Servo control device with function for online optimization of the control gain using an evaluation function

Abstract

Servo control device for a machine tool with a feed axis driven by a servo motor, comprising: - a speed command generation unit (1) generates a speed command value for the servo motor, - a torque command generation unit (2) that generates a torque command value for the servo motor, - a speed sensing unit (3) that detects the speed of the servo motor, - a speed control loop (4) comprising the speed command generation unit, the torque command generation unit and the speed sensing unit, - a speed control loop gain adjustment unit (5) that adjusts a control gain for the speed control loop, - a sine wave disturbance input unit (6) that applies a sine wave sweep to the speed control loop, - a frequency characteristic calculation unit (7) that estimates a gain and a phase of a speed control loop based on an input / output signal ratio of the speed control loop when a sine wave is input into the speed control loop, - a reference characteristic generating unit (8) that generates a frequency characteristic for the feed axis, and - a gain adjustment unit (9) that adjusts the speed control loop gain set by the speed control loop gain adjustment unit (5), - wherein the gain control unit (9) uses the reference frequency characteristic generated by the reference characteristic generation unit (8) and an actual frequency characteristic calculated by the frequency characteristic calculation unit to calculate a weighting function value for each sinusoidal pass with respect to the actual frequency characteristic, and adjusts the speed control loop gain to achieve a desired weighting function value, characterized in that the gain setting unit (9) determines the weighting function value by using the weighting function: S j = ∫ ω min ω max W ( ω ) [ | G model ( ω ) | − | G experimental ( ω ; kv ) | ] 2 d ω (1) calculated, wherein ω is the frequency; kv is the rotary control loop gain; W(ω) is the weight applied to a frequency characteristic. G model(ω) is the characteristic; and G experimental (ω) is the actually measured characteristic.

Description

Background of the invention 1. Field of the invention

[0001] The present invention relates to a servo control device, and more particularly to a servo control device having a function of online optimization of the control gain using an evaluation function. 2. Description of the related prior art

[0002] A variety of devices and methods for measuring the frequency characteristics of a servo control system for a servo control device have been proposed and widely used to measure transfer characteristics in feed axes of machine tools. Generally, a response vibration is measured when an object to be analyzed is vigorously vibrated (frequency sweep) by a vibration generator, and time series data representing the measured vibration is recorded in a mass storage device. Various signal processing techniques are then used to obtain a frequency transfer function. In a simple measurement, a response vibration generated using an actuator as a vibration source is measured and recorded to process signals. An oscillation circuit is used for an electrical circuit or an optical system.

[0003] As a control gain optimization system, a method of fitting a time-series signal to define a base mode function for a reference model and selecting the result of introducing control parameters into the function as a reference model has been proposed (see, for example, document JP 2015-76024 A). In this conventional technique, the weighted sum of a plurality of parameters (overshoot and settling time), which can be obtained and quantified as time-series data, is used as an evaluation function for adjusting the PID parameters. Although the meaning of the evaluation function is not affected, this function typically enables optimization of a compensator for plant control.

[0004] Furthermore, a method for calculating an evaluation value for control characteristics using the error between actually measured time series data and time series data of reference responses has been proposed (see, for example, document JP H09-131087 A). In this conventional technique, an approximate model is stored and adjusted in a setting device, and the adjustment result is applied to an actual controlled object.

[0005] The document US 2011 / 0 074 330 A1 discloses a method and system for determining tuning parameters for use in tuning a controller used in a process control loop for a motor and an associated load.

[0006] Document US 4 675 805 A discloses a closed-loop PID controller for a hydraulic pulse machine.

[0007] Another example of a PID controller is disclosed by LIU, GP and DALEY, S. “Optimal-tuning PID control for industrial systems”, Control Engineering Practice, Vol. 9, 2001, No. 11, pp. 1185-1194.

[0008] However, these conventional techniques rely on a reference model fit for the purpose of curve fitting responses in the time domain. No method has yet been proposed to quantitatively evaluate the difference from a reference transfer function directly in the frequency domain using online frequency characteristics. Summary of the invention

[0009] An object of the present invention is to provide a servo control apparatus that can fundamentally achieve both stability and high-speed response by treating the control characteristics as a mathematically defined optimization problem.

