Portable Stair Lift System Designing a Modular and Mobile Assistive Lift Apparatus 

 A Major Qualifying Project Report:  

Submitted to the Faculty of  WORCESTER POLYTECHNIC INSTITUTE  

In partial fulfillment of the requirements for the  Degree of Bachelor of Science in Mechanical Engineering and the 

Degree of Bachelor of Science in Electrical and Computer Engineering by Madison Brown (ME)  

  

 and Ethan Peters (ECE) 

  

  

The first phase of the project has been completed  in the academic year 2019-2020 by  

 Madison Brown (ME), 2020 Ethan Peters (ECE), 2020 

Chenxi Li (ME), 2021 Robert Peralta (ME), 2021 

Mingzhang Zhu (ME & RBE), 2021  

Completed under the advisement and approval of    

 Selcuk Guceri, Advisor, and 

  

 Stephen Bitar, Co-Advisor 

  

This report represents the work of WPI undergraduate students submitted to the faculty as evidence of a degree requirement. WPI routinely publishes these reports on its website without editorial or peer 

review. For more information about the projects program at WPI, see http://www.wpi.edu/Academics/Projects    

 

Abstract This paper outlines the design and prototyping of a portable stair lift system. 

The system is able to be quickly assembled and disassembled on any straight set of 

stairs to assist in transporting heavy objects between floors of multi-level buildings. 

It is composed of three main parts: rails, cart, and lifting system. Objects to be lifted 

are secured to the cart which is then pulled up the rails by the modular lifting 

system. 

 

 

   

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Acknowledgements  

This project would not have been at all possible without the support of each 

of the following people and groups. For this, we thank you. 

 

 

Worcester Polytechnic Institute 

Professor Selcuk Guceri 

Professor Stephen Bitar 

WPI Mechanical Engineering Department 

WPI Electrical and Computer Engineering Department 

Barbara Furhman 

WPI Library Cafe Staff 

 

   

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Table of Contents 

Abstract 1 

Acknowledgements 2 

Table of Contents 3 

List of Figures and Tables 4 

I. Introduction 5 i. Problem Statement 5 ii. Proposed Solution 5 

II. Methodology 7 i. Mechanical 7 ii. Electrical 8 

III. Design 10 i. Mechanical 10 ii. Electrical 13 

a. Lifting Mechanism 13 b. Power Supply 14 c. Controller 15 

IV. Implementation 20 i. Mechanical 20 ii. Electrical 20 

V. Plans Going Forward 25 

VI. Conclusion 26 

References 27  

   

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List of Figures and Tables 

Figure I.ii.1   SolidWorks assembly of final product 

Figure III.i.1  Free body diagram of the lifting system 

Figure III.i.2  Free body diagram of the critical components of the lifting system 

Figure III.i.3  Product schematic 

Figure III.i.4   Table of calculated significant values 

Figure III.ii.1   12 volt Winch Control Schematic 

Figure III.ii.2  5 volt microcontroller output to 12 volt winch control 

Figure III.ii.3   Program flow chart 

Figure IV.ii.1   H-bridge Motor Controller with MOSFETs 

Figure IV.ii.2   Prototype controller schematic 

Figure IV.ii.3  Prototype controller 

 

   

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I. Introduction According to the United Nations, between 80 and 85 percent of people in the 

United States are living in areas classified as urban. This figure is projected to reach 

approximately 90 percent by the year 2050, which is a stark change from the low 

65 percent in the year 1950 [1]. With this increasing migration of the population to 

urban areas, people are choosing to live in highrise buildings and giving up the 

luxury of owning vehicles. This trend of vertical urbanization has caused a boom in 

the moving company industry, which saw a 2.4 percent increase between the years 

2017 and 2018, with an industry revenue peak of 18 billion dollars [2]. This mass 

exodus into multi-residence buildings with many levels of stairs and no elevators is 

the root of our problem.  

