# Support Operation and Maintenance of Power Wheelchairs with Digital Twins: The IoT and Cloud-Based Data Exchange

Carolina Lagartinho-Oliveira, Filipe Moutinho, Luís Gomes

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Carolina Lagartinho-Oliveira, Filipe Moutinho, Luís Gomes. Support Operation and Maintenance of Power Wheelchairs with Digital Twins: The IoT and Cloud-Based Data Exchange. 14th Doctoral Conference on Computing, Electrical and Industrial Systems (DoCEIS), Jul 2023, Monte Da Caparica, Portugal. pp.191-202, 10.1007/978-3-031-36007-7_14 . hal-05473081

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# Support Operation and Maintenance of Power Wheelchairs with Digital Twins: The IoT and Cloud-Based Data Exchange

Carolina Lagartinho-Oliveira(✉), Filipe Moutinho, and Luís Gomes

NOVA School of Science and Technology, Center of Technology and Systems (UNINOVA-CTS) and Associated Lab of Intelligent Systems (LASI), NOVA University Lisbon, 2829-516 Caparica, Portugal

ci.oliveira@campus.fct.unl.pt, {fcm,lugo}@fct.unl.pt

**Abstract.** Digital twins are becoming popular in a wide range of industries for the monitoring, control, and optimization of physical objects, processes, and systems. Its growing demand is related to its potential to improve efficiency, reduce costs and increase safety in different applications. As a result, a variety of approaches, modeling processes, technologies, and tools have been used to develop and deploy digital twins. The choice of which to use often depends on the specific application area, case study, available resources, and expertise. This paper explores the idea of using the digital twin concept applied to power wheelchair systems, to supervise and improve their operation and maintenance. In particular, it focuses on data flow and connectivity within the digital twin, proposing an IoT and cloud-based data exchange to enable efficient cyber-physical connection and easy data management. For this work, a small-scale prototype of a power wheelchair was built with some sensors and actuators interfaced with a microcontroller, and the data exchange with a ROS-based virtual entity was performed via cloud under the MQTT protocol.

**Keywords:** Data Acquisition · Data Management · HiveMQ Cloud · InfluxDB · MQTT · NXT

## 1 Introduction

Power wheelchairs and related devices are available on the market to assist people with severe mobility impairments [1, 2]. As they often allow for the augmentation and replacement of functions and devices, they can also deal with speech, hearing, and vision problems. This means that two power wheelchairs are unlikely to be alike, making the market relatively limited. This, associated with manufacturers having to comply with all relevant legislation, contributes to the high cost of power wheelchairs, which can be as expensive as a car [3, 4]. So, it is important to properly maintain and preserve a power wheelchair to help to extend its lifespan and guarantee that it continues to provide safe and reliable mobility for its user. This can include regular cleaning and inspections, prompt repairs and replacements of any worn or damaged components, as well as adjustments to some wheelchair parameters, such as acceleration, forward speed, joystick throw or steer correction.
To make this easier, some companies in the sector have begun to explore ways to remotely connect their latest models of power wheelchairs with their service centers. Invacare's MyLiNX app [5] establishes a bluetooth connection with the wheelchair's control unit, and transmits data via Wi-Fi to a cloud accessed by the provider's portal. In the case of Permobil wheelchairs, they have a built in SIM card to communicate data with the MyPermobil app [6] and the fleet management portal [7].

These types of software applications provide users and technicians with valuable data on wheelchair usage and performance, and important information on wheelchair condition to improve remote diagnostics. Examples are battery charge status, number of charging cycles, undertaken drive time, and fault codes. With real-time metrics, technicians can identify and diagnose issues before an appointment, sometimes aided by suggestions to help with troubleshooting. With Quantum Rehab's Interactive Assist app [8], technicians can also access a real-time image of the wheelchair's electronic display to proceed with remote resolutions, as updating software or adjusting settings. Basically, these are the initial steps in employing digital twins for wheelchairs.

The authors of this paper intend to further study the use of digital twins for power wheelchair systems, to supervise them and make their operation and maintenance more efficient. A digital twin allows a physical asset or prototype to be represented in the virtual world, with physical and virtual entities connected and sharing the same properties, characteristics and behavior by means of data and information [9]. In this regard, this paper proposes an IoT and cloud-based data exchange to enable seamless connection between entities and to support data management; the question also posed is: "How can digital twin, along with an IoT and cloud-based data exchange, support the operation and maintenance of power wheelchairs?".

This work builds on the standard MQTT messaging protocol that facilitates communication for IoT devices, and the HiveMQ cloud native IoT messaging broker, which efficiently forwards data to and from a small-scale prototype of a power wheelchair. The prototype was equipped with some sensors and actuators interfaced with a microcontroller, and its virtual entity, responsible for storing data in an InfluxDB database, was based in ROS. Power wheelchairs face some challenges, and this section talked about some innovations on the market for their follow-up and maintenance.

