulation waveform

for high-precision laser direct writing, measuring its performance mainly depends on the stability of speed, responsiveness and accuracy of position. Therefore, in the simulation in Figure 3, the parameters of the control system should be adjusted many times according to the actual situation, and after analysis and comparison, a tracking diagram with stable speed and accurate position can be obtained. The simulation waveform is shown in Figure 4

Figure 4 (a) shows the speed tracking results. The set speed and the actual automatic record coincide with the curve of the moving distance of the collet of the experimental machine (the curve of the force and the stripping length). The speed is stable and controlled within 0.1%. A sudden "jump" in speed occurred near time 0 because there was no adjustment of the integral limit. Therefore, in practice, the integral limit (saturation controller) should be set to o, Ki to the expected value, and then set the integral limit to the expected integral limit

Figure 4 (b) shows the position tracking results, and the output actual position coincides with the set target position. The positioning accuracy is high, and the accuracy is controlled within 0.1%

the simulation results show that there is no "flat top" phenomenon in position tracking and no "dead zone" phenomenon in speed tracking under the condition of friction. The positioning accuracy of position tracking is high, and the steady-state accuracy of speed tracking is high

4 software implementation

the servo unit module is designed by the servo driver, and its parameter adjustment can be set in the servo driver. Please refer to the driver user manual for details. PID digital filter + feedforward compound control system is realized by DSP2812. The operation process is as follows: first, generate the C language in CCS from MATLAB language of PID composite simulation module, then transplant it to CCS software, and modify it according to PID control formula principle and software design process

5 Conclusion

this paper introduces the structure of the servo control system, analyzes the friction links in the system and the actual requirements, and then designs the servo PID digital filter according to the control principle. Matlab simulation verifies that the speed of the digital filter is stable; The position tracking error converges to zero. The experience of adjusting control parameters is obtained. The final actual results show that through the tension deviation experiment of spring compensator and 100000 fatigue experiments (equivalent to 20 years of operation) during the input and output of the whole system, the tension deviation of spring compensator in the life cycle is 5%, that is, the 95% difference of compensation effect is less than 100 ls; No noise, no oscillation; Positioning accuracy error control ± 1 μ M. (end)

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