By Vinh Kim, 3D Modeling and Manufacturing

Introduction:
Here I did a study on Silicone rubber (Mold Max 40 vs OOMOO 30) vs. Polyurethane rubber. The Mold Max 40 was the silicone rubber the fall 2013 Hexapod team used. I want to give our regards to Ramon Luquin (Fall 2013 Hexapod Project Manager) for donating his mold and casting material to our team.

Durability:
Here we want the rubber to be flexible as much as possible, so it does not break when bending. Therefore, we will take a look at the Elongation at Break and Tear Strength number from www.smooth-on.com.

The Elongation at Break is when the material breaks after it being pulled apart and the Tear Strength is the material that resistance to tearing. As shown in the Figure 1 and 2 that I am bending the rubber with my hand and it did not rip or pull apart.

Figure 1: Mold Max 40 Rubber

Figure 2: OOMOO 30 Rubber

Durability Winner:

1. Vytaflex 60
2. Mold Max 40
3. OOMOO 30

 Mixing Viscosity:
Here we will look at the viscosity. I noticed that if the viscosity has a very low Centipoise (CPS) number than it will flow really easy and does not need to be vacuum degassed when mixing. If it has a high CPS number, than it requires vacuum degassing to remove all the air when mixing because it does not flow easily which will create a lot of air bubble.

Figure 3: Mold Max 40 Mixing

Figure 4: OOMOO 30 Mixing 

Mixing Viscosity Winner: 

1. OOMOO 30
2. Vytaflex 60
3. Mold Max 40

What is a Shelf Life?
A shelf life is an unused product in a container that has been opened and exposed to the ambient moisture in the air.

For polyurethane rubber, from Smooth-on.com states, “Unused Part A product exposed to humidity will begin to turn from liquid to solid in as little in a few hours. Part A exposed to relatively low levels of humidity which will last two to three months in the container before showing signs of moisture contamination”.

For silicone rubber, Smooth-on.com states, “Properly stored at room temperature (73°F/ 23°C), silicone rubber will have a shelf life of 6 months from date of manufacture”.

Shelf Life Winner: 

1. Silicone rubber( Mold Max 40 and OOMOO 30)
2.  Polyurethane rubber( Vytaflex 60)

 Cost:
Here we will look at the cost of the 1 gallon unit and the trial size. For our group, we did not need and use a lot of rubber, so we went with 2 trial size of OOMOO 30 to mold the body and the leg of the Hexapod. 

Cost for Trial Size Winner:           

1.  Mold Max 40 and OOMOO 30
2. Vytaflex 60

 Cost for 1 Gallon Unit Winner:

1. Mold Max 40
2. Vytaflex 60
3. OOMOO 30

Conclusion:
Overall, I think the OOMOO 30 is the best rubber to buy because of the trail size cost. Also the shelf life is so much better and last longer than the polyurethane rubber and it does not require vacuum degassing when mixing. Plus the durability is pretty good. I recommend when purchasing molding material, remember look for the durability, mixed viscosity, shelf life and cost.

References:

http://www.smooth-on.com/Silicone-Rubber-an/c2_1113_1135/index.html

http://www.smooth-on.com/Silicone-Rubber-an/c2_1113_1136/index.html

http://www.smooth-on.com/Urethane-Rubber-an/c6_1117_1142/index.html

http://www.smooth-on.com/faq_display.php?faq_id=99

http://www.smooth-on.com/Silicone-Rubber-an/c2_1113_1135/index.html?catdepth=1

http://www.smooth-on.com/Silicone-Rubber-an/c2_1113_1136/index.html?catdepth=1

http://www.smooth-on.com/Urethane-Rubber-an/c6_1117_1142/index.html?catdepth=1

By Tien Dang

Problem Statement
Base on current test and calculations from discharge rate of current (available at https://www.arxterra.com/current-draw), we noticed that the battery provided 7.4V and 5000mAh, while each servo only required 6V (maximum) and 450mAh (with full load) to work. Essentially, the servos could burn without something to step down the voltage.

