Advanced Robotics 25 (2011) 825–848

brill.nl/ar

Full paper
Learning, Generation and Recognition of Motions by
Reference-Point-Dependent Probabilistic Models
Komei Sugiura ∗ , Naoto Iwahashi, Hideki Kashioka and Satoshi Nakamura
National Institute of Information and Communications Technology, 3-5 Hikaridai, Seika, Soraku,
Kyoto 619-0289, Japan
Received 15 December 2009; accepted 1 April 2010

Abstract
This paper presents a novel method for learning object manipulation such as rotating an object or placing one
object on another. In this method, motions are learned using reference-point-dependent probabilistic models,
which can be used for the generation and recognition of motions. The method estimates (i) the reference
point, (ii) the intrinsic coordinate system type, which is the type of coordinate system intrinsic to a motion,and (iii) the probabilistic model parameters of the motion that is considered in the intrinsic coordinate
system. Motion trajectories are modeled by a hidden Markov model (HMM), and an HMM-based method
using static and dynamic features is used for trajectory generation. The method was evaluated in physicalexperiments in terms of motion generation and recognition. In the experiments, users demonstrated themanipulation of puppets and toys so that the motions could be learned. A recognition accuracy of 90%
was obtained for a test set of motions performed by three subjects. Furthermore, the results showed thatappropriate motions were generated even if the object placement was changed.
© Koninklijke Brill NV, Leiden and The Robotics Society of Japan, 2011
Keywords
Imitation learning, robot language acquisition, hidden Markov model, object manipulation

1. Introduction
Adaptability and scalability have been key issues in the realm of machine learning.
In robotics, many studies on the applications of machine learning have been conducted to develop machines able to adapt to various environments and to acquire
new skills. Imitation learning, which explores methods to teach a robot to perform
new tasks by showing them, is one approach adopted in such studies [1–3]. Robotics
research on imitation learning started in the 1990s under names such as learning by
watching, learning from demonstration and programming by demonstration [3, 4].
* To whom correspondence should be addressed. E-mail: komei.sugiura@nict.go.jp© Koninklijke Brill NV, Leiden and The Robotics Society of Japan, 2011

DOI:10.1163/016918611X563328

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From the perspective of scalability, programming humanoid robots to perform newtasks is an attractive application of imitation learning since the cost of designing alarge number of control parameters can be reduced [1].
From the perspective of adaptability, imitation learning has another importance.
Object-manipulating motions such as ‘put the dishes in the cupboard’ are fundamental for robots aimed at home environments, but difficult to program beforehand.
This is because the desired motion depends on elements specific to each home: thesize and shape of the dishes and the cupboard, and whether the cupboard has adoor. In contrast, if a robot can learn motions from multiple observations of theuser’s motions, obtaining a controller for motions adapted to each home is possible.
Moreover, it is important to note that the meaning of learned motions is shared by
the user and the robot in this case. Thus, imitation learning can be a mechanism forsituated communication since the shared meaning is grounded on the sensory-motor
experiences of them [1]. Situated communication between a human and a robot in
object-manipulating tasks has been attempted in Refs [5, 6].
One of the main difficulties in learning object manipulation is that clustering
trajectories in a fixed coordinate system is not effective, since the trajectories arethen not reusable in other conditions. An example of this is learning to ‘place an
object on another’. Initial object placements are usually not fixed within the user’s
demonstrations and, in addition, the robot has to generate appropriate motions undervarious object placements. Therefore, it is necessary to generalize relative motions
between objects in the learning phase and synthesize an appropriate motion trajectory for the given object placement in the generation phase.
Some recent studies have attempted to solve the difficulties in learning object
manipulation [4]. Regier investigated a model describing the spatial relationshipbetween two objects [7]. He proposed to model motions as the time evolution of thespatial relationship between a trajector and a landmark. Ogawara et al. attempted tomodel the relative trajectories between two objects by using hidden Markov models (HMMs) [8, 9]. In their studies, input from a data glove and relative trajectories
between a cup and a bowl were used, and learned motions such as ‘grasping’ and‘pouring’ were performed by a robot. Billard et al. developed a method that extracted important features from task and joint spaces by introducing a cost function,
which was the weighted sum of task- and joint-space-based reproduction errors.
In Ref. [10], relative trajectories were modeled by HMMs and optimal state numbers were determined by model selection. Calinon et al. used multiple Gaussian
distributions to model the time series of spatiotemporal values [11]. Manipulation
trajectories by a couple of robot arms were generated based on Gaussian Mixture
Regression (GMR).
These methods use relative trajectories between two objects under an assumption that the two objects are specified by the user. Therefore, there is a limitationthat the user has to specify which motion needs information on the two objects andwhich does not. For example, the positions of two objects are necessary for learning the ‘place an object on another’ motion, while relative trajectories do not have

