Control method and apparatus using control gain variable with estimated operation of control object

A control method and apparatus which detects a solenoid current as a voltage, and PWM-controls a transistor for a solenoid such that the detection voltage becomes a target voltage. A feedback gain, used for setting PWM signal output time, is set in accordance with the difference between the detection voltage and the target voltage such that the greater the difference becomes, the greater the feedback gain becomes. Further, a detection voltage after a predetermined period is estimated based on the change of detection voltage from the past. If the estimation value overshoots the target voltage, a difference B between the detection voltage and the target voltage and a difference C between the detection voltage and the estimation value are obtained, and the feedback gain is varied by multiplying the feedback gain by the ratio B/C between the these differences.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a control apparatus and method which
 detects operation status of a control object and feedback-controls the
 control object such that the operation status becomes as desired.
 2. Description of Related Art
 A conventional control apparatus detects an operation status of a control
 object by using a sensor or the like, calculates a control amount for
 controlling the control object by using the difference between the
 detected actual operation status and desired status and a predetermined
 control gain (i.e., feedback gain), and feedback-controls the control
 object by the calculated control amount.
 In this control apparatus, when the difference between the actual operation
 status of the control object and the desired status is large, the control
 object can be quickly changed toward the desired status by increasing the
 control gain. This improves the control response. However, when the actual
 operation status is close to the desired status, overshoot or undershoot
 (collectively referred to as overshoot) that the actual operation status
 exceeds the desired status occurs. It takes a significant amount of time
 to converge the operation status to the desired status. On the other hand,
 if the control gain is reduced, the overshoot problem can be prevented, to
 improve the control stability. However, the control response is lowered.
 To overcome those problems, for example, in a constant-speed running
 apparatus which feedback-controls the running speed of a vehicle to a
 target speed, a running resistance (i.e., external load on a control
 object) is determined from an acceleration or the like of the vehicle as
 the control object, and a control gain is varied with the determined
 running resistance to prevent overshoot (U.S. Pat. No. 5,392,215 and JP-A
 6-64461). At the beginning of overshoot, the control gain is changed to
 improve the control convergence after the occurrence of overshoot (JP-A
 3-22734). Overshoot is prevented by setting the control gain in accordance
 with the vehicle acceleration upon start of control (U.S. Pat. No.
 4,725,969 and JP-A 60-71341).
 Further, other controls than those in the constant-speed running device
 have been proposed. For example, in idle rotation control and the like to
 control an idle rotation speed of an internal combustion engine to a
 target idle rotation speed, overshoot can be prevented by setting a
 control gain in accordance with the difference between the desired status
 (target idle rotation speed) and actual operation status (actual idle
 rotation speed).
 That is, in the conventional control apparatuses, to prevent overshoot
 while ensuring control response, the control gain is set based on an
 external load on the control object or the operation of the control object
 upon start of control, or based on the difference between the desired
 status and the actual operation status during control.
 However, as these conventional apparatuses set the control gain from
 operation status or the like of the control object, requirements for both
 the improved control response and control stability cannot be sufficiently
 satisfied.
 SUMMARY OF THE INVENTION
 The present invention has an object to provide a control method and
 apparatus which feedback-control a control object to a desired status and
 more reliably prevents occurrence of overshoot while ensuring control
 response.
 To attain the above object, according to the present invention, an actual
 operation status of the control object is detected, and a control amount
 to control the control object to the desired status is calculated based on
 the difference between the detected actual operation status and the
 desired status and a control gain. Further, an operation status of the
 control object after a predetermined period is estimated based on the
 change of the detected actual operation status from the past to the
 present. It is determined whether the estimated operation status is in
 overshoot status that the estimated operation status exceeds the desired
 status from the present actual operation status. If it is determined that
 the estimated operation status is in the overshoot status, the control
 gain is varied to a value less than a normal value.
