Fall 2016 Biped Trade Off Study- Ankle Servos

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Table of Contents

Ankle Servo Selection

By: Alan Valles (Electronics and Control Engineer)
Approved by: Ijya Karki (Project Manager)

Introduction

The most recent design calls for 4 servos to be utilized. Two servos will function to shift the center of mass onto the planted foot during walking motion. The remaining servos will as the ankle which will allow the robot to turn during walking motion.

Study

Since there are four different servos that have different requirements, the ankle servos will be compared. The other servos are still to be determined so that the necessary torque required to move the CoG to balance while turning is considered.

Servo MFG Voltage Torque(stall) Mass Cost
9g Varies 4.8-6V 18.9 oz-in 9g Free(3.95)
ES08A Emax 4.8-6V 21oz-in 8.5g 4.95
HS-53 Hitec 4.8-6V 16.70z-in 8g 7.19

Under ideal conditions, the CoG will be directly on top of the shaft therefore, torque requirements will be dependent based on the position of CoG and total weight of the robot. Due to these assumption, all of the servos will meet the torque requirements. Therefore, the primary constraint will be cost. This leads BiPed to utilize existing TRC inventory and select the 9g servo. If the servos were to be bought due to stripped gears on existing parts or other issues with inventory, the Emax will be purchased depending on the total budget of the robot due to other subsystems.

Conclusion

In conclusion, the current existing inventory of 9g servos will be used for the ankle servos. This is due to the primary constraint.

Resources

[1]http://www.robotshop.com/en/9g-micro-servo-motor-4-8v.html#description

[2]http://www.emaxmodel.com/es08a-ii.html

[3]http://www.robotshop.com/en/hs-53-feather-nylon-gear-servo-motor.html

Fall 2016 Biped Trade Off Study- Motor

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Table of Contents

Battery Selection

By: Alan Valles (Electronics and Control Engineer)
Approved by: Ijya Karki (Project Manager)

Introduction

The walking Biped robot requires a dc motor to control its walking motion. A DC motor can vary in specifications such as operating voltage, current, RPM, torque, and size. Therefore, it is necessary to pick a DC motor that will operate within our electronic system. Furthermore, the customer requested that the walking motion be produced using DC motors.

Study

Most hobby or toy DC motors operate at a high RPM in the hundreds or even thousands of revolutions per minute. Our current mechanical design has us use a Tamiya gear box in order to step the RPM done and to increase the torque ratio. The Tamiya gearboxes are usually recommended for this since they are flexible in the gear ratio available in one single kit.  For example, the Tamiya 70167 Single gearbox (4-speed) Kit has 4 possible gear ratio configurations, 12.7:1,38:1,115:1, and 344:1. Thus, to meet schedule requirements an all in one Tamiya motor and gearbox subsystem is preferred. However, The Robot Company has existing components that are GM-9 which will also be considered.

Table 1 Motor Comparison

Motor Manufacturer Voltage Stall Current NoLoadCurrent RPM Torque Cost
70167 Tamiya 3V 2100mA@3V 150mA@3V 12300 .5 0z-in 7.55w/gearbox
1117 Pololu 6V 800mA@6V 70mA@6V 11500 N/A 1.99
GM9 SolarBotics 3-6V 400mA@3V 50mA@3V 40RPM 44oz-in Free

The will output 5V so actually testing will need to be done. As Table 1 shows, the motor that comes default with the Tamiya gearbox will be much higher than the required stall current. The PTC fuse is rated for 750mA so this is our driving constraint. Pololu 1117 is just above the PTC rated value so it should be sufficient, but more testing must be done to verify. In conclusion, it is recommended that the Tamiya Gear box be used, but the default motor be replaced by the pololu 1117 which is the same form factor. Secondly, a design which utilizes a single GM9 will also be considered. It is important to note that the DCDC converter outputs 5V. Thus, the motor will be driven at 5V. It has been shown that running Tamiya motors higher than spec voltage is adequate at 5V to meet mission profile[4]. However, this will shorten the life cycle of the DC motor and is not recommend but it is an option.

Conclusion

In Conclusion, The Pololu 1117 DC motor with Tamiya Gearbox in the 70167 will be utilized in the BiPed Design. The gearbox will allow for flexible configurations resulting in adjustable torque and rpm outputs. Furthermore, the motor chosen will utilize a 130 hobby motor form factor which is required to integrate with the Tamiya gearbox.

