SwapStop

Refined Actuator Control

Thursday, January 23, 2020

There have been great strides this past week towards controlled usage of the system's actuators. Both the linear actuators (which handle and transfer the LiPo batteries) and the stepper motors (which drive the timing belts that translate the elevator vertically) have been mounted to their mechanical subsystems and precisely controlled for the first time.

Linear actuator, mounted to Flyn with end effector visible. Linear actuator, mounted to Flyn with end effector visible.

First up, the linear actuators. These two cylinders are mounted to the elevating battery handler mechanism ("Flyn") which is a sandwich between two aluminum sheets. These high speed linear actuators (which can move against 22 lbf of force at an astonishingly fast 5.51" per second) are each bolted to a custom machined cradle, which has been designed to interface with and manipulate our 3D printed battery cartridge mechanism. The active end of each actuator is supported by two heavy-duty drawer slides, which guarantee precise and repeatable actuation.

The actuators are driven by a standard DC motor controller, and are powered by a 96W, 12V power supply. The actuators have built-in limit switches for safety, but an additional pair have been added around each actuator to ensure the system moves to the correct position every time, and the movement profile uses controlled acceleration to minimize current spikes on the 12V line. The movement is controlled by an OpenMV H7 microcontroller running micropython, which receives high-level command from the master controller (an Nvidia Jetson board) via the system's CAN bus.

Linear actuators, moving on Flyn.

The two stepper motors each drive a vertical timing belt, one of which will be attached to either side of Flyn. These stepper motors are responsible for the vertical translation of Flyn, allowing the battery handler to position itself at the level of each of the charging bays and of the drone, allowing for the transfer of LiPo cells back and forth between them.

The impressive mass of Flyn (which clocks in at approximately 9 kg) requires beefy motors to move the mechanism at a high enough speed, and these NEMA 23 motors with a 4:1 planetary gearbox fit the bill perfectly. Driven by powerful stepper drivers and powered by a 24V supply, these motors stay cool even at high speeds and against strong opposing torque.

Stepper motor attached to its timing belt on the frame of SwapStop.

A few more progress updates:

PLA 3D printed parts for mounting on Flyn.
PLA 3D printed parts for mounting on Flyn. Machined aluminium mounting points for belt pulleys, plus engineering drawings. Machined aluminium mounting points for belt pulleys, plus engineering drawings.

There are some pretty significant progress milestones coming up over the next week, so we'll be back soon with some awesome new updates. Stay tuned!