Explaining Robot Actions

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Explaining Robot Actions
Session: LBR Highlights
March 5–8, 2012, Boston, Massachusetts, USA
Explaining Robot Actions
Meghann Lomas, Robert Chevalier, E. Vincent Cross II, R. Christopher Garrett,
John Hoare, and Michael Kopack
Lockheed Martin Advanced Technology Laboratories
3 Executive Campus, Suite 600
Cherry Hill, NJ 08002
1 856.792.9681
{mlomas, rchevali, ecross, rgarrett, jhoare, mkopack}@atl.lmco.com
what action it took, what information it had about the world at
the time, and the logic behind its decision-making process.
Because humans and robots use very different concepts when
thinking about the world, robotic actions, information, and
processes are rarely directly understandable by humans. Robots
typically represent the world in coordinate-based terms (e.g., a
grid representation of occupied areas, or a representation of
detected color blobs in image coordinates); these representations
do not align with human views of the world, which tend to be
semantic representations of objects in space (e.g., the chair is
near the desk). Robotic planning is done using mathematical
formulas whose process and output are not readily expressed in
semantic terminology. Additionally, the actions produced by the
planner are influenced by many factors and based on continuous
mathematical models, and so are not easily discretized.
To increase human trust in robots, we have developed a system
that provides insight into robotic behaviors by enabling a robot
to answer questions people pose about its actions (e.g., Q: “Why
did you turn left there?” A: “I detected a person at the end of the
hallway.”). Our focus is on generation of this explanation in
human-understandable terms despite the mathematical, robotspecific representation and planning system used by the robot to
make its decisions and execute its actions. We present our work
to date on this topic, including system design and experiments,
and discuss areas for future work.
Categories and Subject Descriptors
H.1.2 [User/Machine Systems]
To address these challenges, we have developed the Explaining
Robot Actions (ERA) system, which includes a robotic world
model capable of representing semantic and physically-based
information, models of planning systems, and a query
mechanism for obtaining the desired information from the
robotic planning system to produce real-time, humanunderstandable answers to questions about a robot’s behavior.
For the purposes of this work, we focus on generating a
semantic answer from robot-specific concepts and assume the
use of a natural language system for parsing human questions.
General Terms
Design, Human Factors
Robotic Actions, Natural Communications, Trust, Human-Robot
Partnering, Explanations
As robots become more frequently used in human-inhabited
environments, it is increasingly important for people to maintain
an appropriate level of trust in robots. Research has shown that
no matter how capable an autonomous system is, if human
operators do not trust the system, they will not use it [1].
To improve collaboration and co-existence between humans and
robots, a considerable amount of work has gone into making
robots expressive (e.g., in [3], gestures related to the robot’s
goals make the robot appear more intelligent and approachable).
This supports increased engagement with people, but does not
necessarily help clarify why or how a robot selects actions.
Two key factors that contribute to human trust are predictability
and a mechanism for social exchange, both of which are
frequently reduced or lacking in robotic systems [2]. Without a
mechanism for communicating information and intent, a robot
may appear erratic and untrustworthy when, in fact, it is
following a clear decision-making process. As in human
partnerships, the ability to explain the information and logic
behind decisions greatly increases trust by establishing an
understanding of why a decision was made, and subsequently
provides insight into future decisions.
The idea of explaining behaviors is less developed, but has been
explored in artificial intelligence (AI) research and in robotics.
The area of explainable AI has addressed the challenge of
explaining the state of agents in a simulation but focuses on
after-action analysis to determine behavior models (e.g., [4]).
For robotics—and most directly related to our work— robotic
actions have been explained through visual timelines made up of
action trees that describe the robot’s recent actions [5]. Our
approach differs from this work in that we enable verbal
communication by enriching the robotic world representation
and developing a mechanism for querying the representation and
planning system for task-relevant information.
The focus of our work is on enabling a robot to explain its
actions in human-understandable concepts and terms, including
Copyright is held by the author/owners(s)
HRI ‘12, March 5-8, 2012, Boston, MA, USA.
ACM 978-1-4503-1063-5/12/03.
Session: LBR Highlights
March 5–8, 2012, Boston, Massachusetts, USA
teammates, this capability proved useful for developers
producing and debugging the system and simulator.)
The ERA system (Figure 1) is designed to take in an alreadyparsed human query about the robot’s behavior and output an
answer in sentence form. The ERA system first determines what
information is needed to answer the question. Based on the
“subtext” of the question (i.e., the fundamental question being
posed), ERA selects an answer template, which forms the
framework for the robot’s response. These templates are
designed to be modular and combinable (e.g., “I am in X mode
because Y” where Y is another template: “I detected object A at
position Z.”). To populate the template with the information
specific to the question, ERA queries the world model and
planning system, which have been integrated with and
established as resources for the ERA component.
