Introduction

The Internet of Things (IoT) makes possible communications between people and objects by taking advantage of computing capabilities and hardware accessibility for all types of applications. As a general definition, IoT can be described as an interconnection of many objects through a network that continuously generates information about the physical world. These objects can communicate and be controlled by various agents (other systems or people) to interact and take control of the physical world to manage many services of daily use [1].

IoT devices can generally be classified into controller boards with microprocessors or micro-controllers, sensors that sense data from the physical world, and actuators that connect to controllers and communication modules. However, the development and deployment of IoT applications are confined almost entirely to tech companies, leaving end users with only access to specific functionalities. This approach presents a fundamental problem since it is only possible for the IoT provider to anticipate some of the user’s needs. Unfortunately, it will take a long time before changes are made to an IoT system to fulfill a user’s specific needs. In this work, we argue that for IoT systems to close the gap between users and IoT technology, a different approach is needed where end users have the means to build their IoT systems according to their specific needs. However, for this end, the interface should be user-friendly, using day-to-day instructions as input, such as “If these conditions happened, then do this or that.” Nevertheless, the system should allow users to express complex system behaviors while at the same time verifying that no inconsistencies appear when additional rules are added.

This work proposes that Fuzzy logic (FL) [2] is ideally suited to become the interface between people and objects in IoT systems, modeling logical reasoning with vague or ambiguous statements such as “The temperature is hot (cold or mild).” This logic refers to a family of many-valued logic in which truth values are interpreted as degrees of truth. The truth value of a logically complex proposition such as “Carl is tall, and Chris is rich” is determined by the truth values of its constituents. In other words, truth functions impose on classical logic. This type of logic arises from the need to use daily life statements whose natural language adjectives are used to qualify.

Fuzzy logic has been applied in various ways in sensor networks and IoT, such as energy savings, packet routing, location, and human-sensor interface, among other applications [3–6]. An advantage of fuzzy logic over traditional logic is that the former can reach precise conclusions based on vague, imprecise, noisy, or non-existent arguments common in IoT systems since messages are often lost, collected information is imprecise, or the instructions are vague [7]. Moreover, many IoT applications require human intervention where the user might provide ambiguous inputs to the system, such as higher, smaller, or bigger, that an actuator cannot read directly.

Inconsistent information is recurrent in IoT systems due to many factors, including errors produced by data entry operators, data arriving from multiple sensors, and instructions contradicting each other [8, 9]. Since Fuzzy-IoT systems can only make an action based on the collected information at a given time, it is relevant to verify the consistency of the entire system as soon as new instructions are added to the system; otherwise, the system might present errors when applying these actions. For instance, Let us consider two rules for the activation of warning alarms: “Activate a medium warning alarm when low motion, light, and sound are detected” and “Activate a low warning alarm when high motion, low light, and low sound are detected.” These rules exhibit inconsistency since the second rule implies that a low warning alarm should be triggered by high motion, which contradicts the expectation that a medium or high warning alarm should be activated given the potential presence of a stranger inside the house. Thus, even when some smart IoT systems can enhance daily life, their proper operation could be compromised if the rules inserted into the system are inconsistent. To solve this problem, this paper proposes a novel framework that allows users to interact with the IoT systems using simple instructions and vague language while verifying the system’s overall consistency. For example, a simple rule can be: (1) While a second rule can be: (2)

Fig 1 illustrates the flow diagram of the proposed framework. First, a technician (or even the user) needs to place and connect various sensors and actuators that compose the IoT hardware on which the IoT system operates. Some important contextual concepts may be pre-programmed onto the system, such as really hot and air conditioning set to high. Nevertheless, users may modify these concepts later on. The input is given by simple everyday rules such as instructions 1 and 2, then a fuzzy reasoner will process the user’s input. If the last instruction contradicts another previous rule, the system will alert the user accordingly. Once the system is programmed with non-contradictory (consistent) instructions, it routinely performs its corresponding tasks (e.g., turn on the heater, set the alarm, etc).

