Development of a Building Elevator System

Students: Mike Buck and Bonnie Lawson (in ENSE 621)
Faculty Advisors: Mark Austin and John Baras.
Date: September 2002 -- May 2003.

TABLE OF CONTENTS

  1. Introduction
    Purpose : Setup problem.
    Topics : Overview of elevator design; development process; system framework and boundary.

  2. Goals, Scenarios, and Use Cases
    Purpose : Develop use cases, scenarios and system requirements.
    Topics : Goals and scenarios; actors; initial use cases with activity diagrams.

  3. Models of System Behavior and System Structure
    Purpose : Create simplified models of behavior and structure.
    Topics : Models of system behavior; models of system structure.

  4. Requirements and Specifications
    Purpose : Generate subsystem requirements and specifications.
    Topics : Subsystem requirements; traceability.

  5. Measures of Effectiveness
    Purpose : Establish measures of effectiveness for the elevator design.
    Topics : Reliability; cost; performance; safety.

  6. Tradeoff Analysis
    Purpose : Create framework for trade-off analysis for selection of components in a small subsystem.
    Topics : System performance and economics; decision variables; CPLEX problem formulation; trade-off scenarios and analysis results.

  7. Component - System Testing
    Purpose : Develop procedures of system test, verification and validation.
    Topics : Test plan ..... etc.

  8. References and Web Resources


Introduction

Elevators are prevalent throughout many multi-level structures. They control the flow of foot traffic between various floors of buildings, they allow disabled persons to access upper-level floors, and they facilitate the movement of large items (such as furniture and office equipment) between various levels of the building.


Overview of Elevator Design

Hydraulic and roped elevators are the two main types of elevators in use today.

Factors that govern elevator type and design include cost, speed, capacity requirements, safety, and reliability. There are risks involved in the use of elevators. Many factors must be taken into account in order to ensure that persons are not stuck in elevators for long periods of time, or worse that the elevator does not loose stability and plummet to the basement from a high floor. In the event of fire, elevators are not to be used. The engineers designing the system, as well as the construction company assigned to build the building and installing the elevator system are responsible for the safety and smooth operation of the elevator system. The building architects are responsible for providing space and structural support for the elevator. The engineers designing the elevator must ensure that the elevators will perform as specified in a safe manner (meeting all safety requirements and compliance standards). The engineers are also responsible for designing elevators that can be manufactured easily and feasibly and for designing "fool-proof" systems that will work reliably and safely in the event that they are misused. The elevator users are responsible for operating the elevator according to the safety specifications laid out. The maximum capacity is not to be exceeded by users. The building owners are responsible for maintenance of the elevator and for assuring the elevator is compliant before use (via a certificate). The latter certificate is to be available to users so that they can be assured that the elevator is safe to use. The engineers are to communicate to the owners the specifications of the elevators and are to communicate to the builders the space and support that is needed for the elevator to function in its intended way. The architects and builders are to communicate with the elevator engineers to ensure that the structure and elevator are compatible.


Development Process

In the design, installation, and use of an elevator system much communication goes on between the design engineers, Construction Company, building architects, building owner, and users. The first means of communication is the rules and regulations created by government with the help of various other public organizations. Building codes, fire regulations, the American Disabilities Act (ADA) are a few examples of these rules to which the engineers and architects must submit. As far as the specific rules governing the design of elevators, EN81 and ASME/ANSI A17.1 give detailed documentation of what criteria must be met. These documents are the primary means by which the regulating bodies communicate the regulations to the engineers and architects. The engineers that design the elevator must then communicate to the architects to ensure them that these standards are met. They also convey the features and functions of the elevator system and help them choose the elevator that best fits their needs. The engineers also must convey to the construction company in charge of building the building certain specifications that are critical to the installation of the elevator in the structure. The architects, if they are in fact involved at this point in the design process, then must communicate with the builders what how the building will be built with the elevator system included in the design. The architects communicate with the owner and vice versa to ensure the building design meets all the needs and expectations of the owner as well as meeting all safety standards. The owner then communicates with the user, via some type of certification posted in a public location, that the elevator is safe to use and ?up to code?.


System Framework and Boundary

This is a basic overview of the communications that take place in the design, installation, and use of an elevator system. This communication process is in the adjacent figure.