 That is, when the control object is feedback-controlled in accordance with
 the calculated control amount, the operation status after a predetermined
 period is estimated from the present status of the control object. If the
 estimated operation status overshoots (or undershoots) the desired status,
 the control gain is varied to a small value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring first to FIG. 1, an ECU (electronic control unit) 2 is a control
 apparatus for a vehicle automatic transmission control (AT control) to
 control transmission gears, lock-up and the like of an automatic
 transmission mounted on a vehicle. The ECU 2 has a microcomputer
 comprising a CPU, a ROM, a RAM and the like, as a control circuit 10 which
 performs various calculation processings for AT control. The control
 circuit 10 receives input signals from various sensors such as a vehicle
 speed sensor and a shift position sensor, and an engine status signal (a
 signal indicating rotation, throttle opening angle and the like) from an
 engine control device. The control circuit 10 is programmed to perform the
 various calculation processings for AT control based on the input signals,
 in accordance with a control program stored in the ROM. Based on the
 calculation results, it controls various actuators such as a solenoid
 provided in the automatic transmission.
 In the present embodiment, the control circuit 10 feedback-controls a
 solenoid current Ism which flows through a line-pressure control solenoid
 12 as an inductive load for line-pressure (hydraulic fluid pressure)
 control on the automatic transmission.
 A battery voltage VBT is applied to one end of the solenoid 12 as a control
 object. The ECU 2 has a transistor 14 for switching
 connection/disconnection of the other end of the solenoid 12 to the
 ground.
 The transistor 14 comprises an NPN transistor having a collector connected
 to the solenoid 12 and an emitter connected to the ground via a resistor
 16. When the transistor 14 is turned on, the current (solenoid current)
 Ism flows through the solenoid 12 via the transistor 14 and the resistor
 16. The resistor 16 detects the solenoid current Ism that flows at this
 time (i.e., when the transistor is turned on) as a voltage.
 Further, the collector of the transistor 14 is connected to the anode of a
 diode D. The cathode of the diode D is connected to the one end of the
 solenoid 12 where the battery voltage VBT is applied, via a resistor 17.
 The diode D is used for preventing high voltage from being applied to the
 collector side of the transistor 14 by energy accumulated in the solenoid
 12 when the transistor 14 is turned off. When the transistor 14 has been
 turned off, the diode continues to enable the solenoid current Ism to flow
 until the energy accumulated in the solenoid 12 is discharged. Further,
 the resistor 17 detects the solenoid current Ism that flows at this time
 (i.e., when the transistor 14 is turned off) as a current value.
 Both terminal voltages of the resistors 16 and 17 are amplified by a
 differential amplifier 18 comprising an operational amplifier, further,
 smoothed by a smoothing circuit 20 comprising an integrator or the like,
 and inputted into the control circuit 10 as a current detection voltage
 Vsm.
 Further, the control circuit 10 includes an A/D converter which A/D
 converts the detection voltage Vsm of the solenoid current Ism inputted
 from the smoothing circuit 20. The circuit 10 detects the solenoid current
 Ism from the input voltage value (detection voltage Vsm) via the A/D
 converter, and turns the transistor 14 on/off by way of pulse width
 modulation (PWM) such that the solenoid current Ism becomes the target
 current Itgt necessary for line pressure control.
 That is, as shown in FIG. 2, the control circuit 10 sets a target voltage
 Vtgt in accordance with the detection voltage vsm from the necessary
 target current Itgt to control the line pressure of the automatic
 transmission to the target fluid pressure, generates a pulse width
 modulation signal to control the detection voltage Vsm to the target
 voltage Vtgt based on the difference between the voltages Vtgt and Vsm,
 and outputs the PWM signal to the base of the transistor 14, so as to turn
 on/off the transistor 14 to control the detection voltage Vsm (i.e., the
 solenoid current Ism) to the target voltage Vtgt (i.e., the target current
 Itgt).