i. Problem Statement 

When moving into multi-level buildings, there is a question of how heavy 

furniture and appliances will be carried up to a residence on an upper level without 

the assistance of an elevator. Moving companies charge high rates due to the time, 

liability, and physical stress it takes to move furniture, mattresses, pianos, and 

other heavy objects up multiple flights of stairs. However, moving into a residence 

on any floor above ground is unrealistic without the help of professionals.  

ii. Proposed Solution 

Our solution to this problem is the creation of a powered modular assistive 

lift that can be used by movers of all types and can be installed easily and quickly on 

any straight set of stairs. Our objective is to develop a powered non-permanent lift 

that is easy to install and uninstall, with a simple and modular design that is 

designed to fit any length of straight stairs using a telescopic final rail. It limits the 

problem of movers causing damage to themselves or the items being moved when 

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carrying them manually up a long stairwell, and the liabilities associated therein. 

The following SolidWorks assembly is a detailed depiction of our solution.  

 

Figure I.ii.1: SolidWorks assembly of final product   

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II. Methodology  Below we will discuss both what the requirements are for our product and 

the methods we used to decide on how to fulfill those requirements. 

i. Mechanical 

With our goal of creating a powered non-permanent lift to assist in moving 

burdensome objects up flights of stairs, we had many mechanical aspects and 

functions to consider. There were several requirements that needed to be met in 

order to generate a product that would effectively perform the role of our 

proposed solution. First, we considered the construction of our product, namely 

the three sections involved: embarkation, straight stairs, and disembarkation.  

This year, we mainly focused on the design of the rail section that moves the 

object up the stairs, the straight rail section. This construction included the rails 

themselves, considering the cross-section of the bars and the connection between 

consecutive bars. Then, we considered the cross-bars necessary for stabilization 

and for ensuring that the rails can not translate or rotate when they are bearing a 

load. Finally, we iterated through designs of the cart that would carry the load. This 

included the cart itself as well as the wheels and the connection to the winch.  

Our final consideration was the materials that would be used for each of the 

components mentioned above. For the rails and crossbars, we considered various 

load-bearing metals. For the cart and wheels, we considered many materials, all 

strong enough to withstand the weight of the heaviest loads rated for our product. 

All of the above features were deliberated keeping in mind the safety, 

efficacy, portability, manufacturability, and efficiency of set up and take down time. 

These goals were important in the designs of all of the final products. 

In the following subsection, the methods we used to determine the electrical 

requirements for our product will be discussed.  

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ii. Electrical 

The electrical subsystem for the portable stair lift system is a complex set of 

components. There is no off-the-shelf solution that would fulfill every function 

necessary. The solution needed to be designed from the ground up considering 

many requirements. By evaluating the minimum requirements for the system to 

function and considering the needs of the user, the following list of product 

requirements was developed: safe, robust and reliable, portable, easy to use, low 

cost. 

First and foremost, the lift needs to be safe. No failure of the electrical 

system should result in an unsafe situation, and further, the electrical system 

should not allow for any unsafe operation to occur. The system must be protected 

against reasonable faults but unexpected faults should bring the lift system to a safe 

state (i.e. stopped). 

The lift system should reliably lift and lower the cart up and down the rails. 

Additionally, customers will be using this product in order to avoid lifting the items 

themselves. To be reliable, the electrical system must be robust. A part being 

dropped or a small voltage surge should not impact the operation of the product. 

The electronics must be designed within reasonable limits to handle conditions 

outside the optimal ranges. 

The lift system may be set up, used, taken down, moved, and set up again 

multiple times per day. The electronics must therefore be easily portable. Nothing 

should require extraneous time to connect or set up. It also should be configurable 

in different ways to allow for maximum flexibility in operation. For instance, in the 

case that there is no outlet near where the lift system is needed, it could be run on 

battery power. 

A diverse selection of people with varying skill levels will be using this 

product. Therefore, both the setup and control system should be intuitive and easy 

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to use. A complicated setup or user interface could result in a dissatisfactory 

experience in using the product and turn customers away from future use. 