Figure 1. System Framework and Boundary for Elevator Development


Problem Statement

The purpose of this case study is to apply the fundamentals of systems engineering to the operation and selection of an elevator system.  The final products are a high-level representation of how the elevator system works and a decision of the best elevator system for a given situation


Goals, Scenarios and Use Cases

An elevator system is an easy concept to understand. In its simplest form there is a single elevator, moving vertically, for serving users on all floor levels. When a user desires to ride the elevator they press a button located in the elevator lobby. The lobby is defined as the area of the building, adjacent to the elevator, on a given floor. The user presses the either of two buttons, up or down, correlating to the direction they want to move. The elevator responds to the pressing of this button via the path of predetermined travel routes or cycles. An elevator that is initially idle will dispatch immediately to the floor of the user request. The elevator will stop only for other requests for moving in its current direction of travel. Once all requests have been serviced in one direction of a cycle, the elevator will reverse and begin responding to requests in the same manner as before. An elevator that is idle for several minutes will return to the ground, or bottom, floor.

For the sake of simplicity, the elevator system examined in this case study will be for a small business office.  The office is in a 6 story building that houses typical office things like desks, chairs, computers, and employees.  The details of the office will be further explained through requirements and specifications later.


Goals and Scenarios

Use case development is a prerequisite to the identification of objects/subsystems and object/subsystem operations. A use case describes a single goal and all the things that can happen as the user attempts to reach that goal.

Goal 1. The Elevator System must respond to user requests.

Goal 2. The Elevator System must have a good interface.


Identify Actors

The actors are as follows:

System Boundary

The system boundary is defined by the elevator itself.


Initial Use Case Diagram

Our initial use case diagram has two actors and two use cases.

Figure 2. Initial Use Case Diagram


Use Cases with Activity Diagrams

Use Case 1 : Request Elevator Use Case.

Postconditions : The elevator has arrived in response to a request.
Alternative Flow of Events : None.
Assumptions : None.

Activity Diagram for Use Case 1

Figure 3. Flow of Events for Use Case 1.

Use Case 2: Floor Selection Use Case

Postconditions : Elevator arrives at floor selected by the user.
Alternative Flow of Events : None.
Assumptions : None.


Models of System Behavior and Structure

We view overall behavior of the "elevator system" as the combination of systems -- the users, the elevator system, and the logic control system. The elements of each system are summarized in Figure 4.

Figure 4. Elements of behavior for users of the elevator, the elevator system itself, and the elevator control system.

The behaviors of the elevator user, control system, and system hardware (elevator system) are described therein. For further behavioral analysis this study includes a number of operational scenarios, their context, and sequence and ordering of events, prior requirements, and metrics of performance.


Operational Scenarios

Scenario 1

Scenario 2


Elevator Control System

Every elevator system must be equipped with a control system that will receive the rider input and translate that into a signal that will control the actual elevator hardware.

Logic Controller

These systems include a logic controller that takes the rider input and translates it into meaningful actions. According to Howstuffworks, Inc. the logic controller's central processing unit (CPU) must be given at least three critical pieces of information, namely:

The first input comes directly from the riders. The user pushes the floor number inside the elevator car signaling their final destination. When the floor buttons are pushed the logic controller receives the signal and registers the user's request.

The second input "where each floor is" can often be determined by the addition of holes located on a long vertical tape inside the elevator shaft. The elevator car is equipped with a light or magnetic sensor that reads the number of and which holes are being passed by the elevator car as it ascends and descends. The computer is equipped with a means of varying the speed of electric motor (connected to either of the following system designs) so it can slow down the car when it is approaching a floor at which it is to stop.

Scheduling Elevator Operations

Logic controllers must have some way to determine in what order riders should be picked up and dropped off. Many elevator systems will move in one direction (e.g., upward) and only pick up riders that are also signaling to go in that direction (e.g., upward). When the final floor that has been requested in that direction (e.g., upward) is reached the elevator will turn around and pick up all riders signaling the opposite direction (e.g., downward). Of course, the elevator car also stops at all floors for which riders, already inside the car, have input a requested. A more sophisticated system, often used in hotels and other large buildings with a lot of foot traffic, involves the traffic patterns that reoccur. These systems have logic controllers that are programmed with information about the demand on each floor with respect to the time of day and they route the elevator cars accordingly so as to minimize the wait for all riders. When there are multiple elevator cars, the logic controller bases the movement on each car on that of the others. Often, the elevator car is equipped with a load sensor so that if the elevator is full to capacity it sends a signal to the control system and the logic controller signals the car not to pick up any more passengers until the load is lowered.

