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Multimodal Natural Human–Computer Interfaces for Computer-Aided Design: Comparison
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Please note this is a comparison between Version 3 by Vivi Li and Version 2 by Vivi Li.

Computer-aided design (CAD) systems have advanced to become a critical tool in product design. Nevertheless, they still primarily rely on the traditional mouse and keyboard interface. This limits the naturalness and intuitiveness of the 3D modeling process. Recently, a multimodal human–computer interface (HCI) has been proposed as the next-generation interaction paradigm. Widening the use of a multimodal HCI provides new opportunities for realizing natural interactions in 3D modeling.

  • human–computer interface
  • multimodal interface
  • computer-aided design
  • natural interaction

1. Introduction

Computer-aided design (CAD) systems have traditionally been used to aid the processes of product design and manufacturing [1]. Recent advances in CAD technology have led to a greater efficiency of 3D modeling in multiple industrial domains, such as mechanical and aerospace engineering [2][3]. Yet, there are some significant obstacles preventing further advancements in the research on human–computer interfaces (HCIs) for CAD, slowing the same pace of development. The current CAD systems still heavily rely on the conventional WIMP (Windows, Icons, Menus, Pointer) interface, using keystrokes and pointer movements as input. The behavioral patterns providing the system with input in this way show a mismatch in mapping between the user’s mental model and the computer interface. This mismatch forces the user to perform complex mental transformations between the 3D modeling space and 2D input devices, resulting in a high lead-time and hindered design creativity [3][4]. Moreover, novice users need to master a dense set of toolbars and menus, which takes considerable time to learn [5]. It is therefore necessary to move to user-centered natural HCI technology that can make CAD modeling more natural for users to operate and more intuitive to learn.
Efforts to mitigate this need have led to the development of multimodal interfaces that use eye tracking [6], gesture recognition [7], speech recognition [8], and brain signal recording techniques such as electroencephalograms (EEG) [9]. Standing at the crossroads of several research areas including psychology, computer vision, signal analysis, artificial intelligence, and cognitive science, the field of multimodal HCIs for CAD is in need of a solid theoretical foundation [10]. With the assistance of neighboring disciplines, researchers have developed a variety of novel interfaces for CAD [8][11][12]. Unfortunately, new input modalities, such as gesture, gaze, and EEG signals, are currently processed separately and integrated only at the application stage. Not only does this ignore the many issues regarding the application of each separate modality but also, more importantly, the roles and functions of multiple modalities for CAD and their interplay remain to be quantified and scientifically understood. In addition, there is a one-size-fits all attitude with respect to these modalities, in that an application fails to take individual differences into account.

2. Ways to Implement Natural HCI for CAD

Multimodal interfaces provide a promising solution to realize natural interactions for CAD applications. In this entry, researchers consider the following single-mode components: (1) eye tracking, (2) gesture recognition, (3) speech recognition, and (4) brain–computer interfaces (BCIs). Researchers discuss each of them briefly before turning to the key topic of their multimodal integration in this section. An overview of core references is presented in Table 1.
Table 1. Overview of core references for natural HCIs for CAD.
Category of Interfaces Descriptions References
Unimodal Eye Tracking [13][14][15]
Gesture [5][7][16][17][18][19][20][21][22][23]
Speech [24][25][26][27][28][29][30][31][32]
BCI [9][33][34][35][36][37][38][39]
Multimodal Gesture + Speech [8][11][12][40][41][42][43][44][45][46]
Gesture + Eye Tracking [6][47][48]
Gesture + BCI [49]
Gesture + Speech + BCI [3]
Others [50][51][52][53][54][55]

