1 Introduction
During the past four decades, developments within different fields of robotics have introduced purposefully designed robots into the dynamic environment of humans. Within the past two decades, the emergence of collaborative robots has brought industrial manipulators from fixed enclosed industrial environments into the open proxemics of humans. There is now a multitude of commercial domestic and social robots designed to interact with humans; and, likewise, autonomous collaborative robots, in line with the principles of Industry 4.0 and 5.0, have been applied to assist human workers in industry. As a natural consequence of the interest in introducing robots in applications where non-roboticists face them, the field of Human–Robot Interaction (HRI) has emerged. HRI is concerned with all facets of human–robot interaction, including safety aspects and the design of intuitive interfaces accommodating engagement of inexperienced people with technology as complex as robots.
For some time, the focus has been on designing feasible HRI for when a human initiates and dictates a robot’s actions or the robot reacts to certain input. However, within the past 15 years, the interest in pushing the boundaries and obtaining more intelligent and autonomous robotic behavior has introduced the subfield of Proactive Human–Robot Interaction (Proactive HRI), which has been increasingly researched. The field of Proactive HRI derives from the desire of having more intuitive and natural human–robot interactions inspired by how humans interact with each other. In Proactive HRI, current themes such as human-centered solutions and advancement of socially acceptable intelligent autonomous robots are in focus, making it an important field of the times.
2 Related Works
Proactivity has been mentioned in relation to HRI in a fair amount of research; still it seems there has been no clear definition of what can be considered Proactive HRI. When a new field arises, it is subject to the risk that (mis-)uses of the related terms can result in vague understandings of the field. This can be counteracted by clear definitions based on systematic and intentional studies of the field.
In the past, different reviews have been published in the fields of HRI and, more specifically, Proactive HRI. Goodrich and Schultz [
60] provided an insight into HRI in their survey, where they strive to discuss and promote a unified understanding and handling within the field. Furthermore, they discuss key themes and important challenges that shape this field. Other authors have published works focused on providing an overview of a specific area within HRI or HRI within a specific domain (e.g., metrics within HRI [
169], task planning [
179], social robots [
36],
physical HRI (pHRI) [
38], and more recently proactive robots in HRI [
166]). While Reference [
166] is concerned with Proactive HRI, it is mainly focused on the delimited topic of robots’ emotional intelligence gained from proactivity based on observing nonverbal human cues. Consequently, the article addresses selected aspects of Proactive HRI but does not address Proactive HRI in full as the wide spectrum field it is. Similarly, Bianco and Ognibene [
16] and Brinck and Balkenius [
22] provide a review of the use of Theory of Mind in Proactive HRI and mutually adaptive interaction, respectively. The work by Peng et al. [
140] that studies proactivity in relation to robots offering Decision Making Support presents a division of proactivity into high, medium, and low proactivity levels. However, the given distinctions are context-specific and not generalizable. These observations and the fact that a clear overview of proactivity within HRI is not yet available in the literature motivate this work with the purpose of creating a unified understanding of the nuances of Proactive HRI.
This work reviews previous research studying proactivity in relation to HRI and, based on this, provides a clear definition of what can be considered Proactive HRI. Furthermore, a taxonomy is presented, which can assist researchers to identify and clarify which branch of Proactive HRI they are engaging with, which in turn will result in a more rigorous structure of future work within this field. Last, the state-of-the-art methods described in the literature within each subcategory of the branches are reviewed. This can be summarized in the following contributions:
(1)
A clear definition of Proactive HRI
(2)
A taxonomy for Proactive HRI.
(3)
A review of the state-of-the-art within Proactive HRI with a focus on common tendencies and variation between works.
(4)
An overview of the distribution of the papers within the proposed taxonomy.
The remainder of this article is structured as follows: Section
3 describes the method and procedure used to obtain the relevant literature for this review. Section
4 analyzes the general understandings and structures that can be observed in the included literature, and in Section
5 a taxonomy based on these is provided. Subsequently, highlights in the identified subcategories within the branches of Proactive HRI with respect to focus are provided, and the state-of-the-art within the subcategories is outlined. Section
6 discusses some general cross-category observations, open research questions, as well as the proposed taxonomy. Finally, Section
8 summarizes and concludes this review.
3 The Corpus of Literature
The relevant literature for mapping out the field of Proactive HRI was identified through a structured approach planned and documented using a search protocol. Important considerations from the protocol will be detailed in this section. A block search was conducted on several databases utilizing search terms identified through an initial “Quick and Dirty” free text search on Google Scholar. Since the field to cover is wide, a two-aspect block search with the following terms was constructed: Aspect 1: Proactiv* (where the asterisk ensures inclusion of form of the word in the search), Aspect 2: Human-Robot Interaction, HRI, Human-Robot Collaboration, Human-Robot Cooperation. The searches are constructed in this way to obtain all combinations of proactivity with relevant broad terms related to interactions.
