Ultrasonic Sensors – Field of View

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By: Jose Alcantar, Electronics and Controls Engineer

HC-SR04 Experiment

Data Sheet Values:

Max Range: 4m

Min range: 2cm

Measuring angle: 15 degrees

Purpose:

Testing the field of view on the HC-SR04 to find a suitable mounting position for the two ultrasonic sensors along the front of the rover.

Procedure:

An object was placed in front of the ultrasonic sensor about 25 inches away; the position of the object was marked and moved in increments of 5 inches until the object was out of view. When the object was no longer detected the position was marked. The angle of the field of view was then calculated.

Results:

Based on the experiment, the angle of the field of view was found to be 18 degrees when measuring an object 25 inches away. The position of the sensors was determined by considering the clearance needed on each side of the rover. When considering the solar panels, the two ultrasonic sensors need to detect obstacles at least 5 inches to each side of the chassis. This will allow the pathfinder to avoid obstacles that may bump into the solar panels.

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System Mechanical Design

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By: Nick Lukin (Design and Manufacturing Engineer)

Table of Contents

Introduction

Figure 1: Overall Mechanical Design 

The mechanical design of the Pathfinder utilizes many parts and sub-assemblies in order to achieve all the requirements associated with the overall design. In order to achieve the level 1 requirement of being able to successfully traverse a pre-determined course on campus it was necessary to utilize a proper suspension system. It was also necessary to completely rebuild the pan and tilt smart phone holder in order to fit a Samsung Galaxy S7 edge. The suspension design utilizes a rocker bogie suspension system very similar to the one used on the Spirit rover. Below is a description of the overall mechanical design including all of its assemblies and sub-assemblies.

Initial Design Process

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The Spring 2016 Pathfinder was used as a base reference in order to come up with a usable overall design. Each part was measured and then modeled in solidworks in order to come up with working parts and assemblies. The pictures above shows some sketches of the various parts that needed to be measured, modified and then modeled. It was necessary to change the overall geometry of the suspension and platform in order to achieve some of our desired design outcomes such as wheel clearance and lower center of gravity.

 

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The above pictures are models of the base servo mechanism and the actual motors that are used. It was necessary to properly model these in solidworks in order to get a more accurate overall model of the pathfinder. The base servo mechanism mounts the servo that controls the pan motion of the smart phone holder. Adding the motors gives an accurate measurement of the width of the Pathfinder.

 Overall Design

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The above pictures show a 2-D drawing as well as a 3-D exploded view of the Pathfinder design. The overall dimensions of the design can be seen below.

Height: 21.6 inches

Width: 18.02 inches

Length: 23.60 inches

Floor to Top panel: 11.14 inches

The overall design can be broken into two basic sub-assemblies. These include the rocker bogie suspension system and the pan/tilt smart phone holder. Descriptions of these sub-assemblies can be seen in the next sections.

Rocker Bogie Suspension System

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The rocker bogie suspension system that was utilized in our design is very similar to the one used on the Mars Spirit rover. This suspension system is good for uneven surfaces and requires no springs or dampening mechanisms. Each wheel can move up and down independent of one another. Another benefit of this suspension system is that the main body stays straight and upright while going up of down steep surfaces. This creates good weight distribution and helps prevent the Pathfinder from tipping over. The overall wheel clearance of the suspension system was designed to be 5 inches due to the fact that it will need to go upstairs that are about 5 inches tall. The diameter of the wheels is 6 inches, therefore the height of the Pathfinder from the ground to the bottom of the base platform is 11.14 inches.

Pan/Tilt Smartphone Holder

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The pan/tilt smartphone holder is designed to hold a Samsung Galaxy S7 edge. The dimensions of the phone are 5.94 x 2.86 x 0.30 in. The holder case was designed to have the following dimensions: 6.34 x 3.26 x 0.7 in. The thickness of the case will be 0.2 inches which will allow for the phone to be fully covered. The front cover plate was designed to have cutouts in the appropriate locations. The cutouts can be seen in the picture above. These cutouts are for the camera and for the antennas in the phone. It is necessary that these do not get covered in order to obtain good signal strength. The pan/tilt servos will be able to move 180 degrees in both directions.

