Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1998-11-30
2001-04-24
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S2140RC, C345S165000
Reexamination Certificate
active
06222182
ABSTRACT:
TECHNICAL FIELD
This invention relates to apparatus and methods for sampling the output of a phototransistor, particularly for optical encoding systems employing phototransistors in user input devices such as thumb-wheels, finger-wheels, mice, trackballs, and the like. More particularly, this invention relates to an opto-mechanical encoding system in which a potential difference is applied intermittently across the collector/emitter of a phototransistor to control sampling of the phototransistor output so as to provide an essentially immediate response representative of the light flux at the phototransistor. The apparatus and methods of this invention may also be applied to other transistors.
BACKGROUND OF THE INVENTION
User input devices such as mice and trackballs for use with computers and other electronic devices commonly use opto-mechanical encoding to sense position and/or movement. Motion of the ball of a mouse or trackball, or motion of the wheel and/or shaft of a thumb- or finger-wheel, for example, typically rotates a pair of encoding wheels having light-transmitting and light-blocking regions
Each encoding wheel is typically positioned between one or two light sources in the form of LEDs (light emitting diodes) and two light sensors in the form of PTRs (phototransistors). For each PTR, the surface area of the PTR exposed to the light from the LED(s) is directly correlated with the position of the encoding wheel, and may be approximately represented by a periodic function of the position of the encoding wheel. The signal voltage at the emitter of each PTR is, in typical configurations, directly proportional to the surface area of the PTR exposed to light. The light source(s) and the two PTRs are typically positioned, relative to each other and to the encoding wheel, such that the signal voltage from one PTR varies approximately in quadrature with the signal voltage from the other PTR, as a function of the position of the encoding wheel. The signals from the two PTRs, taken together, are thus representative of the velocity and direction of motion of the encoding wheel: the frequency of the signals indicates the velocity, and the relative phase indicates the direction, of the encoding wheel.
A typical input device includes at least two (one for each of two orthogonal directions of ball rotation), and often three encoding wheels, resulting in the use of four or six PTRs, and a minimum of 2 or 3 LEDs. A microcontroller is employed to control and interpret the sampling of the PTRs and to provide communication with a host device.
Some optical encoding methods require sample times of 50-100 &mgr;s or more. Many traditional opto-mechanical encoding systems, while reliable and low-cost, may require as much as a 3-4 mA average, and 10 mA peak, current. Minimizing this current draw provides an important advantage in battery-operated or parasitically powered devices. Long sampling intervals tend to result in higher average current than short sampling intervals. Long sampling intervals also limit the tracking performance. Shorter intervals allow higher-resolution tracking, or higher-speed tracking without aliasing, or both.
SUMMARY OF THE INVENTION
According to the present invention, a PTR in an optical encoding system is driven or “turned on,” for sampling, by the application of a potential difference across its collector and emitter. A driving signal for producing the potential difference may be supplied, for example, from an output pin of an optical encoding system microcontroller.
An LED associated with the phototransistor may be excited with a low, continuous current. Prior to sampling the PTR, the collector and emitter are held at the same or nearly the same potential. Light from the LED reaching the PTR creates an excess of charge in the electrically isolated base of the PTR. As long as the potential between the emitter and collector of the phototransistor remains zero, transistor action cannot occur. Upon switching the voltage at the collector and/or emitter such that the collector/base junction is back biased and the base/emitter junction is forward biased, electrons can flow, transistor action can occur, and the PTR creates a large and very rapid current flow, in effect causing nearly instantaneous PTR response. For PTRs arranged for sampling of the emitter response, this virtually eliminates the effects of the collector/base capacitance on the emitter response time. Since the current developed by the PTR is proportional to the amount of light received by the PTR, the net effect is that the initial rising edge of the emitter output reaches a voltage level proportional to the modulation level of the encoder wheel, with a subsequent decay.
With the present invention, LED current may be set to a relatively low value, such as 1 to 1.5 mA or even lower, resulting in the advantage of some power savings. The LED may also have a reduced duty cycle, for further power savings.
In applications where high performance is required, the fast emitter response can allow very high sampling rates, in excess of 5 kHz if desired, thus enabling realization of high performance systems with greater tracking velocity and/or higher resolution than most current designs.
Additional features and advantages of the invention will be made apparent from the following detailed description of an illustrated embodiment, which proceeds with reference to the accompanying drawings.
REFERENCES:
patent: 5252968 (1993-10-01), Donovan
patent: 5384457 (1995-01-01), Sommer
patent: 5898170 (1999-04-01), Featherston et al.
patent: 0 310 230 (1989-04-01), None
Featherston Lord Nigel
Lee Mark R.
Mauro, Jr. Charles Salvatore
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Lee John R.
Microsoft Corporation
Pyo Kevin
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