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An innovative straddle monorail track switch design for the personal rapid transit

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An innovative track switch design is proposed for a straddle monorail system that may serve as a PRT system. Previously, a PRT system utilising the monorail design was thought impossible. Here we introduce onboard guide wheels on the monorail vehicles to be the only movable parts that direct the cars to different paths. Each vehicle activates its own switching mechanism and does not need to wait for the movement of the heavy track beams anymore. Guideways and ground rails accommodated by a switch platform are designed to replace the monorail track beams. To verify the system performance, a model monorail loop and a vehicle were built for testing. Test results and analyses show that the monorail vehicles have enough momentum to traverse the switch area without power, and the required headway for the proposed system is much less than the target of 10 seconds.
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I
nt. J. Heavy Vehicle Systems, Vol. x, No. x, xxxx 1
Copyright © 200x Inderscience Enterprises Ltd.
An innovative straddle monorail track switch design
for the personal rapid transit
Chih-Hung G. Li*
Graduate Institute of Manufacturing Technology,
National Taipei University of Technology,
No. 1, Sec. 3, Zhongxiao E. Rd.,
Taipei 10608, Taiwan
Email: cl4e@ntut.edu.tw
*Corresponding author
Zong Jun Lu
General Integration Technology Co, Ltd.,
No. 343, Chongqing Rd., Xitun Dist.,
Taichung City 40751, Taiwan
Email: king92_22@yahoo.com.tw
Abstract: An innovative track switch design is proposed for a straddle
monorail system that may serve as a PRT system. Previously, a PRT system
utilising the monorail design was thought impossible. Here we introduce
onboard guide wheels on the monorail vehicles to be the only movable parts
that direct the cars to different paths. Each vehicle activates its own switching
mechanism and does not need to wait for the movement of the heavy track
beams anymore. Guideways and ground rails accommodated by a switch
platform are designed to replace the monorail track beams. To verify the
system performance, a model monorail loop and a vehicle were built for
testing. Test results and analyses show that the monorail vehicles have enough
momentum to traverse the switch area without power, and the required
headway for the proposed system is much less than the target of 10 seconds.
Keywords: monorail; PRT; personal rapid transit; track switch; headway.
Reference to this paper should be made as follows: Li, C-H.G. and Lu, Z.J.
(xxxx) ‘An innovative straddle monorail track switch design for the personal
rapid transit’, Int. J. Heavy Vehicle Systems, Vol. x, No. x, pp.xxx–xxx.
Biographical notes: Chih-Hung G. Li received his BS in Power Mechanical
Engineering from the National Tsing Hua University, Hsinchu, Taiwan in
1990, and MS and PhD in Mechanical Engineering from Carnegie Mellon
University, Pittsburgh, Pennsylvania, USA in 1994 and 1998, respectively.
He is an Associate Professor in the Graduate Institute of Manufacturing
Technology of National Taipei University of Technology. From 2014 to 2017,
he was the Director of the Automated Vehicle and Equipment Development
Center at the Minghsin University of Science and Technology in Hsinchu,
Taiwan. His research interests are primarily in the areas of deep learning
applications, mechanism innovation, mechatronics and robotics, intelligent
transit system, structural analysis and optimisation, etc.
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Zong Jun Lu received his BS in Mechanical Engineering from Minghsin
University of Science and Technology, Hsinchu, Taiwan in 2013, and MS in
Precision Mechatronics Engineering from Minghsin University of Science and
Technology in 2015. He is currently working at the General Integration
Technology Company as an Engineer.
1 Introduction
Personal rapid transit (PRT) is a highly automated public transportation system,
designed to provide personalised point-to-point transportation service for individuals. The
driverless system received attentions more than five decades ago (Irving et al., 1978).
Since then, researchers continued to study various aspects of the system including recent
progress in the network design (Zheng and Peeta, 2015), empty vehicle management
(Daszczuk et al., 2016), holistic performance assessment (Mascia et al., 2016), waiting
time minimisation (Lees-Miller, 2016), energy optimisation (Mrad and Hidri, 2015),
enhancement of transportation capacity (Chebbi and Chaouachi, 2016), etc. With the
rapid advancement in technology, many features of the PRT system become not only
feasible but also very attractive (Carnegie and Hoffman, 2007).
