Anh Tram Do-Systems Engineer
The weight of the full Hexapod has been calculated at roughly 1694.9 g or 3.74lbs (link to weight report). With that weight estimate we were able to decide on the requirements for the servos. Our tripod gait will require each load bearing servo to lift at least 1/3 the weight of the body. With this requirement, found that the requirement of the servos will be to have at least enough torque to lift 450 grams, or .935lbs each. 450 grams converts to 4.42 Newtons.
The size of the body has been calculated and available at Daniel’s blog (link to Hexapod design). With the size of the servo shoe (our femur) being 8 cm long, the rating of the servo will need to have enough torque to lift the 450 grams, or 4.4145 N at 8cm the torque necessary would be 35 N-cm. So the rating of the servos would need to be 40N-cm, or 4.588kg-cm, which are the units servo manufacturers use to rate servos.
In order to communicate between the Hexapod and the Arxterra Control Panel, an Android phone with a built-in camera must be used. The Arxterra app is designed to take gyroscope and accelerometer and picture data from an Android phone, therefore the Android phone must have control moment gyroscope and accelerometer built-in. The Android smartphone must be able to provide data plan to transmit the data and receive user input over a wireless internet connection. Quick app testing done on a Samsung Galaxy S3 has shown that the Galaxy S3 with Android OS version 4.1 functions fully with Arxterra Control Center and will be the phone used for the build (link to Biped’s Arxterra App tutorial).
In accordance with project level 1 requirement to provide telepresence (link to Project Requirements) and to fulfill level 2 System Requirement of using an Android phone (link to system level 2 system requirements). The control boards used will be the Arduino Mega ADK. The ADK control board provides a build in USB port for easy connectability with the Android phone. The ADK allows for breakout board installation and to allow for all 20 servos to be connected and run individually.
The breakout boards will provide a separate hub to connect all 20 servos. The breakout boards will provide power to the servos independently from the ADK and will protect the ADK from a large surge in current or high voltage demands. The breakout boards used will be the Adafruit 16-Channel 12-bit PWM/Servo Driver Boards which are completely compatible with the ADK and within budget (link to Adafruit Breakout Boards Wiring and Set Up).
Hexapod will be designed and simulated in SolidWorks to test for strength of the parts when fully assembled (link to materials study in SolidWorks). The 3D model will also give us dimensions of the Hexapod and will show us how much clearance we have in the test terrain (link).