Nov 3, 2011

ME3101 Mechanical System Design I

This is the first part of Mechanical Engineering Year 3 Design Module in NUS, which requires paper design of our project.

It's 100% CA and sometimes we questioned the amount of modular credit - "this should be a 6MCs module!!". Thank God, we've gone through this extremely stressful process.

Week 1

We formed a group of six, and all of us are going for SEP. Most of us are getting the second part of this module (ME3102) mapped overseas.

Because of that, we're doing an In-house Design instead of Industrial one.

First sketch
We picked the topic "Gerontechnology" from the available In-house Design options and submitted a proposal of "All Terrain Wheelchair" to Dr Ma Sha.

We previously brainstormed and came out with the idea of a fall detection system, however, more electrical than mechanical components were expected to be used. Hence, we went for All Terrain Wheelchair.

The wheelchair has a pair of tracks on top of the pair of normal wheels. Tracks are used when the wheelchair is in "stair climbing mode", where friction helps it to climb.

Our source of inspiration:

Week 2

We waited for reply from Prof Fuh regarding supervision.

Week 3

We had our first meeting with our supervisor, Prof. Ong Chong Jin at E1-06-02 on 22 Aug (Mon). Calvin and Jared were absent. Wei Zheng presented our project's proposal to Prof Ong and we later discussed and confirmed our availability of future supervision timing and venue (EA-04-06 Control and Mechatronic Lab 1).

Prof Ong requested us to touch on the targeted group of consumers that will use our designed product and expected us to present him a more detailed aspect of our design.

Week 4

We had a regular group meeting at Temasek Hall Block E Lounge on 29 Aug (Mon) and decided the restriction of the weight of the person to be a maximum of 130 kg.

We went more into the elevation of wheels and tracks' mechanism, by driving of two gears. Three wheels are embedded in each tracks to allow some degrees of freedom when it comes to stair climbing.

We met Prof Ong at EA-04-06 on 01 Sep (Thu) and presented to him our design and videos of some similar existing products. We're told that out design might not be realistic and feasible at this level because it requires more than 5 years of development. Besides static analysis, dynamic analysis is important in the aspect of designing.

We ourselves thought that too many mechanisms were involved, so we planned to lower down our expectation and come out with something that is simpler yet realistic.

Week 5

In this regular meeting at Temasek Hall Block E Lounge on 05 Sep (Mon), we decided to use only tracks in our design and limit the ratio and height for the steps of staircase that is climbable by the wheelchair.

Tracks are non-elastic but not 100% tight, to give freedom of sagging and group on corners of the steps.
Stairs are with a ratio of Width:Height ≈ 1.8:1

We met Prof Ong at EA-04-06 on 08 Sep (Thu) and presented to him our more detailed design and static analysis of the wheelchair and consulted the dynamic analysis. We discussed about the capability of our wheelchair to be marketable, and the feasibility.

A few important mindsets addressed by Prof Ong were:
  • Strike for realistic model instead of advanced one
  • Visualize a real-life model instead of a prototype
Considering the matter of safety which elderly are concerning about, we'll need to lower down our expectation again for the utility of the wheelchair. We discussed again and set the goal of the wheelchair to be able to elevate at a one-step kerb/pavement and climbing up slopes without sliding or rolling backwards. This at least gives independence for the elderly to a certain extent.

Week 6

It's another regular group meeting at Temasek Hall Block E Lounge on 12 Sep (Mon) and we went more into comprehensive features of the wheelchair.

We did dynamic analysis of the wheelchair going up a slope and identified the needs of elderly (to improve elderly's quality of life and independence).

We redefined our problem statements:
  • Striking balance for value in relation to cost catered
  • Incorporating safety
  • Technical specifications (comply to LTA regulations)
  • Storage, ergonomics, terrain overcoming ability
We believed that our product will work because we learnt from other countries, cater specially for the elderly, ensure safety. Besides, there is increasing aging population in Singapore.

On 15 Sep (Thu) we met up and practiced presenting the PowerPoint slides we've prepared in E4-04-05.
On 16 Sep (Fri) we presented to Prof Ong our PowerPoint slides at EA-03-06.

Prof Ong mentioned that we might want to consider more other factors that will influence the dynamic analysis, although in our case, area of the track is not governing whether the wheelchair will slide and topple.

Sized-up model, combination of components are things that we as well need to consider.

Before going back for recess week, we had breakfast and group meeting on 17 Sep (Sat) at Temasek Hall Communal Hall and planned the job distribution for current stage's research:
  • Hu Guang: Source for motor and find out price and ask friend about the frame
  • Kai Sheng & Wei Zheng: Proceed with analysis and calculation for slopes, typical pavements, kerbs and plan a rough dimension for the wheelchair
  • Wil Sen: Determine possible materials and course for gears, tracks and gear ratio
  • Jared & Calvin: Determine kinds of ball bearings, dimensions of shafts, frame, gear and detailed suggestion on how the system works

Week 7

Two Slab Frame
On 26 Sep (Mon) at Temasek Hall Block E Lounge, we compiled out information together (the manufacturers of the wheelchair's components) and roughly created a CAD design of the frame body with tentative dimensions.

We presented to Prof Ong our PowerPoint slides on 29 Sep (Thu) at EA-04-06.
He mentioned that we can actually consider Singapore's manufacturers like SKF and Kian Ho Bearing for ball bearings and other components instead of China's manufacturers. Also, he warned us about the load consideration, as the size and materials of the frame seemed to be heavy.

He also described the speed controller that we all need to use in the system by drawing a block diagram.

Week 8

Truss Frame
We had a group meeting on 03 Oct (Mon) at Temasek Hall D4 and planned to cut down some materials on the frame design by redesigning the main frame and using hollow aluminium bars for the top parts of the frame.

