Project Overview
This project employs 13 servo motors, 3  in each leg, and one in its head. The microcontroller is an ESP8266, which has much more code space and RAM than a standard NANO and runs at 80 MHz clock speed. I new I would need that extra space to store the servo movements. Between the eyes is a laser range finder, which give the droid an ability to react to object being placed in front of it. Leg movements and balance are quite complex, so I wrote a custom PC app which helped me to achieve this. The droid is controlled via a  Wii nunchuk, over a 2.4 GHz wireless link using the fast ESP NOW protocol. This in turn provides a bidirectional data channel for telemetry, enabling the movement trainer app to work wirelessly. If you are planning to use the PCA9685 16 channel servo driver board in another project, you might like to try the app I wrote for controlling that too. Click the image to the right to watch a video of my droid in action -->

All of the robots body components were produced in PLA on my 3-D printer, and modelled using the free design package from RS Components, DesignSpark Mechanical. For this design I also constructed an assembly of the complete robot, to ensure that all of the parts aligned. This included simple models of all bought components like the servo motors and driver board. Given the size limitations of my 3-D printer, the chassis consists of a number of parts screwed together using 2 mm countersunk screws. The body top plate has compartments for the two batteries and apertures for the display and push buttons. The finned shoulders allow it to be placed on its back for testing.

The lower legs consist of plates, to provide mounting frames for the servo motors, and joints for hip, and elbow movement. The use of screws makes it easy to maintain the robot, should you wish to disassemble it to replace a part or change its appearance in some way.

RoboDog is powered from two 18650 Lithium batteries, delivering 7.4v to the system. An On/OFF switch is mounted on its body plate, feeding power to the top plate electronics and the PCA9685 servo driver board, mounted underneath. Two push button switches sit either side of the micro, with the OLED 64 x 128 display mounted centre of the top plate. On power-up the robots servos remain inactive for safety reasons. Digital servos, like the MG996R don’t jump when power is first applied.

Programmable RGB LEDs are placed in each eye, and 5 in a strip on its face. The colours provide feedback on the modes of operation, and the front LEDs are animated when the robot is in motion. The servo cables are kept tidy with special clips and in compartments. The head holds a laser range finder, so that autonomous scouting behaviour can be developed with additional code.

The circuit diagram for the robot is shown here on the left, with the ESP8266 micro in the centre, connected to the RGB LEDs below, the OLED display left, VL53L0X laser range finder and MPU6050 gyro above, and PCA9685 servo driver to the right. The I2C bus simplifies the wiring. As the servo motors are only rated up to 6v I have used diodes to drop the battery voltage fed to the driver board. A simple resistor network is used to enable the ESP8266 to measure the combined battery voltage, which should not be allowed to fall below 3 volts per battery; this is monitored in the code, which also predicts battery discharge life.

The built-in WiFi of the ESP8266 is significantly more robust and faster than the serial transceiver I have used in other project. You will need to make the associated wireless controller unit, which will be published separately.

A critical step in building a project like this is in attaching the servo motors leavers and calibrating their respective angles, by recording and storing PWM values in code. To simplify this task I developed a 16 channel controller app which provides adjustable sliders for each servo. Values are then simply cut-n-pasted into your code. Then, as leg movements can be complex, having 3 servos in each, I developed a Move Trainer app which enables you to create a series of movement points through which all 12 servos can be driven. This links with the code in the project, which also allows for linear and non-linear movements, like acceleration and deceleration. Complex moves up to 200 leg positions can be created and simply cut-n-pasted into your code, and copied back into the app for further refinement. This process also works over the wireless interface, so the robot is not tethered by the USB lead. The third app works as a data plotter and PID controller tuner, allowing gyro signals to be viewed over the WiFi link.