Servo control apparatus having function of obtaining frequency characteristics of machine on line

Abstract

A servo control apparatus according to the present invention includes a speed command generator; a torque command generator; a speed detector for detecting the speed of a servomotor; a speed control loop including the speed command generator, the torque command generator, and the speed detector; a sinusoidal sweep input unit for performing a sinusoidal sweep on the speed control loop; and a frequency characteristics calculator for estimating the gain and phase of speed control loop input and output signals from the output of the speed control loop when a sinusoidal disturbance is inputted thereto. The frequency characteristics calculator expresses the output of the speed control loop as the Fourier series having an arbitrary number of terms using a disturbance input frequency as a fundamental frequency, and calculates the amplitude and phase of a fundamental component of the Fourier series in order to calculate frequency characteristics on line.

Claims

1. A servo control apparatus that is a control apparatus for a machine tool having a feed axis driven by a servomotor comprising: a speed command generator for generating a speed command value for the servomotor; a torque command generator for generating a torque command value for the servomotor; a speed detector for detecting the speed of the servomotor; a speed control loop including the speed command generator, the torque command generator, and the speed detector; a sinusoidal sweep input unit for performing a sinusoidal sweep on the speed control loop; and a frequency characteristics calculator for estimating the gain and phase of speed control loop input and output signals from the output of the speed control loop when a sinusoidal disturbance is inputted to the speed control loop of the control device, wherein the frequency characteristics calculator expresses the output of the speed control loop as the Fourier series having an arbitrary number of terms using a disturbance input frequency from the sinusoidal sweep input unit as a fundamental frequency, and calculates the amplitude and phase of a fundamental component of the Fourier series in order to calculate frequency characteristics on line, wherein the frequency characteristics calculator verifies a convergence of the output of the speed control loop to a steady state by monitoring energy of one period of a sinusoidal wave on a periodic basis, and the sinusoidal sweep input unit continues inputting the sinusoidal wave at a constant frequency, until the frequency characteristics calculator determines that the output of the speed control loop has reached the steady state.

2. A servo control apparatus that is a control apparatus for a machine tool having a feed axis driven by a servomotor comprising: a speed command generator for generating a speed command value for the servomotor; a torque command generator for generating a torque command value for the servomotor; a speed detector for detecting the speed of the servomotor; a speed control loop including the speed command generator, the torque command generator, and the speed detector; a sinusoidal sweep input unit for performing a sinusoidal sweep on the speed control loop; and a frequency characteristics calculator for estimating the gain and phase of speed control loop input and output signals from the output of the speed control loop when a sinusoidal disturbance is inputted to the speed control loop of the control device, wherein the frequency characteristics calculator expresses the output of the speed control loop as the Fourier series having an arbitrary number of terms using a disturbance input frequency from the sinusoidal sweep input unit as a fundamental frequency, and calculates the amplitude and phase of a fundamental component of the Fourier series in order to calculate frequency characteristics on line, wherein the frequency characteristics calculator expresses the output of the speed control loop as the Fourier series having a harmonic component corresponding to the disturbance input frequency, and evaluates the nonlinearity of a control system by a characteristic that is the ratio of the contained harmonic component to the fundamental component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The objects, features, and advantages of the present invention will be more apparent from the following description of an embodiment in conjunction with the attached drawings, wherein:

(2) FIG. 1 is a block diagram showing the configuration of a servo control apparatus according to the embodiment of the present invention;

(3) FIG. 2 is a drawing showing the waveforms of an input signal and an output signal of a speed control loop in the servo control apparatus according to the embodiment of the present invention;

(4) FIG. 3 is a drawing showing the waveforms of the input signal and the output signal of the speed control loop before and after updating a frequency, and an energy variation on a periodic basis in the servo control apparatus according to the embodiment of the present invention; and

