【正文】 在執(zhí)行 G指令行期間，用戶界面程序開始計(jì)算、平面運(yùn)動(dòng)、驅(qū)動(dòng)軸、 允許的最大速度和加速度 ,起始位置的減速、方向余弦定理。 圖 5為數(shù)控系統(tǒng)的圖形用戶界面及其簡(jiǎn)要解釋。測(cè)得的固有頻率并不與計(jì)算所得完全一致，但是指定的頻率范圍從沖擊錘測(cè)試的有限元模態(tài)分析來看是類似的。垂直方向的 XY軸由電機(jī)驅(qū)動(dòng)， Z軸由 磁力預(yù)緊空氣軸承和 直流電機(jī)控制，空氣主軸轉(zhuǎn)速可達(dá) 160,000rpm，足以用 于高精度加工。這部分給出了通過沖擊錘測(cè)試得到的有限元分析和固有頻率的結(jié)果。從有限元分析和沖擊錘測(cè)試中，我們已經(jīng)證實(shí)了它有很好的結(jié)構(gòu)剛度和高的固有頻率。 Reset). When a user click the Open Gcode button, a whole Gcode file is read in and saved in a memory area, and then the Gcodes appear at the bottom left list box. When the Start Gcode button is clicked, the user interface program takes out a line from the memory and checks its syntax and identifies all the meaningful tokens. During preprocessing a Gcode line, the user interface program is supposed to pute, a motion plane, a driving axis, maximum allowable velocity and acceleration, the starting position of the deceleration, directional cosines. If the Gcode line is about circular motion, the center point of the arc, the normal direction of the arc, and start and end angles are also puted by the user interface program. All the preprocessed information is entered in the DPRAM and handed to the DSP program. A circular buffer in the DPRAM has rooms for only 4 lines of Gcodes, so the user interface program needs to keep monitoring the circular buffer usage. When the user interface program finds that the DSP program finishes carrying out a Gcode line and empties its space, it fills in the empty space in the circular buffer with a new preprocessed Gcode line in the order in which they occur. DSP Program The DSP program interpolates the preprocessed Gcodes in realtime and generates position mands for multiple axes to follow. It also closes servo control loops. Generally a sampling rate is set to be ten times larger than the bandwidth of a plant to be controlled. The developed CNC system adopted a sampling rate of 2,000 Hz for the servo loops. The DSP program takes out a Gcode line from the circularbuffer and putes the angle between two successive Gcode lines. If the angle is less than a certain (predefined) degree and the contouring is on, it sets a flag so that the tool path does not reduce its velocity when it enters into the next segment. When a ti mer interrupt occurs, the DSP program putes the desired velocity and position of each axis and generates mands for the servo control loop. The puted velocity should be less than the maximum allowable velocity puted by the user interface program and start decreasing when the position reaches the position of deceleration to make a plete stop at the end point if contouring function is not used. If the current motion is linear, all the putations are for the driving axis and mands for the other axes are calculated from straightline equations related to the driving axis. If the current motion is circular, angular velocity and angular acceleration are similarly used as in the linear motion and the final mands are made from the polar coordinate to the Cartesian coordinate transformation. After generating realtime mands for each axis the DSP program drives the servo control loops of the 3axis. The errors which are differences between the mands and the actual feedback positions are fed into a control algorithm such as PID and the control signals for the motor drives are puted. 4. Control System Design To improve the performance of servo control for the 3axis milling machine, several control algorithms have been tested on the 3axis milling machine. They include PID, H∞ control, input shaping control, disturbance observer, and crosscoupled control. These control schemes were digitally implemented on a Daytona DSP board from Spectrumsignal Co. The DSP board has two TI320C6701 chips on it and a sampling rate of 2,000 Hz has been used. The design procedure and experimental results from each control are described as follows. H∞ Optimal Control Design Using a conventional PID controller for the zaxis which has a linear motor and airbearing, it seemed that high gain PID easily started oscillations. As an alternative, an H∞ controller was designed and applied to the zaxis and performance of hand tuned PID and H∞ control is pared. An openloop plant model for control design was obtained from experimental frequency response data. The frequency responses were measured with a dynamic signal analyzer using a swept sine method that generates fixedamplitude sine waves of varying frequencies. From the frequency responses for different input amplitudes, an averaged frequency response wasputed and a nominal continuoustime plant model was fitted. Fig. 6 shows the averaged frequency response and a nominal openloop plant model. A second order plant model was obtained from the curve fit. The identified openloop plant model G(s) forthe zaxis was We can see that the zaxis has a plex pole pair at around . When a PIDtype controller in a typical digital form of where u(k) is controller output, e(k) is error signal, T is samplingperiod, and z is a delay, is applied to the plant, it turns out that a high gain PID can easily excite the oscillatory mode of the plant. To avoid socalled derivative kick, the derivative gain Kd was forced to act on the derivative of the actual position, not on the derivative of the position error, . Kd (1z1)/T is multiplied by the negative position feedback, –y(k) instead of e(k) at Eq. (2). For the zaxis, using the derivative of position instead of that of position error allowed more aggressive PID gains. Based on the plant model at Eq. (1), the control loop employed an H∞ robust controller at 2 kHz sampling frequency. A mixed sensitivity problem was solved to design an H∞ controller in continuoustime and the resulting continuous time controller was converted to a discretetime model. The mixed sensitivity specification for H∞ control design in continuous