Resources

[1]https://www.pololu.com/product/118
[2]https://www.pololu.com/product/1117/specs
[3]https://www.pololu.com/product/188/specs
[4]https://www.pololu.com/docs/0J11/all#3

Fall 2016 Biped Trade Off Study- Battery

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Table of Contents

Battery Selection

By: Alan Valles (Electronics and Control Engineer)
Approved by: Ijya Karki (Project Manager)

Introduction

A Battery must be chosen in order to provide power to the entire system of the Biped. The motors and servos will be the main power sink onboard our system. The rest of the unit will be nominal in comparison to the continuous rotation that will be provided to the motors during normal walking operation.

Study

The default design of the 3Dot board was chosen by the Arxterra team to utilize a 3.7V RCR 123A. However, the team designed the 3Dot board to be flexible and allows for an external battery utilizing a JST connector. In order to meet scheduling requirements, we will assume that the proper discharge rate is used across entire system.

Battery Mfg Voltage Capacity 1hr power Chemistry Mass Cost
RCR123A Varies 3.7V 650mAh 390=>1.4W Li-ion ~50g Free
B0072AEHIC Turnigy 7.4V 1000mAh 600->4.4W LiPo ~23g 9.89
LP-503562 Adafruit 3.7V 1200mAh 720->2.6W LiPo ~23g 9.95
34117 Tenergy 7.4V 2200mAh 1320->9.7W LiPo ~136 14.99

Conclusion

As an estimation, the motors should draw anywhere between 150mA – 300mA at 5V (1.5W)during normal operation. Another estimation is that the cutoff voltage will be reached at .4V capacity. Thus the entire battery capacity cannot be utilized. However, the servos utilized will be have a operating voltage range of 4.8-6V. Although they can be utilized directly at 3.7V if they are connected directly to the battery, the voltage will decrease as the battery voltage lowers over time. Thus, a LDO will be used to regulate the voltage to the servos. Thus, a 7.4V battery will be used in order to raise the voltage delivered to the servos, since the mission profile calls for a worst case of 60 minutes. The battery should be able to supply at a minimum 1.5W continuously. Therefore, the Turnigy B00 will be selected due to its compromise between cost, mass, and capacity. Furthermore, it operates at the necessary voltage. Since it is used for hobby RC cars it also has necessary discharge rates.

Resource

[1]https://www.amazon.com/Turnigy-1000mAh-Lipo-HobbyKing-Battery/dp/B0072AEHIC/ref=sr_1_2?ie=UTF8&qid=1478476586&sr=8-2&keywords=7.4v+lipo

[2] https://www.amazon.com/Olight-Lithium-ion-Rechargeable-Batteries-Flashlights/dp/B01K7I05G8

[3] http://www.robotshop.com/en/lipo-battery-cell—37v-1200mah.html

[4] https://www.amazon.com/Tenergy-2200mAh-Battery-Banana-Connector/dp/B0192AVMGO/ref=sr_1_5?ie=UTF8&qid=1478477368&sr=8-5&keywords=7.4v+lipo&refinements=p_89%3ATenergy

[5] http://web.mit.edu/evt/summary_battery_specifications.pdf

[6] http://www.silabs.com/Support%20Documents/TechnicalDocs/Selecting-the-Optimal-Battery-WP.pdf

Fall 2016 Biped Trade Off Study- String for Lateral Balancing

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Table of Contents

String Material

By: Hector Martinez (Manufacturing Engineer)
Approved by: Ijya Karki (Project Manager)

Introduction

For the Biped Project, we have considered the following materials for our Biped lateral balancing system. The system requires two weights attached to a servo through a lever and string. The purpose of this study is to choose a suitable string material to support the two weights, and allow the proper range of motion to allow the system to work properly, i.e. make the robot balance.

