1. Introduction
The improvement of auxiliary transport efficiency is an important aspect of increasing labor productivity in underground coal mines. Suspended monorails, used for the transportation of materials and the movement of miners to and from the mining face area, are the main transportation means. Increase in the distance to mining panels from the shafts extends the time necessary to cover this distance, with a set speed limit, within which the suspended monorail can move. The speed limit for transportation is defined by the national regulations of each country. In Poland, the maximum speed of people movement is 2 m/s, while the brakes operate automatically after exceeding the permissible speed by 50%, but not more than by 1 m/s [
1]. This provision in the Polish regulations enables setting the trigger of the emergency braking trolley to 3 m/s. A similar limit for the permissible speed of a suspended monorail was adopted in Slovenia. In turn, in Germany, the maximum permissible speed is 3 m/s. In accordance with regulations in Ukraine, the speed of transportation is determined on the basis of the railway’s technical and operational documentation and the manufacturer’s specifications, and the maximum permissible speed, equal to 1 m/s, applies to transportation of long and large-size loads. In the case of countries outside the European Union, Polish producers usually apply Polish regulations regarding the admission of suspended monorails to traffic, which are accepted in countries such as China, Mexico, Russia, and Vietnam. Increasing the speed limit would significantly shorten the miners’ travel time to their workplace, and thus extend the effective working time. Such an approach is economically justified; however, attention should be paid to the safety of the suspended monorail operator and the transported miners [
2]. Currently, the suspended monorail sets used to transport the crew are not equipped with elements protecting people against injuries (e.g., seat belts, energy absorbing linings, and airbags) during emergency situations, such as emergency braking of the suspended monorail. During emergency braking, the braking systems, appropriately selected at the configuring stage of the transportation set, are activated [
3]. This usually results in the stopping of the entire suspended monorail set on a short distance, which does not exceed several meters. In such cases, significant deceleration occurs, which causes dynamic overloads, affecting both people [
4,
5,
6,
7,
8,
9] and the entire suspended monorail set and its route [
4,
5,
6,
7]. Pursuant to the provisions of Polish law, the activation of the brake trolley should ensure a braking deceleration of not less than 1 m/s
2 and not more than 10 m/s
2. A similar value of the maximum allowable braking deceleration for the operator and the transported crew, 9.81 m/s
2, is in force in the Slovenian regulations. These overloads can cause an injury if a miner hits the passenger cabin structure or in the case of failure, e.g., resulting from damage to the railway route or breaking one of its slings [
10,
11,
12].It should also be noted that increasing the speed of a suspended monorail increases its kinetic energy, which is directly proportional to the squared speed, which additionally increases the scale of the problem described [
4,
5,
13,
14,
15,
16,
17,
18,
19,
20,
21]. During the stand tests of emergency braking at the speed of 5 m/s, the formation of a beam of sparks and the exceeding of the permissible temperature of the brake pad surface were observed [
22]. These phenomena significantly increase the risk of methane ignition and, consequently, may cause a serious accident [
23,
24].