A few PRT systems are already operating today, including the famous WVU PRT in
Morgantown, West Virginia, USA. With a headway ranging from 8 s to 15 s, the system
may transport 2000 to 2500 passengers per hour (Juster and Schonfeld, 2014). The Pod in
the London Heathrow Airport is one of the most representative modern PRT systems.
The 3.8 km route is serviced by 21 automated electrical vehicles connecting two business
parking lots and the terminal five (Ultra Global PRT, 2013). A rail-based PRT system
called intelligent people mover (IPM) was built in Suncheon Bay, South Korea, linking
the city to a world famous wetlands and bird reserve in the Suncheon Bay Estuary.
The system employs positive mechanical guidance using on-board switches instead of
conventional track-switches, which are too slow to be practical for PRT headways
(Pemberton, 2013).
Most of the PRT systems developed or operating today adopt the rubber tyre system
like the Heathrow Pod or the dual-track system like the Suncheon IPM; a straddle
monorail PRT (MPRT) system is very rare and thought impossible (Anderson, 2000).
Despite the numerous advantages associated with the monorail systems such as safety,
cost-effectiveness, light weight, space-undemanding, et al., it is believed that the
difficulty in fast track switching is a major obstacle to adopt the straddle monorail design
for the PRT. As the monorail track beams serving as the vehicle track work as the road
structure and support the weights of the vehicles, they must be sturdy and strong, and
usually much heavier than ordinary train tracks. To switch the track beams of a monorail
system, it often takes more than tens of seconds. And as the monorail system is often
elevated, derailing may very likely result in catastrophic falling of the vehicles. This
obviously does not meet the requirements for short headways and fast track switching for
a safe PRT system.
Following our previous efforts in developing a straddle MPRT system (Li, 2015,
2016), in this research, we provide a new approach for designing the track switch system
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for straddle monorails. Inspired by the need of a PRT system, the track switch
system must be fast enough in order to define meaningful headways for a PRT.
The track switch must also be safe, robust, and kinematically reliable. Although the
track beams of a PRT system are expected to be lighter than common monorails which
support high-capacity vehicles, the conventional beam-moving type of switch is not
considered here, due to safety and energy concerns. Instead, a platform type of guideway
design combined with on-board vehicle guide wheels is developed. Details of preliminary
testing based on a small-scaled model are also reported in the experiment section of this
paper.
2 State-of-the-art
Due to its abundant advantages in various aspects, monorail systems have been adopted
by many cities all over the world for mass transportation. In general, the structure is more
compact and requires less land use than a traditional dual-track system (Zhang et al.,
2017). This results in numerous advantages. First of all, the cost on land purchase can be
significantly reduced, as well as material usage and the associated processing cost. It is
believed that the cost of a monorail system is approximately 1/3 to 1/2 of a traditional
metro system (He, 2015). Secondly, the less demand on land use also implies more
environmental friendliness. This is particularly important while planning a transit line
that runs through nature reserves such as forests and mountains. Thirdly, as the structure
is relatively slim, the impact on the existing view is less; also the passengers may have
better ride experiences in this regard. Furthermore, as the vehicles of a straddle monorail
system are guided in two ways – vertically and laterally, the chances of the vehicle falling
off the track beam are nearly zero. And because of such unique characteristics, the
monorail vehicle can negotiate a much tighter turn. The minimum radius of a monorail
line can be smaller than half of that of a traditional metro (He, 2015). This feature is
particularly welcomed for a transit that has to run through rough terrains with
topographic and geological complexity (Zhang et al., 2017). It also makes sense for a
PRT system to adopt the monorail design in this regard, because the PRT system, aimed
to provide the last mile transportation like the micro vessels in the entire blood circulation
system, is deemed to face numerous needs of tight cornering. Normally, the monorail
vehicles use rubber tyres as the main driving and supporting wheels. Compared to
ordinary metro systems that run on steel tracks, the monorail generates less noise and
exhibit better slope performance. It is believed that the monorail noise is 10 db lower than
the metro, and the maximum longitudinal slope of the monorail is 60 %, compared to the
35% of the ordinary metro (He, 2015).
