Abstract:
An object detector which illuminates an area with a pulsating beam of light and measures light reflected back by an object in the beam. The light source is chopped by three different chopping frequencies. The light detector uses the highest frequency to reject out of band ambient signals and noise, uses the middle frequency for synchronously detecting the reflected signal, and uses the lowest frequency to ratio the output to the noise floor. The light detector uses linear gain prior to filtering out of band ambient signals to avoid intermodulation effects. The light detector uses logarithmic gain after the filtering to avoid overload and transient effects, without requiring an automatic level control circuit.

Description:
BACKGROUND 
     The invention relates to optical object detection. 
     Typical optical object detectors illuminate an area with light, usually in the infra-red region of the light spectrum. Any object entering the illuminated area will reflect some of the light. A photodetector circuit detects the reflected light. If the object is larger than a minimum size, the detected signal exceeds a threshold and causes an output signal to be generated indicating the object&#39;s presence. To detect as small an object as possible at the greatest distance possible, the photodetector circuit must be as sensitive as possible. There are three main ways to increase the sensitivity of the photodetector circuit: 
     1. Increase the signal to noise ratio of the signal resulting from the intended light source being reflected by the object. 
     2. Reduce the effect of interfering signals resulting from non-intended light sources directly entering the photodetector. 
     3. Reduce the effects of drift mechanisms, such as temperature fluctuations, on the detection threshold. 
     Non-intended light sources form a varying ambient signal in the photodetector circuit. The desired signal (due to the intended illuminating light source, reflected by the object) rides on top of this ambient signal. As the ambient signal increases, it causes more current to flow in the photodetector. The increased current results in increased detector noise. This is called detector noise floor modulation. For maximum sensitivity, the threshold that the desired signal must overcome is set as low as possible. Both the ambient signal and the increased noise resulting from it force the threshold to be increased, thus reducing sensitivity. Also, as the ambient signal increases, it causes the desired signal to decrease due to non-linear effects in the photodetector. This is called detector saturation. This reduces the signal to noise ratio of the desired signal, again reducing sensitivity. 
     Many techniques have been used to improve sensitivity. One such technique is to chop the intended illuminating light source at a fixed frequency (called the carrier frequency) and filter the intended signal from the photodetector to pass only this frequency. This technique rejects all ambient signals outside the bandwidth of the carrier filter. It also increases the signal to noise ratio of the desired signal by rejecting the noise outside the bandwidth of the carrier filter. An extension of this technique is to use synchronous detection. Synchronous detection rejects ambient signals and noise that are not in phase with the carrier frequency, by using a product detector instead of a diode detector. Synchronous detectors require the carrier frequency signal to be routed into the detector circuitry for the purpose of product detection. The carrier frequency signal must be very well isolated from the optical signal amplifier otherwise the routed signal will swamp the optical detected signal. Such high isolation increases product cost. This carrier frequency filtering technique and its synchronous detection extension do not reduce the effects of detector noise floor modulation or detector saturation caused by ambient signals within the bandwidth of the carrier filter. Since higher frequency ambient signals have lower amplitudes, a higher carrier frequency will reduce the effects of ambient signals. But since photodetector preamp noise increases with frequency, a higher carrier frequency decreases the signal to noise ratio of the desired signal. Thus, there is an optimum carrier frequency. 
     Another technique is to incorporate an automatic level control (ALC) in the photodetector circuit. The ALC maintains the peak of the detected signal at a fixed amplitude by varying the gain of an amplifier. This technique reduces the effects of ambient signal, including detector noise floor modulation, but only if the ambient signal varies at a rate slower than the response time of the ALC. The response time of the ALC must be slower than the carrier period (carrier period is the inverse of the carrier frequency), otherwise the desired signal amplitude will be reduced. The ALC also prevents the photodetector amplifier from being saturated by too strong a signal (saturation is a common problem due to the high gain required for sensitive detection). The ALC can also help reduce the effects of drift mechanisms on the detection threshold. 
     Other techniques which prevent amplifier saturation due to strong ambient signals are electrical ambient cancellation circuitry and optical filtering. The former cancels ambient signals at signal frequencies below the carrier frequency at the input of the preamp rather than at the output (this is done by negative feedback). The latter (optical filtering) is a window or an encasement around the photodetector which filters out all optical energy outside a bandwidth centered around the infra-red radiation frequency of the intended light source. 