Lower Center of Mass

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The goal of the design was to get the center of mass as low as possible. The picture above shows the center of mass in purple. The previous design had a center of mass the was above the main pivot point of the rocker bogie front arm. The design focused on lowering the center of mass below the pivot point. This was achieved by redesigning the top platform and lowing it an inch. Lowering it an inch meant that the clearance would also be 1 inch smaller which was a problem. This was solved by making the rocker bogie arms one inch longer. The electrical box and battery will also be mounted on the bottom too which helps make the center of mass lower.

 

Stress Test (Base platform only)

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The top panel was analyzed to see how it would react to an outside force being applied to it. The top panel will carry the majority of the load which includes the Solar Panels, the Pan/Tilt Smartphone Holder, the Batteries and the Electronics. The above photo is an exaggerated simulation on how the panel may deform under certain stresses.

Solar Panel Interconnection

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It will be necessary to properly interconnect the chassis to the solar panel assembly. The picture above shows the interconnection mounting points and the dimensions. 4 connection points will be used for optimal stability. These connection points will be raised pads with holes drilled through them. The solar panel assembly will then align with the holes and quick release pins win hold them in place. This will allow for quick removal of the panels.

Sensors for no load condition Trade-Off Study

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By Jose Alcantar, Electronics and Controls

A set of different types of sensors were considered for detecting no load conditions on the Pathfinder, these include:

Current Sensors: These type of sensors allow the user to monitor the current draw on each of the motors.

Flex Sensors: This type of sensor measures the deflection caused by bending of the sensor.

Pressure Sensor: This sensor typically measures the pressure of gases.

Micro switch:  This type is a switch that is actuated by physical force

Each of these types of sensors presented both pros and cons when deciding on which to implement on the pathfinder. Starting with the flex sensor, the proposed idea was that as the pathfinder was traveling the suspension system would move and bend as the rover drove along. As soon as one of the wheels came off the ground, the flex sensor would detect this and shut off power to the motor. The biggest problem in implementing this idea is that the rocker bogie system does not bend while it is driving. This makes the sensor in the field unreliable.

The idea with the pressure sensor was that the pressure in the tires would be measured. This would be beneficial if the rover had air-filled tires, which would detect when the tires would come off the ground. With the new design of the rover, this would become an issue due to the use of foam-filled tires.

Similar with the flex sensor, the micro switch would detect any force on the sensor if the suspension lifted a tire off the ground. The problem with this is with the design of the rover, there would be no suitable areas to mount the sensor on the pathfinder.

The best option for the rover were the current sensors. Due to the current draw of each of the motors, a current sensor can be wired in series with the motor to measure the current. Of the different types of current sensors, two were selected as the best options. The two options were the use of a 0.51Ω current sensing resistor and the Adafruit INA219.

When considering the two options the main differentiator between the two was the cost. The Adafruit INA219 has a total cost of $9.95 and multiplying by six (one for each motor) the total comes out to about $60. The shunt resistor has a cost of about .56 cents each, the total coming out to being about $3.50. Another benefit of using the shunt resistor is the fact that the motor shield being used has a pin output specifically for the use of current sensing resistors. Ultimately, the shunt resistor appears to be the best option due to the cost, the easy implementation, and least use of pins.

Motor Shields Trade-Off Study

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By: Nick Lukin (Manufacturing and Design Engineer)

Two motor driver shields were taken into consideration for the Pathfinder Project, the Pololu VNH 5019 and the Adafruit V2.3. Each shield had its own pros and cons and there were a variety of factors that needed to be taken into consideration during the evaluation process. The figure below shows a comparison of both boards.

motorshield-tradeoff

Table 1: Comparison of Polulu VNH 5019 and the Adafruit V2.3

Continuous and Peak current capabilities of each driver shield were evaluated in order to find out what each shield could handle. The motors that we chose to use are 12 volt DC worm gear motors. These motors have a no load current of 0.96 A, a full load current of 1.8 A, and a stall current of 4.8 A. The Pololu VNH 5019 can handle the loads of the motors while the Adafruit V2.3 cannot. The next factor considered was the number of pins used for communication between the Arduino Leonardo and each shield. The Pololu VNH 5019 requires a minimum of 6 pins to control 2 motors. In order to separately control each motor (6 motors) a total of 18 i/o pins would be needed and the Leonardo only has 20 i/o pins total. This creates an issue because pins will also be used to control servos and take in sensor inputs. The Adafruit V2.3 uses i2c for communication and therefore will only require two pins from the Leonardo (SCL and SDA). The next factor taken into consideration was the additional features that each board offered. The Pololu VNH 5019 has built in current sensors that could be used to meet the requirement of stopping each motor under no load conditions while the Adafruit V2.3 has built in servo control that could be used for the pan and tilt mobile phone holder. Each shield has its pros and cons but ultimately we chose to go with the Pololu VNH 5019 because it can handle the load of the DC motors that we plan to use. It is necessary for us to keep the motors that are presently on the Pathfinder because they can handle the heavy weight of the chassis and solar panels. In order to solve the pin issue associated with the Pololu VNH 5019 we plan to use a MCP23017 – i2c input/output port expander. This port expander will give us an additional 16 pins. Our design will use 3 Pololu VHH 5019 motor shields stacked on each other to drive 6 motors. We will also be able to utilize the built in current sensors in order to stop our motors under no load conditions.

