PATENT ABSTRACT
Methods, systems, and computer program products for compensating unstable linear time-invariant due to input nonlinearities are described. In one implementation, compensating a controlled device may include controlling the controlled device using feedforward control. In another implementation, compensating the controlled device may include controlling the controlled device using feedback control.

PATENT DESCRIPTION
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 60/977,498, titled “ANTI-WINDUP SPINDLE SPEED CONTROL DESIGN,” filed on Oct. 4, 2007, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The subject matter as described herein is generally related to servo applications. 
     BACKGROUND 
     One way to maintain a controlled device at a desired level (e.g., as measured by a control output of the controlled device) is to implement a controller which sends a control signal to manage a control input of the controlled device. A conventional controller is generally designed with a limiter to prevent the control input from exceeding a given level. However, when the control signal becomes saturated, the controller and the controlled device can become functionally unstable. If the control signal overshoots a given level, the overshoot can restrict the controlled device from reaching the desired level. 
     One conventional way to resolve this problem is to implement anti-windup control. The aim of anti-windup control is to modify the dynamics of a control loop when the control signal saturates so that a good transient behavior may be attained after de-saturation, while avoiding limit cycle oscillations and repeated saturations. However, because of the complexity of anti-windup control, conventional anti-windup schemes are mostly heuristic in nature, and do not address the saturation problem with stability guarantees and enhanced performance. 
     SUMMARY 
     A new saturation control technique and apparatus is described for compensating unstable linear time-invariant systems subject to input nonlinearities. The control techniques and apparatus can operate to allow regional stability in the existence of input saturation, and provide less conservative performance than conventional anti-windup compensation schemes. 
     In some implementations, a system includes: a control section which operates in a feedback control mode to control a controlled device, a feedforward section which operates in a feedforward control mode to control the controlled device, and a state control section to control the controlled device in a steady state in the feedforward control mode or the feedback control mode. 
     In some implementations, a system includes: a control section to generate a control value for controlling a controlled device, a limiter to receive the control value and generate a regulated value, an adjusting section to adjust the control value through feedback control based on a difference between the control value and the regulated value, and a state control section to control the controlled device based on the regulated value to allow the controlled device to operate in a steady state. 
     In some implementations, a method includes: generating a control value for controlling a controlled device, generating a regulated value from the control value, adjusting the control value through feedback control based on a difference between the control value and the regulated value, and controlling a controlled device based on the regulated value to allow the controlled device to operate in a steady state. 
     In some implementations, a method includes: generating a control value for controlling a controlled device in a feedback control mode, generating a feedforward value for controlling a controlled device in a feedforward control mode, and controlling the controlled device in a steady state by switching between the feedforward control mode and the feedback control mode. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description and drawings, and from the claims 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of a controller  100 . 
         FIG. 2  is an example graph depicting an error difference ω err  relative to an external force. 
         FIG. 3  is an example graph depicting a voltage value being supplied to a controlled device relative to an external force. 
         FIG. 4  is a diagram showing an example of a control range of a limiter. 
         FIG. 5  is a block diagram showing an alternative configuration of the controller  100  shown in  FIG. 1 . 
         FIG. 6A  is an example graph depicting an error difference ωerr relative to an external force being applied to the controller  100  shown in  FIG. 5  when an initial value X c0  is considered. 
         FIG. 6B  is an example graph depicting a voltage value being supplied to a controlled device relative to an external force being applied to the controller  100  shown in  FIG. 5  when an initial value X c0  is considered. 
         FIG. 7A  is an example graph depicting an error difference ωerr relative to an external force being applied to the controller  100  shown in  FIG. 5  when an initial value X c0  is not considered. 
         FIG. 7B  is an example graph depicting a voltage value being supplied to a controlled device relative to an external force being applied to the controller  100  shown in  FIG. 5  when initial value X c0  is not considered. 
         FIG. 8  is a block diagram showing an alternative configuration of the controller  100  shown in  FIG. 1 . 
         FIG. 9  is a block diagram showing an example of a design support apparatus  200 . 
         FIG. 10A  is a diagram showing an example of a stable region ε(P). 
         FIG. 10B  is a diagram showing an example of an initial region Ξ 0 . 
         FIG. 11A  shows an example projection view of a stable region ε(P) projected on a ω err  I plane. 
         FIG. 11B  shows an example projection view of a stable region ε(P) projected on a X c -ω err  plane. 
         FIG. 11C  shows an example projection view of a stable region ε(P) projected on a X c -I plane. 
         FIG. 12  is a flowchart showing an example process performed by the design support apparatus  200  shown in  FIG. 9 . 
         FIG. 13  is a block diagram showing an alternative configuration of the design support apparatus  200  shown in  FIG. 9 . 
         FIG. 14  shows an example flowchart for setting a center value of the control range of the limiter  50 . 
     
    
    
     DETAILED DESCRIPTION 
     System Overview 
       FIG. 1  is a diagram showing an example of the configuration of a controller  100 . The controller  100  may be configured, in some implementations, to control and adjust a controlled device to a desired value using a controlled value. As an example, to configure the controlled device under a steady state, the controller  100  may supply a controlled value to the controlled device, and adjust the controlled value (e.g., continuously or on a periodic basis) so that a physical value of the controlled device can converge to a desired value. In some implementations, the physical value may include, without limitation, the value of rotation velocity of a motor for a spindle or of a hard disk drive. The controller  100  may be an apparatus which controls the physical value of the controlled device to minimize a difference between the physical value and the desired value. The controller  100  may monitor the difference and provide an appropriate controlled value to the controlled device as needed (e.g., based on the rotation velocity of the spindle). 
     As shown in  FIG. 1 , the controller  100  may include a control section  10 , a limiter  50 , an adjusting section  40 , a setting section  80 , and a state control section  60 . The control section  10  may be configured to generate a control value U in  for controlling a controlled device. In some implementations, the control value U in  may be generated based on an error difference ω err  between a physical value of the controlled device and a desired value. The controller  100  may further include a detecting section (as discussed below) operable to detect the physical value of the controlled device, and a calculating section (as discussed below) operable to determine the error difference ω err  between the detected physical value and the desired value. 
     Control Section 
     The control section  10 , in some implementations, may include a first adder  12 , a first delaying section  14 , a first multiplier  16 , a second adder  18 , a second multiplier  20 , a third multiplier  22 , a second delaying section  24 , and a third adder  26 . In these implementations, the first adder  12 , the first delaying section  14 , and the first multiplier  16  can collectively provide an average of the error difference ω err  supplied to the control section  10 , which may be used to generate the control value U in . 
     The first delaying section  14  may delay the error difference ω err  supplied to the control section  10 . In some implementations, the amount of delay used by the first delaying section  14  may be substantially equal to an operation cycle of the controller  100 . In other implementations, the original error difference ω err  and the error difference ω err  delayed by the first delaying section  14  from different operating cycles may be added to produce a combined value. For example, the first adder  12  may add the error difference ω err  supplied in a current cycle and the error difference ω err  delayed by the first delaying section  14  from a previous cycle to generate a first combined value prior to being sent to the first multiplier  16 . 
     Upon receiving the first combined value, the first multiplier  16  multiplies the value from the first adder  12  by ½. With this configuration, the first multiplier  16  can generate an average value based on, for example, the error difference ω err  in the present cycle and the error difference ω err  in the previous cycle. While the first multiplier  16  is illustrated with a value of ½, other values also are contemplated, and the value selection may depend partially on a specific design and application. 
     Digital Filter 
     In some implementations, the average value output by the first multiplier  16  may be output to a digital filter. In these implementations, the digital filter may include the second adder  18 , the second multiplier  20 , the third multiplier  22 , the second delaying section  24  and the third adder  26 . Depending on a specific design and application, the digital filter may include a greater or lesser number of components than those described above. 
     The digital filter, in these implementations, may be configured with an integral characteristic suitable for transferring the average value output by the first multiplier  16  to the limiter  50  based on a transfer function given by [1]:
 