Throughout the paper, the authors also contribute with some considerations about the state-of-the-art in the remaining topics of this work. Section 2 explains how this paper contributes towards the development of connected cyber-physical spaces. Section 3 details the proposal; describes the experimental prototype, and presents some results. Section 4 presents some discussion around the proposal; and Sect. 5 presents the conclusions and future work.

## 2 Contribution for Connected Cyber-Physical Spaces

A digital twin (DT) can simply be composed of three elements [9]: a real space, corresponding to the physical world where physical entities (PEs) exist; a virtual space that exists within the domain of the cyber space and where virtual entities can mirror the PEs; and a bidirectional path that allows data synchronization between the two spaces. In this paper, the focus is on the third element specifically on the connection between a
power wheelchair prototype and a virtual entity, and on the management of related data. This focus is due to two factors: firstly, VE mirroring with PE is driven by real-time or near-real-time data; secondly, the VE relies on the data to compute control actions and send instructions to the PE.

To support data flow and connectivity within DT, internet of things (IoT) transmission technologies can be used together with appropriate network architectures, communication and security protocols, middleware platforms, etc. [10]. A middleware platform for IoT helps manage and process the large amounts of data generated by IoT devices. In particular, message-oriented middleware (MOM) platforms [11] allow devices to communicate asynchronously using a message broker, such as ActiveMQ, RabbitMQ, Mosquitto, VerneMQ, EMQX, and HiveMQ. These popular brokers support the Message Queuing Telemetry Transport (MQTT) protocol, which allows topics to be published and subscribed between distributed client nodes [12]. There are also cloud-based platforms available to manage these brokers' clusters, providing better scalability and accessibility.

Additionally, when accepting an MQTT client or connecting to external resources like a database, these brokers can establish secure connections via SSL/TLS and various authentication mechanisms. The relevant data that is exchanged in the DT – including its context, location and time – must be stored to ensure its future accessibility, so that if the PE changes, it can be analyzed in a specific time frame [13]. The data can be used later to understand and predict the PE behavior within its original context or in a new one. Such data can be collected as time series.

Big data storage technologies such as distributed file storage, NoSQL database and NewSQL database are getting more attention and can also be used to store and manage large volumes of data, including time series data [14]; examples of used tools are: Couchbase, MongoDB, RavenDB, and Cassandra. However, time series databases (TSDBs) are specially designed to store and retrieve large volumes of timestamped data, and provide additional features such as indexing, compression and aggregation. Examples of TSDB are: Prometheus, TimescaleDB, OpenTSDB, kdb+, KairosDB, and InfluxDB.

The following section presents the materials and methods associated with the deployment of the IoT and cloud-based data exchange for two-way connection of cyber-physical spaces, and data management in the context of power wheelchairs. It also includes a description of the experimental prototype, as well as the presentation of results.

## 3 Proposed IoT and Cloud-Based Data Exchange

The aim of this work is to support the research being conducted to use digital twins for power wheelchairs [15–17]. This envisions a seamless and secure remote connection between power wheelchairs and digital counterparts, which could ideally be implemented in service centers of companies in the sector.

The architecture underlying this proposal is represented in the diagram of Fig. 1. PE and VE in this diagram are represented by the prototype and the digital image, respectively. They are connected in a bidirectional manner through the HiveMQ cloud [18] acting as an MQTT broker, which also sends data to an on-premise InfluxDB [19] database using a Telegraf agent. In order to materialize this approach, an experimental prototype of a wheelchair was created with LEGO components, while the digital image was deployed using the ROS framework [20].
<figure><img src="image_4.png" /><figcaption>Fig. 1. Digital twin for a power wheelchair with IoT and cloud-based data exchange.</figcaption></figure>

## 3.1 Power Wheelchair Experimental Prototype and Digital Image

As no real wheelchair was available at the time of this work, a small experimental prototype was developed using LEGO Mindstorms NXT components. This provided a simple yet effective way to test and verify the proposal. The prototype mainly focused on the power system elements that make the wheelchair capable of propelling; that is, the drive and caster wheels, the drive motor, the batteries and the controller.

An L293D motor drive IC interfaced with an ESP32 microcontroller (running micropython) was used to monitor and control the speed and steering of two 9 V DC NXT drive motors using modified RJ-12 cables. A compass was also integrated to monitor changes in tilt and rotation of the wheelchair. To power the prototype, a bank of 8 1.2 V Ni-MH batteries was used, providing a sufficient power source for the motors; as well as an additional bank of 4 batteries for the IC and ESP32. The ESP Wi-Fi module established a connection with the MQTT broker and transmitted the wheelchair motion values and its rotation around each axis. It also received commands from virtual entity to change the speed and state of the motors.

