Introduction to how Elevators work
In the 1800s, new iron and steel production processes revolutionized the world of
construction. With sturdy metal beams as their building blocks, architects and engineers
could erect monumental skyscrapers hundreds of metre in the air. But these towers
would have been basically unusable if it weren't for another technological innovation
that came along around the same time. Modern Elevators are the crucial element that
makes it practical to live and work dozens of stories above ground. High-rise cities like
New York absolutely depend on Elevators. Even in smaller multi-story buildings,
Elevators are essential for making offices and apartments accessible to handicapped
In this module, we'll find out how these ubiquitous machines move you from floor
to floor. We'll also look at the control systems that decide where the Elevator goes and
the safety systems that prevent catastrophes.
1. The main type of Elevators
The concept of an Elevator is incredibly simple it's just a compartment attached to
a lifting system. Tie a piece of rope to a box, and we've got a basic Elevator. Of course,
modern passenger and freight Elevators are a lot more elaborate than this. They need
advanced mechanical systems to handle the substantial weight of the Elevator cage and its
cage go. Additionally, they need control mechanisms so passengers can operate the
Elevator, and they need safety devices to keep everything running smoothly.
There are two major Elevator designs in common use today: hydraulic
Elevators and roped Elevators.
1.1. Hydraulic Elevators
Hydraulic Elevator systems lift a cage using a hydraulic ram, a fluid-driven piston
mounted inside a cylinder. We can see how this system works in the diagram below, see
figure 1.
The cylinder is connected to a fluid-pumping system (typically, hydraulic systems like
this use oil, but other incompressible fluids would also work). The hydraulic system has
three parts:
Figure 1
A tank (the fluid reservoir)
A pump, powered by an electric motor
A valve between the cylinder and the reservoir
The pump forces fluid from the tank into a pipe leading to the cylinder. When the
valve is opened, the pressurized fluid will take the path of least resistance and return to
the fluid reservoir. But when the valve is closed, the pressurized fluid has nowhere to go
except into the cylinder. As the fluid collects in the cylinder, it pushes the piston up,
lifting the Elevator cage.
When the cage approaches the correct floor, the control system sends a signal to
the electric motor to gradually shut off the pump. With the pump off, there is no more
fluid flowing into the cylinder, but the fluid that is already in the cylinder cannot escape
(it can't flow backward through the pump, and the valve is still closed). The piston rests
on the fluid, and the cage stays where it is.
To lower the cage, the Elevator control system sends a signal to the valve. The
valve is operated electrically by a basic solenoid switch (check out How Electromagnets
Work for information on solenoids). When the solenoid opens the valve, the fluid that
has collected in the cylinder can flow out into the fluid reservoir. The weight of the cage
and the cage go pushes down on the piston, which drives the fluid into the reservoir. The
cage gradually descends. To stop the cage at a lower floor, the control system closes the
valve again.
This system is incredibly simple and highly effective, but it does have some
drawbacks. In the next section, we'll look at the main disadvantages of using hydraulics.
1.2. Pros and Cons of Hydraulics
The main advantage of hydraulic systems is they can easily multiply the relatively
weak force of the pump to generate the stronger force needed to lift the Elevator cage
(see How Hydraulic Machines Work to find out how). But these systems suffer from two
major disadvantages. The main problem is the size of the equipment. In order for the
Elevator cage to be able to reach higher floors, you have to make the piston longer. The
cylinder has to be a little bit longer than the piston, of course, since the piston needs to be
able to collapse all the way when the cage is at the bottom floor. In short, more stories
means a longer cylinder. The problem is that the entire cylinder structure must be buried
below the bottom Elevator stop. This means you have to dig deeper as you build higher.
