Mobile robots. Structure of this lecture. Section A: Introduction to AGV: mobile robots. Section B: Design of the ShAPE mobile robot

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Mobile robots

Development of the

ShAPE mobile robot

Ing. A.Tasora Dipartimento di Ingegneria Industriale Università di Parma, Italy [email protected]

Development of the ShAPE mobile robot

A.Tasora,Dipartimento di Ingegneria Industriale, Università di Parma, Italy _{slide n. 2}

Structure of this lecture

• Section

A

:

Introduction to AGV: mobile robots

• Section

B

:

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Section

A

:

Introduction to AGV: mobile robots

Autonomous guided vehicles

• Mobile robots: used for

– surveillance – logistics

– entertainment, etc.

• Solutions are different in terms of

– method of locomotion (wheels, legs, tracks, etc.) – payload & speed (performance)

– navigation system – etc…

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Some ready-to-use AGV

• Esatroll ‘Paquito’

– Max speed 1.3 m/s

– with laser scanner and bumpers

• Proxaut ‘MT10’

– Max speed 1.3 m/s – Max payload 1000 kg

– LGV navigation, with laser and gyroscope

Some ready-to-use AGV

• Skilled ‘MT10’

– Max speed 1.5 m/s – Max payload 2500 kg – LGV navigation, with laser – Repeatability: 10 mm

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Locomotion systems

• Propeller / jet / rocket.. (UAV, unmanned aerial vehicles)

– 6-DOF navigation

– GPS + gyroscopes + magnetic gyrocompass + vision awareness + laser altimeter + accelerometers (and Kalman filter…)

• Legs

– Difficult to control – Useful for uneven

pavements

– Not useful for industrial environments

• Tracks / snakes / etc.

– Mostly for research – not in industry

Locomotion systems

• Three ‘Interroll’ omnidirectional wheels

– No need to turn wheels: direct transmission with 3 motors – All types of 3-DOF manouvers

on 2D plane

– Not suited for high speeds – Not suited for high loads – Possible improvements

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Locomotion systems

• 3 or 4 fully steerable wheels

– All types of 3-DOF manouvers on 2D plane

– Good performances but… – Complex design (more

motors than DOFs)

Locomotion systems

• Two parallel wheels and one steerable wheel

– Simplified design – Only two motors

– Good speed & payload (ex. industrial environments)

– Not all 3-DOF motions in 2D are possible! (non-holonomic constraints) – Two different approaches:

m1 m2

m1

m2 ‘Differential wheels’ ‘Motorized steering wheel’

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Locomotion systems

• Advantages:

– Front wheel never gets stuck

• Disadvantages

– Two sizes for the motors

– One of the two motors works much more than the other – The mechanism for steering requires vertical space

m1

m2 ‘Motorized steering wheel’

Locomotion systems

• Advantages:

– Same size for motors, reducers and controllers

– Both motors are used for accelerating lightweight design – Very simple to build

– Small footprint

• Disadvantages

– The front wheel has passive steering, it can ‘get stuck’..

m1 m2 ‘Differential wheels’

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Navigation systems & sensors

• How to get the absolute position (x, y, θ ) of the robot?

• Odometric data (recordings of wheel

rotation) is not enough! It accumulates errors – it must be integrated with other more ‘absolute’ information..

• Absolute position must be updated

in real-time, as fast as possible

• No need for extreme precision (10 mm repeatability is good)

• Solution? Different systems are used….

G YI XI YR XR θ

Navigation systems & sensors

• Robot on railways / on guides

– Easy solution, but not flexible…

– Requires expensive modifications to the building floor/roof

• Wires in the floor & inductive sensor

– Easy solution, not 100% flexible…

– Requires expensive modifications to the building floor • Optical lanes painted on the floor

– Easy solution, not 100% flexible…

– Cheap modifications to the building floor, but painted lines on the ground can be covered by dirt

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Navigation systems & sensors

• Gyroscopes

– Only rotation information

– Mechanical / Laser ‘Sagnac effect’ / Piezo (MEMS)

– Only piezo gyros are cheap, but easily accumulate drifting..