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to be considered in learning the ‘raise an object’ motion. Thus, overcoming thislimitation enables the learning of various types of object manipulation from naturalinteractions between the user and robot.
The objective of this study is to construct a method for learning object manipulation such as placing an object on another (place-on) or moving it away (moveaway). The proposed method has three key features:(i) The reference point and the type of intrinsic coordinate system, which is thetype of coordinate system intrinsic to a motion, are estimated based on a maximum likelihood criterion. In this process, the number of objects necessary todescribe the motion is determined and the relevant object(s) in each demonstration is estimated as well.
(ii) Manipulation trajectories are modeled by HMMs using dynamic features aswell as static features.
(iii) The optimal number of states of the HMMs is estimated by cross-validation.
With the above first feature, the proposed method can avoid suffering from thelimitation that the user has to specify the relative objects for the motion. Therefore,
it can deal with motions such as raising an object and placing an object on anotherusing the same framework. The second feature has several advantages over othermethods (e.g., Ref. [12]) such as (i) learning object-manipulation motion that isdependent on velocity is possible, and (ii) the maximum likelihood trajectory canbe searched effectively [13]. Although the maximum likelihood trajectory can begenerated from a position-only HMM (HMMs with static features), some sort ofinterpolation or smoothing is needed since the trajectory is discontinuous. Such
trajectory is not guaranteed to be the maximum likelihood trajectory any more.
Figure 1 shows the hardware platform used in this study. The system has multimodal interfaces such as a stereo vision camera and a microphone. Online andoffline learning are possible with the system. In the online learning mode, a user
demonstrates motion trajectories with uttering the corresponding motion label. In
this paper, however, we show experimental results obtained with the offline mode

Figure 1. Hardware platform used in the experiments.

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since the focus of the paper is on the quantitative evaluation of the proposed methodby dividing samples into training and test sets.
The rest of this paper is organized as follows. Section 2 first states the basic concepts and terminology for reference-point-dependent motions. Section 3 describes
the proposed method. The experimental setup and results for the learning, generation and recognition of motions are presented in Sections 4, 5 and 6, respectively.
Section 7 discusses the extensions and problems of the proposed method. Section 8
states related work and Section 9 concludes the paper.
2. Learning Reference-Point-Dependent Motions
In cognitive linguistics, a trajector is defined as a participant (object) being focusedon. A landmark has a secondary focus and a trajector is characterized with respect
to the landmark. Words representing spatial relationships such as ‘away’ and ‘leftof’ are described in terms of a relationship between a trajector and a landmark[14]. An example of this is shown in the left-hand panel of Fig. 2. This depicts acamera image in which the green puppet (Kermit) is moved along the dotted line.
In the example shown in Fig. 2, the trajector is Kermit. On the other hand, there are
multiple interpretations of the landmark. If the blue (right) box is considered as thelandmark, the label ‘place-on’ can be provided to the trajectory. However, the label
can become ‘Let Kermit jump over the green box (jump-over)’ if the green (left)
box is thought of as the landmark.
Motions such as ‘place-on’ and ‘jump-over’ are dependent on landmarks. Other
motions, such as ‘raise an object’, do not depend on a landmark; however, this motion means that the object is moved higher than its original position. Therefore,‘place-on’ and ‘raise’ are both modeled as motions dependent on particular references. In this paper, those motions are called reference-point-dependent motions.
Here, the problem of learning reference-point-dependent motions in a learningfrom-demonstration framework is considered. The reference point of each demonstration is not specified by the user in the learning phase. This setup is based on thefact that it is not always possible to specify the reference point for the given object
placement only through linguistic expressions. Moreover, an easy-to-use interaction

Figure 2. (Left) Example shot of an image stream. (Right) Preprocessed visual features obtained fromthe image stream.

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Figure 3. Relationship between a trajector/landmark, a reference point and an intrinsic coordinate
system. The spheres, ellipsoids and box represent objects, and the arrows represent the axes of the
intrinsic coordinate systems. (Left) ‘Raise’. (Right) ‘Move-closer’.

of teaching motions is possible by using this setup. Another unspecified element is
the intrinsic coordinate system type. Object manipulation involves a spatial relationship between objects, and it is important to select an appropriate intrinsic coordinate
system, which is the frame of reference [15] intrinsic to a motion, to describe theevolution of the spatial relationship.
Two examples of this can be shown by ‘raise’ and ‘move-closer’ (Fig. 3). The
reference point of ‘raise’ can reasonably be assumed to be the trajector’s center ofgravity. The intrinsic coordinate system used can be a Cartesian coordinate system,
as shown in the left-hand panel. However, in the case of ‘move-closer’, another typeof intrinsic coordinate system is necessary. In this case, the x-axis of the coordinate
system passes through the centers of gravity of the trajector and the landmark. The
point is that the reference point and intrinsic coordinate system are unobservable in
the problem trying to be solved, and so they must be estimated.
3. Learning from Observation by Reference-Point-Dependent Probabilistic
Models
3.1. Estimation of Reference Points and Intrinsic Coordinate Systems
Consider that L training samples are given for a kind motion such as ‘place-on’ or‘raise’. Let Vl denote the lth training sample. Vl consists of the motion informationof the trajector, Yl , and the set of positions of the static objects, Ol , as:Vl = (Yl , Ol )Yl = {yl (t)|t = 0, 1, . . . , Tl }yl (t) = xl (t) , ẋl (t) , ẍl (t) ,

(1)
(2)
(3)

where Yl is a time series of vectors yl (t). Here, yl (t) is composed of position xl (t),velocity ẋl (t) and acceleration ẍl (t). For simplicity, the index l is omitted when thecontext is clear.
It is assumed that there are several types of intrinsic coordinate systems and theseare given by the designer. The type of the intrinsic coordinate system is denoted