Since much of the purpose of this project is to lower the cost of moving 

companies’ services, we have decided to prioritize low cost of the system. By 

integrating existing products, the electrical system can minimize cost while still 

maintaining high levels of safety and functionality. 

   

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III. Design In this section, we will illustrate the design choices we made for our product 

and the reasoning behind them. 

i. Mechanical  

As mentioned in section II.i, the mechanical components of our product had 

to be taken into careful consideration. Mathematical calculations and estimations 

were performed by our team members to complete an analysis thorough enough to 

ensure the safety and efficacy of our product. In addition, qualitative and 

observational data was considered and weighed in our design process.  

When performing analyses on our product, we started with the basics of our 

machine. Below, in Figures III.i.1 and III.i.2, free body diagrams of our complete 

product and of its critical components are shown. 

Figure III.i.1: Free body diagram of the lifting system [3] 

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Figure III.i.2: Free body diagram of the critical components of the lifting system 

[3] 

 

Following the completion of several free body diagrams, numerous 

computational calculations were done. These calculations measure various values 

related to our final product. The following table highlights the figures we thought 

most significant, with a schematic of our product to show the variables listed. 

 

 

Figure III.i.3: Product schematic [3]   

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Variable  Value  Units  Meaning 

V cart  0.038  m/s  Velocity of cart 

D 1  44.66  Ns/m  Damping coefficient 

D 2  0.437  Nm/rad/s  Damping coefficient 

D 3  0.25  Ns/m  Damping coefficient 

D A1-D output  0.3  Nm/rad/s  Damping coefficient 

K output  1203.31  N/m  Shaft stiffness 

K winch  1195.31  N/m  Shaft stiffness 

K 1, K 2  1862.03  N/m  Shaft stiffness 

K a  491.13  N/m  Shaft stiffness 

K s  8.08  N/m  Shaft stiffness 

L  1.2  m  Length 

θ  30  degrees  Angle from floor to stair surface Figure III.i.4: Table of calculated significant values [3] 

 

Our next design choice was the material we used in our final product. When 

iterating through choices, we considered many metal options. Eventually, we 

decided on steel and purchased a sample. Upon receiving the sample, we realized 

that the steel was too heavy to support the portability of our product. Because of 

this, we decided on aluminum rails and crossbars.  

For the cart, we also chose aluminum, because of its versatility, low cost, and 

low mass. As a protective layer on the cart and to decrease the chance of damage to 

the load, we chose to add a sheet of rubber. This will give the load a cushioned area 

to rest while being lifted on the rails. As for the wheels, we have chosen to use hard 

rubber wheels. These will be strong enough to hold the weight of the object and the 

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cart, but not so sharp that they would cause damage to the floor during 

embarkation or disembarkation.  

Finally, for the pulleys and the cord attached to the winch, we have decided 

to use steel and steel cable, respectively. These objects need to be completely fail 

safe, and are the most imperative to be strong. This is why we chose steel for each 

of these, despite the cost and the weight associated.  

In the following section, the design choices made for the electrical 

components of our product will be discussed. This section will, in detail, illustrate 

the electrical construction of the product and the basis on which we made these 

decisions.  

ii. Electrical 

The electrical system for our product can be broken down into three main 

pieces: the lifting mechanism, the power supply, and the controller. Each part was 

carefully designed to fulfill the electrical product requirements as outlined in the 

previous section. The lifting mechanism was chosen first, as the requirements for a 

power supply and controller would need to be based on supplying power for and 

interfacing with it. 

a. Lifting Mechanism 

The lifting mechanism, as explained in the mechanical design section above, 

consists of a steel cable running through a pulley system at the top of the rails. The 

cable is pulled from the bottom, moving the cart up the rails. There are a number of 

ways of pulling this cable, the most common being the use of a motor to wrap the 

cable around a drum (i.e. a winch). 

