Abstract:
A feedback switching device and a method allow a drive control loop for a servo motor to actively switch the feedback mode in accordance with the rotating speed of the servo motor. When the servo motor is under a high speed operation, a sensorless position estimation feedback technology is used as the feedback mode; on the other hand, when the servo motor is under a low speed operation, the switching mode is automatically switched to a position sensing feedback technology. Therefore, the development needs for multi-function, high performance and low cost in the field of the servo motor control are met, and the conventional problem is solved that, when being applied to a servo driving system having a wide speed range, the single use of the position sensing feedback technology or the sensorless position estimation feedback technology fails to satisfy the application for a wide speed range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to driving of servo motors, and more particularly, to a feedback switching device and method that allow to actively switch the feedback mode in accordance with the rotating speed of a servo motor. 
     2. Description of Related Art 
     Servo motor drivers have been widely applied in modern industries and developed towards high performance, multi-function and low cost. In some application, a servo motor must operate within a wide speed range, and how to build up a speed control architecture is critical to effectively improving the performance and reducing the cost of the servo motor driver. 
     For example, a designer can use a position sensor such as an encoder or a resolver to build up the speed control architecture for a servo motor. That is, the designer builds up the speed control architecture through a position sensing feedback technology. In such a case, when the servo motor operates at a low rotating speed, the servo motor driver can correctly receive a position signal as long as the bandwidth of the position signal is lower than that of the position sensor. On the other hand, if the servo motor operates at an extra-high rotating speed such that the bandwidth of the position signal is higher than that of the position sensor, the position signal will be distorted. To solve the problem, a sensorless position estimation feedback technology has been developed, which can be applied in a high-speed servo motor to estimate the motor angle, thereby building up the speed control architectures for the high-speed servo motor. 
     However, before a servo motor operates, the user must determine which one of the position sensing feedback technology and the sensorless position estimation feedback technology is to be used for building up the speed control architecture, and during the operation of the servo motor, the speed control architecture preset by the user cannot be flexibly changed in accordance with the rotating speed of the servo motor. As such, the servo motor driver cannot precisely control the servo motor. In particular, if the position sensing feedback technology is used to build up the speed control architecture, when the rotating speed of the servo motor is higher than a certain value, the position signal obtained through the position sensing feedback technology will be distorted. Therefore, the servo motor driver cannot obtain correct position information. On the other hand, if the sensorless position estimation feedback technology is used to build up the speed control architecture, when the rotating speed of the servo motor is lower than a certain value, the position signal obtained through the sensorless position estimation feedback technology is of low precision. As such, the servo motor driver also cannot obtain correct position information. 
     Therefore, there is a need to provide a feedback switching device and method for driving of a servo motor so as to overcome the above-described drawbacks. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a feedback switching device and a method that allow a drive control loop for a servo motor to actively switch the feedback mode in accordance with the rotating speed of the servo motor. 
     In order to achieve the above and other objectives, the present invention provides a feedback switching device applied to a drive control loop for a servo motor, which comprises: a current sensor for sensing a current supplied to the servo motor and outputting a current feedback signal corresponding to the sensed current; a position sensor for sensing a position of the servo motor and outputting a position sensing signal corresponding to the sensed position; a position estimator for receiving the current feedback signal outputted from the current sensor and outputting a position estimation signal corresponding to the current feedback signal; and a feedback switching comparator for receiving the position sensing signal and the position estimation signal and outputting one of the position sensing signal and the position estimation signal as a position feedback signal according to a preset switching condition. 
     Further, the present invention provides a feedback switching method applied to a drive control loop for a servo motor, wherein the servo motor initially operates at a low rotating speed. The method comprises the steps of: (1) using a current sensor to sense a current supplied to the servo motor and output a current feedback signal corresponding to the sensed current, using a position sensor to sense a position of the servo motor and output a position sensing signal corresponding to the sensed position, and using a position estimator to receive the current feedback signal and output a position estimation signal corresponding to the current feedback signal; (2) using a speed calculation module to receive the position sensing signal outputted from the position sensor and the position estimation signal outputted from the position estimator and generate a first speed signal and a second speed signal corresponding to the position sensing signal and the position estimation signal, respectively; (3) using a comparison module to receive the first speed signal and the second speed signal and compare the first speed signal with a preset switching condition, to determine whether the first speed signal is higher than the preset switching condition, if yes, the process goes to step (4), otherwise, the process goes to step (6); (4) using a switching module to output the position estimation signal as a position feedback signal; (5) suing the comparison module to compare the second speed signal with the preset switching condition, to determine whether the second speed signal is lower than the preset switching condition, if yes, the process goes to step (6), otherwise, the process goes to step (4); and (6) using the switching module to output the position sensing signal as the position feedback signal. 