We have presented an approach for enabling a robot to explain
its behavior in human-understandable concepts. Thus far, we
have designed and developed a prototype system and algorithms
that intelligently query semantic information from our layered
world model and construct answers to a set of questions. While
proof-of-concept experiments have shown the merits of this,
future work includes:
1. Supporting more specific answers. Thus far, the responses
from the ERA system have focused primarily on the robot’s
mode and world information. We have developed but not
implemented algorithms that examine changes in the grids used
to produce the cost map. These algorithms look for the most
influential cells on the robot’s route and determine what grid(s)
correspond to the values of these cells. By understanding how
those grids are constructed, we can determine what world
information is most strongly affecting the robot’s behavior.
2. Examining robot routes, not just modes or grids. We
believe that examining multiple possible routes for the robot
might provide a basis for explaining a route choice (e.g., “I took
the left road because the right road is longer.”). To do this, we
plan to generate alternate paths and compare them.
Figure 1: The ERA system determines what information is needed to answer
the question and queries the two-layer world model and planner.
Because of their ubiquity and versatility, we assume the robot
uses a cost map-based planner for low-level actions and a finite
state machine high-level planner that selects the robot’s mode
based on world information (integration with other planners is
left for future work). To represent the world, we use our twolayer world model, which incorporates a grid-based physical
layer for planning and a semantic layer for a richer object
representation and communication with people (see [6] for more
details). This representation provides a translation that enables a
semantic response understandable by people. Each resource
registers the type of information it can provide (e.g., robot mode,
object attributes) which allows ERA to select the information
needed to complete the chosen template. Once completed, the
sentence is output as an answer to the question.
3. Increasing the flexibility of the queries and responses.
Humans tend to think of actions discretely (e.g., “left turn”), but
the continuous nature of robotic motion means that these
discrete actions are not easily associated with timestamps in the
robot’s operation. By integrating with a natural language
understanding system, we plan to use context from the questions
to pair a semantically described action with robotic motion.
[1] Duez, P.P. and Zuliani, M.J. and Jamieson, G.A., “Trust by
design: Information requirements for appropriate trust in
automation.” In Proceedings of the 2006 Conference of the
Center for Adv. Studies on Collaborative Research. 2006.
[2] Lee, J.D. and See, K.A., “Trust in automation: Designing
for appropriate reliance.” Human Factors, Vol. 46, No. 1,
Spring 2004, pp. 50-80.
To test and demonstrate the ERA system, we established a
simulated search and rescue environment in Stage, and tasked a
robot with searching the environment and escorting people to
the nearest exit. “Operators” asked the robot questions about its
behavior using a GUI interface. Two proof-of-concept
experiments were performed. The first focused on retrieving
information from the world model. For example, when queried
“Tell me how you work,” the robot described the type of planner
it used, the grid types that combined to form the cost map, and
the weights on each grid. These fields were populated with data
registered by the world model and planning system. Similarly,
when asked “Tell me what you’ve sensed,” the robot responded
with the information in the semantic layer of the world model.
[3] Takayama, L., Dooley, D., & Ju, W., “Expressing thought:
Improving robot readability with animation principles.”
Proceedings of the 6th International Conference on
Human-Robot Interaction, pp. 69–76. Lausanne,
Switzerland. 2011.
[4] Core, M., Lane, H., Van Lent, M., Gomboc, D., Solomon,
S., and Rosenberg, M., “Building explainable artificial
intelligence systems.” In Proceedings of the National
Conference on Artificial Intelligence, volume 21. 2006.
[5] Brooks, D., Shultz, A., Desai, M., Kovac, P., and Yanco,
H.A., “Towards state summarization for autonomous
robots.” In Dialog with Robots: Papers from the AAAI Fall
Symposium (FS-10-05). 2010.
In the second experiment, questions were asked that required the
robot to intelligently query specific elements of the world model
and planning system. Possible questions ranged from specific
(e.g., “Why did you turn left going out of that room?”) to
general (e.g., “What are you doing?”). The robot used the ERA
system and world model to respond semantically, e.g., “I am in
‘searching for people’ mode, heading to my goal, which is room
20 at [5.34, 20.11].” (N.B., in addition to supporting human
[6] Lomas, M., Cross, E., Darvill, J., Garrett, R., Kopack, M.,
and Whitebread, K., “A robotic world model framework
designed to facilitate human-robot communication.”
Proceedings of the SIGdial 2011 Conference, Portland, OR.
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