Tests related to the possibilities of improving safety during emergency braking, even at increased speeds of suspended monorails, allowed for development of a concept of a control algorithm ensuring a sequential method of braking. In the currently used emergency braking systems, the total braking force is released immediately after the brake is applied. In these systems, it is not possible to modulate the braking force depending on the weight of the transport set or the shape of the suspended monorail route (upward and downward). In practice, this means that in a situation when the transport set is light (without a load) and, additionally, if it moves downward, and there is a need for emergency braking, the maximum permissible braking deceleration values may be exceeded; the dynamic load of the slings of the railway route and the load of the roof support arches increase significantly. This can lead to dangerous situations that could cause serious accidents. This case is particularly dangerous in the context of the increased permissible maximum speed limit. The development of the concept of the sequential braking algorithm is an innovative approach to the emergency braking process. The aim of its use is to reduce the risk of adverse impacts on humans and the suspended monorail route. Thus, the development of a modern method of braking will contribute to the improvement of safety in emergency braking situations. The assumed braking sequence, according to the developed concept, consists in the activation of some braking systems (the first braking stage) at the moment of the control signal activating emergency braking. The first braking stage has a reduced braking force (e.g., 50% of the required braking force). After the activation of the first stage, a decision is made, based on the analysis of the deceleration, whether to activate the second stage braking (maximum available braking force is applied) or not. The braking deceleration occurring after the activation of the first braking stage, regarding the given suspension monorail set, may change depending on the load (number of people transported) or the shape of the monorail route (horizontal, downward or upward inclination) [
13,
25,
26,
27,
28]. The article presents a computational model of a suspended monorail with an algorithm of the sequential emergency braking. The next part of the article presents the results of numerical simulations during which the parameters of the braking algorithm were changed. The aim of these simulations was to identify the impact of changing parameters of the braking process control algorithm on the deceleration as well as the time and the distance of braking in the case of an emergency stop of the transportation set during crew movement. We conducted simulation tests because tests in the real condition were impossible due to the inability to drive at higher speeds (5 m/s) in the mines, as well as the inability to regulate the braking force on the individual braking elements. The observations presented in the article are necessary for the correct adjusting of the braking algorithm. Based on the acquired knowledge, it will be possible to propose how to set parameters in the braking algorithm depending on the transport set or to make changes in the proportion of the braking force values in the first and second stage, e.g., 40% of the total braking force in the first stage and 60% in the second stage.
3. Results
A series of numerical simulations enabled analysis of the impact of changing the parameters in the algorithm of the sequential braking of the monorail set on the emergency braking process. During the simulations, the braking deceleration threshold and the time delay, after which the deceleration was calculated, were changed. The settings of these parameters determined whether the second brake stage was activated or not. Numerical simulations were realized for two speeds at which the emergency braking started. The first of these speeds was 3 m/s and the second was 5 m/s. Similar simulations of the emergency braking were carried out when the monorail set travelled on a route without inclination and on a route with a 30° inclination. In each of the simulations, the braking deceleration, the braking time, and the braking distance were recorded. The maximum deceleration in the entire braking process as well as the maximum deceleration during the first braking stage (the maximum value recorded before the activation of the second braking stage) were analyzed.
Figure 4 shows the maximum braking deceleration recorded during numerical simulations of emergency braking from a speed of 3 m/s, on the horizontal route, as a function of the time delay, after which the condition of activating the second braking stage was calculated. These values are shown in relation to the three brake deceleration thresholds (4 m/s
2 (ACC4), 5 m/s
2 (ACC5), and 6 m/s
2 (ACC6)).
Figure 5 shows the braking distance recorded during numerical simulations of emergency braking from a speed of 3 m/s, on the horizontal route, as a function of the time delay, after which the condition of activating the second braking stage was calculated. These values are shown in relation to the three brake deceleration thresholds (4 m/s
2 (ACC4), 5 m/s
2 (ACC5), and 6 m/s
2 (ACC6)).
Table 1 provides the results recorded during the simulation of emergency braking of the suspended monorail set, from a speed of 3 m/s, on a horizontal route, in relation to different settings of the braking algorithm.
Figure 6 shows the maximum braking deceleration recorded during numerical simulations of emergency braking from a speed of 5 m/s, on the horizontal route, as a function of the time delay, after which the condition of activating the second braking stage was calculated. These values are shown in relation to the three brake deceleration thresholds (4 m/s
2 (ACC4), 5 m/s
2 (ACC5), and 6 m/s
2 (ACC6)).
Figure 7 shows the braking distance recorded during numerical simulations of emergency braking from a speed of 5 m/s, on the horizontal route, as a function of the time delay, after which the condition of activating the second braking stage was calculated. These values are shown in relation to the three brake deceleration thresholds (4 m/s
2 (ACC4), 5 m/s
2 (ACC5), and 6 m/s
2 (ACC6)).
Table 2 provides the results recorded during the simulation of emergency braking of the suspended monorail set, from a speed of 3 m/s, on a horizontal route, in relation to different settings of the braking algorithm.