Solution
To prevent an overload voltage and current, we chose to use a voltage/current step-down regulator to limit the current as well as the voltage that going into ADK board and the servos.

Figure 1: Voltage step down regulator

Figure 2: Equivalent Circuit

With this particular voltage step down regulator, we can limit the voltage and current that flows through the servos and ADK board.

Set up process
This Voltage regulator has a DC input and a DC output. All we need to do is connect the source to the input and use a voltage meter to measure the output voltage. We can adjust the voltage by turning the Adjust Button using a flat screwdriver.

Figure 3: Adjust button and flathead screwdriver.

After adjusting the voltage to a prefer level (in our case we adjusted to 6V), the Voltage regulator is ready to use. All we need to do is connect the output into an outlet and ready to apply power into applications.

 By Chau To

• Introduction:

This blog post will discuss some issues of the servo library (Servo.h) and how coding of the Hexapod was built including functions and subfuctions as well as some tips and troubleshooting.

Note: Hexapod team drives 19 servos directly from the Arduino board (digital I/O pins), and it uses the built-in Servo.h library functions such as “write” and “attach” as the main code to rotate the servo shaft. If your hexapod uses an additional servo controller boards such as the Adafruit etc, some of the materials discussed in the post would not apply in that situation.

• How to create object servo in the Servo.h library:

Since the coding will require “for” loops, creating individual servo object such as servo1, servo2, etc.. Similar to the example in the Ardunino websites is impossible. The only way is to create an array of servo object. How can we do it? We can define a number of servo then call the servo library to create and array of servo object:

#define number_of_servos 19
Servo servos[number_of_servos];

So, we will have an array of servos; in this example, we will have array of 20 servos. Each servo will be appearing like this servo[1], servo[2], etc… NOW! We can put these into for loop. For example: servo[i].write(180).

• Array:

Array is very important. As mentioned above, when we run “for” loop we need to use array because it will optimize our coding. An array of digital I/O pin is a great example since we need to attach each servo to a specific pin on the Arduino. Here is a sample code:

int pins[] = {30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48};

void set_servos() {

  for (int i=0; i<number_of_servos; i++)  {

    servos[i+1].attach(pins[i]);

     }}

The above example is a simple function I wrote to attach the pin to the servo. My digital pins will be from pin 30 to pin 48 of the ADK board.

The same concept applies for the servo’s shaft angle rotation. If user wants a specific servo to rotate at a specific angle, they can make an array of angles and run it in a for loop. More detail about array of C++ can be found at: http://www.cplusplus.com/doc/tutorial/arrays/

• Subfunctions:

Subfunctions are smaller functions that will be called in a main or larger function. Breaking a long and complex function into smaller subfunctions is very useful because it’s easier for the user to double-check and debug the code.  For example, one of the main function of the hexapod is run which includes 2 stages; first stage is moving the femur, tibia, and shoulder to a new position, and second stage is to bring them back to the initial position. So, we can break these 2 stages into 2 subfunctions. And, even smaller subfunctions for controlling and debugging.

• Serial Monitor:

Serial Monitor can be turned on by Ctrl+Shift+M after uploading the code into the Arduino board. The serial monitor is extremely useful when testing the code. It’s used to keep track of the codes and fix all the bugs before uploading to a real model. This is the sample of the serial monitor added in my set_servo function that was discussed in the previous part:

void set_servos() {

 for (int i=0; i<number_of_servos; i++)  {

    servos[i+1].attach(pins[i]);

    Serial.println(“attach pin”);

    Serial.println(i+1,DEC);

    Serial.println(pins[i],DEC);

  }}

With these serial function added in, I can monitor for my loop as well as to see which servo had attached to which pins. The serial monitor is available at: http://arduino.cc/en/reference/serial

Note: In order to run the serial monitor, in the setup you have to call the serial.begin

void setup()

{

 Serial.begin(9600);

Figure 2