A walkthrough of all available databases, to our knowledge, and their coverage was conducted resulting in the identification of a total of eight relevant databases and one net-crawler (see Figure
1). The selected databases were chosen to include archives with an area-specific coverage, e.g., IEEE Xplore, ACM Digital Library, as well as an interdisciplinary coverage; Scopus, Web of Science, and so on. Block searches were constructed on each database and, when possible, predetermined inclusion and exclusion criteria were applied. The inclusion criteria were peer-reviewed articles, only English language, and any publication year was accepted. Technical Standards were excluded. When possible, the searches were limited to the metadata of the articles to obtain only the articles that had the subject as a central focus point. On ACM Digital Library and SpringerLink, the searches were conducted on the full texts. All the hits were examined in a first round of sorting where abstracts were read to determine whether an article was relevant. During this process, the inclusion criteria were that the article included an explanation or definition of what Proactive HRI is, if they claimed to implement Proactive HRI, or if the term “proactive” was used in relation to, in the context of, or describing aspects of HRI. Figure
1 illustrates the date of the searches as well as the amount of hits and the number of relevant entries on the different databases. All searches were carried out in Autumn 2020. A free text search on Google Scholar was conducted as a last check to ensure that the most relevant papers were considered. The first 10 pages of hits were examined, and relevant articles that had not yet been found via the databases were included. The identified relevant articles were combined, and duplicates were resolved in Mendeley. The total amount of different articles found in the block search was 200 articles published in the year span 2001–2020. All papers of relevance that complied with the inclusion criteria available at the time of the search were included to have as much data and as exhaustive a search for this topic as possible. A procedure inspired by the Grounded Theory method (open coding and axial coding) [
186] was used for the analysis of the found literature. The found articles have all been systematically reviewed according to a previously specified set of themes and elements: motivation/inspiration, application context, key methods, contributions, metrics, and so on. This was done through an iterative process where codes for some of the elements were formed and updated throughout the process.
To gain a more elaborate overview of the development of the general field of Proactive HRI, an analysis of the identified articles has been conducted. Figure
2 illustrates the articles plotted according to their publication year in a bar graph. It can be concluded that the field of Proactive HRI indeed is a rather young, however, growing field with most of the articles published after 2010 and with an overall increase in the amount of published works per year.
4 Defining “Proactivity” in HRI
Through inspection of the corpus of literature, it becomes clear that there are large variations in how clearly different works define what they mean by “proactive.” Furthermore, it is evident that there are variations in the use of the term and what authors consider Proactive HRI. Through examination of the explanations provided in some of the works that attempt to clarify what they understand by “proactive” or “proactivity” in the context of the authors’ research, a valuable insight into the variations was attained.
An example of a work that includes a clear definition is the work by Luria et al. [
109]. Although the work is on the boundaries of the scope of this review with respect to HRI, the article provides a clear definition of “Proactivity” in the context of their work as “
initiation of interaction with a user.” Schmid et al. [
153] is less direct in their wording but ascribes the term “proactive” to behavior where initiative is taken to carry out an action that triggers clarifying behavior by the opposite interaction partner. A similar yet less specific meaning of the term is described by Fink et al. [
49], who describe a system that initiates interaction as proactive. Cámara et al. [
26] describe proactiveness decoupled from interaction but as an agent’s ability to “
exhibit goal-directed behavior by taking initiative,” thus, also ascribing to the connection between proactivity and initiative-taking. Jin et al. [
79] provide a slightly different angle by stating that actions can be defined as proactive if they have the ability of creating or controlling a situation. Here, the focus is not only on the robot as the initiator, but also on the purposefulness of the robot actions. Pandey et al. [
136], oppositely, do not focus on the ability to control a situation but rather the ability to contribute and support as proactivity when they define proactive behavior as “
taking the initiative whenever necessary to support the ongoing interaction/task.” In the work by Sirithunge et al. [
166], two specifications are provided. First, they set robot-initiated and proactive HRI alike and describe both as interaction where the robot is responsible for initiating the interaction. Second, proactive robots are defined as “
Robots which identify the requirement of a certain situation and acts instantly without any instructions from outside.” The latter definition points towards an understanding that proactive robots have anticipatory capabilities. This notion is also expressed in the definitions and use of the term “proactive” in other works. An example hereof is a reference made by Peng et al. [
140] to proactivity in occupational psychology from which they derive the following definition of proactivity for service robots: “
The anticipatory action that robots initiate to impact themselves and/or others.” Chinchali et al. [
33] likewise hint at the anticipatory understanding of “proactivity” when they describe how robots both need to proactively decode human intentions and, subsequently, utilize this knowledge for collaborative task satisfaction to do proactive decision-making. Likewise, Mok [
121] clearly connects anticipation to proactivity when he states: “
We defined a reactive robot to be one that responded and executed actions according to the user’s gestures, while a proactive robot anticipated a user’s impending needs and initiated an action to complete a task.” Another definition encountered states that “
proactivity is acting in advance of a future situation, rather than just reacting” [
90].
As can be seen from the presented selection of definitions and uses encountered in articles, there are some agreements on the meaning of “proactive” among the works. Several works express proactivity as taking initiative or controlling a situation rather than being reactive, while another group of works seems to lay out a clear relation between proactivity, anticipation, and acting based on estimations and assumptions. These differences in use of the term might partially stem from a discrepancy between the British and the American dictionaries regarding the word “proactive.” For instance, the American Merriam-Webster describes the meaning of proactive as “
acting in anticipation of future problems, needs or changes” [
118], whereas the British Cambridge Dictionary gives the definition “
taking action by causing change and not only reacting to change when it happens” [
27]. Though there are some similar elements to the two definitions, there are also differences that, again, also are identified in the use of the term in the corpus of literature. While some works can be clearly associated to one understanding or the other, there are also a wide range of works that are seemingly vague in their use of the term or portray recognition and acceptance of aspects of both initiative and anticipation. An example is the mentioned work by Peng et al. [
140], who describe how their definition of robot proactivity indicates three key elements, namely, anticipation, initiation of action, and target of impact. They go on to note how initiation of action is about robot autonomy, thus, hinting at robot control. Tan et al. [
173] discuss proactive behavior in relation to autonomy and specify that the difference of proactivity from mere autonomy is the anticipation of human intention. In these cases, one could argue that both anticipation and initiative play a role. Consequently, it can be noted that although some works can be ascribed to one understanding or the other, proactivity in HRI seems to be a continuum ranging from pure initiative to anticipatory skills. Based on this observation, it is difficult to give a single clear-cut definition of Proactive HRI. Instead, Proactive HRI can be described as
HRI where the robot exhibits some level of either anticipatory or situation controlling capabilities. The two understandings contained in the provided definition of Proactive HRI can be used as a basis for classifying works. Nonetheless, the works can be further categorized based on other observed factors such as research or functional focus. These findings serve as the basis for the taxonomy proposed in Section
5.