Preliminary Project Plan

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Sabina Subedi (Project Manager)

Adan Rodriguez (Mission, Systems & Test)

Jose Alcantar (Electronics & Controls)

Nick Lukin (Design & Manufacturing)

Table of Contents

Work Breakdown Structure (WBS)

By Sabina Subedi (Project Manager)

The WBS shows all the work that is to be completed by the Pathfinder – Chassis group. The WBS is arranged into three main sections or divisions – Mission Systems & Test, Electronics & Controls and Design & Manufacturing, where each section is a responsibility of the corresponding division member. The three sections are then divided into various groups, which include specific sets of tasks that are relevant to the group.

wbs

Figure 1: Work Breakdown Structure

Project Schedule

By Sabina Subedi (Project Manager)

Top Level Schedule

The top level schedule below was created using the generic schedule provided on the class website. This schedule consists of all tasks that are to be completed before the end of the semester, December 15th, 2016. The project milestones are broken down into four phases: Planning, Design, Assembly and Project Launch. The tasks within the different phases are then divided up by the divisions.

top-level-schedule-generic

Figure 1: Top level schedule (Generic)

System/Subsystem level tasks

The generic top level schedule was then modified to include all system/Subsystem level tasks in accordance with the WBS above. All division members are assigned specific tasks that they are responsible for, per “Job Descriptions” document available on the class website. Main tasks then were broken down into sub-tasks, if applicable. All tasks include start and finish dates, as well as percent complete. Blue check mark denotes tasks that are 100% complete.

schedule-1 schedule-2 schedule-3 schedule-4

Figure 2: Schedule including System/Subsystem level tasks

Burn Down and Project Overview

The burn down chart below shows how many tasks are completed and how many are left. The project overview graph shows the percent completed as of today, September 29th 2016.

burndownpercent-complete

Figure 3: Task burndown chart along with project overview graph

System Resource Report

By Adan Rodriguez (Mission Systems and Test Engineer)

Cost Allocation Report

This cost report is a rough estimate of the expected cost of each component on the Pathfinder. Our expected prices for each item excluded tax and shipping price. Tax and shipping prices were included in the Uncertainty category. Some of the items were marked at $0 for expected price because the item was either already on the Pathfinder from previous semester or the item was given to the team. Since there is no budget requirement, the Project Allocation was determined by adding up the expected price of each item and choosing an amount slightly higher than the total expected price.cost-allocation

Figure 4: Cost Allocation Report

Power Allocation Report

This power report displays the expected current drawn by each component that will be drawing power from the battery. The team had trouble identifying the current that would be drawn by the VNH 5019 motor shields. For now we have used rough estimate of the current drawn by the VNH 5019 motor shield by using the same current rating of the Arduino Leonardo. The battery being used on the Pathfinder has a power rating of 10,000 mAh. We considered the 4 hour duration of the mission in order to come up with the Project Allocation value. We simply divided the power rating of the battery by 4 in order to come up with a Project Allocation of 2,500 mAh.

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Figure 5: Power Allocation Report

Mass Allocation Report

This mass report is a rough estimate of the expected weight of each component on the Pathfinder. Some of the expected weight values are rough estimate because some of the item weight values were tough to find. Rough estimates were made relative to similar size of items. For example, the SeedStudio Ultrasonic sensors were estimated to weigh a fraction of the weight of the VNH 5019 motor shield because we had accurate weight values for the motor shields. The Mass Allocation Report will be updated once we actually weigh items with a scale. Since there is no weight requirement, the Project Allocation was determined by adding up the expect weight of each item and choosing an weight slightly higher than the total expected weight.

