WO1995033226A1

WO1995033226A1 - Exposure control system using pulse width modulation

Info

Publication number: WO1995033226A1
Application number: PCT/US1995/006792
Authority: WO
Inventors: Amanda Cooley; Elliott S. Strizhak
Original assignee: Polaroid Corporation
Priority date: 1994-05-27
Filing date: 1995-05-26
Publication date: 1995-12-07
Also published as: EP0731934A1

Abstract

An automatic exposure control system for photographic apparatus utilizing a direct current motor to drive a shutter. The exposure control system evaluates the voltage level determined by a voltage source by driving the direct current motor in a reverse direction to provide a load on the voltage source and, based on the voltage level, transmits a pulse width modulated pulse train. The exposure control system alters the duty cycle of the pulse train to provide a predetermined constant average voltage to the direct current motor. Once exposure is complete, the pulse width of the pulse train is increased to one hundred percent of the period and the direction of current flow through the motor is made negative so that the shutter is driven closed at a maximum speed.

Description

EXPOSURE CONTROL SYSTEM USING PULSE WIDTH MODULATION

Background of the Invention

The present invention relates generally to automatic exposure control systems for photographic devices, and, more particularly, the invention relates to exposure control systems and methods using pulse width modulation (hereinafter "PWM") driven direct current motors for driving a shutter between open and closed positions. Automatic exposure control systems in photographic devices, in their most simplistic form, evaluate scene brightness and weight this evaluation with respect to the sensitometric characteristics of the film being exposed. The result of this evaluation is used to determine one or more variables such as exposure interval or aperture size. The exposure control system then drives open a shutter to a resultant aperture size or for a resultant exposure interval such that an image of the scene is exposed onto film held within the photographic device.

The above-mentioned evaluation is performed by analyzing an exposure design curve relating scene brightness versus aperture size for the photographic device to film exposure characteristics. This information allows each photographic device to capture photographic information in a design specific manner.

Exposure design curves are predetermined by a manufacturer of photographic devices to account for individual characteristics of the photographic device being developed. The more accurate the exposure design curve is to the performance attributes of the photographic device itself, the better the quality of the photographic image can be obtained. Of course, the enhanced quality will then be dependent upon the ability of the exposure control system to adhere to this exposure design curve.

Photographic devices such as low cost 35 mm cameras often use a broadly defined exposure design curve. This broad definition allows for the use of an exposure control system utilizing less stringent tolerances thereby lowering manufacturing costs. This is possible because post-exposure film processing tools may be utilized to make-up for most inadequacies in the photographic image.

Many photographic devices, such as self-developing cameras, require a better photographic image from the moment of exposure. In the case of self developing cameras, the photograph is processed by the camera and, therefore, will not be available for post-exposure processing. Such requirements have necessitated the development of more precise exposure control systems.

Prior art teaches the use of an exposure control system utilizing a solenoid and a spring to drive a shutter in the photographic device, where the shutter controls the aperture size. A version of a camera utilizing such an exposure control system is described in commonly assigned United States Patent No. 3,942,183 entitled "Camera with Pivoting Blades" by George D. Whiteside, issued March 2, 1976. In such exposure control systems, a tractive electromagnetic force generated by energizing the solenoid with energy stored in a battery is used to hold the shutter in a closed position. The spring provides a counter force biasing the shutter toward a fully open position. The shutter blades are then urged open by the spring when then voltage across the solenoid is decreased. Once the exposure is complete, the solenoid is then energized to draw the blades closed.

This type of shutter blade mechanism can not maintain the required precision though. As the battery voltage in the camera decreases, the ability of the solenoid to draw the shutter blades in a fully closed position also decreases. Also, over time the spring looses its elasticity, thus, decreasing the bias to open the blades.

These shutter blade mechanisms have a further drawback of requiring mechanical adjustment to adhere to differing design curves. In other words, to achieve a different rate of opening, the shutter designer must choose a different spring.