First, we had to consider whether to use a commercially available winch or to 

design our own. By designing our own we would have more control over the winch 

and could specify exactly which motor and control electronics to use. However, we 

would also need to design the gear system, brake system, and many other 

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intricacies that have already been determined in commercial products. The winch 

is a critical part of the product. Failure of the motor would result in a stopped cart 

that would be difficult to unload if it were in the middle of the staircase. Failure of 

the brake system could result in substantial damages. Considering this, we decided 

to use a commercially available winch. 

There are two basic types of winches, those that run on AC power and those 

that run on DC power. AC winches require a specified voltage and frequency for the 

motor inside, therefore different winches would be required for operation in 

countries with differing types of power available. AC winches would need to be run 

on power from an outlet, requiring an extension cord for locations that were not 

near one. DC winches are almost universally run on 12 volts DC power which can be 

generated by power supplies (input requirements can have a wide range to account 

for different countries' power) or supplied by a battery. To allow increased 

flexibility and portability, a DC winch will be used. 

DC winches can be based on either a permanent magnet motor or series 

wound motor. In a permanent magnet motor the stator is composed of permanent 

magnets whereas in a series wound motor the stator is made up of field coils 

wound in series with the armature. The permanent magnet motor requires less 

current as it does not need to power the field coils, but magnets may lose their 

strength over time and these motors are generally used for lighter duty 

applications. Given these tradeoffs, a winch with a permanent magnet motor was 

chosen for its lower current requirements. 

b. Power Supply 

The power supply needs to provide power for both the winch and the 

controller. The winch, as discussed above, requires 12 volts DC. The controller will 

run on lower voltage which can be generated using a DC-DC buck converter. A 12 

volt DC winch requires high amounts of current, upwards of 150 amps when loaded 

to full capacity. 

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A standalone AC-DC converter that can supply 150 amps at 12 volts would be 

very expensive. Additionally, without considering power losses, such a power 

supply would require 1800 watts from the AC side. This is equivalent to a 15 amp 

circuit common in houses. Running the winch for a long period of time or using a 

circuit with other loads already on it could result in tripped circuit breakers.  

In general, 12 volt DC winches are designed for use on vehicles where the 

power is supplied by the battery. Vehicle batteries are typically sealed lead-acid 

batteries, are capable of supplying high currents on demand, and come in varying 

capacities. To allow for longer operation, a battery charger could be plugged into 

the wall. The battery charger would require much less power from the outlet as it is 

able to deliver the power to the battery over a long time while the battery supplies 

the high demand of the winch for shorter intervals. This configuration also allows 

for lower cost and more portability. 

c. Controller 

The controller is the part the user will interact with most often. It must be 

robust, reliable, and easy to use. A commercially available winch typically comes 

with a controller that allows the user to operate the winch. It is wired to the winch 

and has two buttons, one for in (up the rails) and one for out (down the rails). 

However, there is not any way to automatically stop the winch when it gets to a 

certain point or add any other automation. To do this, a custom controller is 

required. 

The easiest way to implement a custom controller is to mimic the existing 

controller. To do this, we must first understand how a winch works and how the 

existing controller interfaces with it. 

A winch has a motor which must be run in both directions based on user 

input. To do this, an H-bridge driver is typically used. Due to the extremely high 

current requirements, mechanical relays are used. As seen in Figure III.ii.1 below, 

when relays 1 and 4 are activated, the electricity flows through the motor from 

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positive to negative and it rotates in one direction. When relays 2 and 3 are 

activated, the electricity flows through the motor from negative to positive and it 

rotates in the opposite direction. The controller consists of a single pole double 

throw (SPDT) switch; the pole is connected to 12 volts and the two throws are 

connected to each of the two pairs of relays described above. 

 

Figure III.ii.1: 12 volt Winch Control Schematic 

 

To mimic this switch, transistors can be used to apply 12 volts to the relays. 