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Figure 6: Mass Allocation Report

Project Cost Estimate

By Sabina Subedi (Project Manager)

The total expected cost is $281.24, based on the cost allocation provided above. The cost allocation report consists of rough approximations of the expected cost of each component. The components listed have not been purchased. Further trade-off studies are to be done before any purchases are made. Therefore, the total estimated cost is subject to change as the project progresses.

Source Material:

Preliminary Project Plan: http://web.csulb.edu/~hill/ee400d/Documentation%20Lecture%20Series/05%20Preliminary%20Project%20Plan.pdf

Generic Schedule: http://web.csulb.edu/~hill/ee400d/Lectures/Week%2005%20Project%20Plans%20and%20Reports/c_Generic%20Schedule.pdf

Job Descriptions:

http://web.csulb.edu/~hill/ee400d/Lectures/Week%2001%20Welcome/c_Job%20Descriptions.pdf

Resource Report:

http://web.csulb.edu/~hill/ee400d/Lectures/Week%2005%20Project%20Plans%20and%20Reports/d_How%20to%20Write%20a%20Resource%20Report.pdf

Fall 2016, Preliminary Design Documentation

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Sabina Subedi – Project Manager

Adan Rodriguez – Mission, Systems and Test

Jose Alcantar – Electronics and Controls

Nicholas Lukin – Design and Manufacturing

Table of Contents

Review of Literature

By Sabina Subedi, Project Manager

Level 1 requirements for the Pathfinder from Spring 2016 were reviewed and analyzed per the requirement evaluation rubric provided below.

Requirement Evaluation Rubric:

  1. Is the requirement, Quantitative, Verifiable, and Realizable?
  2. Is the requirement located at the correct level (1 – Program/Project, 2 – System/Subsystem)
  3. Is the requirement response to a higher level requirement or customer’s objective (Requirement Flow Down)? Is the linkage clearly defined?
  4. Does requirement provide links to source material?
  5. Does the requirement move the design process forward?
  6. Are equations used to calculate a requirement provided and are answers correct?
  7. Is language in the form of a requirement?

 Figure 1: Review of requirements from Spring 2016

Program Objectives / Mission Profile

By Sabina Subedi, Project Manager

Program Objective Statement

The Pathfinder is an autonomous rover that is self-sufficient using solar panels. The design of the Pathfinder is inspired by the twin Mars rovers “Spirit and Opportunity.” The Pathfinder will utilize navigation waypoints on Arxterra control panel to traverse through the defined course, articulating ultrasonic range finders and LiDar sensor for obstacle avoidance. Digital slip differential will be implemented for unmanned turning of the rover during it’s course. In order to save battery, the Pathfinder will have sensors that will send signals to the motors to stop the wheels from spinning under no load conditions.

Mission Profile

The mission for the Pathfinder is to complete the same course defined for Spring 2016 AdBot. “The mission profile will be a test course consisting of the 9 steps in front of the USU building, then the concrete path clockwise leading down some inclines back to the bottom of the stairs.” (Spring 2016, AdBot Blogpost) The Pathfinder shall complete its mission in one evening.  The pathfinder shall conduct its mission at night for better operation of the LiDar sensor.

Requirements

Program/Project (Level 1)

By Sabina Subedi, Project Manager

  1. The pathfinder shall be autonomous, demonstrating GPS navigation mode on Arxterra
  2. The pathfinder shall avoid obstacles articulating Ping Ultrasonic Range Finder and a LiDAR sensor
  3. The pathfinder shall implement digital slip differential for turning, i.e., inside wheels turn at a different speed than the outside wheels while turning
  4. The six wheels shall be on a rocker bogie suspension system to allow the Pathfinder to traverse through rough terrain and enable mobility
  5. Each wheel shall have its own motor in order to be able to be controlled independently
  6. Wheels under no load condition shall stop spinning in order to conserve power
  7. The physical design of the Pathfinder shall mimic the Spirit and Opportunity rovers.
  8. The pan and tilt system shall be modified to support an Android phone
  9. The pan and tilt system shall be sealed in order to protect it from weather conditions in December
  10. The Pathfinder shall explore the course defined by the AdBot rover. The mission shall be completed in 4 hours in the evening.
  11. The Pathfinder shall be completed by the last day of 400D course, December 154h, 2016

System/Subsystem (Level 2)

