1. Field of the Invention
The present invention is in the field of electronic sensors and pertains particularly to an improved reflectance sensor usable in various commercial and consumer applications.
2. Discussion of the State of the Art
In the field of sensory devices, more particularly electronic sensors, proximity sensing and motion detection are regimens that provide contact-less control and object detection useful in a large variety of consumer, industrial, and security applications.
Development of various electronic technologies for proximity sensing has occurred and development continues. Each accepted technology has provided one or more advantages depending on the specific application of those technologies. These techniques can be classified in terms of the operating principle of the device versus the detection medium used, whether light, radio waves, or the like.
To provide one example, a well-known proximity sensor based on measuring an echo transit time that uses radio waves as a medium is called a radar. Another echo-principled proximity sensor that uses sound as a medium is called a sonar sensor. Still, another echo-principled proximity sensor that uses light as a medium is called a light-imaging detection and ranging (LIDAR) sensor. Although these classic echo-based sensors provide relatively accurate distance and speed information, they can, depending on the application, be expensive, bulky, may consume high power, and/or may require very high frequency technologies to be successful.
In contrast to the sensor device types described above, there are less expensive and much smaller proximity sensors that use field disturbance techniques to detect proximity. These sensors may be classed as either passive or active sensors. These types of sensors detect proximity base on changes in a field caused by interactions with a detected object.
Of the above-described sensors, passive field-disturbance detectors use background radiation or the emissions of an object as the source of the field. To exemplify, a sound-activated switch may be provided in a “smart toy” to sense when a human is talking nearby. In another case of what would be termed a passive sensor, an infrared motion detector may sense changes in the infrared background due to movement of objects irradiating infrared above the background because of their higher temperature. Such passive infrared sensors are widely used in high-volume applications for alarm systems, automatic light turn-on sensors, and the like.
An active proximity sensor may detect changes in field disturbances caused by an active source. One example of such an active sensor may be a magnetic field-disturbance proximity sensor (metal detector) that senses changes in a local alternating current (AC) magnetic field due to eddy current or magnetic characteristics of metal objects. Another well-known example is that of a “stud finder” used in carpentry. A stud finder is an AC electrostatic field disturbance sensor that senses the change in dielectric constants between air and wood as the finder is slid across overlying plaster.
Still another example of an active field disturbance detector is a proximity sensor that detects changes in optical reflectance, avoiding technical difficulties of nanosecond time resolution necessary for RF or optical echo transit time devices. Optical reflectance proximity sensors have certain key advantages over other types of proximity sensors for sensing objects in the range of 1 to 100 centimeters (cm). The advantage may be due to the fact that they typically use a small LED as a light source and a small photodiode driving a receiver circuit. Another advantage is that they can be small enough to fit in miniature electronic devices. Even these types of active reflectance sensors, although fairly small and robust, have been too expensive for consumer applications and consequently have found applications mostly in the industrial and commercial markets.
Passive infrared sensors are the most common in consumer application due to lower cost. However, they are too large for many consumer products because they require a collecting lens and a photodiode of several centimeters in diameter to gather a sufficient signal to sense changes in the ambient background visible to the sensor. Moreover, a major handicap of such sensors is a lack of reliability earmarked by spurious triggering and by failure to trigger. For example, large, far-away objects or sudden temperature changes may trigger them and they can completely miss radiation-neutral objects.
Reflectance sensors have been developed that overcome the handicaps described immediately above even though they do not inherently measure distance (as echo transit-time devices do). Reflectance sensors can detect an object moving into certain ranges unambiguously because of a strong fourth-power decrease of reflected light from the object sensed. A 20% change in distance will cause approximately a 100% change in reflected signal. For objects with a 10-to-1 variation in reflectance between them, this amounts to less than 50% difference in detection range.
Small, short range (1 cm to 2 m) reflectance proximity sensors are more useful than passive infrared sensors in many applications. However, widespread use in consumer applications is not apparent because of higher cost factors.
