Pulse radar ranging system

A pulse radar ranging system having a measurement channel and a reference channel is described. In the measurement channel containing an antenna and a measurement target, the normal measurement is performed by processing an echo from the target. In the reference channel containing signal delay means, a reference echo is processed to obtain temperature compensation information.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of European Patent Office application No. 07023668.2 EP filed Dec. 6, 2007, which is incorporated by reference herein in its entirety.

FIELD OF INVENTION

The present invention relates to a pulse radar ranging system having:transmit pulse generator means for generating transmit pulses at a transmit clock frequency,sampling pulse generator means for generating sampling pulses at a sampling clock frequency slightly different from the transmit clock frequency,a first signal mixer having a first signal input and a second signal input, said first signal input being coupled to the sampling pulse generator means,directional coupling means having a first port coupled to the transmit pulse generator means, a second port coupled to an antenna and a third port coupled to the second signal input of the first signal mixer,signal processing means,said directional coupling means being designed to convey the transmit pulses to be transmitted via the antenna to a target and to convey echo pulses reflected from the target and received by the antenna to the second signal input of the first signal mixer,said first signal mixer being designed to generate a first intermediate frequency signal by mixing the echo pulses with the sampling pulses, andsaid signal processing means being designed to evaluate the first intermediate frequency signal to determine the target distance.

BACKGROUND OF INVENTION

Such a pulse radar ranging system is known from U.S. Pat. No. 4,521,778.

Pulse radar ranging systems provide distance or level measurement based on the direct measurement of the running time of microwave pulses transmitted to and reflected from a target, e.g. the surface of a fill material in a container. As the running time for distances of a few meters is in the nanosecond range, a special time transformation procedure is required to enable these short time periods to be measured. The microwave pulses generated by a transmit pulse generator means are transmitted to the target at a repetition rate or transmit clock frequency. In a signal mixer, the received echo pulses reflected from the target are sampled by cross-correlation with sampling pulses of the same shape as the transmit pulses but at a sampling clock frequency slightly lower than the transmit clock frequency. The cross-correlation and subsequent integration or low-pass filtering leads to an intermediate frequency (IF) signal corresponding to the received echo pulses but time-expanded relative thereto by a factor T1/(T1−T2), where T1is the transmit pulse repetition period and T2is the sampling period. The time-expansion allows for amplifying, digitizing and further processing of the echo pulses with standard techniques.

One of the sources of errors in level measurement using pulse radar is the temperature drift. Due to variation of the parameters of the semiconductor devices of the pulse radar, the level measurement result will change over the specified temperature range of e.g. −40 to +80° C. without a real change of the measured level. The temperature drift is specified in ppm/° C. and the pulse radars on the market today generally meet +/−30 ppm/° C. specification. A reduction of the temperature drift is desirable as the error can be important for far away targets For example, at +80° C. or −40° C. and at a target distance of 20 m the error can reach 36 mm. The temperature drift is produced by changes of the timing and or shape of the transmitting and sampling pulses over temperature and changes of the time base slope (i.e. the difference between the transmit and sampling clock frequencies) over temperature.

From EP 1 770 409 A1 a pulse radar ranging system is known, where a controllable switch, depending on a control signal, either conveys the transmit pulses to the antenna to be transmitted to the target or to a calibration module, preferably a delay line of known delay and terminated with a pulse reflecting impedance mismatch.

SUMMARY OF INVENTION

It is an object of the invention to allow for continuously correction of errors due to changes in the environment temperature of pulse radar ranging systems.

This object is achieved by the pulse radar ranging system of the type initially mentioned in that the system further has:said directional coupling means having a fourth port and being designed to divide the transmit pulses received at the first port into first transmit pulse portions directed to the second port and second transmit pulse portions directed to the fourth port,a second signal mixer having a first signal input and a second signal input,signal delay means arranged between the fourth port of the directional coupling means and the second signal input of the second signal mixer,splitting means arranged between the sampling pulse generator means and the respective first signal inputs of the first and second signal mixers to divide the sampling pulses into first sampling pulse portions conveyed to the first signal mixer and second sampling pulse portions conveyed to the second signal mixer,said second signal mixer being designed to generate a second intermediate frequency signal by mixing the second transmit pulse portions with said second sampling pulse portions, andsaid signal processing means (2,14,17) being designed to further evaluate the second intermediate frequency signal (IF1) for correcting the determination of the target distance.

The pulse radar ranging system thus has a measurement channel, which contains the antenna and target and performs the normal level measurement, and a reference channel, which contains the signal delay means and is used to obtain temperature compensation information. As the electronic circuits of both channels are at the same temperature, the temperature compensation information can be used to compensate for temperature-related variations in the level measurement result.

To reduce the effect of the pulse shape and timing, the transmit pulses are equally split in two transmit pulse portions, one portion directed along the measurement channel and the other one along the reference channel. To further reduce the effect of the pulse shape and timing the sampling pulses are split in two equal portions, one applied to the first signal mixer in the measurement channel and the other to the second signal mixer in the reference channel.

Other approaches are possible, but they need switches that at high frequencies (e.g. 25 GHz) are expensive and difficult to obtain, design, manufacture and tune. The proposed approach uses low cost components with average performance which are easy to obtain, manufacture and tune.

The second transmit pulse portions are directed through the signal delay means of known delay and stable over temperature to the second signal mixer where they are mixed with the second transmit pulse portions. The thus obtained second intermediate frequency signal is processed in the same way as the first intermediate frequency signal in the measurement channel to determine the delay time of the signal delay means.