2.1. Eye Tracking-Based Interaction for CAD

The human gaze is defined as the direction to which the eyes are pointing in space, which could indicate where the attention of the user is focused [56][57]. Eye tracking is a technique in which eye-trackers record a user’s eye movements to determine where on the screen the user’s attention is directed when looking at the screen [58][59][60]. Ever since 1879, eye tracking has been used extensively in psychological science in the field of reading and image perception, and more recently in neuroscience and computer applications, especially HCI applications [60]. In the area of HCIs, eye tracking offers an important interface for a range of input methods including pointing and selecting [61], text input via virtual keyboards [62], gaze gestures [63], and drawing [64]. By tracking the direction and position of human eyes, the user’s intention could be judged and analyzed directly and rapidly [60][65][66]. For example, Connor et al. [67] developed two eyeLook applications (i.e., seeTV and seeTXT) based on a gaze-aware mobile computing platform. For seeTV, a mobile video player, gaze is used as a trigger condition such that content playback is automatically paused when the user is not looking. seeTXT, as an attentive speed-reading application, can flash words on a display, advancing the text only when the user is looking. Similarly, Nagamatsu et al. [68] integrated an eye-tracking-based interface into a handheld mobile device, through which the user is able to move the cursor and interact with a mobile device through their gaze. Furthermore, Miluzzo and Wang [69] demonstrated the capability of a smart phone to track the user’s eye movements across the phone. They presented a prototype implementation of EyePhone on a Nokia phone, enabling the user to operate a phone in touch-free manner. Incorporating such a facility in the process of interaction enhances the entertainment value and practicability, and benefits the development of natural HCIs. For CAD applications, eye tracking could be used to interpret the user’s intentions for modeling and operating a 3D model effectively in a touch-free manner [13][47]. For example, model selection is one of the fundamental operations and also the initial task for the most common user interactions in CAD modeling [15]. Using eye-tracking technology, the user could select models in 2D/3D environments directly and quickly. Ryu et al. [6] and Pouke et al. [48] adopted eye tracking as an extra channel of input and employed the user’s gaze to find the intersection point with the surface of the target to select 3D objects in a virtual environment. In addition, eye tracking could also be used in certain CAD operation tasks as an independent pointing interface, such as rotation, zooming, and translation. In traditional CAD systems, free rotation is obviously difficult, requiring additional commands for setting the rotation center [47]. With an eye-tracking-based interface, the user in the CAD environment can easily choose the rotation center without much effort just by fixating their gaze on it. For the zooming task, in order to explore detailed information through zooming, the user needs to preset the point of interest to the target that the model view camera moves towards in CAD applications. To ensure intuitiveness of operation, the location of interest is fixated and recorded by the eye-tracker, while the zooming operation could be controlled by another modality, e.g., a hand gesture [47]. Furthermore, the direction of translation operation in CAD could also be designed to be controlled by gazing. During all of the above operations, the application of eye tracking could decrease the user’s effort in position error feedback and reduce the fatigue of users, since it involves moving their eyes instead of hands. Thus, an eye-tracking-based interface in CAD can be a more natural and intuitive interface than traditional mouse and keyboard-based interfaces. In sum, eye-tracking technology presents itself as a potentially natural interface for CAD applications. However, eye tracking has rarely been used independently as an interactional modality for CAD modeling. To achieve more complex modeling tasks, researchers have focused on combining eye tracking with other modalities [48][59][70].

2.2. Gesture Recognition-Based Interaction for CAD

Gestures are a form of body language that offer an effective way of communicating with others. People use a variety of gestures ranging from simple ones (such as pointing and pinching) to more complex actions for expressing feelings [71]. In recent years, the maturity of gesture-recognition technology based on depth vision has promoted the development of gesture interaction technology in applications of CAD [72]. Gestural interfaces aim to offer highly intuitive and free-form modeling modes that emulate interactions with physical products [18] and allow designers to create 3D conceptual models quickly, while just requiring a minimal amount of CAD software experience [19]. Approaches and modes using gestures to execute CAD modeling manipulation tasks will be covered in this section. According to their spatiotemporal characteristics, gestures used in CAD interactions can mainly be divided into static and dynamic ones [16]. Research on static gestures considers the position information of a gesture, while that on dynamic gestures needs to consider not only the change in spatial position of the gesture, but also that in the expression of a gesture during the time sequence. Therefore, static gestures could be mainly used to invoke CAD commands for model-creation tasks while dynamic gestures could be used for model manipulation and modification [17]. In some studies [3][47][73], static gestures and dynamic gestures are combined for CAD manipulation instructions. For example, users can translate a model by closing the left hand into a fist (i.e., static gesture) and moving the right hand in the direction they want to translate it (i.e., dynamic gesture) [73][74]. In this process, the left-hand gesture plays the role of a trigger signal for the translation task while the right-hand gesture controls the direction and distance of the translation. Researchers found that they designed various sets or repositories of gestures for carrying out various CAD model tasks in the past [7][8][16][20]. The gestures in these repositories could, in principle, readily be used to test some gesture recognition algorithms and also directly as resources to select gestures in the application of CAD interfaces. However, most gestures in these repositories are typically chosen by researchers rather than users, often for the sake of ease of implementation with existing technology, while ignoring users’ preferences. It could be difficult for users to remember this information and, as a result, their cognitive loads are enhanced rather than reduced [75]. To advance from this situation, user-based research has been established as an effective gesture elicitation method [7][76]. Vuletic [7] and Thakur [16] developed user-based gesture vocabulary for conceptual design via evaluating the user’s activities in the modeling process in order to achieve a higher adoptability of gestures by inexperienced users and reduce the need for gesture learning. Moreover, some authors explored the use of simple prescribed, even if free-form, gestures, such as pinching or grasping with one’s hand, to quickly create a variety of constrained and free-form shapes without the need for extensive gesture training [21][22][77]. Their evaluations demonstrated that it is possible to enable users to express their intents for modeling without the need for a fixed set of gestures. Therefore, a potential future gesture system for CAD could allow individual designers to use non-prescribed gestures that will support rather than inhibit their conceptual design thinking. Meanwhile, further research into both the procedural structure of CAD-based activities and the ways in which they might change depending on the shape being created could be conducted to explore better gesture-based design patterns that adapt to users and their specific workflows as they are used [7].