Despite the numerous advantages mentioned above, there are critical disadvantages of
the monorail system, too. First of all, switching track of an ordinary monorail system is
usually slow and energy-demanding (Ellzey, 1977; Vance et al., 2011; Mugnier, 2001;
Lamoreaux, 2001; Mihirogi, 1993; Spieldiener et al., 1993). Currently, most of the
straddle monorails use movable track beam sections for track switching. The common
switch systems involve a large platform that bears multiple movable track beams and
some other related equipment such as motors, bearings, hydraulic cylinders, etc.
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The beam replacement type of switch is used on the mainline, and the multi-position
pivot switches are used to provide access to multiple lanes (Timan, 2015). Unlike general
railways or dual-track metros, the track beams of a monorail system are heavy and need a
long time to move. The associated building cost and maintenance fees for the electrical
and mechanical systems are also much higher. Secondly, the structure of the monorail
vehicle is usually more complicated. In addition to the main driving wheels, the monorail
vehicle must be equipped with the guide wheels, the stabilising wheels, and some other
auxiliary mechanisms. Thirdly, the monorail system lacks a natural emergency escape
channel. As the system is usually elevated, and the track beam is narrow and does not
have any protection feature for pedestrians, once an emergency happens, it is difficult to
evacuate the passengers rapidly. To remedy this, some monorail systems introduce an
emergency walkway (Timan, 2015). It was reported that the emergency walkway is
currently the only design that meets the requirement of evacuating a fully loaded train in
15 min., according to the standards of NFPA 130 (Timan, 2015).
Due to the sheer size and weight of the track beam sections, moving them for
switching is slow. It simply lacks the speed and durability that is required for a PRT
system. In fact, if one compares the switch system of common monorails with those of
the PRT systems, one may find that most of the PRT systems adopt on-board track switch
designs, whereas the common monorails rely on the aforementioned movable track
beams. For example, the Heathrow Pod uses self-steering of the vehicles to change lanes,
and the Suncheon IPM utilises an on-board magnetic actuator for track switching
(Pemberton, 2013). In contrast to the movable beams of the monorail systems, the track
switches of these PRT systems do not have any movable parts on the infrastructure of the
switch zone. Every vehicle is equipped with all the necessary mechanisms for track
switching or direction changing. Thus, it does not need to wait for any movable parts in
the switch zone to move, and a great deal of switching time can be reduced. As every
vehicle commands its own track switching, the preparation time is relieved from
constraint of headway. This provides a great potential to improve the headway. Thus, we
conclude that in order to transform the current straddle monorail system to one that can
be applied in PRT, it is critical to develop an on-board switch system for fast and reliable
track switching.
3 Design and analysis
3.1 Design of the track switch system
The MPRT system we propose is depicted in Figure 1, where a small vehicle designed
to accommodate 4–6 people runs on a steel monorail which can be pre-formed and
installed at the site. As discussed above, the current track beam switch design of a
monorail system does not meet the requirements on speed and reliability of the PRT;
thus, we turn to contemplate on-board switch design for the monorail system.
We propose to make the track beam disappear at the switch area. To replace the missing
track beam, we design a platform with fixed guideways and ground rails to support and
lead the vehicles to different paths. In order to move the vehicle on the platform, pairs of
powerless ground wheels are introduced to every vehicle as shown in Figure 2.
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Figure 1 The proposed MPRT has the advantages of compact structure, less land requirement,
lower construction cost, transportation safety, kinematic excellence, etc. (courtesy of
Shan-Hai Recreation Technology Corporation) (see online version for colours)
Figure 2 The proposed structure of the MPRT vehicle. The powerless ground wheels are
designed to carry the vehicle for moving on the switch platform. The guide wheels are
designed to extend beneath the guideways and be tilted to generate both vertical and
lateral force components. Each pair of the guide wheels is connected to a parallel
four-bar linkage for synchronous movement. These concept sketches do not suggest
the actual sizes of components (see online version for colours)
On-board guide wheels are introduced to the vehicles to be coupled with the
guideways along the switch platform. Note that these guide wheels are not the existing
ones that roll along the track beam. When a vehicle approaches the switch platform,
it experiences three entering stages and similar exiting stages (see Figure 3). Before the
first stage, upon determining its next direction, a set of guide wheels of the vehicle will
be activated (lowered). Then at the first stage, the activated guide wheels engage the
guideways while the vehicle is still moving on the track beam driven by the main wheels.