     Finally, temperature compensation techniques reduce the effects of temperature on the detection threshold, allowing the threshold to be set closer to the noise floor. 
     OBJECTS AND ADVANTAGES 
     My optical object detector uses a low gain linear photodetector preamplifier. The gain of the preamplifier is low enough to prevent saturation of the preamplifier output by strong ambient signals, but high enough to maintain good signal to noise ratio. The preamplifier output signal is filtered by a carrier band pass filter to filter out ambient signals and noise which are outside the bandwidth of the carrier filter. The output signal of the carrier filter is detected by a sensitive multistage logarithmic detector instead of a diode junction. Such a detector is typically the signal strength output of an intermediate frequency amplifier/demodulator integrated circuit (the normal demodulator output of the integrated circuit is not used). 
     The illuminating LED is chopped at three different frequency rates. The high frequency chopping rate is the carrier frequency. The medium frequency chopping rate is used to synchronously detect the illuminating signal reflected from an object and detected by the photodetector circuit. The low frequency chopping rate is used to give the photodetector circuit a noise reference to compare to (when the LED is not emitting light), so that the photodetector circuit can produce an output signal which is indicative of the signal to noise ratio of the signal resulting from the reflected light rather than just the amplitude of the signal compared to a fixed voltage level. The carrier frequency could also be used for synchronous detection, but this would be more expensive due to the high isolation required between the carrier signal and the preamp input circuit. 
    
    
     DRAWING FIGURES 
     FIG. 1 is a schematic circuit diagram of a circuit constructed according to one aspect of the invention. 
     FIG. 2 is a schematic circuit diagram of a circuit constructed according to another aspect of the invention. 
    
    
     DESCRIPTION 
     FIG. 1 shows an object detector circuit  7  constructed according to the invention. Generally, it includes a light source circuit  8  and a light receiver circuit  9 . 
     A high frequency oscillator  10  is connected to one side of a switch  11  by a line  12 . The other side of switch  11  is connected to one side of a switch  13  by a line  14 . The other side of switch  13  is connected to the input of a power amplifier  15  by a line  16 . The output of the power amplifier is connected to one side of an LED (light emitting diode)  17  by a line  18 . The other side of the LED is connected to the circuit reference ground. 
     A photodetector  19  is connected to the input of a low noise linear preamplifier  20  by a line  21 . The output of the preamplifier connects to the input of a band pass filter  22  by a line  23 . The output of the band pass filter connects to the input of an IF (intermediate frequency) amplifier  24  by a line  25 . The IF amplifier has a multi-stage logarithmic signal strength detector. The signal strength output  26  of the IF amplifier is connected to one side of a switch  27  and one side of a switch  28  by a line  29 . The other side of switch  28  is connected to one side of a capacitor  30  and to one side of a resistor  32  by a line  34 . The other side of capacitor  30  is connected to the circuit reference ground. The other side of resistor  32  is connected to one side of a resistor  36  and the positive input of an op amp  38  by a line  40 . The other side of resistor  36  is connected to the circuit reference ground. The other side of switch  27  is connected to one side of a capacitor  31  and one side of a resistor  33  by a line  35 . The other side of capacitor  31  is connected to the circuit reference ground. The other side of resistor  33  is connected to one side of a resistor  37  and the negative input of op amp  38  by a line  39 . The other side of resistor  37  is connected to the output of op amp  38  and to one side of resistor  41  and to one side of switch  42  by a line  43 . The other side of switch  42  is connected to one side of a capacitor  44  and the positive input of an op amp  46  by a line  48 . The other side of capacitor  44  is connected to the circuit reference ground. The other side of resistor  41  is connected to one side of resistor  45  and to the negative input of op amp  46  by a line  47 . The other side of resistor  45  is connected to the output of op amp  46  and to the circuit output  49  by a line  50 . 
     A medium frequency oscillator  51  is connected to a line  52 . A low frequency oscillator  53  is connected to a line  54 . 
