city college of new york 1 dr. jizhong xiao department of electrical engineering city college of new...

1City College of New York

Dr. Jizhong Xiao

Department of Electrical Engineering

City College of New York

[email protected]

Advanced Mobile Robotics

Probabilistic Robotics (II)


Outline

• Localization Methods– Markov Localization (review)– Kalman Filter Localization– Particle Filter (Monte Carlo Localization)

• Current Research Topics– SLAM – Multi-robot Localization


Robot Navigation

Fundamental problems to provide a mobile robot with autonomous capabilities:

•Where am I going

•What’s the best way there?

•Where have I been? how to create an environmental map with imperfect sensors?

• Where am I? how a robot can tell where it is on a map?

• What if you’re lost and don’t have a map?

Mapping

Localization

Robot SLAM

Path Planning

Mission Planning


Representation of the Environment• Environment Representation

– Continuos Metric x,y,

– Discrete Metricmetric grid

– Discrete Topologicaltopological grid


Localization Methods• Markov Localization:

– Central idea: represent the robot’s belief by a probability distribution over possible positions, and uses Bayes’ rule and convolution to update the belief whenever the robot senses or moves

– Markov Assumption: past and future data are independent if one knows the current state

• Kalman Filtering– Central idea: posing localization problem as a sensor fusion problem– Assumption: Gaussian distribution function

• Particle Filtering– Monte-Carlo method

• SLAM (simultaneous localization and mapping)• Multi-robot localization


Probability TheoryBayes Formula :

)()|()()|(),(

)(

)()|()(

yP

xPxyPyxP

xPxyPyPyxPyxP

)()|(

1)(

)()|()(

)()|()(

1

xPxyPyP

xPxyPyP

xPxyPyxP

x

Normalization :


Probability Theory

)|(

)|(),|(),|(

zyP

zxPzxyPzyxP

Bayes Rule with background knowledge :

dzzPzyxPyxP

dzzPzxPxP

dzzxPxP

)(),|()(

)()|()(

),()(

Law of Total Probability :


Bayes Filters: Framework

• Given:– Stream of observations z and action data u:

– Sensor model P(z|x).– Action model P(x|u,x’).– Prior probability of the system state P(x).

• Wanted: – Estimate of the state X of a dynamical system.– The posterior of the state is also called Belief:

),,,|()( 11 tttt zuzuxPxBel

},,,{ 11 ttt zuzud


Markov Assumption

Underlying Assumptions• Static world, Independent noise• Perfect model, no approximation errors

),|(),,|( 1:1:11:1 ttttttt uxxpuzxxp )|(),,|( :1:1:0 tttttt xzpuzxzp

State transition probability

Measurement probability

Markov Assumption: –past and future data are independent if one knows the current state


111 )(),|()|( ttttttt dxxBelxuxPxzP

Bayes Filters

),,,|(),,,,|( 1111 ttttt uzuxPuzuxzP Bayes

z = observationu = actionx = state

),,,|()( 11 tttt zuzuxPxBel

Markov ),,,|()|( 11 tttt uzuxPxzP

Markov11111 ),,,|(),|()|( tttttttt dxuzuxPxuxPxzP

1111

111

),,,|(

),,,,|()|(

ttt

ttttt

dxuzuxP

xuzuxPxzP

Total prob.

Markov111111 ),,,|(),|()|( tttttttt dxzzuxPxuxPxzP


Bayes Filter Algorithm 1. Algorithm Bayes_filter( Bel(x),d ):

2. 0

3. If d is a perceptual data item z then

4. For all x do

5.

6.

7. For all x do

8.

9. Else if d is an action data item u then

10. For all x do

11.

12. Return Bel’(x)

)()|()(' xBelxzPxBel )(' xBel

)(')(' 1 xBelxBel

')'()',|()(' dxxBelxuxPxBel

111 )(),|()|()( tttttttt dxxBelxuxPxzPxBel


Bayes Filters are Familiar!

• Kalman filters• Particle filters• Hidden Markov models• Dynamic Bayesian networks• Partially Observable Markov Decision Processes

(POMDPs)

111 )(),|()|()( tttttttt dxxBelxuxPxzPxBel


Localization

• Given – Map of the environment.– Sequence of sensor measurements.

• Wanted– Estimate of the robot’s position.

• Problem classes– Position tracking– Global localization– Kidnapped robot problem (recovery)

Determining the pose of a robot relative to a given map of the environment


Markov Localization

1. Algorithm Markov_Localization

2. For all do

3.

4.