 In FIG. 2, the area 1 of the solenoid current Ism represents the solenoid
 current Ism detected via the resistor 16 when the PWM signal is at a High
 level and the transistor 14 is on; the area 2 represents the solenoid
 current Ism detected via the resistor 17 when the PWM signal is at a Low
 level and the transistor 14 is off.
 In this feedback control, the detection value (detection voltage Vsm) is a
 value obtained as a result of control, and delayed from an actual value
 (the current value Ism that actually flows through the solenoid 12).
 Further, especially in the present embodiment, as the solenoid current Ism
 pulsates due to the PWM control, the detection voltage Vsm is smoothed by
 the smoothing circuit 20 and inputted into the control circuit 10 so as to
 prevent pulsation of the detection voltage vsm inputted into the control
 circuit 10 as a detection value, which further delays the detection value.
 For this reason, the solenoid current (actual current) that flows through
 the solenoid 12 and the solenoid current (detection current) detectable by
 the control circuit 10 substantially correspond when the solenoid current
 Ism is stable around the target current Itgt, as shown in FIG. 3A. On the
 other hand, in the transitional period of the feedback control in which
 the solenoid current Ism follows the target current Itgt, the detection
 current and the actual current do not correspond with each other, which
 causes detection delay as shown in FIG. 3B.
 This detection delay changes in accordance with various control conditions
 and is not uniquely determined. Accordingly, if the control gain (feedback
 gain), used for setting the duty ratio of the PWM signal as a control
 amount, is set only based on the difference between the detection value
 and the target value, overshoot caused due to the above detection delay
 cannot be suppressed well. As shown in FIG. 2, even after the detection
 voltage vsm reached the target voltage from a time point t1 at which the
 target voltage Vtgt was set and the feedback control was started Vtgt, the
 detection voltage Vsm changes due to overshoot. It takes a significant
 amount of time to converge the detection voltage Vsm to the target voltage
 Vtgt.
 Accordingly, in the present embodiment, for example, it is arranged such
 that when the target current Itgt to flow through the solenoid 12 changes
 from 0 mA to 1000 mA, the solenoid current Ism reaches 90% of the target
 current Itgt in 40 msec (the control response is ensured). Further, to
 prevent overshoot that the solenoid current Ism exceeds the target current
 Itgt, a future detection voltage is estimated based on the change of the
 detection voltage Vsm from the past, and it is determined from the
 estimation result whether or not the detection voltage Vsm will overshoot.
 If it is determined that the detection voltage will overshoot, the
 feedback gain used for control is varied to a small value.
 The control circuit 10 controls the line pressure of the automatic
 transmission to a target fluid pressure based on the control program shown
 in FIGS. 4A to 4C.
 The control circuit 10 performs control amount calculation processing (FIG.
 4A) to calculate an output period (i.e., control amount) of the PWM signal
 (High level) to the transistor 14 as interrupt processing at, e.g., every
 5 msec. The control circuit 10 performs detection processing (FIG. 4B) to
 read the smoothed detection voltage Vsm, inputted from the smoothing
 circuit 20, via the A/D converter, as interrupt processing at, e.g., every
 1 msec. Further, the control circuit 10 performs target current
 calculation processing (FIG. 4C) to calculate the target current Itgt to
 flow through the solenoid 12, by using a target current calculation map
 shown in FIG. 5, based on the target fluid pressure (target line pressure)
 Ptgt calculated by control target setting processing (not shown) for AT
 control, as interrupt processing at, e.g., 25 msec.
 Further, in the control amount calculation processing, at step S110,
 target-pressure calculation processing is performed to convert the target
 current Itgt, corresponding to the target fluid pressure calculated by the
 target current calculation processing, into a voltage value (target
 voltage Vtgt) corresponding to the detection voltage Vsm read by the
 detection processing. Then, at step S120, processing as control gain
 setting means is performed to obtain the difference between the calculated
 target voltage Vtgt and the detection voltage Vsm (Vtgt -Vsm), and
 calculate a feedback gain .DELTA.T for feedback control from the voltage
 difference.