(5) FIG. 4 is a flowchart that explains the operation process of the servo control apparatus according to the embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) A servo control apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a servo control apparatus according to an embodiment of the present invention. The servo control apparatus 100 according to the embodiment of the present invention is a control apparatus for a machine tool having a feed axis driven by a servomotor, and includes a speed command generator 1 for generating a speed command value for a servomotor 20; a torque command generator 2 for generating a torque command value for the servomotor 20; a speed detector 3 for detecting the speed of the servomotor 20; a speed control loop 4 constituted of the speed command generator 1, the torque command generator 2, and the speed detector 3; a sinusoidal sweep input unit 5 for performing a sinusoidal sweep on the speed control loop 4; and a frequency characteristics calculator 6 for estimating the gain and phase of speed control loop input and output signals from the output of the speed control loop 4 when a sinusoidal disturbance is inputted to the speed control loop 4 of the servo control apparatus 100. The frequency characteristics calculator 6 expresses the output of the speed control loop 4 as the Fourier series having an arbitrary number of terms using a disturbance input frequency from the sinusoidal sweep input unit 5 as a fundamental frequency, and calculates the amplitude and phase of a fundamental component of the Fourier series in order to calculate frequency characteristics online.

(7) Next, the operation of the servo control apparatus according to the embodiment of the present invention will be described. First the speed command generator 1 generates the speed command value to drive the servomotor 20, and outputs the speed command value to an adder 10. The adder 10 adds the sinusoidal disturbance inputted from the sinusoidal sweep input unit 5 to the speed command value, subtracts a speed detection value of the servomotor 20 detected by the speed detector 3, and outputs a calculation result to the torque command generator 2.

(8) The torque command generator 2 obtains the calculation result from the adder 10, and outputs a torque command to drive the servomotor 20. The servomotor 20 operates a driver (not shown) through a transmission mechanism 30.

(9) The speed control loop 4 is constituted of the speed command generator 1, the torque command generator 2, and the speed detector 3.

(10) The sinusoidal sweep input unit 5 performs the sinusoidal sweep on the speed control loop 4.

(11) The frequency characteristics calculator 6 estimates the gain and phase of the speed control loop input and output signals from the output of the speed control loop 4 when the sinusoidal disturbance is inputted to the speed control loop 4 of the servo control apparatus 100. Furthermore, the frequency characteristics calculator 6 expresses the output of the speed control loop 4 as the Fourier series having an arbitrary number of terms using a disturbance input frequency from the sinusoidal sweep input unit 5 as a fundamental frequency, and calculates the amplitude and phase of a fundamental component of the Fourier series in order to calculate frequency characteristics online.

(12) The servo control apparatus 100 according to the embodiment of the present invention has a loop configuration for speed control in its servo control system. In the speed control loop 4, the mechanical characteristics of the transmission mechanism 30 connected to the servomotor 20 are directly reflected.

(13) When considering a method for calculating the frequency characteristics, the configuration of the speed control loop itself is not important, as long as a broken line portion of FIG. 1 is regarded as a system having one input-output relation. Thus, as shown in FIG. 2, only the corresponding relationship between an input signal and an output signal of the speed control loop is to be considered. When performing a frequency sweep, determining a steady-state response of the output signal, while a stepwise increase of the sinusoidal frequency of the input signal, serves to obtain the frequency characteristics.

(14) When performing the frequency sweep, a slight transient response occurs when switching the frequency. Since the frequency characteristics are defined as the input and output correspondence of a steady-state response when infinite time has elapsed, a steady state has been desirably established to calculate the frequency characteristics with high precision. To realize this, it is necessary to continue inputting a sinusoidal wave of a constant frequency until a steady-state response is established and to verify the establishment of a steady state.

(15) To verify the establishment of a steady-state response to a sinusoidal wave of a frequency F [Hz], it is rational to be determined by a convergence of vibrational energy to a constant value at a period of T=1/F [s]. In other words, when E.sub.1, E.sub.2, . . . , E.sub.n represent the energy of an output signal v(t) at a first period, a second period, . . . , an n-th period, respectively, the following expressions must hold true in a stable control system.

(16) $\underset{n.fwdarw.}{\lim }\frac{E_n}{E_{n+1}}=1$$E_n{}_t^{t+\mathrm{nT}}{\left[v\left(t\right)\right]}^{2}dt$

(17) Wherein, T is a period.