This is an expensive project with buildings over a few stories tall. To install a hydraulic
Elevator in a 10-story building, for example, you would need to dig at least nine stories
The other disadvantage of hydraulic Elevators is that they're fairly inefficient. It
takes a lot of energy to raise an Elevator cage several stories, and in a standard hydraulic
Elevator, there is no way to store this energy. The energy of position (potential energy)
only works to push the fluid back into the reservoir. To raise the Elevator cage again, the
hydraulic system has to generate the energy all over again.
The roped Elevator design gets around both of these problems. In the next section,
we'll see how this system works.
2. The Cable System
The most popular Elevator design is the roped Elevator, Figure 2. In roped
Elevators, the cage is raised and lowered by traction steel ropes rather than pushed from
below to the Elevator cage, and looped around a sheave (3). A sheave is just a pulley with
a grooves around the circumference. The sheave grips the hoist ropes, so when you rotate
the sheave, the ropes move too.
The sheave is connected to an electric motor (2). When the motor turns one way,
the sheave raises the Elevator; when the motor turns the other way, the sheave lowers
In gearless
In geared Elevators, the motor turns a gear train that rotates the sheave. Typically, the
sheave, the motor and the control system (1) are all housed in a machine room above the
Figure 2
Elevator shaft. The ropes that lift the cage are also connected to a counterweight (4),
which hangs on the other side of the sheave. The counterweight weighs about the same
as the cage filled to 40-percent capacity. In other words, when the cage is 40 percent full
(an average amount), the counterweight and the cage are perfectly balanced. The purpose
of this balance is to conserve energy. With equal loads on each side of the sheave, it only
takes a little bit of force to tip the balance one way or the other.
Basically, the motor only has to overcome friction the weight on the other side does most
of the work. To put it another way, the balance maintains a near constant potential
energy level in the system as a whole. Using up the potential energy in the Elevator cage
(letting it descend to the ground) builds up the potential energy in the weight (the weight
rises to the top of the shaft). The same thing happens in reverse when the Elevator goes
up. The system is just like a see-saw that has an equally heavy kid on each end. Both the
Elevator cage and the counterweight ride on guide rails (5) along the sides of the Elevator
shaft. The rails keep the cage and counterweight from swaying back and forth, and they
also work with the safety system to stop the cage in an emergency.
Roped Elevators are much more versatile than hydraulic Elevators, as well as more
efficient. Typically, they also have more safety systems. In the next section, we'll see how
these elements work to keep people from plummeting to the ground if something goes
3. Safety Systems
Elevators are built with several redundant safety systems that keep them in
position, see Figure 3.
Figure 3
The first line of defense is the rope system itself. Each Elevator rope is made from
several lengths of steel material wound around one another. With this sturdy structure,
one rope can support the weight of the Elevator cage and the counterweight on its own.
But Elevators are built with multiple ropes (between four and eight, typically). In the
unlikely event that one of the ropes snaps, the rest will hold the Elevator up.
Even if all of the ropes were to break, or the sheave system were to release them, it
is unlikely that an Elevator cage would fall to the bottom of the shaft. Roped Elevator
cages have built-in braking systems, or safeties, that grab onto the rail when the cage
moves too fast. In the next section, we'll examine a built-in braking system.
3.1. Safety Systems
Safeties are activated by a governor when the Elevator moves too quickly. Most
governor systems are built around a sheave positioned at the top of the Elevator shaft.
The governor rope is looped around the governor sheave and another weighted sheave at
the bottom of the shaft. The rope is also connected to the Elevator cage, so it moves when
the cage goes up or down. As the cage speeds up, so does the governor. The diagram
below shows one representative governor design, see Figure 4.