Other photographic devices use a stepper motor to drive the shutter. A stepper motor uses discrete movement to adjust the aperture size where the rate of change is frequency controlled. For each pulse sent to the stepper motor, the motor will move the shutter a discrete amount; the rate of change is controlled by the frequency of the pulses. Although satisfactory for its intended purpose, the stepper motor itself is costly and substantially increases manufacturing costs of a photographic device.

The above-described stepper motor system additionally requires the use of a voltage regulator. The voltage regulator ensures that the voltage transmitted to the motor is kept constant and, thus, presents sufficient voltage to drive the stepper motor. Problems presented by this additional circuitry include, first, that the circuitry itself presents an additional point of failure in the camera reducing camera reliability. To minimize this problem, additional testing must be performed during the manufacturing process. Second, the circuitry requires valuable space on the circuit board that may otherwise be used for other circuitry or possibly removed to decrease camera size.

Another problem currently encountered is the difficulty of calibrating hardware bound voltage regulation systems during manufacture. Given the variability of the actual resistance of resistors and other circuit components, the actual voltage required to drive the stepper motor may vary accordingly. Calibration in such a case requires time consuming and precise testing by a technician.

Siirnmary

The aforementioned is achieved by the invention which provides, in one aspect, an exposure control system for photographic devices. The invention provides an extremely versatile exposure control system for controlling transmission of image forming light from a scene to a film plane by closely regulating voltage transmission to the exposure control system. The system comprises a shutter; drive means; a voltage source; voltage sensing means; and control means.

The shutter is normally in a blocking position with respect to a film exposure opening such that incident light is blocked. Upon activation of the photographic device, the shutter initiates an exposure interval by moving to an unblocking position unblocking the film exposure opening. Once exposure is complete, the shutter moves to block the film exposure opening to terminate the exposure interval.

The drive means is mechanically connected to the shutter for driving the shutter between the blocking and unblocking positions. The drive means is responsive to a predetermined applied voltage which causes movement in a direction that changes with the voltage sense of the predetermined applied voltage signal.

The entire exposure control system is provided power by the voltage source which, in most cases, is an electrochemical battery.

The voltage sensing means is electrically connected to the voltage source for determining a voltage level of the voltage source. In order to attain an accurate voltage level reading, the predetermined applied voltage is transmitted with a negative voltage sense to the drive means such that the drive means provides a load on the voltage source.

Once the load is applied to the voltage source, the voltage level is determined.

The control means controls operation of the drive means to actuate the shutter. The control means varies a duty cycle of a periodic signal to the drive means in response to the voltage level determined by the voltage sensing means in such a way as to adhere to the design curve of the photographic device. The control means accomplishes this by utilizing several subparts. Among the subparts are evaluation means; modulation means; reversing means; and tractive means.

The evaluation means receives the voltage level of the voltage source and determines an average voltage necessary to propel the drive means. This average voltage is dependent upon the motor used and is modifiable per camera design requirements.

The modulation means then takes the average voltage and alters the duty cycle of the periodic signal, generally a sequence of drive pulses, such that the average voltage transmitted by the sequence of drive pulses equals the predetermined applied voltage necessary to propel the drive means. The periodic signal pulses is then transmitted to the drive means. The duty cycle is varied by the modulation means in a range spanning between zero and one hundred percent of the period of the periodic signal.

The reversing means terminates the exposure interval by signalling the modulation means to increase the pulse width of the sequence of drive pulses to one hundred percent of the period. The reversing means then switches a direction of current flow through the drive means such that the drive means drives the shutter to the blocking position.

The tractive means, which is mechanically connected to the shutter, biases the shutter toward the blocking position. The tractive means thus ensures that the shutter is held in a closed position until the bias of the tractive means is overcome by the drive means. The tractive means itself can comprise an elastic device such as a spring, an inertial plate acting as a clutch, or a mechanical latch.