[0010] A servo control device according to an embodiment of the present invention is a control device for a machine tool having a feed axis driven by a servo motor, and includes a speed command generation unit, a torque command generation unit, a speed detection unit, a speed control loop, a speed control loop gain adjustment unit, a sine disturbance input unit, a frequency characteristic calculation unit, a reference characteristic generation unit, and a gain adjustment unit. The speed command generation unit generates a speed command value for the servo motor. The torque command generation unit generates a torque command value for the servo motor. The speed detection unit detects the speed of the servo motor. The speed control loop includes the speed command generation unit, the torque command generation unit, and the speed detection unit.The speed control loop gain adjustment unit sets a control gain for the speed control loop. The sine disturbance input unit applies a sine sweep to the speed control loop. The frequency characteristic calculation unit estimates the gain and phase of a speed control loop based on an input / output signal ratio of the speed control loop when a sine wave is input to the speed control loop. The reference characteristic generation unit generates reference frequency characteristics for the feed axis. The gain adjustment unit set by the speed control loop gain adjustment unit adjusts the speed control loop gain.The gain adjustment unit uses the reference frequency characteristics generated by the reference characteristic generation unit and actual frequency characteristics calculated by the frequency characteristic calculation unit to calculate an evaluation function value for each sinusoidal sweep with respect to the actual frequency characteristics, and adjusts the speed control loop gain to achieve a desired evaluation function value.

[0011] The gain adjustment unit calculates the evaluation function value by using the following evaluation function: Sj=∫ωminωmaxW(ω)[|Gmodel(ω)|−|Gexperimental(ω;kv)|]2dω

[0012] Here ω is the frequency, kv is the rotary loop gain, W(ω) is the weight applied to a frequency characteristic, G model (ω) is the characteristic and G experimental (ω) is the actually measured characteristic. Brief description of the drawings

[0013] Objects, features, and advantages of the present invention will become more apparent from the following description of an embodiment taken in conjunction with the accompanying drawings. It shows: Fig. 1 is a block diagram illustrating the configuration of a servo control apparatus according to an embodiment of the present invention; Fig. 2A is a diagram illustrating a temporal change of a sine frequency in the servo control device according to the embodiment of the present invention; Fig. 2B is a diagram illustrating a temporal change of an input signal in the servo control apparatus according to the embodiment of the present invention; Fig. 2C is a diagram illustrating a temporal change of an output signal in the servo control device according to the embodiment of the present invention; Fig. 2D is a diagram illustrating temporal changes in the frequency characteristics of the gain and phase in the servo control apparatus according to the embodiment of the present invention; Fig. 3A is a diagram illustrating another temporal change of a sine frequency in the servo control device according to the embodiment of the present invention; Fig. 3B is a diagram illustrating temporal changes of an actual measurement value of the frequency characteristics and a reference frequency characteristic in the servo control device according to the embodiment of the present invention; Fig. 3C is a diagram illustrating a temporal change of an evaluation function value in the servo control apparatus according to the embodiment of the present invention; Fig. 4A is a diagram illustrating the frequency characteristics of the gain when reference characteristics for a large machine and reference characteristics for a small machine are used in the servo control device according to the embodiment of the present invention; Fig. 4B is a diagram illustrating the frequency characteristics of the phase when reference characteristics for a large machine and reference characteristics for a small machine are used in the servo control device according to the embodiment of the present invention; and Fig. 5 is a flowchart for explaining the operation method of the servo control apparatus according to the embodiment of the present invention. Detailed description

[0014] A servo control apparatus according to an embodiment of the present invention will be described below with reference to the accompanying drawings.

[0015] Fig. 1 is a block diagram illustrating the configuration of the servo control device according to an embodiment of the present invention. The servo control device 100 according to the embodiment of the present invention is a control device for a machine tool having a feed axis driven by a servo motor 20, and includes a speed command generation unit 1, a torque command generation unit 2, a speed detection unit 3, a speed control loop 4, a speed control loop gain adjustment unit 5, a sine disturbance input unit 6, a frequency characteristic calculation unit 7, a reference characteristic generation unit 8, and a gain adjustment unit 9. The speed command generation unit 1 generates a speed command value for the servo motor 20. The torque command generation unit 2 generates a torque command value for the servo motor 20. The speed detection unit 3 detects the speed of the servo motor 20.The speed control loop 4 includes the speed command generation unit 1, the torque command generation unit 2, and the speed detection unit 3. The speed control loop gain adjustment unit 5 sets a control gain for the speed control loop 4. The sine disturbance input unit 6 applies a sine sweep to the speed control loop 4. The frequency characteristic calculation unit 7 estimates the gain and phase of a speed control loop input / output signal from the output of the speed control loop 4 when a sine wave is input to the speed control loop 4. The reference characteristic generation unit 8 generates reference frequency characteristics for the feed axis. The gain adjustment unit 9 adjusts the speed control loop gain.The gain adjustment unit 9 uses the reference frequency characteristics generated by the reference characteristic generation unit 8 and the actually measured frequency characteristics calculated by the frequency characteristic calculation unit 7 to calculate an evaluation function value for each frequency sweep with respect to the actually measured frequency characteristics, and adjusts the speed control loop gain online to achieve a desired evaluation function value.