 C ( z )= K   1pg ×( z+b   1 )/( z− 1)  [1]
 
where K 1pg  is a coefficient of the third multiplier  22 , b 1  is the coefficient of the second multiplier  20  and (z−1) is an integral term.
 
     The first multiplier  16  forwards the average value to the second adder  18 . The second adder  18  receives the average value, and adds a delayed value output by the second delaying section  24  to the average value to produce a second combined value. The third multiplier  22  receives the second combined value, and subsequently multiplies the second combined value by the coefficient K 1pg  to produce a control value U in . 
     In some implementations, the first multiplier  16  also forwards the average value to the second multiplier  20 , and the second multiplier  20  then multiplies the received average value by a predetermined coefficient b 1 , the output of which is sent to the third adder  26 . The third adder  26  adds the value output by the second multiplier  20 , the value output by the third multiplier  22 , and the value output by the adjusting section  40  (as will be discussed in greater detail below) to obtain a third combined value. The second delaying section  24  delays the third combined value output from the third adder  26 , the result of which is subsequently fed back to the second adder  18 . 
     Other configurations of the control section  10  are possible. The control section  10  may have a different configuration from that shown in  FIG. 1  as long as such a configuration can yield a similar integral characteristic. 
     Limiter 
     The limiter  50  may receive the control value U in  output by the third multiplier  22  to generate a regulated value U out , which may be regulated within a predetermined control range having an upper limit U max  and a lower limit U min  ( FIG. 4 ). In some implementations, if the control value U in  lies within a predetermined control range, the limiter  50  may output the control value U in  as the regulated value U out . In other implementations, if the control value U in  is larger than the upper limit U max  of the predetermined control range, the limiter  50  may output the upper limit U max  as the regulated value U out . In yet other implementations, if the control value U in  is smaller than the lower limit U min  of the predetermined control range, the limiter  50  may output the lower limit U min  as the regulated value U out . 
     In some implementations, the predetermined control range of the limiter  50  may be established based on a predetermined range value U r . The predetermined range value U r  may be set by, for example, the setting section  80 , as will be discussed in greater detail below. In these implementations, the predetermined range value U r  may be determined to allow for stability at a desired value in a steady state, as will be described in greater detail below with respect to  FIGS. 2 and 3 . 
     In some implementations, the predetermined range value U r  lies within the predetermined control range. In these implementations, an offset may be used to define the relationship between the predetermined range value U r  and the upper limit U max , and between the predetermined range value U r  and the lower limit U min . For example, an offset “a” may be defined, where the difference between the upper limit U max  and the predetermined range value U r  and between the lower limit U min  and the predetermined range value U r  is the offset “a” (e.g., U max =U r +a or U min =U r −a). It should be noted that other offsets may be used, and the offset used for the upper limit U max  and the lower limit U min  may be different. 
     Setting Section 
     The setting section  80  may be configured to set the predetermined range value U r . The setting section  80  also may be configured to set the predetermined control range of the limiter  50 . For example, the setting section  80  may set the value of the upper limit U max  and the lower limit U min , and their associated offset(s) with respect to the predetermined range value U r . 
     To set a suitable range value U r , the setting section  80  may first monitor and detect the output of the limiter  50 . The setting section  80  may determine whether the detected output is sufficient to stabilize the controlled device in a steady state. If the detected output is sufficient to stabilize the controlled device in a steady state, the setting section  80  may set the detected output as the predetermined range value U r , or a center value of the control range of limiter  50 . By setting the range value U r  as the center value of the limiter&#39;s control range while the controlled device is in a steady state, it is possible to maintain the controller  100  in a stable operation. This configuration also allows the controller  100  to provide the controlled device with a larger margin β of a stable region through which the controlled device may operate, as will be discussed in greater detail with respect to  FIG. 9 . 
     Adjusting Section 
     In some implementation, the control value U in  may be fed to the adjusting section  40 . The adjusting section  40  may be configured to adjust the control value U in , generated by the control section  10  based on the difference between the control value U in  and the regulated value U out . In some implementations, if the control value U in  is within the control range of the limiter  50 , the control value U in  may not be adjusted. Because no adjustment has been performed, the control value U in  and the regulated value U out  may substantially be identical. In other implementations, if the control value U in  is outside the control range of the limiter  50 , the adjusting section  40  may perform adjustment to the control value U in  so as to minimize the difference between the control value U in  and the regulated value U out . 
     The adjusting section  40  may include a fourth adder  42  and a feedback coefficient multiplier  44 . The fourth adder  42  subtracts the control value U in  input to the limiter  50  from the regulated value U out  output by the limiter  50 , and outputs a difference value. Subsequently, the feedback coefficient multiplier  44  multiplies the difference value by a preset feedback coefficient K aw  to generate an adjustment value and supplies the adjustment value to the third adder  26 . This process allows the adjusting section  40  to add the adjustment value (e.g., K aw ×(U out −U in )) to the control value U in  to be generated by the control section  10  in the next operating cycle. With this feedback configuration, it is possible to minimize undershoot or overshoot which may occur when the control value U in  reaches saturation. 
     State Control Section 
     The state control section  60  controls the controlled device based on the regulated value U out  output by the limiter  50 . The state control section  60  may include a digital-analog converter which converts the regulated value U out  to an analog voltage value. The state control section  60  also may include a driver circuit (not shown) which generates a voltage or a current to be supplied to the controlled device based on the voltage value output by the digital-analog converter. 
     Example of External Force Measurement 
       FIG. 2  is an example graph depicting an error difference ω err  relative to an external force, where the ordinate axis of  FIG. 2  represents the error difference ω err  between the physical value and the desired value of the controlled device.  FIG. 3  is an example graph depicting a voltage value being supplied to a controlled device relative to an external force, where the ordinate axis of  FIG. 3  represents the level of the voltage value being supplied to a controlled device by the controller  100 . The abscissa axis of both  FIG. 2  and  FIG. 3  represents a time at which values on the ordinate axis are measured. 
     As shown in  FIG. 2 , the controlled device may transition into a steady state if the controlled device is stabilized at ω err =0. If there is no external force (e.g., current or voltage) being applied to the controlled device, the voltage value being supplied from the controller  100  to the controlled device after the controlled device has transitioned into the steady state should essentially be 0. However, as shown in  FIG. 3 , the voltage value after the controlled device has transitioned into the steady state is not zero. Rather, the voltage value at this time corresponds to the external force being applied to the controlled device while the device is in the steady state. For example, if the external force includes a back electromotive force, the external force may be obtained based on the voltage value at that time and the value corresponding to the back electromotive force. 
     As described above, the setting section  80  may detect the control value U in  output by the limiter  50  when the controller device is in a steady state. By setting the range value U r  as a center value of the limiter&#39;s control range with an appropriate offset, the controller  100  can further achieve stability. 
     Control Range of the Limiter 
       FIG. 4  is a diagram showing one example of a control range of the limiter  50 . In  FIG. 4 , the abscissa axis represents the control value U in  supplied to the limiter  50 . The abscissa axis also represents the regulated value U out  output by the limiter  50 . As shown, the control range includes a predetermined range value U r , which is set as the midpoint of the control range. 
     In some implementations, the range value U r  in a steady state of the controlled device may be set as the center value of the limiter&#39;s control range. In other implementations, the range value U r  need not be set as the center value, and a value other than zero may be used as the center value of the limiter&#39;s control range. In yet other implementations, a value that lies within a predetermined range with respect to the range value U r  may be set as the center value of the limiter&#39;s control range, as will be described in greater detail with reference to  FIG. 14 . In yet other implementations, a value that is as close to the range value U r  but smaller than half of the range value U r  may be set as the center value of the limiter&#39;s control range. 
     Feedforward Control 
       FIG. 5  is a diagram showing an alternative configuration of the controller  100 . As shown in  FIG. 5 , the controller  100  includes a feedforward section  70  and an initial value storage section  30 . The remaining components of the controller  100  may be identical to those of the controller  100  shown in  FIG. 1 . 
     Feedforward Section 
     In some implementations, the feedforward section  70  may generate a feedforward value according to a predetermined profile. The profile may include information that provides a suitable feedforward value based on, without limitation, a physical value of the controlled device and time that has elapsed since switching. The state control section  60  may control the controlled device using the feedforward value generated by the feedforward section  70 . The feedforward section  70  may continue to output a feedforward value to the state control section  60  until a point at which the physical value of the controlled device has reached a predetermined switch value. In some implementations, the switch value may be 80% to 90% of the desired value. 
     Once the physical value of the controlled device has reached the switch value, the control section  10  may generate the control value U in . In some implementations, the control value U in  may correspond to the switch value, which may be used as the initial value X c0  of the control value U in . In other implementations, the initial value X c0  may be defined such that the feedforward value and the control value U in  are continuous. For example, the initial value X c0  may be a constant multiple of the feedforward value when the state control section  60  switches from controlling the controlled device using the feedforward value (i.e., feedforward control) to controlling the controlled device using the control value U in  (feedback control). In some implementations, the initial value X c0  may be given by [2]:
 