5 Taxonomy of Proactive HRI
A taxonomy is proposed to emphasize the nuances in Proactive HRI and provide a tool for specifying the focus of future works within this field. Our taxonomy is formed from observations made based on the identified corpus of literature and the use of the term “proactive” with respect to the work conducted within HRI. Please refer to Figure
3 for a selection of representative examples of the systems presented in the corpus. For readability, the taxonomy will be outlined herein, while relevant literature within each category will be provided in their respective subsections. Please see Figure
4 for an overview of the proposed taxonomy.
A basic observed distinction present in the corpus is the question of which agent is to exhibit proactivity. While most of the works focus on endowing a robot with proactive abilities, a minority of works focus on human proactivity and facilitating such behavior through HRI or other means during HRI. For either agent, human or robot, the proactivity can be distinguished based on the two main understandings of proactivity mentioned in Section
4. In relation to robots, this results in the two main branches
Anticipatory Robot Behavior and
Robot Initiative. Furthermore, a small selection of works that do not conform well with this classification introduce a
Miscellaneous category, which will be further elaborated in Section
5.3.
The two branches are subcategorized based on functional focus of the technologies or investigations presented in the analyzed literature. With respect to
Anticipatory Robot Behavior, this gives rise to several subcategories of commonly researched topics that overall relate to a pipeline of identifying intentions of the human or other inferred factors and, subsequently, initiate the adequate robotic actions. As indicated by the two large color blocks within this branch in Figure
4, these topics in general aim to endow robots with anticipatory abilities to determine and plan either necessary robot motion or robot actions or tasks on a more abstract level. Some of the subcategories presented within this branch have relevance for the fulfillment of both objectives while others are specific for one of them. Additional details are given in Section
5.1.
The branch of
Robot Initiative contains the three subcategories
Robot-Initiated Interaction,
Robot-Initiated Actions, and
Robot-Controlled Influence. While these three categories might not always be mutually exclusive in presented systems, there are some clear differences in the functional purposes presented in these categories. These are further explained in Section
5.2.
The branches and subcategories introduced in the taxonomy are substantiated through examples in the literature. The main themes of each branch and subcategory identified in the literature are elaborated and exemplified through a select few relevant articles related in the dedicated sections below. In this fashion, the state-of-the-art within each research topic is outlined.
5.1 Anticipatory Robot Behavior
As the most ramified of the branches in the taxonomy, the works included in
Anticipatory Robot Behavior focus on a range of research topics related to endowing robots with anticipatory capabilities. Though the foci vary, the close correlation of the topics often cause several of them to be addressed in individual publications. Figure
5 provides an overview of all the articles that relate to each of the research topics represented in the subcategories.
5.1.1 Anticipatory Inference.
As anticipation is about acting in advance of a future state or situation, and, in the context of HRI, without explicit input from a human partner; estimating or predicting relevant, possibly “hidden,” factors or information is of central importance. This category covers a range of methods all centered around inferring or estimating such unknown data, which serves as the basis for anticipatory proactive robot behavior. The desired information varies in nature. For anticipation in relation to motions, the information estimated might be humans’ intended trajectories, space occupancy, risk, perturbations, and so on. In the context of actions and tasks, examples of the desired information include human intentions, emotions, wants, needs, goals, human skill levels, human actions, and subsequent robot actions directly. Similarly, many and variate terms are used to describe the disciplines covered in this category dependent on the aim: motion prediction, intention inference, human intention recognition, risk estimation, and so on. The earliest work in the corpus addressing this challenge is by Breazeal et al. [
20]. Their approach builds on Simulation Theory and the idea that we as humans use our own responses to simulate others’ minds to infer their mental states. Utilizing this idea, Reference [
20] presents a robot that infers human intentions based on a mapping between observed human movements and a library of known robotic movements and their corresponding end goals, assuming the found goal mirrors the human intent. A limitation of this method is the fixed finite number of interpretable goals by the robot.
A different approach is to represent each human intention by their corresponding action sequences in the form of
Finite State Machines (FSMs). By comparing an observed action sequence to the actions sequence of the FSMs of a known intention, a probabilistic weight is obtained that indicates how closely the intention represented by the FSM relates to the actual current intention [
9].
The works by Breazeal et al. [
20] and Awais and Henrich [
9] are concerned with deducing some higher-level abstract premises, in this case intentions deduced from measurements or cues, that are of importance to the subsequent robot action selection. However, other works take on a more direct strategy where robot actions are inferred by, e.g., a known behavioral pattern or a sequence of steps that must be carried out to complete a collaborative task [
90].
