Temperature compensation improvements in radar apparatus

0001This invention relates to improvements in radar apparatus using microwave frequency components, and in particular to a radar circuit, which incorporates temperature compensation for microwave components.

0002It is known to use radar in the measurement of distance. In one application radiation in the microwave region of the spectrum is emitted from a source towards a target. The target will reflect some of this radiation back towards a detector, which is sensitive to radiation in the microwave region of the spectrum. An electronic circuit, which may include a digital signal processor, is then used to make a comparison between the emitted signal and the measured, detected, signal from which the range of the target from the source/detector can be estimated.

0003It is known to provide a frequency shift key scheme to determine the range from the signals. In a vehicle application, where the "targets" are other vehicles, which precede the vehicle to which the radar apparatus is fitted, observations are made of the vehicles as they move along the road. The motion of these vehicles is tracked over time to provide a consistent view of their time varying distribution on the road ahead. To achieve this the radar estimates each vehicle's range and relative velocity from the microwave signals.

0004More precisely, distance can be measured from the phase difference that is seen in the returned signal when the emitted signal is modulated by a small step change in frequency as given by: <maths id="math0001"><math display="block"><mi>d</mi><mo>=</mo><mfrac><mrow><mi>c</mi><mo>⁢</mo><msub><mi>φ</mi><mi>s</mi></msub></mrow><mrow><mn>4</mn><mo>⁢</mo><mi>π</mi><mo>⁢</mo><msub><mi>f</mi><mi>s</mi></msub></mrow></mfrac></math><img file="EP1579242B1_D0001.tif" /></maths>

0005Where c is the speed of light, φ, is the relative phase of the Doppler signals returned by two frequencies separated by f_s, the frequency step size. The accuracy of the measurement is related to the frequency step size.

0006Similarly, the relative velocity can be measured by looking at the Doppler shift between the emitted signal and the received signal as given by the following expression: <maths id="math0002"><math display="block"><mi>v</mi><mo>=</mo><mfrac><mrow><mi>c</mi><mo>⁢</mo><msub><mi>f</mi><mi>o</mi></msub></mrow><mrow><mn>2</mn><mo>⁢</mo><msub><mi>f</mi><mi>D</mi></msub></mrow></mfrac></math><img file="EP1579242B1_D0002.tif" /></maths> where c is the speed of light, f₀ is the carrier frequency and f_D is the Doppler shift.

0007As the carrier frequency is regulated by a highly accurate resonator in the microwave components, it is very stable and the velocity measurements are highly accurate.

0008A problem with such apparatus is that variations in the temperature of the microwave frequency generator cause errors in the estimation of the target vehicles distance. This has in the past been solved by providing a heater to regulate the temperature of the frequency generator and in particular to raise its temperature to an ideal or target region in which the temperature co-efficients of the components are linear to allow open loop corrections to be applied to the modulation. A temperature sensor is used to provide the information needed to drive these control circuits.

0009We have realised that the reliance upon a temperature sensor in a scheme of this type can result in unsatisfactory performance after extended use in harsh environments due to significant ageing effects on the temperature sensor.

0010We are aware of <patcit id="pcit0001" dnum="US6317076B1"><text>US 6 317 076 B1</text></patcit> which discloses a radar-based distance measurement apparatus having the features of the pre-characterising portion of claim 1. The disclosed apparatus relies on an accurate output from a temperature sensor. The applicant has appreciated that over time this output may become inaccurate.

0011According to a first aspect the invention provides a radar-based distance measurement apparatus in accordance with claim 1. Optional features are recited in the dependent claims.

0012Because the compensating means modifies the output of the temperature sensor in a closed loop using information derived from an echo signal, the frequency regulating means can regulate the frequency output by the frequency generator- keeping it as close as possible to an ideal constant value- despite changes in its temperature. Additionally, the compensating means enables the apparatus to correct for the effect of ageing of the temperature sensor or the like. This is not possible in prior art open loop systems in which the output of the temperature sensor is simply assumed to be correct throughout its working life.

0013The frequency regulating means may comprise a heater which is adapted to heat the frequency generator by an amount dependent upon the value of the compensated temperature signal. It is well known that microwave frequency generators produce a frequency which varies with temperature. The heater may comprise a resistive heating element. This may be coupled to the oscillator using a thermal matching compound. The corrected temperature may be compared to a reference temperature. If the oscillator is above the reference temperature- as indicated by the corrected temperature value, the heater may be switched off. If below it may be switched on.