 X   c0   =U   0   /K   1pg −ω 0   ×K   vel   [2]
 
where U 0  represents the feedforward value at the time of switching, ω 0  represents the error difference between the physical value and the desired value of the controlled device at the time of switching (e.g., switching from feedforward control to feedback control), and K vel  represents a predetermined coefficient. In some implementations, the predetermined coefficient K vel  may be, for example, a gain when the physical value is detected. The initial value X c0 , in some implementations, may be set such that the difference between the feedforward value at the time of switching and a value obtained by multiplying the initial value X c0  by K 1pg  is smaller than a value obtained by multiplying the initial value X c0  by K 1pg . The initial value X c0  may be a value other than zero.
 
     Initial Value Storage Section 
     The initial value storage section  30  may be used to pre-store the initial value X c0 . The initial value X c0  may be determined, for example, by the user or the controller  100 . In some implementations, the second delaying section  24  may extract the initial value X c0  manually from the initial value storage section  30 , or alternatively may receive the initial value X c0  from the initial value storage section  30 . Upon receipt of the initial value X c0 , the second delaying section  24  outputs the initial value X c0  to the second adder  18 . With this configuration, the controller  100 , which employs a switch based on the feedforward control using the feedforward section  70  and the feedback control using the control section  10 , can be stabilized when switching takes place. 
       FIG. 6A  is an example graph depicting a voltage value being supplied to a controlled device relative to an external force being applied to the controller  100  shown in  FIG. 5  when the initial value X c0  is considered.  FIG. 6B  is an example graph depicting an error difference ω err  relative to an external force being applied to the controller  100  shown in  FIG. 5  when the initial value X c0  is considered.  FIG. 7A  is an example graph depicting a voltage value being supplied to a controlled device relative to an external force being applied to the controller  100  shown in  FIG. 5  when initial value X c0  is not considered.  FIG. 7B  is an example graph depicting an error difference ω err  relative to an external force being applied to the controller  100  shown in  FIG. 5  when an initial value X c0  is not considered. 
     The abscissa axis of  FIGS. 6A-6   b  and  FIGS. 7A-7B  represents the time that has elapsed since switching to the feedback control. The ordinate axis of  FIGS. 6A and 7A , and  FIGS. 6B and 7B  represents the voltage signal (DAC OUT) output by the state control section  60 , and the physical value of the controlled device (which can be expressed as ω err ), respectively. 
     As shown in  FIGS. 6A-6B , with the initial value X c0  set, the controller  100  and the controlled device operate in a stable or steady state at the time of switching. By contrast, as shown in  FIG. 7  where the initial value X c0  is not set (e.g., X c0 =0), the controller  100  and the controlled device exhibit unstableness at the time of switching. 
     Compensation Section 
       FIG. 8  is a block diagram showing an alternative configuration of the controller  100 . As shown in  FIG. 8 , the controller  100  includes a compensating section  90 . The remaining components of the controller  100  may be identical to those of the controller  100  shown in  FIG. 5 . In some implementations, the compensating section  90  may be configured to adjust the regulated value U out , (or the control value U in ) based on the temperature or operation duration time of the controlled device. In these implementations, the controller  100  may further include a detecting section (not shown) for detecting the temperature or operation time of the controlled device. 
     Based on the temperature or the operation duration time of the controlled device, the compensating section  90  also may adjust the feedback coefficient K aw  of the feedback coefficient multiplier  44 , the initial value X c0  of the initial value storage section  30  and the range value U r  of the control range of the limiter  50  accordingly. The adjustment may be performed in a manner that allows the controller  100  to maximize the margin of a stable region through which the controlled device may operate, as will be described below with reference to  FIG. 9 . 
     In some implementations, the compensating section  90  may include a table comprising a list of values for each of the feedback coefficient K aw , the initial value X c0 , and the center value U r . In these implementations, the controller  100  (or the user) can access the table to retrieve an appropriate value based on the detected temperature, operation duration time or other parameters of the controlled device. A user may add or modify the values in the table as desired. For example, the user may add one or more compensation values to the table suitable for compensating fluctuations of the parameters of the controlled device. The parameters may include, without limitation, rotation velocity, offset, temperature and operation duration time. These parameters are exemplary, and are not limiting in nature. 
     Design Support Apparatus 
       FIG. 9  is a block diagram showing one example of a design support apparatus  200 . In some implementations, the design support apparatus  200  may be used to determine the feedback coefficient K aw  of the adjusting section  40 . In some implementations, the design support apparatus  200  may generate an appropriate feedback coefficient K aw  which would maximize the margin of a stable region in which the controlled device operates. As shown in  FIG. 9 , the design support apparatus  200  includes a stable region calculating section  210 , an initial region calculating section  220 , and a gain determining section  230 . 
     Stable Region Calculating Section 
     The stable region calculating section  210  receives a range value U r , an upper limit U max  and a lower limit U min  of the limiter&#39;s control range, and calculates a stable region ε(P). The stable region ε(P) may include a range of parameters including a control value X c  and a physical value within which the controlled device can operate stably based on the range value U r  the upper limit value U max , and the lower limit value U min . In some implementations, the control value X c  and a physical value also may be determined based on an initial value X c0  set for the control section  10 , in addition to the range value U r , the upper limit value U max , and the lower limit value U min . The stable region calculating section  210  may generate a state equation of the controller  100  and the controlled device (as will be explained in greater detail with respect to equation [9]. 