5.1.2 Contextual Awareness.
Contextual Awareness is typically addressed in relation to implementation of anticipatory inference. It can be including context to predict human motion behavior or to support the deduction of human intentions, and so on. While works focused on prediction or inference of “hidden” or future factors do so based on different kinds of data, some works explicitly include additional information that is not considered central to the factor of interest. These are the cases that portray a level of contextual awareness. Liu et al. [
106] proposed a method for intent recognition by domestic assistive robots that combines object affordances and context features related to intentions for the different objects. The objects cup and window are considered where the related context features can be hot weather, low drinking frequency, medicine taking time, and nice weather outside, poor weather outside, quiet outside, noisy outside, respectively. The intentions are recognized using a Bayesian network-based intention recognition model that considers the combination of this information and observations of contextual features. In a later work, a Fuzzy-Naïve Bayesian Network algorithm has been adopted for the same purpose [
105].
5.1.3 Ambiguity Resolution.
Some works that focus on anticipatory inference also explore how a robot handles, e.g., ambiguous intentions. This includes how the robot is to act under such circumstances, but also policies for how the ambiguity can be resolved, and a single intention can be confirmed as the dominant one. Though this is a valuable functionality for Proactive HRI, only a few works have attended to this challenge. The select group of authors behind the majority of the works propose different approaches dependent on the amount of likely human intentions identified. The methods proposed utilize what the authors call “
proactive execution,” where an action is executed under uncertainty in case only a few human intentions are likely. Besides being the “best guess,” the purpose of the proactive execution is to gain more information and determine the “true” human intention [
153,
154,
156,
189].
5.1.4 Task Selection and Planning.
Task Selection and Planning is closely related to the general
Anticipatory Inference and is in some cases inseparable from that category when it comes to task selection. Nonetheless, there are examples of works that present a clear two-stage approach where, e.g., estimates of intentions are obtained in the first stage and then the appropriate robot actions are selected in a second stage based on the estimated intentions. Furthermore, while the category of
Anticipatory Inference mainly focuses on finding the basis for determining
which action should be carried out, a range of works consider methods for determining
when the adequate action should be carried out. This is a question of action or task planning, rather than mere task selection. The question of task planning or timing of the execution of robotic tasks or actions has been mentioned in 8 of the 33 articles in this category. Of the 8 articles, 5 of them are consecutive articles by the same authors, who first propose the use of a probabilistic temporal prediction method based on causal probability and eventually advance a hybrid temporal Bayesian network to a composite node temporal Bayesian network for the prediction of the best robotic actions as well as the best timing of those actions [
90–
94].
Some of the above-mentioned works include the ability to determine which robot action is adequate. This is also explored in the work by Schmid et al. [
153], where actions are selected based on the most probable intentions in an intention estimate and are subsequently filtered using a proposed method inspired by Lorentz’s “psycho-hydraulic model” [
107]. Another example is the work by Pinheiro and Bicho [
143], where a control architecture consisting of coupled Dynamic Neural Fields is proposed for determining goals and executing the appropriate action in an assistive robot. Here, specifically one of the layers in the architecture, the Action Execution Layer, is responsible for selecting the most appropriate behavior. In a more recent work, Wojtak et al. [
190], likewise, propose a method based on DNFs that offers temporal integration of inputs to learn sequences and, thus, enable proactive action selection.
5.1.5 Motion Planning.
The Motion Planning category builds on a selection of papers that use proactive anticipatory skills to improve the planning of robotic motion in relation to HRI. This can be motion planning in 3D space, e.g., for a robot manipulator or the 2D motion planning for a mobile robot navigating an environment.
An example of a 2D application is the use of anticipated human trajectories to determine the appropriate approach for social robots [
81,
177,
178].
One of the themes handled in anticipatory motion planning in 3D are handovers during HRC. Nemlekar et al. [
129] propose a method for anticipating the
Object Transfer Point (OTP) for handovers from humans to robots. The method calculates an offline OTP based on studied human preferences, and a dynamic OTP and combines them to achieve better accuracy. Another interesting theme is collaborative object transportation, for which presented methods anticipate human intended motions and plan robotic motions accordingly [
25,
161]. Similarly, anticipatory motion planning is investigated as a solution in pHRI, where the anticipation of human intentions is used to predict trajectories to improve load sharing. This is relevant in general HRC [
85,
112,
131] but also in relation to exoskeletons [
75,
76], where it can facilitate compliance of the wearable robotics with a reduced interaction force.
5.1.6 Collision Avoidance.
This category is closely related to the research topic of motion planning and might be considered a special case of that category. However, since the topic of collision avoidance is not the only aspect considered in motion planning, it is presented separately. This category also contains examples of both 2D and 3D applications.
The examples of collision avoidance in 2D are found in relation to mobile robots and autonomous vehicles. Leung et al. [
100] propose a traffic-weaving method for autonomous vehicles, which includes a prediction model of human behavior to find collision-free trajectories. The works related to mobile robots are generally concerned with avoiding collision while navigating human-dominated environments [
84,
177,
178]. The works aim at proactively avoiding collisions in these environments by considering human motion as well as social constraints.
The works presenting methods related to collision avoidance in 3D either utilize anticipated motion [
68,
134] or some representation of risk [
151,
152,
172]. The methods are anticipatory of a general nature but can be considered Proactive HRI due to the application in relation to interaction with humans. The aim of the methods is to prevent collisions during close proximity HRI such as during HRC.
5.2 Robot Initiative
The branch of
Robot Initiative is based on the selection of works that are focused on the ability of the robot to take control and initiative to establish interaction, initiate actions during interaction, or create or alter situations proactively according to monitored factors or internal goals. Figure
6 provides an overview of the articles related to each of the subcategories within this branch.
5.2.1 Robot-Initiated Interaction.
A group of works are focused on how a robot can initiate interaction. More specifically, the focus is on the robot actively carrying out interventions to seek out or request interaction with a human. This topic is typically researched in relation to social or service robots, as these can be situated in dynamic environments where the humans might not initiate interaction themselves.
