Build a four-legged robot with knees

Test the power with nothing connected

Charge the LiPo. Connect battery → switch → UBEC and measure the output with a multimeter. It should read between 5 and 6 V. Only connect a servo after that.

Shield on the Uno, SEL jumper removed

Plug in the Sensor Shield. Remove the SEL jumper. With everything off and no battery connected, set the multimeter to continuity mode (the setting that beeps). Touch one probe to a V pin on any servo port and the other to the Uno’s 5V pin (or the shield’s 5V pin): it should not beep. If it beeps, the jumper still connects the two (clones vary). Connect the UBEC to the servo power terminal. The Uno’s logic gets power from the LiPo through the DC jack (the round connector next to USB), with a 5.5 × 2.1 mm P4 plug: positive in the center, red wire to the center pin. Check the polarity with a multimeter before plugging it in.

The Uno’s regulator needs about 7 V at the jack. Set the LiPo alarm to 3.5 V per cell (7.0 V total): it will beep before the Uno starts restarting.

Loose servos set to 90°

Plug the 8 servos into pins D2 through D9 (brown to G, red to V, orange to S), with the arms (horns, the small plastic pieces that fit on the shafts) unattached. With the servo power switch off, upload the assembly sketch over USB (the Uno may have an old program that moves pins D2 through D9). Then turn the switch on: the servos will move to 90° one at a time and hold position. Don’t turn the shaft by hand while the servos are powered.

arduino-cli compile --fqbn arduino:avr:uno firmware/montagem
arduino-cli upload -p /dev/ttyACM0 --fqbn arduino:avr:uno firmware/montagem

Assemble the legs with the servos stopped

Cut the parts using the dimensioned drawing. Screw the hip servos into the openings in the side walls, with the shafts facing out, and attach a horn to the top of each thigh: the thigh should be vertical and point toward the ground. The 2 horn screw holes vary by manufacturer. Hold the horn against the part, centered on the shaft hole, and mark the holes with it (3 mm center hole for the shaft screw, 1.5 mm holes for the horn screws). Fit the knee servo into the opening in the thigh with the shaft facing out (on the same side as the hip) and the body extending up along the thigh, facing inward toward the robot. Attach the shin to the knee horn, on the outside of the thigh, pointing toward the FRONT of the robot at a right angle (for all 4 legs). If it points backward, the knee bends the wrong way and the robot won’t stand. Changing the sign in the code won’t fix it. If the horn won’t fit at an exact right angle, use the closest tooth. You’ll correct the rest in step 7. Optional: to reduce wobble, add an M3 screw with a locknut (a nut with a nylon ring that keeps it from loosening) on the side opposite the shaft, passing through a second plate to act as a bearing.

Close up the body

Mount the Uno + shield on top with 25 mm M3 spacers (so the board clears the hip servos). Attach the battery and UBEC underneath with hook-and-loop tape. Secure the cables with zip ties and leave slack at the knee. Otherwise, the wire will pull when the leg bends.

Upload the firmware

The firmware includes kinematics, gaits, and calibration. Suspend the robot (over a jar, with its legs in the air) the first time you turn it on. If no calibration is saved, it starts in assembly mode with everything at 90°. It will stand only when you tell it to.

arduino-cli compile --fqbn arduino:avr:uno firmware/robo4
arduino-cli upload -p /dev/ttyACM0 --fqbn arduino:avr:uno firmware/robo4
arduino-cli monitor -p /dev/ttyACM0 -c baudrate=9600  # robo4 pronto (sem calibracao: modo montagem, 90 graus).

Calibrate each joint

Type m to set everything to 90°. Correct a crooked joint with the command o <leg> <joint> <degrees>. Type the whole command at once and press Enter (arduino-cli monitor doesn’t show what you type; in the Arduino IDE, set the Serial Monitor to “Newline”). The value replaces the previous offset for that joint; it doesn’t add to it. Legs 0 to 3 (FE, FD, TE, TD), joint 0 = hip, 1 = knee, degrees from −30 to 30. Save to EEPROM with g (the Uno’s memory that doesn’t erase when powered off).

m          # all at 90°
o 0 1 -6   # front-left knee: −6° (if it got worse, use 6)
p          # show the offsets
g          # save to EEPROM (retained when powered off)

Stand up

Type x. The legs will slowly move into a standing pose (each servo moves at most 5° every 20 ms, which also reduces the load on the power supply). All four should bend the same way. If one bends the wrong way, change its sign in SENTIDO[][] in the code, compile, and upload again.

// firmware/robo4/robo4.ino
const int8_t SENTIDO[4][2] = {{1, -1}, {-1, 1}, {1, -1}, {-1, 1}};

First steps: slow walk

With the robot suspended, type c (slow walk, or creep) and check the order: rear left, front left, rear right, front right. The body shifts forward and backward on purpose to keep its weight over the three legs on the ground. Then place it on the floor.

BALANCO only moves the body forward and backward. If it tips forward or backward, increase BALANCO (12 → 18) or decrease PASSO. If it tips sideways, lower its height with - or spread the legs farther apart (base wider than 80 mm).

Trot and commands

Once the slow walk is stable, switch to the trot.

w forward   s backward   a turn left   d turn right
c slow walk   x stop   z lie down   + / - height