This is called the stage of guide wheel engagement. At the second stage, the track beam
that starts to form a small downward slope, gradually drops the vehicle and allows the
ground wheels of the vehicle to touch the ground rails of the platform; the track beam
eventually disappears. This is called the stage of main wheel detachment. At the third
stage, the vehicle now rolls on the platform on the ground wheels and is directed to one
of the two possible paths based on the pairing of the guide wheels and the guideways;
thus, it is called the redirection stage. After the vehicle is directed to the targeted path, the
track beam reappears with a small uphill slope to facilitate the main wheels to climb back
on the track beam. Now the vehicle is moving on the track beam again; eventually it
leaves the switch area and exits the switch platform.
The guide wheels are designed to provide two functions – guiding and balancing.
When the vehicle turns to the right, the right set of guide wheels are lowered to engage,
and vice versa. The lowered guide wheels engage the same side of the guideways and
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lead the vehicle to that side of the paths. As shown in Figure 2, the guide wheels are
designed to be tilted in order to generate both vertical and lateral force components.
The lateral force component provides the centrifugal force needed for turning; the vertical
force component assists to keep the vehicle on the ground rails. Combined with the
gravitational force of the vehicle, the two vertical forces provide a clamping effect to
prevent the vehicle from leaving the ground rails, and enhance the safety of the vehicle
while traversing the switch area.
Figure 3 Illustration of the proposed MPRT switch platform that constitutes three stages:
1. Guide wheel engagement, 2. Main wheel detachment and 3. Redirection (see online
version for colours)
Each pair of the guide wheels is connected to a parallel four-bar linkage for synchronous
movement, powered by an independent motor (see Figure 2). This mechanism is the only
set of movable parts involved in the proposed switch system. Since the structure is much
smaller and lighter compared to the conventional track beam switch design, the proposed
new design requires much less reaction time and energy. And since each vehicle only
relies on its own guide wheels for track switching, it has an unlimited time for
preparation.
3.2 Analysis of momentum requirement
To avoid a complicated design, we attempt to design the system without powering the
ground wheels; hence the vehicle needs to rely on its momentum to roll over the distance
of the switch area until it regains traction on the track beam. The following estimation
verifies the feasibility of the concept. Assuming the vehicle weighs 1200 kgf, and travels
at an initial velocity i
v of 15 km/h (4.17 m/s) when it enters the switch area and touches
the ground rails. The linear momentum is approximately 5000 kg-m/s. When the ground
wheels touch the ground rails, the vehicle loses some of its momentum to the initially still
ground wheels. As suggested by the free body diagram in Figure 4, the equations of force
equilibrium are,
Σ0: 4 0
x
FFma=−= (1)
Σ0: 0
c
MFrI
α
=−= (2)
where
F
denotes the friction force on a wheel, m is the mass of the whole vehicle, r is
the outer radius of the tyre, a is the linear acceleration of the vehicle,
α
is the angular
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acceleration of the wheels, 2
(1/ 2 )
I
mr=, and m denotes the mass of a ground wheel.
Solving equations (1) and (2), the after velocity f
v is arrived at,
12
i
fm
v
v
m
=⎛⎞
+
⎜⎟
⎝⎠
(3)
Assuming the ground wheels are common pneumatic tyres with a mass 17 kgm= and a
radius of 0.33 m, f
v is estimated at 97% of i
v based on equation (3). When the vehicle
travels on the platform without power, the vehicle velocity decreases due to rolling
friction of the tyres. Assuming a typical coefficient of rolling resistance rr
c of 0.015 for
the pneumatic tyres, and let the travel distance Δs be 5 m, the final velocity f
v of the
vehicle is estimated based on energy conservation as,
()
22
1
Δs2
rr f f
mgc m v v−= − (4)
where
g
denotes the gravity. Combining equations (3) and (4), the resulting f
v is
arrived at 93% of i
v. Thus, we conclude that the vehicle still has enough momentum to
ride back on the track beam after losing energy in the switch area. If one assumes f
v as a
constant speed for passing through the switch area, the elapsed time is approximately
1.30 s.