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by k. A schematic illustration is shown in Fig. 3 and such types are explained indetail in Section 4.2.
From the estimation of k and the reference point xr , the intrinsic coordinate
system is obtained in the lth data. The candidate set of reference points is denotedby R. The set of the positions of static objects, O, has to be included in R, sincethese objects are the candidates for the landmark. The first position of the trajector,
x(0), is included in R so that a motion concept that is dependent only on the object’s
trajectory can be described. In addition, the center of the camera image, xcenter , isput in R to describe motion concepts independent from the positions of objects.
Therefore:
R = {O, x(0), xcenter }  {xr | r = 1, 2, . . . , |R|},(4)

where |R| denotes the number of elements contained in R.
r
Let Ck (x l ) Yl denote the trajectory in the intrinsic coordinate system Ck (xrl ),where rl is the index of the reference point in Ck (xrl ). Henceforth, a parameterin a specific coordinate system is represented by using such left superscript. Now,
the optimal k, the optimal set of reference points, r, and the optimal parameters ofa probabilistic model, λ, are searched for using the following maximum likelihood
criterion:(λ̂, k̂, r̂) = arg max
λ,k,r= arg max
λ,k,r

L


log P (Yl |rl , k, λ)

(5)



rl
log P Ck (x ) Yl ; λ ,

(6)

l=1
L

l=1

where ˆ· represents estimation.
The number of candidate solutions for k is generally smaller than for r, whichL
is l=1 |Rl |, so we apply a brute-force method for obtaining the optimal k. Below,
the optimal λ and r are searched for each k.
Thus, we obtain the following equation from (6):(λ̂ , r̂ ) = arg max
k

k

λ,r

L




rl
log P Ck (x ) Yl ; λ ,

(7)

l=1

where ·k means that k is fixed. It is not practical to solve (7) due to its computational
complexity. Therefore, for constraint relaxation, the above discrete optimization
problem is approximated as a continuous optimization problem: |R |
L
l




rl
k
k
k
(λ̂ , ŵ ) = arg max
logwl,rP Ck (x ) Yl ; λ ,
(8)
l
λ,w

l=1

rl =1

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where:
|Rl |


k
wl,r
= 1 (l = 1, 2, . . . , L)
l

rl =1
 k

k
k
, wl,2, . . . , wl,|R
wkl = wl,1|l
wk = wk1 , wk2 , . . . , wkL .

k denotes the weight for selecting the reference point r in the training
Here, wl,rllsample Vl where k is fixed. Thus, r can be thought of as a set of hidden parameters.
Equation (8) can efficiently be solved by applying an EM algorithm [16]. In the
Appendix, a solution to (8) is explained when an HMM is used as the probabilistic
model. The optimal k is obtained by solving: |R |
L
l




rl
k
log
ŵl,r
P Ck (x ) Yl ; λ̂k .
(9)k̂ = arg max
l
k

l=1

rl =1

To estimate the optimal number of states of HMMs, leave-one-out crossvalidation [17] was used. In the first stage of the cross-validation process, training
samples were divided into the real training set (L − 1 samples) and the validation
set (one sample).
A HMM was then trained by using the real training set and evaluated with thevalidation set. The above procedure was repeated for possible combinations of training sets and validation sets, and the optimal number that gave the maximum average
likelihood was selected.
3.2. Motion Generation by Reference-Point-Dependent HMMs
Here we consider the problem of generating trajectories from reference-pointdependent probabilistic models obtained according to the aforementioned method.
Below, an HMM is used as a reference-point-dependent probabilistic model.
Let q = {qt | t = 0, 1, . . . , T } denote a sequence of states, which are unobservable. Now, consider the problem of obtaining a trajectory that maximizes theprobability of Y conditioned by the position of a trajector xrtraj , q, motion label, vi ,and reference point index, r, as:Ŷ = arg max P Y |xrtraj , q, r, vi , R
(10)
Y


= arg max P Y |xrtraj , q, r, ki , λi , R .
(11)Yxrtraj is put in the condition since Ŷ has to start from xrtraj . xrtraj is obtained fromthe index of the trajector and R. The parameters of the HMM corresponding to
the ith verb are denoted by λi = (π, A, b), where π , A and b denote the initialprobability distribution, the state transition probabilities and the output probability
density functions, respectively.

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When a left-to-right HMM is used and T is given, (11) can be solved by applying
Tokuda’s method [13]. The method gives the maximum likelihood trajectory basedon both static and dynamic features. As a matter of practical convenience, the trajectory is generated in the intrinsic coordinate system and then transformed into thecamera/world coordinate system.
3.3. Motion Recognition by Reference-Point-Dependent HMMs
Next we consider motion recognition by reference-point-dependent HMMs. Let V
denote the observed information. Similar to the above, V consists of the trajectoryof the moving object, Y , and the set of positions of static objects, O. Motion recognition is formulated as a problem of obtaining the maximum likelihood probabilistic
model to output Y .
Let V = {vi | i = 1, 2, . . . , |V |} denote a set of learned motion labels, λi denote the HMM parameter set that corresponds to motion label vi , and ki denotethe index of the intrinsic coordinate system of vi . (λi , ki ) is obtained by using the
aforementioned learning method. The maximum likelihood pair of the motion label
and reference point indices, (î, r̂), are searched for as:(î, r̂) = arg max P (Y |r, vi , R)

(12)

i,r

= arg max P (Y |r, ki , λi , R).

