Exposure control apparatus

Exposure control apparatus is disclosed for controlling the amount of light admitted to a photosensitive surface. The apparatus comprises an electromagnetic actuator having a pair of aperture blades which are biased toward a rest position and are movable away from the rest position to define an aperture; the aperture blades also function as a shutter. A control means regulates current supplied to the actuator to control aperture size and exposure time. In order to precisely control the exposure interval, the control means includes a means for continuously sensing the position of at least one of the aperture blades.

FIELD OF THE INVENTION 
The present invention relates to exposure control apparatus for a camera. 
More particularly, the invention relates to such apparatus in which the 
performance of the apparatus is monitored during an exposure interval to 
precisely regulate the amount of light admitted to a photosensitive 
surface. 
DESCRIPTION OF THE PRIOR ART 
Automatic exposure control devices for cameras are known in which a pair of 
aperture blades define the aperture size and also function as a shutter. 
The blades have grooves or cut-out portions which are configured to form a 
range of progressively increasing aperture sizes as they are moved from an 
initial position toward a fully open position. The blades can be 
controlled by an actuator driven by an electrical circuit which is 
responsive both to automatically provided inputs, such as the level of 
scene or image brightness, and to manual inputs provided by the operator 
to control the aperture size or shutter speed. One problem with such 
devices is that a given output from an actuator, such as an 
electromagnetic actuator, does not necessarily always produce the same 
movement of the aperture blades. Thus, it has been recognized that it is 
desirable to monitor the actual blade displacement during the course of an 
exposure interval. 
In U.S. Pat. No. 4,325,614, there is disclosed an automatic exposure 
control in which an aperture blade is driven by a stepper motor which 
drives the blades in accordance with inputs including the lever of scene 
brightness. The exposure control further includes a feedback system for 
monitoring blade displacement during the course of an exposure interval 
and for providing an electrical output which is representative of such 
displacement. The feedback system includes an optical encoder having a 
plurality of vertical slits on the blade which function in conjunction 
with a pair of light emitting diodes to produce a digitally-encoded 
output. One problem with such a device is that the optical encoder is 
relatively complex and requires modification of the aperture blade. 
Further, the encoder does not produce a continuous signal, since a signal 
is only produced when the blade reaches a particular position in which the 
slots are lined up with the diodes. Thus, while the feedback system 
indicates whether the blade is moving, it does not give a continuous 
signal of the precise blade position. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved exposure 
control apparatus in which the position of an aperture blade is 
continuously monitored during an exposure interval. 
In accordance with one aspect of the invention, there is provided exposure 
control apparatus for regulating the amount of light admitted to a 
selected area, the apparatus comprising: light limiting means movable from 
a rest position to admit light to the area: means for moving the light 
limiting means; means for urging the light limiting means toward the rest 
position; means for providing a first output indicative of ambient light; 
means for sensing a change in the urging means which is indicative of an 
instantaneous position of the light limiting means and for producing a 
second output proportional to the change; and means for actuating the 
moving means to admit a predetermined amount of light to the area in 
response to said first and second outputs. 
The present invention provides a means for monitoring the position of an 
aperture blade without changing the structure of the blade and without 
making major modifications to the exposure control apparatus. A further 
advantage of the present invention is that it provides a continuous signal 
which is indicative of the instantaneous position of the blade at all 
times during the exposure interval. 
Other features and advantages will become apparent from reference to the 
following description of the preferred embodiment when read in light of 
the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The exposure control apparatus of the present invention includes an 
electromagnetic actuator 100, shown in FIG. 1, and a control means 199, 
shown in FIG. 4. Actuator 100 includes a mechanism plate 101 which has a 
fixed aperture 102 adapted to overlie a photosensitive surface (not 
shown). Mechanism plate 101 is made from mild steel and is covered with a 
layer of dielectric material, such as a porcelain enamel. Four spacer 
posts 104 are supported on plate 101, and four conductive mounting pads 
106, 108, 110 and 112 are formed on the enamel surface of plate 101. 
Conductive mounting pads 106 and 112 are adapted to be connected to 
electrical contacts of a power source, and pads 108 and 110 are 
electrically connected by a conductor strip 114. 
An armature 116 includes an aperture blade 118 having a tapered aperture 
120 therein, and two coil support portions 122 and 124. Spiral coils 126 
and 128 are formed on the coil support portions 122 and 124 respectively. 
As will be apparent from the discussion which follows, blade 118 is 
movable relative to aperture 102 to function as a light limiting means 
which regulates the amount of light passing through aperture 102. 