     Typically, all resistors except  41  and  45  would have the same value. Resistor  41  would typically have a value of 100 kilo-Ohms and resistor  45  would typically have a value of 500 kilo-Ohms. Capacitors  30  and  31  would typically have a value of 1000 pico-Farads and capacitor  44  would typically have a value of 1 micro-Farad. 
     Operation 
     Oscillator  51  produces a signal vm(t) on line  52 . Signal vm(t) closes and opens switches  13 ,  27 , and  28  at a frequency rate of fm, such that whenever switch  13  is closed, switch  28  is also closed but switch  27  is open, and whenever switch  13  is open, switch  28  is also open but switch  27  is closed. Oscillator  53  produces a signal vr(t) on line  54 . Signal vr(t) closes and opens switches  11  and  42  at a frequency rate of fr, such that whenever switch  11  is closed, switch  42  is open, and vice versa. Oscillator  10  produces a signal vc(t) on line  12 . When both switches  11  and  13  are closed, signal vs(t) on line  16  is the same as signal vc(t) and causes LED  17  to emit light energy in pulses which occur at a frequency rate of fc. This is the intended light energy. 
     If an object  55  is present in the area illuminated by the LED light energy, the object reflects some of the light energy back to photodetector  19 . The resulting signal from the photodetector is amplified by preamplifier  20  and filtered by filter  22 . IF amplifier  24  amplifies the signal from filter  22  and detects it, producing a signal on line  29  which is proportional to the logarithm of the peak absolute value of the amplified signal. The logarithmic relationship holds over a very wide range of signal levels because the signal strength detector inside amplifier  24  is composed of many sections of diode detectors with a crossover addition circuit. 
     Switch  28 , being closed, charges capacitor  30  to a voltage which is indicative of the detected light energy during the period when the LED is emitting light. When switch  13  is open, no light energy is emitted by LED  17 . Switch  27 , now being closed, charges capacitor  31  to a voltage which is indicative of the noise level on line  29 . Op amp  38 , being configured as a differential amplifier, produces a noise ratioed signal on line  43  which is proportional to the ratio of the signal on line  29  when the LED is emitting light to the noise on line  29  when the LED is not emitting light. When switch  11  is closed, switch  42  is open and the noise ratioed signal on line  43  is amplified by op amp  46 . When switch  11  is open, LED  17  does not emit any light regardless of the state of switch  13 . During this time, switch  42  is closed and capacitor  44  charges to a voltage which is indicative of the noise level on line  43 . If not for circuit offsets, this voltage would be 0 volts. Op amp  46 , being configured as a differential amplifier, thus only amplifies the deviation from the noise. It is important to cancel circuit offsets in this way, because op amp  46  has high gain for maximum detection sensitivity. The voltage at output  49 , when filtered, is indicative of how much light energy has been reflected back by an object in the illuminated area. 
     For sensitive detection, conventional object detectors use a low detection threshold. This also lessens the requirements on their ALC circuit by providing more dynamic range so the ALC does not need to be activated as often. But if the threshold is too low, it will be affected by noise modulation due to strong ambient signals and it will be too temperature sensitive. Therefore, it will need to be temperature compensated and frequently adjusted. The ALC circuit, being susceptible to transients, will also limit how low a threshold can be used. 
     Since my object detector circuit provides an output which is ratioed to the noise floor instead of a signal level, it is insensitive to temperature and noise floor modulation effects. Therefore, the detection threshold at output  49  can be fixed and never needs adjustment. The actual threshold can be set at any value since circuit gain can be adjusted so that noise produces an output just below the threshold when no objects are in the illuminated area. As the noise floor changes due to ambient signals, temperature effects, aging, etc., the signal at output  49  will remain unchanged and just below the detection threshold as long as there is no object in the illuminated area. Thus maximum sensitivity is preserved without the need for recalibration. 
     The preamplifier being linear, does not translate ambient signals from outside the carrier filter bandwidth to inside the carrier filter bandwidth. The preamplifier gain can be kept to a low transconductance value of only 100,000 (thus preventing saturation) because the following stage, IF amp  24 , is a very sensitive, low noise amplifier. The wide dynamic range of 10,000 (80 decibels) of the amplifier and logarithmic detector in IF amp  24  insures that the photodetector circuit never saturates, even in broad daylight, and without requiring an ALC circuit. 