Figure 4 The free body diagram for the estimation of the velocity reduction in the switch area
(see online version for colours)
4 Experiment
To verify the design concept, we build a 1/20 model for testing (Lu, 2015). The main
goal of this experiment is to demonstrate that the track switch system meets the desired
headway requirement with a certain level of reliability. As the headway of a PRT system
generally ranges between 8 and 15 s, we expect the elapsed time of a PRT vehicle rolling
on the switch platform to be sufficiently short. Thus, if for any reason, a vehicle stops
inside the switch area, the following vehicles have sufficient time to react to the
situations.
The model includes a closed-loop monorail track, two track switch platforms, a
section of through-lane, a section of auxiliary lane, and a model vehicle as shown in
Figure 5. At the first switch platform, the vehicle is directed to either go straight or turn
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right. At the second switch platform, vehicles from two different paths are merged into a
single track. The main wheels of the model vehicle are driven by a DC motor powered by
a carry-on battery. The guide wheels are driven by servo-motors, which are programmed
to switch between two states, and commanded wirelessly by an RF remote control.
The monorail tracks and the main frame of the vehicle are fabricated using a 3D printer
with a precision of 0.2 mm.
Figure 5 The 1/20 MPRT model for testing. The model includes a closed-loop monorail track,
two track switch platforms, a section of through-lane, a section of auxiliary lane, and a
model vehicle (see online version for colours)
The parallel four-bar linkage of the guide wheels was made by steel rods connected by
slit rubber sleeves as shown in Figure 6. The middle member of the linkage is driven by
the servo-motor to ensure that the front and the rear guide wheels act synchronously.
Figure 6 The parallel four-bar linkage of the guide wheels was made by steel rods connected by
slit rubber sleeves to ensure that the front and the rear guide wheels act synchronously
(see online version for colours)
We let the vehicle continuously run on the track loop, and randomly chose the switching
direction, when the vehicle reached the switch platform. The total time to finish a loop is
measured as well as the elapsed time of the vehicle traversing the switch platform.
Results show that the vehicle moves at an average speed of 0.25 m/s, equivalent to 5 m/s
(18 km/h) for a real vehicle which is 20 times larger than the model vehicle.
For simplicity, only the right-turn guide wheels were installed on the model vehicle.
When we command the vehicle to turn right, the guide wheels are lowered. When the
guide wheels remain retracted, the vehicle goes straight. To better guide the vehicle in
straight movement, a guard rail was added to the straight ground rails. This will not be
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necessary in reality, as there will be a set of guide wheels and guide rails for the straight
path as well. In Figure 7(a), it shows the right-turn guide wheels being retracted, and the
vehicle continues on the straight path. In Figure 7(b), the guide wheels are lowered, and
the vehicle is guided to the right path.
Figure 7 The position of the guide wheels and its effect on the switching direction: (a) the guide
wheels are retracted; the vehicle goes straight; (b) the guide wheels are lowered;
the vehicle goes to the right (see online version for colours)
Figure 8 The total loop time of each measurement. The variations are less than ±1 s, showing
that the model monorail system operates steadily. Upper data: loop through the
auxiliary lane; lower data: loop through the through lane (see online version for colours)
By collecting more than 200 measurements and taking the average of the data, the
elapsed time for traversing a switch platform was found to be 1.50 s, close to our
previous estimate of 1.30 s. During the more than 200 times of test runs, we did not
experience any malfunction of any kind. The performance of track switching was steady,
and the time variance in track switching was very small. As shown in Figure 8, the
variations in the total loop time are less than ±1 s. The average loop time associated
with the through lane is 57.3 s, and that associated with the auxiliary path is 61.7 s.
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A clip showing the operation of the track switch model can be found at
https://youtu.be/tzA_3jR-ueM.
