Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to pulsed electromagnetic sensors, and more particularly to a pulse center detector (PCD) for pulse echo radar and time-domain reflectometry (TDR) sensors. These sensors can be used for rangefinding, for automation, or for determining the fill-level of a tank. 
     2. Description of Related Art 
     Radar echo pulses exhibit large amplitude variations, depending on target size, range and dielectric constant, and these variations produce range measurement errors when the pulses are detected with a fixed threshold detector. Echo amplitude variations also occur to a lesser extent with TDR-based tank level sensors, mainly being limited to dielectric constant variations of the liquid in the tank. However, accurate TDR-based tank level sensors require accurate, amplitude-independent pulse detectors. 
     Detectors with amplitude-tracking thresholds or other means to achieve amplitude independence are generally termed automatic pulse detectors and several automatic pulse detectors have been in existence for more than 30 years. U.S. Pat No. 5,610,611, High Accuracy Material Level Sensor, to McEwan, describes the well-known constant fraction discriminator, or CFD, for use in a TDR-based tank level sensor. The CFD in the &#39;611 patent uses a peak detector to determine the peak amplitude of repetitive equivalent-time pulses and sets a trigger point that is a fraction of the peak amplitude, such as the half-way point on the rise of the pulse (half-max detection). Unfortunately, the CFD exhibits latency errors caused by slow peak tracking when the pulses decrease in amplitude. Latency is a particular problem when the CFD is used in a TDR level sensor for sloshing liquids in a tank, such as an automotive gas tank. Another potential problem with the peak detector is that it will lock-on to the strongest peak in a radar or TDR waveform, such as the main bang peak, and thus create errors in echo pulse detection—unless the CFD is provided with analog gating, or separate, gated CFD&#39;s are used for the transmit and echo pulses. Yet another problem with the CFD is that it will trigger on baseline noise when no echo pulses are present, so a threshold detector is needed to inhibit operation on weak pulses. To fully overcome all the limitations of a CFD, substantial additional circuitry is needed. 
     Another well-known automatic pulse detector is the time-of-peak (TOP) detector. The TOP detector differentiates pulses and triggers on the resulting zero axis crossings. To prevent false triggering on baseline noise, the desired pulses must be above a threshold before zero axis detection is enabled. This standard detector is utilized in an application to TDR in U.S. Pat. No. 5,457,990, Method and Apparatus for Determining a Fluid Level in the Vicinity of a Transmission Line, to Oswald, 1995. However, the TOP detector is less accurate than the CFD for the simple reason that a pulse peak is somewhat flat and has a low rate of change, making accurate time-of-peak detection difficult. Small baseline perturbations, such as baseline ringing or radar clutter, will sum with the pulse and substantially displace the exact time-of-peak. In contrast, a CFD detects at a fast slewing point of the pulse risetime, so the detection time is much less sensitive to baseline perturbations. 
     An amplitude-independent pulse detector is needed that (1) triggers on a high-slew point of a pulse like the CFD to avoid the inaccuracies of the TOP detector, (2) does not have the latency of the CFD, and (3) does not have the complexity of the prior automatic detectors. 
     SUMMARY OF THE INVENTION 
     According to the invention, a pulse center detector (PCD) threshold detects radar or TDR baseband transmit and echo pulses using a single fixed threshold comparator to produce transmit and echo detection pulses. The leading edges of the detection pulses are then formed into a leading-edge PWM (pulse width modulation) pulse (or “leading-PWM pulse”) having a width proportional to the time between the leading-edge of the transmit pulse and the leading-edge of the echo pulse, i.e., the transmit-echo range. Similarly, a trailing-edge PWM pulse (or “trailing-PWM pulse”) is formed from the trailing edges of the detection pulses. 
     The leading- and trailing-PWM pulses gate separate range counters. The range counts are added and divided by two to produce an average—or midpoint—count that corresponds to a PWM pulse centered on the baseband transmit and echo pulse centers. Thus, pulse center detection is achieved, i.e., the range is measured from the center of the transmit pulse to the center of the echo pulse. 
     By definition, a pulse center is the midpoint between its leading and trailing edge. The center of a pulse does not necessarily correspond to the peak of the pulse, but it often does. The PCD relates the pulse center to the time average of the leading and trailing edge times of the pulse. 
     An analog alternative to digital count summation involves simple resistive summing and integration of the leading-PWM and trailing-PWM pulses to produce a DC value corresponding to the time difference between the centers of the baseband transmit and echo pulses. This DC value is a range-proportional analog output voltage. Analog summation and integration is a simple approach suitable for driving analog “gas gauges.” With either analog or digital summation, the final range indication is related to the centers of the transmit and echo pulses and does not vary with pulse amplitude. Although baseband pulses generally exhibit symmetric rise and fall times, asymmetric rise and fall times can be compensated by weighted PWM addition. 