Two P-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) will 

be connected to the relays similar to how the SPDT switch is connected. When 12 

volts are applied to the gate of the MOSFET, the “switch” is off. When 0 volts is 

applied, the “switch” is on. To drive the gates to 12 volts using a microcontroller 

(which operates at 5 volts), a NPN bipolar junction transistor (BJT) is used as seen 

below in Figure III.ii.2. Now we have a system that can be connected to the winch in 

place of the existing controller. This is useful as it requires no modification of the 

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winch and allows for sensors to be connected to the microcontroller for more 

advanced operation. 

 

Figure III.ii.2: 5 volt microcontroller output to 12 volt winch control 

 

When operating, the winch should only run between the limits of the top and 

bottom of the rail. It should not continue to pull the cart once the cart has reached 

the top or continue to let out cable when the cart is at the bottom. To know when 

the cart is at the top and bottom, limit switches will be attached to the rails. When 

the limit switch at the top is activated the controller will only allow the winch to 

lower the cart and when the limit switch at the bottom is activated the controller 

will only allow the winch to raise the cart. 

The limit switches used will be normally closed switches, that is when 

unpressed, the circuit is closed and when pressed, the circuit is open. The limit 

switches will be connected to 5 volts on one side and an input on the 

microcontroller on the other. When the microcontroller senses 0 volts on the input 

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(open circuit from the limit switch being pressed), it will not allow the winch to 

move in the respective direction. This configuration is fail-safe as any break in the 

wire would look the same as the limit switch being pressed instead of no longer 

being able to register it being pressed. 

The controller will have a user interface consisting of two buttons, a rotary 

encoder with a switch, and a small LCD. The two buttons will act like a normal 

winch control, one for up and one for down. By using a SPDT switch, only one 

direction will be able to be activated at a time. The LCD will be used to show the 

status of the device to the user and the rotary encoder will be used for user input 

such as acknowledging fault conditions or changing settings. The custom controller 

will be housed in a protective plastic case. A flow diagram of the program control 

can be seen in Figure III.ii.3 below.  

 

Figure III.ii.3: Program flow chart 

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The three parts of the electrical system work together to provide a low cost, 

safe, and user friendly portable stair lift.   

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IV. Implementation The following section will be an explanation of our prototype and the 

realization we completed on this phase of our project. 

i. Mechanical 

For our initial prototype, we chose to build a small-scale model of our 

machine. This model was not made completely to scale because we used the 

actual-sized aluminum rails as well as crossbars and fixtures. However, the cart and 

the length and width of the model were made scaled down. This way, we could 

analyze the ability of the cart to navigate up the rails, without building a full-size 

model. In this model, we used inexpensive and accessible materials in order to 

fashion our cart. This included birch plywood as the base of the cart, rubber drawer 

protectors, small rubber wheels, and piano hinges to hold the wood together and to 

increase ease of embarkation and disembarkation. From what was completed, the 

model design was successful. The section below discusses the electrical 

components of our semi-scaled prototype.  

ii. Electrical 

An electrical system for the small-scale mode was designed and built. The 

model electrical system consists of a motor, a motor control circuit, the 

microcontroller, and buttons for user input. The motor control circuitry was 

designed to be functionally equivalent to a winch that would be used in the final 

product to help more accurately design the microcontroller software. 

A small 12 volt DC motor was purchased for the model. The type of hobby 

motor used is a “geared motor”, it is a high speed brushed DC motor connected to 

an internal planetary gear system to reduce the spindle speed to 50 RPM 

(revolutions per minute). To control the motor, an H-bridge driver was built using 

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MOSFETs. Two P-channel MOSFETs make up the high voltage side of the bridge 

(the side between the motor and the 12 volt supply) and two N-channel MOSFETs 

make up the low voltage side (between the motor and ground). By alternating which 

pair of MOSFETs is “on” at a time the motor can be controlled to rotate in either 

direction. 