The system as thus far described is configured for open loop operation which provides a level of performance that is well suited for use in a wide variety of photographic devices. For use in more sophisticated photographic devices, the performance level of the system can be extended by configuring it for closed loop operation. In this case, the system further includes exposure evaluation means; integration means; and terminating means.

The exposure evaluation means views the scene to be photographed and generates an output value representative of scene brightness. It then passes the output value on to the integration means which integrates the output value over time to quantify an amount of light energy passing through the shutter. Once sufficient light energy has passed indicating proper exposure, the terminating means discontinues said sequence of drive pulses and then signals to the reversing means to close the shutter.

In further aspects, the invention provides methods in accord with the apparatus described above. The aforementioned and other aspects of the invention are evident in the drawings and in the description that follows.

Brief Description of the Drawings

The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which:

Figure 1 shows a schematic diagram of a basic photographic device utilizing an exposure control system;

Figure 2 shows an block diagram of an exposure control system in accordance with the invention;

Figure 3 shows a series of timing diagrams for a camera utilizing the exposure control system of Figure 2.

Figure 4 shows a series of timing diagrams in accordance with the exposure control system of Figure 2.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Detailed Description

While the present invention retains utility within a wide variety of photographic devices and may be embodied in several different forms, it is advantageously employed in connection with a fully automatic singular lens self-developing type of camera. Though this is the form of the preferred embodiment and will be described as such, this embodiment should be considered illustrative and not restrictive.

Figure 1 is a basic illustration of a photographic device 10 employing the exposure control system of the invention. In the Figure, the sun is presenting a source of natural illumination that is reflected off a subject of a photograph and its background. The reflected iUumination, or scene brightness, is then collected by a lens 12 in the photographic device 10 as indicted by dashed lines. The reflected light passes through an aperture 13 created by opening a shutter 14 which is normally held closed. The shutter is opened by a motor 30 which is driven by an exposure control system 18. The exposure control system 18 uses preprogrammed knowledge of the photographic system to govern the amount of light through the aperture 13 during an exposure interval and an appropriate signal is used to direct the motor 30 to drive open the shutter 14 until the proper amount of light enters.

The reflected light then strikes film 16 held within the photographic device thus exposing the film 16 creating a photographic image thereon. The shutter 14 is then driven closed by the motor 30 ending the exposure interval.

The quality of the photographic image created during this exposure interval is affected by many variables. Some of the variables, such as lens quality, become fixed during the design and manufacture of the photographic device. Other variables must change in accordance with environmental conditions.

The fixed variables are used by the designers of the photographic devices to determine an exposure design curve that, when followed, creates an optimal photographic image for the particular photographic device. The exposure design curve generally relates scene brightness and aperture to film exposure characteristics for the photographic device.

The variables that are determined at the time of exposure are controlled through an exposure control system 18. The exposure control system 18 takes collected scene data and predetermined design curves as well as sensitometric characteristics of the film being exposed and uses this information to adjust such variables as the size of the aperture 13 at the moment the photograph is being taken. In the preferred embodiment, the design curve is based on an assumption that the shutter will open at a constant speed. This assumption is a design concept that allows the designers to form a relationship among the other variables.

The ability of the exposure control system to adhere to this exposure design curve directly affects the quality of the photographic image. As the system is allowed to vary from the curve, the photographic image may be adversely affected. For example, if the shutter opens too quickly then the depth of field of the image will be reduced and, in some cases, the image may blur due to collection of light passing through an outer portion of the lens. Alternatively, if the shutter opens too slowly, the exposure will be too long and camera or subject motion may blur the image.

Figure 2 portrays generally a block diagram of the exposure control system 18 according to the invention. The exposure control system 18 is activated by the photographic device 10 via a start signal 26. This start signal 26 indicates to the processor 20 that a photograph is to be taken by exposing the film 16 during the exposure interval.