Applications presented in the works within this category vary in respect to who is the beneficiary. In some works, the purpose of initiating interaction is to provide the human with a service, while other works investigate how to establish interaction with the purpose of the human assisting the robot. In the cases where a human is the beneficiary, the robot might initiate interaction to provide additional information about exhibitions in a museum [
77,
144–
146] or offer decision-making support, e.g., in relation to a potential purchase [
104,
140]. Rosenthal et al. [
149] present a work where a robot navigating an office space is the beneficiary. The work presents a model for seeking out occupants in the environment to request assistance for the completion of the robot’s task.
The works in this category cover themes such as how a mobile robot should approach potential users, how different levels of proactivity are perceived, how a robot should initiate interaction, and so on.
5.2.2 Robot-Initiated Actions.
Works in this category focus on the initiation of robotic actions. Whereas
Robot-Initiated Interaction is concerned with robots establishing interaction where there previously was none, this category of works is more focused on how and when robots should initiate actions in the context of ongoing HRI. Naturally, this category is closely linked to the research topics in the branch of
Anticipatory Robot Behavior, as robot-initiated actions are realized by means of the methods presented in those topics. However, this group of works is focused on the robot initiative and the effects of this behavior rather than how this behavior is achieved. Examples of works in this category include the work by Baraglia et al. [
13], who proposed and compared three initiative models for a robot collaborating with a human partner on a tabletop manipulation task. Tan et al. [
173] present their work with a slightly different aim, namely, to investigate the relationship between proactively initiated actions by a social robot and the perceived anthropomorphism of the robot. In their work, they carried out Wizard-of-Oz experiments where subjects interacted with the robot with five different levels of robot proactivity to determine the effects of the proactivity level.
5.2.3 Robot-Controlled Influence.
This category covers examples of robots enforcing their own “intentions” during HRI with the purpose of actively influencing the situation or circumstances of the interaction in a certain way. This can be to improve certain aspects of the interaction or to expedite the robot’s “internal goals” or “interests.” The work by Gray and Breazeal [
63] is an example of what could be perceived as a robot taking initiative to facilitate achievement of its autonomous internal goals during HRI. In this work, the correlation between action perceived visually by humans and the change in mental states is learned. This information is then utilized by the robot to manipulate the human mental state for its own advantage. A different example is presented by Lazar et al. [
95] in their work related to autonomous vehicles, where they investigate how robot-controlled influence through planned interactions with human-driven cars can be used, purposefully, to improve road configurations. This could be done with the goal of facilitating positive influence on social objectives, e.g., fuel consumption.
5.3 Miscellaneous
Some works in the corpus simply have a too vague usage of the term proactive in relation to their work to give merit to a placement in one of the suggested categories [
8,
32,
62,
126,
184], the notion they convey seems to be not proactive at all [
47], or the relation between the proactive behavior and HRI is questionable [
142,
168]. An example of the latter is the work by Cámara et al. [
26] that addresses robot proactivity as a characteristic of self-aware computing systems but not specifically in the context of HRI. Such works have been grouped in
Miscellaneous. Some of these works could, arguably, have been excluded from the corpus altogether but have been included due to the criteria set for the structured sorting process.
5.4 Human Proactivity
This category consists of a group of works concerned with handling, inducing, or facilitating proactive behavior by humans through HRI. With a different focus from most works (endow robots with human-like proactivity; see Section
6.2), this category can be viewed as an outlier to the general tendencies in research within Proactive HRI.
Examples of works that fall into this category are works concerned with people struggling with social shortcomings, such as children with
Autism Spectrum Disorders (ASD). The work by Feil-Seifer and Matarić [
46] focuses on development of systems for interventions for children with ASD to improve on social deficiencies. More specifically, addressing reduced self-initiation using robots is suggested, and the effect of a robot on the proactive social interaction is observed. François et al. [
52] conducted similar work investigating the effects of robot-assisted play for children with ASD with the purpose of encouraging proactivity and initiative-taking. Robins et al. [
148] hypothesized that repetitive exposure of children with ASD to a small interactive humanoid robot will increase basic social interaction skills. They observed a more social response to a human dressed and acting as a robot compared to a human and, subsequently, discovered that repetitive exposure to a small humanoid robot facilitated the emergence of spontaneous and proactive interactions.
Exemplifying differently are works that strive to encourage humans to be proactive against potential dangers during HRI or HRC. In Reference [
115], two classes of techniques for ensuring safe HRC are described, one of which is focused on cultivating proactive human behavior by raising awareness through cognitive aids. In the article, these techniques are investigated in a
Virtual Reality (VR) environment where the cognitive aids during HRC are implemented as audio and visual cues and continuous adaption of these based on environmental conditions.
Yet other works deal with how some proactive human behaviors can be a beneficial element in HRI [
158,
159] or how unfavorable human proactivity can be handled [
71]. Figure
7 provides a condensed overview of the articles in this category.
6 Discussion
Reflections related to the proposed taxonomy, as well as a selection of noteworthy observations made based on general tendencies in the analyzed literature, are next discussed.
6.1 Discussion of Literature Search and Taxonomy
During the literature search process, the focus in the block search on open terms directly describing interaction scenarios has consequences for the outcome. A test search conducted to examine these consequences including more field-specific terms has indicated that only the inclusion of “autonomous vehicles” gave a higher yield. In a future taxonomy, this term could be included. A natural consequence of the search process is also that only works that relate proactivity to their research topics are obtained, and more literature exists on, e.g., intent recognition, which otherwise is closely related to proactive behavior, than found in this study. Similarly, sorting the hits based on metadata only means that works are excluded if they only address proactivity in the full text.