In general, proximity-sensor applications break down into two broad functional groups: (1) those that provide an on-off function and (2) those that provide analog or digital proportional information. Examples in the first category include: automatic flushing for a public lavatory; an automatic doorbell that detects a person passing through a door; or an object sensor on an automatic production line. Examples in the second category are: an automobile bumper warning indicator that puts out an audible warning signal whose pitch is proportional to the distance from an obstacle; a toy car that slows down when it approaches an object and steers away from it; or a light switch that can be activated and dimmed by waving one's hand near it.
Proximity sensors that provide on-off functions can often serve as replacement for switches that are either operated manually or by some other machine function. In both cases, the electronic proximity switch will be more reliable than a standard mechanical switch especially if a very high number of cycles occur over the life of the switch. But since most proximity sensors require significant amounts of power, they are not normally used for power switches on battery-powered products. Of course, like normally open or normally closed switches, a proximity sensor can provide either an “on” or “off” function when an object moves in or out of proximity.
Further to the above, some proximity-sensor applications also need to measure the ambient background light such as a proximity-activated security light. Proximity sensors that provide analog proportional information can functionally replace analog controls allowing smarter processing of proximity information for more complex applications. However, unlike on-off proximity sensors, proportional sensors generally need to interface to a microprocessor.
The inventors are aware of a method taught by Holcombe (U.S. Pat. No. 5,864,591) for using circuitry to reduce feedback in an infrared data receiver. The method includes a circuit that is configured as an infrared receiver including an automatic gain control (AGC) circuit where the AGC is isolated from the input to the receiver in response to the output signal from the receiver in order to suppress the effect of feedback from the output signal to the input of the receiver.
The inventors are also aware of an enhancement to the method taught by Holcombe (U.S. Pat. No. 6,240,283). The enhancement includes a method and apparatus for controlling the input gain of a receiver whereby the input gain is controlled by sampling an amplified data signal during a time interval when a positive-going feedback transient from an output terminal of the receiver to an input terminal of the receiver is not present in the amplified data signal.
In this enhanced circuit, an input amplifier has variable gain determined by a gain control signal, a comparator which compares the amplified data signal from the input amplifier to a detection threshold voltage to produce a demodulated data signal and an analog delay circuit which delays the amplified data signal by a predetermined time interval to produce a delayed data signal. The method is enabled by a switch that is driven by the demodulated data signal to sample the delayed data signal for input to an automatic gain control circuit. The automatic gain control circuit compares the sampled delayed data signal to an automatic gain control threshold potential and rectifies and integrates the resulting waveform to produce the gain control signal.
In one application, the data signal is amplified by a gain factor and the signal is then compared to a detection threshold voltage to produce a demodulated data signal. The amplified signal is also delayed to produce a delayed data signal. The delayed signal is sampled using the demodulated data signal to produce a sampled data signal that is used to adjust the gain factor in the amplifier. Although the technique described by Holcombe may provide some feedback immunity from a detected infrared receiver output to a photodiode in a IRDA communications receiver, there may also be significant feedback from the LED driver to the photodiode. For example, the LED driver produces both a voltage and an inductive current transient when it initially turns on. The voltage transient can couple to the photodiode input via wire-bond and PCB-trace capacitances. The inductive transients can couple to the photodiode input through ground and power-supply traces. These transients (voltage and inductive) may produce spurious signals that mask low-level reflectance signals, thus limiting the minimum detectable signal levels in the communications receiver circuit.
It has occurred to the inventor that with some innovative enhancement to the feedback immunity techniques described in U.S. Pat. No. 5,864,591, and in U.S. Pat. No. 6,240,283, a low cost optical reflectance proximity sensor could be provided that could overcome the problems associated with the relevant art described above.
Therefore, what is clearly needed in the art is a reliable and sensitive optical reflectance proximity sensor that is very small and inexpensive to manufacture. Such a sensor would consume very little power, would not be required in all applications to interface with a microprocessor, and could be implemented in some analog output applications without the complexity and cost of using a standard digital-to-analog (DAC) converter.