(13)

i,r

4. Experiment 1: Motion Learning
4.1. Hardware Platform
The experiments were conducted with the platform shown in Fig. 1. The user’s
movements were recorded by a Bumblebee 2 stereo vision camera at a rate of
30 frame/s. The size of each camera image was 320 × 240 pixels. The left-handpanel of Fig. 2 shows an example shot of an image stream, and the right-hand panel
shows the internal representation of the image stream. The transformation matrix
between the camera coordinate system and the world coordinate system was fixedsince the camera was fixed in the environment. The recorded data were used for thelearning and recognition of motions.
Motion generation experiments were conducted with a Mitsubishi Heavy Industries PA-10 manipulator with 7 d.o.f. The manipulator was equipped with a Barrett
Hand, a 4-d.o.f. multifingered grasper. Motion generation results were examinedin an environment using the manipulator and physical objects such as puppets andtoys. The hardware platform was used for motion recognition/generation experiments as well.

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4.2. Experimental Setup
In this subsection, the setup of the experiments conducted to obtain trainedreference-point-dependent HMMs for subsequent motion generation and recognition experiments is described.
A subject was asked to demonstrate motions that manipulated objects placedon a desk. Each motion was given to the subject in the form of a motion such as‘raise’ or ‘rotate’. The environment used in the experiments is illustrated in the lefthand panel of Fig. 2. A training sample was obtained from the manipulation by
preprocessing the recorded image stream of the manipulation (see the right-handpanel of Fig. 2).
For the tracking of objects, we used a heuristic algorithm, which tracks objects
on/above the table between the user and robot. The start and end points of object
trajectories were determined a priori. Specifically, if an object remained in the same
position for five frames, the trajectory of the object was segmented. The object
placement was changed after each demonstration.
All trajectories were obtained from the demonstration by the same subject, Subject A. The following motion labels (verbs) were used in the experiments: jumpover, move-away, move-closer, place-on, put-down, raise, rotate. The trajectories
corresponding to each motion label were used to train the corresponding HMM.
Seven different HMMs were obtained from the trajectory sets. The duration of eachstate of an HMM was also recorded. The average duration of each state was used
when a trajectory was generated from an HMM.
The training sets obtained in the experiments are summarized in Table 1. TA-2 toTA-9 were obtained by increasing the number of training samples, L. The average
number of objects contained in a sample in TA-9 was 3.1, which means a typicalscene contained one trajector and two landmark candidates on average.
The following types of intrinsic coordinate systems were defined:
C1 A translated camera coordinate system with its origin at the landmark position.
The direction of the y-axis is always vertical. The x-axis is inverted in the casethat the x coordinate of xl (0) is negative after translation.
Table 1.
Training sets
Training set

Subject

No. of motion
labelsNo. of trajectories
per motion label

TA-2
TA-3
..
.
TA-9

A
A
..
.
A

7
7
..
.
7

2
3
..
.
9

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C2 An orthogonal coordinate system with its origin at the landmark position. The
direction of the x-axis is from the landmark position towards xl (0).
C3 A translated camera coordinate system with its origin at xl (0).
C4 A translated camera coordinate system with its origin at xcenter .
In C3 - or C4 -type intrinsic coordinate systems, the reference point does not have tobe estimated since it is fixed to xl (0) or xcenter . Therefore, the proposed estimation
method (8) was not applied.
The weight for selecting the reference point r, wl , was initialized to wl = 1/|O|,where |O| denotes the number of static objects. Note that l, k are omitted here forsimplicity. The number of states for the initial HMMs was either of {6, 10, 14, 18}.
The average computation time for learning each motion was 9.8 s, where thetraining set of a motion contained nine samples. The computation was carried out
on a quad-core 2.66-GHz computer running the Linux operating system. The HMM
Toolkit (HTK [18]) was used for training HMMs. In the HTK, HMM parameters
are initialized based on iterative application of the Viterbi algorithm, which follows
the association of uniformly segmented training data with each state.
4.3. Results
Figure 4 illustrates examples of the internal representations for the training samples. In Fig. 4, motion labels are shown below the corresponding panels. The final
positions of the trajectors are illustrated as the painted ellipses and the sequences of
the contours represent the trajectories.
Figure 5 shows the output probability density functions (OPDFs) for HMMs
trained with trajectories corresponding to the motion label ‘place-on’. The HMM
for which the optimal state number was estimated as 18 was obtained using TA-9.
The left-hand, middle and right-hand panels of Fig. 5 illustrate distributions corresponding to the position, velocity and acceleration, respectively. The center andsemimajor (or semiminor) axis of each ellipse stands for the mean and standard
deviation of each distribution. The direction of the state transition is indicated by
the darkness of shading. From Fig. 5, the HMM is shown to have a large variancein the first state and small variance in the last state for position. This fact indicatedthat the trajectory of ‘place-on’ started from an arbitrary point, but converged to acertain point in the end.
The results regarding the estimation of the optimal state number by crossvalidation are shown in Fig. 6. Trajectories regarding ‘place-on’ were transformed
into coordinate system C1 and used for training HMMs. Figure 6 plots the likelihood of four different HMMs against the number of training samples. In this paper,
likelihood values shown in the figures are normalized by the number of frames.
Instead of likelihood, L, −log | log L| is shown so that the reader can easily findwhich HMM has the maximum likelihood. In Fig. 6, the average log-likelihood ofdifferent numbers of states are compared by cross-validation. Each line shows theK. Sugiura et al. / Advanced Robotics 25 (2011) 825–848‘place-on’‘jump-over’‘move-closer’‘move-away’‘raise’‘put-down’835‘rotate’Figure 4. Example internal representation of training samples.

