Defining the Number of Mobile Robotic Systems Needed for Reconfiguration of Modular Manufacturing Systems via Simulation

Abstract

The European vision of the Factory of the Future is based on increasing competition and sustainability by transformation from cost orientation to high-adding value with technical and organisational innovations. One of the expected outcomes is an increase in modularisation, i.e., the reconfigurability of the technical system in manufacturing conditions. Modular manufacturing systems (MMS), will consist of modular platforms (MP) that are capable of rapid rebuilding, and reconfiguration performed by adding or removing a module by Mobile Robotic Systems (MRS). In the conditions of MMS, to make the most efficient use of reconfiguration MRS capacities, it is necessary to know the optimal ratio of these MRS to the number of modular platforms (MP) used in MMS, which does not exist today. This ratio will help industrial companies that are deploying MMS-based solutions to plan the number of MRSs needed to reconfigure deployed systems. As a method of determining this optimal ratio, an experimental approach via simulation was chosen, using data from custom MRS and MP prototypes with testing different layouts of modular platforms with the view of warehouse layout, manufacturing island, manufacturing island power supply, and MRS. Based on the results, it can be determined that the MP-MRS limit ratio is 2:1, where the further increase in MRS has only a minimal impact on the reconfiguration period. With the reduction of MP transferred to one MRS, there is a gradual decrease in the time required for reconfiguration. When the ratio of 1:1 is attained, the time required for reconfiguration lowers, but not as dramatically as in bigger ratios.

1. Introduction

The production environment as we know it is undergoing significant changes, with emphasis beginning to be placed on the ability to adapt quickly to change. Traditional manufacturing systems as we know them are being replaced by new ones that are significantly more autonomous and can better respond to the changes that are emerging around them. Development in the field of autonomy is made possible, in particular, by technological developments. Here we can see an exponential increase, particularly in control and communication technologies [1,2]. Traditional systems are still dominant in today’s industries. The development of new systems is influenced by technological developments, globalisation, and the efforts of manufacturers to cope to the maximum extent possible with changes in customer demand through the adaptability of the system [3]. It is possible to approach the adaptability requirement in several ways, which is why scientists have developed and tested a whole range of new production concepts, such as reconfigurable manufacturing systems, manufacturing competency islands, and multi-agent control systems [4]. The characteristics to which these new systems will have access may include the modularity of the system [5] or the high interconnection with the virtual system environment [6]. This modularity will be supported by the development in the field of modular self-reconfigurable robotic systems that will allow the connection and disconnection of modules [7,8]. The modularity of the system will allow a quick change of capacity configuration for the production of the chosen family of products. New production systems should therefore be designed as small, highly flexible production units that can be deployed where there is sufficient real demand [9,10]. Such production systems will be designed for the production of the selected product family, which requires their concept to be built on the principles of reconfigurable manufacturing systems. From a performance standpoint, this style of manufacturing will benefit craft and small-scale production, but it will not exclude its applicability for greater volume production [11]. The applicability of such systems has certain limits, in particular the size of the product as well as its weight [12]. Modular manufacturing systems are not effective for use in mass production. The issue of quality is no less important in the area of new systems, which include reconfigurable manufacturing systems. The Zero-Defect Manufacturing (ZDM) concept also has its place to ensure the sustainability of this type of production [13]. ZDM offers a holistic approach, aiming at greater manufacturing sustainability, which ensures both process and product quality by reducing product defects using corrective, preventive, and predictive techniques made possible by data-based technologies, and guarantees that no defective products leave the production site, and reach the customer [14]. For these manufacturing systems, new logistics systems were developed that offer the characteristics required by these systems [15,16,17]. Such characteristics may include transporting the required quantity as efficiently as possible, which necessitates a high degree of flexibility on the part of these means [18]. The design of the Automated Guided Vehicles (AGV) logistics system assumes that basic operational parameters and factors need to be addressed [19]. The authors [20] point to the impact of the use of modularity in the design of the AGV system. Several authors point out the various problems with AGV design. These design problems are to be solved by the Intelligent Logistics Systems (ILS) of the new generation. The primary goal of the next generation of ILS is to solve the flaws of existing logistics systems by enhancing autonomy, flexibility, fault resistance, and reconfiguration capabilities [21]. The ILS system consists of distributed entities that break down complex problems into several minor problems, such as material security, storage, packaging, handling, and transportation, each of which belongs to one or more parts of the intelligent system—control units [22,23,24]. Each of these units has its own knowledge, capabilities, and objectives and can be bound to a physical logistics entity. Global logistical decisions are made through communication, coordination, cooperation, and negotiation between several management units [25]. The new generation of ILS systems no longer only serves to move materials and products. It can also be used to move the manufacturing facilities themselves. The possibility of moving appropriately designed manufacturing facilities brings with it the possibility to quickly respond to current market requirements and, in a short time, change the layout of the manufacturing line so that it can produce the desired product. Many of the newly developed manufacturing approaches are based on Reconfigurable Manufacturing Systems (RMS) because they enable a rapid response to market change as well as direct customers. With reconfigurable manufacturing systems, two types of systems can be encountered: dynamic and static reconfigurable manufacturing systems. Both are expected to use a modular composition system [26,27]. In dynamic and static RMS, their composition and configuration require systems capable of changing their structure. It also depends on how they are supplied and powered. Systems based on RMS will have active communication between all members of that system while actively using the Internet of Things (IoT) [28]. The newly developed Mobile Robotic Systems (MRS) and Mobile Automated Platform (MAP) will be capable of communicating, negotiating, and synchronisation so that the requirements imposed by Factory of the Future reconfiguration systems are met [29,30]. We can expect that in the near future, the number of MRS deployed in manufacturing will increase and the demands on their tasks will become more complex [31]. Flexible assembly lines necessitate versatile robots with a wide range of capabilities [32]. For modular manufacturing systems, it will be necessary to set the MRS ratio given the tasks carried out by these robots, including the reconfiguration of modular platforms from which the modular manufacturing system will be composed.