Figure 4
In this governor, the sheave is outfitted with two hooked flyweights (weighted metal
arms) that pivot on pins. The flyweights are attached in such a way that they can swing
freely back and forth on the governor. But most of the time, they are kept in position by a
high-tension spring. As the rotary movement of the governor builds up, centrifugal
force moves the flyweights outward, pushing against the spring. If the Elevator cage falls
fast enough, the centrifugal force will be strong enough to push the ends of the flyweights
all the way to the outer edges of the governor. Spinning in this position, the hooked ends
of the flyweights catch hold of ratchets mounted to a stationary cylinder surrounding the
sheave. This works to stop the governor. The governor ropes are connected to the
Elevator cage via a movable actuator arm attached to a lever linkage. When the governor
ropes can move freely, the arm stays in the same position relative to the Elevator cage (it
is held in place by tension springs). But when the governor sheave locks itself, the
governor ropes jerk the actuator arm up. This moves the lever linkage, which operates the
brakes. In this design, the linkage pulls up on a wedge-shaped safety, which sits in a
stationary wedge guide. As the wedge moves up, it is pushed into the guide rails by the
slanted surface of the guide. This gradually brings the Elevator cage to a stop.
3.2. Safety Systems: More Backups
Elevators also have electromagnetic brakes that engage when the cage comes to a
stop. The electromagnets actually keep the brakes in the open position, instead of closing
them. With this design, the brakes will automatically clamp shut if the Elevator loses
Elevators also have automatic braking systems near the top and the bottom of the
Elevator shaft. If the Elevator cage moves too far in either direction, the brake brings it to
a stop.
If all else fails, and the Elevator does fall down the shaft, there is one final safety
measure that will probably save the passengers. The bottom of the shaft has a heavyduty shock absorber system typically a piston mounted in an oil-filled cylinder. The
shock absorber works like a giant cushion to soften the Elevator cage's landing.
In addition to these elaborate emergency systems, Elevators need a lot of machinery just
to make their stops. In the next section, we'll find out how an Elevator operates under
normal conditions.
4. Control system
Many modern Elevators are controlled by a computer. The computer's job is to
process all of the relevant information about the Elevator and turn the motor the correct
amount to put the Elevator cage where it needs to be. In order to do this, the computer
needs to know at least three things:
Where people want to go,
Where each floor is,
Where the Elevator cage is.
Finding out where people want to go is very easy. The buttons in the Elevator cage
and the buttons on each floor are all wired to the computer. When we press one of these
buttons, the computer logs this request.
There are lots of ways to figure out where the Elevator cage is. In one common
system, a light sensor or magnetic sensor on the side of the cage reads a series of holes on
a long vertical tape in the shaft. By counting the holes speeding by, the computer knows
exactly where the cage is in the shaft. The computer varies the motor speed so that the
cage slows down gradually as it reaches each floor. This keeps the ride smooth for the
In a building with many floors, the computer has to have some sort of strategy to
keep the cages running as efficiently as possible. In older systems, the strategy is to avoid
reversing the Elevator's direction. That is, an Elevator cage will keep moving up as long as
there are people on the floors above that want to go up. The cage will only answer "down
calls" after it has taken of all the "up calls." But once it starts down, it won't pick up
anybody who wants to go up until there are no more down calls on lower floors. This
program does a pretty good job of getting everybody to their floor as fast as possible, but it
is highly inflexible.
More advanced programs take passenger traffic patterns into account. They know
which floors have the highest demand, at what time of day, and direct the Elevator cages
accordingly. In a multiple cage system, the Elevator will direct individual cages based on
the location of other cages.
In one cutting-edge system, the Elevator lobby works like a train station. Instead
of simply pressing up or down, people waiting for an Elevator can enter a request for a
specific floor. Based on the location and course of all the cages, the computer tells the
passengers which cage will get them to their destinations the fastest.
Most systems also have a load sensor in the cage floor. The load sensor tells the
computer how full the cage is. If the cage is near capacity, the computer won't make any
more pick-up stops until some people have gotten off. Load sensors are also a good safety
feature. If the cage is overloaded, the computer will not close the doors until some of the
weight is removed.
In the next section, we'll look at one of the coolest components in an Elevator:
the Automatic Doors.
