Spring 2017 SpiderBot

Material Trade of Study

By Daniel Matias – Manufacturing Engineer

 

Requirements:

A level 1 requirement states that SpiderBot shall lift itself off the ground. To achieve this, and still maintain a stable, mobile robotic platform, SpiderBot will be made of an inexpensive, lightweight material with a high tensile strength.

 

Introduction

Traditionally walking robots have proven to be a challenge. Proper walking mechanism selection can either means success or failure. Walking robots experience large amounts of shock and vibration that can cause stress fractures and material deformations. The study aims to present all materials considered for SpiderBot’s use.

Materials:

The materials considered in this trade-off are aluminum, acrylic, and plywood. A fixed volume of 1/8’’ thickness, 6’’ wide, and 12’’ long sheets are being used when comparing the materials. Also considered is the strength of the material to ensure our robot will not break under the weight of the components.

Calculations:

 

The following table shows the cost and mass of the set volume

Material

Cost

Mass

Aluminum

$17.14

398 g

Acrylic

$5.07

174.5 g

Plywood

$2.50

98.8 g

The following is a table of the tensile and compressive strength of the three materials.

Material Tensile Strength Compression Strength
Aluminum 310 MPa 20 GPa
Acrylic 69 MPa 124 MPa
Plywood 31 Mpa 36 MPa

Conclusion:

Although plywood is the cheapest and the lightest, acrylic is more suited for our robot due to the inexpensive, toy requirement. Aluminum could be reduced in size since it is stronger, but the material and machine shop costs would exceed our budget when all the qualities of acrylic can satisfy requirements.

Resources:

 

https://www.mcmaster.com/#acrylic-sheets/=16uqxrr

https://www.mcmaster.com/#standard-aluminum-sheets/=16uqxy0

http://www.woodworkerssource.com/shop/product/18balpack3.html

http://www2.glemco.com/pdf/NEW_MARTERIAL_LIST/Alumina%206061-T6.pdf

http://www.engineeringtoolbox.com/wood-density-d_40.html

https://www.metalsdepot.com/catalog_search.php?search=s318T6

http://edge.rit.edu/edge/P14418/public/4-Subsystems%20Design/Plywood%20Materials.pdf

http://www.builditsolar.com/References/Glazing/physicalpropertiesAcrylic.pdf

Spring 2017 SpiderBot

Motor Trade Off Study

By Shaun Pasoz – Electronics & Control Engineer

 

 

Requirements:

Based on the level 1 requirement that all power shall be supplied via the 3DoT board, the motor choices have certain limitations including: must draw 450mA or less, and have an operating voltage of 5V or less.  

UPDATE

Following the Preliminary Design Review the customer requests that SpiderBot’s overall size needs to be reduced. Our initial build had a mass of 858 grams. The SpiderBot team has decided to reduce the overall mass to 500 grams or less. Since our mass has changed, so has the motor torque requirement to make SpiderBot walk. In a future post we will present a systematic process used to determine the torque requirement.

 

Introduction:

Due to the delicate nature of servos, SpiderBot’s walking mechanism will be driven by two DC motors. DC motors are basic electromechanical devices. They consist of two wires, one for V+ and one for V. The speed, or power level, of the motor is controlled via pulse width modulation (PWM). PWM controls the motor by allowing, for example, full voltage for 50% of a duty cycle to create 50% of the rated RPM. Since the DC motor only has two wires, it provides no feedback. When compared to a servo motor, the servo has: a DC motor, a position sensing unit, and feedback control. However, servos are prone to gear degradation under heavy loads, and size is proportional to cost. This post attempts to deliberate the factors in choosing the correct dc motor for SpiderBot. 

 

Types of DC Motors:

 

  • Stepper Motor: A type of DC motors that moves in a predefined step size. They are much more precise, and allow for more control than say a brushless motor. This will not be used in the robot as they are much too bulky for our desired size and weight.
  • Brushless Motor: A motor that utilizes electronic communication to control the current flow through the motor. This will not be used in our robot as it is more complicated to control.
  • Brushed Motor: These are the most common motors and are useful for many applications. They can be used in geared motors which will provide more torque. The increased torque and price are what makes the brushed motor ideal for our robot.

 

 

Discussion:

The three main factors in choosing the correct motor comes down to: cost, size, and operating parameters. Out of the three types of DC motors, brushed motors became the immediate choice for the reasons listed under types of DC motors. After the PDR review, it became evident that we needed to shrink the robot and cut as much weight as possible, therefore we will be trying to choose motors that weigh less than 20g. However, smaller motors tend to have less power. Our operating parameters require that the motor operate at 6V or less and have an operating current of under 450mA as this is what the 3DoT can power. After conducting research this is the list of motors that most closely suit what we are looking for:

 