 In the feedback gain .DELTA.T calculation, a feedback gain calculation map
 having a characteristic as shown in FIG. 6 is used. The feedback gain
 .DELTA.T is set by selection or correction by using the map to be smaller
 as the difference (Vtgt-Vsm) is smaller, and to be larger as the
 difference (Vtgt-Vsm) is larger. As is apparent from the map, in the
 present embodiment, the feedback gain Dt is set to a maximum value when
 the voltage difference exceeds a predetermined upper limit value.
 Further, as the duty ratio of the PWM signal upon PWM control on the
 solenoid current is determined by adding the feedback gain .DELTA.T to the
 PWM signal output period (High level time), if the target voltage Vtgt is
 higher than the detection voltage Vsm and the difference (Vtgt-Vsm) is a
 positive value, the feedback gain .DELTA.T is set to the same positive
 value as the difference to increase the duty ratio of the PWM signal
 (i.e., the solenoid current). On the other hand, if the difference
 (Vtgt-Vsm) is a negative value, the feedback gain .DELTA.T is set to the
 same negative value as the difference to decrease the solenoid current.
 Next, when the feedback gain .DELTA.T has been set, processing is performed
 to estimate a detection voltage Vsm(i+m) after a predetermined period,
 based on the change of detection voltage Vsm from a past detection voltage
 Vsm(i-n), read a predetermined period ago (more specifically, a
 predetermined sampling period ago) by the detection processing to a
 present detection voltage Vsm(i), at step S130.
 The suffix "(i)" of the detection voltage Vsm indicates that the detection
 voltage Vsm is a latest value obtained by the detection processing. The
 suffix "(i-n)" indicates that the detection voltage Vsm is a value
 detected by detection processing n times ago. The suffix "(i+m)" indicates
 that the detection voltage Vsm is an estimation value estimated to be
 obtained when m-th voltage detection processing from the present if the
 present feedback control is continued.
 In the present embodiment, for example, as shown in FIG. 7, the detection
 voltage Vsm(i+m) to be obtained upon m-th (m=2) detection processing (at a
 time point t(i+m); e.g., after 10 msec.) is estimated by adding a
 difference A between the detection voltage Vsm(i) at the present time
 point t(i) and the detection voltage vsm(i-n) obtained by the detection
 voltage n times ago (time point t(i-n); e.g., after 10 msec.), to the
 detection voltage Vsm(i).
 Thus, when the estimation value Vsm(i+m) of the detection voltage vsm has
 been obtained, the process proceeds to step S140, at which processing is
 performed to determine whether or not the estimation value Vsm(i+m)
 exceeds the target voltage Vtgt from the present detection voltage Vsm(i)
 in overshoot (more specifically, overshoot or undershoot) status.
 If it is determined at step S140 that the estimation value is in overshoot
 status, the process proceeds to step S150 at which the feedback gain
 .DELTA.T is varied to a small value based on the result of determination.
 Then the process proceeds to step S160. The process returns and proceeds
 to step S160 until it is determined at step S140 that the estimation value
 is not in overshoot status.
 In the present embodiment, as shown in FIG. 7, a difference B between the
 detection voltage Vsm(i) at the present time point t(i) and the target
 voltage vtgt (=vtgt-vsm(i)) and a difference C between the detection
 voltage Vsm(i) at the current time point t(i) and the estimation value
 Vsm(i+m) (=Vsm(i+m)-Vsm(i)) are obtained, and the feedback gain .DELTA.T
 is multiplied by the ratio B/C between these differences B and C, so as to
 correct the feedback gain .DELTA.T to a value smaller than a normal value.
 At step S160, processing is performed to obtain the PWM signal output time
 T(i) by adding the above feedback gain .DELTA.T to the PWM signal output
 time T(i-1) obtained by previous processing, and store the obtained time
 T(i) as a control amount for line pressure control. Then, the processing
 temporarily ends.