Several models can be considered for VE to replicate PE [21]. In order to deploy the digital image of the prototype, the virtual entity was modeled using a combination of geometric and physical models. While the geometric model represents the overall solid appearance of the VE using data structures that contain topological and geometric information, the physical model reflects the physical characteristics of the PWC. Concretely, a unified robot description format (URDF) XML file was described to represent the visual and physical aspects of the wheelchair digital image in the ROS environment. This model was visible in 3D in Gazebo and closely resembles a real wheelchair.
Then, and most importantly, a python script node was implemented to establish the MQTT connection with the broker and manage the transmission and reception of data. It was also used to monitor the status of the prototype so that any changes could be replicated and animated by the VE in the Gazebo simulator; as well as used to manage the commands to affect and control the physical prototype.

## 3.2 Data Exchange and Data Storage

A private HiveMQ cloud cluster was used to connect MQTT clients to a cloud native IoT messaging broker hosted on Amazon Web Services. At no cost, the cluster provides a maximum of 100 MQTT client sessions and can handle up to 10 GB of data traffic per month; however, in this particular scenario, it was only necessary to configure the access credentials of 3 clients, so that the power wheelchair, the digital image, and the database could publish and subscribe to the cluster.

The broker effectively gathers information published on specific topics and distributes it to the clients who have subscribed to those topics. The wheelchair prototype sends messages to the digital image by publishing to topics "prototype/#" and receives messages from the digital image by subscribing to "image/#". For bidirectional communication between the prototype and the digital image, each client sends messages to the other by publishing on the topic that the other client is subscribed to. To receive the data from both clients, the InfluxDB database simply subscribed to the prototype and digital image topics using the Telegraf agent.

All communications were secured using the TLS protocol, which provided encryption and authentication to ensure data was transmitted securely; and messages were sent with a quality of service (QoS) level of 1 to ensure they arrived at least once, even in the presence of network outages. In this setup, the InfluxDB and the digital image were installed on the same computer, as the cloud-based version of InfluxDB only has a maximum data retention period of 30 days. Yet, data needs to be saved for a longer period for potential future services.

## 3.3 Outcome

To validate the proposal, two types of procedures were considered. Firstly, a test web client was defined at HiveMQ cloud cluster to publish and subscribe to all topics. This allowed to test the communication with each client individually under the different topics, as well as determine the impact of the messages on each client. Secondly, all the clients were launched and establish communication with the HiveMQ cloud broker.

From top to bottom, Fig. 2 presents the successful connection of Telegraf, ROS, and ESP, with the subscription of topics from both prototype and digital image. In the ESP terminal, it is possible to see the first publish of data, "Publishing status", with data from compass, namely the pitch ("p") and roll ("r"), and data speed and motion values of each motor. In turn, we see in ROS terminal, that the messages were correctly parsed.

From InfluxDB it is possible to better inspect the results obtained. Figure 3, 4 and 5, depict all the messages received during approximately 1 min. Figure 3 corresponds to the values from compass. Here we see the first values from pitch and roll to be the same as those shown in terminals from Fig. 2.
<figure><img src="image_5.png" /><figcaption>Fig. 2. Connection, publish and subscribe of clients.</figcaption></figure>

Since no remote controller like a joystick was used, the prototype waited for commands from ROS node to change its state. So, for Fig. 4 and 5 the results focused are the ones in which we can see that the commands sent to “prototype/#” affected the status of motors, updated previously with the status send by “image/#”.

Finally, it was also possible verify the correct mirroring between that both entities, which functioned identically, in Gazebo and in real prototype.
<figure><img src="image_6.png" /><figcaption>Fig. 3. Messages from ESP in prototype/sta_compass topic.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Messages from ESP and ROS in prototype/sta motor left and image /cmd /motor left.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. Messages from ESP and ROS in prototype/sta and image/cmd motor right.</figcaption></figure>

4 Discussion

The increasing complexity of current systems has been a motivation for the use of
appropriate techniques to verify and validate their dependability, especially with regard to
safety-critical systems (SCSs). A SCS is any kind of system where non-desired properties
can endanger people's lives and the environment, as well as lead to financial losses. Power
wheelchair systems fall precisely within the scope of SCSs.

As stated before, power wheelchairs have their own set of issues and problems and
despite the efforts of companies to provide follow-up and maintenance services for
them, there is still room for improvement. For instance, Quantum Rehab's Interactive
Assist app [8] enables technicians to remotely access a real-time mirror image of the
wheelchair's electronic display to resolve issues such as updating software or adjusting
settings. However, they have limited control over the actual wheelchair. In many cases,
they rely on customers and caregivers to follow technicians' instructions on how to
proceed.