Elevator Door Control

The control system's computer also controls the movement of the elevator car doors. The amount of time for which the doors are held open when a floor is reached is programmed into the logic controller. The elevator car doors also have a sensor that detects if someone or something is caught in the door and stops the door closing mechanism from closing the door with the large force that is required. This is also part of the safety system since it ensures that people are not hurt when trying to enter or exit the elevator car.


Statecharts for System Behavior

Statechart for "User" Behavior

Figure 5. Statechart for User Activity

Statechart for "Elevator System"

Figure 6. Statechart for Elevator System.

The elevator system is comprised of two major subsystems: the control system and the mechanical hardware. After the features and functions of each subsystems have been described, we will generate and analyze design alternatives.

Statechart for "Controls System"

Figure 7. Statechart of Controls Systems Behavior.


System Structure

MECHANICAL HARDWARE -- BASIC ELEVATOR TYPES

Three of the most prevalent types of elevator system structures will be examined and analyzed in the following sections.  The types will consist of hydraulic elevator systems, as well as geared and gearless ‘roped’ elevator systems.  A brief description of each elevator type follows along with system structure diagrams and design analysis.

SAFETY SYSTEMS

Both hydraulic and roped elevator systems should be equipped with a complete safety system. Some of the safety systems that most elevators currently on the market utilize are: built in braking systems, a governor, electromagnetic brakes, and a shock absorber system. These subsystems are easier to install in roped elevator systems -- see: http://www.howstuffworks.com/elevators.htm.

Another safety feature in most elevators is a sensor on the door that ensures that nothing is caught in the doorway when the doors are closing.

When motion or the presence of an object is sensed the doors go to an open position for a specified number of seconds. Then the doors again try to close.

These safety systems are included in the system structure diagrams but will not be looked at in any great detail.  Readers are directed to the various references for more information on these and other safety systems.

PROS AND CONS OF EACH MECHANICAL SYSTEM OPTION

Hydraulic elevators expend a large amount of energy each time the elevator moves upwards.  They are extremely inefficient as well as expensive to operate and install.  The actual cylinder must be buried very deep under the building.  The taller the building, the farther under ground the cylinder must be buried.  The main benefit of using hydraulics is that a more powerful motor can easily be integrated to generate more pump power to lift the elevator.  Even then, this type of design is often infeasible, especially for tall buildings.

Roped elevators are considerably more efficient.  Potential energy is stored in the elevator car and counterweight so there is much less energy needed to raise the elevator.  They are versatile and easily supplemented with numerous safety systems.  Much research and development has been done on this elevator type and it is certainly the most prevalent throughout industry.


Basic System Structure Diagram

Figure 8. Basic System Structure


Elevator Car Subsystem Structure

Using the basic system structure as a starting point, we now expand the system structure to lower levels of detail and add attributes and functions relevant to the correct subsystem operation.

Figure 9. Elevator Car Subsystem Structure.

Points to note:

  1. The "power source" for the elevator is the device providing electric service for all the subcomponent needs.

  2. The "framework/structural component" is the physical elevator unit that holds the passengers, typically made from various materials and incorporating one or more doors.

  3. The "motor/driver" is the device providing the force to move the elevator unit. Finally, the "car movement" components are how the "motor/driver" is connected to the elevator car.


Requirements and Specifications

Having generated the baseline textual use cases and the scenarios for the current problem we can now generate the requirements for the Elevator System. Requirements are derived from various goals and scenarios, use cases so it is important to trace back the source of requirement. Per instructor approval, only a part of the entire system structure will be looked at from this point on. Creating a requirements list for every aspects of the entire elevator system structure would be time consuming and in some cases redundant. The aspects of the elevator car type will be investigated from a requirement and specifications perspective, for the sake of learning and demonstrating proper project procedures.

Since the focus of this project is now only the elevator car and its components, only these items are developed in further detail. Structure components such as the elevator controls system and/or users will not be developed any further.