[0016] The operation of the servo control device according to the embodiment of the present invention is described below. The speed command generation unit 1 generates a speed command value for driving the servo motor 20 and outputs it to an adder 10. The adder 10 adds a sine disturbance input from the sine disturbance input unit 6 to the speed command value, subtracts the detected speed value of the servo motor 20 detected by the speed detection unit 3 from the speed command value, and outputs the calculated result to the speed loop gain adjustment unit 5.

[0017] The speed control loop gain setting unit 5 sets a control gain for the speed control loop 4 set by the gain setting unit 9.

[0018] The torque command generation unit 2 receives the product of the calculated result obtained by the adder 10 multiplied by the speed control loop gain and outputs a torque command to drive the servo motor 20. The servo motor 20 activates a driven body (not shown) via a transmission mechanism 30.

[0019] The speed control circuit 4 is configured to include the speed command generation unit 1, the torque command generation unit 3 and the speed detection unit 3.

[0020] The sine disturbance input unit 6 applies a sine sweep to the speed control loop 4. In other words, the sine disturbance input unit 6 inputs a sine disturbance to the speed control loop 4.

[0021] The frequency characteristic calculation unit 7 estimates the gain and phase of a speed control loop input / output signal from the output of the speed control loop 4 when a sinusoidal disturbance is input to the speed control loop 4 of the servo control device 100. The frequency characteristic calculation unit 7, while defining the disturbance input frequency from the sinusoidal disturbance input unit 6 as the fundamental frequency, represents the output of the speed control loop 4 using a Fourier series comprising any number of terms. The frequency characteristic calculation unit 7 calculates the amplitudes and phases of the fundamental wave components of the Fourier series to calculate the frequency characteristics online.

[0022] The reference characteristic generation unit 8 generates reference frequency characteristics for the feed axis.

[0023] The gain adjustment unit 9 adjusts the speed control loop gain. The gain adjustment unit 9 preferably determines the value of the speed control loop gain adjustment based on the difference between the desired evaluation function value and an evaluation function value for the actual sweep.

[0024] The servo control device 100 according to the embodiment of the present invention has a speed control loop structure incorporated into a servo control system. The speed control loop 4 directly reflects the mechanical characteristics of the transmission mechanism 30 connected to the servo motor 20.

[0025] As far as a method for calculating frequency characteristics is concerned, the structure of the speed control loop is not important. It is sufficient to define a Fig. 1 as a system with an input / output relationship. Therefore, only the correspondence between the input and output signals of the speed control loop 4 can be considered. Frequency sweeping obtains frequency characteristics by verifying a steady-state response of the output signal as the sine frequency of the input signal is gradually increased.

[0026] The operation of the servo control apparatus according to the embodiment of the present invention will be described below. Fig. 2A shows a temporal change of a frequency of a sine wave, which represents a disturbance input applied by the sine disturbance input unit 6 to the speed control loop 4. Referring to Fig. 2A, the sine frequency is increased from time t0 to time t1 and reset to the initial value at time t1. Similarly, the sine frequency is increased from time t1 to time t2 and reset to the initial value at time t2. The sine frequency is then varied in the same way. The time period from time t0 to time t1 is defined as the first time period, the time period from time t1 to time t2 as the second time period, and the time period from time t2 to time t3 as the third time period.