5. Endfor

6. Return Bel(x)

),,),(( 1 mzuxBel ttt

111 )(),,|()( tttttt dxxBelmxuxPxBel

)(),()( tttt xBelmxzPxBel

tx


Localization Methods• Markov Localization:

– Central idea: represent the robot’s belief by a probability distribution over possible positions, and uses Bayes’ rule and convolution to update the belief whenever the robot senses or moves

– Markov Assumption: past and future data are independent if one knows the current state

• Kalman Filtering– Central idea: posing localization problem as a sensor fusion problem– Assumption: Gaussian distribution function

• Particle Filtering– Monte-Carlo method

• SLAM (simultaneous localization and mapping)• Multi-robot localization


Kalman Filter Localization


Introduction to Kalman Filter (1)• Two measurements

• Weighted leas-square

• Finding minimum error

• After some calculation and rearrangements

2

1

iiw

Gaussian probability

density function

Take weight as:

Best estimate for the robot position 2

22

1

22

212

2

1

i

iw


Introduction to Kalman Filter (2)• In Kalman Filter notation

1kXBest estimate of the state at time K+1 is equal to the

best prediction of value before the new measurement

is taken, plus a weighting of value times the difference between

and the best prediction at time k

1ˆ

kX

kX 1kZ

1kZkX


Introduction to Kalman Filter (3)• Dynamic Prediction (robot moving)

u = velocity w = noise

• Motion

• Combining fusion and dynamic prediction


Kalman Filter for Mobile Robot Localization

Five steps: 1) Position Prediction, 2) Observation, 3) Measurement prediction, 4) Matching, 5) Estimation



•Robot Position Prediction– In the first step, the robots position at time

step k+1 is predicted based on its old location (time step k) and its movement due to the control input u(k):

))(,)(ˆ()1(ˆ kukkpfkkp

Tuuu

Tpppp f)k(ff)kk(f)kk( 1

f: Odometry function


Robot Position Prediction: Example

b

ssb

ssssb

ssss

k

ky

kx

kukkpkkp

lr

lrlr

lrlr

)2

sin(2

)2

cos(2

)(ˆ)(ˆ

)(ˆ

)()(ˆ)1(ˆ

Tuuu

Tpppp f)k(ff)kk(f)kk( 1

ll

rru sk

skk

0

0)(

OdometryOdometry



•Observation– The second step is to obtain the observation Z(k+1) (measurements)

from the robot’s sensors at the new location at time k+1

– The observation usually consists of a set n0 of single observations zj(k+1) extracted from the different sensors signals. It can represent raw data scans as well as features like lines, doors or any kind of landmarks.

– The parameters of the targets are usually observed in the sensor frame {S}.

• Therefore the observations have to be transformed to the world frame {W} or

• the measurement prediction have to be transformed to the sensor frame {S}.

• This transformation is specified in the function hi (seen later).


Observation: Example

j

r j

line j

Raw Date of Laser Scanner

Extracted Lines Extracted Lines

in Model Space

jrrr

rjR k

)1(,

j

j

R

j rkz )1(

Sensor (robot) frame



• Measurement Prediction– In the next step we use the predicted robot position

and the map M(k) to generate multiple predicted observations zt.

– They have to be transformed into the sensor frame

– We can now define the measurement prediction as the set containing all ni predicted observations

– The function hi is mainly the coordinate transformation between the world frame and the sensor frame

kkp 1

kkp,zhkz tii 11

ii nikzkZ 111


Measurement Prediction: Example• For prediction, only the walls that are in

the field of view of the robot are selected.• This is done by linking the individual

lines to the nodes of the path


Measurement Prediction: Example• The generated measurement predictions have to be transformed to

the robot frame {R}

• According to the figure in previous slide the transformation is given by

• and its Jacobian by


Kalman Filter for Mobile Robot Localization• Matching

– Assignment from observations zj(k+1) (gained by the sensors) to the targets zt (stored in the map)

– For each measurement prediction for which an corresponding observation is found we calculate the innovation:

and its innovation covariance found by applying the error propagation law:

– The validity of the correspondence between measurement and prediction can e.g. be evaluated through the Mahalanobis distance:


Matching: Example


Matching: Example• To find correspondence (pairs) of predicted and observed

features we use the Mahalanobis distance

with


Estimation: Applying the Kalman Filter• Kalman filter gain:

• Update of robot’s position estimate:

• The associate variance


Estimation: 1D Case

• For the one-dimensional case with we can show that the estimation corresponds to the Kalman filter for one-dimension presented earlier.