(18) In essence, determining the convergence of a sequence {E.sub.n} allows the verification of the establishment of the steady-state response. As shown in FIG. 3, energy is calculated in each period, and it is regarded that a steady state has been established, when the ratio E.sub.n/E.sub.n+1 of the energy in the present period to that in the preceding period comes to 1. In this manner, the frequency characteristics calculator 6 preferably verifies the convergence of the output of the speed control loop 4 to the steady state by monitoring the energy of one period of the sinusoidal wave on a periodic basis. The sinusoidal sweep input unit 5 preferably continues inputting the sinusoidal wave of the constant frequency until the output of the speed control loop 4 reaches the steady state that is determined by the frequency characteristics calculator 6. In actuality, a steady-state determination threshold value may be defined within the order of E.sub.n/E.sub.n+1=10.05 in consideration of variations in measurement points.

(19) When the steady-state response has been verified to be established, only one period of v(t) is taken out and expanded into the Fourier series with the assumption that the one period repeats infinitely. The Fourier series is given as follows.

(20) $v\left(t\right)=V_0+\underset{n=1}{\overset{}{.Math.}}\left(a_n\cos nt+b_n\sin nt\right)$$a_n=\frac{2}{T}{}_0^{T}v\left(t\right)\cos ntdt$$b_n=\frac{2}{T}{}_0^{T}v\left(t\right)\sin ntdt$$V_0=\frac{1}{T}{}_0^{T}v\left(t\right)dt$

(21) Wherein, [rad/sec] represents the angular frequency of a fundamental wave of the signal, V.sub.0 represents a direct current component of the signal, T represents the period of the fundamental wave, and the others represent an n-th harmonic component. Coefficients a.sub.n and b.sub.n are obtained as the results of extraction of harmonic components that are equivalent to base signals cos(nt) and sin(nt) of the Fourier series, and represent the magnitude of a cosine component and a sine component of the corresponding harmonic component, respectively.

(22) The Fourier coefficients a.sub.n and b.sub.n may be obtained by integrating a product of the output signal and a cosine wave, and a product of the output signal and a sine wave throughout one period, respectively. The integration throughout only a fixed period requires much less calculation than the Fourier transform, and therefore can be performed much more easily by a DSP online.

(23) By calculating the Fourier series with an arbitrary number of terms N, whenever switching the frequency, the amplitude c.sub.1() and phase .sub.1() of the fundamental wave are obtained as the frequency characteristics in the following form.

(24) $v\left(;t\right)=V_0+c_1\left(\right)\sin \left(t+{}_1\left(\right)\right)+\underset{n=2}{\overset{N}{.Math.}}{c}_n\left(\right)\sin \left(nt+{}_n\left(\right)\right)$

(25) Wherein, c.sub.n represents the amplitude of an n-th harmonic, and .sub.n is the phase of the n-th harmonic.

(26) The output signal manifests itself as a strain wave. Assuming that how close the strain wave is to a sinusoidal wave represents nonlinearity, the nonlinearity can be evaluated whenever switching the frequency on a frequency-by-frequency basis. In terms of a comparison between the fundamental wave and the harmonics, it is rational to use the following distortion factor from a physical viewpoint.

(27) $\mathrm{distortion}\mathrm{factor}=\frac{\mathrm{root}\text{-}\mathrm{mean}\text{-}\mathrm{square}\mathrm{of}\mathrm{amplitude}\mathrm{of}\mathrm{harmonics}}{\mathrm{amplitude}\mathrm{of}\mathrm{fundamental}\mathrm{wave}}=\frac{\sqrt{\underset{n=2}{\overset{N}{.Math.}}{c}_n^2}}{c_1}$

(28) As a method for evaluating the strain wave, a form factor or a crest factor may be used instead of the distortion factor. The frequency characteristics calculator 6 may express the output of the speed control loop 4 as the Fourier series having a harmonic component corresponding to the disturbance input frequency, and evaluate the nonlinearity of the control system by a characteristic that is the ratio of the contained harmonic component to the fundamental component, such as the distortion factor.

(29) Next, the operation process of the servo control apparatus according to the embodiment of the present invention will be described with reference to a flowchart shown in FIG. 4. First, in step S101, the sinusoidal sweep input unit 5 (see FIG. 1) inputs a sinusoidal disturbance to the speed control loop 4. After that, in step S102, the speed detector 3 detects the speed of the servomotor 20.