[0027] Fig. Figure 2B shows a temporal change of an input signal of the speed control loop 4. Fig. 2C shows a temporal change of an output signal of the speed control loop 4. The frequency characteristic calculation unit 7 estimates the gain and phase of a speed control loop input / output signal from the output of the speed control loop 4 when a sine wave is input to the speed control loop 4. Fig. 2D shows temporal changes in gain (solid line) and phase (dotted line) estimated by the frequency characteristic calculation unit 7. In this embodiment, since the gain setting unit 9 sets the speed control loop gain to a higher value, the gain increases during the first, second, and third periods, respectively.

[0028] A method for sequentially measuring the frequency characteristics of the servo control system and optimizing the control parameters online using an evaluation function including reference characteristics and actual measured characteristics according to the present invention is described below. Fig. 3A shows a temporal change of a frequency of a sine wave representing a disturbance applied from the sine disturbance input unit 6 to the speed control loop 4.

[0029] The frequency characteristic calculation unit 7 estimates frequency characteristics for the frequency of interest, that is, the gain and phase of a speed control loop, based on an input / output signal ratio of the speed control loop when a sine wave is input to the speed control loop 4. The reference characteristic generation unit 8 calculates the frequency characteristics of the feed axis. Fig. Figure 3B shows temporal changes of an actual measured value of the frequency characteristics (solid line) and a reference frequency characteristic (dashed line). When the sine frequency of the input signal reaches a maximum value, the gain adjustment unit 9 calculates an evaluation value function value for sweeping.

[0030] The gain adjustment unit 9 uses the reference frequency characteristics generated by the reference characteristic generation unit 8 and the actual frequency characteristics calculated by the frequency characteristic calculation unit 7 to calculate an evaluation function value for each sinusoidal sweep with respect to the actual frequency characteristics. The evaluation function S j is indicated by: Sj=∫ωminωmaxW(ω)[|Gmodel(ω)|−|Gexperimental(ω;kv)|]2dω where ω is the frequency, kv is the speed control loop gain, W(ω) is the weight applied to the frequency characteristics, G model (ω) are the characteristics and G experimental (ω) are the actually measured characteristics.

[0031] Equation (1) is based on the weighted least squares method. A speed control loop gain kv, which S j minimized, is calculated by sequential sweeping. This allows for the optimal speed control loop gain kv to be obtained.

[0032] Fig. 3C shows a temporal change of an evaluation function value. An evaluation function value for the first time period (time: t0 to t1) is calculated at time t1. At time t1, the evaluation function value in question is compared with the evaluation function value obtained one time period before the previous one, and their difference is reflected in the gain setting value. Evaluation function values ​​for the second and third time periods are similarly calculated, and such calculation is repeated until the evaluation function value reaches a target value. In this way, the gain setting unit 9 adjusts the speed control loop gain online to achieve a desired evaluation function value. The gain setting unit 9 preferably determines the speed control loop gain setting value based on the difference between the desired evaluation function value and an evaluation function value for sweeping. Fig. Figure 3C shows an example where the evaluation function value reaches a target value at time t3.

[0033] The reference characteristic generation unit 8 preferably generates characteristics that are variable according to the configuration of the feed axis. For example, the reference characteristics are variable depending on whether the machine to be controlled is a large or small machine. Fig. 4A shows the frequency characteristics of the gain based on reference characteristics for a large machine and the frequency characteristics of the gain based on reference characteristics for a small machine. Fig. Figure 4B shows the phase frequency characteristics based on reference characteristics for a large machine and the phase frequency characteristics based on reference characteristics for a small machine. To accurately estimate the evaluation function value, reference characteristics (ideal characteristics) may be preferentially adopted considering the magnitude of the resonance frequency. The ideal characteristics are defined herein as characteristics exhibiting a narrow control band for a large machine and characteristics exhibiting a wide control band for a small machine. For a small machine, since the resonance frequency, as nominal characteristics, is comparatively high, the band can be set relatively easily to be wide (the gain can be set high).For a large machine, since the resonance frequency is comparatively low, the band width can hardly be set wide (the gain cannot be set satisfactorily high). Thus, changing the reference characteristics based on whether the machine to be controlled is large or small allows an evaluation function value, which includes a criterion for higher gain, to be more accurately calculated given the difference in resonance characteristics, as mentioned above.

[0034] As described above, the servo control device according to the embodiment of the present invention functions as a sequencer capable of vibration input to measure frequency characteristics online. At each measurement repetition, the evaluation function calculation is performed to sequentially adjust the parameters of the control device to bring the calculation result close to the characteristics. Therefore, the control device can obtain parameters that achieve optimal control characteristics online.

