Armchair Case Study
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Table of Contents
Activity Overview
This Life Cycle Analysis activity is based around an IKEA Poang armchair. In this activity, forces are applied to the chair, and results such as stress, strain, and chair displacement are recorded. Next, students will refine some aspect of the chair to improve the performance, depending on the focus of the course.
This activity is designed as an in-class project for materials or design-based courses, with a focus on iterating through the process of design-testing-analysis.
Objectives
Due to the length of school terms, many students do not get to experience an iterative design in their school career and experience their first large-scale iterative process after graduation. Because of this, many students do not understand how much design, testing, and analysis are dependent on each other. A need was identified to expose students to an iterative design process in a school environment.
With that being said, the Life Cycle Analysis of an Armchair was an activity formed with multiple goals in mind.
The primary goal of this activity was to showcase the interconnectivity of design, testing and analysis, as mentioned. The students will be constantly improving their design, teaching students that how the chair is designed will impact how it is tested, which will impact the analysis results, which will in turn impact the next design, etc.
The secondary goal for this activity was introducing students to Finite Element Analysis (FEA) software and getting them familiar with it. Many students are aware of FEA and understand what the software accomplishes, but very few actually know how to use it. This provides challenges as some co-op positions, especially in mechanical or mechatronics engineering, assume that a student knows how to use the software before their co-op term.
The third goal of this project was to demonstrate the concepts of stress, and strain, as well as different failure modes in a more practical environment. Many students learn about stress, strain and failure modes such as yielding failure or fatigue failure, however the examples provided for students are usually not applicable to real applications, such as stress and strain being demonstrated on a specific tensile test specimen. Using an easily recognizable object that is commonly used, shows students how these concepts can apply in a practical scenario.
Project Specifications
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Conceptual Design
From the set of generated objectives, a number of possible structures to use were selected that would accomplish these goals. An additional constraint used was the complexity of the structure. The desired structure needed to be somewhat complex, as being too simple would not accomplish objective number three, but should not be incredibly complex, as the course is intended for undergrad to grad level MME students and the project may stretch on for too long.
Using this added constraint, 2 possible structures were identified: a bike frame, and an IKEA Poang Armchair. The bike frame was identified as meeting all requirements but was on the more complex end of the spectrum, and was decided to be used in upper year undergrad or grad level courses. As this page focuses on the armchair, the bike frame will not be discussed in this page, see the Bicycle Project Wiki Page for more on the bicycle.
The armchair was selected for many reasons. Mainly, the design is simple enough to analyze as the entire load of the chair is transferred through the 2 legs into the ground. This makes any simulations or loading of the chair relatively simple, and more importantly makes it relatively easy to identify the highest load area on the chair.
There are several other reasons the IKEA armchair was selected. Firstly, it is mechanically different enough from a chair to add needed complexity to the project, while still achieving the same goal. Secondly, an IKEA armchair is very recognizable and many people have seen or used one before the project. By analyzing the chair, the project gets students to analyze an everyday object that they have likely seen before, encouraging students to continue analyzing everyday objects.
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| IKEA Poang Armchair |
Testing System
After the chair was selected, the actual testing system needed to be designed. Early on it was decided to use 40mm hollow aluminum extrusions as the base for the testing rig. A square of extrusions would fit snugly around the base of the chair and all apparatus and instruments would be mounted to the extrusions.
Afterwards, the testing system was split into two separate sections based on their functions: applying the load and measuring the results.
Applying the Load
Multiple criteria and constraints were put in place to determine the best method of applying the downward force to the chair. The primary constraint is that is must fit on or around the chair and the aluminum extrusion of the testing rig base. This constraint is in place as all pieces of the rig must attach to the extrusions, and it was desired to limit the working space of the chair, such that the rig does not add a large amount of space to the exterior of the chair.
Another functional constraint put in place was that the solution must be able to apply the exact same amount of force repeatedly. Ideally this testing rig could be used to simulate the entire life cycle for the chair, so repeatability is needed to simulate hundreds or thousands of cycles. In addition, loading time of each option was considered as testing time is a consideration in the project, especially when fatigue testing is concerned.
The selected solution is a linear actuator located under the seat of the chair. Two beams of extrusion are arranged in the shape of a "T" across the middle of the base to house the loading mechanism. The design of the loading mechanism is fairly straightforward. First, a load of up to 200 lbf (890N) is inputted into the system using a linear actuator. As this actuator extends, the load is converted from a roughly horizontal force into a vertical force through the rotation of the bell crank. This causes the linkage arm connected to the bell crank to move downwards, pulling along with it the aluminum extrusion sitting across the seat of the chair which serves as the load applicator.
