Position control device for movable member

A position control device for controlling a position of a movable member has high durability and is free from a hunting phenomenon. Electric contacts arranged in a row at a given pitch are respectively connected to the nodes between series-connected resistors and first and second brushes interlocked with the movable member can contact the adjoining two of the electric contacts at a time such that discrete two voltage signals are generated at the first and second brushes. A pair of comparators compare these voltage signals with a voltage signal corresponding to a target position where the movable member is to be stopped and a driving means for the movable member stops its operation when the former voltage signals sandwitch the latter voltage signal. A third brush capable of contacting index terminals in response to the movement of the movable member and a pair of memorizing means capable of memorizing the outputs of the comparators each time the third brush contacts one of the index terminals are provided for minimizing the error in the position where the movable member is actually stopped.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a movable member control device for 
stopping a movable member at a position corresponding to an analog signal. 
2. Description of the Prior Art 
In order to move a movable member and stop it at a position corresponding 
to an analog signal, it is necessary that the instantaneous position of 
the movable member is converted to a signal that can be compared with the 
analog signal. A conventional device for this purpose includes a slider 
contact slidable on a resistor band or strip in a linked relationship with 
the movable member whereupon a signal representative of the position of 
the movable member is derived through the slider contact. However, such a 
conventional device is subject to the following disadvantages. As the 
slider contact is adapted to slide on a resistor strip, the latter is 
subject to wear which changes the resistance and accordingly the 
relationship between the position of the movable member and the output 
from the slider contact. In short, the conventional device has poor 
durability. Additionally, if a servomechanism is employed to continuously 
drive the movable member, a target position signal representing a target 
position at which the movable member is to be stopped is compared with a 
continuously changing signal derived from the slider contact whereby 
hunting is likely to occur, wherein the movable member oscillates about 
the target position. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a position control device 
for a movable member which device is free from the above-mentioned 
disadvantages of the conventional device, in particular the occurrence of 
hunting. 
Another object of the present invention is to provide a position control 
device for a movable member which device has good durability. 
To attain those objects, the present invention adopted the following 
construction: 
(1) The device of the invention is provided with a means for converting the 
position of a movable member into two analog signals having different 
values from each other and a means for comparing those two analog signals 
with a target position signal corresponding to a target position where the 
movable member is to be stopped. The device drives the movable member 
until the above two signals sandwich the target position signal 
therebetween, thereby eliminating the hunting discussed above. 
(2) To obtain the above-mentioned two analog signals relating to the 
position of the movable member, voltages of different values (or measures) 
are supplied to respective fixed contacts arranged in a row and a pair of 
movable contacts separated by a given distance in the direction of the row 
slidably move along the fixed contacts in a linked relationship with the 
movable member. The distance between the movable contacts is selected so 
that they may engage adjoining fixed contacts. With this arrangement, two 
analog signals different from each other are obtained depending upon the 
position of the movable contacts. The voltage signals obtained through the 
movable contacts will not change even if the fixed contacts are worn out 
by sliding contact with the movable contacts, whereby the durability of 
the device is improved. 
The above and other objects and advantages of the present invention will 
become more apparent from the following description of a preferred 
embodiment of the present invention taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENT 
In the embodiment that will be described hereinafter, the present invention 
applies to an objective lens driving device for a camera automatic 
focusing device. A signal derived from a range finding device or a 
camera-to-object distance measuring device serves as the target or stop 
position signal which represents the position where the movable member is 
stopped. The objective lens is driven with the stop signal being compared 
with electric signals corresponding to the position of the objective lens. 
With reference to FIG. 1, movable contacts B1 and B2 slidingly move along 
the row of fixed contacts T1 and T8 which correspond to discrete positions 
of the objective lens. The movable contacts B1 and B2 are arranged to 
engage the adjoining pair of fixed contacts so that from the movable 
contacts two signals are obtained corresponding to the two adjoining 
positions of the discrete positions of the objective lens. Those two 
signals are respectively compared with the output of distance measuring 
device 2 by comparator AC1 and AC2. As will be described below, both 
outputs of comparators AC1 and AC2 have "High" levels to stop objective 
lens driving motor M when the output of distance measuring device 2 has a 
value between the values of the voltages derived from the two adjoining 
fixed contacts, e.g. T2 and T3 in FIG. 1. 