From literature sources that are focused on modular manufacturing systems, we can identify that none of the sources take into account how much MRS, is needed for performing various tasks of reconfiguration. The deployment of MRS itself has its limitations in terms of the smooth running of manufacturing. The condition of effective reconfigurability in modular manufacturing systems is the requirement to minimise manufacturing made effort and maximise time reduction needed for the realisation of changes [33]. Their ratio must not be too high or too low, because each of these extremes will lead to unsatisfactory results. Modular manufacturing systems in the Factory of the Future will be composed of modular platforms (MPs), which must be reconfigured to a certain configuration. To reconfigure them, MMS will be used, which will work within intelligent manufacturing systems. It is critical to understand the ratio between reconfigured MP and reconfigured MRS in order to maximise MRS capacity utilisation while minimising investment expenditures. It is critical to understand the ratio between reconfigured MP and reconfigured MRS in order to maximise MRS capacity utilisation while minimising investment expenditures.

The subject of the paper is a description of the results of experiments conducted to determine this ratio of MP to MRS. The novelty of the article consists in determining this ratio, which has not yet been analysed in the literature and defined from the perspective of MMS. Time characteristics for conducting experiments are carried out on their MRS and MP prototypes with and without superstructure. This prototype is designed specifically for systems that can respond quickly to changing market requirements by changing the distribution of manufacturing lines. The characteristics of the prototype have been used as inputs for simulations that tested different MP layouts, like MP warehouse layouts, manufacturing island layouts, manufacturing islands, and MRS powering. Based on experiments, considering different layouts, it is found that the number of MRS needed to reconfigure a system composed of modular platforms is 1:2.

2. Materials and Methods

The first part of the chapter is devoted to the description of mobile robotic systems and modular platforms. The second part consists of research in which the acquisition of MRS time characteristics for further experimentation is described. This part also contains the definition and description of different layouts of a modular system that are taken from different perspectives of warehouse layout, manufacturing island, manufacturing island power supply, and MRS.

2.1. Mobile Robotics System

Mobile Robotic System (MRS) is most often used to handle loads in manufacturing or warehouses to locate an MRS prototype using colour tape navigation. The MRS prototype consists of several parts, such as mechanical construction, power supply system, control system, location, and guidance system, communication system, sensory system, safety system, and drive system. The MRS is equipped with programmed software to choose a destination, choose the most suitable route, and avoid collisions. The MRS is computer-controlled with its computing unit, which communicates with the central control system. The shortcomings of the existing MRS system is the need to design a different solution system that can cooperate with modular platforms and that would eliminate the disadvantages of existing systems regarding their mutual mechanical-electrical connection, mutual accessibility, centering, and dynamics.

In MRS that are used for MMS, the control unit of the MRS must be able to respond appropriately to sudden changes in the environment as well as respond appropriately in the event of an unexpected event (for example, an impassable route or equipment failure) so that no damage is caused. The management of the intelligent system is divided into two main levels. A higher level of control takes place in the central control unit, the computer, and is based on multiagent control. The lower level of management takes place in individual elements of the intelligent logistics system and is subordinated to the central control. See Figure 1.