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Fig 1. The architecture of fuzzyDL reasoner.

https://doi.org/10.1371/journal.pone.0296655.g001

The structure of this work is organized as follows: In the first section, we discuss previous research to provide context for our study. Then, in the IoT Systems section, we define the system and introduce the concept of consistency. Next, in the Fuzzy Description Logic Verification section, we explain the basics of fuzzy logic and present a result that shows how consistency relates to both the IoT system and fuzzy description logic. Ultimately, in the Fuzzy Control for an IoT Security System section, we outline the following steps: providing context, setting system rules, and putting the system into action. Finally, we present the program’s syntax and share the experiments we conducted to support and illustrate the concepts we have discussed.

Related work

Fuzzy logic has been used in a wide variety of systems such as the automatic focus of digital cameras [10], control and optimization of industrial processes and systems [11], improving the efficiency of fuel-running engines [12], environmental improvement [13], expert systems [14], robotics [15], vehicles and autonomous driving [16], computer technology [17], Fuzzy databases [18], artificial intelligence, control systems for air conditioners [19], family appliances [3, 20], wireless sensor networks [3–6], and cellular automata [21–23].

Concerning formal verification of IoT systems, the authors in survey [24] present various works focused on verifying security properties [25–27]. Some other IoT works studied the settings of formal verification, including communication protocols [28], healthcare and environmental monitoring systems [29, 30]. Even when all these approaches focus on verifying data, protocols, and security consistency, these proposals work over static variables. On the contrary, the proposal presented in this paper can modify the system’s behavior by adding new rules on running time while the system verifies consistency. Furthermore, none of the above proposals interact with end users.

Input data in IoT systems is usually collected from heterogeneous sensor devices that need more interoperability since data values are based on proprietary formats. Similarly, IoT systems can accumulate poor-quality data since events such as offset data, missing data, wrong time stamps, and wrong attribute values can occur. Verifying the consistency of collected data has traditionally used machine learning and point-based calibration algorithms. For instance, authors in [31] proposed a data consistency method based on neural networks to reduce data errors by approximately 4%. However, this approach cannot interact in real-time with end users since it verifies consistency before the system starts.

Logical data inconsistencies have also been studied in the description logic (DLs) setting comprising a family of knowledge representation languages [32]. The balance between computational complexity and the expressiveness of DLs has allowed efficient reasoning tools to be constructed. These tools have enabled the application of DLs in several domains successfully [33]. Notably, the Web Ontology Language (OWL), a standard for Web Semantics technologies, is based on DLs [34]. Fuzzy extensions of DLs have also been developed [35]. These extensions have found application in human activity modeling for ambient intelligence systems [36], diabetes diagnosis systems [37], and database systems [38], to mention a few. Authors in [9] proposed a consistency data representation for IoT healthcare systems, transforming health data obtained from heterogeneous IoT devices into a semantic data model that supports logical reasoning using OWL. Even when the authors used a logic reasoner, they only focused on creating a unified static data model in which new rules cannot be introduced on running time. In [8], the authors proposed a reasoning framework to guarantee the consistency of the data stream produced by physical sensors in smart spaces. However, this framework does not interact with end users.

In summary, the proposed framework sets apart from previous works in the literature in two directions, mainly in the context of IoT applications. Firstly, it separates which tasks in the IoT system belong to an expert and which ones are the end user’s responsibility, thus freeing end users from dealing with the most complex part of building and operating an IoT system. Most related works do not make this task distinction, providing little freedom to users wanting to implement their own IoT applications. However, the interaction of expert and user-related tasks will likely generate inconsistencies in the instructions introduced by end users and the data being collected and processed by sensors and actuators. Secondly, this framework verifies the dynamic properties of these task interactions and detects inconsistencies resulting from end-user instructions and wrong data that can be detected in real-time. This may allow end users to identify contradictory instructions so they can be modified to guarantee the IoT system’s correctness. About this point, most related works dealing with consistency focus on verifying static properties defined for the design of the IoT system only, but they do not consider the dynamic aspect once the system is running.

