P16421: Robo-Composter
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Systems Design

Table of Contents

Team Vision for System-Level Design Phase

During this second design phase, the team planned to completely map the composting process settle on our parameters and steady state for the device, select main systems to be used in the design and visit the classroom in order to obtain specific space requirements and make observations regarding the operating environment.

This phase actually, yielded most of the above accomplishments but also allowed for us to accomplish much more than that. We have identified seven functional levels for the stages that will be implemented within the device and that was entirely before deciding on any specific designs. The group also developed an extensive list of possible methods that can be used to accomplish each task required for successful composting and then developed seven system concepts from this list. Comparing these concepts against each other, we settled on the best design possible and then justified that selection through bench marking for each stage and each method to completing each stage. Additionally, our process was mapped out in a fairly generic systems architecture while also breaking that down into a black box input/output for each of our selected designs. Finally, the group asked some very involved questions with respect to the feasibility of the selected designs and gained a very good understanding of the actual requirements that are required in order to satisfy the customer through the use of our designs.

Functional Decomposition

It was determined that the entire project has one primary goal, which is to fertilize plants via compost created easily and efficiently. After hours of deliberation, and multiple iterations, the functional decomposition has been broken down below. It will change and evolve as the group determines specifics on types of mechanisms that will be employed with in the designs, but this satisfies all of the questions the group faced regarding how certain steps will be performed and why specific tasks will be performed.
Functional Decomposition

Functional Decomposition

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Benchmarking

Insertion

Insertion

Grinding

Grinding

Agitate

Agitate

Transfer

Transfer

Transport

Transport

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Morphological Table and Concept Selection

The team used a morphological table to generate designs for several aspects of the composter. Several core functions of the device were pulled from the Functional Decomposition, and different means of accomplishing those functions were identified. In this step, the validity of the concept is not taken into consideration.
Morphological Table

Morphological Table

Concept Selection

Using the morphological table, several system designs were produced by combining one element from each of the categories in the table. Those system designs were then evaluated in a Pugh Chart with respect to several import factors (ie. cost and safety) with respect to an arbitrarily chosen "Datum" design." Then, the designs were evaluated a second time with a different design serving as the datum.
Pugh Chart

Pugh Chart

The results from both Pugh charts were nearly identical. In both cases, Concept 4 scored the highest.

Pugh Chart Comparison

Pugh Chart Comparison

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Systems Architecture

On a high level, our composter should perform the following actions regardless of final design concept:

Organic material will flow in from the entrance of the machine through each station in order until usable and fertile compost exits at the end of the machine. Each station's sensor outputs and control signal inputs will be routed through a single central processing station to ensure that all systems remain coordinated with each other and any error or interrupt signals are processed and executed quickly to ensure a safe operating machine.

System Architecture Flowchart

System Architecture Flowchart

System Design Flowcharts

System Design Flowcharts

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Feasibility: Prototyping, Analysis, Simulation

How fast should the cart move?

In order to get a good idea of the motor, power and stability requirements of the cart, we first had to establish how fast the cart could move without endangering anyone in the vacinity. The minimum amount of collision energy required to bruise was found to be about 8 Joules (source).

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Control, Power and User Interface

Control Power User Interface

Can agitation and grinding be powered via the same motor?

Options:

  1. Selectable belt drive actuated via electromagnets (similar to deck drive belt on lawn tractors)
  2. Transfer Case

Is it feasible?

No. While it may be the smaller and possibly cheaper option, the additonal complication added by a transfer case or belt drive would put us at risk of not being able to complete the machine by the end of MSD. Both options are very complex and cost inefficient at performing the task. Two separate motors should instead be used, totaling less than $150.

Possible motor candidate.

What is an acceptable operating volume?

Comparing our product with the current loudest class of home kitchen appliance (the dishwasher), we determined that an acceptable maximum range for operating volume is between 50 dB and 64 dB.
GE LG Hotpoint Frigidaire

What motor speed will be required for the shredder?

What motor torque will be required to shred worst case input?

How will carbon and nitrogen be measured?

Several Cornell composting sites showed excessive nitrogen compared to carbon in a composting system led to increased amounts of both methane and ammonia. There was also data that showed that if there was an excessive amount of carbon to nitrogen there would be little methane and ammonia produced. From these findings an experiment with a worm bin using methane and ammonia sensors can be conducted to find out the ppm for both gases that would be produced when:

  1. The composting system is balanced.
  2. There is excessive nitrogen in the composting system
  3. There is excessive carbon in the composting system.

The two sensors that would be used are a methane sensor that only costs around $7.99 and a general Air Quality Detection Sensor for around $3.31 that can be configured to sense ammonia.

What will control the composter?

public/Systems Level Design Documents/Microcontroller and Microprocessor Comparison.png

How will the sensors be used to speed up the composting process?

public/Systems Level Design Documents/Feasibility -- How to use the sensors.png

Source: The Art and Science of Composting

Additional prototyping required

We were unable to accurately gauge how long a batch of compost had to react before being considered "done", so in order to figure out compost reaction time we will be setting up several compost batches using variable food particle size.

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Risk Assessment

Risk Assessment

Risk Assessment

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Design Review Materials

Photo gallery from our Rochester Roots visit

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Plans for next phase

Throughout the next 3 weeks, our team will continue to analyze our selected concept and adjust it as we see fit. Our goal is to have completed drawings and specific designs for our components and flow charts defining the interactions between the specific systems. We will also have a bill of materials ready to present for approval and much more specific design parameters with associated sensitivity analysis.

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