Armature 116 is supported for pivotal movement by a flexure hinge 130 
comprising a pair of leaf springs 132 and 134 formed from two mil 
beryllium sheet. Flexure hinge 130 defines a virtual pivot at a point 
where a projection of the springs 132, 134, would intersect, and thus, 
hinge 130 serves as a pivotal support as well as a means for urging blade 
116 toward a rest position. Spring 132 has a tab 138 soldered to an end of 
the coil 128. Similarly spring 134 has a tab 142 attached to coil 126. 
A strain transducer 135 is mounted on spring 132 to provide a means for 
sensing a change in spring 132 and thereby monitor the position of 
aperture blade 118. Transducer 135 contains a wire 137 which deforms in 
exactly the same manner and to the same degree as spring 132 when stress 
is applied to the spring 132. The deformation, or strain, in the wire 137 
is directly proportional to the resistance of the wire. As will be 
explained in detail hereinafter, this change in resistance is used to 
produce a signal indicative of the instantaneous position of blade 118. 
One example of a strain transducer suitable for use in the present 
invention is a strain gage, part number FAE-12S-12SGL, manufactured by BLH 
Electronics, 42 Fourth Ave., Waltham, Mass. 
Coil 128 includes windings on both sides of blade 118 which are connected 
through a hole 144 in blade 118, as shown in FIG. 1. In a similar manner, 
coil 126, which is connected to coil 128, includes windings on both sides 
of blade 118 which are connected through a hole 146 in blade 118. As 
viewed in FIG. 1, coil 126 has two legs 143 and 145 which extend generally 
radially of the virtual pivot point defined by flexure hinge 130 and also 
generally perpendicular to the direction of movement of the armature 116. 
Likewise, coil 128 has two legs 147 and 149 which extend generally 
radially of the virtual pivot point defined by the flexure hinge 130. Tabs 
136 and 140 are formed on the outer ends of the springs 132 and 134 
respectively. Tab 136 is soldered to mounting pad 112, and tab 140 is 
soldered to pad 110. 
An armature 116' is generally similar to armature 116, and parts of 
armature 116' which are similar to parts in armature 116 have the same 
reference numeral with a prime added. Tabs 136' and 140' are soldered to 
mounting pads 108 and 106 respectively. As shown in FIG. 1, a permanent 
ceramic magnet 148 is arranged to produce the magnetic fields in the 
vicinity of coils 128 and 128'. A second permanent ceramic magnet 150, 
similar to magnet 148, produces the magnetic fields in the vicinity of 
coils 126 and 126'. The magnets 148 and 150 are cemented to the underside 
of a plate 152 made from mild steel. Plate 152 has an aperture 154 therein 
for receiving a lens 156. Plate 152 has tabs 158 which receive posts 104, 
and fasteners 105 hold the actuator 100 together. In the assembled form of 
actuator 100, a gap is formed between the magnets 148, 150, and the 
mechanism plate 101, and the armatures 116 and 116' are adapted to move in 
this gap. 
With reference to FIG. 2, the positions of the armatures 116 and 116' are 
shown in the initial, or rest positions. In FIG. 3, the positions of the 
armatures 116 and 116' are shown in the fully open positions. In 
operation, when the positive terminal of an electrical power supply is 
connected to the terminal adjacent mounting pad 112, and the negative 
terminal of the power supply is connected to the terminal adjacent 
mounting pad 106, current flows counterclockwise in coils 128 and 126 and 
clockwise in coils 128' and 126'. The forces generated, due to the 
currents flowing in the magnetic fields produced by magnets 148 and 150, 
cause armature 116 to pivot in a clockwise direction, and armature 116' to 
pivot in a counterclockwise direction, thereby moving blades 118 and 118' 
to uncover aperture 102, as shown in FIG. 3. The forces on armatures 116 
and 116' can be controlled by controlling the current supplied to the 
actuator 100. 
It has been found that the effects of static friction are overcome, and 
reliable, repeatable position control is achieved by applying the driving 
current to armatures 116 and 116' in pulses of constant amplitude. The 
frequency of the pulses in the control signal is chosen to be somewhat 
above the cutoff frequency, but not so far above the cutoff frequency that 
the system does not respond, since the armatures 116 and 116' must vibrate 
enough to break the static friction. The cutoff frequency is the point in 
a frequency vs. response curve (not shown) of actuator 100 where the 
response is down about 3 dB from the DC response. In the preferred 
embodiment of the invention, the pulse duration is modulated to control 
aperture size, and the number of pulses applied to the actuator controls 
the shutter time. The amount of light admitted through aperture 102 during 
an exposure interval depends on the effective aperture size and the time 
blades 118, 118', remain open. Various exposure apertures plotted against 
time are shown in FIG. 6 where the area under each curve represents the 
total quantity of light admitted during an exposure interval. A more 
complete description of the use of pulses to drive an exposure control 
actuator is contained in commonly-assigned U.S. Pat. No. 4,333,722, issued 
June 8, 1982, and this patent is expressly incorporated herein by 
reference. 