     In the following description, the error difference ω err  between the rotation velocity ω of a spindle of a hard disk and its desired value, and the current I consumed by the spindle will be used as examples of physical values of controlled device. However, one of ordinary skill in the art would readily appreciate that these parameters are not limiting in nature, and other parameters also are applicable. 
     In some implementations, the stable region ε(P) may be represented as a function whose variables include parameters (X c , ω err , I) indicating a state of the controlled device. In these implementation, the stable region ε(P) may be defined as a region where the state of the controlled device converges to a certain state when the controlled device is in this region. 
     The coefficient of each parameter of the function representing the stable region ε(P) may be defined based on the feedback coefficient K aw . In some implementations, if the feedback coefficient K aw  is changed, the region enclosed by the stable region ε(P) also changes in an n-dimensional space defined by n axes of parameters. The stable region calculating section  210  may determine the function of the stable region ε(P) based equation [20] as will be discussed in greater detail below. 
     Initial Region Calculating Section 
     The initial region calculating section  220  may store one or more initial values of a current, rotation velocity, and control value at the time of switching, and calculates an initial region Ξ 0  based on these values. The initial region Ξ 0  may include a range of values suitable for extraction when the controller switches from feedforward control to feedback control. For example, the initial region calculating section  220  may calculate a current range −i 0 &lt;I&lt;i 0  (where −i 0  is the lower limit and i 0  is the upper limit of the current I), a rotation velocity range −ω 0 ≦ω≦ω 0  (where −ω 0  is the lower limit and ω 0  is the upper limit of the rotation velocity ω), and an initial region range −X c0 &lt;X c &lt;X c0  (where −X c0  is the lower limit and X c0  is the upper limit of an initial value X c ). In some implementations, the ranges −i 0 &lt;I&lt;i 0 , −ω 0 &lt;ω&lt;ω 0  and −X c0 &lt;X c &lt;X c0  may be used to describe the initial region Ξ 0 , as will be described later with respect equation [33]. 
     Gain Determining Section 
     The gain determining section  230  may be used to determine the feedback coefficient K aw  of the adjusting section  40  such that the stable region ε(P) may include the initial region Ξ 0 , and the margin β of the stable region ε(P) with respect to the initial region Ξ 0  can be maximized. In some implementations, determining the feedback coefficient K aw  may include maximizing the margin β such that β×Ξ 0  lies within the stable region ε(P). By maximizing the margin β of the stable region ε(P), the controller  100  may be optimized to achieve stability and efficiency. The process for determining the feedback coefficient K aw  which maximizes the margin β of the stable region ε(P) will be described with respect to, for example, equations [1]-[33]. 
       FIG. 10A  is a diagram showing an example of a stable region ε(P), and  FIG. 10B  is a diagram showing an example of an initial region Ξ 0 . As described above, the stable region ε(P) and the initial region Ξ 0  may be represented as a function with variables that include, without limitation, the initial value X c , error difference ω err , and current I. In some implementations, the stable region ε(P) may be expressed as an n-dimensional ellipse. For example, as shown in  FIG. 10A , the stable region Ξ(P) may be expressed as a three-dimensional ellipse. Similarly, as shown in  FIG. 10B , the initial region Ξ 0  may be expressed as an n-dimensional rectangular parallelepiped. 
       FIG. 11A  shows an example projection view of a stable region ε(P) projected on a ω err -I plane,  FIG. 11B  shows an example projection view of a stable region ε(P) projected on a X c -ω err  plane, and  FIG. 11C  shows an example projection view of a stable region ε(P) projected on a X c -I plane. 
     As shown in  FIGS. 11A-11C , the gain determining section  230  may determine a feedback coefficient K aw  which maximizes the margin β. In some implementations, determining a feedback coefficient K aw  includes determining a feedback coefficient K aw  such that each vertex, represented by β×Ξ 0 , may situate within the stable region ε(P). 
     The process of determining the feedback coefficient K aw  which maximizes the margin β will now be described. An error system is a system that defines the difference between the original dynamics of a system and the dynamics of the system in its steady state. In some implementations, the error system may be defined using a state equation and an output equation, which may be given by [3] and [4]:
 
 x   p   [k+ 1 ]−x   pr   =A   P ( x   p   [k]−x   pr )+ B   p sat( v   c   [k]−v   t )  [3]
 
 y[k]−r=C   p ( x   p   [k]−x   pr )  [4]
 
where k indicates a k-th sample, x p  [k] indicates a state vector of the controlled device represented by an n p -dimensional real-number matrix, v c  [k] indicates an input vector represented by an m-dimensional real-number matrix (this input vector may function as the control signal output by the controller  100 ), y[k] indicates an output vector represented by a p-dimensional real-number matrix (this output vector may function as a physical value of the controlled device), v r  indicates a vector of an external force represented by an m-dimensional real-number matrix, x pr  indicates a state vector of the controlled device in its steady state represented by the n p -dimensional real-number matrix, r indicates an output vector in a steady state represented by the p-dimensional real-number matrix (this output vector may function the desired value of the physical value), and A p , B p , and C p  each indicate a constant matrix. The function sat(v) may be defined by [5]:
 
     
       
         
           
             
               