A battery 24 is used to power the electrical components within the photographic device 10. But, the battery 24 is a depletable power supply. Over time the voltage across the battery decreases due to use of the photographic device 10, and, less significantly, internal leakage. Therefore, the voltage available to drive the internal systems of the photographic device 10 is dependent upon such variables as the battery's usage and age. The processor 20 must, therefore, interrogate the battery 24 to determine its current voltage level. This voltage level determination is used by the processor 20 to set a duty cycle of a pulse width modulation ("PWM") signal 28 that is transmitted to the motor 30. The PWM signal 28 conveys a predetermined applied voltage to the motor 30 which causes the motor 30 to rotate accordingly. As the motor 30 rotates, it drives open a shutter 14 initiating film exposure as previously described.

When the processor interrogates the battery, a load must be placed upon the battery in order to obtain an accurate accounting. Therefore, a direction of current flow through the motor 30 is reversed to provide the load. Using the motor 30 itself for the load substantially guarantees accurate results due to the fact that the motor 30 will ultimately be the load.

The motor 30 used in the preferred embodiment is a limited angle motor. This means that the angle of rotation is limited to a fixed rotational range. In the preferred embodiment, this range of rotation is zero to thirty degrees. This movement is restricted mechanically by two fixed terminals that extend from the motor housing. As the motor

30 progresses, a peg on the armature rotates according to the angle of the motor 30 until either the voltage to the motor 30 is terminated or the peg strikes a terminal and is mechanically stopped.

When the previously described reverse current is driven through the motor 30 to check the load voltage on the battery, the motor 30 attempts to rotate to a negative angle of rotation. This rotation is restricted by the peg striking the terminal and, therefore, the shutter will be restricted to a fully closed position during interogation of the battery 24.

The motor 30 itself converts electrical energy into mechanical energy. Generally, rotational speed of a DC motor is directly related to applied voltage, VA. The relationship varies depending upon the type DC motor but is clearly exemplified in a common type which has a fixed armature, or electromagnetic loop-carrying assembly, which maintains a constant electromagnetic field current. Assuming minimal armature winding inductance, the relationship is as follows:

or

(2)^ = (VA - RA* lA)/ ^J dt where dθ/dt Angular velocity, or speed of rotation

RA Armature winding resistance

IA Armature winding current A Applied armature voltage k Motor constant

The second term of equation (1) represents the induced electromotive force, or back emf, which depends on the speed of rotation of the motor. Solving for speed of rotation, it can be seen from equation (2) that as the armature voltage, VA, fluctuates, the dθ speed of rotation, — , of the DC motor will also vary proportionally. Consequently, in dt a system where there exists no voltage regulation, fluctuations in the voltage level of the battery 24 would directly affect the speed at which the motor 30 rotates and, in turn, governs the speed at which the shutter 14 opens and closes. Since, as previously stated, the exposure control system relies on the shutter opening at a predetermined constant speed, a regulated constant voltage must be presented to the motor 30.

The invention achieves a regulated voltage by transmitting a periodic signal to the motor 30 wherein the average voltage is the regulated voltage value. To compensate for the varying input voltage from the battery, the processor 20 utilizes a PWM signal 28. The PWM signal 28 is a series of pulses with a constant period and a variable duty cycle, or pulse width. The duty cycle of the PWM signal 28 governs the average voltage to the motor 30, by adjusting the percentage of the period that the pulse is high, or "on." The percentage can be varied between zero and one hundred percent of the period. The longer the pulse is high, the higher the average voltage will be. The motor

30 responds to the average voltage of the PWM signal 28 by actuating the shutter 14 at a speed corresponding to the average voltage.