Figure 5. Output probability density functions for HMMs corresponding to motion ‘place-on’. The
distributions for x, ẋ and ẍ are shown in the left-hand, middle and right-hand panels, respectively. The
vertical and horizontal axes show the C1 coordinate system (pixel).

log-likelihood of an n-state HMM given a validation set. The trajectories for the
motion ‘place-on’ in C1 were used as training samples.
Figure 6 illustrates that the validation set likelihood increased with the increasein the number of training samples. Specifically, the likelihood was very small when
the sample number was two, while the likelihood converged after six samples wereobtained. This indicates that the learned models suffered from over-fitting in theearly stage of learning, but appropriate models were finally obtained.

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Figure 6. Validation set likelihood of n-state HMMs.

Figure 7. Evolution of training set likelihood.

From Fig. 6, the likelihood of a six-state HMM can be seen to be maximum
when the number of samples is less than five; otherwise that of an 18-state HMM is
maximum. Therefore, the 18-state HMM was selected as a result of cross-validation
when TA-9 was used.
Figure 7 shows the evolution of the training set likelihood. In Fig. 7, the average
log-likelihood of intrinsic coordinate systems for the motion ‘place-on’ is plotted.
The likelihood of C3 and C4 was not obtained when TA-2, which contained two
samples, was used since the likelihood was too small due to lack of data. The average log-likelihood of each coordinate system was re-estimated by using L samples,so the likelihood of C1 was not identical with the data in Fig. 6.
Figure 7 shows that the likelihood decreases with an increase in the number oftraining samples. This can be explained by a general fact that estimated probabilistic
models suffer from over-fitting in the early phase of statistical learning. Therefore,
the training set likelihood of an n-state HMM given a small number of samples
was larger than the likelihood of the HMM given more samples. From Fig. 7, thelikelihood of C1 increased when the sample number was five. This indicates that

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the 18-state HMM, which fits the observations better than others, was trained witha sufficient number of samples after five samples (cf., Fig. 6) were obtained.
Figure 8 compares the log-likelihood of intrinsic coordinate systems under thecondition that TA-9 was used for training. Figure 8 shows that appropriate coordinate systems were selected for the motions. Table 2 summarizes the relationshipof motions and the corresponding types of their intrinsic coordinate systems. Table 2 reveals that C4 was not selected for any motion, indicating that the learning
trajectories of these motions in the camera coordinate system was inappropriate.
The results regarding the parameter estimation described in the Appendix are
shown in Fig. 9. Figure 9a shows the evolution of weight wl,rl for the C2 -type intrinsic coordinate system. In this case, three candidate landmarks were in the sample

Figure 8. Average log-likelihood of intrinsic coordinate systems: (a) ‘place-on’, (b) ‘jump-over’,(c) ‘move-closer’, (d) ‘move-away’, (e) ‘raise’, (f) ‘put-down’, (g) ‘rotate’.
Table 2.
Selected type of intrinsic coordinate system
Type

Motion labels

C1
C2
C3
C4

jump-over, place-on
move-away, move-closer
put-down, raise, rotate
NA

(a)

(b)

Figure 9. (a) Evolution of wl,rl against iteration. (b) Evolution of the average log-likelihood.

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and wl,rl converged after three iterations. wl,rl was confirmed to be converged after
less than three iterations for all training samples. Figure 9b reveals how the average
log-likelihood increased with the increase in the number of iterations. These two
results clearly indicate that the proposed method successfully estimated the correct
object as the landmark.
5. Experiment 2: Motion Generation
5.1. Experimental Setup
The objective of the motion generation experiments was to show how the generated
trajectories improved when the size of training set increased.
To avoid generating motions in any of trained object placement, another set was
obtained from Subject A. The test set RA-5 consisted of samples of seven kindsof motions and each motion has five trajectories. To investigate a generation error
decrease with the increase in training set size, trajectories were generated fromthe models obtained from training sets TA-2 to TA-9. A trajectory performed by
Subject A, Y , and generated trajectory, Ŷ , can be compared since both Y and Ŷ
were performed/generated given (rtraj , vi , r).
To evaluate generation error D(Y , Ŷ ), Euclidean distance was used since it is a
widely used criterion for time series data [19]. Let x(t) and x̂(t) denote the position
sequences of Y and Ŷ , respectively. The generation error D(Y , Ŷ ) was defined as:
1 |x(t) − x̂(t)|2 ,T +1T

D(Y , Ŷ ) =

(14)

t=0

where D(Y , Ŷ ) is normalized by the frame length T + 1 and x̂(t) is resampled sothat x̂(t) contained the same number of points with x(t). Note that D(Y , Ŷ ) evaluates similarity of two position sequences but does not explicitly evaluate similarity
in terms of velocity and acceleration. The same is equally true of most standard
techniques for measuring similarity between two time series such as simple dynamic time warping.
The Speech Signal Processing Toolkit was used for generating trajectories.
5.2. Results
The qualitative results regarding the relationship between the generated trajectories
and the training set size are shown in Fig. 10. The objects were placed as in Fig. 10,
and trajectories for placing Object 3 on Object 1 were generated. In Fig. 10, ‘Subject A’ represents the trajectory performed by Subject A under the same conditionand ‘TA-n’ represents trajectories generated from models trained with n samples.
Figure 10 shows that the last position of TA-2’s trajectory was not close to theposition where it was supposed to be; however, the last positions of TA-8’s andSubject A’s were almost the same. This is explained by the variances of the laststates of the trained models. Indeed, the variances of vertical positions in the models

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Figure 10. Example generated trajectories from models trained by two, four, six and eight samples.