5. Automatic Doors
The automatic doors at grocery stores and office buildings are mainly there for
convenience and as an aid for handicapped people. The automatic doors in an Elevator,
on the other hand, are absolutely essential. They are there to keep people from falling
down an open shaft. Elevators use two different sets of doors: doors on the cages and
doors opening into the Elevator shaft. The doors on the cages are operated by an electric
motor, which is hooked up to the Elevator computer. You can see how a typical dooropener system works in the Figure 5 below.
Figure 5
The electric motor turns a wheel, which is attached to a long metal arm. The metal
arm is linked to another arm, which is attached to the door. The door can slide back and
forth on a metal rail. When the motor turns the wheel, it rotates the first metal arm,
which pulls the second metal arm and the attached door to the left. The door is made of
two panels that close in on each other when the door opens and extend out when the
door closes. The computer turns the motor to open the doors when the cage arrives at a
floor and close the doors before the cage starts moving again. Many Elevators have
a motion sensor system that keeps the doors from closing if somebody is between them.
The cage doors have a clutch mechanism that unlocks the outer doors at each floor
and pulls them open. In this way, the outer doors will only open if there is a cage at that
floor (or if they are forced open). This keeps the outer doors from opening up into an
empty Elevator shaft.
In a relatively short period of time, Elevators have become an essential machine.
As people continue to erect monumental skyscrapers and more small buildings are made
handicap-accessible, Elevators will become an even more pervasive element in society. It
is truly one of the most important machines in the modern era, as well as one of the
6. Design and calculation of hardware
This section discuss about the design of an elevator by the calculation of certain
parameters which defines the operation of the elevator.
6.1. Block diagram
Based upon the need the blocks have been developed and drawn as block diagram,
Figure 6. After the designing of the Elevator prototype, calculation has to be done for
each and every component present in the model.
Figure 6
Figure 6 depicts the block diagram of a PLC based Elevator. Supply is given to both the
PLC controller as well as to the motor. This is attached with the elevator cabin with the
use of proximity sensor (LDR) the position of the cabin will be determined and by using
the door sensor (LDR) the position of the door is found out and the cabin is made to work
accordingly. The drive system for the motor may have inverter with a rectifier or it may
depends upon the type of power converter that the designer choose.
6.2. Calculation of the Elevator lifting mechanism
Calculation of the Elevator lifting mechanism includes three characteristic parts.
Initial data of traction calculation are:
destination and kinematic scheme of the elevator,
load capacity,
the basic size and the cage speed,
the cage mass,
the design of doors,
number of stops and the height of cage lifting,
operating mode of the Elevator
Duration Factor of the Elevator.
One of the factors determining the choice of the drive motor power is of the
elevator kinematic scheme, which is a scheme of interaction of lifting mechanism with
moving parts of the Elevator - cabin and counterweight.
Consider the calculation of the motor power for a traditional kinematic scheme of
the Elevator:
1. Efforts of F1 and F2 in the ropes on either side of the rope leading pulley (RLP) are
F1  (m  m0  qx) ,
F 2  mcw  q( H  x)g
where m0 is mass of an empty cage, kg; m is mass of lifting net load, kg; q is the specific
mass of a rope, kg/m; x is the length of a rope from the cage, m; mcw is the mass of a
counterweight, kg; H is the lifting height of a cage, m; g is the acceleration of gravity,
m/c2. Taking m0  150kg , m  400kg , q  1,5kg / m , x  10m , mcw  180kg , H  25m , we
can calculate correspondingly efforts of F1 and F2
F1  (150  400  1,5 10)  9,8  5537 N ; F2  180  1,5  (25  10)9,8  1984,5N
2. Effort on the RLP is determined
F  F1  F2  m  m  q(2 x  H ) ,
where α is the balance coefficient equal to 0,4-0,6. Using the obtained values of F1 and
F2, we can calculate effort on the RLP
F  5537  1984,5  3552,5N .