     The invention also provides another feedback switching method applied to a drive control loop for a servo motor, wherein the servo motor initially operates at a high rotating speed. The method comprises the steps of: (1) using a current sensor to sense a current supplied to the servo motor and output a current feedback signal corresponding to the sensed current, using a position sensor to sense a position of the servo motor and output a position sensing signal corresponding to the sensed position, and using a position estimator to receive the current feedback signal and output a position estimation signal corresponding to the current feedback signal; (2) using a speed calculation module to receive the position sensing signal outputted from the position sensor and the position estimation signal outputted from the position estimator and generate a first speed signal and a second speed signal corresponding to the position sensing signal and the position estimation signal, respectively; (3) using a comparison module to receive the first speed signal and the second speed signal and compare the second speed signal with a preset switching condition, to determine whether the second speed signal is lower than the preset switching condition, if yes, the process goes to step (4), otherwise, the process goes to step (6); (4) using a switching module to output the position sensing signal as a position feedback signal; (5) using the comparison module to compare the first speed signal with the preset switching condition, to determine whether the first speed signal is higher than the preset switching condition, if yes, the process goes to step (6), otherwise, the process goes to step (4); and (6) using the switching module to output the position estimation signal as the position feedback signal. 
     Compared with the prior art, the present invention effectively reduces the cost, and improves the performance and functionality in the servo motor control. The present invention also increases the flexibility of the application of a servo motor driver in a servo driving system having a wide speed range, and improves the overall operation efficiency of the servo motor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating the architecture of a feedback switching device for driving of a servo motor according to an embodiment of the present invention; 
         FIG. 2  is a schematic circuit diagram of a feedback switching comparator of  FIG. 1 ; 
         FIG. 3  is a speed-time diagram illustrating the operation of the feedback switching device of the present invention; 
         FIG. 4  is a flow diagram illustrating a feedback switching method for driving of a servo motor according to an embodiment of the present invention; and 
         FIG. 5  is a flow diagram illustrating a feedback switching method for driving of a servo motor according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following illustrative embodiments are provided to illustrate the disclosure of the present invention, these and other advantages and effects can be apparent to those in the art after reading this specification. 
       FIG. 1  is a block diagram showing the architecture of a feedback switching device  100  for driving of a servo motor according to an embodiment of the present invention. In the present embodiment, the feedback switching device  100  is applied to a drive control loop for a servo motor  113 . The feedback switching device  100  at least comprises a current sensor  111 , a position sensor  115 , a position estimator  117  and a feedback switching comparator  119 . Further, the feedback switching device  100  can selectively comprise a speed feedback generator  121 , such as a differentiator. 
     The drive control loop can further comprise proportional-integral-derivative controllers (PID controllers)  103 ,  105 ,  107  and a power module  109 , wherein the PID controller  103  can receive a position command  102 , the PID controller  105  can receive a speed command  104 , and the PID controller  107  is connected to the power module  109  and can receive a current command  106 . 
     The current sensor  111  is disposed on a power supply path for the servo motor  113  for sensing a current supplied to the servo motor  113  and outputting a current feedback signal  108  corresponding to the sensed current. 
     The position sensor  115  is disposed on the servo motor  113  for sensing a position of the servo motor  113  and outputting a position sensing signal  115  corresponding to the sensed position. In the present embodiment, the position sensor  115  can be an encoder or a resolve that encodes the position of the servo motor  113 . 
     The position estimator  117  is connected to the current sensor  111  for receiving the current feedback signal  108  and outputting a position estimation signal  117   a  corresponding to the current feedback signal  108 . 
     The feedback switching comparator  119  is used to receive the position sensing signal  115   a , the position estimation signal  117   a  and a switching condition preset by the user, so as to output one of the position sensing signal  115   a  and the position estimation signal  117   a  as a position feedback signal  110  according to the preset switching condition. 
     Therefore, when the servo motor operates at a low rotating speed, the feedback switching comparator  119  automatically chooses the position sensing signal  115   a  as the position feedback signal  110 , so as to meet the demand for high efficiency. On the other hand, when the servo motor operates at a high rotating speed, the feedback switching comparator  119  automatically chooses the position estimation signal  117   a  as the position feedback signal  110 , so as to meet the demand for wide speed range operation. 
     Further, the position feedback signal  110  outputted from the feedback switching comparator  119  is combined with the position command  102 , so as to be received by the PID controller  103  for generating the speed command  104 . The position feedback signal  110  outputted from the feedback switching comparator  119  can be transmitted to the speed feedback generator  121  for generating a speed feedback signal  112 . The speed feedback signal  112  is further combined with the speed command  104 , so as to be received by the PID controller  105  for generating the current command  106 . The current command  106  is further combined with the current feedback signal  108 , so as to be received by the PID controller  107 , thereby causing the power module  109  connected with the PID controller  107  to perform a relative speed control on the servo motor  113 . 