The field of Proactive HRI is a field interwoven with elements from several disciplines and where topics interconnect. The process of defining a taxonomy in relation to Proactive HRI is not straightforward and has a series of related challenges. As is indicated in Section
4, giving a single unanimous definition of proactivity in relation to HRI is even a challenge because of the continuum nature of the meaning and use of the term. It should be noted that this also affects the proposed taxonomy. While it would be preferable to be able to assign a presented method resolutely to a single category, the reality is that works often engage with research topics from several of the categories of the taxonomy simultaneously. Even more so, some works clearly present topics that contain elements from both understandings of proactivity, pointing at the continuous and interconnected nature of the spectrum, which can hardly be portrayed to its full extent in a categorical taxonomy.
The work by Rashed et al. [
145,
146] is an example hereof. While the main focus is to endow a museum guide robot with the ability to proactively approach visitors and initiate interaction by providing extra information about an exhibition piece, the work also depends on anticipatory aspects of estimating the directed attention of the visitor as well as the interest in additional information about the piece.
Although works might not fit solely into a single subcategory in the taxonomy, it is the strong belief of the authors that the taxonomy can serve as a valuable tool for clarifying the intent and the focus points of a specific work.
Another, challenge when forming a taxonomy based on the corpus of literature is the differences in the aim of the works. While some works focus on the technical methods and challenges in relation to endowing a robot with the desired proactive abilities, other works are more concerned with evaluating the effects of the proactive robot behavior. This means that a category such as Anticipatory Robot Behavior contains works concerned with enabling the prediction of, e.g., needs and corresponding supportive robot actions as well as works investigating whether anticipatory behavior is desired by human partners. These investigations are, for example, carried out using Wizard-of-Oz experiments simulating the proactive robot behavior.
In relation to the definition provided in Section
4, it is important to note that even with an agreed characterization of the term, determining whether a specific work can be considered Proactive HRI can be a matter of interpretation. One of the commonly presented notions of proactive behavior is that it stands in contrast to reactive behavior. Robots taking initiative might clearly comply with this notion; however, when it comes to anticipatory robot behavior, the lines become more blurred. In practice, anticipatory behavior might be achieved by acting according to predictions, or it might be achieved by action upon early cues or precursors of certain events. With respect to predictions, it may be clear that this practice is in fact not simply reactive; but for action upon cues, one might argue that this is simply “early reaction” or an adjustment of what is the triggering observation.
Another discussion point is whether robotic actions carried out on own initiative with no anticipation or adjustment to the situation in fact can be considered proactive or simply a case of autonomous behavior. The work by Huang et al. [
73] serves as an example of such a discussable case. In their work, they propose a “proactive coordination” method for human–robot handovers. The description provided for this coordination method explicitly states that the robot does not consider the user’s task demand and simply proactively fetches a new item as soon as it has an empty gripper. In this context, the word “proactively” seemingly could be a synonym for either “autonomously” or “in advance” and seems to describe a behavior that is automatic rather than “reflective” or intelligent in the way most proactive behaviors appear to be.
6.2 Motivation behind Proactive HRI
As is custom in general, many of the included articles start off by motivating their work. While some are focused on the background for their novel technical ideas, a selection of articles argues for the general need for robot proactivity. By examining these arguments, an insight into the line of thought and assumed benefits motivating the introduction of proactive abilities to the robot in HRI can be gained. These motivations will be addressed in relation to the two main understandings/branches identified in the taxonomy.
6.2.1 Source of Inspiration.
Works addressing topics related to anticipatory robotic behavior often argue for this behavior based on observations from human-human interaction. Awais and Henrich [
9] point out the importance of proactivity for effective cooperation, stating how humans collaborating on a task are required to be proactive toward each other to secure intuitive collaboration. They continue, “
It is equally important in the human-robot interaction that the robot should be proactive according to the collaborating human depending on the current situation.” Similarly, Kwon and Suh [
94] mention how interacting humans predict each other’s intentions and synchronize their reactions accordingly with the benefit of ensuring seamless interactions through proactive responses. It is further argued that delays in human–robot interactions caused by frequently occurring rigid request-and-react patterns can be reduced and more seamless interaction be facilitated by robots’ abilities to predict future events based on observed human activities.
Similarly, articles engaged with robot initiative often point to human behavior for the inspirational source of the behavior they strive to establish in robots. In Reference [
2] the desired proactive behavior of initiating actions for humans without prior interaction is compared to the spontaneous behavior people exhibit.
Some works design proactive robotic behavior in HRI based on inspiration gathered by studying human-human interaction directly [
73,
199].
6.2.2 Argued Advantages.
The main advantages argued for in relation to Proactive HRI are intuitiveness and understandable or natural interaction seen from the human partners’ point of view [
136,
153]. Another important point is the argumentation related to efficiency. This is expressed in different ways in the literature; in some cases mentioned as fluency and in other cases, more concretely, as the minimization of delays or idle time for either partner [
110,
199]. Finally, some works argue for more specific advantages such as, e.g., the ability to efficiently execute tasks while avoiding collision [
68] or the benefit for the human in being freed from supervising and commanding a proactive robot [
3].