2.3. Speech Recognition-Based Interaction for CAD

With the rapid development of speech synthesis and recognition technologies in recent years, speech-based HCIs have been extensively employed in a variety of household, automobile, office, and driving applications [25][78]. In particular, voice-enabled intelligent personal assistants (IPAs), such as Amazon’s Alexa, Apple’s Siri, Google Assistant, and Microsoft Cortana, are widely available on a number of automatic devices [78]. Using a speech-based interface, routine operations can be efficiently executed with intuitive voice commands and the ease of use of the system can be improved. As well as in the household, speech-recognition technology can be applied in the context of CAD. CAD modeling involves the development of a virtual object, always using step-by-step commands of instantiating primitives and using operations such as move, copy, rotate, and zoom for model manipulation and assembly [11]. These CAD commands could be realized naturally and directly through a speech-based interface. Therefore, some studies tried converting the names of menus and icons into voice-controlled commands [25][26][27][28][29][30]. Most of these studies were aimed at using fixed expressions to perform modeling operations. For example, Gao et al. [79] developed a prototype system with more than 80 speech commands which were primarily used to activate different CAD operations, for instance, selecting various operations of the system or switching from one mode to another. Similar research was also carried out by [27][29][30]. However, for all these speech-based CAD systems, CAD commands can only be activated when users utter the exact corresponding words or expressions [28]. So, novice users still have to devote considerable time to familiarize themselves with these systems and remember all the fixed vocabularies or preset expressions, which limits the utility of these speech-based CAD systems [31]. In order to implement a more flexible modeling operation, X.Y. Kou and S.K. Xue [24][31] proposed integrating a semantic inference approach into a speech-based CAD system, so users will no longer be constrained by predefined commands. In such a system, instead of using one-to-one mapping from word expressions to model actions, users can, for example, employ various expressions, such as “Rectangle”, “Draw a Rectangle” and “Create a Rectangle”, all with the same effect, namely, to generate a rectangle. This frees users from the need to memorize a host of predefined voice commands [24]. Whereas it is plain to see how this would work for simple commands, recognition methods should be further advanced for a speech-based CAD system to deal flexibly and intelligently with complex verbal commands [24][32]. Note that, as an effective interaction modality for CAD modeling, speech has been mostly used in conjunction with other interaction interfaces, such as “speech and gesture” [40], “speech and EEG” [3], and “speech and eye tracking” [70]. For this topic, more detailed studies will be reviewed in Section 4.5.