Figure 8 shows the maximum braking deceleration recorded during numerical simulations of emergency braking from a speed of 3 m/s, on an inclined downward route of a 30° angle, as a function of the time delay, after which the condition of activating the second braking stage was calculated. These values are shown in relation to the three brake deceleration thresholds (4 m/s
2 (ACC4), 5 m/s
2 (ACC5), and 6 m/s
2 (ACC6)).
Figure 9 shows the braking distance recorded during numerical simulations of emergency braking from a speed of 3 m/s, on an inclined downward route of 30°, as a function of the time delay, after which the condition of activating the second braking stage was calculated. These values are shown in relation to the three brake deceleration thresholds (4 m/s
2 (ACC4), 5 m/s
2 (ACC5), and 6 m/s
2 (ACC6)).
Table 3 provides the results recorded during the simulation of the emergency braking of the suspended monorail set, from a speed of 3 m/s, on a route inclined downward at a 30° angle, in relation to different settings of the braking algorithm.
Figure 10 shows the maximum braking deceleration recorded during numerical simulations of emergency braking from a speed of 5 m/s, on a route inclined downward at a 30° angle, as a function of the time delay, after which the condition of activating the second braking stage was calculated. These values are shown in relation to the three brake deceleration thresholds (4 m/s
2 (ACC4), 5 m/s
2 (ACC5), and 6 m/s
2 (ACC6)).
Figure 11 shows the braking distance recorded during numerical simulations of emergency braking from a speed of 5 m/s, on an inclined route of 30°, as a function of the time delay, after which the condition of activating the second braking stage was calculated. These values are shown in relation to the three brake deceleration thresholds (4 m/s
2 (ACC4), 5 m/s
2 (ACC5), and 6 m/s
2 (ACC6).
Table 4 provides the results recorded during the simulations of the emergency braking of the suspended monorail set, from a speed of 5 m/s, on a route inclined downward at a 30° angle, in relation to different settings of the braking algorithm.
Figure 12,
Figure 13 and
Figure 14 show a comparison of the braking distance at different threshold values of the first stage of braking deceleration, the delay in activation of the second stage of braking, the speed from which the set was braked, and the inclination angle of the route.
In the next stage of the work, another group of simulations, in which the forces of pressing the jaws against the rail web and the number of actively braking jaws were changed, were aimed at observing the impact of the first degree braking force changes on the deceleration during emergency braking, the braking distance, and the braking time. Braking with two pairs of jaws and with only one pair of jaws was simulated. In each case, the braking was simulated with the full clamping force of the jaws (12,500 N), with a force 50% lower (6250 N), and a force lower by 75% (3125 N). The time delay for the activation of the second stage of braking was 0.5 s. Moreover, for reduced forces (50% and 25% of the initial value), simulations were carried out with the activation time delay of the second stage equal to 1 s. All simulations calculated braking from a speed of 5 m/s. The simulation results are presented in
Table 5.
Figure 15 shows the relationship between the braking distance and the braking force of the first braking stage and the delay in the activation of the second braking stage.
Figure 16 shows the maximum deceleration recorded during the simulation of emergency two-stage braking as a function of the braking force and the time delay in the activation of the second stage braking.
Figure 17 shows the accelerations recorded when simulating braking from a speed of 5 m/s with one pair of jaws, different forces pressing the jaws against the rail, and a different delay in the activation of the second braking stage.
Figure 17.
Waveforms of changes in the acceleration of the monorail cabin during emergency braking at different pressing forces of jaws and the time delay in the activation of the second stage of braking using one pair of jaws. Distances covered by the monorail during those simulations are given in
Figure 18.
Figure 17.
Waveforms of changes in the acceleration of the monorail cabin during emergency braking at different pressing forces of jaws and the time delay in the activation of the second stage of braking using one pair of jaws. Distances covered by the monorail during those simulations are given in
Figure 18.
Figure 18.
Distances covered by the monorail during simulations of emergency braking at different pressing forces of jaws and the time delay in the activation of the second stage of braking using one pair of jaws.
Figure 18.
