This is the second part of a post series on the design, manufacturing and software design of a low cost quadruped robot using cheap servos closed-loop control and force-sensing resistors for ground force feedback. The first post focused on the design of the quadruped leg, which is the most essential hardware part of the robot. Similarly this post moves on to discuss the robot frame that holds the four legs together. In addition to the frame, the post shortly discusses robot electronics.
An X-shaped, flat frame 3D printed in two parts to fit printer dimensions holds the four legs together. Legs mount on a surface with a slight inclination, which helps bring the tips of the legs closer together in the lateral direction. The legs themselves may not be positioned close to each other as the servos are protruding on both sides.
In addition to the leg mounts, the central frame also includes holes for securing the electronics via nylon spacers, as well as for attaching other modules. For instance I’ve currently mounted a volt meter on the rear of the robot to easily read battery charge, but modules such as camera gimbals, arms etc could also find place there. A single 3.7v Li-Ion battery is located under the frame, secured by a single zip tie.
The main robot controller is a Teensy 3.5 board. Teensy sends PWM signals to each of the eight leg servo actuators. Similarly, on the sensing side the Teensy receives analog input from four FSRs and orientation from an MPU9250 IMU on a Pololu Minimu v3 carrier. The IMU interfaces directly to one of the board’s I2C buses. On the contrary, FSR signals are first amplified by a TLC2272 op-amp and then fed into one of Teensy’s analog inputs.
A two-layer PCB acts as a base board for the different modules, and provides the necessary input/output and power supply. Servos function at a reduced voltage of 3.7V in order to reduce wear and tear, as well as to minimize jittering. 5V are supplied to the rest of the electronics by means of a 1A Pololu buck/boost converter. This dual-power supply setup allows different input voltages to be used, but also acts to isolate the control and actuator parts of the circuit for interfacing e.g. with USB.
Design focus now slowly shifts to software. My first goal is to have the PCT code used in the single leg working for all four legs and then make the robot assume static poses (crouch, stand, lean etc.). The video below demonstrates a first software prototype shaking out the electronics:
Research on gaits and stability will come right after this part is finalized.
This post is the second part of a series of posts on the design, manufacturing and control of a low-cost quadruped robot. The post focused on the design of the robot chassis and electronics. In the next and final post, we will focus on the software side of things, and finally make the robot come alive!
Got any questions or suggestions? Share your experience in the comments below!
Read the first part of this post series here.