Robotics arm for charging plug

When autonomous car gets to packing lot it’s nice to plug it to power automatically. This project is to build such charging post – operated remotely or automatically.


The goal is to connect the charging cable between a car and a power supply plug. At initial stage assuming the plug is on the same height from the ground as the charging connector – to simplify the solution eliminating need to move in vertical direction (axis Z).


  • Plug on the wall, arm is mounted on the car.
  • Plug on the car, arm is mounted on the wall.
  • Segmented arm
  • Telescopic arm

Segmented arm

Initial position Plugged Plugged (top view)
PlugOnWallSegmentedArmSchema01 PlugOnWallSegmentedArmSchema02 PlugOnWallSegmentedArmSchema03

The arm has 3 legs (segments) with a connector on the top leg. Legs are rotated on axises by step-motors. The step-motor should rotate legs over gear-set because the motor has not enough torque to rotate it directly.

Version 1

The gear is fixed on an axis, this gear is rotated by a smaller gear-set on the axis of the step-motor. To make the construction more compact – the step motor is set horizontally and bevel gears are used.


The gear ratio is 1:2 – it is not enough to rotate the axis with sufficient torque. More gears need to be added.

The step-motor weights 0.4 kg – it is better to mount it closer to the base axis to reduce the load on the leg and inertia.


Version 2

Additional pair of gears have been added. The total gear ratio between the step-motor and the axis is 1:8. With such ratio the axis is rotated with the torque 6 kg/cm.


The targeting design looks pretty complex. One leg weights 1.2kg – where 30% is a mass of the step-motor.


PlugOnWallSegmentedArmDesignWithBevelGears-side PlugOnWallSegmentedArmDesignWithBevelGears-top

Version 3

The lack of torque encourage to use a gear-set with more ratio. The planetary gear is a good choice when the size and torque are valuable an the pretty large noise can be ignored.

The planetary gear was acquired from the electric screw-driver. The total gear-ratio in one leg was increased to 1:40 – now the step-motor confidently managed to rotate the axis with the torque 0.7 kg with 27 cm leg (up to 18 kg/cm).



The red part on the model is a planetary gear.


Version 4

Combination of solutions above with improvements:

  • spur gears drives the first leg
  • planetary gear drives the second leg
  • timing belts can keep the top leg in one direction while another legs are moving because the charger connector need to be always directed to the plug



Because the first pulley for timing belts is fixed on the base, second pulley is freely rotating on the axis and the third pulley is rotating the axis with the “charger contact” – this contact is always directed towards the “charger plug”. Two step-motors is enough to move the arm.


Test of the arm. The arm is controlled with gamepad by the program on C# which sends commands via serial port to Arduino board, which controls two step-motor drivers.