In making the determination of the proper duty cycle, the processor 20 evaluates design constants such as the period, T, of the PWM signal and the voltage required by the motor, VM. The processor 20 then examines the voltage across the battery as determined under load, Vβ, as previously described. The processor 20 endeavors to make the average voltage seen by the motor 30, VA, equal the voltage required by the motor 30, A = VM. The relationship is illustrated in the following algebraic expressions:

For Vβ ≥ V , the pulse width, τ, in the PWM signal 28 can be expressed as

τ = (V_M * T) /V_B

and the duty cycle for the PWM signal 28 would be

duty cycle (%) = (τ / T) * 100

The average voltage would then be

V_A = (τ * V_B) /T

These simple relationships allow the processor 20 to quickly calculate the required duty cycle for the PWM signal 28 such that the motor will see the constant voltage that it requires. The motor 30 is then enabled to open the shutter 14 at a constant rate of speed. In practice, the shutter 14 begins in a blocking position and is held firmly in that position by a tractive device. The tractive device is any device commonly used in the art to bias or latch the shutter in a closed position. The tractive device can be an elastic device, such as a spring, biasing the shutter closed, or it can be an inertial plate or latch to hold the shutter closed until released by the control system.

The shutter 14 blocks substantially all light from entering the photographic device 10 while in this closed position. Once the exposure control system 18 determines the proper duty cycle, the processor transmits a PWM signal 28 to the motor 30 that drives the shutter 14 open against the force of the spring, for example, from a fully closed position to a fully open position or any position therebetween. Light then passes through the shutter aperture 13 to expose the film 16 contained within the photographic device 10 creating a photographic image thereon.

With the speed of rotation of the motor, and therefore the shutter speed, known, the exposure control system 18 is then able to calculate the time it must transmit the PWM signal 28 to keep the shutter open to properly expose the film.

The system as thus far described is configured for open loop operation which provides a level of performance that is well suited for use in a wide variety of photographic devices. For use in more sophisticated photographic devices, the performance level of the system can be extended by configuring it for closed loop operation. The preferred embodiment operates in closed loop by adding an additional feedback system which is set up to ensure quality photography in the face of a potentially changing scene environment.

In the preferred embodiment, this feedback is accomplished by utilizing a photometer 22 that monitors an amount of light entering the photographic device 10 as the shutter 14 opens. The photometer has blades that open in direct relation to the shutter

14. The photometer then transmits a signal to the processor indicating an amount of light passing through to the film. The processor integrates this amount of light over time determining the amount of light energy that has been incident upon the film 16. When sufficient light energy has entered, the processor ceases propagation of the PWM signal 28.

Upon completion of the exposure interval as described above, the processor reverses the direction of current flow to the motor to drive the shutter 14 closed. The full voltage direct current signal has a constant duty of one hundred percent to drive the motor 30 at a maximum velocity in a reverse direction back to the motor's initial angle of rotation. The elastic device, previously described, also aids in returning the shutter to this normally closed position to end the exposure interval by contributing an elastic bias toward the closed position. In the preferred embodiment, the direction of current flow is reversed by utilizing an Η bridge where the motor is centered in the bridge and transistors being used as switches are on each leg of the Η. Normal current flow is down the upper left through a closed upper left switch.

The bottom left switch, along with the upper right switch, is open in this case forcing the current through the motor 30 and down through the bottom right switch. For illustration purposes, the bottom of the TT can be assumed to be ground though this may not always be the case.

To reverse the current flow, and therefore, reverse the rotational direction of the motor 30, the upper left switch and the lower right switch are opened, and the upper right switch and the lower left switch are closed. The current flow will now be from upper right to lower left through the motor in a direction opposite of that previously described.

Figures 3 and 4 show timing diagrams which correspond to what has been previously described and more particularly illustrates how the pulse width modulation is used based upon the amount load voltage across the battery 18.

Figure 3 is a timing diagram for a typical self-developing camera system utilizing the invention. It should be noted that the dashed vertical lines showing positions in the timing line are not meant to be to scale but only are meant to show the various stages in the photographic progression.