The MRS must be able to communicate continuously not only with the control system, but also with other MMS components such as other MRS, modular platforms, and their superstructures, thanks to the communication unit.

In systems such as MMS, the mobile robotic system constantly monitors its surroundings and inner state. In the event of an unexpected obstacle on the route or in the event of a malfunction of the MRS, it immediately stops the task being performed and sends detailed information about the current location and the reason for the stop to the control system. Subsequently, the MRS is waiting for further instructions from the control system. The MRS is also able to respond to the change in the task performed, when it completes the just performed action after receiving the new task, sends the location information to the control system, and then waits for further instructions.

During the task, the MRS provides information about crossing to the control system, allowing it to govern the job and gain an overview of all system pieces and their current position in space.

2.2. Modular Platform

The modular platform consists of individual modules from which the desired manufacturing island is assembled. The modular platform must be able to perform several tasks:

• hosting of the superstructure—the modular platform must be sufficiently rigid to accommodate the necessary superstructure
• mechanical fixation—the modular platform can firmly connect to other modular platforms and thus create a manufacturing island
• power transmission—connectors that serve as mechanical fixation also transmit electricity, which is then used to power the superstructure
• connectivity with the Mobile Robotic System—the modular platform can connect to the Mobile Robotic System using an appropriate type of connector
• Communication—part of the modular platform is a communication module that is used to communicate with the Mobile Robotic System and control system. The platform must also be able to communicate with its superstructure

Superstructure

The superstructure is a solid part of the modular platform that uses sensors to sense its current state and ambient state and can use actuators to influence the environment in which it is located. The superstructure offers the product the services it needs. Platform algorithms and their controls being implemented can be divided into several basic categories:

• The pallet provides logistical service for an intelligent product (semi-product) and materials for a manufacturing modular line, and it returns to storage after use. If the material resources that are being carried are used up, the pallet requests logistical service from MRS, which then takes it to the storage area. There, it refills and puts the pallet back in its place on the line.
• A modular conveyor—consists of several conveyor belt parts. Each of these can be controlled separately. It provides transport for the intelligent products and materials on the modular line.
• CNC milling machine—offers operations for grinding, drilling, cutting, and machining of intelligent products and materials.
• The Delta robot—helps to sort out and arrange the material.
• A robotic arm—is controlled by inverse kinematics and provides transport of the intelligent product as well as other services based on its defined task: loader loads a product or material on the manufacturing line; a CNC operator provides transport into a machine tool and back to a conveyor; a screwdriver, welder, or assembler; an unloader unloads an intelligent product from a conveyor to a pallet.

A robotic arm is able to change its current tool (electrode, gripper, screwdriver, suction gripper). The superstructure can communicate with the control system through the communication module of the modular platform. The selected superstructures must meet the requirements for the accuracy of the manufacturing and assembly specified by the product and be capable of at least basic self-diagnosing. Superstructures may be constructed in such a way that they can perform several of the above services within a single superstructure.

2.3. Experimental Time Characteristics Prototypes

For the realisation of experiments in order to obtain reliable results for simulation and final statement, tests with their own prototype MRS and MP were performed to obtain time characteristics. Since the main task of the MRS is to move modular platforms, it is necessary to adapt it for this task. This is achieved by designing a suitable connector, through which the modular platform is grasped. Figure 2 depicts the interconnection of a custom Mobile Robotic System and an empty modular platform. The resulting connector solution for the MRS prototype consists of four folding parts. In the disconnected state, these parts of the connector are folded down. This prevents a collision between the MRS and the modular platform when the MRS enters the platform. In the connected state, parts of the connector are erected, ensuring the grip of the modular platform. Since the parts of the connector are made up of conductive material, they also provide electricity transmission to the platform. A single servo motor lifts all four pieces of the connection, reducing the mechanism’s energy demands as well as the MRS’s space requirements. To locate a prototype of a MRS, navigation using colour tape is used. In the test area, a square network of possible routes is created, consisting of dark tape glued to a light base for high contrast. At the bottom of the MRS are located a series of infrared diodes and photodiodes. Because the light base reflects more infrared light than dark tape, it is easy to tell which of a set of sensors the tape is now positioned beneath.

Start-up and running characteristics were measured when the Mobile Robotic System itself was moving, in the combination of a Mobile Robotic System and an empty modular platform, and using different loads of the modular platform to simulate different types of superstructures. All characteristics were measured for all kinds of movements within the engine power range, from 40 to 100%. The following

Table 1. Duration and types of movements.

