GB2378833A

GB2378833A - Object Dectection System

Info

Publication number: GB2378833A
Application number: GB0120032A
Authority: GB
Inventors: Adrian George Garrod; Gareth Liam Harris; Michael Richard Richardson; Bryan Stephen Rickett
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-08-17
Filing date: 2001-08-17
Publication date: 2003-02-19
Anticipated expiration: 2021-08-17
Also published as: GB0120032D0; GB2378833B

Abstract

An object detection system comprises at least two sensors (1, 2), each sensor being adapted to transmit and receive signals and a control device (3) to control the sensors. A first sensor derives direct range measurements from reflections off an object of a signal transmitted by the first sensor (1) and derives indirect range measurements from reflections off the same object of a signal transmitted by the second sensor (2). The first and second sensors are phase locked to a common reference oscillator and in transmit mode, each sensor transmits an FM CW signal sweep starting at a first frequency. In receive mode, each sensor starts an FM CW signal sweep at a second frequency, the first and second sensors being time synchronised to start the transmit and receive sweeps together. The first and second frequency are offset by a predetermined amount; such that each sensor can receive whilst continuing to transmit. The invention is particularly applicable to use on motor vehicles as an object detector for a vehicle parking aid, or for use with a cruise control or for collision avoidance.

Description

OBJECT DETECTION SYSTEM This invention relates to an object detection system, in particular for use with moving vehicles.

FMCW radar sensors have been proposed for use in collision detection for moving vehicles. An example of one such system is described in US 5,949, 366 which is concerned with obtaining detailed information on a potential collision between a motor vehicle and an object. This system uses a single sensor to transmit an FMCW signal and receives reflections of that signal from an object. An active time profile is derived of amplitude values or relative velocity values, then compared with a stored time profile to determine a lateral offset between the motor vehicle and the object.

However, if the range of the object is such that the path length of the direct signal and an indirect signal (for example a reflection of the target via the ground) differ in length by (n +1/2) wavelengths, where n is an integer, then the two signals will arrive out of phase and cancel each other at the receiver. This is a well known problem in radar referred to as a multipath null.

In accordance with a first aspect of the present invention, an object detection system comprises at least two sensors, each sensor being adapted to transmit and receive signals; and a control device to control the sensors; wherein a first sensor derives direct range measurements from reflections off an object of a signal transmitted by the first sensor and derives indirect range measurements from reflections off the same object of a signal transmitted by a second sensor; wherein the first and second sensors are phase locked to a common reference oscillator; wherein in transmit mode, each sensor transmits an FM CW signal sweep starting at a first frequency; wherein in receive mode, each sensor starts an FM CW signal sweep at a second frequency; wherein the first and second sensors are time synchronised to start the transmit and receive sweeps together; and wherein the first and second frequency are offset by a predetermined amount; such that each sensor can receive whilst continuing to transmit.

The present invention overcomes the problems of poor isolation capability in a system which attempts to derive data from reflections of a signal which has been transmitted from a remote sensor, (i. e. the need to use high quality switches to switch off the transmitter effectively when receiving), by continuing to transmit, but offsetting the frequency at which the receive sweep starts relative to the frequency at which the

transmit sweep starts so that the source of the signal can be determined. By using two sensors, as both monostatic and bistatic sensors, then there are three possible signal paths to the reflecting object, so it is unlikely that the reflecting object will be in a multipath null for all three signal paths, thus overcoming the problem of the prior art.

In accordance with a second aspect of the present invention, a vehicle parking aid comprises an object detection system according to the first aspect.

In accordance with a third aspect of the present invention, an automotive adaptive cruise control system comprises an object detection system according to the first aspect.

In accordance with a fourth aspect of the present invention, an automotive collision avoidance system comprises an object detection system according to the first aspect.

An example of an object detection system according to the present invention will now be described with reference to the accompanying drawings in which :- Figure 1 illustrates a general arrangement of the system; Figure 2 illustrates an FMCW radar sensor for the system of Fig. 1; Figure 3 illustrates conventional FMCW radar operation; and, Figure 4 illustrates FMCW radar operation in the system of the present invention.

The general arrangement for an object detection system according to the present invention is shown in Fig. 1. This comprises two sensors 1,2 and a central unit 3. The central unit controls the sensors and could be a microprocessor, electronically programmable logic device (EPLD), field programmable gate array (FPGA) or other control device. The sensors are frequency modulated continuous wave (FM-CW) radar sensors which operate by sweeping the transmit frequency up and down to obtain range information. A linear sweep means that transit time and hence range is proportional to the difference in frequency between the transmit and receive signals. An example is shown in Fig. 2. A transmit antenna 4 and a receive antenna 5 are provided. A signal from the receive antenna 5 is input to a mixer 6 along with a signal from a transmitter 7.