The first two horizontal lines (A-B) indicate the basic start-up events for initiating the photographic process. Line 3(A) shows that a switch has been triggered indicating to the camera to initialize and prepare for photography. This signal was previously illustrated in Figure 2 as the start signal 26. Line 3(B) shows that the exposure control system is now enabled.

Line 3(C) illustrates voltage to the motor. During the battery check stage, the reverse current is directed through the motor to provide a load for the exposure control system thus enabling the exposure control system to determine the load voltage level across the battery. After the battery check is compete, the reverse current is discontinued and the exposure control system proceeds to evaluate scene data to determine such variables as strobe requirements.

Employed in the evaluation of scene data is a wink. A wink, in simplest of terms, is a short burst of strobe energy (see 3(D)) used to determine reflectivity of the scene at various wavelengths of light, including infrared. Strobe technology is well known as seen in commonly assigned U.S. Patent No. 4,785,322, entitled "Infrared Wink Ranging System" by Harrison et. al., issued November 15, 1988, and is now specifically incorporated herein by reference.

This scene data having been gathered, the photographic device can proceed to initiate the exposure interval. The processor establishes the duty cycle of the PWM signal 28 based upon the load voltage level determined during the battery check. The PWM signal 28 is then transmitted to the motor to initiate the exposure interval.

During this exposure interval, the strobe is fired as shown in line 3(D) to augment ambient light levels. Line 3(E) illustrates a light exposure profile indicating the light energy incident upon the film due to the shutter opening and the strobe firing. Once the exposure control system determines that sufficient light energy has been allowed to pass through to the film, the direction of current flow to the motor is reversed forcing the shutter to close rapidly.

The final stages of the process are more specific to the self-developing camera used herein as an example. The post-exposure processing stage includes moving the exposed film though the internal film developing system in order to produce a final photograph. The strobe regulation system then keeps the camera ready to take another photograph for an additional thirty seconds before shutting down the system.

Figure 4 is a more detailed illustration of how the pulse width modulation is used to control the exposure interval and provide voltage regulation without a hardware regulator.

Figure 4(A) illustrates the shutter aperture opening, or aperture 13, increasing over time. This is a parabolic curve that will vary according to the shutter design. The point at which the shutter 14 has opened to allow sufficient light energy, as determined by the processor, is represented by the small letter tau, τ. In the preferred embodiment, the time from fully closed to fully open is forty-five (45 ms) milliseconds when the motor 30 is provided an average voltage of 3 Volts.

Figure 4(B) shows an exposure design curve for the photographic device used as an example herein. Note that there is a delay from time zero when the motor 30 is initiated until first light is attained. This delay compensates for non-linear motor start-up which then allows the motor 30 to achieve essentially linear movement.

Figures 4(C) and 4(D) show two possible examples of PWM signals 28 transmitted by the processor 12 to the motor 20. The period, and therefore frequency, of the PWM signal 28 is constant and is represented by the capital letter "T". In the preferred embodiment, the period is one millisecond (1 ms.). In order to determine the duty cycle of the pulse train, the processor looks at the voltage across the battery 18 while under load and changes the duty cycle to force the motor to operate a predetermined voltage level thereby regulating the voltage to the motor.

By way of example, Figure 4(C) shows a pulse train where the duty cycle was determined to be fifty percent. This means that the pulse width, τrj, is equal to one half of the period, T. This being the case, the average voltage seen by the motor 30 is fifty percent of the pulse, as signified by VA- For example, if the battery 24 indicated a voltage

level of Vi = 6V then this duty cycle would provide an average voltage of three volts,

V_A = 3V.

Figure 4(D) shows a depleted battery that can not provide the pulse height as shown in Figure 4(C). The processor 12 compensates by increasing the duty cycle and, therefore, the pulse width, τ\. For example, if the battery indicated a load voltage level of

V2 = 4V and the design requires an average voltage of 3 V then the duty cycle would be

seventy-five percent making VA ⁼ 3 V.