	No Modular Platform [s]	With Modular Platform [s]
Start-up of MRS from static position	3	3.12
Uniform movement of MRS	2.8	2.8
Slowing down and stopping MRS at a crossroads	3.1	3.18
MRS rotation by 90°	1.4	1.42
MRS rotation by 180°	2.78	2.81

results were obtained from the measurements.

Furthermore, the duration of the basic actions: was reduced by 1.2 s, releasing the platform takes 1.22 s, and linking platforms takes 2 s.

By setting different engine outputs for different modes of modular robotic systems, the characteristics of the operation of a separate Mobile Robotic System and for the operation of a Mobile Robotic System with different types of modular superstructure platforms were achieved.

For separate movement of the modular robotic system without a platform, the power of the engines was used by 50%. For different types of platforms, engine power ranged from 55 to 70%. Since the increase in the weight of the superstructure increases the kinetic energy of the entire device in motion, it was necessary to use the electric brake movement mode to successfully stop at the desired point. This allowed for an accurate stop of the Mobile Robotic System at the crossroads.

2.4. Description of Manufacturing Systems Interconnection Scheme

To determine the modular platform—Mobile Robotic System (MP-MRS) ratio, several layouts of the work schemes of modular manufacturing systems were tested. The determination of the optimal ratio is generalised to this background, and therefore, several variations have been tested from the point of view of the MP warehouse layout, the distribution of manufacturing islands, as well as the MP and MRS charging. The following symbols used in the figures:

Table 2. Symbols and their interpretations.

Symbol	Interpretation	Symbol	Interpretation
	A mobile robotic system		Conveyor-equipped modular platform
	modular platform—with pallet		modular platform—with battery
	modular platform—empty		modular platform—with traction line
	modular platform with robotic arm superstructure—type 1		CNC—type 1
	modular platform with robotic arm superstructure—type 2		CNC—type 2
	modular platform with robotic arm superstructure—type 3		CNC—type 3
	modular platform -with superstructure combination of robotic arm with conveyor		fixed obstacle in the space with the possibility of connecting the platforms to the power supply

.

2.4.1. Modular Platform Warehouse Layout

When developing a modular platform warehouse layout, there are four key factors to consider.

Single-row random layout of modular platforms—this layout consists of placing unused modular platforms in one row at the edge of the manufacturing space (see Figure 3).

The main advantage of this solution is that it allows easy access of the Mobile Robotic System to each of the available mobile platforms. However, with a larger number of unused platforms, there is an increased demand for dedicated space for modular platform storage since platforms can only be placed at the edges of the manufacturing space. If unused platforms are positioned other than on the edge, the time required to reconfigure the production line may be lengthened owing to the necessity to avoid idle platforms located in space.

Multi-row random layout of modular platforms—unlike the previous design, in this case, it is possible to store idle modular platforms in multiple series (see Figure 4).

The advantage of this solution is that it allows you to place the unused platform in the nearest free space in the storage space. This technique also allows for the concentration of all idle modular platforms in a dedicated location, eliminating the need to deploy them solely around the perimeter of the manufacturing area.

In the event that only modular platforms from the first row of the warehouse are used to create a new manufacturing island, this method of positioning platforms shows the same results as the previous single-row solution. If platforms from different ranks are required, however, the time required to release the platform from storage rises dramatically owing to the need to shift platforms in front of the desired platform and then return the relocated platforms to the storage area.

Multi-row random layout of platforms with access corridors—this warehouse space solution means that every second position is occupied, and between the occupied positions, there is a space, which helps to access the platforms of the Mobile Robotic System in all the series (see Figure 5). Such storage will allow, in certain cases, the reduction in the time taken to remove the modular platform with the superstructure from stock and thus also a reduction in the total time needed for reconfiguration.

However, as compared to the multi-row random distribution of modular platforms, the creation of corridors increases the demands on the dimensions of dedicated storage space.

Multi-row platform layout by platform type—this solution consists of placing the same type of platform in one series, see Figure 6. With such a layout, direct access to any type of modular platform is possible without the need to move other platforms. The layout also eliminates the need for access corridors, thus reducing the spatial demands on the storage space. This solution is especially beneficial for fewer modular platforms. However, as the number of distinct specialised modular platforms grows, the dedicated storage space also grows, resulting in wasteful utilisation.

