Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Patent
1994-06-17
1995-10-03
Westin, Edward P.
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
353101, G03B 2153
Patent
active
054554110
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to an autofocus device for still projectors, comprising a controlled measuring radiation source emitting a beam of radiation which is directed onto the surface of a slide to be focussed and is reflected therefrom to a dual photoreceiver mechanically coupled to a focussing device, the signals of said photoreceiver activating an electronic control means such that a servomotor connected thereto influences the focussing device until the measuring radiation reflected by the slide impinges on the central portion of the dual photoreceiver.
BACKGROUND OF THE INVENTION
Autofocus devices for still projectors using controllable measuring radiation sources are known.
EP-PS 0131 794 discloses an autofocus device for still projectors in which an infrared light emitting diode is used as a measuring radiation source which is operated by means of the half-wave voltage of a power rectifier and whose emission is controlled in response to the signal obtained from a dual photoreceiver. The device also includes a differential amplifier which amplifies and rectifies only the alternating portion of the signals received from the dual photoreceiver. An additional amplifier amplifies the rectified signal to activate a d.c. motor of a focussing drive.
It is the object of the present invention to provide an autofocus device of the generic type which effects fast and reliable focussing of a slide and which, in particular, is insensitive to interference signals and component tolerances and has a short reaction time and high positioning accuracy. Moreover, it should be guaranteed that slides which have different reflection factors are also quickly and reliably focussed.
SUMMARY OF THE INVENTION
The above object is attained by a generator circuit for controlling the measuring radiation source by means of variable current pulses and a signal processing circuit for digital signals.
In a preferred embodiment of the invention, the generator circuit is provided with a controllable ramp generator to produce sawtooth current pulses, and with a control circuit to control their leading edge slopes in response to the signals obtained from the dual photoreceiver. Furthermore, the signal processing circuit is provided with a threshold circuit to produce digital input signals from the signals obtained from the dual photoreceiver, and with a signal decoding circuit for separating digital direction signals and digital speed signals from the input signal.
Expediently, the autofocus device is designed such that the ramp generator is provided with a clock control circuit to control the current pulses in their time-related sequences and duration, and with a limiting circuit to control amplitude and duration of the current pulses. Moreover, the signal processing circuit is provided with a signal conversion circuit for the speed signal to produce a motor control voltage for the servomotor, and with a signal control circuit to centrally control the circuits of the measuring radiation source and the servomotor. The measuring radiation source is designed as a diode emitting I.R. radiation.
Advantageously, the embodiment according to the invention includes an interface circuit within the signal processing circuit by means of which the circuits of the measuring radiation source and the servomotor are controlled by digital control signals of a microprocessor unit and/or a remote control unit.
Further features and advantages will be apparent from the description of an embodiment of the invention shown in the drawing as well as from the subclaims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the autofocus device according to the invention in a schematic overall representation,
FIG. 2 shows the autofocus device of FIG. 1 in a block diagram,
FIG. 3 is another embodiment of the autofocus device of FIG. 2, and
FIG. 4 is a functional diagram of the autofocus device of FIG. 1.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS
The autofocus device shown in FIG. 1 includes a measuring radiation source 1 having a measur
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patent: 4198138 (1980-04-01), Parker et al.
patent: 4220850 (1980-09-01), McEachern
patent: 4494839 (1985-01-01), Brueckner
patent: 4743109 (1988-05-01), Harvey
patent: 4748469 (1988-05-01), Tamura
patent: 5379082 (1995-01-01), Staiger
Boos, Jr. Francis H.
Eastman Kodak Company
Lee John R.
Westin Edward P.
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