Project Number: P16421

Robo-Composter

Jacob Daniels
Mechanical Engineering / Tyler Jones
Electrical Engineering
James Oddy
Electrical Engineering / Colton Scott
Mechanical Engineering
Jake Vavricka
Electrical Engineering / Rob Wieber
Mechanical Engineering

Abstract

Global warming and the energy crisis has sparked a large interest in clean energy and sustainability movements. Our product attempts to simplify and automate composting in order to make it easy, efficient and attractive to mass consumers. This in turn would reduce landfill waste and eventually result in a greener environment. This design also incorporates learning tools and an attractive classroom aesthetic in order to assist with elementary education of decomposition. However, eventually our design would be refined under different operating conditions in order to make it much more feasible and desirable by daily consumers for implementation in their homes.

Introduction

It is no secret that typical consumers throw out tons and tons of food waste each day. However, recently using this food waste as plant nutrient through compost has become a growing trend with the large emphasis on sustainability and environment preservation that exists today.

This project takes that even further, attempting to automate the process and make it much simpler for consumers who may not have room or time to maintain a compost pile. This device also attempts to speed up the typical months long process of achieving finished compost by monitoring environment conditions and adjusting the environment in order to cut this time down to less than a month hopefully.

Design Process

The customer asked that we design a device that would both speed up the composting process as well as making it more “fun” and “entertaining” for students in a classroom. There were also very unique space requirement that the device had to fit into for this specific application. Additionally, it must be safe and easy for young children to operate because it is intended to help assist with the learning process in the classroom. The project was allotted $3500 in order to reach these goals and assemble a final working prototype.

Many design options were put together prior to seeing the space in which the device needed to fit. Unfortunately, after getting the measurements all of those were thrown out mainly due to the height restrictions. The final design settled on included a shallow drawer input and a grinder in order to increase surface area and normalize the particle size. Finally in order to function with continuous input the process has to be done in two stages, one in which the material sits and the other in which it actually composts with no outside disturbances other than mixing.

Drawer Input

Input materials to the Auto-composter are placed in a drawer at the top of the machine. The drawer features a safety interlock that acts as a hard disconnect to all motor power in the system, which ensures the grinder and other motors can’t be turned on while the drawer is open. To add material to the system, the drawer is pulled open by the handle, which also connects to a push rod and plate. When the rod is fully retracted and the drawer is open, materials can be inserted into the drawer. Then, the drawer should be pushed closed. Lastly, the push rod should be reinserted to advance the materials into the grinder hopper for shredding.

Grinder

After the drawer, the composting materials need to be ground into smaller pieces before heading into the holding tank. The grinder has two counter-rotating shafts with toothed discs that feed the materials into the grinder and shear them into smaller particles. One shaft is directly driven by the gearbox, and power is transferred to the second shaft by two gears. The shafts have a hexagonal cross section, and the grinding discs and spacers have internal hex. This design allows the grinding discs to be indexed when installed to stagger the shearing instances along the shaft while only requiring a single disc design. Fingers mounted on the grinder side walls scrap material from between the discs to keep them clean. A clear acrylic shield allows grinding to be observed through the top of the device.

AC Motor

The 1.8 stepper motor with the customized gearhead from Harmonic Drive is used for the grinder due to its high torque operation. This motor with the gearhead does not have a high speed of operation but does not need the speed to be effective. The motor has the ability to operate in both directions enabling the option to go in reverse if an object that does exceed the torque capabilities slips into the system. This is important when there is a user error that would otherwise cause the entire device to cease to work.

Big Motor Controller (AC Motor Controller)

The Anaheim Automation DPD75601 motor controller was chosen for its ability to work with the AC motor and perform complex movements. This controller is programmable making it easy to have the motor perform the desired motions. The Arduino Mega sends a signal out that pulls one of the controllers pins down which is how the two communicate. When a pin is pulled down the controller performs an action that is written into its code so that the Arduino Mega does not need to send multiple messages to the controller.

Interlock

The limit switch with 120 Volts and 10 Amps used in series with the power strip is used for its ability to cut power to the motors when necessary. It is graded to withstand the amount of current and voltage going through it at a given time. It was also chosen for its side adjustable rotor that will work with the drawer to determine when it is open or not and cut power to the power strip.