(30) After that, in step S103, the torque command generator 2 generates a torque command value from a speed command value and a speed detection value. After that, in step S104, the frequency characteristics calculator 6 integrates the square of the speed detection value detected by the speed detector 3.

(31) After that, in step S105, whether or not an integration period has reached one period of the sinusoidal wave is determined. When the integration period has not reached one period of the sinusoidal wave, the operation goes back to step S104, and the frequency characteristics calculator 6 continues integrating the square of the speed detection value.

(32) On the other hand, when the integration period has reached one period of the sinusoidal wave, in step S106, the frequency characteristics calculator 6 calculates the vibration energy E.sub.n+1 of one period of the speed detection value. After that, in step S107, it is determined whether or not vibration energy E.sub.n in the preceding period corresponds with the vibration energy E.sub.n+1 in the present period within the confines of a threshold value. When the vibration energy E.sub.n in the preceding period does not correspond with the vibration energy E.sub.n+1 in the present period within the confines of the threshold value (steady-state determination threshold value), the operation goes back to step S104, and the frequency characteristics calculator 6 continues integrating the square of the speed detection value.

(33) On the other hand, when the vibration energy E.sub.n in the preceding period corresponds with the vibration energy E.sub.n+1 in the present period within the confines of the threshold value, in step S108, the frequency characteristics calculator 6 regards that the vibration of the speed detection value at a corresponding frequency has become a steady-state response. After that, in step S109, the frequency characteristics calculator 6 calculates Fourier coefficients a.sub.n and b.sub.n of one period of the vibration of the speed detection value at the corresponding frequency.

(34) After that, in step S110, the frequency characteristics calculator 6 calculates the amplitude c.sub.1 and phase .sub.1 of a fundamental wave of the Fourier series at the corresponding frequency. After that, in step S111, the frequency characteristics calculator 6 calculates frequency characteristics at the corresponding frequency.

(35) After that, in step S112, the sinusoidal sweep input unit 5 updates an input frequency. After that, in step S113, it is determined whether or not the sinusoidal frequency of an input signal has exceeded a maximum value. When the sinusoidal frequency of the input signal has exceeded the maximum value, the sequential operation is completed. On the other hand, when the sinusoidal frequency of the input signal has not exceeded the maximum value, the operation goes back to step S101, and the sequential operation is restarted.

(36) As described above, according to the servo control apparatus of the embodiment of the present invention, it is possible to provide a servo control apparatus having the function of precisely obtaining the frequency characteristics of the feed axis in real time online by a vibration test that is easily performed using a motor attached to the feed axis.

(37) The present invention has high significance in the following three terms, with respect to conventional technologies.

(38) 1. A control loop structure for a feed axis is provided, and the output of the loop is expressed as the Fourier series having an arbitrary number of terms. Although the Fourier transform is not based on the assumption of the periodicity of objects, the Fourier series is usable only for periodic signals. When applying a frequency sweep, a signal is determined to be a periodic signal, so that it is possible to measure frequency characteristics from a fundamental component of the Fourier series (the invention according to claim 1).

(39) 2. Since the original meaning of the frequency characteristics is the amplitude and phase difference of a steady-state vibration, it is preferable to make sure of the establishment of a steady state. To verify a convergence of a vibration to the steady state, the energy of the vibration is calculated on a periodic basis. The vibration continues to be applied at a constant frequency until the convergence to the steady state is established, thus improving a measurement precision (the invention according to claim 2).

(40) 3. Nonlinearity does not directly manifest itself in the frequency characteristics even with the use of the Fourier transform. However, by calculating the Fourier series at each frequency and comparing the amplitude of a fundamental component with the amplitude of a harmonic component, it is possible to quantitatively evaluate the nonlinearity at each frequency (the invention according to claim 3).

(41) According to the servo control apparatus of the embodiment of the present invention, it is possible to obtain the frequency characteristics of the feed axis with high precision in real time by the vibration test that is easily performed using the motor attached to the feed axis.