3. The torque and power to the motor shaft at its operation are calculated according to
Tcal.m 
2ired red
Pcal.m 
 red
where DRLP is RLP diameter, m; ired is the reducer gear ratio; vrat is the rated speed of
cage movement, m/s;  red is the reducer coefficient of performance.
Taking DRLP  0,575m , ired  48 , red  0,57 , vrat  0,71m / s , motor torque and power are
Tcal.m 
3552,5  0,575
 37,33Nm ;
2  48  0,57
Pcal.m 
3552,5  0,71
 4425,04W
4. The necessary frequency of rotation of the motor is determined using the equation
m 
vrat  ired
where RRLP is the RLP radius, m. Substituting known values we obtain
m 
 48  118,54s 1 .
5. Based on the conditions of Pcal.m  Prat and m  rat a type of motor from the catalog
are selected.
6.3. Principles of Elevator control
Operation mode of an Elevator is characterized by frequent switching on and
switching off. At that the following stages of Elevator movement can be marked out
acceleration of the motor up to a steady speed v ,
movement at a steady speed,
decrease in speed when approaching the destination floor (directly to zero or to a
low speed of reaching),
Braking and stopping of the Elevator cage at the destination floor with the
required accuracy.
Necessity to limit the acceleration and jerk and provision maximal performance of
an Elevator requires that during the transient processes electric drive provides
acceleration and deceleration of the cage with the maximum permissible values of
acceleration and jerk. Corresponding to fulfillment of this condition graph of the cage
movement is shown in Figure 7. It is usually considered to be optimal because it ensures
the minimum duration of the cage overclocking and accelerating modes. At this graph the
values of acceleration and jerk at certain intervals of the transition process are kept
constant and equal to the maximum permissible values.
Figure 7
6.3.1. Functional scheme of Elevator Control System
Scheme of the Elevator control system in normal operation mode is shown in Figure 8 in
that the following notations are adopted:
SCC– Signals of Calls and Commands; PMD - Positionally-Matching Device;
SDM – Signal of Downward Movement; SUM - Signal of Upward Movement;
SHS – Signal of a High Speed; SLS – Signal of a Low Speed; SDC – Signal of a Door
SDO - Signal of a Door Opening; SCL - Signal of a Cage Location; SPPC - Signal of
presence of a passenger in the cage; SFLC - Signal of a Fully Loaded Cage;
SCO - Signal of a cage overloading; SCEO - Signal of control of the Elevator overclocking;
Figure 8
7. Allocation scheme of the Elevator Control Devices
Allocation scheme of the Elevator control devices is shown in Figure 9. The main
control board is installed in the controller box. Sequential line of data transfer RSL is
divided into channels of the cage and a hoistway. Cage channel to which is connected the
terminal box of the cage is an overhead cable.
The scheme of sensors allocation in the cage is presented in the Figure 10a, where
1LV and 2LV are sensors of precise stopping; terminal sensors 1LS and 2LS, and the
magnetic shunts are shown in Figure 10b. Position sensors, door contacts and a motor of
the door drive are switching on to the discrete inputs of the controlling board through
overhead cable. Service Tool (ST) is used for the installation and maintenance as well as
for programming and reprogramming the program areas of the controlling boards.
Figure 9
1- controller box; 2 - positional indicator; 3 - storey buttons; 4 - position sensor.
Figure 10
8. A microcontroller based Elevator Control System.
Nowadays Elevators Control Systems are mainly designed on the basis of a
microcontrollers. Consider operation of the control system on the example of “Otis”
company Elevator. This approach is justified because the Elevators control systems
practically do not differ each other, except for the number of serviced floors and loadcarrying capacity.
The considered control system (see Figure 11) is realized on a modular scheme on
the basis of MCS 220 (300) controller with LCB-II controlling board and includes the next
OCSS operational control,
MCSS movement control,
DBSS controlling of the main drive and braking,
DCSS controlling of the door drive.