IoT systems

Fuzzy logic is a multi-valued logic whose statements can take truth values associated with the interval [0, 1] [39]. In deductive logic, inferences have the following structure:

From the premises P₁, P₂, …, P_n, the conclusion R is reached. Consider, for example, the following inferences in classical logic:

Notice that in these examples, the degrees of membership are Boolean: Aristotle is a man, and Mishi is a cat. However, in many contexts, the degrees of membership are not Boolean. For example, the route to the airport meets with much traffic, and the wind chill is hot. In fuzzy logic, the structure of the inferences is conserved concerning classical reasoning. The degrees of membership are those that are considered fuzzy. Consider the following example:

Let us assume a set of sensors and a set of actuators in an IoT security system. For example, sensors might detect movement, light, and sound, while actuators make an action, such as set the alarm, phone_call, and call_the_police. Domains of sensors and actuators are considered to be fuzzy. This occurs because data obtained from sensors and actuators could be more accurate in practice due to many factors. A fuzzy domain D is a finite set of closed intervals d₁, d₂, …, d_n, such that d_i ⊆ [0, 1] and . For instance, a domain for an alarm actuator can be defined by the intervals [0, 0.35] (low), [0.35, 0.7] (medium), and [0.7, 1] (high). Note that human language often uses adjectives instead of particular intervals.

To associate fuzzy domains with sensors and actuators, we consider fuzzy interpretation functions, that is, , for a fuzzy domain D. Consider the example of an alarm actuator; if the alarm is set high, we formally write f_z(alarm) ∈ [0.7, 1]. It also can be written f_z(alarm) ∈ high.

The following grammar defines a system expression: where . The system expression f_z(alarm) ∈ high stands when the alarm is set to high. We may also write f_z(alarm) ∉ high to show that the alarm is not high. Other Boolean combinations of these atomic expressions may also form a system expression: f_z(alarm) ∈ high ∧ f_z(lamp) ∈ on.

Definition 1 (IoT System). Consider a set of fuzzy interpretation functions. We then define an IoT System as a finite set of rules of the following form: where SystExp are system expressions and f_z are fuzzy interpretation functions of actuators a.

Example. Consider three sensors: movement, light, and sound. Domains for these sensors are composed of three intervals: few, some, and much. As for actuators, the system comprises alarm and phone_call. Alarm domain is defined by low, medium, and high. The domain for phone_call is Boolean; it can be set on or off. Now, the IoT system can be programmed to call the police if the sensor perceives much sound, light, and movement. The following expression can express this behavior: (3)

If the system perceives some movement but low sound and some light, instead of calling the police, turn the alarm on to medium. This behavior can be written as follows: (4)

Before providing semantics for IoT System expression, we must precisely define when a sensor or actuator is set to a particular interval domain. This is not immediate since intervals may intersect; for instance, this might be the case for medium and low intervals for a sound sensor. We then introduce the fuzzy membership function m_f : D ↦ [0, 1], provided a fuzzy domain D. Fuzzy membership functions may be defined according to the application context of the IoT System. Some standard Fuzzy membership functions are (a) trapezoidal, (b) triangle, (c) rectangular, (d) right-shoulder, and (e) left-shoulder as depicted in Fig 2.

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Fig 2. Fuzzy membership functions.

https://doi.org/10.1371/journal.pone.0296655.g002

The membership of a sensor or actuator to a particular domain is then defined as the maximum of the fuzzy membership functions. This is formalized by a Boolean structure, which is defined as a function from expressions f(e) ∈ d to {0,1} as follows:

Recall the example of the sound sensor intervals intersecting: if the sensed value falls within this intersection, our system employs a special structure to determine which interval is closer to the sensed value.

We are now ready to provide precise semantics of an IoT System. The interpretation of an IoT system rule concerning a Boolean structure B is then defined as follows:

• ⟦f_z(e) ∈ d⟧^B = 1, if and only if, B(f_z(e) ∈ d) = 1;
• ⟦f_z(e) ∉ d⟧^B = 1, if and only if, B(f_z(e) ∈ d) = 0;
• ⟦SystExp₁ ∨ SystExp₂⟧^B = 1, if and only if, ⟦SystExp_i⟧^B = 1 for some i ∈ {1, 2};
• ⟦SystExp₁ ∧ SystExp₂⟧^B = 1, if and only if, ⟦SystExp_i⟧^B = 1 for all i ∈ {1, 2};
• ⟦If SystExp then f_z(a) ∈ d⟧^B = 1, if and only if, ⟦SystExp⟧^B = 0 or ⟦f_z(a) ∈ d⟧^B = 1.