Also illustrated in Figure 4(D) is the substantial control exercised by the processor to adhere to the exposure design curve. A complete final pulse in the pulse stream would provide more voltage to the system than is necessary. This would cause the shutter 14 to open more than required which, in turn, would be exaggerated by the parabolic nature of the shutter aperture. Therefore, the pulse is discontinued after transmission of only the portion of the pulse thereby attaining the proper blade position.

The invention as herein described also further simplifies the design and manufacturing process. The invention may be used with a wide variety of photographic device designs. In each such design, the design curve of the new device is simply programmed into the processor. Such constants as the average voltage transmitted to the motor can be adjusted as per the design with minimal effort. The calibration step of the manufacturing process is also further simplified. Given the variability of the actual resistance of resistors and other circuit components, the actual voltage required to achieve a constant motor speed may vary. Calibration of the invention in such a case would simply require that changing of a processor variable.

Claims

1. An apparatus for controlling exposure in a photographic device comprising

a shutter having a blocking position normally blocking a film exposure opening being movable during an exposure interval to an unblocking position to effect film exposure and then being movable back to said blocking position to terminate the exposure interval;

drive means connected to said shutter and responsive to a predetermined applied voltage for driving said shutter between said blocking and unblocking positions;

a voltage source;

voltage sensing means electrically connected to said voltage source for determining a voltage level of the voltage source; and

control means responsive to said voltage level of the voltage source for controlling operation of said drive means by generating a periodic signal having a duty cycle which is adjusted to apply said predetermined applied voltage to said drive means.

2. The exposure control system according to claim 1 wherein said control means comprises reversing means for terminating said exposure interval by reversing a direction of current flow to the drive means such that the drive means drives the shutter to said blocking position.

3. The exposure control system according to claim 2 wherein said control means further comprises exposure evaluation means for generating an output value representative of scene brightness;

integration means electrically connected to said exposure evaluation means for receiving said output value and integrating the output value over time to quantify an amount of light energy passing through said film exposure opening; and

terminating means responsive to said integration means for discontinuing said sequence of drive pulses upon determination of sufficient light energy having passed through the film exposure opening, and for signaling to said reversing means to terminate said exposure interval.

4. The exposure control system according to claim 1 wherein said control means comprises means for varying the duty cycle of said sequence of drive pulses between zero and one hundred percent.

5. The exposure control system according to claim 1 wherein the control means comprises means for transmitting said applied voltage signal with a reverse current to the drive means while the drive means is in the blocking position such that said drive means provides a load on the voltage source.

6. The exposure control system according to claim 1 wherein the exposure control system further comprises tractive means mechanically connected to said shutter for biasing the shutter toward the blocking position.

7. The exposure control system according to claim 6 wherein said tractive means comprises an elastic device having an end fixed with respect to the shutter.

8. The exposure control system according to claim 6 wherein said tractive means comprises an inertial plate responsive to said control means to release tension from said shutter.

9. The exposure control system according to claim 6 wherein said tractive means comprises a mechanical latch responsive to said control means to unlatch said shutter allowing the shutter to move.

10. An exposure control system comprising:

shutter means selectively movable between a first position in blocking relation to an exposure opening and a second position defining a given aperture value in unblocking relation to the exposure opening, said shutter means for defining a progressive variation of aperture values over the exposure opening when so moved between said first and said second positions inclusive;

drive means responsive to an input voltage for effecting movement of said shutter means between said first and said second positions;

a voltage source;

voltage sensing means for determining a voltage level of the voltage source, said voltage sensing means comprising

load means for reversing a voltage sense of said input signal to the drive means such that the drive means provides a load on said voltage source without moving said shutter means from said first position; and

voltage evaluating means for determining a load voltage representative of voltage across said voltage source while under the load provided by said load means;

control means for regulating the input voltage to said drive means in response to said load voltage.