Also, we imported a downloaded human model to check whether the dimensions of the wheelchair are suitable in real life situation. Two small wheels were to be mounted on each side of the suspension bars to allow some degrees of freedom for them to move up and down when the wheelchair encounters a step.

Prototype 1.0

We didn't manage to meet Prof Ong that week because he fell sick.

Week 9

We compiled and finalized out presentation slides during group meeting on 10 Oct (Mon) at Temasek Hall D4.

We presented out ideas to Prof Ong on 13 Oct (Thu) at EA-04-06 and we're warned that our progress was too slow - too many sources of materials but lack of analysis.

He mentioned that ton top of finding suitable type of motors, we should size it up, in terms of voltage and power rating. Also, the centre of mass of the wheelchair has to be located (by using SolidWorks itself).

Week 10

Prototype 2.0
During our regular group meeting on 17 Oct (Mon) at Temasek Hall Block E Lounge, we improved on the design with two sets of spring dampers with a pair of small wheels each hung and fixed on the horizontal bars of the frame, to prevent the track from overstretching inwards when the wheelchair encounters obstacles like step or kerb.

Stress analysis was performed on the main frame in SolidWorks by applying Aluminium Allow 7075-O (SS) as the material of frame and it didn't fail under von Mises criterion.

Weight of the frame is approximately 30kg and the maximum stress experienced is 3MPa, while the yield strength of the frame is 95MPa. Analysis for the required motor was done and angle of inclination for slope that the wheelchair can take was 20.5° with factor of safety of 1.25.

Upon meeting with Prof Ong on 21 Oct (Fri) at EA-04-06, he mentioned that, on the stress analysis part, we have to redesign to save materials from the blue regions of the main frame.

Rotational inertia was needed to be experimented in calculation of motor requirement. We have to account for loss in resistance, and 50A is too big for current design, hence speed of going up slope has to be decreased as a compromise. We're also reminded about the mistakes that we did in the feedback control analysis.

Week 11

Prototype 3.0
On our group meeting on 24 Oct (Mon) at Temasek Hall Block E Lounge, we redesigned the frame to be connections of aluminium bars with smaller circular cross sectional area to even out the stress when stress analysis is performed.

New foot rest, smaller spring dampers, two 24V batteries (7.5kg each) and a pair of motors with gear boxes are modelled into the CAD drawing.

Analysis of sliding and toppling on a slope was done with acceleration parameter eliminated for simplification.

The feedback control system analysis was redone.

We went on a field trip to Mustafa Centre near Little India on 25 Oct (Tue) to find out some specifications of their electric wheelchair as reference.

Market Wheelchair Ours
Total maximum mass 187kg 150kg
Speed 1.67m/s (6km/hr) 1.4m/s (5km/hr)

We presented our latest frame design and motor sizing to Prof Ong on 28 Oct (Fri) at EA-04-06.
We're questioned about the shape of the planks where wheels are mounted on and there's a need to substantiate the reasons we use the three rectangular shapes instead of others.

Forces that come from different scenarios, like pressing on arm rest while getting up and leaning backwards, apart from the standard weight and forces on the wheelchair, needed to be considered as well. Besides, we might have to ensire that the stress analysis plot from SolidWorks is correct by perhaps doing manual stress analysis on some parts.

It's also better of the dynamic analysis of sliding and toppling can be figured out even when acceleration is present.

For feedback control system, graphs of frequency curves are to be plotted together. Encoder feedback (where end of the motor there's a plactic sheets with colored markings) can be considered.

Voltage amplifier or current amplifier required within the range of the motor has to be found out. We could also consider summing amplifier to be implemented on operational amplifier (can be found off the shelf for ±24, ±15 etc).

If possible, we can find Kt from some brochures as reference.

Week 12

We prepared our PowerPoint slides individually to be compiled later for our final presentation, which will be on 03 Nov (Thu).

Final Frame Design

Stress Analysis (Max: 40MPa)

Factor of Safety (Min: 5.65)

Suspension System

Seat with Arm Rest

Foot Rest

Back Wheels

Exploded View of Final Product

Final Product

Open Loop Motor Response

Motor Response with Proportional Feedback

Motor Response with Proportional-Integral Feedback

Circuit Implementation

We compiled all our slides and findings on 02 Nov (Wed) at Temasek Hall D4. A few videos and animations were included in the presentation slides and we thoroughly rehearsed through the slides and timed ourselves.

We're at EA-06-05, our final presentation venue on 03 Nov (Thu), thinking that Prof Fuh or Dr Ma Sha will sit through the presentation but they didn't turn up, ending up realizing that we only had to present to our supervisor, Prof Ong because it's an In-House Design Project.

Everything went quite smooth I supposed, this marked the end of this super stressful module after taking a group photo with Prof Ong.

Week 13

Oh ya, it's still not the end because we still had to submit our group report, individual log books and peer evaluation form.

A lot of my group members did the log books last minute, as they didn't keep track every week.

As for peer evaluation form, it's the evilest component of this module - encouraging one to back stab his/her group mates. What if everyone did the same amount of work?! It's just an unfair decision to make and we actually had a discussion session on this before submitting everything to Prof One on 08 Nov (Tue).
Indeed, we felt sorry for each other...

Grade is one thing for the module, but the result we got from this module is friendship. At least we're not doing it in the hierarchical or managerial way. Instead, every time we had group meeting, we joked and laughed at each other, making it so fun to work together.

Knowledge can never replace friendship.
- Patrick Star

For more photos, please check out my photo album ME3101 Mechanical System Design I on Facebook.

Javin Tham MSc, PMP, PMI-ACP, CSSBB, CQPA, CQIA, MSOE, MIPlantE Industrie 4.0 Consultant | Pop Piano Coach | YouTube Coverist

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