Estimation: Example

• Kalman filter estimation of the

new robot position : – By fusing the prediction of

robot position (magenta) with the innovation gained by the measurements (green) we get the updated estimate of the robot position (red)

)(ˆ kkp


Monte Carlo Localization

• One of the most popular particle filter method for robot localization

• Current Research Topics

• Reference: – Dieter Fox, Wolfram Burgard, Frank Dellaert,

Sebastian Thrun, “Monte Carlo Localization: Efficient Position Estimation for Mobile Robots”, Proc. 16th National Conference on Artificial Intelligence, AAAI’99, July 1999


MCL in action

“Monte Carlo” Localization -- refers to the resampling of the distribution each time a new observation is integrated


Monte Carlo Localization• the probability density function is represented by

samples• randomly drawn from it• it is also able to represent multi-modal distributions,

and thus localize the robot globally• considerably reduces the amount of memory required

and can integrate measurements at a higher rate• state is not discretized and the method is more

accurate than the grid-based methods• easy to implement


“Probabilistic Robotics”

• Bayes’ rule p( A | B ) = p( B | A ) • p( A )

p( B )

• Definition of marginal probability

p( A ) = p( A | B ) • p(B) all B

p( A ) = p( A B )all B

• Definition of conditional probability

- Sebastian Thrun


Setting up the problem

The robot does (or can be modeled to) alternate between

• sensing -- getting range observations o1, o2, o3, …, ot-1, ot

• acting -- driving around (or ferrying?) a1, a2, a3, …, at-1

“local maps”

whence?






We want to know rt -- the position of the robot at time t

• but we’ll settle for p(rt) -- the probability distribution for rt

! What kind of thing is p(rt) ?






We want to know rt -- the position of the robot at time t

We do know m -- the map of the environment

• but we’ll settle for p(rt) -- the probability distribution for rt

! What kind of thing is p(rt) ?

p( o | r, m )

p( rnew | rold, a, m )

-- the sensor model

-- the accuracy of desired action a


Robot modeling

p( o | r, m ) sensor modelmap m and location r

p( | r, m ) = .75

p( | r, m ) = .05

potential observations o


Robot modeling

p( o | r, m ) sensor model

p( rnew | rold, a, m ) action model

“probabilistic kinematics” -- encoder uncertainty

• red lines indicate commanded action

• the cloud indicates the likelihood of various final states

map m and location r

p( | r, m ) = .75

p( | r, m ) = .05

potential observations o


Robot modeling: how-to

p( o | r, m ) sensor model

p( rnew | rold, a, m ) action

model

(0) Model the physics of the sensor/actuators(with error estimates)

(1) Measure lots of sensing/action resultsand create a model from them

theoretical modeling

empirical modeling

• take N measurements, find mean (m) and st. dev. () and then use a Gaussian model

• or, some other easily-manipulated model...

(2) Make something up...

p( x ) = p( x ) = 0 if |x-m| >

1 otherwise 1- |x-m|/ otherwise

0 if |x-m| >



Start by assuming p( r0 ) is the uniform distribution.

take K samples of r0 and weight each with an importance factor

of 1/K




Get the current sensor observation, o1

For each sample point r0 multiply the importance factor by p(o1 | r0, m)


of 1/K







of 1/K

Normalize (make sure the importance factors add to 1)

You now have an approximation of p(r1 | o1, …, m) and the distribution is no longer uniform







of 1/K


You now have an approximation of p(r1 | o1, …, m)

Create r1 samples by dividing up large clumps

and the distribution is no longer uniform

each point spawns new ones in proportion to its importance factor







of 1/K





The robot moves, a1


For each sample r1, move it according to the model p(r2 | a1, r1, m)


MCL in action

“Monte Carlo” Localization -- refers to the resampling of the distribution each time a new observation is integrated


Discussion: MCL


References

1. Dieter Fox, Wolfram Burgard, Frank Dellaert, Sebastian Thrun, “Monte Carlo Localization: Efficient Position Estimation for Mobile Robots”, Proc. 16th National Conference on Artificial Intelligence, AAAI’99, July 1999

2. Dieter Fox, Wolfram Burgard, Sebastian Thrun, “Markov Localization for Mobile Robots in Dynamic Environments”, J. of Artificial Intelligence Research 11 (1999) 391-427

3. Sebastian Thrun, “Probabilistic Algorithms in Robotics”, Technical Report CMU-CS-00-126, School of Computer Science, Carnegie Mellon University, Pittsburgh, USA, 2000

city college of new york 1 dr. jizhong xiao department of electrical engineering city college of new...

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