Material Chart

Material Costs Safe Load Pro Con
Manila Rope (Fiber Rope) $0.05 – 2.23 /ft .15kN (5mm dia)

~15.2kg

 1. Durable

2. Flexible

3. Biodegradable

4. Cheap

 1. Shrinks when wet

2. Difficult to unknot when wet

3. Fibrous

4. Only found in braided strands

Nylon $0.22 – 0.68 / ft .326kN (5mm dia)

~33.2kg

 1. Elastic and shock absorbing

2. Long lasting

3. Rot proof

4. Chemical resistance

5. Can be bought in single strand

 1. Weak against UV rays

2. Susceptible to heat

3. Loses strength when wet
Polyester $0.10 – 0.36 / ft .284kN (5mm dia)

~28.9kg

 1. Rot and UV resistant

2. Retains properties wet or dry

3. Abrasion resistant

 1. Not suitable for heavy duty applications

2. Susceptible to chemicals

3. not elastic, not very shock absorbent

Conclusion

Based on the research we have decided to use Nylon. The elastic and shock absorbing properties of Nylon make it perfect for a toy robot that will inevitably be used and played with and may be put through some rough times. Nylon also has the highest Safe Load rating of the three materials, and even though it is highly unlikely the rope will be used anywhere near this rating, it’s only a few cents more per square foot. The biggest reason for choosing Nylon is the ability to purchase it in a single strand. This allows us to use it in areas where space is an issue, having to use string/rope that is braided makes it difficult to use in such applications.

Resources

1. http://www.knotandrope.com/store/pc/Manila-Rope-c3.htm?pageStyle=m&ProdSort=19&page=2&idCategory=3&viewAll=yes
2. http://www.engineeringtoolbox.com/manila-rope-strength-d_1512.html
3. http://www.engineeringtoolbox.com/nylon-rope-strength-d_1513.html
4. https://knotandrope.com/store/pc/home.asp?gclid=Cj0KEQjwqfvABRC6gJ3T_4mwspoBEiQAyoQPkf52P_avp4P23eVobtda_agXx5x95YtQhVcXsBSvCBsaAkE-8P8HAQ
5. https://survivalblog.com/ropes_and_rope_making_by_be/https://www.usnetting.com/rope/selection-guide/#polyester

Fall 2016 Biped Material Trade-Off Study

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Table of Contents

Building Material

By: Hector Martinez (Manufacturing Engineer)
Approved by: Ijya Karki (Project Manager)

Introduction

For the Biped Project, we have considered the following materials for manufacturing prototypes as well as for our final robot. The following chart shows advantages and disadvantages for that material.

Material Comparison Chart

Material Price Pro Con
PLA $20-165 /kg 1. Ideal for small parts2. Higher printing speed

3. Hard, soft, and flexible  variants

4. Wide range of colors

5. Plant based

 1. Odor while printing2. Requires additional support while printing.

3. Some printers incapable to print PLA

4. Risks of cracks in finished product
ABS $19-175 /kg 1. Strong2. Quick solidification

3. Flexible

4. Higher temperature resistance

 1. Requires heated printer bed2. Fumes while printing

3. Weak against moisture
Acrylic $3-122 /sq ft 1. High impact resistance2. Chemical resistance

3. High heat resistance

4. Weather resistance

 1. Poor solvent resistance2. Subject to stress cracking

3. Low continuous service temp
Aluminum $7 / cm3 1. Strong/lightweight2. Non Magnetic

3. Electrically conductive

4. Recyclable

 1. Variation in surface texture2. Expensive

3. Abrasive to tooling

Conclusion

Through careful consideration we have decided to move forward and build the Biped robot with a mix of acrylic and ABS. The use of acrylic will be more time efficient. Acrylic will allow us to quickly laser cut less intricate parts such as linkages that simply extrude outwards. The time frame to laser cut is minimal compared to the time frame to 3D print. For example, laser cutting can be accomplished in little as a minute while 3D printing could last for multiple hours. We would use ABS to produce more complicated pieces, such as the foot. The foot is considered a more complicated piece because it consists of a cut that will house the color sensor. Thus, substituting laser cutting for 3D printing intricate pieces is a more practical approach.

 

 

References

1.   http://www.3ders.org/pricecompare/
2.   http://www.absplastic.eu/pla-vs-abs-plastic-pros-cons/
3.   http://www.jwwinco.com/engineeringtips/prosconsaluminum.html
4.   http://elementaluminum.tripod.com/id8.html
5.   https://all3dp.com/metal-3d-printer-guide/
6.   https://all3dp.com/best-3d-printer-filament-types-pla-abs-pet-exotic-wood-metal/#carbon
7.   http://www.makergeeks.com/me3dprfi.html
8.   http://www.3dxtech.com/firewire-carbon-fiber-peek-3d-printing-filament/
9.   https://plastics.ulprospector.com/generics/3/acrylic
10. http://www.sdplastics.com/sdplas2.html