                 
                   
                     sat 
                     ⁡ 
                     
                       ( 
                       v 
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             - 
                             
                               μ 
                               0 
                             
                           
                         
                         
                           for 
                         
                         
                           
                             v 
                             &lt; 
                             
                               - 
                               
                                 μ 
                                 0 
                               
                             
                           
                         
                       
                       
                         
                           v 
                         
                         
                           for 
                         
                         
                           
                             
                               - 
                               
                                 μ 
                                 0 
                               
                             
                             &lt; 
                             v 
                             &lt; 
                             
                               μ 
                               0 
                             
                           
                         
                       
                       
                         
                           
                             μ 
                             0 
                           
                         
                         
                           for 
                         
                         
                           
                             
                               μ 
                               0 
                             
                             &lt; 
                             v 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   5 
                   ] 
                 
               
             
           
         
       
     
     The state equation and output equation of the error system of the controller  100  may be given by [6] and [7]:
 
 x   c   [k+ 1 ]−x   cr   =A   c ( x   c   [k]−x   cr )+ B   c ( r−y[k ])+ E   c ( sat ( v   c ( t )− v   r )−( v   c ( t )− v   r ))  [6]
 
 v   c   [k]−v   r   =C   c ( x   c   [k]−x   cr )+ D   c ( r−y[k ])  [7]
 
where x c [k] indicates a state vector of the controller  100  represented by an n p -dimensional real-number matrix, v c [k] indicates an output vector represented by an m-dimensional real-number matrix, x cr  indicates a state vector of the controller  100  in its steady state represented by an n c -dimensional real-number matrix, E c  indicates a matrix representing a feedback coefficient K aw , and A c , B c , C c , and D c  each indicate a constant matrix. The function sat(v c (k)−v r ) may be defined by [8].
 
     
       
         
           
             
               
                 
                   
                     sat 
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             v 
                             c 
                           
                           ⁡ 
                           
                             [ 
                             k 
                             ] 
                           
                         
                         - 
                         
                           v 
                           r 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             - 
                             
                               μ 
                               0 
                             
                           
                         
                         
                           for 
                         
                         
                           
                             
                               
                                 
                                   
                                     v 
                                     c 
                                   
                                   ⁡ 
                                   
                                     [ 
                                     k 
                                     ] 
                                   
                                 
                                 - 
                                 
                                   v 
                                   r 
                                 
                               
                               &lt; 
                               
                                 
                                   U 
                                   min 
                                 
                                 - 
                                 
                                   v 
                                   r 
                                 
                               
                             
                             = 
                             
                               - 
                               
                                 μ 
                                 0 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                               
                                 v 
                                 c 
                               
                               ⁡ 
                               
                                 [ 
                                 k 
                                 ] 
                               
                             
                             - 
                             
                               v 
                               r 
                             
                           
                         
                         
                           for 
                         
                         
                           
                             
                               
                                 - 
                                 
                                   μ 
                                   0 
                                 
                               
                               &lt; 
                               
                                 
                                   
                                     v 
                                     c 
                                   
                                   ⁡ 
                                   
                                     [ 
                                     k 
                                     ] 
                                   
                                 
                                 - 
                                 
                                   v 
                                   r 
                                 
                               
                               &lt; 
                               
                                 
                                   U 
                                   max 
                                 
                                 - 
                                 
                                   v 
                                   r 
                                 
                               
                             
                             = 
                             
                               μ 
                               0 
                             
                           
                         
                       
                       
                         
                           
                             μ 
                             0 
                           
                         
                         
                           for 
                         
                         
                           
                             
                               μ 
                               0 
                             
                             &lt; 
                             
                               
                                 v 
                                 c 
                               
                               ⁡ 
                               
                                 [ 
                                 k 
                                 ] 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   8 
                   ] 
                 
               
             
           
         
       
     
     A state equation of a system obtained by merging the controller  100  and the controlled device may therefore be obtained from [3], [4], [6] and [7] to establish [9]:
 
ξ[ k+ 1 ]=Aξ[k ]−( B+RE   c )φ( Kξ[k ])  [9]
 
where:
 
     
       
         
           
             
               ξ 
               ⁡ 
               
                 [ 
                 k 
                 ] 
               
             
             = 
             
               
                 [ 
                 
                   
                     
                       
                         
                           
                             x 
                             p 
                           
                           ⁡ 
                           
                             [ 
                             k 
                             ] 
                           
                         
                         - 
                         
                           x 
                           pr 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             x 
                             c 
                           
                           ⁡ 
                           
                             [ 
                             k 
                             ] 
                           
                         
                         - 
                         
                           x 
                           cr 
                         
                       
                     
                   
                 
                 ] 
               
               ⊆ 
               
                 
                   R 
                   
                     
                       n 
                       p 
                     
                     + 
                     
                       n 
                       c 
                     
                   
                 
                 : 
                 
                   extended 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   state 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   vector 
                 
               
             
           
         
       
         
         
           
             φ( v )=v−sat(v): dead zone function 
           
         
       
    
     
       
         
           
             A 
             = 
             
               [ 
               
                 
                   
                     
                       
                         A 
                         p 
                       
                       - 
                       
                         
                           B 
                           p 
                         
                         ⁢ 
                         
                           D 
                           c 
                         
                         ⁢ 
                         
                           C 
                           p 
                         
                       
                     
                   
                   
                     
                       
                         B 
                         p 
                       
                       ⁢ 
                       
                         C 
                         c 
                       
                     
                   
                 
                 
                   
                     
                       
                         - 
                         
                           B 
                           c 
                         
                       
                       ⁢ 
                       
                         C 
                         p 
                       
                     
                   
                   
                     
                       A 
                       c 
                     
                   
                 
               
               ] 
             
           
         
       
     
     
       
         
           
             B 
             = 
             
               [ 
               
                 
                   
                     
                       B 
                       p 
                     
                   
                 
                 
                   
                     0 
                   
                 
               
               ] 
             
           
         
       
     
     
       
         
           
             R 
             = 
             
               [ 
               
                 
                   
                     0 
                   
                 
                 
                   
                     
                       I 
                       
                         n 
                         c 
                       
                     
                   
                 
               
               ] 
             
           
         
       
         
         
           
             K=[−D c C p C c ] 
           
         
       
    
     A matrix G may be defined as [10]:
 
 G ⊂ R   mx(n     p     +n     c     )   [10]
 
     A polyhedron J also may be defined as [11]:
 
 J={ξ ⊂ R   n     p     +n   c ;−μ 0 ≦( K−G )ξ≦μ 0 }  [11]
 
     Establishing [12]:
 
ξ ⊂   J   [12]
 
     a positive constant matrix T may be given by [13]:
 
φ( K ξ)′ T [φ( K ξ)− Gξ]≦ 0  [13]
 
     where T may be defined as [14]:
 
 T ⊂ R   m×m   [14]
 
     Establishing the following relationship [15]:
 
