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A Microwave Radar Sensor for the Control of Autonomous Vehicles
A MICROWAVE RADAR SENSOR FOR THE CONTROL OF AUTONOMOUS VEHICLES M CBSMITH MobiJe Robots Ruearcir Group, IHpartment ofElectrical and Electronic Engillurilfg. University of ust London. Longbridge Road, Dagenham, usa, RM8 2AS, United Kmgdom Abstract . This paper describes work that is being carried out to develop a miniatvre microwave radar sensor and signal processing system for use on autonomous vehicles. The sensor is intended for use in collision avoidance . proximity detection , medium resolution vision sensing, navigation and applications requiring continuous real time feedback control. The system is capable of measuring range , relative velocity and direction of relative motion . thus it will operate while the sensor and/or target obstacle is moving. By using a high gain aerial a sufficiently fine resolution is obtained to enable a simple 3-D vision system to be realised . Key Words . Sensors; transducers; guidance systems; navigation; obstacle avoidance; target tracking; robots ; vehicles; intelligent machines; warehouse automation
1. INTRODUcnON
in ideal conditions, good range accuracy and resolution. The ideal conditions are that the transmit beam must be perpendicular in both planes to a hard flat faced object which is between 1 m and 10 m distant. In the hands of a human who can ensure that these conditions are met, reasonable reliability can be achieved particularly if grossly inaccurate results are ignored. When mounted on a mobile robot, which has a limited ability to compensate for the shortcomings of the sensor, typically over 70lff of the readings are found to be dangerously misleading (Smith, 1992). The errors are potentially dangerous when the sensors indicate that an obstacle is further away than it really is. This is the normal case with sonar because multiple reflections travel via longer paths, and reflections from objects not at right angles to the transmit beam generally give too little power at the receiver to exceed the noise level. This normally results in the sonar reading maximum range . Techniques for avoiding this problem include :- providing the robot with a map of the room (Drumheller, 1987), and choosing only those measurements where three adjacent readings agree to within a few percent (Kanesalingam, 1993). A comparative review of all the sensors of practical value for mobile robots including television, sonar and radar is given in (Bradshaw, 1990).
Radar has rarely been considered as a suitable technique for range measurement at short ranges because the speed of light makes the time of flight of the transmit signal so short as to be extremely difficult to measure. In addition the range and angular resolutions are generally low except in relatively expensive systems . The purpose of the work described here has been to try to overcome these difficulties to a sufficient degree that short range radar can compete on cost and performance with sonar and television . The potential advantages of radar include :- 1) Simplicity and low cost (relative to television based vision systems), 2) Discrimination of metallic objects from non-metallic objects, 3) Simple tracking and following of moving objects , 4) Lower background noise level, 5) Relative immunity to erroneous range readings in comparison with sonar, 6) Long range, typically 30 m for a safe level of transmit power, 7) Usable in total darkness, dust, smoke, and dirty environments.
2. THE PERFORMANCE OF TELEVISION AND SONAR The predominant sensor systems for Autonomous Guided Vehicles (AGV's), television and sonar have particular disadvantages. Television has good angular resolution but the poor range resolution makes image recognition a difficult and expensive task, particularly in near real-time applications. Sonar has poor angular resolution but can produce,
3. THE PERFORMANCE OF THE RADAR For safe use in close proximity to humans the maximum permissible microwave transmitter power level is 10 mW. At this level sufficient power is
513
received from most common objects to be able to detect them even when the beam strikes the objects at glancing angles of incidence. 1 m diameter obstacles may be readily detected at ranges of around 30 m with wide beam antennas and at considerably greater distances with narrow beam antennas. These results are achieved partly because the background noise level at microwave frequencies is very low and partly because the conversion efficiency of microwave detectors is high. The predominant source of noise is due to reflections from the ionised gas in fluorescent lights at harmonics of the power line frequency . ,
amplitudes of the harmonics of the modulation frequency in the frequency domain. Olrrently various transforms are being investigated to enable frequency domain processing of the harmonic amplitudes. From this the range data must be extracted with sufficient accuracy and resolution but without undue bulk and cost. The difficulty is in achieving acceptable ran~ ~racy and resolution at reasonable cost.