A more detailed description will now be given with respect to the 
embodiment of FIG. 1. Movable member 1 is provided with brushes B1, B2 and 
B3 and is linked with driving motor M through an objective lens 1 driven 
by the motor. Distance measuring device 2 generates a distance signal in 
the form of an analog signal. An exemplary arrangement of the distance 
measuring device is shown in FIG. 2. 
With reference to FIG. 2, pulse generator or oscillator F generates output 
signals at proper intervals which drive light emitting element LE. The 
light emitted from light emitting element LE is converged by lens L2 into 
a narrow beam and directed to an object to be photographed which reflects 
the light. The light reflected from the object is focused by lens L3 on 
light detector PD on which terminals t1 and t2 are disposed. Terminal t1 
is located at or near a point where light reflected from an object at a 
distance that may be considered as optically infinite is focused. Terminal 
t2 is located at or near the point where light reflected from an object at 
the shortest available distance is focused. The focusing position of the 
incident light changes between terminals t1 and t2 in accordance with the 
camera-to-object distance. Thus, the ratio of currents obtained through 
terminals t1 and t2 represents the camera-to-object distance. The ratio is 
obtained in the FIG. 2 circuit by the subtraction of the logarithms of the 
currents. In FIG. 2, amplifiers AM1 and AM2 amplify respectively the 
currents from light detector PD. Logarithmic compressors LG1 and LG2 
generate outputs respectively proportional to the logarithms of the 
amplified currents. Subtractor SU generates an output as a function of the 
difference between the outputs of logarithmic compressors LG1 and LG2. The 
output of subtractor SU is supplied through switch element SW to capacitor 
C to charge the latter. Switch element SW is controlled by the output of 
pulse generator F so that it is conductive only while light emitting 
element LE is emitting light. Thus, capacitor C always stores the 
information of the last mentioned camera-to-object distance in the form of 
an analog signal. The details of this range finding device may be found in 
a copending U.S. patent application of Matsuda et al. (Ser. No. 362,033) 
field on Mar. 25, 1982 with the title "DISTANCE MEASURING DEVICE" and 
assigned to the same assignee. 
Returning to FIG. 1, distance signal a from distance measuring device 2 is 
compared, by comparators AC1 and AC2, with the voltage signals 
corresponding to the instantaneous advanced position of objective lens L1. 
Resistors R1 through R8 are connected in series with each other to 
constant current source IS and have their nodes respectively connected 
with fixed contacts T1 through T8 along which brushes B1 and B2 slidingly 
move. The resistances of resistors R1 through R8 are determined so that 
the voltage at the fixed contact corresponding to the position of 
objective lens L1 is equal to the distance signal produced by distance 
measuring device 2 for an object at one of the discrete distances to be 
focused by the objective lens at that position. Brushes B1 and B2 are 
arranged to engage adjoining fixed contacts. In the embodiment, objective 
lens L1 moves in the direction to focus on a nearer object as brushes B1 
and B2 move towards fixed contact T8, while objective lens L1 moves 
towards the infinity focused position. Assuming that distance measuring 
device 2 generates a distance signal a having a voltage between the 
voltages at fixed contacts T4 and T5 and that movable member 1 assumes the 
position shown in FIG. 1, the voltages picked up by brushes B1 and B2 are 
both lower than the distance signal a since brushes B1 and B2 respectively 
engage fixed contacts T2 and T3. Accordingly, output d of comparator AC1 
is a "Low" level since the voltage at the negative input terminal of the 
comparator is higher than the voltage at the positive input terminal of 
the same. Comparator AC2 generates a "High" level as output e since its 
positive input terminal receives higher voltage than its negative input 
terminal. Then, if brushes B1 and B2 engage fixed contacts T4 and T5, the 
distance signal a is a voltage between the voltages at fixed contacts T4 
and T5 so that the positive input terminal of comparator AC1 becomes a 
"High" level to make output d a high level, while output e of comparator 
AC2 is also a "High" level since the input to the positive input terminal 
of comparator AC2 is higher than that of the negative input terminal of 
the same. When brushes B1 and B2 move further towards resistor R8 i.e. 