The combined modular platform layout solution—this case is a combination of multi-row platform layout by platform type and multi-row random distribution of platforms with access corridors (see Figure 7).

The solution combines the benefits of these two solutions when a certain number of types of modular platforms are placed in the warehouse in a multi-row layout by platform type and unique modular platforms are located in a dedicated section, separated by a created corridor to facilitate Mobile Robotic System access.

2.4.2. Optimal Manufacturing Island Layout

In terms of the layout of the manufacturing island and access to individual modular platforms, we can determine two basic types of manufacturing islands.

This manufacturing island with direct access to modular platforms allows instant access to any of the modular platforms. See Figure 8.

The downside of this technique is that it requires more space to establish a manufacturing island. However, if one of the platforms fails, it can be rapidly replaced by another without disrupting other modular platforms.

Manufacturing island without direct access to modular platforms—this arrangement of modular platforms does not allow immediate access to all modular platforms. See Figure 9. Modular platforms around the perimeter of the manufacturing island are directly accessible, but in case of failure on modular platforms in the centre of the manufacturing island, it is necessary to disconnect and move several platforms without breakdowns, thus extending the service time.

2.4.3. Method of Supplying Power to the Manufacturing Island

When developing a reconfigurable manufacturing system, three methods of powering the production islands are examined.

Specialised modular platforms with high capacity battery—used to power the manufacturing island. See Figure 10.

With this type of power supply, all manufacturing island platforms are powered by battery platforms. This solution does not require any further manufacturing area changes and does not restrict the manufacturing island to a specific position inside the manufacturing area.

The disadvantage of this solution is the need to replace the battery platform after a certain period of operation due to the discharge of batteries. Since batteries have a certain service life, their frequent charging reduces capacity and thus the need for more frequent replacement of the modular platform. The batteries must be changed if the required number of charging/discharging cycles is surpassed, which raises the cost of operating a reconfigurable manufacturing system.

Power supply using platforms connected to the traction line—power supply of the manufacturing island is used by specialised modular platforms that can connect using the pantograph to the traction line. See Figure 11.

With this solution, the battery platform is replaced by a platform with a current collector. With the aid of this pantograph, it is possible to supply the production island with mains voltage, eliminating the need to change the battery platform regularly.

With this method of power supply, it is necessary to build a traction line in the manufacturing area. The dense network of traction lines above each of the possible points of location of the modular platform does not limit the deployment of manufacturing islands in space, but the initial cost of its creation and maintenance costs are higher than when creating a lower density traction network. However, because the power supply platform must be within range of the traction line, the dispersion of the manufacturing island is constrained by the lower density traction network.

Power supply using fixed power stations—power stations can be formed by fixed modular platforms connected to the electrical network in the manufacturing area or support columns can be used in the area of the manufacturing hall, see Figure 12.

When compared to prior techniques, this option for powering the manufacturing islands saves both initial and ongoing expenses, but it also has drawbacks.

The main disadvantage of this solution is the need to adapt the layout of the manufacturing island to fixed power stations. These power stations also create obstacles to the possible pathways of Mobile Robotic Systems and can increase the time needed to reconfigure the manufacturing system.

2.4.4. Method of Charging Mobile Robotic Systems

Due to the dispersion of production areas, we may evaluate two options for charging station placement.

Separate charging stations—there is a dedicated space for charging Mobile Robotic Systems in the manufacturing area. These charging positions are only accessible for Mobile Robotic Systems without a modular platform attached.

The advantage of this solution is the possibility of sizing the charging station precisely for the needs of one Mobile Robotic System. The disadvantage of the solution is the need to reserve a special space for charging and a limited number of simultaneously charged Mobile Robotic Systems with the number of charging stations shown in Figure 13. The yellow lines represent that this are two separate stations.

The use of modular platforms connected to fixed power stations—in this case, it is possible to charge the Mobile Robotic System by connecting to the powered modular platforms. See Figure 14.

Because the Mobile Robotic System may take energy from any powered modular platform, the number of charging sites is only limited by the number of such platforms.

The disadvantage of this solution is the need to dimension power stations also for the charging function, which increases the initial cost of creating a system.

3. Results

To verify the functionality of the proposed properties and to obtain input data for simulation, it was necessary to construct prototypes of selected system elements. Based on the measured parameters of the prototypes, it was possible to perform a simulation of a system with a larger number of elements over a longer period. Tests were carried out at different layouts of the MP warehouse, manufacturing islands and power supplies for manufacturing islands, and MRS. The efficiency and applicability of a certain scheme of operation of the reconfigurable manufacturing system were also validated during the testing.

