Patent Publication Number: US-2010109639-A1

Title: Frequency spectrum analysis system and method

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to analysis systems and methods, and particularly to a frequency spectrum analysis system and method. 
     2. Description of the Related Art 
     In the machining field, machining performances of a servo system not only depend on driver, motor, and characteristics of the servo system itself, but also optimum control parameters of the machine. It is important to get the optimum control parameters for the servo system working according to different machining conditions. When the servo system has optimum control parameters, the frequency range of the output signal of the servo system is greatest when the servo system is stable. The greatest frequency range is taken as a desired frequency bandwidth. Generally, a simulation module of the servo system is employed for a frequency spectrum analysis to determine the optimum control parameters. However, this method is not as precise as desired in practice. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an exemplary embodiment of a frequency spectrum analysis system including a servo system. 
         FIG. 2  is a flowchart of an exemplary embodiment of a frequency spectrum analysis method. 
         FIG. 3  shows exemplary waveform graphs of a speed input signal and a speed output signal in time domain of a speed control loop of the servo system in  FIG. 1 . 
         FIG. 4  is an exemplary magnitude-frequency characteristic curve of the speed input signal and the speed output signal in  FIG. 3 . 
         FIG. 5  shows exemplary waveform graphs of a current input signal and a current output signal in time domain of a current control loop of the servo system in  FIG. 1 . 
         FIG. 6  is an exemplary magnitude-frequency characteristic curve of the current input signal and the current output signal in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an exemplary embodiment of a frequency spectrum analysis system  10  includes a control platform  100 , such as a personal computer, and a servo system  400 . The servo system  400  includes a driver  200  and a motor  300 . In one exemplary embodiment, a proportional-integral (PI) plus function acting as a control parameter of a control loop of the servo system  400  is adjusted to obtain a frequency bandwidth of the control loop. Hereinafter, a speed control loop and a current control loop of the servo system  400  will be used as examples. 
     The control platform  100  includes a first transmission device  102 , such an such as an RS-232 interface, an RS-485 interface and so on The driver  200  includes a second transmission device  202 , a signal source  204 , and a data logger  206 . The first transmission device  102  of the control platform  100  is connected to the second transmission device  202  of the driver  200 . The second transmission device  202 , such as an RS-232 interface, an RS-485 interface and so on, is connected to the signal source  204 . The signal source  204  is connected to the data logger  206  and the motor  300 . The data logger  206  is connected to the motor  300  and the second transmission  202 . The signal source  204  is capable of providing different frequencies, such as a chirp source (e.g., a short, sharp frequency source) for example, but the disclosure is not limited thereto. In one exemplary embodiment, the attenuation value of output signals of the motor  300 , functioning as output signals of the servo system, is −3 dB. That is to say, the output signal divided by the input signal is 0.707, and the logarithm of 0.707 multiplied by 20 dB is −3 dB. When an output signal is 0.707 times an input signal of the motor  300 , the frequency bandwidth of a control loop of the servo system  400  can be obtained by adjusting the PI plus of the control loop if the servo system  400  is stable. 
     Referring to  FIG. 2 , one embodiment of a method of frequency spectrum analysis method for the servo system  400  is provided, which includes the following blocks. Depending on the embodiment, certain blocks described below may be removed, others may be added, and the sequence of the blocks may be altered. 
     In block S 1 , the control platform  100  sets a frequency response condition of a control loop of the servo system  400 , such as an offset value, a magnitude value, and a sampling time of the input signal of the control loop of the servo system  400 . 
     In block S 2 , the control platform  100  transmits a trigger command to the signal source  204  via the first transmission device  102  and the second transmission device  202  to trigger the signal source  204 . 
     In block S 3 , the signal source  204 , according to the trigger command, provides a frequency response trigger signal as the input signal of the control loop of the servo system  400 , and transmits the input signal to the motor  300  and the data logger  206 . 
     In block S 4 , the motor  300  outputs an output signal according to the input signal, and the output signal of the motor  300  functioning as an output signal of the servo system  400  is transmitted to the data logger  206 . 
     In block S 5 , the data logger  206  records the input signal and output signal of the motor  300 , and transmits the input signal and output signal of the motor  300  to the control platform  100 . 
     In block S 6 , the control platform  100  stores the input signal and the output signal of the motor  300  in a memory (not shown). 
     In block S 7 , the input signal and the output signal of the motor  300  are processed by the control platform  100  by processing the input signal and the output signal using a fast Fourier transform. 
     In block S 8 , the control platform  100  draws a magnitude-frequency characteristic curve of the input signal and the output signal of the motor  300  using the fast Fourier transform, and determines an attenuation value of the output signal thereby to obtain the frequency range of the output signal of the servo system  400 . 
