3D Printed Custom Fingertech Style Kill Switches for Combat Robotics (with anti-spark precharge resistor)

Introduction

As part of the World Science Festival, Brisbane hosted Robowars Australia's 2024 Sportsman Cup. For this competition, I wanted to enter a creative, yet mechanically simple robot that would be easy to construct and maintain. I recently designed a rather unique combat robot (although inspired from an NHRL beetleweight called ‘Noob Tube’ as well as my previous ‘Bladerunner’ Robots)

I won't go into full detail on the robot's design in this post, the core complexity is that every part of the robot's electronics and tubular body spins at approx 500RPM. You can see this in the photo below, Bladerunner is the blurry one!

Existing Killswitch Solutions

The round spinning walls make using a standard XT60 killswitch “link” impractical - it will be immediately destroyed, or loosened and fall out due to the vibrations and centripetal force at play. There is no protected location for it on the robot.

The typical solution if you’re taking the next step up from these links, is a fingertech power switch, which uses a bolt as the contact between two electrical connections. They are proven reliable over years of use and are available here:

Australia: https://botbitz.com.au/products/fingertech-mini-power-switch

Global: https://www.fingertechrobotics.com/proddetail.php?prod=ft-mini-switch

Safety Note Regarding Fingertech Switches

The mode of operation of the fingertech switch is not always clear so I will try to outline it here.

There are two copper contacts within the switch housing. The one furthest from the screw head is threaded to match the screw, while the one closest to the screw head has a larger opening. This larger opening is not meant to make contact with the length of the bolt (although I have concerns that over time the copper contact can become loose and make unintended intermittent contact). The electrical connection is actually made in a planar manner, between the underside face of the screwhead and the top face of the copper contact.

This is good because the connection has a large area and is immediate, meaning very few rotations are needed to turn the switch on and off. The small amount of flex in the contact also helps, as it maintains a positive pressure between the faces, ensuring reliable electrical contact during the mechanical shocks of combat.

With this mode of operation in mind, note the following recommendation from fingertech:

“Connect the battery to the terminal closest to the screw head. The screw head is part of the circuit (constant connection to the far terminal), so this will ensure it is not live when the robot is off.”

The screwhead, and therefore allen key used for fastening, being live when the robot is off, is indeed an electrical safety hazard. This only occuring when the screw is fully inserted in its insulating housing (in the on position) is hugely preferential.

A live, protruding battery connection of an unscrewed screwhead can easily lead to short circuits, as a metal robot chassis can easily become shorted to ground. I have also seen builders from an automotive electrical background intentionally ground their robot chassis as in a car (not recommended), meaning a dead short across battery is only a millimeter away.

In a combat robotics environment a millimeter's worth of damage is more likely to happen than not in any given fight.

With that out of the way, I felt there was plenty of scope for an open source DIY design that can be easily adapted to any robot geometry and mounting constraints.

Existing Designs Used as a Starting Point:

There are a couple of files out there on the main 3D printing sites, that serve as a good starting point. The main one I used for inspiration was:

https://www.thingiverse.com/thing:3226776

This design is for smaller robots, but apart from adjusting that, the existing designs share a few weaknesses:

• No integrated connectors.

• Standard steel nuts that do not allow strong solder connections

• Lack of pre-charge resistor to avoid sparking (the importance of this is exacerbated when common steel screws are used, as they will rapidly accumulate oxidisation leading to increased impedance, intermittent contact and even more sparking).

Combat robots are inherently space-constrained, and the overall footprint of any component should always include the wiring and connector that goes with it. A typical 10cm of 12AWG wire and 2 XT60s add a lot bulk and tangled mess that is often underestimated when laying out a design.

Key Design Choices

Brass inserts: The core aspect of my killswitch design is the transition to brass inserts. Compared to standard nuts, these are don’t corrode during electrical contacting, and so maintain low impedance, solder well, and are be permanently affixed in their required postion to hold alignment with the screw.

Integrated Connectors for inline plug-and-play: The killswitch has integrated XT60 plugs and sockets, with the switch inline of the positive wire, and the negative connection simply passthrough. This means it can just be dropped in between battery and load. I would like to design similarly compact battery series connectors and load breakouts that can be connected in a stacking manner.

Anti-Spark: A simple through-hole resistor can be inserted into the screwpath to provide a higher impedance initial connection. This prevents sparking and corrosion in the system. I ran this at 12S with a 200Ohm resistor, so it has been tested up to 50.2V.

3D Printed Housing

The housing provides insulation, mounting points and alignment for the brass inserts, cabling and XT60s. The side shown below is where the brass inserts are exposed for soldering to the positive battery and load connections.

The opposite side provides routing for a simple passthrough of the negative power connection. Those cables are wrapped through the semicircular notches on the corresponding sides of the XT60.

Component Layout

As you can see, the switch is remarkably simple. Simply install the 2 brass inserts using a soldering iron, and slot the pre-charge resistor into the hole so its lead protrudes into the bolt path by approx 2mm. Then solder the other end of the resistor to the lower brass insert.

From here, all you need to do is hotglue in the 2 xt60s, solder the main power wires, and seal the assembly up with more hotglue. For my geometric requirements, a regular m4x25mm screw was used to complete the switch.

As with the fingertech switch, there is a correct side to connect the battery. It is vital for optimum electrical safety that you connect the positive battery terminal to the lower brass insert (the one with the precharge resistor).

Assembled Killswitch (Post Combat)

My robot, Bladerunner, performed reliably throughout the 2024 Robowars sportsman cup. As it was the first time the design had seen combat, this was a huge success for me. Bladerunner was robust, but not particularly destructive, and so all 6 fights went the full 3 minutes for a total of 18 minutes of combat.

The switch performed flawlessly from an electrical and useability perspective and I will continue to use adaptations of this design going forward. I intend to integrate things like batteries series connections, and ESC parallel connections into a block structure to further simplify my wiring going forward.

Even the stone-age precharge resistor design was functional after close to 50 cycles.