The intuition of interpreting a system rule is that Boolean structures provide a particular context for sensors and actuators.

Definition 2 (IoT System Consistency). We say an IoT System is consistent, if and only if, for all rules of the system R and any Boolean structure B, have that ⟦R⟧^B = 1.

For instance, consider a system comprised of expressions 3 and 4. It is evident that this configuration is consistent, as both rules are interpreted as 1 under any Boolean structure (within the context of sensors and actuators). Additionally, if we include the following rule: (5)

Then the system is inconsistent, as there is a Boolean structure considering sensors detecting some movement, low light, and sound, and setting the alarm to medium, such that the interpretation of rule 5 is 0. Note that there is another structure considering the alarm set to low, which causes rule 5 to hold. However, under this structure, rule 4 does not.

Fuzzy description logic verification

In this Section, we describe a fuzzy description logic. Description logics form a family of Knowledge Representation languages, vastly and successfully known among several other domains in the Semantic Web community. The description logic language described in this work allows us to model IoT expert systems in the form of Knowledge Bases (KB). The fuzzy part of language allows us to model ambiguous notions such as “much movement”. We will first describe the syntax and semantics of the logic. Then, the notion of KB consistency is introduced. We next show that the consistency of IoT systems can be tested in terms of KB consistency.

Zadeh [39] proposed fuzzy set theory and logic to manage fuzzy and ambiguous knowledge. In classical set theory, either one of the elements belongs to the set, or it does not. In fuzzy set theory, the elements belong to a certain degree. Let X be a set of elements called the reference set. A fuzzy subset A of X is defined by a membership function μ_A(x) (or simply A(x)), which assigns any x ∈ X to a value in the real interval between 0 and 1. In the classical case, 0 is no membership, and 1 is full membership. It should be noted that the fuzzy interpretation functions defined in the previous section and the membership functions exhibit similar behavior. The distinction lies in the domain that each considers. To avoid confusion, the former functions pertain to the IoT system, while the latter pertains to fuzzy description logic. A value between 0 and 1 indicates how x is considered an element of X. The crisp set operation is extended to fuzzy sets. The intersection, union, complement operation, and implication set are interpreted as a t-norm ⊗, a t-conorm ⊕, a negation ⊖, and an implication ⇒. Let α, β ∈ [0, 1], we define the fuzzy operators’ negation, t-norm, t-conorm, and implication; moreover, the fuzzy implications are used to describe the Zadeh logic: α ⊗ β = min{α, β}, α ⊕ β = max{α, β}, ⊖α = 1 − α, and α ⇒ β = max{1 − α, β}. It is possible to define more operators to define Łukasiewicz logic, Kleene-Dienes logic, and Classical logic (for more details see [35]). The Mamdani model is a particular and usual case of reasoning in the literature. This is a fuzzy If-Then system that includes the basic rule, that is, a set of rules of the form: (6) where for each i = 1, …, n, A_i, and B are linguistic values defined by language-wide fuzzy sets. X_i and Y, respectively. The inference rule (Generalized Modus Ponens). Note that the Mamdani model is a particular case of Definition 1.

We introduce the fuzzy description logic behind fuzzyDL inference engine. fuzzyDL is defined on a discrete set for such that there is a machine representable .

Definition 3. Fuzzy Description Logic’s main elements are concepts, denoting unary predicates, and roles, denoting binary predicates. Finally, connectives allow the construction of complex concepts. The syntax of fuzzyDL is as follows: where C, D denote concepts, R denotes abstract roles names, T denotes concrete roles names, d denotes membership functions, m denotes modifiers, and ∘ = {L, G}.