The N-channel MOSFETs act similarly to relays; when 0 volts are applied to 

the gate they are off and when 12 volts are applied they are on. For the P-channel 

MOSFETs, however, this is inverted. To allow the P-channel MOSFETs to be 

controlled as relays would be, an inverter using complementary MOSFETs was 

added to the gate input. With the inverter, a 0 volt input would result in a 12 volt 

output to the MOSFET’s gate, turning it off and a 12 volt input would result in a 0 

volt output, turning the MOSFET on. Opposing pairs of MOSFETs are connected 

together, resulting in two control lines. Now, the H-bridge driver and motor system 

seen below in Figure IV.ii.1 are functionally equivalent to a winch. 

 

Figure IV.ii.1: H-bridge Motor Controller with MOSFETs 

 

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An Arduino, a popular microcontroller development board, was used for the 

prototype. Four pushbuttons are connected to inputs on the Arduino, two to 

simulate the SPDT switch and two to act as the limit switches. Two Arduino outputs 

drive NPN transistors which in turn drive P-channel MOSFETs connected to the 12 

volt supply to turn on and off the two control lines. The microcontroller is 

programmed almost the same as it would be on the product with two main 

differences: it checks the two user input buttons exclusively (would not be 

necessary on the product as the SPDT switch does this by design) and there is no 

LCD (the Arduino is connected to a computer and the Serial Monitor function is 

used instead). The complete electrical prototype can be seen in the figures below. 

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Figure IV.ii.2: Prototype controller schematic 

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Figure IV.ii.3: Prototype controller 

 

The current prototype worked as a test of the 5 volt to 12 volt driver circuit 

and the microcontroller code. It can be used in the future as a test bed for 

additional features, such as more sensors or a different user interface. 

   

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V. Plans Going Forward The team this year was composed of both third-year and fourth-year 

students. This year, we designed a basic, straight rail system, a cart, a lifting 

mechanism, and a controller. Next year, the remaining students will continue 

working on the project, developing it further and adding more features. The main 

goal for next year is to design the system so it will work on stairs with a corner or 

turn in the middle of them. Additional design goals include simpler assembly and 

disassembly, increased load capacity, and reduction of cost to manufacture. 

   

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VI. Conclusion In recent decades, there has been a mass migration of the U.S. population 

from rural to urban areas, especially presenting itself in younger generations. With 

so many people moving into cities that, comparably, have a lot less space for 

residences, the solution has been to build vertically. People are moving into homes 

built in highrises, many of them lacking elevators. This has caused an upsurge in the 

moving company industry, which routinely charges high fees for moving heavy 

objects up multiple flights of stairs. These high prices are due to the time, effort, 

physical strain, and liability associated with the moving process. Our solution is to 

create a product that could aid movers in transporting cumbersome items up 

flights of stairs efficiently, effectively, and at a low cost.  

To accomplish this, we designed a powered portable assistive lift. This 

machine will be able to be set up and taken down in less than an hour, will move 

objects up to 800 pounds up any straight set of stairs, and will use the power of an 

electric winch to do so. This product will be modular, and hence will be able to fit 

on any set of stairs without turns or curves. This design is made for movers to bring 

to multiple establishments per day to make moving into multi-level buildings many 

times easier.  

This project will continue into next year, with members of our team 

continuing on. We hope to improve upon our product until it is able to perform 

many types of stairs, including ones with turns. We also hope to make the design 

simpler in set-up and take-down and more affordable. This year, our completed 

project is merely the base model of what our team hopes to accomplish in the 

future.  

 

 

   

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References  

[1] “World Urbanization Prospects - Population Division,” United Nations . [Online]. 

Available: https://population.un.org/wup/Country-Profiles/ . [Accessed: 

15-May-2020]. 

[2] M. C., “Moving Trends & Relocation Industry Analysis,” Movers Development , 

07-May-2020. [Online]. Available: 

https://moversdev.com/moving-trends-analysis/. [Accessed: 15-May-2020]. 

[3] Z. Huang, L. Zhang, and M. Zhu, “Portable Heavy Load Lifting Mechanism.” 

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