farther distance focused position, output d of comparator AC1 is a "High" 
level while output e of comparator AC2 becomes a "Low" level. The 
relationship between the outputs of comparators AC1 and AC2 and the 
position of brushes B1 and B2, i.e. the focusing position of the objective 
lens, is shown in the following Table. 
TABLE 
______________________________________ 
Position of the Objective Lens 
AC1 AC2 
Nearer Focused Position 
Low High 
In-focus Position High High 
Farther Focused Position 
High Low 
______________________________________ 
The outputs of comparators AC1 and AC2 are supplied through D-flip-flops 
DF1 and DF2 to motor drive circuit 3. 
The operation of the motor drive circuit will be explained with reference 
to FIG. 3. When input k (see FIG. 3) is a "Low" level and input l is a 
"High" level transistors BT5, BT6, BT3 and BT4 are conductive and 
transistors BT1, BT2, BT7 and BT8 are non-conductive, whereby the current 
flows from power source +V through transistor BT4, motor M and transistor 
BT5 to energize motor M. In this case, as the input k is a "Low" level and 
input l is a "High" level showing that the objective lens is at a position 
to focus on an object at a nearer distance than that of the target object, 
objective lens 1 and movable member 1 are shifted by motor M in a 
direction where the distance on which the objective lens focuses becomes 
larger. When the objective lens 1 is at a position to be focused on an 
object farther than the target object, input k is a "High" level and input 
l is a "Low" level, making transistors BT1, BT2, BT7 and BT8 conductive so 
that motor M is driven in the opposite direction to shift objective lens 
L1 amd movable member 1 in a direction where the distance on which the 
objective lens focuses become shorter. When the objective lens is at a 
position to focus on the target object, both inputs k and l are "High" 
levels which cause transistors BT2, BT5, BT1 and BT6 to be conductive and 
transistors BT3, BT4, BT7 and BT8 non-conductive to stop motor M. It can 
be understood that when inputs k and l are both "High" levels, transistor 
BT2 and BT5 are conductive to short-circuit across motor M, which is 
braked and stops abruptly. 
When considering the situation where the objective lens has been stopped, 
it will be seen in FIG. 4 that the electric condition does not change at 
whatever the position from point .beta. to point .gamma.. Brush B 
(representing brush B1 or B2) is in contact with fixed contact T 
(representing any one of fixed contacts T1 through T8). However, assuming 
that objective lens 1 is optically best focused on the target object with 
brush B engaging fixed contact T at point .gamma., an error of 
approximately 1 step will occur in the case when brush B engages fixed 
contact T at point .beta.. If objective lens L1 is arranged to be stopped 
at any time with brush B engaging fixed contact T at the central point 
.alpha., the above mentioned error will be reduced by half and will be 1/2 
step at the maximum. In short, if the objective lens control mechanism 
described above is arranged so that motor M always stops with brush B at 
the center of fixed contact T, it is unlikely that objective lens 1 will 
be stopped at a position significantly deviating from its optically best 
focus position. Brush 3 and index terminals J1 through J8 are provided as 
a measure to attain that purpose. 