Layout

The entire system was designed to move from left to right due to the physical limitations of the operating space in which the device will be delivered and positioned.

2-Stage Design

Due to its nature composting has to be done in batches. However, in order for the composting machine to handle continuous material input the decision was made to design the process so that there were two main stages in which the organic matter would spend most of its time. This was done using was is called a holding stage and reacting stage. The holding stage is exactly that; once the material is put into the machine it is deposited into this area until it is ready to move onto the isolated reacting area. In this area, new material is unable to be added in order to help make the output a uniform substance.

Auger

In order to standardize the design of the two main chambers, an auger was chosen to accomplish the task of moving the matter from the holding area to the reacting area as well as mixing and final output of completed compost. The auger satisfied our needs because it is able to effectively move small uniform sized particles of varying consistency. The first tank uses an auger to move the pre-compost into the reactor. The second tank, however, uses it to both lift the composting material to the top of a tube thus dropping it down and mixing the compost as well as turn the opposite way in order to push the finished product out of the unit.

DC Motor

The two Molon permanent magnet DC gearmotors with the torque of 50 in-lbs were chosen to operate the two augers. The 50 in-lbs for torque exceed the required amount to move the augers with composting material moving through them. The two motors are bidirectional allowing them to go in reverse if it is necessary for the augers to move that way at a given time. The two motors also are the right voltage that the power supply operates at with 24 Volts.

Motor controller (DC)

The sabertooth dual 12A motor driver was chosen for its ability to drive both of the auger motors in both directions. This is essential for having the auger move in the opposite direction if it is needed to instead of just the one direction forward. Each motor only draws 1 Amp each so the range given by the sabertooth exceeds the requirements. The sabertooth also communicates with the Arduino Mega easily using digital PWM connection.

Microcontroller

The Arduino Mega 2560 microcontroller is essential for collecting sensor data and sending signals to the motor controllers. The first reason why this device was chosen is due to the fact it has many digital and analog I/O pins for the analog based sensors and for the digital sensors and motor controllers. The next reason is because the device operates at both 3.3V and 5V which enables all of our I/O pins based connections to be powered if they need power.

Micro-Computer

Our project requires a medium for making, modifying, and updating software which controls the composter's hardware. The tasks performed are not very CPU heavy and require rather basic programing (turn a motor on, read a sensor value, etc). The Raspberry Pi 2 Model B not only allows this but is also extremely well documented online. The community surrounding the Pi was a major selling point for us as no one on the team had any “real” programming experience. The Pi is the center for control of our project. An Arduino is used to collect data from all sensors (gas levels, temperature, etc) and sends this data directly to the Pi. In the final product, the Arduino will be sending this data autonomously with no input form the user. The Pi takes this data and logs it to a database. This database is then used to plot trends and inform the user on the condition of the compost. The Pi also hosts the GUI on a website which anyone can reach. This allows anyone in the world to see how the compost is doing. In addition to that, users with the correct log in credentials (users physically near the composter in the classroom) will be able to turn the grinder on as well as the two augers. The Pi will output through HDMI to a monitor and be controlled with a keyboard and mouse. If time and funds permit, this will be upgraded to a touchscreen tablet which will instead be used as the GUI (since the GUI is a website, any device can host it).

Communications

The communications of this device are rather complex. We are communicating between a microcontroller (arduino), micro-computer (Raspberry Pi), and a user interface (webiopi/web-browser). In addition to these, there is a database being used to store all of the sensor information and food log information. There are python and arduino scripts running at all times that are creating graphs, sending commands to motors, and pulling/pushing sensor information. We are using a serial communication between the Arduino and Raspberry PI. This kind of communication only sends one value at a time. The Arduino collects all of the sensor information and sends it to the Raspberry Pi through this serial communication. Once the Raspberry Pi receives this information it stores it on a SQL database. Once stored on the database it can be called by the user face (webiopi/website) or be used to create a graphical image. For motor controls we turn on and off GPIO pins using the webiopi/user interface. We have python scripts running on the RPI to tell the arduino if the raspberry pi has turned the motor on or off. In conclusion, the communications deals with the passage of information from the Webiopi to the Raspberry Pi through web-based files and then from the Raspberry Pi to the Arduino using Python and Arduino scripts.

