Patent Publication Number: US-2012041573-A1

Title: Dynamic gain controller and control method for a control plant

Description:
FIELD OF THE INVENTION 
     The present invention is related generally to a linear control system and, more particularly, to a dynamic gain controller and control method for a control plant. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  shows a traditional fixed gain close loop linear feedback control system, in which a unit gain controller  10  provides an adjust value Sc for a control plant  18  to control the output value OV of the control plant  18 . In the unit gain controller  10 , a feedback circuit  16  detects the output value OV to generate a feedback value FB related to the output value OV, an operational circuit  12  has two input terminals receiving the feedback value FB and an input reference value Ref, respectively, and an output terminal outputting the error value Err between the reference value Ref and the feedback value FB, and an amplifier  14  amplifies the error value Err with a gain A to generate the adjust value Sc. Typically, a general digital control system is set with a proper gain A as the control parameter according to the target it is applied to. However, most of users desire a control system to have its output value reaching a stable set value in a settling time as short as possible when the input or the load is changed. For a general fixed gain close loop linear feedback control system, the reference value Ref sets the output value OV, and the gain A changes the settling time. However, while setting a large gain A helps the output value OV to reach the set value sooner, it may also lead to unstable output. On the other hand, setting a small gain A ensures a stable output, yet this makes the settling time excessively long or even, when the gain is small to an extent, makes the output become stable before the output value OV reaches the set value, which brings about a static error between the output value and the set value. 
     For example, in a fixed gain single phase digital brushless direct current (BLDC) fan control system, the reference value Ref is the set reference revolutions per minute (RPM), the feedback value FB is the fed back output RPM, the control plant  18  is a BLDC fan, and with the error value Err between the reference RPM Ref and the fed back output RPM FB and through the linear operation of the amplifier  14 , the RPM of the BLDC fan  18  is controlled.  FIG. 2  is a functional block diagram of a single phase digital BLDC fan control system, in which output terminals OUT 1  and OUT 2  are connected to a fan motor  34 , and by setting the on time and on/off order of four switches M 1 , M 2 , M 3  and M 4  in an H bridge, it can control the level and direction of the currents at the output terminals OUT 1  and OUT 2 , thereby varying the RPM of the fan motor  34 . The RPM of the fan motor  34  is fed back by a feedback circuit  23  which includes a Hall component  24  for detecting the RPM of the fan motor  34 , and the output of the Hall component  24  is converted into a current RPM FB. A reference RPM assign circuit  19  provides the reference RPM Ref, which includes a duty-to-RPM converter  20  and a lookup table  22 . The duty-to-RPM converter  20  decodes the duty cycle of the signal at an input terminal PWMIN, and the decoded value is converted into the reference RPM Ref with the lookup table  22 . Alternatively, the value of a configuration register may be used to set the reference RPM. A programming interface  36  is provided for updating the lookup table  22  for the reference RPM assign circuit  19 . An operational circuit  26  calculates with the reference RPM Ref and the current feedback RPM FB to generate the error value Err, a compensator  28  having a gain A generates the adjust value Sc according to the error value Err, and a digital pulse width modulator (DPWM)  30  generates the output value OV according to the adjust value Sc for the driver  32  to control the four switches M 1 , M 2 , M 3  and M 4  in the H bridge. 
     Assuming that the reference RPM Ref is changed from 3000 RPM to 4000 RPM, the RPM of the fan motor  34  will vary correspondingly. As shown in  FIG. 3 , the RPM of the fan motor  34  starts to change at about 7.5 seconds, and becomes stable at the set value 4000 RPM at about 12.3 seconds.  FIG. 4  shows the relationship between the varying RPM and time under different gains, in which waveforms  40 ,  42  and  44  represent the curves of the output RPM versus time when the gain of the compensator  28  is A 1 , A 2  and A 3 , respectively, where A 3 &gt;A 1 &gt;A 2 . As shown by the waveforms  40 ,  42  and  44 , under all the gains, the RPM of the fan motor  34  starts to change at about 7.5 seconds, while the compensator  28  having the gain A 1  makes the fan motor  34  reaches 4000 RPM at about 12.3 seconds, namely the settling time of 4.8 seconds; the compensator  28  having the gain A 2  makes the fan motor  34  reaches 4000 RPM at about 13.8 seconds, namely the settling time of 5.5 seconds; and the compensator  28  having the gain A 3  makes the fan motor  34  reaches 4000 RPM at about 12 second, namely the settling time of 4.5 seconds. As shown in  FIG. 4 , the higher the gain is, the shorter the settling time is, yet the gain excessively high can make the RPM of the fan unstable, as shown in  FIG. 5 . 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a controller and control method for a control plant to have a shorter settling time. 
     Another objective of the present invention is to provide a controller and control method for a control plant to have a stable output value. 
     A further objective of the present invention is to provide a dynamic gain controller and control method. 