Example. Consider again the IoT system rule 4 In terms of a fuzzy description logic expression, this rule is written as follows:

Definition 4. A fuzzy knowledge base (KB) consists of: is a fuzzy ABox, is a fuzzy TBox, and is a fuzzy RBox denoted by , where:

• A fuzzy ABox consists of a finite set of fuzzy concepts and fuzzy role assertion axioms of the form 〈x : C, α〉 and 〈(x, y):R, α〉, where α ∈ (0, 1]_D.
• A fuzzy TBox is a finite set of fuzzy General Concept Inclusion axioms (GCIs) 〈C ⊑ D, α〉, where C, D are concepts and α ∈ (0, 1]_D. Informally, it states that all instances of concept C are instances of concept D to degree α; that is, the subsumption degree between C and D is at least α. We write C = D as a shorthand of the two axioms 〈C ⊑ D, 1〉 and 〈D ⊑ C, 1〉.
• A fuzzy RBox is a finite set of role axioms of the form: (funR) a role R is functional, (transR) a role R is transitive, R₁ ⊑ R₂ role R₂ subsumes role R₁.

In the setting of an IoT system, the information commonly provided by an expert, such as fuzzy interval for sensor and actuator (much movement, low light, etc.) is described as an ABox. Whereas the information corresponding to rules (instructions) provided by the end user are codified in terms of TBox expressions.

We now introduce the semantic notions of the fuzzy description logic.

A fuzzy data type D = 〈Δ_D, ⋅_D〉 is such that ⋅_D assigns to every n-ary fuzzy relation over Δ_D. For instance, the predicate ≥₀ may be a crisp unary predicate over , denoting the set of reals smaller or equal to 0.

Definition 5. A fuzzy interpretation relative to a fuzzy data type theory D = (Δ_D, ⋅)_D consists of a nonempty set disjoint from Δ_D and of a fuzzy interpretation function that coincides with on every data value, data type, and fuzzy data type predicate, and it assigns:

• To each abstract concept C a function .
• To each abstract role R a function .
• To each abstract feature r a partial function such that for all there is a unique on which is defined.
• To each concrete role T a function
• To each concrete feature t a partial function such that for all there is a unique v ∈ Δ_D on which is defined.
• To each modifier m the modifier function f_m : [0, 1]_D → [0, 1]_D.
• To each abstract individual x an element in .
• To each concrete individual v an element in Δ_D.

Definition 6. The mapping is extended to roles and complex concepts as follows:

• ,
• 
• where ∘ = {C, G, L}
• where ∘ = {C, G, L}
• where * = {C, G, KD, L}
• .

Definition 7. The satisfaction of a fuzzy axiom E by a fuzzy interpretation , denoted , is defined as:

• if and only if .
• if and only if , .
• if and only if , .
• if and only if , .

The concept C is satisfiable if and only if there is an interpretation and an individual such that .

Let F be a set of axioms and E be a fuzzy axiom. We will say that satisfies F if and only if satisfies each axiom in F. is a model of E, if and only if, and is a model of F, if and only if, . is a model of KB, if and only if is a model of each component and , denoted by . An axiom E is a logical consequence of a knowledge base K, if and only if every model of K satisfies E, denoted by K ⊧ E. Finally, a fuzzy knowledge base K is consistent iff a model of K satisfies each axiom. This notion of consistency for fuzzy description logic knowledge base is equivalent to the notion of consistency of IoT systems. In order to formally prove this equivalence, we first define a translation function (⋅)* from IoT systems to knowledge bases.

Definition 8. Let (⋅)* : SystExp → FDL be a star-interpretation from SystExp(system expressions) to FDL (fuzzy description logic) defined as:

• (f_z(a) ∈ d)* = ∃ A.D
• (f_z(a) ∉ d)* = ¬(∃ A.D)
• (SystExp₁ ∧ SystExp₂)* = (SystExp₁)* ⊓ (SystExp₂)*
• (SystExp₁ ∨ SystExp₂)* = (SystExp₁)* ⊔ (SystExp₂)*
• (If SystExp then f_z(a) ∈ d)* = (SystExp)* → (∃ A.D)

Example. Note that IoT system rules 4 and 5 are translated as follows: This Knowledge Base is inconsistent since no model satisfies both axioms.

We now state the main formal result of the article: IoT system consistency can be tested in terms of Knowledge Base consistency. In practice, consistency is tested before the execution of the IoT system. The following Theorem provides a mathematical guarantee that the system is free of inconsistencies under any real-time scenario (any sensor inputs).

Theorem 1. An IoT System R₁, R₂, …, R_n is consistent, if and only if there is a Knowledge Base K, such that .

