USING UWB TECHNOLOGY AND SENSOR FUSION FOR NAVIGATION OF AN AGV

Risang Yudanto

Flanders Make

Master Class Seminar ‘Indoor and Outdoor Positioning’

28 March 2017

2

Application case

Autonomous navigation of an agriculture AGV in a green house

Requirements:

1-2 Centimeter accuracy

Precise and smooth positioning

High update rate (>25 Hz)

GPS signal is not always available

Other technology is needed to localize the AGV

Visual beacon accurate but may suffer from environment (fogs, etc)

UWB positioning interesting, but still need to be validated

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Ultra-Wide Band (UWB) Technology

UWB signals are typically modulated pulse train, with very short pulse duration (in the order of nanosecond, or sub-nanosecond)

• This time resolution leads to precise ranging capability

The very short pulse duration leads to large signal bandwidth

• Normally ≥ 500 MHz

Images source: I. G. Zafer Sahinoglu, Sinan Gezici. Ultra-wideband Positioning Systems: Theoretical Limits, Ranging Algorithms, and Protocols. Cambridge University Press, October 6, 2008.

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Decawave UWB Technology

Frequency: 3.4 – 4.8 GHz and 6 – 8.5 GHz

Bandwidth 499.2 MHz or 900 MHz

Enables two-way UWB communication to measure range (Two-Way-Ranging TWR)

No synchronization is required

±10 cm ranging accuracy (bias error)

Max distance, outdoor line-of-sight:

200 m @ 110 kbps PHY data rate

60 m @ 6.8 Mbps PHY data rate

Tag Anchor

poll

response

final

report

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UWB Localization Architecture using Two-Way-Ranging

Mobile platform

Tag

(x0,y0,z0) (x1,y1,z1)

(x3,y3,z3) (x2,y2,z2)

Anchor 1Anchor 0

Anchor 2Anchor 3

R1

R2R3

Processing Unit

R0

R0,R1,R2,R3

Visualization & configuration

interface

R0,R1,R2,R3

3x or more ToF (Ranges) 3D localization

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Factors contributing to position error

Range measurement error (bias & random part)Non line-of-sight between tag and anchor(s)

Multipath signal

Internal source of error:Clock noise and clock drift in the receiver

Signal delay in the antenna

…

Dilution of Precision (DOP)Anchors configuration and tag relative position to the anchors

Direct signal

Multipath signal

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DOP Concept

High Uncertainty

Low Uncertainty

Poor Geometry

(high DOP)

Good Geometry

(low DOP)

Tag

Tag

Anchor Anchor

Anchor Anchor

Anchor

Anchor

Anchor

Anchor

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DOP & Position Error

DOP can be used to analyze the expected variation of position error of different anchors configurations of a localization algorithm under ideal condition (no multipath, no signal blocking). Anchor

Anchor

Anchor

AnchorUEREyxD

UEREzyxD

HDOP

PDOP

uu

uuu

222

2223

Ranging error

mUERE 1.0

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Anchors configuration in the greenhouse

• Horizontal (XY) dilution of precision is ≤ 1.2 in most of covered area

Anchor Anchor

Anchor Anchor

• Anchors height: 0.385 m• Tag height: 0.88 m

AnchorAnchor

AnchorAnchor

Tag

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Using only UWB for vehicle navigation is not enough

• Advantages

• No position drift over time

• No need for recalibration over time

• Accurate for long periods

• Disadvantages

• Low update rate

• Considerable system delay

• Not precise for short periodsnot smooth enough for controller

• Need for external infrastructure

UWBReference

UWBReference

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Other sensors (IMU, encoders) have also their own advantages and disadvantages

• Advantages

• High update rate

• Limited system delay

• Precise for short periods

• No need for external infrastructure

• Disadvantages

• Position drift over time

• IMU sensitivity and offset drift

• IMU needs recalibration over time

• Not accurate for long periods

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Combine UWB, Encoders and Inertial Measurement Unit for AGV Navigation

Low cost IMU (Accelerometer & Gyroscope)

Decawave anchor

Acceleration at tangential direction

Acceleration at normal direction

Decawave tag

Yaw rate

Wheel velocity (Left)

Wheel velocity (Right)

x

Orientation

y

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Implementation with multi-rate Extended Kalman Filter

Measurements @ different rates:

• UWB positioning (position: x,y) • low rate: < 20 Hz

• Encoder (velocity: v) • high rate: 100 Hz

• IMU (angular rate: ω, acceleration: at, an)• high rate: 100 Hz

UWB positioningx, y

IMU Measurementsω, at, an

Sensor Fusion (EKF)

Low rate < 20 Hz High rate 100 Hz

Encoder Measurements vL, vR

x, y, θ, v, ω, at, an, aω

To controller

State estimate @ high rate

• Position: x, y

• Orientation: θ

• Linear velocity: v

• Angular rate: ω

• Acceleration: at, an, aω

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Multi rate estimator

Low rate UWB

High rate IMU

Time

Non synchro: fused in the next sample time

High rate Encoder

System time

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Hardware platform comparison for sensor fusion implementation

Zed-board

RPi 1 B+ RPi 2 B XMC 4500

Hercules STM32F7 SoM

Processor Cortex A9 Cortex ARM1176JZ

Cortex A7 Cortex M4F Cortex R4F Cortex M7F

Clock freq. 667 MHz 700 MHz 900 MHz 120 MHz 220 MHz 216 MHz

Floating pt. Yes Yes Yes Yes Yes Yes

Multicore 2 cores 1 core 4 cores 1 core 1 core 1 core

DMIPS/core 1667 875 1710 150 356 462

RAM 512 MB(?) 512 MB 1 GB 128 kB 256 kB 512 kB

I/O ?x SPIEthernet

3(?)x SPIEthernet

3(?)x SPIEthernet

4(?)x SPIEthernet

?x SPI Ethernet

2 x SPI Ethernet

Form factor PCB PCB9 x 6 cm

PCB9 x 6 cm

PCB PCB PCB (in developmen

t in FM)

Developmentenvironment & tools

Xilinx tools Linux Linux Eclipse, debugger

Code Composer

Studio

Eclipse, debugger

Module price(EUR)

435 30 30 30 200 From 27 USD

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Experimental validation on the AGV prototype

1σ : 0.0105 - 0.0175 [m]

Some signal drops

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Experimental validation - AGV Trajectory

• The fusion output is smooth enough forthe controller

• Position difference to reference < 20 cm

Anchor

Anchor

Anchor

Anchor

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Positioning without systematic ranging error (simulation)

• True position is within thedistribution of the estimates

Anchor Anchor

Anchor Anchor

• Ranging error 1σ: 0.0175 [m] • Ranging error mean: 0

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Positioning with systematic ranging error (simulation)

• True position is outside thedistribution of the estimates

Anchor Anchor

Anchor Anchor

• Ranging error 1σ: 0.0175 [m] • Ranging error mean: +0.1 [m]

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Conclusions

A low-cost localization system for autonomous navigation of agriculture AGV has been developed

Fusion of UWB positioning system, encoders and inertial measurement unit

It has been implemented on low cost embedded platform and validated on a prototype of agriculture AGV

The sensor fusion improves positioning precision and gives other information important for AGV navigation such as orientation, velocity, angular rate and acceleration

Sensor fusion with EKF, however, doesn’t entirely remove bias error

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