3.1. Layout of the Modular Platform Warehouse

The results of the simulation point to a significant impact on the warehouse layout of modular platforms. The type of production hall in which the intelligent logistics system will be implemented determines the best configuration.

A single-row random distribution of modular platforms (SRRDMP) is a good idea to use for a smaller number of idle platforms. In this warehouse solution, it does not depend on the number of unique platforms, but only on the total number of platforms that must be stored fundamentally only at the edges of the manufacturing space in one row. Due to the layout of the warehouse in a wide space, this type has a slight impact on the reconfiguration time due to the need to move the Mobile Robotic System throughout the space, and in the case of a greater distance, there is an increase in the necessary time to move the modular platform.

The multi-row random layout of modular platforms (MRRLMP) reduces the demands on the available space by allowing for higher density storage of modular platforms in one location. In this layout, the system performance also does not depend on the number of unique modular platforms. However, this layout has a significant impact on the reconfiguration time when the pallets being moved are blocked by other stored pallets, as the space in front of them must be freed up before the desired pallet is moved. This approach is appropriate for confined locations where frequent reconfiguration and a longer start-up time are not vital to production.

The multi-row random layout of modular platforms with access corridors (MRRLMPWAC) allows for the concentration of idle modular platforms in one place, but by creating access corridors, the space for platform storage is doubled compared to the previous method. However, this warehouse solution allows instant access to all platforms, so the impact on reconfiguration time is low. The number of various modular platforms has no bearing on the correct operation of this layout.

The multi-row platform layout by platform type (MRPLPT) provides direct access to each type of modular platform. The demands on storage space are growing with the number of unique types of platforms. This type is suitable for a low-to-medium number of unique types of modular platforms. Because of the fast access to platforms, the layout has a negligible impact on reconfiguration time.

The combined modular platform layout solution (CMPLS) is suitable for operations that contain a large number of unified modular platforms but also contain an increased number of unique modular platforms. Unified platforms are sorted by type to allow access to each of the types, and unique platforms are stored in the warehouse so that direct access to them is created by the access corridor. The effect of a modular platform warehouse layout is shown in

Table 3. The effect of a modular platform warehouse layout.

How to Distribute a Platform Warehouse	Space Requirements	Number of Platforms	Impact of Reconfiguration Time
SRRDMP	High	Any	Moderate
MRRLMP	Low	Any	Tall
MRRLMPWAC	Medium	Any	Low
MRPLPT	Low–Medium	Medium	Low
CMPLS	Low	Medium–High	Low

.

The results of the simulation and subsequent comparison show that the choice of the type of modular platform warehouse depends on the spatial possibilities of creating a modular platform warehouse, the number of platform types, and the total number of platforms. The layout of the warehouse mainly affects the time it takes to reconfigure. The ideal solution appears to be the creation of a simulation with the conditions of a particular manufacturing hall, but, if necessary, it is also possible to choose the layout of the warehouse of modular platforms based on the above-mentioned characteristics of individual layouts.

3.2. Manufacturing Island Distribution

The configuration of the production island is determined by the complexity of the goods, the available space, and the failure rate of the various modular platforms, as shown in

Table 4. The impact of the distribution of the manufacturing island.

Direct Access to Platforms	Space Requirements	Complexity of the Manufacturing Island	Service Time
Yes	Medium–High	Low–Medium	Low
Not	Low	Medium–High	Medium–High

.

A manufacturing island with direct access to modular platforms has higher space requirements as it allows the creation of a maximum of two series of modular platforms. If the product requires many operations to be made, the size of the island will be increased. However, because all platforms are accessible, the problematic platform may be rapidly replaced, resulting in minimum disruption.

A manufacturing island without direct access to modular platforms can be sorted more compactly and thus reduce spatial requirements. In this way, it is also possible to increase the complexity of the manufacturing island while maintaining smaller dimensions and achieving more compact manufacturing islands. However, in the event of a failure of the modular platform, which is in the middle of the manufacturing island, the time of its replacement is increased. This configuration is appropriate for the production of more complicated items when modular platforms with a low failure rate are employed.

3.3. Power Supply to the Manufacturing Island

The way manufacturing islands are powered has a significant impact on costs as well as on restrictions on the distribution of the manufacturing island. The impact of how the manufacturing island is powered is illustrated in

Table 5. The impact of how the manufacturing island is powered.