Figure 11. Example generated trajectories for various combinations of relevant objects. The following
sets of [motion label, trajector ID, landmark ID] are used: (a) [jump-over, 3, 2], (b) [jump-over, 3, 1],(c) [move-closer, 4, 1], (d) [move-closer, 1, 3] and (e) [rotate, 1].

trained by TA-2, TA-4, TA-6 and TA-8 were 26.6, 23.5, 5.41 and 7.44, respectively.
This clearly supports the result shown in Fig. 10.
From Fig. 10, we can see that there were differences in the height dimension
between the test sample (Subject A’s trajectory) and the generated trajectories. Thiscan be explained by the fact that the subject’s trajectory was affected by Object
1; however, the generated trajectories were not. In other words, the subject performed object manipulation, avoiding object collision, while the proposed method
simply generated trajectories from probabilistic models representing appearancelevel object manipulation. Figure 11 also qualitatively illustrates that appropriate
trajectories were generated for various combination of relevant objects. For example, trajectories (a) and (b) were originally generated from the same HMM, but they
ended in opposite directions.

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K. Sugiura et al. / Advanced Robotics 25 (2011) 825–848

Figure 12. Evolution of generation error D(Y, Ŷ) plotted against the number of training samples.

Figure 12 shows the quantitative evaluation of the proposed method. In Fig. 12,
D(Y , Ŷ ) averaged by the number of trajectories is plotted against the number oftraining samples. Figure 12 reveals that D(Y , Ŷ ) for the motions ‘place-on’, ‘jumpover’, ‘move-closer’ and ‘move-away’ decreased with the increase in the number oftraining samples. The generation error converged after seven samples were used fortraining. This indicates that seven samples were sufficient for the proposed method
to learn motions performed by Subject A in the experiments.
Figure 12 illustrates that the ‘move-away’ trajectory has larger error than othertrajectories. This can be explained as follows. In the training and test set, each trajectory shown by the subject started from the trajector’s initial position and ended atvarious points, increasing the distance from the landmark. Therefore, the difference
between the generated and Subject A’s trajectory became large, particularly at the
final position. The large variances in the trained OPDFs of the ‘move-away’ HMM
also support this explanation. On the other hand, the error of ‘move-closer’ wassmallest among other motions when the sample number was nine. This indicatesthat the OPDFs of the ‘move-closer’ HMM had smaller variances.
Figure 13 shows sequential photographs of the manipulator executing trajectories generated by the proposed method. The grasping motions were not obtained
through learning, but implemented by the designer.
6. Experiment 3: Motion Recognition
6.1. Experimental Setup
The objective of the motion recognition experiments was to investigate the recognition accuracy of the proposed method. The probabilistic models trained by TA-9
were tested with RA-5 (see Section 5.1). In order to evaluate the recognition accuracy for other persons’ motions, we obtained two more test sets, RB-5 and RC-5,
from Subjects B and C in the same manner as RA-5.

K. Sugiura et al. / Advanced Robotics 25 (2011) 825–848

841

(a)

(b)

(c)

Figure 13. Sequential photographs of the manipulator executing manipulation trajectories. Grasping
behavior was implemented by the designer. (a) ‘Place-on’, (b) ‘move-closer’ and (c) ‘raise’.

Figure 14. Example of motion recognition results. (∗ 1) [move-away, 2]: −19.59. (2) [move-closer,1]: −30.59. (3) [jump-over, 1]: −31.62.

6.2. Results
First, a qualitative result is shown in Fig. 14. Figure 14 shows the trajectory thatstands for moving Object 3 away from Object 2. The top three recognition results

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K. Sugiura et al. / Advanced Robotics 25 (2011) 825–848
Table 3.
Number of correct recognition results and accuracy
Training set:
Test set:

TA-9
RA-5

TA-9
RB-5

TA-9
RC-5

place-onjump-over
move-closer
move-awayraise
put-down
rotate

5
5
5
5
5
4
5

5
5
4
5
5
3
4

5
4
3
5
5
3
5

Total

34 (97%)31 (89%)30 (86%)and their scores (log-likelihood) are shown in the legend. Each recognition result
stands for the motion label and the index of the landmark. The result with an asterisk
represents the correct recognition. Figure 14 shows that successful recognition wasachieved.
Next, the quantitative results are shown in Table 3. The recognition accuracies
of RA-5, RB-5 and RC-5 were 97, 89 and 86%, respectively, for an average recognition accuracy of 90%. The average number of objects contained in a test sample
was 3.11 and the average chance performance was 3.03%.
7. Discussion
Motion learning, if achieved, can be a considerable step towards language acquisition by robots. In this paper, motions and objects were specified in the form ofindices since the focus was on the evaluation of motion learning; however, motions
can be mapped with utterances if the proposed method is combined with the learning of phoneme sequences. In fact, the proposed method has been integrated in aprobabilistic framework with other modules for learning phoneme sequences, visual features, simple grammar and motion–object relationships [6]. Furthermore,
we are currently investigating the generation and recognition of sequential motions
by combining reference-point-dependent HMMs [20, 21].
There have been arguments on a number of key issues in imitation learning [1,4]. Among them, what to imitate and how to imitate are two fundamental questions.
The first issue is on determining what perceptual aspects are relevant to a task whenattempting to imitate a human [1]. The proposed method focused on the estimationof relevant objects in object-manipulation tasks based on the idea that referencesof motions performed by humans, were implicit. On the other hand, the correspondence problem [22], or the ‘how to imitate’ question, poses a problem when a user
is trying to transfer skills to robots with different bodies.