5 Discussion
5.1 Headway analysis
As the PRT vehicle is powerless while rolling on the switch platform, it must possess
sufficient momentum when it leaves the track beam and enters the switch area. Thus, if
for any reason the vehicle needs to stop before the switch platform, it must have enough
distance ahead of it to accelerate and regain sufficient speed on the track beam before
entering the beamless area. If the headway between two vehicles is too short, the
following vehicle may not have sufficient time or distance to respond and resume. Here
we contemplate the normal sequences of a vehicle entering the switch platform, which is
followed by another vehicle, separated by a headway of t
Δ
s. Through the analysis,
we try to estimate the required headway due to the constraints of the switch platform, and
see whether the estimate is less than 10 s.
Assuming vehicle B trailing behind vehicle A by a headway of t
Δ
, when vehicle A
reaches the switch area, corresponding to the main wheel detachment stage as shown in
Figure 9(a), the distance between vehicle A and vehicle B is
s
Δ
. Then vehicle A spends
a period of time 1
tΔ to cross the switch area; in the meantime, vehicle B continues
moving forward by a distance 1
s
Δ
(see Figure 9(b)). Considering the worst scenario
when vehicle A breaks down at the end of 1
t
Δ
, vehicle B must receive the signal and
react to the emergency promptly. Assuming vehicle B needs a period of time 2
tΔ to
receive and process the signal and react to the situation, during that time, vehicle B
continues moving forward by a distance of 2
s
Δ
(see Figure 9(c)). Then, vehicle B starts
to brake and decelerate. It takes another period of time 3
t
Δ
for vehicle B to come to a
full stop; during the time, vehicle B moves forward by another distance of 3
s
Δ
(see Figure 9(d)). At this moment, vehicle B is at a position which is some metres away
from the end of the track beam; this distance must be long enough for vehicle B to
accelerate to a sufficient speed in order to have the required momentum to cross the
switch area. Assuming the acceleration takes a time of 4
t
Δ
, and at the end of it, vehicle B
reaches the entrance of the switch area as shown in Figure 9(e).
Assuming the vehicles travel at a speed of 30 km/h on the track beam, and reduce to
15 km/h while traversing the switch area, the associated times and distances are given in
Table 1. Thus, the distance between vehicle A and vehicle B when vehicle A is entering
the switch area can be expressed as a summation of various distances according to
Figure 9 as,
1
8.33 21.35 ; 1, 2, 3, 4.
i
i
ss t miΔ= Δ = Δ+ =
(5)
In a normal situation, this distance should be travelled by a vehicle in a total time of tΔ,
within which a percentage (
α
) is spent at a constant speed of 30 km/h, and the rest is
spent on decelerating to 15 km/h. With a deceleration of 2
6 m/
s
, the relationship
between
s
Δ, tΔ and
α
is,
()
(
)
4.17 8.33 1
8.33 m
2
t
st
α
α
+−Δ
Δ= Δ+ (6)
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Figure 9 Illustration of the scenario where the following vehicle performs an emergency stop and
tries to regain sufficient speed for traversing the switch area. The times and distances
are used to estimate the required headway (see online version for colours)
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And the equation of deceleration is,
()
4.17 6 1 m/st
α
−=Δ (7)
Combining equations (5)–(7), one can arrive at the relationship of the headway tΔ and
the platform traverse time 1
tΔ as,
12.74 sttΔ=Δ + (8)
This result shows that the required headway is the switch area traverse time plus 2.74 s.
In other words, as long as the traverse time is less than 7.26 s, our goal of keeping the
headway under 10 s can be achieved. According to our experiment result, it only takes
1.5 s to cross the switch area. Thus, the required headway is just 4.24 s, much shorter
than our target of 10 s.
Table 1 Elapsed times and travel distances associated with various steps
Index i Time period i
tΔ (s) i
s
Δ
(m) Note
1 12
τ
τ
Depends on the
platform traversing time
8.33 i
t
Δ
Vehicle B moves forward at 30 km/h
2 23
τ
τ
1 8.33
Assume a reaction time of 1 s
3 34
τ
τ
1.39 5.79
Assume a deceleration of 2
6 m/s
4 45
τ
τ
3.47 7.23
Assume an acceleration of 2
1.2 m/s
5.2 Ground rail gap and transition smoothness
As the straight ground rail and the right-turn guideway cross each other, it inevitably
results in a gap in the straight ground rail (see Figure 10). The gap must be large enough,
so the supporting structure of the guide wheels can pass through it. However, if the gap is
too large, it becomes a pothole for the ground wheels and may cause discomfort or even
jeopardise dynamic stableness. A large gap may decrease more of the vehicle momentum
as well. Thus, it is recommended that the supporting structure is optimised for a small
cross section near the guide wheel; thus the gap may be designed smaller.
