Method for measuring the distance and/or the relative velocity between two ...

United States Patent tw]

Hane

[ii] Patent Number: 4,851,851 [45] Date of Patent: Jul. 25,1989

4,307,397 12/1981 Holscher 342/125

4,388,622 6/1983 Fletcher, Jr 342/112

4,464,662 8/1984 Tomasi 342/125

4,577,150 3/1986 Schreuder 342/125

4,646,092 2/1987 Schreuder 342/125

FOREIGN PATENT DOCUMENTS

0096558 12/1988 European Pat. Off. .

0096559 12/1988 European Pat. Off. .

OTHER PUBLICATIONS

The above art, copies attached, were cited in the International Search Report mailed on Feb. 10, 1987. *NOTE: A U.S. Pat. No. 4,577,150 to Jan Schreuder corresponds to EPA No. 0096558.

Primary Examiner—Thomas H. Tarcza

Assistant Examiner—Mark Hellner

Attorney, Agent, or Firm—Nies, Webner, Kurz &

Bergert

[57] ABSTRACT

A method of measuring the distance between two objects and/or the speed of one object in relation to the other, the objects incorporating respectively a transmitter-receiver unit and a transponder or reflector, in which method a phase comparison is made between a signal transmitted by the transmitter-receiver unit and a signal received in the transmitter-receiver unit and transmitted from the transponder or reflector. In accordance with the invention the transmitter-receiver unit is caused to transmit signals of microwave frequency, preferably about 2450 MHz, this transmission comprising the transmission of a first signal having a first frequency, the transmission of at least one second signal having a higher or lower frequency, and the transmission of a third signal having the same frequency as the first mentioned signal, wherewith the phase differences <f> between the transmitted signals are formed, these phase differences corresponding to the distance between the transmitter-receiver unit and the transponder.

7 Claims, 1 Drawing Sheet

METHOD FOR MEASURING THE DISTANCE AND/OR THE RELATIVE VELOCITY BETWEEN TWO OBJECTS

5

The present invention relates to a method of measuring the distance and/or the relative velocity between two objects. More specifically, although not exclusively, the invention relates to the measurement of the distance and/or the relative velocity between a first 1° object and a second object, of which the first object incorporates a transmitter-receiver unit and the second object incorporates a transponder. The transmitterreceiver unit is constructed to transmit a signal to the transponder and to receive a signal emanating there- 15 from.

The invention relates specifically to the measurement of distance and/or velocity by making a phase comparison in the transmitter-receiver unit between the signal transmitted to the transponder and the signal received 20 therefrom.

The concept of the phase difference method in distance measuring processes is well known per se and can be applied with various types of transmitter-receiver apparatus and transponders or reflectors.

When practising the present invention there is preferably used the method and apparatus for creating phase differences described, and illustrated in the Swedish patent specification No.... (corresponding to Swedish 3Q patent application No. 8505888-1), although it will be understood that the present invention is not at all dependent on the use of this described and illustrated method and apparatus.

Since a phase difference can only be determined 35 within the range 0—2ir, the greatest unambiguous distance R for a given transmitted frequency Fl is

R=c/(2-Fl)

25

and the prevailing velocity between the objects can be measured.

Thus, the present invention relates to a method for measuring the distance between two objects and/or the speed at which they move relative to one another, said two objects incorporating respectively a transmitterreceiver unit and a transponder or reflector, in which method a phase comparison is made between a signal transmitted by the transmitter-receiver unit and a signal received thereby from the transponder or reflector, the method being characterized by transmitting from the transmitter-receiver unit signals of microwave frequency, preferably a microwave frequency of about 2450 MHz; transmitting a first signal having a first frequency; transmitting a second signal of higher or lower frequency; transmitting a third signal of the same frequency as the frequency of the first signal; and forming the phase differences <J> between the transmitted signals, these phase differences corresponding to the distance between the transmitter-receiver unit and the transponder.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to various diagrams shown on the accompanying drawings, in which

FIGS. 1, 3, 4 and 6 are diagrams in which phase difference is plotted against frequency;

FIG. 2 is a diagram in which frequency is plotted against time; and

FIG. 5 is a diagram in which phase difference is plotted against prevailing frequencies when carrying out a multiple of measuring operations (i).