6.3 Common Applications in Proactive HRI
As a large part of the motivational arguments provided in Section
6.2 suggest that a desirable benefit of robot proactivity in HRI is the more human-like interaction, it is expectable that robot proactivity is applied in contexts with many human–robot interactions and in contexts where the robot operates in human-centered environments. This is confirmed by the contexts and applications observed within the Proactive HRI corpus. Figure
8 reports findings related to applications and interaction characteristics. The classification draws inspiration from Reference [
41]. A large amount of the applications are social robots, and more specifically service, assistive and domestic robots navigating in human centered environments. An example hereof is the assistive robot developed in the RAMCIP project, which is focused on assisting elderly in daily living activities with increasing levels according to the elderly’s level of impairment [
48]. The idea is that the robot proactively adjusts the level of assistance offered in accordance with the need based on the estimated skill level of the elderly. Another example is the museum guide robot proposed by Iio et al. [
77], which proactively offers information about the displayed exhibitions based on the human’s estimated focus of interest. Truong and Ngo [
177] present a work focused on a service robot situated in a crowded human-ruled environment, where the challenge is to navigate safely and sociably. For this purpose, they propose a proactive social motion model used for proactively adjusting the robot’s navigation according to social cues and anticipated trajectories.
Another common field in which Proactive HRI is applied and investigated is HRC, where a large number of interactions and a still rising need for flexibility of the robot calls for more analytical, adaptive capabilities in the collaborative robots. Robots can collaborate with humans in different ways under different conditions. Tabletop HRC is a condition often explored in articles. Schulz et al. [
157], e.g., investigates the human experience of different robot behaviors, proactive among others, during a collaborative tabletop building task.
These two application areas are the dominating ones within the corpus. Nonetheless, other areas such as the aforementioned “autonomous vehicles” [
71,
95], “wearable robotics” [
19,
75,
76], “robotic objects” (“RObjects”) [
49,
121,
123], etc. are represented by a few examples.
6.4 Measurements and Evaluation in Proactive HRI
As mentioned earlier, Proactive HRI is a field that is characterized by the intersection of multiple disciplines. In the corpus of literature, this is reflected by the wide variety of focuses and aims of the works. Some works propose general concepts or frameworks, other works are focused on technical development and in some cases evaluation of the implemented systems, and yet again another group of works investigates the psychological effects of different robotic behaviors. One difficulty that arises from this variety is that it is hard to compare works and results across the field. Dependent on the aim of the work and which scientific discipline the work takes its onset in, different methods with each their own measures are used to obtain and evaluate the findings.
Works addressing prediction or recognition of factors usually evaluate their performance with traditional machine learning and deep learning measures such as confusion matrices [
45,
128], recall and precision, or measures based hereon [
30,
119,
199].
For evaluating the effect and efficiency or performance of proposed systems yet another set of metrics are used. Here, measures such as completion time, idle times and number of actions carried out by the robot and human, respectively, during a collaborative task are considered [
13,
44,
196].
Other works use metrics or elements for their evaluation that are very specific to their works. An example is the interaction diagrams used by Moulin-Frier et al. [
125] as a visualization of the interactions between test subjects and their proactive robot to portray the processes, their time of occurrence, and their duration. The road capacity measure mentioned in Reference [
95] is, similarly, a specific measure used to evaluate how successful the proposed strategy for proactively influencing humans to optimize conditions.
One could argue that this is a necessary evil of the field of Proactive HRI due to its wide spectrum and that different focal points naturally call for different evaluation methods and measures. However, even works with the same disciplinary focus sometimes prove hard to compare due to individualized and handcrafted measures. An example of this issue is the different studies investigating attitudes towards robot proactivity that use Likert scales to evaluate different measures. Some studies utilize existing questionnaires or scales directly or in modified versions, while others entirely compose their own set of scales. Mok et al. [
123] evaluated subjects’ perceptions of interaction with a system, respectively, with or without proactive and expressiveness by using an adapted version of Dillard’s Relational Message Scale that consists of a set of 7-point Liket scales addressing different relational factors: immediacy, affect, similarity, dominance, among others. In contrast, Zhang et al. [
196] and Schulz et al. [
157] both evaluated the human experience of interaction using self-compiled, non-standard 7-point Likert-scale questionnaires.
6.5 Human Perception of Robot Proactivity
With a widespread advocation for the desirability of robot proactivity with the purpose of improving HRI, it is relevant to investigate how robot proactivity influences the interaction and the users and examine how robot proactivity is perceived by humans. In the literature, investigative works are found that address these topics. As expected, observations with respect to these topics may be closely connected to the kind of robot proactivity and the context of application of the robot and vary accordingly.
The improved efficiency that is used as an argument for implementing proactive behaviors has in fact been confirmed by several works. Anticipatory abilities have been proven to reduce waiting time in a collaborative assembly task [
110], reduce reaction time in human–robot handovers [
129], and reduce completion time [
196].
Baraglia et al. [
13] found in their study that human collaboration with a proactive robot resulted in high ratings of the system and better team fluency compared to collaboration with a reactive robot. Zhang et al. [
196] have investigated whether proactive supportive actions based on inferred human goals and intentions carried out by a robot during a collaborative urban search and rescue task are desirable. They found that subjects preferred the robot with the ability to provide proactive support compared to one without this ability. However, their results also suggested that the proactive support led to a higher cognitive load.
Although many works present results that argue that robot proactivity is desirable or preferable compared to other reference behaviors, some studies also highlight that proactivity can have negative effects. Humans may have reservations toward proactive robotic behavior. The results obtained by Deutsch et al. [
39] in their study of older adults’ attitudes toward proactivity as a design aspect for home robotic devices revealed that the majority of the utterances collected related to proactivity were negative. Test subjects expressed concerns about the influence on their own activity level, the degeneration of their abilities, and the loss of independence and control; and some stated that they preferred explicitly telling the robot what to do rather than it taking initiative. Cesta et al. [
31] studied elderly peoples’ attitudes toward a robot mediator offering cognitive support. The robot, which was evaluated offering both on-demand and proactive support in relation to a specific set of daily living scenarios, was overall reported useful and indicated as an acceptable solution by the test subjects. Furthermore, the preference for the robot’s support and its usefulness was significantly higher when it offered proactive support related to emergency and healthcare situations compared to on-demand and proactive suggestions.
