Tapped delay line high speed register

Details Tapped delay line high speed register Tapped delay line high speed register

2000-11-28

2002-12-10

Shah, Kamini (Department: 2857)

Data processing: measuring, calibrating, or testing

Measurement system

Time duration or rate

C327S261000

Reexamination Certificate

active

06493653

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to timing systems to measure short time intervals, and more particularly to timing systems suitable for time of flight pulse measurements such as found in systems used to guard protected equipment.
BACKGROUND OF THE INVENTION
In many applications it is necessary to know the distance between two points. Although knowledge of distance per se can be used to make a range finder, in other applications knowledge of distance can be used to protect a zone against intrusion. A factory may have robotic or potentially hazardous equipment that is to be protected from outsiders. A system that can measure the distance between such equipment and a perimeter region around the equipment can sound an alarm or turn-off the equipment if anyone approaches closer than the periphery of the protected zone. In this fashion, outsiders are protected against harm from the equipment, and any operators using the equipment are protected from harm by being startled or otherwise disturbed by outsiders.
FIG. 1
depicts a generic so-called time-of-flight system
10
used to calculate the distance X between system
10
and a target (TARGET). System
10
may be located adjacent robotic or perhaps hazardous machinery in a factory where an alarm is to be sounded or the machinery turned-off if anyone approaches closer than distance X.
Typically system
10
includes a trigger generator
20
that creates a pulse train that is input to a transmitter (XMTR)
30
, such as a high speed laser, that broadcasts a pulse via a suitable lens
40
. The broadcast pulse
50
radiates outward at the speed of light, and at least a portion of the radiation may contact the surface of the target, and be reflected back toward system
10
. The reflected-back radiation
55
, which also travels at the speed of light, is detected by an appropriate transducer
60
(e.g., an optical lens) and photodetector
70
. In a zone protection application, a mirror within system
10
mechanically rotates in a plane such that transmitted pulses scan the protected region, and return pulses are detected from this region. The protected region may be defined as a swept arc centered on the equipment to be protected, and extending outward with a radius of at least X. Typically the laser transmitter is triggered or pulsed with a known frequency in synchronism with mirror rotation such that detected return pulses can be correlated with an angle of emission, to locate the angular position and range of the intruding object. In such applications, any target (TARGET) within range X within the swept protection zone is presumed to be an intruder. Note that X may be a function of scan angle in that the guarded perimeter need not be defined by a swept arc.
As indicated in
FIG. 1
, there will be a phase or time shift between corresponding portions of the radiating pulse energy
50
and the return or reflected back radiation
55
. Thus, at time t
0
a first radiated pulse transitions 0-to-1, but the same pulse upon detection (denoted now P
1
′) will have its leading edge transition 0-to-1 at time t
1
+Tw later than t
0
. A high speed counter logic unit
80
within system
10
then attempts to calculate the difference in time between t
1
+Tw and t
0
. Tw is a signal strength dependent term that is sometimes called “timing walk”.
Within unit
10
, detected return pulse P′ is amplified and coupled to a comparator to determine the return pulse transition timing. Return pulse transition timing is typically dependent on the strength of the return pulse, which in turn is determined by object reflectivity and range. In
FIG. 1
, T
1
is the delay corresponding to the physical separation between system
10
and the object or target, whereas Tw is the timing walk strength dependent term.
Typically unit
80
includes a high speed master clock
85
(CLK) and a high speed counter
90
(COUNT). At time t
0
, as determined by a START pulse associated with the beginning of an output emission
50
, counter
90
begins to count clock pulses. At time t
1
+tw, when pulse P
1
′ is detected, counter
90
is halted upon receipt of a STOP pulse, and the count value is determined.
Typically Tw is strongly dependent upon the signal response of the transmitter and receiver circuitry and must be characterized. Correction values are determined over a range of P
1
′ signal strengths and are stored in a table. The values stored in the correction table are indexed by detected signal strength and may be used by a system control circuit to extract the value t
1
. Thus, prior art systems that employ time-interval counters typically will use a peak-detector or signal integrator.
Once t
1
is known, a measure of distance x given At &Dgr;t=(t
1
−t
0
) is determined by the following equation:
x
=
c
·
Δ
⁢
 
⁢
t
2
where c=velocity of light (300,000 km/sec).
Within system
10
, generating, transmitting, and receiving pulses can be straightforward. But it can be challenging for system
10
to resolve the distance X within a desired measurement granularity or tolerance. For example, to measure distance with a resolution granularity of about ±5 cm requires a 3 GHz counter. Such high speed devices are expensive and typically consume several watts of electrical power.
An alternative approach would be to replace the function of high speed clock
85
and high speed counter unit
90
with a high speed analog-to-digital converter. However high speed analog-to-digital converters are relatively expensive.
Yet another approach would be to replace units
85
and
90
with a transient recorder, perhaps inexpensively implemented using common CMOS fabrication processes. Transient recording could be extremely fast yet would not consume excessive electrical power. One prior art transient recorder technique is described in a Univ. of Calif. At Berkeley 1992 M. Sci. thesis entitled “A Multi-Gigahertz Analog Transient Recorder Integrated Circuit” by S. A. Kleinfelder. Kleinfelder's thesis described a tapped, active delay line using an array of storage capacitors. The capacitors stored samples of the detected return pulse P
1
′ at specific delay times that were set by the delay of each element in the delay line.
Keinfelder's approach appears ideal in that it presents a fully digitized representation of the delayed pulse (or multiple pulses), at relatively minimal cost. Further, no thresholding of the analog return pulse is necessary, and range distance may be computed using an algorithm that takes into account the full pulse shape. The latter is important in determining target range, independently of the strength of the return pulse P
1
.
Unfortunately, in practice Kleinfelder's system is difficult to implement because of the large amount of data that must be processed in a relatively short time. Further, it is necessary to characterize performance of the active delay line and particularly the storage capacitors and analog-to-digital converter circuitry over process, temperature, and voltage variations.
What is needed is a high speed time interval measurement system for use in applications such as time-of-flight systems, especially in systems used to guard machinery or the like. Such measurement system should be inexpensive to fabricate, preferably using existing CMOS processes, should exhibit low power consumption, and should provide timing and strength information for one or more return pulses. Such measurement system should rapidly detect multiple return pulses, preferably within time intervals of less than about 500 ns, with a sub-nanosecond timing resolution that can provide spatial resolution of ±5 cm or less. Further, the system should measure return pulse signal strength with sufficient precision for use as an index to a lookup table to correct for timing walk. The system should communicate range measurements with a minimal amount of data. Finally, the system should exhibit reduced sensitivity to variations in ambient temperature, operating vo

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