0014Alternatively, the frequency regulating means may modify the modulating signal applied to the frequency generator according to the value of the compensated temperature signal. For example, if the compensated temperature is at or close to an ideal temperature then no modification of the drive signal may be applied, the amount of modification of the modulating signal increasing as the compensated temperature gets further away from the ideal. Thus, it may alter the size of the frequency steps requested from the frequency generator. This is especially useful if the generator is too hot and above the reference temperature, a condition which cannot be corrected by a heater. In changing the requested step sizes it is possible to cause the generator to produce the correct step sizes in the modulating signal even when temperature causes the frequency generator to work incorrectly.

0015Of course, the invention may include a combination of heating/cooling and modification of the modulating signal as a way of regulating the frequency output from the frequency generator.

0016The apparatus may include a first processing means adapted to determine the distance of a target from the apparatus by comparing the phase shift between regions of the modulating signal either side of the step change in frequency and the corresponding regions in an echo signal reflected from the target detected by the detector. A reading may be produced every computational cycle of the radar.

0017The apparatus may also include a second processing means adapted to determine the relative velocity of the apparatus and the target by measuring the Doppler shift between the modulating signal and an echo signal returned from an object. A reading may be produced every computational cycle of the radar.

0018A third processing means may determine a first change-in-distance value using the directly obtained distance measurements taken at these two moments in time.

0019A fourth processing means may be provided which integrates the measured velocity between two different moments in time and from this determines a second change-in-distance value indicative of the change in distance between the radar apparatus and the source of an echo between these two times.

0020The first, second, third and fourth processing means may form part of a single integrated circuit. Alternatively, the apparatus may include a processor and a memory which stores program instructions that causes the processor to carry out the functions of the first, second, third and fourth processing means.

0021A comparison means may be provided which is adapted to compare the first and the second change-in-distance values to produce an error value. This may be fed to the compensating means which in turn is adapted to correct the temperature signal output from the temperature sensor by an amount dependent on the magnitude and sign of the error value.

0022The temperature sensor may comprise, for example, a thermister or a silicon temperature sensor. The temperature sensor may measure the temperature of the oscillator, since the sensitivity of this component to temperature is the main source of errors.

0023The frequency generating means may comprise an oscillator and may comprise a voltage-controlled oscillator. These are known to be sensitive to temperature changes. It may be provided as an integrated circuit, and the temperature sensor may be embedded within the oscillator.

0024The apparatus may produce a reported distance value whenever a distance reading is made by the first processing means by combining the reading with the error value produced by the comparator. For example, the error value may be calculated by dividing the first change-in-distance value by the first change-in-distance value. The reported distance could then be formed by multiplying the distance reading from the first processing means by the error value.

0025The apparatus may include a microwave source which generates a microwave signal when driven by the drive signal, and a microwave detector which produces an echo signal in response to microwave radiation incident upon the detector. They may be provided in a common housing, or may be located separately.

0026According to a second aspect the invention provides a radar apparatus substantially as described herein with reference to and as described in the accompanying drawings.

0027There will now be described, by way of example only, on embodiment of the present invention with reference to the accompanying drawings of which: <ul id="ul0001" list-style="none"><li><figref idref="f0001">Figure 1</figref> shows the installation of an apparatus which is in accordance with the present invention on a vehicle;</li><li><figref idref="f0002">Figure 2</figref> is a schematic of a radar apparatus in accordance with one embodiment of the present invention; and</li><li><figref idref="f0001">Figure 3</figref> illustrates a typical FSK drive signal used by the circuit of <figref idref="f0002">Figure 2</figref>.</li></ul>

0028As shown in <figref idref="f0001">Figure 1</figref>, a vehicle 10 is fitted with an example of a radar apparatus in accordance with the present invention. The apparatus comprises a combined microwave source and detector 12. These are mounted towards the front of the vehicle, perhaps at bumper level as shown, where there is a clear and unobstructed (in the microwave range of the spectrum) view ahead of the vehicle as it travels along a road. The source/detector are connected to a processor through a suitable electrical cable.