     In practice, the current sensor  111  disposed on the power supply path for the servo motor  113  senses the current of the servo motor  113  and outputs the current feedback signal  108  corresponding to the sensed current. The position sensor  115  senses the position of the servo motor  113 , so as to output the position sensing signal  115   a  corresponding to the sensed position. The current feedback signal  108  is further transmitted to the position estimator  117  and also transmitted through another path, so as to be combined with the current command  106 . When receiving the current feedback signal  108 , the position estimator  117  outputs the position estimation signal  117   a  corresponding to the current feedback signal  108 . 
     Subsequently, the feedback switching comparator  119  receives the position sensing signal  115   a  and the position estimation signal  117   a , so as to output one of the position sensing signal  115   a  and the position estimation signal  117   a  as the position feedback signal  110  according to the preset switching condition. The operation of the feedback switching comparator  119  will be detailed later. 
     Based on the position feedback signal  110 , the speed feedback generator  121  generates the speed feedback signal  112 , which is further combined with the speed command  104 . 
     Referring to  FIG. 2  in combination with  FIG. 1 ,  FIG. 2  is a schematic circuit diagram of the feedback switching comparator  119 . The feedback switching comparator  119  comprises two speed calculation modules  201 , a comparison module  203  and a switching module  205 . 
     The speed calculation modules  201  can be differentiators for receiving the position sensing signal  115   a  outputted from the position sensor  115  and the position estimation signal  117   a  outputted from the position estimator  117 , so as to generate a first speed signal  201   a  and a second speed signal  201   b , respectively. In practice, the two speed calculation modules  201  can be integrated into one module. 
     The comparison module  203  is used to receive the preset switching condition  202 , the first speed signal  201   a  and the second speed signal  201   b , and further compares one of the first speed signal  201   a  and the second speed signal  201   b  with the switching condition  202 , so as to output a comparison result  203   a . It should be noted that in the present embodiment, the comparison module  203  can be controlled through an additional controller, logic circuit or software (not shown). The comparison module  203  can have a hysteresis characteristic within a specified range, such that the comparison module  203  can use the specified range and the switching condition  202  as a comparison standard, thus avoiding oscillation of the comparison result  203   a  around a threshold value. The switching condition  202  can be set at any point on the characteristic curve of the servo motor  113 , as long as the position feedback result is correct. 
     The switching module  205  is used to receive the position sensing signal  115   a  outputted from the position sensor  115  and the position estimation signal  117   a  outputted from the position estimator  117 , so as to output one of the position sensing signal  115   a  and the position estimation signal  117   a  as the position feedback signal  110 . 
     Referring to  FIG. 3  in combination with  FIG. 2 ,  FIG. 3  is a speed-time diagram illustrating the operation of the feedback switching device  100 , wherein the hysteresis range is combined with the switching condition  202  for forming a flexible comparison standard. 
     Before time point  302 , the position sensing signal  115   a  can be set by the user as the position feedback signal  110  for the speed control of the servo motor  113 . At this time, the comparison module  203  can also determine that the second speed signal  201   b  is lower than the preset switching condition  202 , i.e., the servo motor  113  operates at a low rotating speed. Therefore, the comparison module  203  outputs the comparison result  203   a  to the switching module  205 , so as to cause the switching module  205  to output the position sensing signal  115   a  as the position feedback signal  110  for the speed control of the servo motor  113 . 
     At time point  302  (the upper limit of the hysteresis range), the comparison module  203  can determine that the first speed signal  201   a  is higher than the switching condition  202 , i.e., the servo motor  113  operates at a high rotating speed. Therefore, the comparison module  203  outputs the comparison result  203   a  to the switching module  205 , so as to cause the switching module  205  to output the position estimation signal  117   a  as the position feedback signal  110  for the speed control of the servo motor  113 . 
     At time point  304  (the lower limit of the hysteresis range), the comparison module  203  can determine that the second speed signal  201   b  is lower than the preset switching condition  202 , that is, the servo motor  113  operates at a low rotating speed. Therefore, the comparison module  203  outputs the comparison result  203   a  to the switching module  205 , so as to cause the switching module  205  to output the position sensing signal  115   a  as the position feedback signal  110  for the speed control of the servo motor  115   a.    
     Referring to  FIG. 4  in combination with  FIGS. 1 and 2 , a feedback switching method  400  for driving of a servo motor according to an embodiment of the present invention is shown. The method  400  can be applied to a drive control loop for the servo motor. In the present embodiment, the servo motor initially operates at a low rotating speed. 