Figure 11
Modules are intended for fulfillment definite functions by the control system. OCSS and
MCSS subsystems are united by LCB-II board that has two independent channel of
remote RSL sequential line of data transfer. Modules of the microprocessor control system
are united each other by sequential lines of data transfer. Calls and commands buttons,
alarm lamps, indicators of movement direction, floor indicators and complimentary keys
are connected to the remote stations located at the stopping ground and in the cage.
Communication between controller and remote stations is realized by means of sequential
line of data transfer. That kind of system configuration provides simple installation,
detection and replacement of faulty components as well as protection against
unauthorized access to the system. All system information and system signals can be
accessible only after connection of the Service Tool.
9. Functions of the Control System Subsystems
Operational control subsystem OCSS interacts with the subsystem MCSS and in
case of group control with other groups of OCSS via serial communication. This
subsystem performs the following functions;
receiving commands from the cage or from the floor,
issuing commands on the movement to the subsystem MCSS,
control of indication of movement direction and cabin position
obtaining information from other subsystems of OCSS group.
The subsystem contains in the memory addresses of the remote stations, the
parameters of their inputs and outputs, as well as other parameters and modes of
operation of the elevator, which are installed at the factory, but can be changed at the
installation site.
Movement control MCSS subsystem is an element of the modular Elevator control
system. This subsystem is connected via a sequential communication to the following
OCSS operational control,
DCSS door drive,
DBSS Elevator drive and brake control.
Software of subsystem MCSS is intended for issuing commands to the subsystem of
the drive and braking control needed for Elevator movement control under the set
program after request coming from operational control subsystem OCSS and from the
various interfaces of manual control. This subsystem issues commands to the interface
subsystem of the door drive to move the doors after request coming from OCSS.
Subsystem MCSS performs the following functions:
movement control that is control by the sequence of logic states of movement,
ensuring functions of preparation to movement and correction in the height,
generation of distribution graphs of speed and acceleration,
providing control of electric drive brake during normal operation,
determination stopping distance and breakpoint, information transfer on the state
of the cage into subsystem OCSS,
determination of the speed and direction of the cage movement, parameters of the
absolute position, location of the cage in relation to the next specified floor,
protection, ensuring current control over the safety devices and emergency
the current control of data, input of the setting parameters, event registration,
Drive and braking control subsystem DBSS is intended to provide movement
control of the Elevator cage after receiving commands from the movement control
subsystem. DBSS performs the following functions:
coupling with a winch,
braking control,
coupling with PVT speed encoder,
coupling with subsystem MCSS,
precise tracking of the specified profile of speed distribution,
independent verification of the speed.
This subsystem, being a system of frequency control of the asynchronous electric
drive speed, designed on the basis of PWM that consists of two main components: the
control board and power unit. Functional scheme of the power unit is shown in Figure
12. The power unit consists of a circuit connection to electric network and the inverter
consisting of an uncontrolled three-phase full-wave rectifier, and a three-phase inverter.
Three-phase electric network voltage is rectified and smoothed by the filter in the DC
communication link. After this, the PWM-controlled transistor inverter by means of a
designated sequence of IGBT transistors commutation converts the DC voltage into threephase AC one with variable frequency. Transistors provide a high speed switching with a
carrier frequency of 10 kHz. Information on output values is accepted from the speed
sensor BR located on the motor shaft.
Controller MCS exchanges signals with DBSS (control signals V1 - V4, encoded by
four bits; UIB, DIB, NOR are signals, each of which is encoded by one bit; signals of the
current state of the Elevator DS1-DS3 that are encoded by three bits. Signals UIB, DIB
and NOR are data defining the initial state of the system DBSS before work, i.e. Elevator
works in the testing mode or normal one. Closed-loop speed control provides precise and
comfortable drive behavior at each moment of work. The measured speed of the motor is
input into PI speed regulator. The system provides high dynamic speed accuracy.
Figure 12