By Jose Alcantar, Electronics and Controls Engineer

  1. The Pathfinder shall use an Arduino Leonardo implementing the latest version of the 3DoT library. Due to customer request, the current Arduino Mega will be replaced with the Arduino Leonardo. The Leonardo will be programmed using the 3DoT library.
  2. The Pathfinder shall use GPS coordinates from its sensors to communicate position with the Arxterra app. This will allow GPS navigation and the use of waypoint navigation.
  3. For obstacle avoidance, a LIDAR sensor and two SeedStudio ultrasonic range finders (SEN136B5B) shall be implemented onto the design of the Pathfinder. The ultrasonic range finder will be able to detect the range of obstacles in front of the pathfinder from 3cm to 400 cm. The LiDAR sensor will be able to detect the obstacle itself, allowing the rover to maneuver around that obstacle. A trade off study was done prior to choosing this specific model of ultrasonic range finder.
  4. The design of the pathfinder shall resemble the Spirit and Opportunity rovers. The Spirit and Opportunity rovers are each 1.5 meters high, 2.3 meters wide and 1.6 meters long, and weigh about 400 lbs.
  5. Due to the removal of the Google Tango, the pan, and tilt platform shall be redone to accommodate solely an Android phone, specifically a Samsung Galaxy S7 Edge. The dimensions of the phone to be used are 9 x 72.6 x 7.7 mm (5.94 x 2.86 x 0.30 in). The phone weighs 157.6 g.
  6. The encasement for the Android phone shall be sealed to protect the device from outdoor conditions.
  7. This requirement will vary depending on the weather conditions on the day of the final demonstration in December. The forecasted load for the day of the final, December 15th, 2016, is partially cloudy with a high of 65 degrees Fahrenheit and a low of 47 degrees Fahrenheit. No rain is forecasted for that week. Historical average for the day is 67 °/47 °.
  8. In order for the Pathfinder to implement electronic slip differential 6-wheel drive, three separate VNH 5019 motor shields shall be used to allow the control of each independent DC motor. Testing will be done to find a suitable voltage ratio between the wheels to ensure slip differential turning.
  9. The ACS712 (SEN-08883) current sensor shall be implemented to get feedback on the amount of current passing through the load. This current sensor will allows us to measure load up to 5A of AC or DC current.

Source Material

The links below consist of sources that were used in developing the Level 2 requirements.

PINGER and Arduino:

https://www.arduino.cc/en/Tutorial/Ping

Ultrasonic range finder (SEN135B5B) Fritzing diagram:

http://fritzing.org/media/fritzing-repo/projects/3/3pi-robot-atmega-328p-and-ultra-sonic-range-measur/other_files/ULTRASONIC%20SEN136B5B.pdf

SeedStudio UltraSonic range finder and Arduino interface:

https://www.youtube.com/watch?v=y8ox5NPvcPc

SeedStudio Ultra-sonic range finder:

http://www.eio.com/p-40804-seeedstudio-sen136b5b-ultra-sonic-range-measurement-module.aspx

Ultrasonic Field of View test, Fall 2015 Pathfinder:

http://arxterra.com/pathfinder-ultrasonic-field-of-view-test/

 Specifications of the Spirit and Opportunity:

http://hobbiton.thisside.net/rovermanual/

Waypoint navigation on Arxterra:

https://www.arxterra.com/arxterra-now-supports-waypoint-navigation/

Performance of Arduino Leonardo:

https://www.arduino.cc/en/Main/ArduinoBoardLeonardo

Performance of LiDar Lite V2:

https://www.sparkfun.com/products/retired/13680

Weather Forecast for December 2016:

http://www.accuweather.com/en/us/los-angeles-ca/90012/daily-weather-forecast/347625?day=86

Spring 2016 AdBot Preliminary Design Documentation:

http://arxterra.com/preliminary-2/

Low Current Sensor:

https://www.sparkfun.com/products/8883

Preliminary Budget

The preliminary budget was created based on the new upgrades required to the existing rover in order to meet the new Level 1 and Level 2 requirements for this semester, Fall 2016. This budget will be verified by our customer before any parts are purchased. Therefore, the total estimated cost is subject to change.