 A timer for the PWM signal output is set to the output time T(i) in
 synchronization with the output period (e.g., 3 msec.) of the PWM signal.
 The timer outputs a signal which is at a High level during the set output
 time T(i). As a result, the transistor 14 is turned on/off in accordance
 with the PWM signal periodically outputted from the timer, to control the
 solenoid current Ism to the target current Itgt.
 In the present embodiment, as the operation status of the solenoid 12 as
 the control object, the detection voltage Vsm converted from the solenoid
 current Ism as a voltage value is read, and the transistor 14 that
 determines the solenoid current Ism is feedback-controlled such that the
 detection voltage Vsm becomes the target voltage Vtgt as desired status of
 the control object. Upon execution of feedback control, first, the
 feedback gain .DELTA.T is set in accordance with the difference between
 the detection voltage Vsm and the target voltage Vtgt such that the
 greater the difference becomes, the greater the feedback gain .DELTA.T
 becomes. Further, a detection voltage after a predetermined period is
 estimated based on the change of the detection voltage Vsm from the past,
 and it is determined whether or not the estimation value (estimation
 operation status recited in the claims) is in an overshoot status where
 the estimation value exceeds the target voltage Vtgt from the present
 detection voltage vsm. If it is determined that the estimation value is in
 the overshoot status, the feedback gain .DELTA.T is varied to a small
 value by multiplying the feedback gain .DELTA.T by the ratio B/C between
 the difference B between the present detection voltage Vsm and the target
 voltage Vtgt and the difference C between the present detection voltage
 Vsm and the estimation value.
 Accordingly, as shown in FIG. 8, overshoot can be more reliably prevented
 in comparison with a case where the feedback gain .DELTA.T is simply set
 in accordance with the difference between the detection voltage Vsm and
 the target voltage Vtgt to prevent overshoot while maintaining the control
 response.
 That is, in a case where the feedback gain .DELTA.T is set in accordance
 with the difference between the detection voltage Vsm and the target
 voltage Vtgt, if the difference is large, the feedback gain .DELTA.T is
 increased to quickly change the detection voltage Vsm toward the target
 voltage Vtgt, while if the difference is small, the feedback gain .DELTA.T
 is reduced to slowly change the detection voltage Vsm toward the target
 voltage Vtgt. In this case, occurrence of overshoot can be suppressed to a
 certain degree in comparison with a case where the feedback gain .DELTA.T
 is fixed. However, if the estimation of future detection voltage Vsm and
 determination of possibility of overshoot, as described in the present
 embodiment, are not performed, the feedback gain .DELTA.T is too large
 when the detection voltage Vsm is closer to the target voltage Vtgt,
 further, the PWM signal (High level) output time as a control amount is
 long, and as a result, the detection voltage Vsm may overshoot the target
 voltage Vtgt, as indicated by a dotted line in FIG. 8.
 However, in the present embodiment, the future detection voltage Vsm is
 estimated, and if it is determined that the estimation value in overshoot
 status, the feedback gain .DELTA.T is varied to a small value in
 correspondence with the degree of overshoot of the estimation value (i.e.,
 ratio B/C). By this arrangement, the occurrence of overshoot can be more
 excellently suppressed.
 According to the present embodiment, the control response to quickly bring
 the detection voltage Vsm close to the target voltage Vtgt and the control
 stability to quickly converge the detection voltage to the target voltage
 Vtgt if the detection voltage Vsm is closer to the target voltage Vtgt,
 can be satisfied. Further, the line pressure of the automatic transmission
 can be quickly and highly precisely controlled to the target fluid
 pressure Ptgt.
 As described above, in the present embodiment, the overshoot of the
 detection voltage Vsm from the target voltage Vtgt can be suppressed in an
 excellent fashion. However, the overshoot occurs due to the delay of
 feedback control such as detection delay from an actual detection value,
 and the delay changes due to various control conditions. Accordingly, even
 when the result of control is estimated and the feedback gain is varied as
 described above, overshoot cannot be completely prevented without
 difficulty, and overshoot may occur depending on the control conditions.
