Need some guidance on designing a protection PCB for a drone hangar - Page 1
Forum section: electronics / projects

author: jb82
author_rank: Contributor
timestamp: February 11, 2025, 07:03:02 pm
content: I'm currently working on a hangar for an autonomous drone. It has four upward facing contacts that the drone should land on. These are:Two contacts connecting a charger to '+' and '-' of the drone's batteryTwo contacts powering a relay on the drone that switches said drone on or off (relay on = drone off). The relay is currently 24V but may change to 12V in the near future.The principle was tested and works, however, when the drone lands incorrectly, several misconnection events are possible that I need to account for. I consider battery and charger reasonably safeguarded, but the relay contacts have no protection whatsoever.So the task is: I want to develop something that sits in front of the two relay contacts and recognizes, handles and reports the following cases:The relay is not connected to the contacts although it should be (open load)Overload or short circuit between the contacts (for whatever reason)The drone landed too far to the side and the battery poles on the drone are now connected to the relay contacts (so, the contacts should be connected to a relay, but instead are connected to a voltage source). The battery voltage is between 22 and 26VThe drone landed at the correct spot but is pointing in the wrong direction (180 degrees reversed). This is the same situation as the prior one, but with + and - reversed.My thoughts so far:I thought it easiest to look for a so called "intelligent" load switch, which already fulfills as many of these requirements as possible. I'm planning to use the BTS71040 which should handle 1. and 2., and should look something like this (work in progress, may be missing components):The screw terminal on the right is where the two relay contacts would go. The SPI lines connect to an STM32F04K4T, which operates the load switch, reads it's status and communicates with the hangar controller via CAN.My current problems:I'm not sure how to best approach 3. and 4.. I completely ignored these problems in my above schematic and would be grateful for any tips on how to handle them.Also, are there any general issues with my approach so far?Thanks in advance

author: benseno
author_rank: Regular Contributor
timestamp: February 11, 2025, 09:03:47 pm
content: 3. put 3 load cell on bottom of landing space and discover the mass center shift.4. magnetometer on your drone will tell you the orientation of your drone.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 11, 2025, 09:54:00 pm
content: Quote from: jb82 on February 11, 2025, 07:03:02 pmIt seems difficult to tolerantany skew and any rotationwith that many signals.Even with a smart footprint, that can auto sense which contact it has, you have a small but finite chance a contact exactly aligns with the gap.If you make the contacts spring loaded and wider than any gap, you then need even smarter and more footprints to accommodate the two-zones contact case.If you have control over the drone design, it may be simpler to have just 2 contacts, non polarized, and use some modulation to control the relay.

author: jb82
author_rank: Contributor
timestamp: February 12, 2025, 06:17:39 pm
content: I think there is a little misconception: The drone will not actually land on contacts on the board. There are four nickel plates at the bottom of the hangar that will be wired to the charger and the board I'm talking about (The drone is 4,5kg heavy and 1m in diameter).Also, the hangar is a funnel with the bottom shaped to the exact outline of the drone's landing gear. So a skewed drone is guided into position, and even if it gets stuck at an awkward angle, it's landing gear with the contacts won't fit over the contacts on the hangar bottom.When my board would deal with the cases 1-4 (open load, short circuit, voltage source connected, reverse voltage source connected). I'd consider myself reasonably protected.