Navigation problem Define internal navigation sensors for a ground robot with car type kinematics (4 wheels + ackerman steering + rear wheel drive) Sensors? Where? Why? ~ 15-20 min. Describe your system shortly.
Navigation (internal) Sensors To sense robot s own state Magnetic compass (absolute heading) Gyro (angular speed => change of heading) Acceleration sensors (acceleration) tako, encoder (speed, distance) syncro, resolver (speed, position)
Magnetic Compasses Based on the detection of earth s magnetic field~60μt Absolute heading, coarse accuracy Available magnetic compasses: Mechanical magnetic compasses Fluxgate compasses Magnetoinductive compasses Hall-effect compasses Magnetoresistive compasses Magnetoelastic compasses
Magnetic Compasses Magnetic field ~ 60μT About from south to north Declination = the angle between true and magnetic north Deviation = the angle between the indicated and actual bearing to magnetic north Inclination = the vertical component of the magnetic field (magnetic dip) Variation = local errors
Mechanical Magnetic Compasses Marine navigation device (the first written reference: China 2634BCE, commonly in use 1300) Fluid damping and gimbal mounting is adequate for marine applications problems in rough terrain Starguide miniatyre compass permanent-magnet rotor low-friction jeweled bearing internal damping 8 led display or analog output
Fluxgate-Compass Fluxgate = trade name of the first commercial saturablecore magnetometer High permeability core non-saturated (A) and saturated (B) controls the magnetic flux Saturation is controlled with sinusoidal or quadratic wave in the drive-coil The expanding and collapsing magnetic flux induces to the sense coil an emf. relative to the existing magnetic field
Fluxgate-Compass In rod type structure the field of caused by the drive coil affects to the sense coil undesired phenomena a rod is replaced with two rods where drive coils are wound oppositely opposite magnetic fluxes compensate each other
Fluxgate-Compass To measure the direction of the (earth s)magnetic field two perpendicular sense coils are needed Toroidal structure is suitable because magnetic fluxes (by drive coils) compensate each other demagnetization effects are smaller than in the rod-type structure
Fluxgate-Compass An other popular design is three-legged spider configuration Three horizontal sense coils (120 apart each others) Common vertical drive coil
ZEMCO Fluxgate-Compass Two perpendicular sense coils on a toroidal drive coil Used in ROBART II Electrical cabling of the robot and metal object near by caused errors Relatively high power consumption (250 ma, 12V)
ZEMCO Fluxgate-Compass Analog compass was later replaced with digital one by same manufacturer Smaller power consumption (94 ma) Typical accuracy ± 6º
WATSON Fluxgate Gyro Compass Fluxgate-Compass and gyro data is fused with micro processor More stable less sensitive to enviromental noise Toroidal ring-core fluxgate sensor internally gimbal mounted Piezoelectric tuning-fork gyro (next chapter) Analog output and serial 12-bit digital output Accuracy ± 2º (with ModBot on flat storage floor)
WATSON Fluxgate Gyro Compass
KVH Fluxgate-Compasses Different type of fluxgate-compasses from inexpensive to sophisticated systems intended for military applications Example: C100 Compass Engine developers kit Micro processor controlled toroidal ring-core fluxgate sensor Different type of internal mountings for different tilt angles Resolution ± 0,1º, accuracy ± 0,5º, repeatability ± 0,2º System damping is user selectable Automatic compensation algorithm (SW) to correct the magnetic anomalities associated with the host vehicle Analog/digital output
Miniatyre Orientation sensor Applied Physics Systems Three axis (pitch, roll, yaw) orientation sensor The combination of three axis acceleration sensor and non-gimballed three axis fluxgate-sensor Interface: RS-232 port Output: either angles (pitch, roll, yaw) or individual acceleration values and magnetic field values Accuracy ± 0,5º
Magnetoinductive Compass One sense coil for each axix Sense coil serves as an inductive element in low-power LRoscillator The relative permeability of the coil core material varies as a function of the magnetizing force => output signal is relative to the existing magnetic field = orientation in the earth s field power consumption is smaller than with fluxgate-sensors
Magnetoinductive TCM-Compass Three axis magnetoinductive sensor for the X-, Y- ja Z- components of the magnetic field Two axis inclinometer for tilt and roll measurement Microprocessor corrects the error from inclinations Automatic distortion detection algorithm raise warning when disturbances (metallic objects, cables) are detected Accuracy ± 1º 5...25 VDC, 6...12 ma For mobile robots like (ROBART III, MDARS)
Hall-effect Compasses A voltage is generated in Hall-sensor by the presence of external magnetic field (Chapter 3) Experimental compasses have been built Problems with sensitivity Both chips and compasses available
Magnetoresistive Compass The resistivity of magnetoresistive material changes as the function of the external magnetic field Magnetoresistive AMR (anisotropic magnetoresistive) ja GMR (giant magnetoresistive) - sensors are discussed in chapter 3 Magnetoresistive compasses are perfect for mobile robots: Excellent sensitivity Small power consumption Small size Decreasing price => future technology!
Magnetoresistive Compass Everett represents three magnetoresistive options: Philips AMR-compass Restricted sensitivity Space Electronics AMR-kompassi Micro-Mag-sensor with Wheatstone-bridge Honeywell digital intelligent HMR-magnetometer Three perpendicular sensors Accuracy ± 1% from full scale Development Kit available Honeywell has a wide variety of magnetoresistive sensors: http://www.ssec.honeywell.com/magnetic/products.html And Philips: http://www.nxp.com/pip/kmz51.html#description
Magnetoelastic Compass Based on magnetoelastic (= magnetostrictive) material as sensor element Magnetostriction = all ferromagnetic materials shrink or expand in the direction of magnetization Very accurate displacement sensor needed Interferometric measurement Tunneling-tip sensing (tunneling microscopy) Prototypes manufactured
Magnetoinductive TCM2 Compass (demonstrated) Basic structure like in TCM1 Better sensitivity More advanced compensation algorithm Accuracy +/- 0,2 deg Resolution +/- 0,1deg Repeatibility +/- 0,1 deg http://www.pnicorp.com/