Several sensor systems each consisting of a transmitter, receiver and common antenna (see Fig.l) have been built and evaluated. They weigh approximately 250 grammes each and measure 60mm x 30mm x 20mm with a wide beam width antenna. The sensors operate at frequencies of 10 GHz and 13 GHz with a current consumption of 500 mA at 12 V.
+7V
10 mW Gunn source
The radar fronl-end
Fig.2
Fig.1
The microwave section
'''~
AM PLITUDE
pV (\ f\
1\
A
A.
v l j \J VV
The principle of operation is that the sensing area is illuminated with a frequency modulated continuous wave (FMCW) microwave beam . Obstacles within the beam reflect part of the signal to a receiver which detects the reflected signal .
eST
FREOVENCV
TRANSMIT
d.F
T· F
RE CENiOD
't;44J??1""" 1.1/=
T
I
~., '"''"'~'.
The time delayed received signal is mixed with a small sample of the transmit signal. The mixing process produces sum and difference frequencies . The sum frequency is absorbed in a filter and the difference frequency, which is proportional to the range. is processed (see Fig. 2). As the modulation is cyclic the difference frequency is interrupted cyclically at the same rate . This frequency does not therefore exist other than as a tone burst in the time domain (see Fig. 3) or as variations in the
Fig.3
514
n
1\
~
The transmit and receive signals
! ""!'"
~. ~ =
3.1 Range measpremeDt
range of 2.5 unacceptable.
If the transmit frequency is increased in a linear sweep from FO to F1, a frequency deviation of F, in time T, and the transmitted frequency signal is delayed, on its flight from transmitter to the obstacle in its path and back to the receiver, by time dT then:-
dT
T
--F elF
where T is the time for one cycle of the modulating signal and dF is the frequency difference between the transmitted signal and the received signal. The time delay is proportional to the range R, from:-
dT _ 2R
= the velocity of light
of which
are
4. CONCLUSION The simple transmitter/receiver modules that have been made give very good performance when measuring velocity and direction of relative motion. As expected the simplest method of modulating the transmitter frequency (by altering the bias supply to the Gunn dic;KIe) only permits limited frequency deviation and gives poor linearity. Only limited range resolution has therefore been achieved . Methods of improving the range resolution with minimal increase in cost are being investigated.
This work is being undertaken as a part-time PhD research project under the supervision of Dr. H.A. Fatmi PhD., DIC, CEng, MIEE, CPhys, MlnstP . of the Department of Electrical and Electronic Engineering, King's College, Strand, London, WC2R 2LS.
thus R _ c T dF 2F If the modulating frequency is f then
6. REFERENCES
1 f
T-and
R _ c dF
2Ff If the zero crossings of the difference frequency are counted then the smallest range resolution obtainable occurs when the number of cycles of difference frequency changes by one half. If less than one half cycle of difference frequency cannot be measured then the shortest range that can be measured is when the difference frequency is equal to half the modulating frequency, i.e . when
R- -
both
S.ACKNO~DGEMENT
c where c
Metres,
C
4F
The transmit frequency may be modulated by a) varying the Gunn diode bias supply voltage, b) by adding a varactor diode to the microwave circuit or c) by using a YlG oscillator. Option a) adds nothing to the cost, option b) adds approximately 50 pounds, and option c) adds approximately 1000 pounds . Options a) and b) have been tried. Option c) has been shown to work well (Lange and Detlefsen, 1989), (Young et al., 1992) but at considerable bulk and cost. Using method a), a 10 GHz transmitter gives a deviation of about 30 MHz. This gives a range resolution of 2.5 Metres and closest measurable
515
Bradshaw, A. (1990). Sensors for Mobile Robots. Measurement and Control Vol. 23. March. pp. 48-52. Drumheller, M. (1987). Mobile Robot Localisation Using Sonar. IEEE Trans. on Pattern AJlalysis and Machine Intelligence, Vol. PAMI-9, No. 2 . March. pp . 325-332. Kanesalingam, c., Dodds S.J., and Smith, M.C.B. (1993). A Simple Mapping and Path Planning Algorithm for Mobile Robots. 1st IFAC lilt. Workshop on Intelligent Autonomous Vehicles, Southampton, April (submitted). Lange, M., and Detlefsen, J. (1989). 94GHz 3D Imaging Radar for Sensor Based Locomotion. IEEE MTT-S Digest. pp. 1091-1094. Smith, M .C.B. (1992). A Microwave Radar Transceiver System for use as an External Robot Control Sensor. Inst. Meas. Con. Symp. 011 Sensory Systems for Robot Control, London, October. pp. 2/1-2/4. Young, E., Tribe, R., and Conlong, R. (1992). Improved Obstacle Detection by Sensor Fusion . lEE Coli. on Prometheus and Drive, London, October. pp. 2/1-2/6.