[0035] The operation method of the servo control device according to the embodiment of the present invention will be described below with reference to a Fig. 5 is described. In step S101, the sinusoidal disturbance input unit 6 (see Fig.1) The input frequency. In step S102, the sine disturbance input unit 6 inputs a sine disturbance to the speed control loop 4. In step S103, the speed detection unit 3 detects the speed of the servo motor 20.

[0036] In step S104, the torque command generation unit 2 generates a torque command value based on a speed command value, a speed detection value, and a speed loop gain. In step S105, the frequency characteristic calculation unit 7 calculates frequency characteristics for the frequency of interest. In step S106, the reference characteristic generation unit 8 calculates frequency characteristics for each frequency. In step S107, the sinusoidal disturbance input unit 6 updates the input frequency.

[0037] In step S108, it is determined whether the sine frequency of the input signal has reached a maximum value. If the sine frequency of the input signal has not reached a maximum value, steps S101 to S108 are repeated. If the sine frequency of the input signal has reached a maximum value, the gain adjustment unit 9 calculates an evaluation function value for sweeping in step S109. An evaluation function value is calculated using the method described above.

[0038] In step S110, it is determined whether the evaluation function value has reached a target value. If the evaluation function value has not reached a target value, the gain adjustment unit 9 varies the speed control loop gain in step S111 to bring the evaluation function value close to the target value, and steps S101 to S110 are repeated.

[0039] When the evaluation function value reaches a target value, the gain adjustment unit 9 determines a final speed control loop gain in step S112. In the above manner, the difference from a reference transfer function can be quantitatively evaluated directly in the frequency domain using frequency characteristics available online.

[0040] As described above, the servo control apparatus according to the embodiment of the present invention can sequentially measure the frequency characteristics of a servo control system and optimize the control parameters online using an evaluation function including actually measured characteristics.

[0041] The servo control device according to the embodiment of the present invention can basically achieve both stability and high-speed response.

Claims

[1] Servo control device for a machine tool with a feed axis driven by a servo motor, comprising: - a speed command generation unit (1) generates a speed command value for the servo motor, - a torque command generating unit (2) which generates a torque command value for the servo motor, - a speed detection unit (3) which detects the speed of the servo motor, - a speed control circuit (4) comprising the speed command generation unit, the torque command generation unit and the speed detection unit, - a speed control loop gain setting unit (5) which sets a control gain for the speed control loop, - a sine disturbance input unit (6) which applies a sine sweep to the speed control loop, - a frequency characteristic calculation unit (7) which estimates a gain and a phase of a speed control loop from an input / output signal ratio of the speed control loop when a sine wave is input to the speed control loop, - a reference characteristic generating unit (8) which generates a frequency characteristic for the feed axis, and - a gain setting unit (9) which sets the speed control loop gain set by the speed control loop gain setting unit (5), - wherein the gain adjustment unit (9) uses the reference frequency characteristic generated by the reference characteristic generation unit (8) and an actual frequency characteristic calculated by the frequency characteristic calculation unit to calculate an evaluation function value for each sinusoidal sweep with respect to the actual frequency characteristic, and adjusts the speed control loop gain to achieve a desired evaluation function value, characterized by , that the gain adjustment unit (9) determines the evaluation function value by using the evaluation function: Sj=∫ωminωmaxW(ω)[|Gmodel(ω)|−|Gexperimental(ω;kv)|]2dω (1) where ω is the frequency; kv is the rotary control loop gain; W(ω) is the weight applied to a frequency characteristic G model (ω) is the characteristic; and G experimental(ω) is the actually measured characteristic. [2] A servo control device according to claim 1, wherein the reference characteristic generating unit (8) generates a reference characteristic that is variable according to a configuration of the feed axis. [3] A servo control device according to claim 1, wherein the gain setting unit (9) determines a value of setting the speed control loop gain based on a difference between the desired evaluation function value and an evaluation function value for sweeping. [4] The servo control device according to claim 1, wherein the frequency characteristic calculation unit (7) estimates the gain and phase of a speed control loop input / output signal by representing the output of the speed control loop (4) using a Fourier series while defining the disturbance input frequency from the sine disturbance input unit (6) as the fundamental frequency, and calculates amplitudes and phases of the fundamental wave components of the Fourier series to calculate the frequency characteristics.

Images