     According to the present invention, a dynamic gain controller for generating an adjust value for a control plant to control an output value of the control plant, includes a gain selector detecting an error value between an input reference value and a feedback value related to the output value to dynamically select different gains. When the error value is larger than a threshold, the gain is selected to be larger for a shorter settling time, and when the error value is smaller than the threshold, the gain is selected to be smaller to maintain the output value stable. 
     According to the present invention, a control method for a control plant to control an output value of the control plant includes monitoring the output value to generate a feedback value related to the output value, and selecting different gains according to an error value between the feedback value and a reference value. When the error value is larger than a threshold, the gain is selected to be larger for a shorter settling time, and when the error value is smaller than the threshold, the gain is selected to be smaller to maintain the output value stable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a traditional fixed gain close loop linear feedback control system; 
         FIG. 2  is a functional block diagram of a single phase digital BLDC fan control system; 
         FIG. 3  is a diagram showing the relationship of the varying RPM of a fan motor versus time; 
         FIG. 4  is a diagram showing the relationship between the varying RPM of a fan motor and time under different gains; 
         FIG. 5  is a diagram showing the relationship between the varying RPM of a fan motor and time when the gain is excessively large; 
         FIG. 6  is a system including a dynamic gain controller according to the present invention; 
         FIG. 7  is a circuit diagram of an embodiment for the gain selector shown in  FIG. 6 ; 
         FIG. 8  show experimental data of the control system shown in  FIG. 6 ; 
         FIG. 9  show experimental data of the control system shown in  FIG. 6 ; and 
         FIG. 10  is a single phase digital BLDC motor control system using a dynamic gain controller according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 6  is a system which includes a dynamic gain controller  50  according to the present invention to provide an adjust value Sc for a control plant  18  to control the output value OV of the control plant  18 . In addition to the operational circuit  12 , the amplifier  14  and the feedback circuit  16  as shown in  FIG. 1 , the dynamic gain controller  50  shown in  FIG. 6  further includes a gain selector  52  and a multiplexer  54 . The gain selector  52  monitors the error value Err generated by the operational circuit  12  and generates a select signal CP according to the error value Err and a preset threshold Err_thresh, and the multiplexer  54  selects one of gains A 0  and A 1  according to the select signal CP for the amplifier  14  as a control parameter.  FIG. 7  is a circuit diagram of an embodiment for the gain selector  52  shown in  FIG. 6 , which has a comparator  56  for comparing the error value Err with the threshold Err_thresh to generate the select signal CR When the error value Err is larger than the threshold Err_thresh, the select signal CP is low, and the multiplexer  54  selects the larger gain A 0  for the amplifier  14  to have a shorter settling time. When the error value Err is smaller than the threshold Err_thresh, the select signal CP is high, and the multiplexer  54  selects the smaller gain A 1  for the amplifier  14  to ensure a stable output value OV. The control system shown in  FIG. 6  may be either a digital system or an analog system. 
       FIGS. 8 and 9  show experimental data of the control system shown in  FIG. 6 . As shown in  FIG. 8 , at 151.5 seconds, the reference value Ref of the control system changes from 3000 RPM to 4000 RPM, causing the RPM of the output value OV beginning increasing. At this time, since the error value Err between the feedback value FB and the reference value Ref is larger than the threshold Err_thresh, the larger gain A 0  is selected for the amplifier  14 . Once the error value Err becomes smaller than the threshold Err_thresh, the smaller gain A 1  is selected for the amplifier  14 . At last, the RPM of the output value OV stays stably at 4000 RPM at 153 seconds. As shown in  FIG. 9 , at 391.8 seconds, the reference value Ref of the control system is changed from 4000 RPM to 3000 RPM, causing the RPM of the output value OV beginning decreasing. At this time, since the error value Err between the feedback value FB and the reference value Ref is larger than the threshold Err_thresh, the larger gain A 0  is selected for the amplifier  14 . Once the error value Err becomes smaller than the threshold Err_thresh, the smaller gain A 1  is selected for the amplifier  14 . At last, the RPM of the output value OV stays stably at 3000 RPM at 393.6 seconds. As shown in  FIGS. 8 and 9 , a system using a dynamic gain controller according to the present invention will have a settling time less than two seconds and ensures the output value OV stable. 
       FIG. 10  is a single phase digital BLDC motor control system using a dynamic gain controller according to the present invention. In addition to the circuit shown in  FIG. 2 , this system further includes a gain selector  52  and a multiplexer  54 . The reference RPM generated by the reference RPM assign circuit  19  in  FIG. 10  is equal to the reference value Ref in  FIG. 6 . In  FIG. 10 , the gain selector  52  has two input terminals to receive a preset threshold Err_thresh and the error value Err generated by the operational circuit  26 , and generates a select signal CP accordingly, and the multiplexer  54  selects a gain A 0  or A 1  for the compensator  28  according to the select signal CP. When the error value Err is larger than the threshold Err_thresh, the multiplexer  54  selects the larger gain A 0  for the compensator  28  for a shorter settling time. When the error value Err is smaller than the threshold Err_thresh, the multiplexer  54  selects the smaller gain A 1  for the compensator  28  to ensure the output value OV stable. 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.