The study by Peng et al. [
140], which investigated the effects of different levels of robot proactivity on user perception during a decision-making support process, found that the medium level of proactivity was more helpful and preferable compared to the highest level of proactivity. Kraus et al. [
87] investigated the effects of proactive dialog strategies on the human trust toward the system. The level of proactivity was varied through the degree of initiative or freedom to actively influence a given situation. They introduce three levels of proactivity in the robot’s behavior in addition to a reactive baseline behavior. The three levels of proactive behavior are “Notification,” simply notifying the user that the robot has a solution to offer to the problem at hand, “Suggestion,” actively providing information on a solution, and “Intervention,” take control and solve the problem using the found solution. As in Reference [
140], they found that the medium level proactive strategies were preferred. In both Reference [
140] and Kraus et al. [
87], the medium levels of proactivity are characterized by the robot proactively offering intelligent services to the user but requiring the user to take action to utilize the robot’s service, thus maintaining some level of user control. In contrast to [
87,
196] found that proactive system strategies, in their experiments, did not result in a significant increase in cognitive load.
To summarize, both the effects and the perception or desirability of proactivity in HRI vary according to the application, the way the proactive behavior is realized, and who the user of the system is. As such, it can be concluded that the benefit of proactive robotic behavior is not absolute, and that more research is needed to fully understand the factors influencing the success of proactive robot behavior.
7 Open Research Questions
With Proactive HRI being connected to many disciplines, there is rich potential for further research in several directions. The studies related to proactivity in children with ASD reveal that robots have potential to facilitate proactive behaviors. The study by Robins et al. [
148] further suggests that the appearance of the robot has influence, and the studies of this connection between factors such as appearance and the evoked proactivity could be further investigated.
Similarly, numerous works have studied robot proactivity and investigated how a robot can anticipate or recognize a need and act accordingly. However, shortcomings are the rigid context-specific programming of the robot behaviors, which also requires further investigation. True potential lies within flexible solutions that are able to learn connections between observations and feasible robot contributions. This also emphasizes the need for contextual inclusion and the ability to autonomously extract and weigh relevant contextual information from, possibly, several modalities. This could support further explorations of how ambiguity can be resolved in proactive robot behavior.
The effects of interaction between humans and proactive robots also hold potential for further discovery. Though researchers have investigated human preference of robot proactivity, there are still insights to be gained from evaluating real experienced interactions between a subject and a robot. The results in both References [
31,
39] are based on elderly people observing video materials and providing answers in interviews and questionnaires. Theoretical and video sample-based studies play important roles; however, measuring subjects’ experiences, opinions, and preferences through evaluations obtained in the context of as realistic scenarios as possible can improve ecological validity. As such, for a greater understanding of
in situ experiences, HRI researchers are encouraged to conduct more studies with real robots like in Reference [
44], measuring, e.g., elderly peoples’ opinions towards assistive robots. Furthermore, it should be noted that preference of robot proactivity might be one thing when discussed in theoretical scenarios, something else when experienced in a real encounter with a novel and unknown system, and something entirely different when experienced throughout long-term practical use. Thus, for gaining beneficial knowledge of these relationships and the preferences in long-term interactions, more longitudinal studies would be valuable.
The results presented in both References [
39] and [
31], furthermore, highlight that not all robot proactivity is equally desired. Consequently, the need for more research into when the robot should or should not engage, e.g., to support human abilities and autonomy [
86], and methods that enable robots to do such reasoning correctly, is apparent. In general, extensive studies of the effects and desirability of proactive robot behavior is lacking. Although the current research indicates that there are differences between applications, levels of proactivity, the types of proactivity, user profile, and so on, too little research has been conducted that is fine-grained enough to map out the correlation between these variables and the resulting effects. Investigating different proactive behaviors, rather than a single proposed proactive behavior, and comparing to a baseline could also prove valuable to offer an improved, more fine-grained insight into what is positive and what has negative consequences. This will decrease the likelihood of robot proactivity being falsely rejected in its entirety due to an inappropriate design.
While it is argued that robots working more independently and proactively will relieve humans and reduce their cognitive load [
125], investigation suggests that this behavior in human–robot teaming increases the cognitive load [
196]. This indicates that there could be a discrepancy between the expected and actual effect on cognitive load or that it differs between setups. With new robot abilities and behavior come new unknown effects on the users. One important research question is how does proactivity in, e.g., HRC, affect the assumptions made by the human partner about the robot’s intelligence. Can proactivity, for example, cause misplaced trust?
The vast majority of the works in the collected corpus focus on proactivity in a one-to-one human–robot interaction, which is expectable, as interaction in environments with multiple agents often end up as several one-to-one interactions. However, research into proactive behavior in multi-agent interaction with more than one human and/or robot is scarce. For this perspective, it could be interesting to expand the knowledge on the effect of robot proactivity on team dynamics or investigate the potential and effects of a collective proactivity by a robot team, e.g., in a human-dominated environment.
Finally, a great challenge in this diverse field is the large variety of measures used. As seen in other fields, commonly accepted metrics and benchmark datasets can ensure a rapid development. Also, objective measures for evaluating HRI can facilitate clear and precise results. A push in this direction is highly recommended.