Terminals J1 through J8 are positioned so that brush B3 engages one of the 
terminals when brushes B1 and B2 are at the centers of adjoining ones of 
fixed contacts T1 through T8. Terminals J1 through J8 are connected to the 
positive terminal of power source +V, while brush B3 is connected to pulse 
generator OS1 such as a one-shot circuit. Accordingly, when brushes B1 and 
B2 reach the centers of adjoining ones of fixed contacts T1 through T8, 
brush B3 comes into contact with either one of terminals J1 through to J8 
to trigger pulse generator OS1 for causing the same to produce a pulse at 
point f. The pulse is supplied through OR gate OR1 to clock input 
terminals CL of D-flip-flops DF1 and DF2, which in turn transmit their 
inputs at the time of pulse reception, i.e. the outputs of comparators AC1 
and AC2 at that time, to motor drive circuit 3 as its inputs. Thus, 
D-flip-flops DF1 and DF2 constitute a memorizing means for memorizing the 
comparison results by comparators AC1 and AC2 in response to the pulse 
from trigger pulse generator OS1. As described above, when distance signal 
a has a level between the voltages at fixed contacts T4 and T5, the 
outputs of comparators AC1 and AC2 are both "High" levels. However, those 
outputs are transmitted to motor drive circuit 3 to stop motor M only when 
brush B3 comes into contact with terminal J3. That is, the objective lens 
is stopped only when brush B3 engages any one of terminals J1 through J8. 
Next, a description will be given concerning the case when the distance 
signal changes for any reason, with objective lens L1 having been stopped. 
When objective lens L1 has been stopped, motor M is at a stop condition so 
that the outputs of D-flip-flop DF1 and DF2 are both "High" levels. 
Accordingly, the output of AND gate AN1 is a "High" level. When distance 
signal a changes at this state, the outputs of comparators AC1 and AC2 
change from both "High" levels to "High" and "Low" or "Low" and "High" 
levels. Then, the output of exclusive OR gate EO1 becomes a "High" level 
with the output of AND gate AN1 being a "High" level as described above, 
so that the output of AND gate AN2 becomes a "High" level so as to trigger 
pulse generator OS2 such as a one-shot circuit. The output of pulse 
generator OS2 is supplied through OR gate OR1 to clock input terminals CL 
of D-flip-flops DF1 and DF2 which transmit the outputs of comparators AC1 
and AC2 at the time of pulse reception, to motor drive circuit 3 and 
actuate motor M. Motor M is driven until objective lens L1 again reaches a 
position to focus on a target object, and is stopped when brush B3 comes 
into contact with any one of terminals J1 through J8. 
It is to be understood that index terminals J1 and J8 need not necessarily 
be disposed adjacent to fixed contacts T1 through T8, but may be provided 
on any rotary member movable in response to the movement of movable member 
1 so that a slider contact corresponding to brush B3 relatively slides 
over the terminals. 
The present invention has been described with reference to FIGS. 1 to 4 in 
connection with an embodiment applied to an objective lens driving 
mechanism for a camera automatic focusing device. The present invention 
may also apply to an automatic adjusting or control device for an 
objective lens diaphragm. In this case, block 2 of FIG. 1 is substituted 
by a light measuring and exposure calculation circuit which produces at 
terminal a an analog signal corresponding to a diaphragm aperture value to 
be adjusted, while motor M may be arranged to drive the diaphragm and 
movable member 1 may be arranged to move in response to the change of the 
diaphragm aperture. 
According to the present invention as described above, a movable member is 
stopped when two signals corresponding to the position of the movable 
member sandwich a signal for a target position at which the movable member 
is to be stopped, whereby the hunting phenomenon will not occur. 
Additionally, error regarding the stop position of the movable member will 
be reduced if the device of the invention is arranged so that the movable 
member is stopped in accordance with the comparison of the signal for the 
target position with the above-mentioned two signals when the movable 
member reaches a particular position in any one of the continuously 
dividing ranges. Further, the wear of a resistor which changes the 
resistance and lowers the accuracy of the position control will not occur 
if the means for converting the position of the movable member to an 
analog signal is arranged so that the intermediate taps are derived from a 
resistor and respectively connected with fixed contacts over which movable 
contacts slidably move in response to the movement of the movable member. 
While a preferred embodiment has been described, variations thereto will 
occur to those skilled in the art within the scope of the present 
inventive concepts which are delineated by the following claims.