11. The exposure control system according to claim 10 wherein said control means further comprises

pulse width modulation means for generating a periodic pulse train with a duty cycle which is variable in response to the voltage level from said voltage sensing means, said periodic pulse train actuating said drive means to move said shutter means from said first position to said second position establishing a film exposure interval; and

blocking means for actuating said drive means to move said shutter means from the second position to the first position to block said exposure opening and terminate said exposure interval.

12. The exposure control system according to claim 11 wherein the drive means comprises a direct current driven electric motor where the input voltage is an average voltage of the periodic pulse train.

13. The exposure control system according to claim 12 wherein said control means further comprises reversing means for terminating said exposure interval by reversing a direction of current flow through the drive means such that the drive means drives the shutter to the first position.

14. The exposure control system according to claim 11 wherein said control means further comprises means for varying the duty cycle of said sequence of drive pulses between zero and one hundred percent.

15. The exposure control system according to claim 10 wherein the exposure control system further comprises tractive means mechanically connected to said shutter for biasing the shutter toward a blocking position.

16. An exposure control system comprising a shutter having a blocking position normally blocking a film exposure opening being movable during an exposure interval to an unblocking position to effect film exposure and then being movable back to said blocking position to terminate the exposure interval;

drive means energizable by a sequence of drive pulses to move said exposure means between said blocking and unblocking positions;

a voltage source; and

control means for governing said drive means to control said exposure interval, said control means comprising

voltage evaluation means for receiving a signal from the voltage source and determining an average voltage necessary to propel the drive means at a predetermined speed;

generator means for driving said sequence of drive pulses with a constant period to provide said average voltage to said drive means, and for altering a pulse width of the sequence of drive pulses in response to the voltage evaluation means where said pulse width may vary from zero to one hundred percent of the period and where a direction of current flow through the drive means can be reversed such that said drive means operates in a reverse direction.

17. The exposure control system according to claim 16 wherein the drive means comprises direct current driven electric motor responsive to the average voltage of the pulse train.

18. The exposure control system according to claim 16 wherein said control means further comprises reversing means for terminating said exposure interval by reversing the direction of current flow through the drive means such that the drive means drives the shutter to blocking position.

19. The exposure control system according to claim 16 wherein said control means further comprises

exposure evaluation means for generating an output value representative of scene brightness;

integration means electrically connected to said exposure evaluation means for receiving said output value and integrating the output value over time to quantify an amount of light energy passing through the exposure opening; and

terminating means responsive to said integration means for discontinuing said sequence of drive pulses upon determination of sufficient light energy having passed through the exposure opening, and for signaling to said reversing means to move the shutter to the blocking position.

20. The exposure control system according to claim 16 wherein the exposure control system fUrther comprises tractive means mechanically connected to said shutter for biasing the shutter toward a blocking position.

21. The exposure control system according to claim 15 wherein said voltage sensing means comprises load means electrically connected to said drive means for actuating said drive means and placing a load on said voltage source.

22. A method for controlling exposure in photographic devices comprising

determining a voltage level of a voltage source; governing an exposure interval by receiving the voltage level of the voltage source and determining an average voltage necessary to propel a motor at a predetermined speed; and

driving a periodic sequence of drive pulses and altering a duty cycle of the sequence of drive pulses to transmit said average voltage;

receiving said periodic sequence of drive pulses by the motor to govern said exposure interval;

opening a shutter having at least one light regulating element movable during the exposure interval to derive a dynamic definition of aperture values unblocking an exposure opening, and closing the shutter to block said exposure opening to terminate said exposure interval.

23. The method according to claim 21 comprising the additional step of evaluating a scene and returning an output value which corresponds with an exposure evaluation.

24. The method according to claim 21 comprising the additional steps of monitoring exposure light voltage and reporting a digital representation corresponding thereto; and terminating the pulse train in response to said digital representation.