Subsystem Requirements

Requirements for the elevator car system and its subcomponents are given below.

Motor / Driver Requirements

Index Requirement
MD1 Be at least 99% reliable.
MD2 Be capable of operation in ambient temperatures.
MD3 Weigh 2000 pounds or less.
MD4 Cost less than $10,000
MD5 Be reversible.
MD6 Develop a upward force of at least 8000lbs
MD7 Move the elevator in the up direction a distance of 6 stories in less than 50 seconds.
MD8 Move the elevator in the down direction a distance of 6 stories in less than 70 seconds.
MD9 Support the "dead" weight of the elevator car in the case of failures.

Framework / Structural Component Requirements

Index Requirement
FS1 Be at least 99% reliable.
FS2 Weigh less than 20,00 lbs.
FS3 Cost less than $15,000.
FS4 Occupy a footprint of less than 12' x 12'
FS5 Consume less than 40 amps.
FS6 Have required technology/fire safety features.
FS7 Have a useful life of at least 20 years.

Power Source Requirements

Index Requirement
P1 Be at least 99% reliable.
P2 Weigh less than 500lbs.
P3 Be capable of 10,000 activations.
P4 Cost less than $4,000.
P5 Capable of handling 360VAC and 100 amps.

Car Movement Requirements

Index Requirement
CM1 Be at least 99% reliable.
CM2 Have a range of 6 stories or more.
CM3 Stop under max load.
CM4 Reverse under max load.
CM5 Weigh less than 1,500 lbs.
CM6 Be powered by system motor/driver.
CM7 Cost less than $25,000.
CM8 Incorporate a counterbalance of 40% total weight for roped design.

Often these higher-level requirements, which are supplied by the user, are ambiguous and cannot be properly validated. So we take these requirements and break them down to come up with lower level requirements, which are more specific, unambiguous and which could be traceable to system objects / functions in the UML diagrams. This synthesis is a top down approach in the verification and validation. Later with the final system design these requirements are again traced back to higher level requirements supplied by the end user to see if the given system design meets the user specified criteria.


Traceability

Use Case
Requirement#
Description
Components
1. Request Elevator
MD1-9
PS1-7
FS1-5
CM1-4,6.
Elevator car must respond to user requests at a given floor.  Elevator moves within the structure along a given path.

Motor/Driver
Power Source
Framework/Structure
Car Movement.
2. Floor Selection.
MD 1-9
PS 1-7
FS 1-5
CM 1-4,6
Elevator must move within structure to the floor requested by user. Elevator moves within structure along a given path.
Motor/Driver
Power Source
Framework/Structure
Car Movement.


Measures of Effectiveness

There are many ways of analyzing how well an elevator system works.  To ensure that the elevator will behave as expected for the functions it was intended for an analysis of the elevator ‘effectiveness’ for various systems should be done before the purchase of an elevator system.  After this initial study is done to test the validity and to verify the system behavior should be undertaken.  The latter analysis is outside the scope of this paper so the measures of effectiveness are all that will be looked at in this section.  A small sample of the measures that could be used to describe the elevator system’s effectiveness include reliability, cost, efficiency and safety.  These four measures will be described in more detail in the following paragraphs.


Reliability

Time and use is the test of an elevator system's reliability. The system should continue to operate in the same way it did when it was first installed. The users do not want surprises, all components must continue to function for their intended function and in the intended way. This is important for the building owner and clientele. If the users have a bad experience with the elevator, they will likely get a bad impression of the business or businesses that are located in the building with the faulty elevator. Besides losses of customers, unreliable elevators will need costly and timely maintenance and will lead to major financial burdens.

Cost

As noted above, cost of elevators includes the purchase price, installation, and maintenance over the system’s lifetime.  To minimize the cost, the elevator should be designed, manufactured, and marketed in a cost effective way.  The reliability of the elevator, the quality of parts and installation, also affect the price of the system over the long run.  If the price tag does not match the product, customers will take their business elsewhere.

Performance

Performance is the measure of a components ability to do its job.  How powerful is the motor, how long will the structures last, how much current can the power source deliver, etc.  An effective elevator system has the best performing parts with respect to certain constraint thresholds.   