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The proposed method did not deal with the correspondence problem since thelearning was done in task space since the main focus is on the estimation of relevant
objects. Unlike most related work in the area of imitation learning, the joint space
information is not used in this study. Although using joint space information mightenable the method to learn various motions, our approach has a practical advantage
in that no motion capture system is needed. With this feature, the proposed method
can be introduced to service robots even if a motion capture system is unavailable
or the estimation of the user’s motion from visual information is unreliable.
For future work, it would be an interesting research topic to improve the proposed
method so that it can deal with the correspondence problem for learning more various motions such as ego-centric motions [4]. Some studies have attempted to solve
the correspondence problem by extracting important features from task and joint
spaces [10].
The basic idea of clustering trajectories by estimating reference points and intrinsic coordinate systems is not dependent on specific methods for modeling timeseries data, so other methods such as linear autoregressive models, recurrent neuralnetworks or probabilistic models could be used instead of HMMs. If such a method
were not based on a probabilistic framework, the likelihood-based criterion used inthis study could be replaced with other criteria, such as prediction errors.
8. Related Work
This section presents related work on imitation learning from a broader perspective,
so here the topic is not limited to object manipulation. Early studies on imitation
learning in robotics dealt with an assembly task and a kendama (ball-in-cup) task
[3, 23]. Recent studies and terminology on imitation learning were presented inRef. [1]. In Ref. [4], machine learning methods used in imitation learning were reviewed, and different approaches taken within the computer vision, robotics andartificial intelligence communities were analyzed. Reference [2] is an extensive
overview of neurocomputational approaches to motor learning by imitation.
Tani et al. took a connectionist approach to imitation learning [24]. They used
the recurrent neural network with parametric bias (RNNPB) to train the controller
of a 4-d.o.f. robot arm. Reference [25] presents an application of RNNPB to the
problem of handling many-to-many relationships between motion sequences andlinguistic sequences. Okada et al. also stressed the importance of self-organizing
characteristics for dynamical systems [26]. The recognition and generation of periodical motions are possible with their method.
Synthesizing human-like animation and programming humanoid robots to perform new tasks are attractive applications of methods based on the learning-fromobservation framework [1]. Inamura et al. used HMMs to model motion patterns
such as ‘swing’ and ‘walking’ [12]. The trained HMMs were mapped in a space
by applying multidimensional scaling to Kullback–Leibler divergences between
the parameters of the HMMs. Novel motions were generated by the interpolation

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K. Sugiura et al. / Advanced Robotics 25 (2011) 825–848

and extrapolation of the HMMs in the mapped space [27]. A hidden semi-Markov
model, which is an HMM with state duration probability distributions, was used forlearning walking patterns in Ref. [28]. In Ref. [29], a Gaussian process dynamical
model (GPDM) was proposed and applied to human motion capture data. A GPDM
comprises of a low-dimensional latent space with associated dynamics and a map
from the latent space to an observation space.
These studies focused on the learning motions in a fixed coordinate system. In
contrast, the proposed method in this study is able to estimate the optimal intrinsic
coordinate system to generalize motion trajectories.
9. Conclusions
This paper presented a method for learning object-manipulation motions from multiple demonstrations by humans. Conventional methods for learning object manipulation from demonstration had trouble dealing with motions that did and did notdepend on a landmark with the same learning framework. In contrast, one of thecontributions of the paper was to provide an algorithm for estimating reference
points to overcome such a limitation.
The proposed method was applied to the learning of object manipulation andevaluated its generation errors and recognition accuracy. While the results in thispaper are promising, the learned motions were limited to an appearance level. In
the future the application of machine learning to the problem of understanding theintent of an action will hopefully be explored, as that is necessary to develop a robotthat imitates the goal of an action.
Acknowledgements
This research was partially supported by the Ministry of Education, Science, Sports
and Culture, Grant-in-Aid for Scientific Research (C), 20500186, 2008, and the
National Institute of Informatics.
Note
An earlier version of this work was presented in the Proceedings of the 2007 Workshop on Multimodal Interfaces in Semantic Interaction [19].
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Appendix: Parameter Estimation of Reference-Point-Dependent HMMs
This appendix explains a parameter estimation method to solve (8). The superscript
k is omitted for the sake of simplicity since k is fixed in the following equations.
First, (8) is simplified before applying an EM algorithm. Since wl,rl = P (rl |w),
this results in:rl
wl,rl P Ck (x ) Yl ; λ = P (rl |w)P (Yl |rl , k, λ)= P (Yl , rl |k, λ, w)= P (Yl , rl |k, ),(A.1)
where:  (λ, w).

(A.2)

Applying (A.1) and (A.2), (8) can be rewritten as:
 |R |Llˆ = arg max
log
P (Yl , rl |k, )= arg max


l=1
L


rl =1

log P (Yl |k, ).

