P18347: Baby Stroller with HVAC
/public/

# Systems Design

During the Systems Design Phase, the Climate Constroller team began to work on the idea of how the stroller would operate and what would be the best type of design to use. The team worked on a functional decomposition table to understand what the primary functions of the product would be. Based on the functions, the team then worked to develop different design ideas that would be combined to accomplish a very basic, early-on design for the product.

## Team Vision for System-Level Design Phase

Team Vision Accomplishments
Functional Decomposition
• Understand stroller functionality for the project
• Identified functionality to pursue
Benchmarking
• Research product reviews
• Test different strollers
• Observed products with similar systems, like a heated blanket, fan, heated car seat
• Bought, tested, and disassembled a stroller
Concept Generation
• Make product use sketches
• Product use sketches completed
Engineering Analysis
• Review heat transfer notes
• Calculate work to heat/cool stroller
• Determine 0-3 year old homeostasis
• Developed system model of work done to heat the baby stroller
• Created a document with 0-3 year old homeostasis values for use in further calculations
• Calculated an estimated battery size for 8-10 hour continuous use
Systems Design
• Determine if stroller will be heated, cooled, or both
• Concluded that stroller will be heated for this revision
• Design choice: heated stroller seat with closed plastic cover

## Functional Decomposition

HVAC Stroller Functional Decomposition

## Benchmarking

In order to expand our knowledge of how similar systems operated, various products were benchmarked. They include a heated blanket, a heated car seat, a fan, and a space heater. By observing different aspects of the products, such as the price, power output, weight, and size, the team was able to get familiar with the types of systems that could be modeled.

Heated Blanket

Heated Car Seat

Fan

Space Heater

## Feasibility: Prototyping, Analysis, Simulation

Each team member posed a question to understand the feasibility of our potential designs:
Question Assumptions Approach Conclusion
What are the specifications/necessities for the battery?
• System is running for 12 hours continuously
• Battery is made from environmentally friendly materials

Battery Specifications

It will be most feasible for our team to create a custom battery pack to fit our power needs. We should also proceed forward with seperate batteries for the HV/AC system and the low voltage components.
Will the HVAC system be able to run all day without battery being too big/heavy making the stroller hard to move?
• Est 22.4W for baby sized seat. (45W for full size car seat (3.25A 13.8V))
• Would be cycled on and off. (battery life would actually be approximately doubled)
• Assuming an all day use is 16 hours

For a heated seat:

• 3.25 * 8 = 26 Ampere hour (Ah) or 26000 mAh

For a cooling seat:

• 12v, 23W ---> 1.9A
• 3.8Ah for 2hour use
• 30.4Ah for 16 hr
• Fans needed too
• 12V .1A ---> 1.6Ah for 16 hr use.

Total power draw for a heating and cooling the seat for 16 hours:

• Total = 26 Ah + 30.4 A h +1.6 Ah = 58 Ah
• Dimensions of the battery: 9.84 inches x 6.30 inches x 7.20 inches
• Weight: 39 lbs

Pump Specs for a small water cooling loop

We expect that the battery will be too heavy with the rough assumptions used to calculate. Moving forward, we will need to increase the efficiency of our design.
Will the stroller be able to fold properly with all of the components attached?
• Dimensions of components are known
• Dimensions of stroller standing and collapsed are known
• Prototype with foam, cardboard, and/or PVC pipe models made to see placement options on stroller frame
• Systems Level: look at spacing and mounting for connections
• Follow ASTM F2549-14a (No exposure to sharp points, small parts, lead paint, scissoring, shearing, pinching, etc.)
This analysis is currently TBD until component sourcing. Because we are unsure of the power source and size of components, this feasibility analysis cannot yet be completed.
Will the frame of the stroller be able to hold various brands of baby car seats? Due to the system design the team has selected, having a stroller that can have a carrier attach is no longer a customer requirement. Therefore, this feasibility analysis was not relevant.
How much power will the HVAC system need to run in the coldest temperatures?
• Closed system - Plastic cover zips around baby space
• Steady State - internal air recirculation to avoid pressurizing space
• Baby is a heater producing a given amount of heat

HVAC Power Calculations

It will take a lot of power (626.81 Watts) to heat the stroller in -40 degree weather. These calculations do not take into account Qsolar which will assist in heating the stroller and reduce this value.
How much heat does the baby lose to the environment?
• Calculations are based off the baby being 3 months to 3 years old
• These are worst case calculations at the lowest possible temperatures with the stroller open (no screen), baby having no clothes/blankets etc.
• The baby’s body temperature is at 37 [degrees C] (310 K)
• Emissivity of the skin is 98%
• Thermal conductivity of human skin is 0.02352 [W/m*K] at -10 [degrees C]
• Boy gives off more heat than a girl, therefore these calculations were done based on a boy's projected heat loss
• Calculate the heat lost due to convection and radiation
• Calculate heat loss at an environmental temperature of -10 degrees C (263 K)

Heat loss by baby

The HVAC device needs to produce more heat than the baby is losing by radiation and convection.