• Magnetic gyrocompasses

– Only rotation information

– Extremely cheap (two IC fluxometers)

– Measure the magnetic field of Earth absolute, but low precision – Affected by disturbs

Navigation systems & sensors

• Satellite GPS

– Only x,y position

– Not precise enough (but cheap) – Requires open air

• MEMS gyroscopes + MEMS accelerometers ( + gyrocompass + …) – 3 DOF rotation without drifting

– Useful for attitude of UAV, drones, etc – Redundant sensors: exploit Kalman filters – Adding GPS for translation too: full 6 DOF

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Navigation systems & sensors

Example: a quadcopter drone with autopilot (Ilmenau University , DE)

Navigation systems & sensors

• Laser navigation (LGV)

– Both x,y position and rotation – Very used for industrial AGV – Rotates a laser and sees when

it hits some fixed reflective markers in the building – Problems with occluded

markers / bad illumination – Not that cheap…

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Navigation systems & sensors

• Feedback with artificial vision

– 1) One or more camera on the roof ‘see’ the AGV

– 2) Image analysis software can extract features from camera views – 3) Position of AGV is obtained in view field, then trasformed to abs.space – No need to put the computer on the robot

– Often used for small robots (soccer robot games, etc)

– Robots must have recognizable symbols on their top (problems with bad illumination, etc.)

• Artificial vision awareness (SLAM approach)

– The camera is mounted on the robot the robot ‘looks’ at the

environment which it navigates, while an AI software with artificial vision can understand the position respect to known objects (walls, windows). – Very complex sw, low robustness not ready for industrial applications.

Section

B

:

Design of the ShAPE

mobile robot

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Operating environment

• The robot must carry small boxes filled with plastic materials

• Small footprint is required (max 1m length)

• No need to buy large commercial AGV

• We developed a custom AGV, with simple navigation method based

on feedback from fixed videocameras and image analysis

Operating environment

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Operating environment

• The storage system: how the load/unload buffer works

Locomotion system

• We choose the ‘differential’ system because, among other

advantages, allowed us to keep the vertical size of the load plane under the strict requirement (150 mm)

m1 m2 ‘Differential wheels’

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Overall sizing

Choosing motors and transmissions

Requirements:

• Speed: 1 m/s

• Ramps: 8%

• Accelerations: as from various

benchmark for typical duty cycles.. Results:

• Reducers ratio: 1/20

• Wheel diameter: 120 mm

• Brushless motors LENZE Fluxxtorque 931E

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Choosing motors and transmissions

• Lenze brushless motors (24V)

Choosing motors and transmissions

• The worm reducer

– Low precision (some backlash) and low efficiency but… – ..fits into budget constraints

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Mechanical design

• The aluminum truss

Mechanical design

The box for drive controllers, electronic devices and accumulators

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Mechanical design

Details

Mechanical design

The wheel: it must touch the ground in a point (i.e. the smallest possible area) Bearings must be resistant (1000N of radial force)

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Mechanical design

The pivoting wheel must be as stiff as possible, with toroidal surface, so that it does not create unwanted frictional effects during changes of direction.

We tried different types of materials. Cast polyurethane is worse than hard polyammide. Cylindrical tire is worse than beveled or toroidal surface.

Electrical design

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Electrical design

The accumulators: 4 x 27Ah standard lead batteries

Predicted continuous operating time without need to recharge: 2h.

Control

• The AGV is controlled by a remote computer using Wi-Fi ethernet

• The remote computer is fixed to ground (it does not waste electric

power) while on the AGV there are only simple controllers for the simplest tasks

• The remote computer is also responsible of complex image analysis

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Control

• Remote computer • Router wireless • Bridge wireless • Converter Ethernet CAN Ethernet Wireless IEEE 802.11g Ethernet CAN bus • MCU real-time controller • Drives of the two motors • Hi-Res Videocamera firewire

Control

• Note!!! This CAN-over-WiFi scheme is enough for the prototype, but NOT for hard-real-time environments (an embedded controller should take care of RT)

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Control

The two drives for the control of the brushless motors

Software

The software updates the state of the robot each 20ms

Acceleration / speed / rotation ramps for the two wheels are calculated on-the-fly, so the speed setpoint is continuously passed to the two controllers with CAN telegrams:

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Software

The user interface Allows: - jogging - storing a position list - programming

Software

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Repeatability

Test: good results even with open-loop feed-forward only

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Conclusions

• The ShAPE mobile robot is a custom AGV with good

performance and low cost

• Global positioning comes from artificial vision

• CPU-intensive operations are performed on a computer

that is fixed to ground