1. INTRODUcnON
in ideal conditions, good range accuracy and resolution. The ideal conditions are that the transmit beam must be perpendicular in both planes to a hard flat faced object which is between 1 m and 10 m distant. In the hands of a human who can ensure that these conditions are met, reasonable reliability can be achieved particularly if grossly inaccurate results are ignored. When mounted on a mobile robot, which has a limited ability to compensate for the shortcomings of the sensor, typically over 70lff of the readings are found to be dangerously misleading (Smith, 1992). The errors are potentially dangerous when the sensors indicate that an obstacle is further away than it really is. This is the normal case with sonar because multiple reflections travel via longer paths, and reflections from objects not at right angles to the transmit beam generally give too little power at the receiver to exceed the noise level. This normally results in the sonar reading maximum range . Techniques for avoiding this problem include :- providing the robot with a map of the room (Drumheller, 1987), and choosing only those measurements where three adjacent readings agree to within a few percent (Kanesalingam, 1993). A comparative review of all the sensors of practical value for mobile robots including television, sonar and radar is given in (Bradshaw, 1990).
Radar has rarely been considered as a suitable technique for range measurement at short ranges because the speed of light makes the time of flight of the transmit signal so short as to be extremely difficult to measure. In addition the range and angular resolutions are generally low except in relatively expensive systems . The purpose of the work described here has been to try to overcome these difficulties to a sufficient degree that short range radar can compete on cost and performance with sonar and television . The potential advantages of radar include :- 1) Simplicity and low cost (relative to television based vision systems), 2) Discrimination of metallic objects from non-metallic objects, 3) Simple tracking and following of moving objects , 4) Lower background noise level, 5) Relative immunity to erroneous range readings in comparison with sonar, 6) Long range, typically 30 m for a safe level of transmit power, 7) Usable in total darkness, dust, smoke, and dirty environments.
2. THE PERFORMANCE OF TELEVISION AND SONAR The predominant sensor systems for Autonomous Guided Vehicles (AGV's), television and sonar have particular disadvantages. Television has good angular resolution but the poor range resolution makes image recognition a difficult and expensive task, particularly in near real-time applications. Sonar has poor angular resolution but can produce,
3. THE PERFORMANCE OF THE RADAR For safe use in close proximity to humans the maximum permissible microwave transmitter power level is 10 mW. At this level sufficient power is
513
received from most common objects to be able to detect them even when the beam strikes the objects at glancing angles of incidence. 1 m diameter obstacles may be readily detected at ranges of around 30 m with wide beam antennas and at considerably greater distances with narrow beam antennas. These results are achieved partly because the background noise level at microwave frequencies is very low and partly because the conversion efficiency of microwave detectors is high. The predominant source of noise is due to reflections from the ionised gas in fluorescent lights at harmonics of the power line frequency . ,
amplitudes of the harmonics of the modulation frequency in the frequency domain. Olrrently various transforms are being investigated to enable frequency domain processing of the harmonic amplitudes. From this the range data must be extracted with sufficient accuracy and resolution but without undue bulk and cost. The difficulty is in achieving acceptable ran~ ~racy and resolution at reasonable cost.
Several sensor systems each consisting of a transmitter, receiver and common antenna (see Fig.l) have been built and evaluated. They weigh approximately 250 grammes each and measure 60mm x 30mm x 20mm with a wide beam width antenna. The sensors operate at frequencies of 10 GHz and 13 GHz with a current consumption of 500 mA at 12 V.