 Further, in a case where overshoot actually occurs, if the feedback gain
 .DELTA.T is varied as described above, the control after the occurrence of
 overshoot is delayed as represented by a solid line in FIG. 9, and the
 convergence of the control may be degraded.
 Accordingly, if this problem occurs, the control amount calculation
 processing shown in FIG. 4A may be performed as shown in FIG. 10.
 That is, in the control amount calculation processing in FIG. 10, at step
 S210, the target voltage Vtgt is calculated from the target current Itgt
 (conversion of Itgt to Vtgt), and at step S220, it is determined whether
 or not the detection voltage Vsm has overshot the target voltage Vtgt. If
 it is determined that the detection voltage Vsm has overshot the target
 voltage, the process proceeds to step S260 at which an estimation
 permission flag F is reset (to a value "0" ) such that the processings
 performed for subsequent feedback gain correction (i.e., the estimation of
 future detection voltage, the determination of overshoot status of the
 estimation value and the feedback gain correction) are inhibited.
 Further, if it is determined at step S220 that the detection voltage Vsm
 has not overshot the target voltage Vtgt, the process proceeds to step
 S230, at which it is determined whether or not the absolute value of the
 difference between the target voltage Vtgt(i) set at step S210 and the
 previously set target voltage Vtgt(i-1) is a predetermined value K1 or
 greater. If the difference is the predetermined value K1 or greater, as
 the target voltage Vtgt has greatly changed, feedback control must be
 newly started to control the detection voltage Vsm toward the target
 voltage Vtgt. The process proceeds to step S240 at which the estimation
 permission flag F is set to a value "1" to permit the processings
 performed for the subsequent feedback gain correction.
 On the other hand, if it is determined at step S230 that the absolute value
 of the difference between the target voltage Vtgt(i) and the target
 voltage Vtgt(i-1) is less than the predetermined value K1, as the target
 voltage Vtgt has not greatly changed, i.e., the target voltage Vtgt is the
 same as the previously set target voltage or even if it has changed, they
 are substantially the same, the process proceeds to step S250. At step
 S250, it is determined whether or not the absolute value of the difference
 between the present detection voltage Vsm and the target voltage Vtgt is a
 predetermined value K2 or less. If it is determined that the absolute
 value of the difference between the detection voltage Vsm and the target
 voltage Vtgt is the predetermined value K2 or less and the detection
 voltage Vsm (i.e., the solenoid current Ism) is substantially controlled
 to the target voltage Vtgt (i.e., the target current Itgt), the process
 proceeds to step S260 to reset the estimation permission flag F.
 If the estimation permission flag F is set or reset at step S240 or S260,
 or it is determined at step S250 that the absolute value of the difference
 between the detection voltage Vsm and the target voltage Vtgt is over the
 predetermined value K2, the process proceeds to step S270. At step S270,
 similarly to step S120 of the above embodiment, the feedback gain .DELTA.T
 is calculated from the difference between the target voltage Vtgt and the
 detection voltage Vsm (Vtgt-Vsm).
 Further, as the feedback gain .DELTA.T is calculated at step S270, the
 process proceeds to step S280, at which it is determined whether or not
 the estimation permission flag F is set. If it is determined that the
 estimation permission flag F is set, the process proceeds to step S290. At
 step S290, similar to steps S130 to S150 of the above embodiment, the
 detection voltage Vsm(i+m) after a predetermined period is estimated based
 on the present detection voltage Vsm(i) and the past detection voltage
 Vsm(i-n), then it is determined at step S300 whether or not the estimation
 result overshoots the target voltage vtgt. If it is determined that the
 estimation result overshoots the target voltage, the process proceeds to
 step S310, at which the series of processings for feedback gain .DELTA.T
 correction to change the feedback gain .DELTA.T to a small value is
 performed. Thereafter, the process proceeds to step S320.