Proof.

“Only if” part. Suppose that the IoT system R₁ is consistent. Note that , then by applying the star-interpretation, we obtain

Claim: There is an interpretation and such that . Indeed, otherwise for each interpretation and such that . Without loss of generality suppose that for each j ∈ [1, n − 1], and , consequently SystExp_n = f_z(a) ∉ d. Then, given a Boolean structure B, we have that , thus the IoT system is not consistent, which is a contradiction. We conclude that there is a knowledge base K that satisfies .

Suppose that the IoT system R₁, R₂ is consistent, where R₁ is as in the previous step and R₂ = If R₁ then A.

Claim: K ⊧ A*. Indeed, otherwise for each interpretation and such that consequently, A = (f_z(a) ∉ d). Given a Boolean structure B, we have that ⟦R₂⟧^B = ⟦If R₁ then A⟧^B = 0, which is a contradiction. Therefore, the knowledge base K that satisfies . Following the previous construction, it is concluded that if R₁, …, R_n, R_n+1 is consistent, then K ⊧ R₁* ⊓ ⋯ ⊓ R_n* ⊓ R_n+1*.

“If” part. Suppose there is a knowledge base K that satisfies K ⊧ R₁* ⊓ R₂* ⊓ ⋯ ⊓ R_n* and that the IoT system R₁, R₂, …, R_n is not consistent. Since K ⊧ R₁* ⊓ R₂* ⊓ ⋯ ⊓ R_n* then every model and each verify that: .

So for each i ∈ [1, n], and when considering the inverse interpretation, we have that for each such that ⟦R_i⟧^B = 1, where B is any Boolean structure. Therefore, the IoT system R₁, R₂, …, R_n is consistent, which is a contradiction.

Once the IoT system consistency is verified in terms of Knowledge Base consistency, the system can be executed with the guarantee that no errors can be computed, no matter the inputs from sensors. Values for actuators can be computed from the instructions in the Knowledge Base by means of a defuzzification process. Defuzzification is the output value for the membership function m on the values of the variables in x using the specified defuzzification method. Some examples of defuzzification methods can be seen in the following definition.

Definition 9. Let B be a fuzzy set to be defuzzified, and let x be an arbitrary element of the universe. Then for all x:

• x_LOM is the largest of maxima (LOM), if and only if, μ_β(x_LOM) ≥ μ_β(x), and if μ_β(x_LOM) = μ_β(x) then x_LOM > x.
• x_SOM is the smallest of maxima (SOM), if and only if, μ_β(x_SOM) ≥ μ_β(x), and if μ_β(x_SOM) = μ_β(x) then x_SOM < x.
• x_MOM is the middle of maxima (MOM), if and only if, .

Consider for instance, in the following axiom: (∃move.SomeMove⊓∃light.LowLight⊓∃sound.FewSound) → ∃alarm.MediumAlarm. If input sensors for movement, light, and sound correspond to some, low, and few, respectively, then the alarm should be set to medium. The numerical value corresponding to medium is computed by the defuzzification process.

Fuzzy control for an IoT security system

Smart-home systems are challenging when implementing security systems since wireless sensor devices used in IoT can be heterogeneous and use various communication protocols having different coverage areas when detecting intruders or risk situations. Moreover, even when intelligent IoT devices can monitor their sensors to notify users about potential issues or risks in smart homes, most applications operate without knowledge-based consistency, provoking the system to take wrong actions or presenting failures since more than one rule can contradict each other. For instance, suppose a smart-security home application with sensors that measure light, movement, and sound, and a user is looking for security against an intruder. This system, guided by (a) blueprint, strategically places sensors in the (b) hall, (c) dining room, (d) library, and (e) living room (see small black sensors placed on the walls in Fig 3). The goal is to create a secure and comfortable (f) Home (refer to its configuration). A fuzzyDL reasoner is a system in which a user establishes the rules to take actions, for example, calling the police, sending a notification message, or turning on the alarm.