Safety

A major measure of effectiveness is the safety of the elevator system.  Lack of an effective safety system could lead to injury, or possibly death of users or maintenance personnel.  The elevator system must be designed so as to protect riders with multiple backup systems for use in the event of failure of one or more than one critical mechanism.  The elevator should also be easy to maintain with the control room and electric motor in an easily accessible location.  The various safety systems often utilized are described in the above system structure section.  For the sake of this project and simplicity, we will assume the various types and manufacturers under consideration make equally safe elevators.


Trade-Off Analysis

In the elevator industry there are a few decisions that need to be taken before setting up an elevator system. These include the performance characteristics and the decision variables to achieve that performance characteristic. This is largely a multi objective optimization problem with competing objectives with respect to the decision variables.

We now develop a multiobjecive trade-off problem for the elevator design that includes the following measures of effectiveness and decision variables.


System Performance and Economics


Decision Variables

We assume that the elevator system design can be adequately modeled with four decision variables:

Furthermore, we assume that each decision variable is discrete, corresponding to selection of an alternative from a library of choices. The available options are summarized in the following table.

Component
Performance
Normalized
Performance
Reliability
Normalized
Reliability
1000*log()
Reliability
Cost
Motor/Driver (X) Force - lbs    
MD1
11000
1
0.99995
1.0000
0.000
$10,000
MD2
10000
.909
0.998
0.998
-.869
$9,000
MD3
9000
.818
0.994
0.994
-2.613
$5,500
MD4
8000
.727
0.99
0.99
-4.364
$4,500
Structural/Framework Component (Y) Life - Years    
FS1
35
1.000
0.99995
1.0000
0.000
$15,000
FS2
30
0.857
0.998
0.998
-.869
$12,000
FS3
25
0.714
0.994
0.994
-2.613
$11,000
FS4
20
0.571
0.99
0.99
-4.364
$10,000
Power Source (Z) AMPS    
P2
140
1
0.99995
1.0000
0.000
$4,000
P2
120
.857
0.998
0.998
-.869
$3,500
P3
120
.857
0.994
0.994
-2.613
$3,250
P4
100
.714
0.99
0.99
-4.364
$3,000
Car Movement (A) Range - Floors    
CM1
12
1.000
0.99995
1.0000
0.000
$25,000
CM2
10
0.833
0.998
0.998
-.869
$20,000
CM3
8
0.667
0.994
0.994
-2.613
$15,000
CM4
6
0.500
0.99
0.99
-4.364
$10,000


CPLEX Problem Formulation

We now cast the multiobjective optimization problem in a format suitable for analysis with CPLEX. The basic goal is to minimize cost, while maximizing performance and reliability.

TOTAL SYSTEM COST

Formula for System Cost

System Reliability

System performance:

Design Objective

Constraints

CPLEX PROBLEM WITH SOLUTIONS

C = Cost / 1000 Performance is normalized

Minimize

Subject to:

Bounds

Value Constraints

We change the value of K1 and K2 to get the different points with various cost, reliability and performance.

The following table is the different combination of K1 and K2.

Point#
K 1
K 2
1
-29
3
2
-19
3
3
-19
3.25
4
-15
3.25
5
-15
3.5
6
-13
3.5
7
-12
3.6
8
-10
3.6
9
-8
3.6
10
-8
3.7
11
-7
3.7
12
-6
3.7
13
-4
3.8
14
-3
3.9
15
-2
3.9
16
-1
4

Each combination of K 1 and K 2 corresponds to each set of solution variables.

Point#
X 1
X 2
X 3
X 4
Y 1
Y 2
Y 3
Y 4
Z 1
Z 2
Z 3
Z 4
A 1
A 2
A 3
A 4
Cost
Reliability
Performance
1
 
 
 
1
 
 
 
1
 
 
 
1
 
 
 
1
Cost = 27.5 Reliability = -17.46 Performance = 2.51
2
 
 
 
1
 
 
 
1
 
 
 
1
 
 
 
1
Cost = 27.5 Reliability = -17.46 Performance = 2.51
3
 
 
 
1
 
 
 
1
 
 
 
1
 
 
 
1
Cost = 27.5 Reliability = -17.46 Performance = 2.51
4
 
 
 
1
 
 
 
1
 
1
 
 
 
 
 
1
Cost = 28 Reliability = -13.96 Performance = 2.66
5
 
 
 