 If it is determined at step S280 that the estimation permission flag F is
 reset, as the feedback gain .DELTA.T correction is inhibited, the series
 of processings at steps S290 to S310 is not performed, and the process
 proceeds to step S320.
 At step S320, similarly to step S160 of the above embodiment, the PWM
 signal output time T(i) is obtained by adding the feedback gain .DELTA.T
 to the previously obtained PwM signal output time T(i-1). The obtained
 output time T(i) is stored as a control amount for line pressure control,
 and the process temporarily ends.
 As described above, in the control amount calculation processing shown in
 FIG. 10, if the detection voltage Vsm has already overshot the target
 voltage Vtgt, the estimation permission flag F is reset so as to inhibit
 the feedback gain .DELTA.T correction until the target voltage Vtgt
 changes to the predetermined value K1 or greater. Accordingly, as
 represented by a dotted line in FIG. 9, the time to converge the detection
 voltage Vsm to the target voltage Vtgt after the overshoot of detection
 voltage Vsm can be reduced. Thus, the control performance is improved.
 Further, if the target voltage vtgt has changed to the predetermined value
 K1 or greater, the estimation permission flag F is set so as to remove the
 inhibition of the feedback gain .DELTA.T correction. Accordingly, if the
 detection voltage Vsm must be greatly changed toward the target voltage
 Vtgt, the detection voltage Vsm can be quickly changed toward the target
 voltage Vtgt while the detection voltage Vsm is prevented from
 overshooting the target voltage Vtgt.
 Further, in the control amount calculation processing shown in FIG. 10, if
 the target voltage Vtgt has not changed to the predetermined value K1 or
 greater, it is determined whether or not the absolute value of the
 difference between the target voltage Vtgt and the detection voltage Vsm
 is the predetermined value K2 or less (i.e., the detection voltage Vsm is
 substantially controlled to the target voltage Vtgt). Even if it is
 determined that the detection voltage Vsm is substantially controlled to
 the target voltage Vtgt, the estimation permission flag F is reset so as
 to inhibit the feedback gain .DELTA.T correction until the target voltage
 Vtgt changes to a predetermined value or greater. This prevents the
 problem of the feedback gain .DELTA.T being varied to a small value even
 though the detection voltage Vsm is controlled around the target voltage
 Vtgt and the convergence of control is lowered.
 In the control amount calculation processing shown in FIG. 10, it is to be
 noted that the estimation is inhibited by a series of processings at steps
 S220 to S260 and the processing at step S280.
 The embodiment has been described as a case where the present invention is
 applied to the method and apparatus that controls the solenoid 12 for line
 pressure control of the automatic transmission. However, the present
 invention is not limited to the above embodiment but applicable in various
 ways.
 For example, in the above embodiment, to feedback-control the current
 amount (solenoid current) to flow through the solenoid 12 as the control
 object, the feedback gain .DELTA.T, set in accordance with the difference
 between a target value and an actual value, is added to the PWM signal
 output time as a previous control amount, so as to update the control
 amount. However, even in use of an apparatus to calculate a control
 amount, if the present invention is applied to the apparatus, similar
 advantages to those in the above embodiment can be obtained by multiplying
 the control gain (feedback gain) by the difference between the target
 value and the actual value.
 Further, in the above embodiment, the feedback gain .DELTA.T is set in
 accordance with the difference between the target value and the actual
 value. However, even in use of an apparatus where the feedback gain
 .DELTA.T is fixed to a predetermined value, if the present invention is
 applied to the apparatus, occurrence of overshoot can be prevented without
 lowering the control response, and the control stability can be improved.
 As many apparently widely different embodiments of the present invention
 can be made without departing from the spirit and scope thereof, it is to
 be understood that the invention is not limited to the specific
 embodiments thereof except as defined in the appended claims.