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| SolidWorks model of the load application setup | Load path of the chair testing system |
Measuring the Results
In terms of measuring the results, multiple different instruments and measurements were considered. The primary measurements desired were stress, strain, and displacement of the chair. Between the three of these measurement results, a large amount of analysis could be conducted and the design could be easily changed to improve at least one of these measurements. Overall, this function of the test rig has very little limitations and constraints as more results are always helpful. The only limiting factor for this function is the space limitations, as previously stated.
The selected instruments used to measure the results were a load cell, multiple strain gauges, cameras and possibly microphones.
The purpose of the load cell would be to measure the force applied to the chair, and by extension the stress in the chair. The planned location for the load cell was directly underneath the chair seat, as that was the only location where it could accurately measure load. At this time, issues with fitting the load cell alongside the linear actuator were not investigated as redesigning the actuator placement and position was deemed too difficult and that the load cell would likely be small enough to fit.
The purpose of the strain gauges would be to measure the strain of different sections of the chair. Two strain gauges were planned to implemented on the front face of each chair leg, as these locations faced the highest stress and likely faced the highest strain.
The purpose of the camera was to measure the displacement of the chair. It was planned to position the camera perfectly on one side of the chair, far enough away to capture the entire chair. The goal for the positioning was to get a 2D view of the chair profile. This way, any displacement would be easily viewable by the camera. It was also discussed that lights or markers dotting the side of the chair might be needed in order to more accurately measure the displacement at multiple points along the chair.
It was also discussed that a microphone might be involved to gather audio from the tests. The purpose of the microphone would be to capture any noise the chair makes, whether it be consistent noise, or occurring irregularly.
Test System Building
Linear Actuator
Construction of the test system began in the F20 term, focusing on fully implementing the linear actuator. Several parts needed to be manufactured such as the bell crank, and connecting arm. In addition, a large amount of electrical work was required in order to control the linear actuator.
Upon first test of the system, the force applied by the actuator lifted the entire testing rig base into the air, since it was not held or secured down. The devised solution was to add in three corner brackets along the inside perimeter of the base of the machine and have the leg of the chair sit on these. That way, when the base of the machine wants to lift upwards, the corner brackets will grip onto the bottom of the legs keeping it on the ground.
By the end of the term the linear actuator was fully installed and could be operated to apply load on the chair.
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A close-up of the load mechanism. The linear actuator, bell crank, and connecting arm can be seen in this image. | First Test of the linear actuator | Demonstration of the fully functioning system. |
Instrumentation
In the W21 term, implementation of the measuring devices began.
The first instrument planned to be installed were the strain gauges. A strain gauge in a flat circuit that adheres to a surface and measures the change in length. After much discussion and careful planning, it was eventually determined that strain gauges would likely not be used on the legs of the chair. The primary reason for this is that almost all strain gauges on the market are only designed to stick to metal or plastic. Because of this, any strain gauge applied to the wooden chair would require extra preparation and would possibly not be fully functional when installed. Although this was discussed, it was ruled out when the other strain gauge issues were considered. The second major reason the strain gauges were not used was because strain gauges have a very limited number of uses. Strain gauges can be reused, only as long as they remain undamaged, and they cannot be removed and reapplied. When these two issues were combined, the result was that applying the strain gauges would be a meticulous, time consuming process, which would likely only result in a few test results before they would stop functioning.
The next implemented measuring device was the load cell. A load cell can be simplified as a block that is attached to the chair at one end and the fixed extrusion base at the other end. For the load cell to work optimally, the direction of force applied from the chair must be perpendicular to the load cell (ie. tension or compression). Because of this, the load cell Needs to be placed directly under the chair, close to where the downforce is applied, to receive the maximum compression possible.
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Testing and Design Verification
No testing or design verification has been completed yet. Please fill this section out when testing has been completed.
Development of the Digital Twin
Alongside the physical chair development, a digital twin of the chair was created. The intention behind this was to run a project revolving around FEA analysis of the chair, instead of the physical model. This aspect of the project required significantly less work, as no testing rig was needed, since forces can be simulated in an FEA.
Once the digital model was created in SolidWorks, all that was required was to set up the initial FEA forces and materials. Most of the planning for the class project would be done by the professor running the project, as it was desired to keep the project as open-ended as possible to encourage a larger number of professors to customize the project to suit their needs.
For more information on running an FEA simulation, see the Finite Element Analysis page.
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| Digital model of the chair | Stress results of initial FEA forces |