When measuring distances with the aid of phase-difference measuring techniques, the phase difference between transmitted and received signals is

40

where c is the speed of light.

The present invention particularly recommends the use of microwave frequencies. Rmax is only 6 cms when using the frequency 2500 MHz.

Another problem associated with measuring methods 45 that rely on phase-differences resides in the difficulties which occur when the two objects move in relation to one another, since the phase relationships then change with time.

It is often necessary at times, however, to measure 50 distances under dynamic conditions.

It is also desirable, in many contexts, to be able to determine the location of an object, e.g. a motor vehicle, within a restricted area with a high degree of accuracy, inter alia so as to be able to navigate the vehicle 55 within this area. One common method of determining the position of an object in relation to a reference system is to measure the distance between the object and a number of reference points in the system. The position of the object can be readily calculated from these mea- 60 sured distances, with the aid of trigonometrical functions. The accuracy to which the position of the object is determined is directly proportional to the accuracy to which the distance(s) is (are) measured.

These drawbacks and problems are not found with 65 the method according to the invention, which enables distances to be determined very accurately, even under dynamic conditions, and with which both the distance

Where v is the relative velocity of the objects and t is the time interval between measurements.

This error is eliminated in accordance with the invention by carrying out a further phase measuring process, in addition to the two phase measuring processes aforementioned, this further phase measurement being made at a lower frequency than the highest frequency F2. It is assumed, by way of example, that the third phase measuring operation is carried out at the frequency Fl and at the time 2t.

Thus, there is transmitted a first signal having the frequency Fl, a second signal having the frequency F2, which differs from the frequency Fl, and a third signal having the same frequency Fl as the first signal.

Assume that the distance R is changed uniformly with time, in accordance with the expression

R(i)=R0+v-t

15

20

25

30

(6)

2 ■ W2 ■ R0 2 ■ W2 ■ v • t (7)

Similar to the expression (7), <J>3 can be expressed as

2 • wi ■ R0 2 H>i • v • 2 • / (8)

The above is illustrated in FIG. 2, in which straight lines have been drawn between co-ordinates which constitute the frequency F transmitted at times o, t and 2t. 50

FIG. 3 illustrates the phase differences cj> which occur, as a function of the transmitted frequency, in which straight lines have been drawn between the measuring points.

The slope of the line LI in FIG. 3, drawn between the 55 phase differences which occur when measuring with the frequency Fl at time o, and with the frequency F2 at time t, differs from the slope of the broken or discontinuous line L2. The discontinuous line L2 corresponds to the line in FIG. 1. This difference in slope is due to go the relative speed of one object to the other. The line L3 connects the phase differences which occur when measuring with the frequency F2 at the time t, and with the frequency Fl at the time 2t.

The distance at time t can be determined from the 65 slope of the broken or discontinuous line L4.

For this reason there is formed a median value of <J>i and <J>3, referred to as <f>4

The distance can be readily determined, even when the transmitter-receiver unit and the transponder move relative to one another, by utilizing three phase angles, of which two occur at the same frequency. The first and the third signal conveniently have a frequency of preferably 2450 MHz, and the frequency difference between the frequencies of these signals and the frequency Fl of said second signal is preferably much lower, preferably from 50 kHz to 50 MHz.

According to one preferred embodiment, there are transmitted several series of said first, second, and third signals in successive order, where the frequency differences between the second signal F2 and the remaining two signals Fl, Fl increase, which corresponds to a progressively decreasing unambiguous-distance range.

According to another preferred embodiment, at least three series of said first, second, and third signals are transmitted, where the aforesaid frequency differences increase in accordance with a series which is an even multiple of the lowest frequency, preferably the series 50 kHz, 500 kHz and 5 MHz, as exemplified below.

A high degree of accuracy and a long or wide unambiguous range can be had by carrying out a multiple of measurements according to the aforegoing with progressively decreasing distances, i.e. with a progressively increasing difference between the frequencies F2 and Fl.

A first series of the first, the second, and the third signal can be used, for example, to determine the distance within a range of 300 meters, when the effective