Power Method	Input Costs	Maintenance Costs	Constraints
Battery	High	High	Slightly
Traction line	Medium	Low	Minimally
Power stations	Low	Low	High

.

Battery power comes with high input costs due to the purchase of a large number of batteries and charging stations. Due to the limited battery life, replacement is required after a certain number of charging and discharging cycles, which increases maintenance costs. This method of supply limits the possible distribution of the manufacturing island only minimally. This is because, in the event of the need for continuous operation, it is necessary to allocate a place for two battery modular platforms on the manufacturing island. In this manner, the replacement of drained battery platforms may be planned such that the production island is continually powered by at least one battery platform.

The input costs of traction lines consist of the creation of a traction line network. Their expenses/costs depend on the density of the traction line. High density will increase input costs but minimise restrictions on the location of the power platform. Lower traction line density allows the power platform to be placed only at certain points in space. Maintenance expenditures are incurred as a result of frequent inspections of the condition of the traction line and the upkeep of points of contact between the traction line and the power modular platform.

Creating fixed power stations is the most affordable. However, because the production islands must always be in close communication with the supply stations, this arrangement severely limits their ability to be dismantled.

3.4. MRS Charging

The charging technique used for MRS impacts the amount of charging area required, the cost of building charging boxes, and the maximum number of MRS that can be charged at the same time.

Table 6. Impact of MRS charging stations.

Charging Method	Space Requirements	Costs	Number of MRS Charges
Charging stations	Higher	Low	Limited by number of stations
Modular platforms	Minimal	Higher	Limited by the number of modular platforms

.

When creating separate charging stations, it is necessary to allocate space for charging and create charging stations with the possibility of automatic MRS interconnection. However, the number of MRS charged at the same time is limited by the number of charging stations.

When charging using modular platforms, the initial costs are increased, as it is necessary to dimension the power stations not only for powering modular platforms and their superstructures but also for charging MRS. In this solution, modular platforms must be equipped with a separate circuit designed for the distribution of charging voltage and current for MRS, in addition to the circuit for the distribution of supply voltage and other connected platforms. However, the maximum number of MRS currently charged is limited in this case only by the number of modular platforms and the current throughput of the distribution circuit for MRS charging.

3.5. The Required Number of Mobile Robotic Systems

The simulation results reveal that the ratio of transferable modular platforms (N_MP) to available mobile robotic systems (N_MRS) affects the time it takes to reconfigure the manufacturing line.

As the number of modular platforms transferred to one Mobile Robotic System is reduced, the time required for reconfiguration is also gradually reduced. The curve of the graph is almost linear to the point where each of the Mobile Robotic Systems must move two mobile platforms. Time is fixed after this point, and its further reduction occurs only with the use of the same number of modular platforms and Mobile Robotic Systems (see Figure 15).

From a given simulation, it appears as if there is a boundary N_MP:N_MRS ratio of 2:1, i.e., each Mobile Robotic System moves exactly two modular platforms when reconfigured. The subsequent increase in the number of Mobile Robotic Systems has only a minimal impact on the reconfiguration period. When the ratio of 1:1 is reached, the necessary time for reconfiguration decreases, but this decrease is not as pronounced as in larger ratios. The main reasons for this less significant decrease in time are the busyness of routes by Mobile Robotic Systems, their mutual blocking, and the emergence of downtime in grabbing modular platforms from the warehouse and placing them at the target position.

For individual iterations, the initial state of the warehouse layout and the required final distribution of platforms on the production line were the same. For each round of simulations, there were several simulation runs where the resulting evaluations were then averaged. Only the number of MRS available varied in the simulation.

3.6. The Effect of the Manufacturing Island’s Size on the Time Required to Reconfigure It

The simulation of the impact of the size of the manufacturing island on the time needed for its reconfiguration was carried out with a fixed number of 2 Mobile Robotic Systems and a changed number of mobile modular platforms (N_MP). From the resulting reconfiguration time, depending on the number of modular platforms, it is clear that by increasing the number of platforms being moved, there is a non-linear increase in the necessary time. See Figure 16. The reason for this non-linearity is the requirement to create a manufacturing island in a specific order, which increases the time required to select individual modular platforms from the modular platform warehouse, as well as to influence the Mobile Robotic System’s route selection due to the distribution of already interconnected parts of the manufacturing island.

For each iteration, the initial state of the warehouse layout and the number of MRS used were the same. During the simulation, the number of MP moved was changed. For each round of simulations, there were several simulation runs where the resulting evaluations were then averaged.