(A.3)

l=1

Here, an HMM is employed as the probabilistic model. Let π = {πi } denote theinitial probability vector, A = {aij } denote the state transition probability matrix,b = {bj (y)} denote the OPDFs and ql = {ql,t } denote the state transition vector of
the lth learning data. When an EM algorithm is applied by regarding rl and ql as
hidden parameters, the following auxiliary function Q(,  ) is derived:|Rl | L P rl , ql |Yl , k,  log P (Yl , rl , ql |k, ).
Q(,  ) =

l=1 rl =1 ql

(A.4)

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K. Sugiura et al. / Advanced Robotics 25 (2011) 825–848

Each term in the above equation is represented with HMM parameters as:

P (rl , ql |Yl , k,  ) =


wl,r
l

Pl



rl
P Ck (x ) Yl , ql |λ(A.5)

log P (Yl , rl , ql |k, )= log wl,rl + log πql,0 +Tl

log aql,t ql,t+1 +

t=1

Tl


log bql,t+1 zl (t),

(A.6)

t=1

r
where zl (t)  Ck (x l ) yl (t).

From (A.4)–(A.6), the following parameter re-estimation equations are derived,
which maximize Q(,  ) in terms of :w̄l,rl =wl,rl αl,rl (Tl , i)
Pl

L


|Rl |


N

(A.7)

i=1

π̄i =
āij =

L
l=1 1/Pl

1
L

1
Pl

(A.8)

wl,rl αl,rl (0, i)βl,rl (0, i)

l=1
rl =1
|Rl |
Tl
t=1 αl,rl (t, i)aij bj zl (t + 1)βl,rl (t + 1, j )rl =1 wl,rl
|Rl |
Tl
L
l=1 1/Plt=1 αl,rl (t, i)βl,rl (t, i)
rl =1 wl,rl

Pl  P (Yl |k, ),(A.9)(A.10)

where N , αl,rl (t, i) and βl,rl (t, i) denote the number of the states, and the forward
and backward parameters [30] for the HMM, respectively.
When bj is single Gaussian, the following re-estimation equations of the mean
vector μj and variance–covariance matrix  j are obtained:

¯j =


|Rl |
Tl
L
l=1
t=1 γl (t, j )zl (t)
rl =1
μ̄j =
|Rl |
Tl
L
l=1
t=1 γl (t, j )
rl =1
|Rl |
Tl
L

l=1
t=1 γl (t, j )[zl (t) − μj ][zl (t) − μj ]
rl =1
L
l=1

|Rl |
rl =1

Tl
t=1 γl (t, j )(A.11),(A.12)

where:
γl (t, j ) wl,rl
αl,rl (t, j )βl,rl (t, j ).
Pl

(A.13)

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K. Sugiura et al. / Advanced Robotics 25 (2011) 825–848

About the Authors
Komei Sugiura is an Expert Researcher at the Knowledge Creating Communication Research Center, National Institute of Information and Communications
Technology (NICT). He received his BE degree in Electrical and Electronic Engineering, and MS and PhD degrees in Informatics from Kyoto University, in 2002,
2004 and 2007, respectively. From 2006 to 2008, he was a Research Fellow, Japan
Society for the Promotion of Science, and he has been with NICT since 2008. His
research interests include robot language acquisition, spoken dialogue systems,
machine learning, sensor evolution and service robots.
Naoto Iwahashi received the BE degree in engineering from Keio University,
Yokohama, Japan, in 1985. He received the PhD degree in Engineering from
Tokyo Institute of Technology, in 2001. In 1985, he joined Sony Corp., Tokyo,
Japan. From 1990 to 1993, he was at ATR Interpreting Telephony Research Laboratories, Kyoto, Japan. From 1998 to 2003, he was with Sony Computer Science
Laboratories Inc., Tokyo, Japan. In 2003, he joined ATR Spoken Language Communication Research Laboratories. In 2006, he joined the National Institute ofInformation and Communications Technology. His research areas include interactive speech system, language acquisition and human–robot interaction.
Hideki Kashioka received his PhD in Information Science from Osaka University, in 1993. From 1993 to 2009, he worked for ATR Spoken Language Translation Research Laboratories. From 2006, he has worked for the National Instituteof Information and Communications Technology (NICT). He is currently the Research Manager of the Spoken Language Communication Group at the Knowledge
Creating Communication Research Center, NICT. He is also Visiting Associate
Professor of the Graduate School of Information Science at the Nara Institute of
Science and Technology, from 1999.
Satoshi Nakamura received his BS from Kyoto Institute of Technology, in 1981,
and PhD from Kyoto University, in 1992. He was an Associate Professor of the
Graduate School of Information Science at Nara Institute of Science and Technology, in 1994–2000. He was Director of an ATR Spoken Language Communication
Research Laboratories, in 2000–2008. He is an ATR Fellow. He launched theworld’s first network-based commercial speech-to-speech translation service for3G mobile phones, in 2007. He is currently the Director General of Keihanna
Research Laboratories and the Executive Director of the Knowledge Creating
Communication Research Center, National Institute of Information and Communications Technology,
Japan. He received the Yamashita Research Award, Kiyasu Award from the Information Processing
Society of Japan, Telecom System Award, AAMT Nagao Award, Docomo Mobile Science Award,
in 2007, Telecom System Award, ASJ Award for Distinguished Achievements in Acoustics, and theCommendation for Science and Technology by the Minister of Education, Culture, Sports, Scienceand Technology.