2.4. BCI-Based Interaction for CAD

Brain–computer interfaces (BCIs) provide a novel communication and control channel from the brain to an output device without the involvement of users’ peripheral nerves and muscles [80]. Typically, brain activity can be detected using a variety of approaches, such as EEG, magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), electrocorticography (ECoG), and near-infrared spectroscopy (NIRS) [81]. Among them, the EEG signal is considered as the input of choice in most BCI systems to relate patterns in brain signals to the users’ thoughts and intentions [33]. The very first advancements in this respect were made by Pfurtscheller [82], who for the first time demonstrated that it is possible for users to move through a virtual street just by imagining their feet movements. Lécuyer [83] and Zhao [84] also developed some prototypes allowing users to navigate in virtual scenes and manipulate virtual objects just through EEG signals. Furthermore, Trejo [34] and Li [85] further extended the application of BCI to cursor movement. These studies show that BCI, as a new non-muscular channel, could be used in many applications involving human cognition, such as computer games, vehicle driving, assistive appliances, and neural prostheses [80]. In CAD systems, BCI could offer a more intuitive and natural pattern of interaction between the user and CAD application. Esfahani and Sundararajan [35] conducted the first study that investigated the possibility of using BCI for geometry selection. They used the evoked potential component P300-based BCI for selecting different target surfaces of geometrical objects in the CAD systems. Some other important functions for CAD application such as creating models or manipulating models via BCI have been studied by other researchers [9][33][36][37][86]. Esfahani and Sundararajan [36] also carried out experiments to distinguish between different primitive shapes based on users’ EEG activity, including cube, sphere, cylinder, pyramid and cone shapes. The EEG headset was used to collect brain signals from 14 locations on the scalp and a linear discriminant classifier was trained to discriminate between the five basic primitive objects with an average accuracy of about 44.6%, significantly above the chance level of 20%. Postelnicu [38] conducted similar studies where the user was able to create and modify geometrical models by using EEG signals. As an important task in CAD applications, model manipulation-based BCI was also realized by [9][37][38]. In another recent work, Sree [39] used EEG and electromyogram (EMG) signals from facial movement using the Emotiv headset to enable users to carry out certain tasks in the CAD environment of Google SketchUp. Meanwhile, a human-factors study assessing the usability of the EEG/EMG interface was performed with participants from different backgrounds. The results suggested that the BCI-based system could help to lower the learning curve and has high usability as a generalized medium for CAD modeling [39]. In sum, BCI shows great potential in allowing users in CAD applications to create, modify and manipulate models directly. However, there are still some common limitations on the practical applicability of BCI resulting from the difficulty of brain signal acquisition and the limited accuracy of instruction discrimination. Future, lightweight EEG acquisition equipment and high-precision classification algorithms will become the new frontier for developing BCI-based CAD applications. Additionally, detecting the emotional state and user’s satisfaction from EEG signals could be used in CAD systems to correct for errors and to strengthen proper classifications, which will make the system more reliable [87].