During the factory calibration of the pulse radar, the delay time along the reference path at a preset temperature can be measured and stored. During the operation, the pulse radar will measure the signal delay over the reference channel and use this value and the stored reference value to calculate the correction for the measured level.

The measurement on the reference channel can be done for each level measurement sequence, which, however, may slow down the level measurement process. Preferably, the measurement on the reference channel is done after a preset number of level measurements or when the temperature changes by a preset number of degrees. Thus, the temperature drift can be reduced below +/−10 ppm/° C.

In summary, the following advantages are obtained:The correction is based only on the propagation along the signal delay means in the reference channel using the same measurement techniques as those used in the measurement channel. This reduces the possibility of new errors or additional corrections and does not require added software for measurement, but only for computation of the correction.The same transmit pulses are applied on both the measurement and the reference channels. Further, the same sampling pulses are applied on both the measurement and the reference channels. This way, the change of the pulse shape and timing over temperature is minimized. Contrary to this, using two different transmit pulse generator means or sampling pulse generator means would introduce supplementary errors due to the differences between the pulse generator means.The correction is made continuously during the operation of the pulse radar.There is no need to use lookup-tables or time consuming factory calibration.

DETAILED DESCRIPTION OF INVENTION

A time base control circuit1, which is under control of a microcontroller2, contains a transmit clock generator3for generating a transmit clock CLKTXat a transmit clock frequency fTXin the MHz range. The transmit clock CLKTXtriggers a transmit oscillator4for generating microwave transmit pulses TX with a pulse repetition rate equal to the transmit clock frequency fTX. The transmit pulses TX may have a duration of 1 ns and a frequency in the GHz range. The transmit clock generator3in combination with the transmit oscillator4constitute transmit pulse generator means5which is coupled to a first port6aof a directional coupler (hybrid)6. First transmit pulse portions TX1of the transmit pulses TX are transmitted through the directional coupler6and an antenna7coupled to a second port6bof the coupler6to a target8, e.g. the surface of a fill material in a container. The target8reflects the transmit pulse portions TX1back as echo pulses RX1which are received by the antenna7. The received echo pulses RX1are passed through the directional coupler6via a third port6cto the second one,9b, of two signal inputs9a,9bof a first signal mixer9.

The time base control circuit1further contains a sampling clock generator10for generating a sampling clock CLKSat a sampling clock frequency fSwhich is slightly lower (for instance by a few kHz) than the transmit clock frequency fTX. The sampling clock CLKStriggers a local oscillator11for generating sampling pulses S of the same shape and frequency as the transmit pulses TX and with a pulse repetition rate equal to the sampling clock frequency fS. The sampling clock generator10in combination with the transmit oscillator11constitute sampling pulse generator means12. The sampling pulses S are divided by means of a splitter13into equal first and second sampling pulse portions S1, S2. The first sampling pulse portions S1are conducted to the first signal input9aof the signal mixer9which generates an intermediate frequency signal IF1by multiplying the received echo pulses RX1by the first sampling pulse portions S1. As the pulse repetition rate of the sampling pulses S or first sampling pulse portions S1is slightly lower than that of the transmit pulses TX or first transmit pulse portions TX1, the first sampling pulse portions S1will sweep in small increments per measuring cycle over the transmit or echo pulse interval so that the received echo pulses RX1are sampled by cross-correlation with the first sampling pulse portions S1. The cross-correlation and subsequent integration and amplification by an IF amplifier14lead to a signal SRX1which is expanded in time and in shape corresponds to the received echo pulses RX1. This signal SRX1is further processed in the microcontroller2for determining the running time of the first transmit pulse portions TX1to the target8and thus the distance of the target8from the antenna7.

The directional coupling means6has a fourth port6dand is designed to divide the transmit pulses TX received at the first port6ainto the first transmit pulse portions TX1directed to the second port6band second transmit pulse portions TX2directed to the fourth port6d. The second transmit pulse portions TX2are transmitted through a delay means15to the second one,16b, of two signal inputs16a,16bof a second signal mixer16. The first signal input16areceives the second sampling pulse portions S2from the splitter13, and the second signal mixer16generates a second intermediate frequency signal IF2by correlating the delayed second transmit pulse portions TX2with the second sampling pulse portions S2. The second intermediate frequency signal IF2is integrated and amplified in a second IF amplifier17and further processed in the microcontroller2for determining the delay time ΔT of the second transmit pulse portions TX2.

The pulse radar ranging system thus has a measurement channel and a reference channel. In the measurement channel containing the antenna7and target8, the normal level measurement is performed by processing the echo RX1from the target8. In the reference channel containing the signal delay means15, a reference echo RX2represented by the delayed second transmit pulse portions TX2is processed to obtain temperature compensation information. During the factory calibration of the pulse radar ranging system, the delay time ΔT along the reference path at a preset temperature is measured and stored in an internal register18of the microcontroller2. During the operation the pulse radar ranging system will measure the ΔT delay of the second transmit pulse portions TX2over the reference channel and the microcontroller2will use this value and the stored reference value to calculate a correction for the measured distance of the target8.

As the microwave components are not ideal, there is a leakage of the signal between the measurement channel and the reference channel due to the limited isolation of the directional coupler6. Because of that, when the target8is close to the antenna7a second echo will be present in the reference channel, close to the reference echo RX2and may affect the measurement if it is of comparable amplitude with the reference echo RX2. Also reflection at the input16bof the second signal mixer16in the reference channel will be leaked into the measurement channel and produce there an echo at approximately two times the length of the reference channel. The influence of the leakage between the measurement and reference channels can be reduced by appropriate leveling the signals in the channels. For that purpose, the signal delay means15should be lossy and is therefore preferably configured as a line19or cable.