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Fig 3. Smart home (black dots are wall sensors).

https://doi.org/10.1371/journal.pone.0296655.g003

Let us assume that an expert in security systems has pre-programmed the IoT System with certain restrictions on the RBox, TBox, and Datatypes (Step 1). Since the user can define the TBox with basic rules according to their preferences (Step 2), the objective of the fuzzyDL tool is to verify that the TBox rules are consistent. At the same time, the actuators operate according to the rules established by the user (Step 3). This process involves checking the program syntax and conducting experiments (Program syntax, Experiments) to ensure the smooth running of the system.

Step 2. System rules

The system’s rules may be pre-programmed, but ideally, non-expert users are expected to define particular rules for the system. Considering the smart home security system, examples are: if movement, light, and sound are low, then do nothing; or if movement, light, and sound are high, then call the police. These instructions correspond to the TBox of the corresponding Fuzzy Description Logic Knowledge Base. A user-friendly interface is also considered for this step: voice or text instructions directly from the users, and information about the system (sensors, actuators, etc.) are depicted to help users define these instructions. At this step, a logic reasoner analyzes the instructions to detect inconsistencies: if the user provides an instruction contradicting an already loaded instruction, the interface provides a warning. In other words, the logic reasoner guarantees that the rules the user introduces are consistent under any system setting. On the other hand, the user’s role is to indicate the rules (the TBox) that satisfy an ideal security system. Furthermore, the consideration of system rules. The number of permutations determined by the labels of each sensor corresponds to the following arithmetic operation 3 × 4 × 4 (for the previous example). Therefore, the system rule-set is 48 rules. Let us assume that the user determined the following four rules:

1. R1. IF light IS low AND movement IS low AND sound IS low, THEN code AlertGreen.
2. R2. IF light IS low AND movement IS low AND sound IS middle, THEN code AlertYellow.
3. R3. IF light IS low AND movement IS low AND sound IS high, THEN code AlertOrange.
4. R4. IF light IS low AND movement IS low AND sound IS very-high, THEN code AlertRed.

The interfaces for step 1 (Contextual information) and step 2 (System rule) can be seen in Fig 4, with (a) representing the expert interface and (b) representing the user interface.

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Fig 4. Interfaces.

https://doi.org/10.1371/journal.pone.0296655.g004

Program syntax

The internal architecture of the code of our security system programmed in fuzzyDL reasoner is the following:

• System sensors. For each system variable (sensor), we define some specific characteristics that represent it. We also specify its range as a closed subset of the real ones [k₁,k₂], for example:
  (functional sensor-1)
  (functional sensor-2)
  (functional code)
  (range sensor-1 *real* k₁ k₂)
  (range sensor-2 *real* k₃ k₄)
  (range code *real* k₅ k₆)
  We define the linguistic labels to describe the value of these variables (using the triangular membership function, see Fig 5), such as:
  For sensor-1:
  (define-fuzzy-concept label1 triangular(k₁ k₂ q₁ q₂ q₃))
  (define-fuzzy-concept label2 triangular(k₁ k₂ q₄ q₅ q₆))
  For sensor-2:
  (define-fuzzy-concept label1 triangular(k₃ k₄ q₁ q₂ q₃))
  (define-fuzzy-concept label2 triangular(k₃ k₄ q₄ q₅ q₆))
• Actuator system. We define linguistic labels to describe the value of actuators.
  For actuator-1:
  (define-fuzzy-concept label1 triangular(k₁ k₂ q₁ q₂ q₃))
  For example, AlertGreen, the label representing that the actuator green is in order, can be defined as triangular (q₁,q₂,q₃).
  We represent the system input as fuzzy statements involving a single digest.
  (instance individual (= sensor-1 q′) α), where q′ ∈ [k₁, k₂]
  (instance individual (= sensor-2 q″) β) where q″ ∈ [k₁, k₂]
• System rules. The system defines each concept from the rules given by the user, for example:
  R1. (define-concept Rule1(g-and(some sensor-1 label1)(some sensor-2 label2)))
  R2. (define-concept Rule2(g-and(some sensor-1 label1)(some sensor-2 label2)))
  RuleMamd. (define-concept Mamd (g-or Rule1 Rule2))
• Defuzzification. The output of the system is resolved through the use of queries.
  (defuzzify-mom? Mamd individual sensor-3)
  (sat RuleMamd)

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Fig 5. Partitioning a domain using fuzzy membership functions.

https://doi.org/10.1371/journal.pone.0296655.g005

The complete internal algorithm of the security system is depicted in Fig 6.