2.5. Multimodal HCI for CAD

Multimodal interfaces aim to construct interactive systems that leverage natural human capabilities to communicate through different modalities such as speech, gesture, gaze, and others, bringing more accurate and robust intention recognition methods to human–computer interaction [88]. Thanks to advances in the development of pattern-recognition techniques (in natural language processing, machine vision, etc.) and hardware technologies of input and output (cameras and sensors, etc.), there has been a significant increase in multimodal HCI research [89]. Turk [88], Jaímes [90], and Dumas [91] performed valuable surveys on the current status of multimodal HCI research. From these studies, researchers can easily see that the main goal of research on multimodal interactions is to explore a more transparent, flexible, efficient, and natural interface that can remove existing constraints on what is possible in the field of HCIs, and progress towards the full use of human communication and interaction capabilities. The realization of this aim depends on collaboration among researchers from different disciplines, such as computer scientists, mechanical engineers, cognitive scientists, and other experts [88]. Meanwhile, multimodal interfaces are considered to offer an improved user experience and better control than a unimodal input in the application of CAD modeling. The pioneering study by Bolt [41] in creating a “Put That There” system showed the potential and advantages of combining gesture and speech inputs for the selection and displacement of virtual models. This system proposed the use of commands such as “Move that to the right of the green square” and “Delete that”, allowing users to employ vague language to activate CAD functions while disambiguating them with gestures. It is noteworthy that none of these commands can be interpreted properly from just a single input modality—at least two were required—but this multimodal interface created simple and expressive commands that are natural to users. The work by Bolt [41] led to a rapid development of many multimodal interfaces that typically used gesture, speech, eye-tracking, or BCI-based input modals for creating and manipulating CAD models, as is shown in Figure 1. From the survey results, “Gesture and Speech” is the most widely used interface in the application of CAD and drove the majority of multimodal interface research. Alternative formulations have also been followed, bringing new multimodal interfaces such as “Gesture and Eye Tracking” [47][48], “Gesture and BCI” [49], “Gesture, Speech and BCI” [3] and others [50][51][52][53][54][55]. For these multimodal interface-based CAD applications, there are three main types of combined modalities, as shown below.
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Figure 1. Survey on multimodal interfaces for CAD.
  • Different modalities can be used to execute different CAD commands. For example, in [48], eye tracking is used to select CAD models and gesture is used to activate different types of manipulation commands.
  • Different modalities can be used to execute the same CAD commands at the same time. In another words, there is overlap in the set of CAD commands that can be implemented through different modalities. For example, the task of creating models can be completed both by BCI-enabled commands and speech commands [3]. In this case, users can choose different interfaces according to their preferences. Moreover, to a certain extent, this kind of overlap in commands improves the robustness and flexibility of the interfaces.
  • Different modalities can be combined to execute a CAD command synergistically. In this respect, the advantages of different interfaces can be leveraged to work together on a model manipulation task. For example, in most CAD prototype systems combined with speech and gesture [8][12][42], speech is used to activate specific model manipulation commands (translation, rotation, zooming, etc.), while gestures are used to control the magnitude and direction of specific manipulations.
Most studies on multimodal interaction have concluded that using eye tracking, gesture, speech, and BCI for conceptual CAD modeling is easy to learn and use [3][8][12]. However, they mainly focused on the feasibility of the multimodal interface developed, considering aspects such as the accuracy and efficiency of user intent recognition, rather than user experience [3]. Notably, one of the most important goals of multimodal interfaces is to provide a natural interactive experience for CAD users. Hence, it is essential to carry out human factor studies for multimodal systems. Song [47] tested the intuitiveness and comfort levels of a multimodal system, i.e., GaFinC, by means of user interviews. Participants reported that it was very natural to pick the point of interest using their own gaze. However, they felt more uncomfortable and fatigued using two hands as opposed to one during the process of CAD modeling. In addition, in most gesture-based multimodal systems, the gestures used are always prescribed in advance and users have to remember complex gesture vocabulary before modeling, which significantly increases the additional cognitive load imposed on users [7]. All the issues mentioned above will benefit from the development of future multimodal-based CAD systems.

3. Devices for Natural HCI

Although these interfaces are unlikely to completely replace traditional WIMP-based interfaces, the importance of multimodal natural HCIs is growing due to advances in hardware and software, the benefits that they can provide to users, and the natural fit with the increasingly ubiquitous mobile computing environment [92]. In the process of CAD modeling, signals that can represent the design intention should first be obtained to identify the user intention. Thus, signal acquisition and identification devices are important for achieving a natural HCI. In this section, the devices used for eye tracking, gesture recognition, speech recognition, and BCI are introduced in detail, as presented in Table 2. In additional, some other devices related to natural HCIs are also briefly described.
Table 2. Overview of devices for natural HCIs.
Signal Modalities Categories Devices
Eye tracking Head-mounted Dikablis Glass 3.0 [93], Tobii Glass 2 [94], Pupil Labs Glasses [95]
Tabletop Gazepoint GP3 [96], Tobii Pro Spectrum 150 [97], EyeLink 1000 Plus [98]
Gesture Sensor-based MoCap Pro Glove [99], Cyber Glove [100], Vrtrix Glove [101]
Vision-based Kinect [102], Leap Motion [20][103]
Speech DP-100 Connected Speech Recognition System (CSRS) [41], Microsoft Speech API (SAPI) [31], CMUSphinx [104]
EEG Saline electrode Emotiv Epoc+ [3][36]
Wet electrode Neuroscan [34]
Dry electrode Neurosky MindWave [105], InteraXon Muse [106]
Others Haptic SensAble PHANTom [107], SPIDAR [108]
VR/AR HTC Vive [109], Oculus Rift [110], HoloLens 2 [111]