CO2 Sensor

The MG-811 sensor module is highly sensitive to CO2 and less sensitive to alcohol and CO, low humidity & temperature dependency. On board heating circuit brings the best temperature for sensor to function. This sensor has an on board conditioning circuit for amplifying output signal. In order to produce “perfect” compost, the ratio of carbon to nitrogen must be monitored. Since determining the precise amount of Carbon and Nitrogen in the compost would require an entire laboratory and/or sending samples to an actual lab, the team decided to instead monitor CO2, CH4, and NH3 levels as a means of estimating the amount of carbon in the system. This sensor will tell us what CO2 levels are normal (using a control) and also when things are going wrong. This sensor gives a digital output which corresponds to the amount of CO2 in the system. Using the data sheet we can convert that to either an amount in ppm or %. This sensor will be connected to and powered by the Arduino.

Oxygen Sensor

The ME-O3 Oxygen sensor module uses ME series O2 sensor, and has low consumption, small size, high sensitivity, wide range of linearity, and better anti-jamming capacity, good reproducibility, stability and reliability. Compost must be aerated in order for the proper organisms to grow and decompose the organic material. Without enough oxygen in the system these organisms will die and the material will not compost. This sensor will ensure we have enough oxygen in the system. This sensor provides an analog output signal which corresponds to the amount of oxygen in the system.

Methane Sensor

The MQ-2 sensor module is highly sensitive to methane (CH4) gas. In order to produce “perfect” compost, the ratio of carbon to nitrogen must be monitored. Since determining the precise amount of Carbon and Nitrogen in the compost would require an entire laboratory and/or sending samples to an actual lab, the team decided to instead monitor CO2, CH4, and NH3 levels as a means of estimating the amount of carbon and nitrogen in the system. This sensor will tell us what CH4 levels are normal (using a control) and also when things are going wrong. This sensor gives an analog output which corresponds to the amount of CH4 in the system. Using the data sheet we can convert that to either an amount in ppm or %. This sensor will be connected to and powered by the Arduino.

Ammonia Sensor

The MQ-137 sensor module is highly sensitive to ammonia (NH3) gas. In order to produce “perfect” compost, the ratio of carbon to nitrogen must be monitored. Since determining the precise amount of Carbon and Nitrogen in the compost would require an entire laboratory and/or sending samples to an actual lab, the team decided to instead monitor CO2, CH4, and NH3 levels as a means of estimating the amount of carbon and nitrogen in the system. This sensor will tell us what NH3 levels are normal (using a control) and also when things are going wrong. This sensor gives an analog output which corresponds to the amount of NH3 in the system. Using the data sheet we can convert that to either an amount in ppm or %. This sensor will be connected to and powered by the Arduino.

Temperature sensor

The temperature sensor we used was the Waterproof DS18B20 Digital temperature sensor. This sensor is durable and can reach high temperatures. It is also has a long sensor so we can feed it into the soil, which will tell us the temperature of the soil and not just the temperature of the chamber.

Temp Sensor

Soil moisture sensor

The sensor we used was the VH400 Soil Moisture Sensor Probe. This sensor could handle high temperatures and is durable which is the type of sensor we were looking for. This sensor is stuck into the soil and will tell us the amount of moisture in soil. This will be displayed on the graphs.

Soil Moisture Sensor

Distance Sensor

The four infrared analog distance sensors are able to see through the acrylic walls that the device is made of to measure how far the product is from overflowing. These four sensors were chosen for their cheap and easy use with the Arduino Mega. They simply provide an analog voltage level based off of the distance the product is away from the sensor. This range that the sensor can see is between 80-10 cm which is more than reasonable for sensing the product. The infrared sensors are also able to view the product when there is an abundance of water present on the acrylic which is a possible outcome while composting.

Motor Interface

The motor interface page on the user interface has the same structure as most of the other user face pages. It is located in a web browser as well as the other displays. In this page there is motor controllers for the three motors. The Grinder motor, the Holding Chamber motor, and the Reacting Chamber motor. Each motor has two push buttons for turning the equipment in a forward or backward direction. The Grinder motor controls a grinder, which is the first step of the process. The Holding Chamber motor drives the first auger in the process to move the material into the Reacting Chamber. The Reacting Chamber motor drives the second motor located in the Reacting Chamber and this motor will dump material on itself if moving one way and push material out of the chamber if moving the backward way.