     The synchronous detection performed by switches  27  and  28  and capacitors  30  and  31  improves the signal to noise ratio by rejecting out of phase ambient signals and noise. Because the synchronizing signal is not the carrier (fc) but rather a modulation of the carrier (fm), it cannot couple into the highly sensitive high gain preamp  20  and associated circuitry. Thus there are no costly signal isolation requirements. 
     Setting the object detection threshold to be a predetermined signal to noise ratio (noise ratiometric) rather than a predetermined signal level reduces noise modulation effects caused by all ambient signals (even those inside the carrier filter bandwidth) without requiring an ALC circuit. A noise ratiometric detection threshold also eliminates the effects of temperature and other drift mechanisms on the detection threshold since no stable absolute voltage reference level is required. 
     Not requiring an ALC circuit, my object detector is not susceptible to saturation and other effects due to transients. Thus, maximum sensitivity is not only preserved in broad daylight, but also in a fast moving vehicle that moves quickly into and out of shady areas. 
     Alternative Embodiments 
     Other switching circuits can be constructed according to the invention. Modifications and combination circuits can be made by one of ordinary skill in the art without necessarily departing from the spirit and scope of the invention. For example, FIG. 2 shows an object detector circuit  100  constructed according to the invention. Generally, it includes a carrier oscillator  110 , a chopping oscillator  112 , a modulation oscillator  114 , a light source  116 , a light detector  118 , a filter  120 , a logarithmic amplifier  122 , a synchronous demodulator  126 , and a signal to noise ratioing circuit  129 . The carrier oscillator is connected to the chopping oscillator by a line  111 . The chopping oscillator is connected to the modulation oscillator by a line  113  and to circuit  129  by a line  128 . The modulation oscillator is connected to the light source by a line  115  and to circuit  126  by a line  125 . The light detector is connected to the filter by a line  119 . The other side of the filter is connected to amplifier  122  by a line  121 . The logarithmic detection output  123  of amplifier  122  is connected to synchronous demodulator  126  by a line  124 . Synchronous demodulator  126  is connected to circuit  129  by a line  127 . Circuit  129  has an output node  130 . 
     Oscillators  110 ,  112 , and  114  modulate light source  116 . Light originating from the light source and reflected by an object  117  is detected by detector  118 , filtered by filter  120 , and logarithmically detected by amplifier  122 . Circuit  126  synchronously demodulates the logarithmically detected output signal by multiplying it with a signal from the modulation oscillator. Circuit  129  ratioes the resulting signal by comparing it to the noise floor when a signal from the chopping oscillator disables light source  116 , and provides the ratioed output at output node  130 . 
     Other object detector circuits can be made by omitting circuit blocks from FIG.  2 . For example, circuit  129  and chopping oscillator  112  can be omitted. As another example, circuit  126  and modulation oscillator  114  can be omitted. As yet another example, all four circuits  112 ,  114 , 126 , and  129  can be omitted. As still another example, synchronous demodulator  126  can use the carrier oscillator instead of the modulation oscillator with which to form the demodulation product. 
     Other object detectors can also be made by modifying the circuit blocks in FIG.  2 . For example, filter  22  in FIG.  1  and filter  120  in FIG. 2 can be either a band pass filter which passes signals only at the carrier frequency or a high pass filter which passes signals at the carrier frequency or higher frequencies. 
     Other object detectors can also be made by adding circuit elements. For example, in FIG. 1, a buffer amplifier can be added between switch  28  and capacitor  30  and between switch  27  and capacitor  31 . 
     CONCLUSION, RAMIFICATIONS, AND SCOPE 
     The use of a wide dynamic range logarithmic amplifier/detector following a linearly amplified and filtered signal greatly reduces ambient effects while preventing saturation in strong ambients. Synchronous detection at a modulation frequency rate (rather than at the carrier frequency rate) greatly increases signal to noise without introducing a signal isolation cost penalty. Ratioing the output to the noise floor further reduces ambient effects and greatly reduces drift mechanisms. 
     Although the description above contains many specifities, these should not be construed as limiting the scope of the invention, but as merely providing illustrations of some of the preferred embodiments of this invention. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.