3.1. Devices for Eye Tracking

Eye tracking technology is used to track the eye movement state of the user and recognize where the user is looking on a computer screen through some advanced optical recognition methods, such as the pupil–corneal reflection method [112], pupillary-canthus method [113], HN method [114], and so on. The first eye-tracker that could measure eye movements quantitatively was developed by Dodge and Cline in 1901 [115]. With the dramatic improvement in eye-tracking technology, more advanced and easy-to-use hardware has been developed on the market in recent years [116]. Currently, there are two main categories of eye-trackers available as interactive input devices, including head-mounted devices and tabletop devices. The head-mounted eye-tracker, which usually needs to be attached to the eyes as special contact lenses or headsets, is composed of a scene camera, which records the user’s first-person view, and an eye camera, which continuously records the changes of the sight line by using the infrared ray of cornea and pupil reflection [117]. Some common commercial head-mounted eye-trackers can be used for HCIs, as shown in Figure 2. Obviously, these head-mounted devices are inconvenient and involve some physical load for users. Therefore, the lightweight design of head-mounted eye-trackers is needed to make them more widely used in natural HCIs.
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Figure 2. Commercial head-mounted eye-trackers: (a) Dikablis Glass 3.0 [93]; (b) Tobii Glass 2 [94]; (c) Pupil Labs Glasses [95].
Compared to the head-mounted tracer, the tabletop eye-tracker is more comfortable and flexible for users as it can be integrated with a monitor and does not require the user to wear any equipment, as shown in Figure 3. However, most of these commercial eye-trackers are designed to target the PC environment, with a short distance of 50 to 80 cm between users and eye-trackers, which limits their application in the field of HCIs [70]. Moreover, complex calibration between the eye-tracker and display coordinates is required and the user’s freedom of head movement is limited owing to the fixed camera. All the mentioned drawbacks need to be improved and optimized to promote the future development of tabletop eye-trackers.
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Figure 3. Commercial tabletop eye-trackers: (a) Gazepoint GP3 [96]; (b) Tobii Pro Spectrum 150 [97]; (c) EyeLink 1000 Plus [98].

3.2. Devices for Gesture Recognition

Gesture recognition involves algorithms to detect and recognize the movement of fingers, palms, arms, or the entire body so as to interpret the user’s interaction intent. Currently, the devices for gesture recognition can be broadly divided into sensor-based devices, such as data gloves, and vision-based devices, such as normal video cameras or depth-aware cameras. Sensor-based devices can directly capture the motion and position of hand gesture by using data gloves. Commonly used sensors mainly include EMG sensors [118], bending sensors [119], pressure sensors [120], and acceleration sensors [121]. By using these glove sensors, researchers can easily obtain various gesture signals and accurately identify the hand pose and movement. Representative products in data gloves for gesture recognition are MoCap Pro Glove, Cyber Glove, and Vrtrix Glove, as shown in Figure 4. However, these data gloves are too expensive and their wired sensors restrict natural hand movement [23]. To overcome these limitations, the vision-based approach came into existence [74].
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Figure 4. Commercial data gloves for gesture recognition: (a) MoCap Pro Glove [99]; (b) Cyber Glove [100]; (c) Vrtrix Glove [101].
The vision-based device is less constrained and moves more naturally than the sensor-based one, and does not require users to wear anything over the hands. This device is relatively cheap, simple, natural, and convenient to use in the application of CAD. At present, representative vision-based products used in the process of CAD are Kinect and Leap Motion, as shown in Figure 5. Vinayak [21] developed a novel gesture-based 3D modeling system and used Kinect to complete the recognition and classification of human skeletal and hand posture in the process of modeling. However, the recognition performance of vision-based devices is sensitive and easily affected by environmental factors such as lighting conditions and cluttered backgrounds [122]. Therefore, the illumination change, multiuser cluster, partial or full occlusions are important challenges for vision-based gesture input devices to be addressed in the future study [74].
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Figure 5. Commercial vision-based devices for gesture recognition: (a) Kinect [102]; (b) Leap Motion [103].