0029As shown in <figref idref="f0002">Figure 2</figref> of the accompanying drawings, the source is driven by a stable voltage controlled oscillator (VCO) 24 which is modulated by a drive signal 23. The drive signal is produced by a modulation voltage waveform generator that is synchronised by a clock 22. The difference in frequency of each of the frequency steps F, produced by the changing voltage of the drive is known as the FSK step size. A typical drive signal 23, in which four steps are present, is shown in <figref idref="f0001">Figure 3</figref> of the accompanying drawings and this voltage signal is applied to the VCO. The source in turn sends out a narrow beam of microwave radiation to the front of the vehicle.

0030If there is a vehicle in the road in front of the source, a portion of the transmitted signal will be reflected back towards the detector which converts the reflected signal into an electrical signal. The detected echo signal will also be a FSK signal with the four steps. Its strength will depend on the size of the vehicle, how reflective the vehicle is and also how far from the detector it is. In addition, a Doppler shift will be present between the emitted and the received signals which indicates the relative velocity of the vehicle carrying the radar and the vehicle ahead of it on the road. This relative velocity is given by the expression: <maths id="math0003"><math display="block"><mi>v</mi><mo>=</mo><mfrac><mrow><mi>c</mi><mo>⁢</mo><msub><mi>f</mi><mi>o</mi></msub></mrow><mrow><mn>2</mn><mo>⁢</mo><msub><mi>f</mi><mi>D</mi></msub></mrow></mfrac></math><img file="EP1579242B1_D0003.tif" /></maths>

0031The drive signal and the echo signal may be combined to extract the Doppler signal 25.

0032Similarly, a phase shift, φ_s will be present between the Doppler signals produced by each of the frequency steps that are <i>f_s</i> apart and this can be used to determine a distance value according to: <maths id="math0004"><math display="block"><mi>d</mi><mo>=</mo><mfrac><mrow><mi>c</mi><mo>⁢</mo><msub><mi>φ</mi><mi>s</mi></msub></mrow><mrow><mn>4</mn><mo>⁢</mo><mi>π</mi><mo>⁢</mo><msub><mi>f</mi><mi>s</mi></msub></mrow></mfrac></math><img file="EP1579242B1_D0004.tif" /></maths>

0033To determine the velocity and the distance the apparatus passes the Doppler signal 25 through an analogue to digital converter (A/D) 26. The output of the A/D 26 is then fed to a digital signal processor (DSP) 27 together with a reference signal from the clock 21. The DSP 27 generates a target velocity signal 28 which changes over time as the velocity changes and a distance signal 29.

0034The target velocity signal 28 passes to an integrator 30 which integrates the velocity over time to determine a first change-in-distance over time value 31. This is then compared with the change in distance estimated "directly" using the distance output 29 of the DSP 27. The ratio of the "direct" estimate to the integrated estimate is used to form an error correction coefficient 32. Whenever distance is reported from the radar-which is produced in real time using the direct output signal 29 based on the FSK phase shift, the correction is applied so as to keep the reported distance accurate.

0035The increased accuracy is achieved because the velocity measurement 28 is not affected by temperature to the same extent as the direct distance measurement 29. It is known that the main error in the direct distance measurement is the effects of temperature on the microwave components, and in particular the VCO 24. The Doppler based velocity measurement is dependent on the base frequency of the VCO which is controlled by a ceramic resonator.

0036Clearly, it is best to keep the frequency step constant throughout the life of the radar; i.e. the distance error coefficient should be as small as possible. Therefore, to reduce the effects of temperature drift a heater 33 is provided within the VCO module and a temperature sensor 34 is provided which measures the temperature of the VCO module. The output of the temperature sensor 34 is passed to a heater controller 35 and the controller applies a voltage to the heater 33 until the measured temperature is within a target range.

0037Before the output of the temperature sensor 33 is passed to the heater controlled it is corrected by subtracting a compensation value that is dependent upon the error correction coefficient 32 applied to the reported distance values.

0038The correction of the temperature value ensures that drift of the temperature sensor with age is compensated for. The output from the temperature sensor is no longer assumed to be correct, and it is possible to compensate for changes in the temperature sensor's characteristics over time. Also, since the heater 33 will do its best to keep the temperature of the VCO 24 in the ideal range the magnitude of the correction that must be applied to the "direct" distance measurements will tend towards an ideal unitary value over time..