 G=YP  
 
 Y ⊂ R   m×(n     p     +n     c     )   [15]
 
     the term μ 0   2  may be defined as [16]:
 
μ 0   2 &gt;ξ′( K′−W   −1   Y ′)( K−YW   −1 )ξ  [16]
 
where the following definition [17] is used:
 
 P=W   −1   [17]
 
     A linear matrix inequality (LMI) problem may be assumed, which may be defined by the following inequality [18]: 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           W 
                         
                         
                           
                             
                               WK 
                               ′ 
                             
                             - 
                             
                               Y 
                               ′ 
                             
                           
                         
                       
                       
                         
                           
                             KW 
                             - 
                             Y 
                           
                         
                         
                           
                             μ 
                             0 
                             2 
                           
                         
                       
                     
                     ] 
                   
                   ≥ 
                   0 
                 
               
               
                 
                   [ 
                   18 
                   ] 
                 
               
             
           
         
       
     
     Based on a Schur&#39;s complementary matrix, the following inequality [19] may be defined as:
 
ξ′ W   −1 ξ·μ 0   2 &gt;ξ′( K′−W   −1   Y ′)( K−YW   −1 )ξ  [19]
 
     An ellipse ε(P) also may be assumed, which may be represented by [20]:
 
ε( P )={ξ ⊂ R n     p     +n     c     ;ξ′Pξ≦ 1}  [20]
 
     If the inequality [18] is satisfied, the ellipse ε(P) defined in equation [20] may be included in the polyhedron J defined by equation [11] based on equation [16] and equation [19]. The stable region calculating section  210  may calculate the stable region ε(P) based on equation [20]. 
     A symmetric positive matrix P will now be assumed, which may be defined as [21]:
 
 P ⊂ R   (n     P     +n     c     )×(n     p     +n     c     )   [21]
 
     Further, a quadratic Lyapunov function given by the equation [22] will be assumed:
 
 Vξ[k ])=ξ[ k]′Pξ[k]   [22]
 
     The variation of the function defined in equation [22] may be represented using equation [9] to create [23]:
 
Δ V (ξ[ k ])= V (ξ[ k ])− V (ξ[ k+ 1])=ξ[ k]′Pξ[k]−ξ[k ]′( A′PA )ξ[ k] 2 ξ[k]′A′P ( B+RE   e )φ( Kξ[k ])−φ( Kξ[k ])′( B+RE   c ) P ( B+RE   c )φ( Kξ[k ])  [23]
 
     Using equation [13] and equation [23], the following inequality [24] may be defined:
 
Δ V (ξ[ k ])≧ξ[ k]′Pξ[k]−ξ[k ]′( A′PA )ξ[ k]+ 2 ξ[k]′A′P ( B+RE   c )φ( Kξ[k ])−φ( Kξ[k ])′( B+RE   c )′ P ( B+RE   c )φ( Kξ[k ])+2φ( kξ[K ])′ t [φ( kξ[K ])− gξ[K]]   [24]
 
     Based on equation [23] and equation [24], the following relationship may be established: 
                     Δ   ⁢           ⁢     V   ⁡     (     ξ   ⁡     [   k   ]       )         ≥         [       ξ   ′     ⁢           ⁢     φ   ′       ]     ⁡     [           X   1           X   2               X   2   ′           X   3           ]       ⁡     [         ξ           φ         ]               [   25   ]               
where:
         X 1 =P−A′PA   X 2 =A′P(B+RE c )−G′T   X 3 =2T−(B+RE c )′P(B+RE c )
 
and:
 
Δ V (ξ[ k ])≧0
       

     In this case, the energy of the system represented by the state equation in equation [9] will gradually decrease along with the transition of the state, and the system will subsequently converge to a predetermined state. 
     To establish:
 
Δ V (ξ[ k ])≧0  [26]
 
     the following equation [27] also should be established from equation [25]: 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             X 
                             1 
                           
                         
                         
                           
                             X 
                             2 
                           
                         
                       
                       
                         
                           
                             X 
                             2 
                             ′ 
                           
                         
                         
                           
                             X 
                             3 
                           
                         
                       
                     
                     ] 
                   
                   ≻ 
                   0 
                 
               
               
                 
                   [ 
                   27 
                   ] 
                 
               
             
           
         
       
     
     Based on a Schur&#39;s complementary matrix, the following relationship [28] may be established from the equation [27]: 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           W 
                         
                         
                           
                             - 
                             
                               Y 
                               ′ 
                             
                           
                         
                         
                           
                             - 
                             
                               WA 
                               ′ 
                             
                           
                         
                       
                       
                         
                           
                             - 
                             Y 
                           
                         
                         
                           
                             2 
                             ⁢ 
                             S 
                           
                         
                         
                           
                             
                               SB 
                               ′ 
                             
                             + 
                             
                               
                                 Z 
                                 ′ 
                               
                               ⁢ 
                               
                                 R 
                                 ′ 
                               
                             
                           
                         
                       
                       
                         
                           
                             - 
                             AW 
                           
                         
                         
                           
                             BS 
                             + 
                             RZ 
                           
                         
                         
                           W 
                         
                       
                     
                     ] 
                   
                   ≻ 
                   0 
                 
               
               
                 
                   [ 
                   30 
                   ] 
                 
               
             
           
         
       
     
     where:
         W ⊂ R (n     p     +n     c     )×(n     p     +n     c     )      Y ⊂ R m×(n     p     +n     c     )      S ⊂ R m×m      Z ⊂ R n     c     ×m      G=YP   P=W −1      E c =ZS −1          

     From the above, if equation [28] and equation [18] are satisfied, the stable region ε(P) will include the initial region Ξ 0 . Note that the initial region Ξ 0  may be defined by [29]:
 
Ξ 0   =Co{μ   r   ⊂ R n     p     +n     c     ;r= 1,2 , . . . ,n   r }  [29]
 
     The initial region calculating section  220  may calculate the initial region Ξ 0  based on equation [29]. E c , which may be used to maximize β and represented by the following relationship, will now be assumed.
 
β·Ξ 0 ⊂ε( P )  [30]
 
     That is, E c , which satisfies the following relationship, may be obtained based on LMI. 
     