The monorail vehicle experiences two transition periods on the switch platform,
the first being from the track beam to the ground rail, and the second from the ground rail
back to the track beam. Smooth transition from one support to another is important for
safety and comfort. It is particularly so when transitioning from the ground rail to the
track beam, as the main wheels are driven at a certain speed and may not act in concert
with the track beam. To enhance the transition smoothness, it is better to control the spin
rate of the tyres to be in accord with the linear speed of the vehicle. So when the main
wheels enter contact with the track beam, relative sliding and the associated friction do
not occur. When the vehicle transitions from the track beam to the ground rail, the impact
force from the friction of the ground wheels is inevitable, as the ground wheels are
powerless and can not be pre-spun. The only way to reduce the impact force is by
decreasing the moment of inertia of the ground wheels as suggested by equation (2).
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Figure 10 The gap in the ground rail due to the guideway crossing over (see online version
for colours)
6 Conclusion
This paper presents a novel design of the straddle monorail track switch system, which is
motivated by the need of a modern PRT system. The new design adopts a concept
entirely different from the current monorail track switch design that involves movable
track beam sections. By introducing small on-board switching mechanisms to every
vehicle, and eliminating all movable parts on the switch platform, every vehicle is in
control of its own track switching and the waiting time is eliminated. Within the switch
area on the switch platform, track beams are removed and replaced by ground rails that
support the powerless ground wheels on the car that transport the vehicle temporarily. In
a momentum analysis, the result shows that the vehicle speed is reduced by 7% due to
ground wheel impact and friction loss. However, the remaining momentum should be
sufficient for the vehicle to traverse the switch area and ride back on the connecting track
beam. To verify the proposed design concept, a 1/20 model including a monorail loop, a
vehicle, and two switch platforms was built for performance testing. The model vehicle
moves at an average speed of 0.25 m/s, equivalent to 5 m/s for a real vehicle. The test
results reveal that the track switch system performs reliably with an average track
switching time of 1.5 s. In an analysis investigating the required headway for the
proposed system, we contemplated the worst scenario when the leading vehicle breaks
down near the end of the switch area. By considering the needed time and distance for the
following vehicle to receive the warning signal, react, brake to stop, restart, and regain
sufficient speed, an estimate of 4.24 s on the headway was arrived at. This result is
shorter than our target of 10 s with a large margin. We achieve the goal of developing a
new straddle monorail switch system which can be applied to PRT systems with short
headway, robust performance, and affordable technology.
14
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H
.G. Li and Z.J. Lu
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... For the majority of small-and medium-sized cities, it is difficult to meet the conditions of subway construction because of their small passenger flow demand and weak local finance. e medium-carrying-capacity rail transit, represented by the straddle monorail ( Figure 1), can meet the passenger transport needs of smalland medium-sized cities, and its construction investment is only 1/3 to 1/4 of that of subway, so it is a reasonable choice of rail transit system [1][2][3]. e track beam of straddle monorail is not only the load-bearing structure of the monorail bridge, but also the track of the stabilizing wheel and running wheel of monorail vehicle, so the manufacturing precision of millimeter level is required. When using concrete beam, the construction technology requirements for the manufacturing and erection of track beam are very high. ...
... (2) When the load is 100 kN, the theoretical value, FEM value, and measured value of the maximum compressive stress of concrete slab were −4.9 MPa, −5. (1) . α′ was the stiffness correction coefficients calculated by FEM values, and α′ � (W) (2) /(W) (1) . ...
... (1) . α′ was the stiffness correction coefficients calculated by FEM values, and α′ � (W) (2) /(W) (1) . (W) (1) was the vertical displacement obtained by analyzing the measured data of the sensors. ...
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