Figure 2: Preliminary Budget for the Pathfinder, Fall 2016

Design Innovation

By Sabina Subedi, Project Manager

A brainstorming exercise was performed in order to come up with creative solutions to the problems found in the existing Pathfinder. Some of the problems with the existing Pathfinder were:

  • It cannot climb up the stairs. This problem occured because the weight on the existing rover is not distributed properly. It could be fixed by lowering the center of gravity. Possible ways of lowering the center of gravity include but are not limited to redistributing the weight on top of the chassis, or lowering the entire platform and pushing the wheels further back.
  • The rover draws excess current merely due to its weight. The weight could be reduced by milling excess metal off or by using lighter components. This could potentially increase the battery life.
  • The existing pan and tilt system is designed for a Google Tango and is controlled by two servo motors. The existing encasement a bulky design made out of plastic. The bulkiness of the design limits the Pathfinder in its range of movement and makes it harder to navigate. Possible solution to this problem is to design a new pan and tilt system to hold an Android phone and utilize stepper motors instead of servos to reduce bulkiness as well as increase functionality.

Systems/Subsystem Design

By Jose Alcantar, Mission Systems and Test Engineer

Product Breakdown Structure

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Figure 3: Product Breakdown Structure

The product breakdown structure demonstrates the hierarchical breakdown of the products that will be used in the overall design of the Pathfinder chassis subsystem. At the highest level, the Pathfinder chassis subsystem splits into two separate main subcategories, the hardware and software aspect. The hardware consists of three subcategory items, the chassis, pan and tilt platform, and the Arduino microcontroller. The chassis subcategory was organized by first starting at the bottommost level, where the manufacturing engineer is responsible. This consists of the six wheels and six DC motors that will be installed onto the rocker bogie suspension system and ultimately onto the chassis. The pan and tilt platform is the responsibility of the manufacturing and electronics engineer. At the lowest level, the category consists of the Android device, motors for pan and tilt, and the two sensors. These bottom level components will be installed onto the manufactured pan and tilt platform allowing obstacle detection and platform control. The Arduino subcategory is the physical microcontroller used on the pathfinder. This category is further split into two categories, the motor shield, and Bluetooth module. The electronics engineer is responsible for these components. The use of the motor shields allow for DC motor control and the Bluetooth module allows for communication between Arduino and Android.

The other main subcategory is the software block. This block is then further split into other functions needed for the control of the pathfinder. The first sub component is the software-implemented control of the pan and tilt motors. The second component is the software controlled DC motors used for the wheels of the pathfinder. Third is the control of the LIDAR and Pinger sensors used for obstacle avoidance. The fourth component is the software integration for communication between Android and the Bluetooth.

Electronic System Design

By Adan Rodriguez and Jose Alcantar

System Block Diagram

By Adan Rodriguez, Mission, Systems and Test Engineer

Figure 4: System Block Diagram

Interface Definitions

By: Jose Alcantar, Electronics and Controls Engineer

Figure 5: Interface Matrix

The interface matrix for the Pathfinder chassis subsystem tabularizes the pin connection layout of the electronic system. The embedded system is comprised of the ATMEGA32U4, Arduino Leonardo board, Motor Shield and finally sensors and actuators on the rover. The Arduino and motor shield use up some pins from the ATMEGA and in exchange get an extension in capabilities for the components used on the rover.

Fritzing Diagram

By: Jose Alcantar, Electronics and Controls Engineer

Figure 6: Fritzing Diagram 

Source Material

The following sources were used in developing the interface definition.

https://cdn.sparkfun.com/datasheets/Sensors/Proximity/lidarlite2DS.pdf

https://www.arduino.cc/en/uploads/Main/arduino-leonardo-schematic_3b.pdf

https://www.arduino.cc/en/Main/ArduinoBoardLeonardo

https://www.adafruit.com/product/1438

https://cdn-shop.adafruit.com/datasheets/TB6612FNG_datasheet_en_20121101.pdf

Mechanical Design

By Nicholas Lukin, Design and Manufacturing Engineer

Overall Design

Figure 7 : Overall design of the new chassis

Figure 8: Exploded view of the new chassis

The above pictures are models of what the Pathfinder should look like in order to satisfy the level 1 and 2 requirements. In order to satisfy the mission objective of traversing the same course/path that was outlined for the “Spring 2016 Adbot” the Pathfinder will utilize a rocker bogie suspension system that is very similar to the design used on the Spirit mars rover. This suspension system gives the Pathfinder a wide and long stance allowing it to climb up and around obstacles such as rocks and stairs. The length from the front to the back wheel will need to be longer than the length of the base platform in order to produce a wide stance. This wide stance will prevent the Pathfinder from tipping backwards when going up steep surfaces. Many components will be mounted to the base platform such as the pan/tilt mobile case, solar panels, electronics box, battery and sensors. These components will be mounted in a way that will create even weight distribution on the base platform. This even weight distribution will also help the Pathfinder remain upright when going up steep surfaces. Using a rocker bogie suspension system and having even weight distribution will help achieve the goal of traversing the course.