     Then, a determination is made whether the frequency range is the frequency bandwidth according to state performance of the servo system  400 . The state performance of the servo system  400  may be according to steady state performance of the servo system  400 , and the PI plus of the control loop of the servo system  400  can be adjusted as needed. In one embodiment, the determination may be done by an operator of the servo system  400 . Hereinafter, the speed control loop and the current control loop of the servo system  400  are used to provide a detailed explanation about the frequency spectrum analysis system  10  and the method for the same. 
       FIG. 3  illustrates exemplary waveform graphs of a speed input signal and a speed output signal in the time domain of the speed control loop of the servo system  400 . Further details of the speed input signal and the speed output signal will be explained in further detail below. In the illustrated embodiment of  FIG. 3 , a frequency response condition of the speed control loop is set as: speed offset value S 1 =300 rpm, speed magnitude value M 1 =200 rpm, and sampling time t 1 =1 ms. The signal source  204  provides an input signal V 1  for the motor  300  after the frequency response condition of the speed control loop is set by the control platform  100 . The motor  300  outputs a corresponding speed output signal V 2  after receiving the speed input signal V 1 . 
     The control platform  100  stores the speed input signal V 1  and the speed output signal V 2  in the memory, and processes the speed input signal V 1  and the speed output signal V 2  using the fast Fourier transform for transforming the speed input signal V 1  and the speed output signal V 2  from the time domain to the frequency domain. Then, the control platform  100  draws a magnitude-frequency characteristic curve of the speed input signal V 1  and the speed output signal V 2  as shown in  FIG. 4 . It can be seen in  FIG. 4  that the frequency range of the speed output signal V 2  of the speed control loop of the servo system  400  is 130 Hz when the attenuation value of the speed output signal V 2  is 3 dB. 
     If the servo system  400  is determined to be stable, that is to say, noise and vibration of the motor  300  are in a permitted range, the frequency range may be less than the frequency bandwidth. Therefore, the frequency range of the speed control loop of the servo system  400  can be increased by increasing the PI plus of the speed control loop so as to obtain the frequency bandwidth. However, the servo system  400  may become unstable when the PI plus of the speed control loop is increased to a certain value. When the servo system  400  is unstable, that is to say, the noise and the vibration of the motor  300  exceed the permitted range, the frequency range may be more than the frequency bandwidth. Therefore, the frequency range can be decreased by reducing the PI plus of the speed control loop so as to obtain the frequency bandwidth. When the frequency bandwidth is determined, the PI plus, functioning as a control parameter of the speed control loop of the servo system  400 , is determined. 
       FIG. 5  illustrates exemplary waveform graphs of a current input signal and a current output signal in the time domain of the current control loop of the servo system  400 . Further details of the current input signal and the current output signal will be explained in further detail below. In the illustrated embodiment of  FIG. 5 , a frequency response condition of the current control loop is set as: current offset value S 2 =0 mA, current magnitude value M 2 =1500 mA, sampling time t 2 =0.05 ms. The signal source  204  provides an input signal I 1  for the motor  300  after the frequency response condition of the current control loop set by the control platform  100 . The motor  300  outputs a corresponding current output signal I 2  after receiving the current input signal I 1 . 
     The control platform  100  stores the current input signal I 1  and the current output signal I 2  in the memory, and processes the current input signal I 1  and the current output signal I 2  by the fast Fourier transform for transforming the current input signal I 1  and the current output signal I 2  from the time domain to the frequency domain. Then the control platform  100  draws a magnitude-frequency characteristic curve of the current input signal I 1  and the current output signal I 2  as shown in  FIG. 6 . It can be seen in  FIG. 6  that the frequency range of the current output signal I 2  of the current control loop of the servo system  400  is 1400 Hz when the attenuation value of the current output signal I 2  is 3 dB. 
     If the servo system  400  is determined to be stable, that is to say, noise and vibration of the motor  300  are in a permitted range, the frequency range of the current control loop of the servo system  400  may be less than the frequency bandwidth. Therefore, the frequency range can be increased by increasing the PI plus of the current control loop so as to obtain the frequency bandwidth. However, the servo system  400  may become unstable when the PI plus of the current control loop is increased to a certain value. When the servo system  400  is unstable, that is to say, the noise and the vibration of the motor  300  exceed the permitted range, the frequency range may be more than the frequency bandwidth. Therefore, the frequency range can be decreased by reducing the PI plus of the current control loop so as to obtain the frequency bandwidth. When the frequency bandwidth is determined, the PI plus functioning as a control parameter of the current control loop of the servo system  400 , is determined. 
     In one exemplary embodiment, the control parameters such as PI plus values of other control loops of the servo system  400 , such as a pressure control loop of the servo system  400 , can be determined by using the frequency spectrum analysis system and method mentioned above. The selection of attenuation value of 3 dB can be other values. 
     It is to be understood, however, that even though numerous characteristics and advantages of the present disclosure have been set forth in the foregoing description, together with details of the structure and function of the disclosure, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.