     Timing jitter is lower in the PCD compared to prior automatic detectors since the addition of the leading and trailing PWM pulses averages the noise and yields a 3 dB reduction in noise, or equivalently, timing jitter. 
     The PCD can enhance the accuracy and reduce the cost of TDR-based electronic dipsticks for automotive gas gauges, industrial vat level sensors, automatic swimming pool regulators, and toilet tank level controllers. It can also be used to improve the performance of radar rangefinders for robotics or for automotive backup warning. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  depicts a radar system having a pulse center detector (PCD). 
     FIG. 1 b  depicts a TDR system having a PCD. 
     FIG. 2 a  is a timing diagram of the PCD. 
     FIG. 2 b  is a timing diagram of the PCD with reduced echo pulse amplitude. 
     FIG. 3 a  is a block diagram of a PCD with a digital adder. 
     FIG. 3 b  is a block diagram of a PCD with an analog summer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A detailed description of the present invention is provided below with reference to the figures. While illustrative circuits are given, other embodiments can be constructed with other circuit configurations. All U.S. Patents and copending U.S. applications cited herein are herein incorporated by reference. 
     FIG. 1 a  depicts a pulse radar system  10  employing a pulse center detector (PCD)  20  of the present invention. Pulse radar  12  is generally a pulse-echo radar employing equivalent time sampling techniques. RF transmit pulses are radiated from transmit antenna  14  to target  16  and returning echoes are received by antenna  14  and detected by radar  12  to produce, in combination with the transmit pulses, baseband transmit and echo pulses on line  18 . In the preferred embodiment, the baseband output is a sampled, equivalent time replica of the transmit and echo pulses passing through antenna  14 . Also in the preferred embodiment, the baseband pulses are envelope detected unipolar pulses. Details of a pulse radar  12  are described in U.S. Pat. No. 6,137,438, Precision Short-Range Pulse-Echo Systems with Automatic Pulse Detectors, to McEwan. Line  18  is input to PCD  20 . Pulse center detector  20  detects the baseband transmit and echo pulses at their centers and produces a range output on line  22  that is proportional to range, as defined by the time difference between the center of the baseband transmit pulse on line  18  and the center of the echo pulse on line  18 . It should be understood that, in a less preferred mode, the transmit pulse on line  18  may be substituted with a reset pulse or a digital start pulse rather than the detected transmitted main-bang RF pulse. Also, the transmit pulse may be in the form of a close-in reflection from antenna  14  or other nearby reference reflector, (i.e., a fiducial pulse). A readout or processor  24  responsive to range output on line  22  may process and/or display range data, or provide an output on line  26  to control a parameter of another system such as a toilet valve or a vehicle braking system. 
     FIG. 1 b  depicts a TDR system  30  employing the pulse center detector (PCD)  20  of the present invention. TDR  32  is generally a pulse-echo time domain reflectometer employing equivalent time sampling techniques. Details of a sampling TDR  32  are described in U.S. Pat. No. 5,610,611, High Accuracy Material Level Sensor, to McEwan. Transmit pulses are propagated on transmission line  34  to a transmission line probe  36  which is at least partially immersed in a liquid  38  in a tank  39 . Liquid  38  reflects the transmitted TDR pulses back as echo pulses to TDR  32 , which produces baseband transmit and echo pulses on line  18 . The function of references blocks  20  and  24 , and line  26  are the same as for FIG. 1 a . It should be understood that, in a less preferred mode, the transmit pulse on line  18  may be substituted with a reset pulse or a digital start pulse rather than the detected transmitted main-bang RF pulse. Also, the transmit pulse may be in the form of a tank-top reflection from probe  36  or other reference reflector, (i.e., a fiducial pulse). 
     FIG. 2 a  plots the waveforms associated with pulse center detector  20 . BASEBAND PULSES  40  are input to the pulse center detector on line  18  and comprise a transmit pulse T and an echo pulse E. BASEBAND PULSES  40  preferably occur on a millisecond equivalent-time scale. DETECTION PULSES  42  result from threshold detecting the BASEBAND PULSES with a fixed threshold V th , having an exemplary amplitude illustrated by dashed line  41 . LEADING-PWM pulse  44  and TRAILING-PWM pulse  46  result from triggering a first flip-flop (or logical equivalent) on the leading edges (shown as edge circles  1  and  2 ) of the DETECTION PULSES  42  to produce LEADING-PWM pulse  44 , and from triggering a second flip-flop (or logical equivalent) on the trailing edges (shown as edge circles  3  and  4 ) of DETECTION PULSES  42  to produce TRAILING-PWM pulse  46 . PWM pulses  44  and  46  are then added and scaled to produce, in effect, a center triggered response, as will be discussed with respect to FIGS. 3 a  and  3   b . An EFFECTIVE CENTER-DETECTED PWM pulse is plotted with dashed lines to illustrate its centering on the T and E pulse centers. It should be understood that the EFFECTIVE CENTER-DETECTED PWM is a computed result of adding the LEADING-PWM and TRAILING-PWM digital counts, or of adding the corresponding analog voltages, as will be discussed with reference to FIG. 3 a  and  3   b ; it does not exist as a real pulse. 