4. Discussion and Conclusions

The results of the simulation show that the ideal ratio of the number of modular platforms and Mobile Robotic Systems transferred in a modular manufacturing system is 2:1. When reducing this ratio, there is no significant decrease in the total time of reconfiguration of the manufacturing line. When the 1:1 ratio is reached, there is a slight decrease in time, but this reduction is not significant due to the mutual blocking of reserved routes by several moving Mobile Robotic Systems. The defined ratio will assist undertakings considering the deployment of modular manufacturing systems for the production of their products at the planning stage in determining a sufficient number of reconfigured MRS since this ratio has not yet been described in the literature. The defined ratio will help enterprises that are deploying modular manufacturing systems for the production of their products, especially in the planning phase, to determine a sufficient number of reconfiguration MRS, as this ratio has not yet been described in the literature. The current state of the intelligent logistics system prototype provides enough data to verify its functionality and advantages over traditional logistics systems. However, because of its small size and absence of realistic superstructures, accurate results in the real system cannot be obtained and compared to simulation findings.

The limitations of the modular manufacturing systems use are mainly that the system is designed as a priority for craft production and small-batch production of one family of products. Thus, it is the production of products that have many similarities in the production process and many identical parts. The main advantage of such a system is the personalization of the product, tailored to the customer. The main limits of the modular manufacturing system include the dimensions of the modular platform, which do not allow the processing of very large products. Therefore it is especially suitable for products or semi-finished products of smaller sizes. Also, the dimensions of the platforms do not allow the use of larger Computer Numerical Control (CNC) machining centres. However, these CNC can be placed in a stable place where transport between the modular manufacturing system and the fixed CNC would be provided by MRS. Fixed CNC will produce semi-finished products for later assembly and processing on the production line. There will be communication between the CNC and the production line control system. For example, the production line control system sends a request to produce a specific type of blank. After receiving the order and after the end of production, the CNC sends the control line information about the possibility of picking up the desired part. In the area of such systems, the long-term sustainability of quality will undoubtedly be important. It will be necessary to ensure that each part produced by any system configuration meets the specifications. This will be done by Zero Defect Manufacturing, whose future use in such systems has been described in [13].

The current situation offers several directions for further research. The main priority is the creation of a real prototype at a scale of 1:1, on which it will be possible to find out real data from the operation of such a system. Another option for research is to change the way we locate and navigate. The current prototype uses colour tape for guidance. The use of other more advanced technologies was not possible due to the dimensions of the prototype and the performance of its control unit. Therefore, in future research, it is advisable to focus on the development of more advanced navigation using laser or visual navigation. This type of navigation eliminates the need to modify the environment by creating a square network of routes and increases the possibility of Mobile Robotic System movement. By adding visual navigation, it is also possible to accurately recognise obstacles in the route of a Mobile Robotic System and determine whether they are moving or static parts of the system (another Mobile Robotic System, modular platform) or an unexpected obstacle that is not part of the system, such as the presence of a person in the manufacturing space or a part of the system with a malfunction that makes it impossible to communicate between the object and the control system.

From the measurements made on the created prototypes, the time characteristics of individual movements and actions were obtained, which were subsequently used in the simulation of an intelligent logistics system with a larger number of components. The simulation focused mainly on the necessary number of mobile robotic platforms and the impact of the layout of the created manufacturing island and the warehouse of unused modular platforms for the necessary time to reconfigure the manufacturing line. The simulation also included the issue of available solutions for powering the created manufacturing islands and deploying charging stations for Mobile Robotic Systems.

The simulation results show that a combined solution for modular platform distribution outperforms other solutions, which combine multi-row platform layout by platform type and multi-row random distribution of platforms with access corridors, where the best ratio of warehouse dimensions and time required to pick up a modular platform occurs.

The results of the manufacturing island layout simulation with direct access to modular platforms and no direct access to modular platforms are almost identical at low platform failure rates. More significant differences between the two solutions occur only when the failure rate of modular platforms increases and there is a significant increase in the time needed to replace the faulty platform in the layout of the manufacturing island without direct access to the platforms.

Each of the simulated solutions for powering the manufacturing island and charging Mobile Robotic Systems has its advantages and disadvantages, and therefore it is necessary to choose specific solutions in the context of the used manufacturing space, its layout, size, and possible fixed obstacles that are part of the construction of the building itself.

5. Patents

Protection of parts of the solution by utility model No. 8443—Connector system of connecting/disconnecting Mobile Robotic System and modular platforms; and 8444 Mobile robotic configuration and robotic dynamic configuration of the working space.