+7V
10 mW Gunn source
The radar fronl-end
Fig.2
Fig.1
The microwave section
'''~
AM PLITUDE
pV (\ f\
1\
A
A.
v l j \J VV
The principle of operation is that the sensing area is illuminated with a frequency modulated continuous wave (FMCW) microwave beam . Obstacles within the beam reflect part of the signal to a receiver which detects the reflected signal .
eST
FREOVENCV
TRANSMIT
d.F
T· F
RE CENiOD
't;44J??1""" 1.1/=
T
I
~., '"''"'~'.
The time delayed received signal is mixed with a small sample of the transmit signal. The mixing process produces sum and difference frequencies . The sum frequency is absorbed in a filter and the difference frequency, which is proportional to the range. is processed (see Fig. 2). As the modulation is cyclic the difference frequency is interrupted cyclically at the same rate . This frequency does not therefore exist other than as a tone burst in the time domain (see Fig. 3) or as variations in the
Fig.3
514
n
1\
~
The transmit and receive signals
! ""!'"
~. ~ =
3.1 Range measpremeDt
range of 2.5 unacceptable.
If the transmit frequency is increased in a linear sweep from FO to F1, a frequency deviation of F, in time T, and the transmitted frequency signal is delayed, on its flight from transmitter to the obstacle in its path and back to the receiver, by time dT then:-
dT
T
--F elF
where T is the time for one cycle of the modulating signal and dF is the frequency difference between the transmitted signal and the received signal. The time delay is proportional to the range R, from:-
dT _ 2R
= the velocity of light
of which
are
4. CONCLUSION The simple transmitter/receiver modules that have been made give very good performance when measuring velocity and direction of relative motion. As expected the simplest method of modulating the transmitter frequency (by altering the bias supply to the Gunn dic;KIe) only permits limited frequency deviation and gives poor linearity. Only limited range resolution has therefore been achieved . Methods of improving the range resolution with minimal increase in cost are being investigated.
This work is being undertaken as a part-time PhD research project under the supervision of Dr. H.A. Fatmi PhD., DIC, CEng, MIEE, CPhys, MlnstP . of the Department of Electrical and Electronic Engineering, King's College, Strand, London, WC2R 2LS.
thus R _ c T dF 2F If the modulating frequency is f then
6. REFERENCES
1 f
T-and
R _ c dF
2Ff If the zero crossings of the difference frequency are counted then the smallest range resolution obtainable occurs when the number of cycles of difference frequency changes by one half. If less than one half cycle of difference frequency cannot be measured then the shortest range that can be measured is when the difference frequency is equal to half the modulating frequency, i.e . when
R- -
both
S.ACKNO~DGEMENT
c where c
Metres,
C
4F
The transmit frequency may be modulated by a) varying the Gunn diode bias supply voltage, b) by adding a varactor diode to the microwave circuit or c) by using a YlG oscillator. Option a) adds nothing to the cost, option b) adds approximately 50 pounds, and option c) adds approximately 1000 pounds . Options a) and b) have been tried. Option c) has been shown to work well (Lange and Detlefsen, 1989), (Young et al., 1992) but at considerable bulk and cost. Using method a), a 10 GHz transmitter gives a deviation of about 30 MHz. This gives a range resolution of 2.5 Metres and closest measurable
515
Bradshaw, A. (1990). Sensors for Mobile Robots. Measurement and Control Vol. 23. March. pp. 48-52. Drumheller, M. (1987). Mobile Robot Localisation Using Sonar. IEEE Trans. on Pattern AJlalysis and Machine Intelligence, Vol. PAMI-9, No. 2 . March. pp . 325-332. Kanesalingam, c., Dodds S.J., and Smith, M.C.B. (1993). A Simple Mapping and Path Planning Algorithm for Mobile Robots. 1st IFAC lilt. Workshop on Intelligent Autonomous Vehicles, Southampton, April (submitted). Lange, M., and Detlefsen, J. (1989). 94GHz 3D Imaging Radar for Sensor Based Locomotion. IEEE MTT-S Digest. pp. 1091-1094. Smith, M .C.B. (1992). A Microwave Radar Transceiver System for use as an External Robot Control Sensor. Inst. Meas. Con. Symp. 011 Sensory Systems for Robot Control, London, October. pp. 2/1-2/4. Young, E., Tribe, R., and Conlong, R. (1992). Improved Obstacle Detection by Sensor Fusion . lEE Coli. on Prometheus and Drive, London, October. pp. 2/1-2/6.