Chassis Lightening

Figure 9: Rocker-Bogie Suspension system

In order to conserve battery life and limit the stress put on the driver motors the chassis will be made as light as possible. The suspension components of the current Pathfinder will need to modified in order to achieve this requirement. The above picture shows the suspension components and how they may look in order to achieve the lightest possible design. About half an inch may be cut off from the inside of the suspension legs. Stress simulations and calculations will need to be performed to ensure that the new lightened chassis can handle the load of all the components that will be mounted to the base platform.

Mobile Phone Pan and Tilt System Design

Figure 10: Pan and Tilt System design

The use of a Pan/Tilt mobile encasement is required in order to help the Pathfinder navigate through the course. The holder can be designed for an Android or Iphone. The above picture shows a proposed design model for an Iphone 6. Measurements were taken from an Iphone six and these exact measurements were used in the above model. The Pan/Tilt system will be driven by servos that will be mounted directly to the Pan/Tilt chassis. The above model does not include the servo that controls the “pan” movement. Future design will include the second servo. Wiring for the servo motors will be run through the support tube and then ran to the electrical box via the bottom side of the base platform.

New Wheels

Figure 11: New wheels, McMaster-Carr 6″

In order for the Pathfinder to complete the course it will need to be equipped with new wheels. These wheels will have to be strong enough to support the load of the entire pathfinder and rugged enough to withstand the environment that the course will provide. The picture above is of a wheel sold by McMaster-Carr that may be used on the Pathfinder. The wheel is 6 inches in diameter and 1.5 inches wide. The axel has a diameter of 0.5 inches. The wheel hub is made of smooth steel and the actual wheel is made of solid rubber with hollow treads. The capacity of each wheel is 55 lbs. The wheel hub has ball bearings pressed into it to ensure smooth operation. Having a 6 inch diameter will allow the Pathfinder to get up stairs easier. Having a steel wheel hub may be too heavy therefore an aluminum wheel hub may be used instead.  A wheel mounting hub will be designed in the future. This hub will properly connect the DC motor to the wheel. A link to the wheel specs can be found in the source material section.

Electrical, Wiring, AC Unit


Figure 12: Electrical box and top view of the chassis

It is necessary that all electronics be protected during the mission. An electrical box will be used to house all of the electronics such as the micro-controller and motor driver shields. The electrical box may be made out of metal or plastic and will be mounted in the best place possible to ensure even weight distribution. A cooling fan might be used in case the heat produced by the electronics gets too high. This fan will be built into the electrical box along with vents to ensure proper air flow. Future tests will need to be run with the electronics engineer to see how hot the electronics get. All wiring will be neatly mounted and covered for protection. The battery is the heaviest part of the electric system and will need to be mounted in the center of the base platform in order to maintain even weight distribution. The picture above shows a general idea of where the electronics will be mounted on the base platform relative to everything else.

Solar Panel Interconnection

The design of the actual solar panel support is not in our scope of work but it is necessary for us to know how it will look in order for us to figure out how it will be attached to the chassis. It is crucial that the solar panels have good weight distribution and that they are mounted on the base platform in the correct location. Working with the Solar Panel Group in the future will be necessary in order to figure out what the best location is.

Source Material

Spirit Rover:

https://en.wikipedia.org/wiki/Spirit_(rover)

Wheels:

http://www.mcmaster.com/#2331t11/=149w7jd

General Information on spring 2016 Pathfinder:

http://arxterra.com/spring-2016-pathfinder-preliminary-design-documentation/

Design and Unique Task Description

The following tests and experiments shall be conducted.

Solidworks simulations will need to be performed in order to figure out if the new lighter chassis is strong enough and to see if the center of mass is in the appropriate location. These tests include load and shear analysis (stress test) and various animations

Rolling tests will need to be performed on the new wheel to make sure they do not wobble and have a smooth rotation.

Pan/tilt platform may be animated to show proper mobility.

Weight distribution test with solar panel connected.