Geometrical instruments – Straight-line light ray type – Level
Reexamination Certificate
1998-08-25
2001-02-06
Fulton, Christopher W. (Department: 2859)
Geometrical instruments
Straight-line light ray type
Level
C033S294000, C033S293000
Reexamination Certificate
active
06182372
ABSTRACT:
TECHNICAL FIELD
The present invention relates to survey instrumentation. In particular, the present invention pertains to a total station.
BACKGROUND ART
Modern total stations are typically required to electronically measure distances with an accuracy of five (5) mm or better. One method used to measure these distances is the time-of-flight method, where a short (<10 nano-seconds) pulse of light is emitted by the instrument, and a careful measurement is made of the time-of-flight between when the pulse leaves the instrument and when an echo pulse is received from the object to be measured.
The speed of light (which is 3×10{circumflex over ( )}8 meters/second), coupled with the desire to measure distances to within 5 mm, leads to a requirement that the time-of-flight be measured with a precision of less than 40 pico-seconds. This, in turn, leads to a requirement in the hardware to either have a temperature-stable, precision reference oscillator with a frequency of 25 gigahertz (which is currently not practical), or a method of interpolating between the clock pulses of a slower reference oscillator.
In existing total stations using time-of-flight to measure distance, this interpolation is done by using a combination of analog and digital electronics to form time-to-voltage converters. A typical embodiment of these circuits is as follows.
Referring to Prior Art
FIG. 1
, the leading edge of start signal
100
activates a constant current charge cycle of a capacitor (not shown), leading to the voltage-versus-time ramp
101
. The first leading edge of reference oscillator waveform
102
that occurs after this start signal terminates the charge cycle. The voltage across the capacitor is then amplified, converted using an analog-to-digital converter (not shown), and then further processed by a microprocessor (not shown). For reference purposes, the time represented by the voltage of this first ramp is called t
A
.
The time between the leading edge of stop signal
103
and the next leading edge of the reference oscillator waveform that occurs after stop pulse
104
is then measured using a similar circuit. For reference purposes, the time represented by the voltage by this second ramp is called t
B
.
The time of flight is then calculated as nT+t
A
−t
B
, where nT is defined as the number of rising edges of the reference oscillator that occurs between the rising edge of the start signal and rising edge of the stop signal multiplied by the period of the reference oscillator.
Variations of the circuit described above are possible, including (but not limited to):
using inductors rather than capacitors for the storage components,
using current or voltage sources that are not constant, or otherwise generating ramps that do not increase linearly with time, or
using the falling edges of the various waveforms or the center of the start and stop signals instead of their rising or falling edges.
However, all existing total stations doing time-of-flight measurements use some variation of this analog-based timing mechanism to interpolate the intervals between the clock pulses of the reference oscillator.
Because of the temperature-dependent and inherent unit-to-unit variations of the analog electronics used, extra analog and digital circuitry must also be provided in these circuits to measure and compensate for the temperature and unit-to-unit variations of the analog electronics.
Like all commercial products, there is an ongoing need in total stations to reduce the cost and increase the reliability of the system wherever possible. Thus, there is a need to eliminate the analog electronics and the digital electronics associated with them from the interpolation circuit.
DISCLOSURE OF THE INVENTION
The present invention provides a digital method of interpolating between the clock pulses used to measure the time of flight of a pulse of light, eliminating the costly and temperature-sensitive analog electronics currently used in similar total stations. The invention provides this functionality by accumulating and averaging the number of clock periods that elapse between the transmission of each pulse of light and the reception of its corresponding echo pulse over multiple pulses of light. The average value of clock periods obtained over the total number of pulses is equal to the interpolated value.
With the clock used to make the time-of-flight measurements operating asynchronously with the timing of the transmission of the light pulses, the resolution of the interpolated value is:
1/(Nf),
where:
N=the number of light pulses averaged; and
f=the frequency of the master clock.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.
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patent: 5745442 (1998-04-01), Herscher
patent: 5815095 (1998-09-01), Yamamoto
patent: 5877641 (1999-03-01), Ziegler et al.
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patent: 5991706 (1999-11-01), Tsukamoto et al.
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Doan Quyen
Fulton Christopher W.
Trimble Navigation Limited
Wagner , Murabito & Hao LLP
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