3.3. Devices for Speech Recognition

Speech recognition-based interfaces provide a natural method to operate and modify the CAD models for users. Along with the development of signal input devices and natural language-processing technology, the accuracy and effectiveness of speech recognition has been significantly improved, which further promotes the application of speech-based interfaces for CAD. Generally, the devices for speech recognition consist of two main modules, i.e., an input microphone and speech recognition module. After converting the analog signal with speech information into a digital signal through the input microphone, the speech recognition module integrated with natural language-processing algorithms completes the final recognition of semantics [123]. Obviously, the speech recognition module is a cord part of this system as an internal algorithm processor. Additionally, many companies have developed a variety of speech-recognition modules. For example, NEC (Nippon Electric Company) developed a system called DP-100 Connected Speech Recognition System (CSRS) [41] which is able to recognize a limited number of connect voices without pause. Microsoft also introduced its own voice-recognition interface for computers, Microsoft Speech API (SAPI) [31]. Carnegie Mellon University developed a speech-recognition toolbox, CMUSphinx, that can be used for microphone speech recognition [104]. In addition, machine learning and deep learning algorithms such as the Hidden Markov Model (HMM) [124], Convolutional Neural Network (CNN) [125] and Long Short-term Memory (LSTM) [126] have been widely used in the core part of speech recognition modules to improve the accuracy of speech recognition.

3.4. Devices for EEG Acquisition

The EEG acquisition device is a piece of equipment that can record the electrical activity of the brain through some specific electrodes placed on a user’s scalp [127]. In 1924, the first EEG detection device in the world was developed by Berger in Jena, Germany [128]. Following the development of signal processing and microelectronics technology, the structure and function of EEG-acquisition devices have gradually matured. There are numerous EEG devices from different companies that are able to cater to the specific needs of EEG users, such as Neuroscan, Emotiv, and Neurosky MindWave. Generally, EEG-acquisition devices can be divided into invasive, semi-invasive, and non-invasive categories according to the connection with the brain [129]. Among them, a non-invasive EEG device is mainly used in the area of natural HCIs due to the damage of the scalp caused by the invasive device and the semi-invasive device. For non-invasive EEG devices, the types of electrodes used can be categorized into saline electrode, wet electrode, and dry electrode [129]. Saline electrodes use salt water as a conductive medium, which is easy to carry and low in cost [130]. Wet electrodes usually need to use gel as a conductive medium and can collect high-quality EEG signals. Dry electrodes can directly contact the user’s scalp to achieve conduction without any conductive medium which is more natural and convenient to the user, but the signal acquisition accuracy is limited [131]. In current studies in the literature, Emotiv Epoc+ is commonly used as an EEG-acquisition device in the application of CAD [3][35][36][37]. Emotiv Epoc+ is a small and low-cost device used to record and recognize EEG and facial muscles’ movements (EMG) with 14 saline electrodes. This device can communicate wirelessly with computer via a USB dongle and provide the Emotiv API, a C++ API, which can be used to easily obtain the EEG/EMG data and transform the user-specific action command into an easy-to-user structure [3]. Additionally, some other lightweight and low-cost EEG devices, such as Neurosky MindWave and InteraXon Muse, may also be used to build future BCI for CAD.

3.5. Other Devices for Natural HCI

In addition to the main devices mentioned above, some other devices for natural HCIs are briefly introduced in this part. A haptic device is an interaction device which can deliver force feedback to a user, while interacting with the virtual environment (VE) or real environment (RE) [132]. This device could provide a more realistic interaction experience by making users feel the movement and touch the interaction models. The SensAble PHANTom [107][133][134] is one of the most common haptic devices used in CAD applications, which consists of a mechanical robotic arm with joints and a pen-shaped end-effector as manipulator. The arm follows the movement of the manipulator and is capable of generating a force equivalent to that applied to the manipulator [10]. Additionally, the wearable haptic device, SPIDAR, is also used to interact with virtual mock-ups [108]. Compared to SensAble PHANTom, SPIDAR has a more flexible workspace and allows the user to grasp the virtual models with natural experience. From the studies of [10][53][132], haptic devices are always applied in a virtual environment (VR) for CAD. In another word, VR devices are also needed and important for natural HCIs of CAD. Currently, the most popular VR/AR devices on the market are HTC Vive, Oculus Rift, HoloLens, and so on, as shown in Figure 6.
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Figure 6. VR/AR devices: (a) HTC Vive [109]; (b) Oculus Rift [110]; (c) HoloLens 2 [111].
Additionally, some other interactive devices such as touchscreen and EMG-based devices, are not listed here due to their limited application in the area of CAD.

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