Distances covered by the monorail during simulations of emergency braking at different pressing forces of jaws and the time delay in the activation of the second stage of braking using one pair of jaws.
Figure 19 shows the accelerations recorded when braking from a speed of 5 m/s with two pairs of jaws, with different forces pressing the jaws against the rail, and a different delay in the activation of the second braking stage.
Figure 19.
Waveforms of changes in the acceleration of the monorail cabin during emergency braking at different pressing forces of jaws and the time delay in the activation of the second stage of braking using two pairs of jaws. Braking distances of the monorail during those simulations are given in
Figure 20.
Figure 19.
Waveforms of changes in the acceleration of the monorail cabin during emergency braking at different pressing forces of jaws and the time delay in the activation of the second stage of braking using two pairs of jaws. Braking distances of the monorail during those simulations are given in
Figure 20.
Figure 20.
Distances covered by the monorail during simulations of emergency braking at different pressing forces of jaws and the time delay in the activation of the second stage of braking using two pairs of jaws.
Figure 20.
Distances covered by the monorail during simulations of emergency braking at different pressing forces of jaws and the time delay in the activation of the second stage of braking using two pairs of jaws.
Table 6 presents the maximum forces recorded in each suspension during the simulation of emergency braking from a speed of 5 m/s, with different forces pressing the jaws against the rail and different time delays in the activation of the second braking stage, as well as braking with one or two pairs of jaws.
Figure 21 shows the markings of the suspensions of the suspended monorail route.
Table 6 shows the maximum values of the forces in the suspensions. During emergency braking, the whole monorail route is able to move in the direction of the travel. During the movement of the route, some suspensions, especially those stabilizing the position of the route, become overloaded and others loosen. This is one reason for the dynamic overload of the suspension. Depending on the location of individual components on the rail, the character of the displacement of rails and loads on the suspensions may change. From this point of view, it seems that ensuring the appropriate condition of the route on the sections with an increased speed limit is necessary.
5. Conclusions
The article presents an innovative approach to emergency braking of the suspended monorail in underground hard coal mines. The innovation of this approach consists in the novel use of sequential activation of braking elements that enables modulation of the braking force depending on the boundary conditions during emergency braking. During the braking process, the deceleration affects the operator and the personnel. The maximum deceleration can be seen in the acceleration diagrams as a peak. Use of the sequential emergency braking algorithm enables minimizing the braking decelerations acting on people in the monorail during emergency braking. In this way, the operator or the personnel injury risk is limited. For the best operation of the algorithm, it is necessary to select the appropriate settings of its parameters (decisive for the activation of the second braking stage), which are: the deceleration threshold and the delay. On the basis of the numerical simulations, it can be estimated that setting the delay time in the range from 0.3 s to 0.5 s, and the deceleration threshold of 4 m/s2 should allow a satisfactory braking process during an emergency stop in relation to the analyzed monorail set. However, the analysis of emergency braking under such different conditions as braking on a horizontal route and on a downward 30° angle, leads to whether the algorithm should include information on the position/incline of the route. Then, based on the indications of the measuring system, e.g., an inclinometer installed on one of the monorail components, and the determined ranges of the route inclinations, the settings of the braking algorithm parameters could take different values.
Another area of testing should be related to the method of selecting the proportions of braking forces on the first and second stage of emergency braking to meet the total deceleration required by regulations. The selection of these parameters is very important and must be performed responsibly, ensuring the possibility of stopping the monorail in the most unfavorable variant of emergency braking, i.e., with the maximum permissible load to the monorail set during a downward travel with the greatest permissible angle of an inclination at the highest speed permissible for a given monorail. This process should be performed in such a way that the braking at the first stage during empty running of the monorail set on a horizontal route or running upward does not generate excessive dynamic overload. A braking system selected by this method, together with a properly adjusted control algorithm and perhaps a necessary system of additional safety components (i.e., safety belts, energy absorbers in the form of foam liners or vibration and energy dampers, used in links or suspension components of each cabin), should significantly improve the safety of the monorail passenger cabin, even at the higher speeds.