Motor: Rated Voltage Speed @Rated Voltage Free-run Current   @Rated Voltage Stall Current @Rated Voltage Stall Torque @Rated Voltage Weight: Size  (mm)
Pololu 298:1 Micro Metal Gearmotor MP 6V 75 RPM 40 mA 700 mA 125 oz-in 10.5 g 10x12x26
SparkFun Micro Gearmotor 6V 45 RPM 30 mA 360 mA 40 oz-in 17 g 26x12x10
Solarbotics GM3 224:1 Gear Motor 90 deg. Output 3V-6V 46 RPM 50 mA 733 mA 57 oz-in 31 g 69.4×22.2×18.6
Hobby Motor 3V 6600 RPM 110 mA 800 mA Not Specified 26 g 27x27x33

Table 1: Motor Trade Off Study Data

 

Conclusion:

In conclusion, the most suitable type of DC motor for our robot is the brushed DC motors. From there the guiding factors for deciding which motors to look at were decided from the limitations of the 3DoT board. For the time being, the motors that are going to be the most viable are the SparkFun Micro Gearmotor, or the Pololu 298:1 Micro Metal Gearmotor. To conduct our initial testing of the miniaturized SpiderBot we will be using the SparkFun Micro Gearmotors as it meets the correct parameters even with a load large enough to stall. Some of the testing will include average current draw under load, and max power test.

 

Motor 1: https://www.pololu.com/product/2371

Motor 2: https://www.sparkfun.com/products/12285

Motor 3: https://solarbotics.com/product/gm3/

Motor 4: https://www.sparkfun.com/products/11696

Preliminary Project Plan

By Martin Diaz (Project Manager)

Adan Rodriguez (Mission System and Test)

Moses Holley ( Electronics and Control)

John Her (Manufacturing)

Edgardo Villalobos (Solar Manufacturing)

 

WBS

By Martin Diaz (PM),

Adan Rodriguez(Mission,Systems)

 

The work breakdown structure organizes the work needed to complete the project by putting task under each engineer. For our WBS the work of the system engineer was organized into 3 blocks, System Design, Software, and system tests. The work of the ENC engineer was organized into 4 blocks, Electronic Design, Research/Experiments, Microcontroller and PCB, and finally MCU Subsystem and control. The work of the Manufacturing engineer was broken down into  5 blocks, Mechanical Design, Research, 3D simulations, and manufacturing parts and assemblies, and assemble Mini-Pathfinder.

 

Schedule

By Martin Diaz (PM)

The project schedule was created by using Project Libre. Each Task in the WBS was put as task into Project Libre and then the start and end dates were assigned. When a task depended on a other task to be finished first dependencies were assigned. This can be done by clicking and dragging arrows to other boxes. The program will automatically adjust the task.

Burndown

By Martin Diaz (PM)

The Burndown is a chart that shows how much work is left to complete the project vs time.  The Burndown was calculated by moving the task in the schedule to columns in excel and then assigning the percent completion for each task. The ideal percent completion and real percent completion were then plotted vs time.

Power Allocation

By Adan Rodriguez (Mission and Systems)

John Her (Manufacturing)

Moses Holley (ENC)

Edgardo Villalobos (Solar-Manufacturing)

 

The power rating for all of the components on the Power Allocation Report list were calculated using specification sheets of corresponding components. We approximated our mission duration to be 1 hour (one fourth the duration of the Pathfinder’s mission due to our rover being one fourth scale in size). Using the specification sheets to find current ratings of the components, we multiplied the current ratings by one hour to calculate power ratings in milliamp-hours. For the motor drivers, we looked at the power consumption of the IC and divided the operating voltage to get the current draw (0.78W/6V=130mA). The I2C I/O Expander is rated for output of 25 milliamps per pin and we will be utilizing 6 pins implying a total power consumption of 150 milliamp-hours. The Project Power Allocation was set to be slightly higher than the total Expected Power. Note that the Project’s Power Allocation value was used to aid in determining which battery to choose for our mission.

Mass Allocation

By Adan Rodriguez (Mission and Systems)

John Her (Manufacturing)

Moses Holley (ENC)

Edgardo Villalobos (Solar-Manufacturing)

 

Estimates of the 3Dot Board and Custom SMD Board were based off the fact that they are similar in size to the Raspberry-Pi Board (31 millimeters x 66 millimeters). The chassis, solar panel and suspension system were weighed with a scale. Corresponding Sources of expected weights is provided under the Source column. The Project Mass Allocation was set to be slightly higher than the total Expected Weight.

Cost Allocation

By Adan Rodriguez (Mission and Systems)

John Her (Manufacturing)

Moses Holley (ENC)

Edgardo Villalobos (Solar-Manufacturing)

 

The current battery that we intend to buy may be switched out for a different battery after the Mini Pathfinder is built and tested for its power efficiency. About one third of the products that will make up the Mini Pathfinder will be free of charge due to our team members already possessing certain products. Corresponding sources of expected pricing is provided under the Source column. Because the Mini Pathfinder didn’t have a cost requirement the Project Cost Allocation was set to be slightly higher than the total Expected Cost.