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Fig 6. The internal architecture of a security system code programmed in fuzzyDL.

https://doi.org/10.1371/journal.pone.0296655.g006

Experiments

We conducted a comprehensive analysis of defuzzification and consistency within our security system. In Experiment one (see Table 1), we carried out the following tasks:

1. Introducing three distinct inputs for light, movement, and sound within the range defined by the expert. We then evaluated the defuzzification (MOM) and consistency (sat) queries. The outcomes aligned with our expectations.
2. For the fourth set of inputs for light, movement, and sound, we deliberately exceeded the predefined range, resulting in an inconsistency flagged by the sat query.
3. Lastly, we introduced an additional rule aimed at creating a contradiction within the system, which also resulted in inconsistency as indicated by the sat query.
4. Additionally, it is worth noting that the execution times in these experiments exhibited variability. This variability can be attributed to the specific configurations tested, particularly the introduction of contradictory rules that could either halt or significantly extend the program’s execution. These time discrepancies underscore the influence of experimental factors on the system’s performance.

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Table 1. Defuzzification and consistency: Experiment one.

https://doi.org/10.1371/journal.pone.0296655.t001

Finally, in Experiment two (see Table 2), we programmed the three rules outlined in the IoT system section. In the program syntax, R2, R4, and R6 correspond to Eqs 3, 4 and 5, respectively. Our findings are as follows:

1. The (sat) query for rules R2 and R4 yielded a consistent outcome.
2. However, the (sat) query involving rules R2, R4, and R6 yielded inconsistency, primarily due to a contradiction between R4 and R6.
3. Additionally, the execution times in Experiment two exhibited variation. Notably, The execution of rule R7 showed consistency within 3 seconds. However, when involving rules R8, the program’s execution time was extended to 10 minutes, reflecting inconsistency primarily caused by the interaction between rules R4 and R6.

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Table 2. Defuzzification and consistency: Experiment two.

https://doi.org/10.1371/journal.pone.0296655.t002

The initial and final times were programmed on a computer with the following specifications: Computer Model: [Compaq nc6400]. Processor: [Genuine Intel(R) CPU T2300 1.66Ghz]. RAM: [2.50 GB]. Operating System: [Windows 7 Home Premium].

Conclusion

The conclusions drawn from the analysis presented in the preceding sections fall into two main categories: Fuzzy Logic and IoT systems. First, we derived a theorem that establishes a relationship between the consistency of Fuzzy Logic and IoT systems, specifically in the context of the FuzzyDL reasoner. Second, we developed an algorithm for a security system that employs fuzzy logic as its primary language and identifies inconsistencies between rules. This system can find practical applications in smart homes, however, many other systems can be applied.

Our experiments with the fuzzyDL reasoner provide compelling evidence that the algorithm functions correctly according to the defined problem, even as more rules are added to the system. As an additional step towards the validation and applicability of our system, we plan to conduct evaluations in real-world scenarios. This will involve implementing our system in smart home environments and collecting real-world usage data. By doing so, we will be able to measure the performance and effectiveness of our system in real-world situations and fine-tune it as needed. This real-world evaluation will also allow us to gather feedback from end users, helping us further tailor the system to meet their needs and ensure practical utility. Additionally, we are open to collaborations with the research community and industry to test and validate our system in various applications and scenarios. These additional steps will strengthen our research’s practicality and real-world relevance while fostering collaboration and external validation.

In future work, we plan to explore how the methodology presented in this study can be applied to other IoT systems with varying logic types. Furthermore, we intend to enhance the user interface based on feedback from non-technical users and implement security measures to prevent unintended consequences resulting from user-defined rules.

The system is currently in a prototype stage. The main scalability challenge relies on the fuzzy logic reasoner tool. Other research perspectives include developing and optimizing reasoning algorithms for fuzzy description logic. Similarly, the accuracy of translating natural language instructions into fuzzy logic formulae depends on the accuracy of the corresponding NLP algorithms. We also want to develop NLP interfaces for the proposed frameworks using large language models.