       
         
           
             
               
                 
                   
                     
                       
                           
                       
                     
                     
                       
                         
                           ( 
                           i 
                           ) 
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 W 
                               
                               
                                 
                                   - 
                                   
                                     Y 
                                     ′ 
                                   
                                 
                               
                               
                                 
                                   - 
                                   
                                     WA 
                                     ′ 
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   Y 
                                 
                               
                               
                                 
                                   2 
                                   ⁢ 
                                   S 
                                 
                               
                               
                                 
                                   
                                     SB 
                                     ′ 
                                   
                                   + 
                                   
                                     
                                       Z 
                                       ′ 
                                     
                                     ⁢ 
                                     
                                       R 
                                       ′ 
                                     
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   AW 
                                 
                               
                               
                                 
                                   BS 
                                   + 
                                   RZ 
                                 
                               
                               
                                 W 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             min 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             μ 
                           
                           
                             W 
                             , 
                             Z 
                             , 
                             S 
                             , 
                             Y 
                             , 
                             μ 
                           
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         subject_to 
                       
                     
                     
                       
                         
                           
                             ( 
                             ii 
                             ) 
                           
                           ⁡ 
                           
                             [ 
                             
                               
                                 
                                   W 
                                 
                                 
                                   
                                     
                                       W 
                                       ′ 
                                     
                                     - 
                                     
                                       Y 
                                       ′ 
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       
                                         K 
                                         ′ 
                                       
                                       ⁢ 
                                       W 
                                     
                                     - 
                                     Y 
                                   
                                 
                                 
                                   
                                     μ 
                                     0 
                                     2 
                                   
                                 
                               
                             
                             ] 
                           
                         
                         ≥ 
                         0 
                       
                     
                   
                   
                     
                       
                           
                       
                     
                     
                       
                         
                           
                             
                               
                                 ( 
                                 iii 
                                 ) 
                               
                               ⁡ 
                               
                                 [ 
                                 
                                   
                                     
                                       μ 
                                     
                                     
                                       
                                         η 
                                         r 
                                         ′ 
                                       
                                     
                                   
                                   
                                     
                                       
                                         η 
                                         r 
                                       
                                     
                                     
                                       W 
                                     
                                   
                                 
                                 ] 
                               
                             
                             ≥ 
                             
                               0 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               r 
                             
                           
                           = 
                           1 
                         
                         , 
                         2 
                         , 
                         … 
                         ⁢ 
                         
                             
                         
                         , 
                         
                           n 
                           r 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   32 
                   ] 
                 
               
             
           
         
       
     
     where:
 
β=1/√{square root over (μ)}
 
     The gain determining section  230  may calculate the feedback coefficient K aw  based on equation [32]. By the process described above, the feedback coefficient K aw (E c ), which maximizes the margin (β) of the stable region ε(P), can be obtained. 
       FIG. 12  is a flowchart showing an example process performed by, for example, the design support apparatus  200 . As shown, the design support apparatus  200  estimates an external force applied to the controlled device while the device is in a steady state (S 300 ). For example, the design support apparatus  200  measures the regulated value U out  output by the limiter  50  as the controlled device enters the steady state, as described with respect to  FIG. 3 . 
     Next, the design support apparatus  200  sets the control range of the limiter  50  based on the estimation performed in step S 300  (S 302 ). The design support apparatus  200  may set the range value U r  as the center value Ur of the control range as well as the values for the upper limit U max , the lower limit U min , as described with respect to  FIG. 4 . 
     The design support apparatus  200  sets the initial value X c0  of the control value U in  generated by the control section  10  when the controller  100  switches from feedforward control to feedback control (S 304 ). In some implementations, the design support apparatus  200  may set the feedforward value output by the feedforward section  70  at the time of switching as the initial value X c0 , as described with respect to  FIG. 5 . 
     Subsequently, the initial region calculating section  220  calculates the initial region Ξ 0  (S 306 ). In some implementations, the initial region calculating section  220  may calculate the initial region Ξ 0  based on equation [29]. For example, the initial region Ξ 0  calculated by the initial region calculating section  220  may be represented by [33]: 
     
       
         
           
             
               
                 
                   
                     Ξ 
                     0 
                   
                   = 
                   
                     Co 
                     ⁢ 
                     
                       { 
                       
                         
                           { 
                           
                             
                               
                                 
                                   + 
                                   
                                     i 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   + 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   + 
                                   
                                     X 
                                     c0 
                                   
                                 
                               
                             
                           
                           } 
                         
                         , 
                         
                           { 
                           
                             
                               
                                 
                                   - 
                                   
                                     i 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   + 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   + 
                                   
                                     X 
                                     c0 
                                   
                                 
                               
                             
                           
                           } 
                         
                         , 
                         
                           { 
                           
                             
                               
                                 
                                   + 
                                   
                                     i 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   + 
                                   
                                     X 
                                     c0 
                                   
                                 
                               
                             
                           
                           } 
                         
                         , 
                         
                           { 
                           
                             
                               
                                 
                                   + 
                                   
                                     i 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   + 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   
                                     X 
                                     c0 
                                   
                                 
                               
                             
                           
                           } 
                         
                         , 
                         
                           { 
                           
                             
                               
                                 
                                   - 
                                   
                                     i 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   + 
                                   
                                     X 
                                     c0 
                                   
                                 
                               
                             
                           
                           } 
                         
                         , 
                         
                           { 
                           
                             
                               
                                 
                                   + 
                                   
                                     i 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   
                                     X 
                                     c0 
                                   
                                 
                               
                             
                           
                           } 
                         
                         , 
                         
                           { 
                           
                             
                               
                                 
                                   - 
                                   
                                     i 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   + 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   
                                     X 
                                     c0 
                                   
                                 
                               
                             
                           
                           } 
                         
                         , 
                         
                           { 
                           
                             
                               
                                 
                                   - 
                                   
                                     i 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                             
                               
                                 
                                   - 
                                   
                                     X 
                                     c0 
                                   
                                 
                               
                             
                           
                           } 
                         
                       
                       } 
                     
                   
                 
               
               
                 
                   [ 
                   33 
                   ] 
                 
               
             
           
         
       
     
     where:
 
 i   0 =1.2 i*   0 ,ω 0 =1.2ω* 0   ,X   c0 =1.2 X*   c0   [34]
 
     The following expression [35] may be used to indicate the initial values of the current consumed by the spindle of the hard disk, the rotation velocity of the spindle, and the control value generated by the control section  10 , when the controller  100  switches to feedback control:
 
 i*   0 ,ω* 0   ,X*   c0   [35]
 
     Next, the gain determining section  230  calculates the feedback coefficient K aw  (S 308 ). The gain determining section  230  may calculate the feedback coefficient K aw  based on E c  which can be determined via equation [32]. Using the calculated E c , the gain determining section  230  may calculate the feedback coefficient K aw  based on the following equation [36]:
 
 K   aw   =E   c   /K   lpg   [36]
 