## Morphological Chart and Concept Selection

The team created a morphological chart using the functionality from our functional decomposition and exploring the different options available to implement each function.

Morphological Chart

CONCEPT 1 CONCEPT 2 CONCEPT 3 CONCEPT 4 CONCEPT 5

Plastic Cover with Fan

Closed Pod with Fan

Panoramic Visor with Heated Seat

Open Stroller with Heated Seat

Plastic Cover with Heated Seat

Sketches were designed to visualize and ensure uniform team comprehension of the different conceptual system options. Sketches provided by Anna Keyes.

CONCEPT 1 CONCEPT 2 CONCEPT 3 CONCEPT 4 CONCEPT 5

Plastic Cover with Fan

Closed Pod with Fan

Panoramic Visor with Heated Seat

Open Stroller with Heated Seat

Plastic Cover with Heated Seat

## Concept Selection

SELECTION CRITERIA
• Safety: Weather Shielding
• Safety: Burn Risk
• Safety: Suffocation Risk
• Cost
• Portability: Transportation
• Mobility: Ease of Moving Stroller
• Size of Components
• Required Power- Battery Capacity
• Weight (Lightweight is ideal)
• Effectiveness: How well it heats/cools
• Efficiency: Time to heat/cool and power use
• Usability: HVAC System
• Ease of Design

The team created a concept screening matrix to compare the five possible design concepts. The conclusion is that a plastic cover paired with a heated seat is the best design choice to heat the baby.

Screening Matrix #1

Screening Matrix #2

Final Concept Morphological Chart

Final Concept Selection

## Systems Architecture

Basic heat transfer analyses were done to understand the bounds of our system model.

Heat Transfer Analysis for Open Stroller with Heated Seat/Blanket

Heat Transfer Analysis for Plastic Cover with Heated Seat/Blanket

Closed HVAC System Required Work Calculations and Results

## Designs and Flowcharts

Flow Chart 1: HVAC System breakdown which gives cold air and hot air as output.

Flow Chart 2: Stroller input breakdown which gives velocity and distance travelled as output.

Flow Chart 3: Stroller Insulation breakdown which gives harsh and noise free environment as output.

Flow Chart 4: HVAC System breakdown which gives hot air as output.

## Risk Assessment

The team has updated our risk assessment to account for our final design concept choice.
Rows highlighted in GREEN show new additions to the risk assessment.
Rows highlighted in YELLOW show changes made to the importance of the event.
Rows highlighted in RED show events that no longer apply to our chosen stroller concept.

Updated Risk Assessment for Phase 2

## Plans for next phase

Category Questions to be Answered in the Next Phase
Mechanical Design
• How will the stroller be ventilated?
• What is an achievable stroller efficiency?
• What CAD software will we use to model the system?
• How will the heating elements affect the materials used in the purchased stroller seat?
• How do we ensure uniform heat distribution?
• Where will we locate the battery and other electrical components on the stroller?
• How much component weight is acceptable to add to the stroller?
Material Selection
• What material will be used for:
- Insulation
- Plastic cover
- Stroller with heated blanket
• How thick will material be for insulation?
• Will the plastic cover be tinted/coated to absorb solar gains?
Electrical Design
• What will be the battery size?
• Will we use a single battery for the entire system or will we split the system into high-voltage (HVAC) and low-voltage (sensors)?
• Will we use an off-shelf battery or customizing a battery pack?
• How does the change in solar gains affect the battery usage?
• How will the battery be charged?
• How will we power down the system in an emergency?
Control Selection
• Will it be open or closed loop (feedback wise for system)?
• What type of microcontroller will we be using to control the system?
• Will we buy an off-the-shelf remote or would we install buttons on the stroller handle?
• How difficult will it be to program a remote?
Physiological Questions
• Max temperature/time it takes to (1st, 2nd, 3rd degree) burn baby?
• What is the heat loss (per unit time) by conduction between baby and heated blanket?
Ergonomic Considerations
• Where will the controls be located on the stroller for the HVAC system?
• How do different diameter wheels affect max weight the average user can push, and how will this affect our stroller selection?

Team's Plan:

• Refine and add detail to the system-level proposal
• Come to agreement with guide on the PRIORITY of the work done
• Seek input on design ideas (From faculty, guide, and customer)