As shown in FIG. 4, control means 199 comprises a microprocessor 200, a 
reference oscillator 202, a light-controlled oscillator 204, a sensing 
control 230, an output driver 206, a power supply 208 and an actuator 
switch 210. The light-controlled oscillator 204 functions as a means for 
providing an output indicative of ambient light and includes a 
photosensitive element, such as a photodiode 212, a logarithmic feedback 
diode 214, an operational amplifier 216, and a voltage controlled 
oscillator 218. These elements are responsive to scene light to produce a 
voltage proportional to the log of the intensity of the scene light; this 
voltage is applied to the voltage controlled oscillator 218 to produce a 
pulse train having a repetition frequency proportional to the log of the 
intensity of scene light. 
When switch 210 is closed by an operator, reference oscillator 202 starts 
to produce clock pulses used to time the operations of the microprocessor 
200, microprocessor 200 is initialized, and the light-controlled 
oscillator 204 starts to produce a pulse train having a frequency 
proportional to the log of the scene light. The microprocessor 200 is 
programmed to sample the output of the light-controlled oscillator 204 and 
to count the number of pulses received during a predetermined time 
interval. The final count is proportional to the log of the scene 
brightness. The microprocessor 200 employs a look up table, using the 
pulse count as an address, to retrieve the required pulse duration and 
total number of pulses needed to produce the proper aperture and shutter 
time. The microprocessor 200 then constructs the pulse train from the 
information retrieved from the table. The driver 206 buffers the pulse 
train from the microprocessor and applies the pulse train to the armatures 
116 and 116'. 
The control means 199 is easily adapted for total program control, 
including a shutter preferred operation or an aperture preferred 
operation. If a shutter preferred operation or an aperture preferred 
operation is employed, in addition to measuring the scene light, the 
microprocessor retrieves desired aperture or shutter settings from 
manually set inputs; such inputs are indicated in dotted lines in FIG. 4 
by a block 220 for aperture settings and a block 222 for shutter settings. 
The microprocessor 200 uses the manual inputs to compute, or retrieve from 
a look-up table, the proper actuator pulse control train. The 
microprocessor 200 can be programmed to override any manual input that 
would result in improper exposure. 
In the present invention, microprocessor 200 is adapted to adjust the pulse 
train applied to armature 116, 116' in accordance with an output from 
sensing control 230. This output is proportional to a change in spring 
132, and it is indicative of the position of blade 118. As shown in FIG. 
5, transducer 135 is connected in a conventional amplifier circuit having 
a differential amplifier 233 and a buffer amplifier 234. The output of 
buffer amplifier 234 is connected to a suitable analog-to-digital 
converter 235 which provides an output to microprocessor 200. A 
calibration resistor 236 is provided to adjust the offset so that the 
signal from control 230 is zero when the armatures 116 and 116' are in the 
position shown in FIG. 2. 
The displacement of the aperture blade 118, attached to leaf spring 132, 
produces a change in the resistance of transducer 135 which results in an 
analog signal at the output of amplifier 234 that is representative of the 
displacement of blade 118. The signal from amplifier 234 is processed by 
the analog-to-digital converter 235 which supplies a digital signal to the 
microprocessor 200. The microprocessor 200 is programmable to control the 
timing and the amplitude of the pulses supplied to armatures 116 and 116' 
to produce a desired exposure in accordance with the signal from control 
230. Thus, the signal from control 230 is compared by microprocessor 200 
with values representative of the desired positions of blade 118 for a 
particular scene brightness. If such a comparison indicates that blade 118 
is not moving as desired, the pulse train will be corrected to obtain the 
proper exposure. 
It will be apparent that the present invention would be used to reflect the 
adjustment of an exposure setting mechanism, such as the aperture 
adjustment ring (not shown) common on cameras used with 35 mm film. In 
such an arrangement, the aperture blades 118 and 118' would define a 
selected aperture, and a separate shutter blade (not shown) could be 
actuated to initiate and terminate the exposure at a predetermined time. 
The present invention is also useful with a camera having a fixed aperture. 
In this case, the signal produced by a strain transducer would be used to 
monitor the movement of a shutter blade (not shown) to produce a desired 
exposure time. 
The disclosed invention provides a simple, reliable means of continuously 
monitoring the position of an aperture blade during an exposure interval. 
It will be apparent that a pair of strain transducers could be connected 
as complementary elements in a bridge circuit to provide temperature 
compensation for the transducers, as is well known in the transducer art. 
The invention has been described in detail with particular reference to a 
preferred embodiment thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.