     Next, the gain determining section  230  checks whether the stable region ε(P) defined by the feedback coefficient K aw  includes the initial region Ξ 0  (S 310 ). The gain determining section  230  may alternatively check whether the margin β of the stable region ε(P) with respect to the initial region □ 0  (which in one implementation corresponds to the calculated feedback coefficient K aw ) is larger than a predetermined value. In some implementations, determining whether the stable region ε(P) includes the initial region Ξ 0  includes determining whether β is larger than 1. 
     Where the stable region ε(P) includes the initial region Ξ 0 , in some implementations, the gain determining section  230  may utilize the calculated feedback coefficient K aw  as the coefficient for the feedback coefficient multiplier  44 , and complete the process. Thereafter, the gain determining section  230  sets the initial value X c0 , the center value U r , the upper limit U max , and lower limit U min  of the limiter&#39;s control range as the initial parameters for the controller  100 . 
     Where the stable region ε(P) does not include the initial region Ξ 0 , in some implementations, the design support apparatus  200  modifies the initial value X c0 , the center value U r , the upper limit U max , and lower limit U min  of the limiter&#39;s control range, and repeats the process beginning from step S 302 . For example, where the margin  3  corresponding to the feedback coefficient K aw  calculated at step S 308  is smaller than 1, the design support apparatus  200  can modify the initial value X c0 , and repeats the process shown in  FIG. 12 . 
     If the upper limit U max , and lower limit U min  of the control range are changed, the range enclosed by the stable region ε(P) may be affected as shown by equations [3], [4], [6], [7], [9], [11], [13], [16], [18] and [19]. Thus, in some implementations, before repeating the process, the design support apparatus  200  may re-set the upper limit U max , and lower limit U min  such that the difference between the upper limit U max , and lower limit U min  becomes a larger value than the difference before re-setting so that the region enclosed by the stable region ε(P) may become larger. 
     Further, if the initial value X c0  is changed, the initial region Ξ 0  also may change as shown by equations [29] and [30]. Thus, in some implementations, before repeating the process, the design support apparatus  200  may re-set the initial value X c0  to be a value closer to the desired value, so that the initial region Ξ 0  may become smaller. 
     Alternative Configuration of Design Support Apparatus 
       FIG. 13  is a diagram showing an alternative configuration of the design support apparatus  200 . As shown, the design support apparatus  200  further includes a range determining section  240  and an initial value determining section  250 . The remaining components may be essentially identical to those shown in  FIG. 9 . 
     Range Determining Section 
     The range determining section  240  may be configured to determine the control range of the limiter  50 . As described with respect to  FIG. 12 , the range determining section  240  may be used to re-set the upper limit U max , and lower limit U min  of the control range when the margin β corresponding to the feedback coefficient K aw  (generated by the gain determining section  230 ) is smaller than a predetermined value. For example, the range determining section  240  may initially supply a predetermined value, and change the upper limit U max , and lower limit U min  when the margin β is smaller than the predetermined value. Further, a variable range within which the upper limit U max , and lower limit U min  can be changed may be set in advance by the range determining section  240 . In some implementations, the range determining section  240  may modify the upper limit U max , and lower limit U min  such that the upper limit U max , and lower limit U min  are equidistant from the center value U r . 
     Initial Value Determining Section 
     The initial value determining section  250  may be configured to determine the initial value X c0  of the control section  10 . As described with respect to  FIG. 12 , the initial value determining section  250 , in some implementations, may be used to re-set the initial value X c0  of the control value U in  when the margin β corresponding to the feedback coefficient K aw  (generated by the gain determining section  230 ) is smaller than a predetermined value. For example, the initial value determining section  250  may initially identify the predetermined value, and change the initial value X c0  when the margin β is smaller than the predetermined value. Further, a variable range within which the initial value X c0  can be changed may be set in advance by the initial value determining section  250 . 
     In some implementations, the range determining section  240  and the initial value determining section  250  may simultaneously modify associated settings to change the control range (U max , U min ) and the initial value X c0  when the margin β is smaller than a predetermined value. In these implementations, a priority scheme may be employed for prioritizing such modifications. For example, the range determining section  240  may effectuate changes to the control range (U max , U min ) before the initial value determining section  250  modifies the initial value X c0 . As another example, if the initial value X c0  has a greater margin than that of the control range (U max , U min ), the initial value determining section  250  may modify its associated setting before the range determining section  240  changes its associated setting. 
     The design support apparatus  200  can set the feedback coefficient K aw  of the controller  100  described with respect to  FIGS. 1 to 8  to an optimum value. Where there is no feedback coefficient K aw  available that satisfies a given margin condition with a particular control range (U max , U min ) and the initial value X c0 , the design support apparatus  200 , in some implementations, may re-set these parameters to appropriate values to allow for an optimum feedback coefficient K aw . 
       FIG. 14  shows an example flowchart for setting a center value of the control range of the limiter  50 . Hereinafter, the control value U r  in the steady state of the controlled device will be referred to as an ideal value. Also, the parameter as described in  FIG. 12  may be set such that the margin β of the stable region with respect to the initial region Ξ 0  is at maximum (β=β max ). Where a small margin β is tolerated (e.g., βp&lt;β max ), the design support apparatus  200  may set a center value within a range corresponding to this tolerated value βp. This tolerated value βp may be obtained, for example, from the specification of the controlled device. Alternatively, the tolerated value  13   p  may be obtained based on one or more operating parameters associated with the controlled device. 
     Referring to  FIG. 14 , the design support apparatus  200  calculates a width d max  of the control range associated with a maximum margin β when the ideal value is set as the center value of the control range (S 320 ). For example, the design support apparatus  200  may successively change the width d max  of the control range and calculate the stable region ε(P) for each width of the control range. Then, the design support apparatus  200  calculates the margin β of each calculated stable region ε(P) with respect to the initial region Ξ 0 , and identifies the largest β as d max . 
     In some implementations, the feedback coefficient used for calculating the stable region ε(P) may be calculated in advance by the design support apparatus  200  (or may be determined by user). For example, the design support apparatus  200  may calculate the feedback coefficient used in step S 320  by performing the process described in  FIG. 12  using the range value U r  and the width d max  of the control range. 
     The design support apparatus  200  obtains a tolerated value βp of the margin β (S 322 ). Again, the tolerated value βp may, for example, be obtained from the specification of the controlled device. Alternatively, the tolerated value βp may be obtained based on one or more operating parameters associated with the controlled device. In some implementations, the design support apparatus  200  may be given a tolerated value βp whose absolute value is smaller than the maximum margin β max . 
     The design support apparatus  200  calculates a width d p  of the control range which corresponds to the tolerated value Pp when the ideal value is set as the center value of the control range (S 324 ). For example, the design support apparatus  200  may successively change the width of the control range and calculate the stable region ε(P) for each width of the control range. Subsequently, the design support apparatus  200  may calculate the margin of each calculated stable region ε(P) with respect to the initial region Ξ 0 , and identify the width that corresponds to the tolerated value βp as the width d p . 
     The design support apparatus  200  sets the ideal value (range value U r ) as the center value of the control range of the limiter  50  and which corresponds to the difference between d max  and d p , (S 326 ). For example, assuming that L=d max −d p , the range determining section  240  may set the center value within a range of Ur−L to Ur+L. Of course, the ideal value need not be set as the center value but one which is bound within the range. In such a case, the center value may be determined based on the range defined by the margin γ. 
     While this specification contains many specifics, these should not be construed as limitations on the scope of what being claims or of what may be claimed, but rather as descriptions of features specific to particular embodiments. For example, the above descriptions also are applicable for controlling the controlled device where an output to the controlled device has reached saturation. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understand as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the scope of the invention.