     First, in step S 402 , a current sensor disposed on the power supply path for the servo motor senses the current supplied to the servo motor and outputs a current feedback signal corresponding to the sensed current; a position sensor senses the position of the servo motor and outputs a position sensing signal corresponding to the sensed position; a position estimator receives the current feedback signal and outputs a position estimation signal corresponding to the current feedback signal; and the position sensing signal is used as the position feedback signal. Then, the process goes to step S 404 . 
     In step S 404 , a speed calculation module receives the position sensing signal outputted from the position sensor and the position estimation signal outputted from the position estimator, so as to generate a first speed signal and a second speed signal corresponding to the position sensing signal and the position estimation signal, respectively. Further, the comparison module receives the preset switching condition, the first speed signal and the second speed signal. Then, the process goes to step S 406 . 
     In step S 406 , the comparison module compares the first speed signal with the preset switching condition, to determine whether the first speed signal is higher than the preset switching condition, if yes, the process goes to step S 408 , otherwise, the process goes to step S 412 . 
     In step S 408 , the switching module outputs the position estimation signal as the position feedback signal. Then, the process goes to step S 410 . 
     In step S 410 , the comparison module compares the second speed signal with the predetermined switching condition, to determine whether the second speed signal is lower than the preset switching condition, if yes, the process goes to step S 412 , otherwise, the process goes to step S 408 . 
     In step S 412 , the switching module outputs the position sensing signal as the position feedback signal. Then, the process goes to step S 406 . 
     In the feedback switching method  400 , the position sensor is an encoder or a resolver that encodes the position of the servo motor. The comparison module can have a hysteresis characteristic within a specified range. Therefore, in step S 406  the comparison module can use the hysteresis range and the preset switching condition as the comparison standard for determining whether the first speed signal is higher than the preset switching condition; and in step S 410  the comparison module can use the hysteresis range and the preset switching condition as the comparison standard for determining whether the second speed signal is lower than the preset switching condition. 
     Referring to  FIG. 5  in combination with  FIGS. 1 and 2 , another feedback switching method  500  for driving of a servo motor according to an embodiment of the present invention is shown. The method  500  can be applied to a drive control loop for the servo motor. In the present embodiment, the servo motor initially operates at a high rotating speed. 
     In step S 502 , a current sensor disposed on the power supply path for the servo motor senses the current supplied to the servo motor and outputs a current feedback signal corresponding to the sensed current; a position sensor senses the position of the servo motor and outputs a position sensing signal corresponding to the sensed position; a position estimator receives the current feedback signal and outputs a position estimation signal corresponding to the current feedback signal; and the position estimation signal is used as the position feedback signal. Then, the process goes to step S 504 . 
     In step S 504 , a speed calculation module receives the position sensing signal outputted from the position sensor and the position estimation signal outputted from the position estimator, so as to generate a first speed signal and a second speed signal corresponding to the position sensing signal and the position estimation signal, respectively. Further, the comparison module receives the preset switching condition, the first speed signal and the second speed signal. Then, the process goes to step S 506 . 
     In step S 506 , the comparison module compares the second speed signal with the preset switching condition, to determine whether the second speed signal is lower than the preset switching condition, if yes, the process goes to step S 508 , otherwise, the process goes to step S 512 . 
     In step S 508 , the switching module outputs the position sensing signal as the position feedback signal. Then, the process goes to step S 510 . 
     In step S 510 , the comparison module compares the first speed signal with the predetermined switching condition, to determine whether the first speed signal is higher than the preset switching condition, if yes, the process goes to step S 512 , otherwise, the process goes to step S 508 . 
     In step S 512 , the switching module outputs the position estimation signal as the position feedback signal. Then, the process goes to step S 506 . 
     In the feedback switching method  500 , the position sensor is an encoder or a resolver that encodes the position of the servo motor. The comparison module can have a hysteresis characteristic within a specified range. Therefore, in step S 506  the comparison module can use the hysteresis range and the preset switching condition as the comparison standard for determining whether the second speed signal is lower than the preset switching condition; and in step S 510  the comparison module can use the hysteresis range and the preset switching condition as the comparison standard for determining whether the first speed signal is higher than the preset switching condition. 
     Therefore, the present invention provides an automatic feedback switching mechanism, so as to improve the functionality of the drive control loop, increase the flexibility of the application of a servo motor driver in a servo driving system having a wide speed range, and improves the overall operation efficiency of the servo motor. 
     The above-described descriptions of the detailed embodiments are intended to illustrate the preferred implementation according to the present invention but are not intended to limit the scope of the present invention. Accordingly, all modifications and variations completed by those with ordinary skill in the art should fall within the scope of present invention defined by the appended claims.