1
 
 
 
1
 
1
 
 
 
 
 
1
Cost = 28 Reliability = -13.96 Performance = 2.66
6
 
 
 
1
 
 
1
 
 
1
 
 
 
 
 
1
Cost = 29 Reliability = -12.21 Performance = 2.80
7
 
 
 
1
 
 
1
 
1
 
 
 
 
 
 
1
Cost = 29.5 Reliability = -11.34 Performance = 2.94
8
 
 
1
 
 
 
1
 
1
 
 
 
 
 
 
1
Cost = 30.5 Reliability = -9.59 Performance = 3.03
9
 
 
1
 
 
1
 
 
1
 
 
 
 
 
 
1
Cost = 31.5 Reliability = -7.85 Performance = 3.18
10
 
 
1
 
 
1
 
 
1
 
 
 
 
 
 
1
Cost = 31.5 Reliability = -7.85 Performance = 3.18
11
 
1
 
 
 
1
 
 
 
1
 
 
 
 
 
1
Cost = 34.5 Reliability = -6.97 Performance = 3.12
12
1
 
 
 
 
1
 
 
1
 
 
 
 
 
 
1
Cost = 36 Reliability = -5.23 Performance = 3.36
13
1
 
 
 
 
1
 
 
1
 
 
 
 
 
1
 
Cost = 41 Reliability = -3.48 Performance = 3.52
14
1
 
 
 
1
 
 
 
1
 
 
 
 
 
1
 
Cost = 44 Reliability = -2.61 Performance = 3.67
15
1
 
 
 
 
1
 
 
1
 
 
 
 
1
 
 
Cost = 46 Reliability = -1.74 Performance = 3.69
16
1
 
 
 
1
 
 
 
1
 
 
 
1
 
 
 
Cost = 49 Reliability = 0 Performance = 4.0


Trade-Off Scenario

The distribution of the points in terms of “cost and reliability”, “cost and performance”, and “reliability and performance” are shown in the following charts, some of the points overlap because some different combinations of K1 and K2 produced the same variable values.

Trade-Off between Cost and Reliability

Figure 10. Trade-Off between Cost and Reliability

Trade-Off between Cost and Performance

Figure 11. Trade-Off between Cost and Performance

Trade-Off between Performance and Reliability

Figure 12. Trade-Off between Performance and Reliability

Together, these three figures define the cause-and-effect trends in performance for the elevator design.

POINTS OF Interest

Points of interest are adjacent designs where an incremental increase in one desirable variable creates a much larger increase in another desirable variable. The later represents a point that will be studied later on, to determine optimality.

The goal is to find a collection of 4-5 points common to all three graphs and perform the optimality analysis on them alone. Infeasible combinations will also be omitted at this time. Specifically, instances where hydraulic motor/drivers (X1, X2) are not matched with hydraulic car movement components (A1, A2). This affects data points 11-14 only; all other combinations of components are permitted.

Results

For each of the three trade-off curves, the points of interest are as follows:

Trade-Off Curve
Points of Interest
Cost versus Reliability
3, 5, 7, 8, 10, 15
Cost versus Performance
3, 5, 7, 8, 10
Performance versus Reliability
3, 5, 8, 10, 15
All
3, 5, 8, 10

The optimal solution lies at either point 3, 5, 8, or 10.

Point Number
Performance

Normalized Total
Cost

$1000's
Reliability

1000*log%
3
2.51
27.5
-17.46
5
2.66
28
-13.96
8
3.03
30.5
-9.59
10
3.18
31.5
-7.85

Points to note:

FINAL RESULT

According to the analysis, the final choice is "point 10" which has best results for the given constraints.

The de-normalized results of the analysis conclusion are as follows:

The system cost and reliability are $31,500 and 0.9821, respectively.


System-Component Testing

To be completed in ENSE 623.


References and Web Resources
  1. Tom Harris, Howstuffworks, Inc., "How Elevators Work." See: http://www.howstuffworks.com/elevator.htm (2002).


Developed by Mike Buck and Bonnie Lawson, Spring Semester 2003
Reformatted and slightly modified by Mark Austin, May 2003
Copyright © 2003, Mike Buck, Bonnie Lawson, John Baras and Mark Austin. All rights reserved.