     FIG. 2 b  illustrates the amplitude independent nature of the PCD. Echo pulse E′ is shown with reduced amplitude compared to E of FIG. 2 a . DETECTION PULSES exhibit shifted edges  2 ′ and  4 ′ due to pulse E′ barely exceeding threshold  41 . However, edges  2 ′ and  4 ′ have shifted in equal and opposite directions as can be seen with reference to edges  2  and  4  (dashed lines of FIG. 2 b ). After addition of PWM pulses  44  and  46 , the effect of edge shifts from  2  to  2 ′ and  4  to  4 ′ cancel each other, thereby illustrating the amplitude independence of the apparatus and method. 
     FIG. 3 a  is a block diagram of a digital implementation of pulse center detector  20 . BASEBAND PULSES are input on line  18  to a fixed threshold comparator  60 , which compares the pulses with a threshold V th  and outputs DETECTION PULSES on line  62  to leading flip-flop  64  and to inverter  66 , whose output coupled to trailing flip-flop  68 . Leading flip-flop  64  and trailing flip-flop  68  produce LEADING-PWM and TRAILING-PWM pulses on lines  72  and  74 , respectively. Flip-flops  64 ,  68  may be other logic elements. The width of the LEADING-PWM and TRAILING-PWM pulses correspond to the transmit-to-echo time delays of the leading and trailing edges of the corresponding T to E DETECTION PULSES of FIG. 2 a . A reset is provided on line  70  to the flip-flops after each operation cycle (i.e., range sweep). It should be understood that flip-flops  64  and  68  may incorporate additional logic, such as a lock-out function, to prevent re-toggling if additional pulses beyond the two DETECTION PULSES are input within one operation cycle. 
     The LEADING-PWM and TRAILING-PWM pulses gate counters  76  and  78 , respectively, which produce range counts on lines  80  and  82 . Adder  84  adds the counts from lines  80  and  82  to produce an average count on line  86  which effectively corresponds to a count from a single PWM pulse centered on the T and E pulses, i.e., extending from center to center of the two pulses (neglecting a scale factor of 2). An optional divide-by-two circuit  88  scales the digital count from line  86  to compensate for the addition operation of element  84 . However, the output on line  22  is usually in an unscaled binary format that requires processing by readout/processor  24  to present meaningful data scaled to some measurement quantity, for example, a fluid volume in a tank. Thus the scaling provided by divide-by-two element  88  can be incorporated in display/processor  24 . 
     FIG. 3 b  is a block diagram of an analog implementation of pulse center detector  20 . Elements  60 ,  64 ,  66  and  68 , and lines  18 ,  62 ,  70 ,  72 , and  74  have the same function as described with reference to FIG. 3 a . The LEADING-PWM and TRAILING-PWM pulses on lines  72  and  74 , respectively, are input to an analog summer  90  via resistors R L  and R T , respectively, which are both connected to one input of op amp A. A feedback connection comprising R F  and C F  across op amp A provides voltage scaling and pulse integration or smoothing to the PWM pulses input to R L  and R T . The time constant set by R F C F  is much longer than the PWM duration to smooth the PWM pulses. A lowpass filter may be connected to line  22  for further smoothing. Operational amplifier A outputs a voltage on line  22  that is the sum of the voltages on lines  72  and  74 , integrated over time to a smooth DC value. Thus, summer  90  adds and smoothes the LEADING-PWM and TRAILING-PWM pulses to produce a proportional analog voltage that corresponds to a single smoothed PWM pulse centered on the T and E pulses, thereby implementing an amplitude-independent pulse center detection function. 
     If the leading or trailing edges of the baseband T or E pulses have different transition times, which would result in reduced amplitude independence for the PCD, either R L  or R T  can be scaled, i.e., weighted, to compensate the asymmetry and regain amplitude independence. For example if the trailing edges of the baseband T and E pulses are 2× slower than the leading edges, resistor R T  would need to be doubled to regain amplitude independence. Weighting can also be accomplished digitally by simple means known in the art with respect to FIG. 3 a.    
     Although the invention has been described with reference to an equivalent time radar or TDR, the principles of the PCD apply to other pulse-echo embodiments as well, such as a realtime radar or TDR. Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.

Technology Category: g