In this sense, we acknowledge the potential benefits in the use of digital twins in
the power wheelchair industry, like what already happens in other sectors, such as the
automobile industry. As an example, a digital twin can be useful for wheelchair prepara-
tion and fitting, as it can show how the wheelchair functions, support its customization
and adaptation, and help in planning and testing different scenarios. The DT can also be
applied to justify, design, and validate new or existing wheelchair features, services, and
parts, as well as simulating the health conditions of the wheelchair, to detect and prevent
the sources of problems and unwanted situations. The digital twin can also predict the
performance of the power wheelchair, including the occurrence of unexpected scenarios.
As the digital twin can collect real-time operational data from the power wheelchair, it can provide insights into possible outcomes of its use, or trigger alerts during follow-up, maintenance, and repair so that a problem can be diagnosed. With the DT, wheelchair repair can be facilitated through remote analysis and real-time power wheelchair intervention. Here are some ways digital twin, along with an IoT and cloud-based data exchange, support the operation and maintenance of power wheelchairs.

The architecture depicted in Fig. 6 presents how digital twins can be used by service providers in real-time connectivity with their wheelchairs.

<figure><img src="image_9.png" /><figcaption>Fig. 6. Monitoring of multiple wheelchairs with IoT and cloud-based data exchange.</figcaption></figure>

The proposed architecture facilitates flexible communication among multiple devices, with each client capable of communicating with one or more other clients as required. It can also be applied in a scenario where a provider center monitors multiple wheelchairs to support their operation and maintenance.

In future, this proposal can be improved to enhance the following services:

* Real-time monitoring of power wheelchair locations;

* The collection of information about each user's driving style and use;

* Engaging with users by sharing tips about their driving;

* Access wheelchair data and information on how to optimize its performance;

* Sending instructions or updates directly to the power wheelchair;

* Establish a maintenance schedule and perform pre-diagnostics;

* Generation of cost simulations based on required maintenance;

* Based on the information from used wheelchairs, configure new or replacement wheelchairs;
* Automatic report generation or downloaded on demand.

In the following section, the concluding remarks of this study will be presented, and
emphasis will be given to future research.

**5 Conclusions and Further Work**

This paper explores a solution to support the operation and maintenance of power wheelchairs. One solution can be based on sensors that gather data to monitor important parameters of power wheelchairs. The data can be transferred into digital copies of power wheelchairs, to supervise and improve their operation and maintenance, ensuring they are used correctly and providing reliable diagnoses. This concerns the use of digital twins for power wheelchairs. So far, there is no evidence of its application in this sector.

The bidirectional link in a digital twin is a crucial element; it is only with the exchange of data and information through cyber-physical spaces that DT allows the convergence and synchronization of physical and virtual entities. With the proposed IoT and cloud-based data exchange, it was deployed a seamless and secure way for data sharing between a wheelchair prototype composed of LEGO Mindstorms NXT components and a virtual entity.

Regarding the experimental results, they depend on the dynamic interaction between
the physical and virtual entities, and it is challenging to present results that capture their
change and effect over time and space. However, we have provided a comprehensive
description of how we did the experiment and try to present results that show the con-
nection and synchronization between the prototype and the digital image, through the
data flow and updates between them (and the database); we used screenshots and tables
to illustrate our results more clearly.

The prototype's motors and a compass were interfaced with an ESP32, so that the
ROS-based virtual entity could monitor and control the speed and direction of the PE via
MQTT. The MQTT messages were centralized using a HiveMQ Cloud MQTT broker,
and the relevant data was stored in an InfluxDB time-series database to support future ser-
vices. This facilitates distributed communication between nodes that are located remotely
and opens up unexplored possibilities for the wheelchair market.

The next phase of this study involves the use of an actual power wheelchair, an
Invacare Fox [22]. Among other things, it will be necessary to modify the digital twin
to accommodate the LiNX electronics found in Invacare's wheelchairs [23]. In addition,
other sensors will be assembled to provide useful data in the given context.

On the virtual entity side, the authors also plan to extend and use a new high-level
Petri net [24] to specify both the behavioral state model and the rule model. It is intended
that this model can meet specific requirements, such as defining and establishing secure
MQTT connections. The improvement of the VE geometric model is also a point to
work on; and finally, real use case scenarios will be defined so that the digital twin has
validation in the real wheelchair.

**Acknowledgments.** This work had the support of the Portuguese Agency FCT ("Fundação para a Ciência e a Tecnologia"), in the framework of project UID/EEA/00066/2020, and under the PhD scholarship 2020.08462.BD.
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