The signals are separated in band pass filters 8,9 and then pass to a processor 10 for baseband processing.

Conventional FM-CW operation is described with respect to Fig. 3. The frequency is swept with a linear profile, at a known sweep rate fo. If a portion of the transmit signal is mixed with the receive signal, the beat note-assuming no Doppler shift-from a target range R is:

If the frequency is modulated over the frequency range Af, in time, the beat frequency becomes:

This is illustrated in Fig. 3 for an upsweep, i. e. frequency increasing with time.

The resolution to which the beat frequency can be measured is proportional to the time over which the measurement is made, which equals the sweep time,, T. As the beat frequency itself is inversely proportional to the sweep time the resulting capability to resolve target returns is related to the sweep bandwidth.

If the target is not stationary, the beat frequency, fb will be a combination of the frequency due to range, fr and the Doppler shift, fd and will have different values depending on the direction of the FM sweep.

Equations 3 and 4 above show that in principle the Doppler shift and range frequency can be separated by combining the results from an upsweep and a downsweep. For more complex targets or multiple targets this a complex processing task.

The sensors of the present invention make direct range measurements as described above. The sensors also need to make indirect range measurements (bistatic operation), i. e. where one sensor receives and another, separate, sensor transmits. The frequency range of the transmitting sensor is offset from that of the receiving sensor by an amount much greater than the expected frequency shift due to target range and Doppler. This is illustrated in Fig. 4. Given an offset frequency Foffset, a sweep

bandwidth B and a centre frequency Fc, the transmitting sensor sweeps from :

Ftl = Fc - (BI2) to Ft2 = Fc + (B/2)

The receiving sensor sweeps from :

Fri = Fc + Foff, e.- (B/2) to Fr2-F, + Foffset + (B/2) t

When the received signal is mixed down, the signals due to monostatic operation would be in the frequency ranges 0 to maximum frequency shift due to target range and Doppler. Signals due to bistatic operation are in the range Forget +/-maximum frequency shift due to target range and Doppler. Therefore, if Foffset is set appropriately, the two signals can be separated easily with filters.

The offset frequency method has other advantages. The signal output from the mixer includes a large unwanted signal close to d. c. This is due to self detection of the local oscillator. This causes considerable degradation of the short range performance of the radar. Offsetting the frequencies of the sensors moves the signals for bistatic operation away from this low frequency noise.

The present invention is not limited to a pair of sensors, but is equally applicable to an array of sensors. For example, consider an array of n + 1 sensors. n of these make identical sweeps, and one makes an offset sweep as described above. In this example, the offset sensor acts as the transmitter, and detects the signal transmitted by itself, and reflected from the target. The other three sensors also detect the signal transmitted by the offset sensor and reflected by the target. Note that although the signals transmitted by the three identical sensors can also be detected, they are not used since, for n > 1, they cannot be uniquely identified.

For automotive radar applications typical sweep parameters would be: Centre Frequency, Fc = 77.5 GHz Sweep Bandwidth, B = 450 MHz Sweep time, t = 2 ms Offset frequency, Foffset = 1 MHz

Claims

CLAIMS 1. An object detection system, the system comprising at least two sensors, each

sensor being adapted to transmit and receive signals ; and a control device to control the Z-1 t : l sensors; wherein a first sensor derives direct range measurements from reflections off an object of a signal transmitted by the first sensor and derives indirect range measurements from reflections off the same object of a signal transmitted by a second sensor; wherein the first and second sensors are phase locked to a common reference oscillator; wherein in transmit mode, each sensor transmits an FM CW signal sweep starting at a first frequency; wherein in receive mode, each sensor starts an FM CW signal sweep at a second frequency; wherein the first and second sensors are time synchronised to start the transmit and receive sweeps together; and wherein the first and second frequency are offset by a predetermined amount; such that each sensor can receive whilst continuing to transmit.
2. A vehicle parking aid comprising an object detection system according to claim 1.
3. An automotive adaptive cruise control system comprising an object detection system according to claim 1.
4. An automotive collision avoidance system comprising an object detection system according to claim 1. ^
5. An object detection system as hereinbefore described with reference to Figs. 1,2 and 4.