Hot swap Power Module
This page outlines the power system that is being utilized in many of our MAVs.
Note: no one with power electronics experience / qualifications has ever looked at the presented circuit. It seems to work but we never leave the MAVs plugged in unattended in case it fails and causes a fire.
The features of the power module are as follows:
- Allows running the system off a battery or power-supply
- Provides power at the required voltages for the Pixhawk, motors, ESCs, computer and any sensors/actuators.
- Allows the PX4 to monitor the battery voltage and current.
- Allows swapping batteries without losing power to the computer.
The system has two connectors.
- The Full MAV Connector. Everything is powered via this connector. This means that the motors may draw large currents through this connector or discharge into it during regenerative breaking. Both of these situations make it unsuitable for use with a wall power supply and so this plug should only be used with LiPo batteries.
- The Computer only Connector. As the name implies only the computer (plus other sensors on the 12V rail) are powered via this connector. Note it bypasses the Pixhawk power converter and so the Pixhawk cannot measure the current drawn through this plug.
There should be no issue with connecting both plugs simultaneously. However we recently had an issue with a commercial system that utilized a similar setup that resulted in a Lipo fire. Because of this we only plug both connectors in while swapping which one is powering the system. When both are plugged in the computer will be powered off the one with the highest voltage. A NuC power supply runs at 19V, so the system will always prefer to draw from it over 4S batteries. Note that if the computer is connected to the Pixhawk through USB, it will also not lose power. However, it will momentarily display a low voltage warning during the switch.
The power module consists of the following parts
- 12V 4.5A UBEC 2-5S Lipoly (7.2-21V)
- Pixhawk Power Module
- 19V 65W Nuc Power Supply
- XT90 Male Connectors x2
- XT90 Female Connector
- Schottky diodes (2 in 1 pack)
- 12 AWG wire
Note that if active breaking on the ESCs is used the system voltage may spike as high as 2x the battery voltage. Take this into account when selecting power components.
The power circuit is very simple and shown in the image below.
The Pixhawk is powered through the Pixhawk power module, the computer is powered through a 12V BEC and the power to the ESCs/motors passes through the current sensor in the Pixhawk power module. Shottky diodes are used between the connectors and the BEC to allow both to be connected at the same time without one power source discharging into the other.
The real life version of this circuit is shown in the image below.
In the case of the NuC we get both the connector needed for the computer and the ground power supply by taking a standard Nuc power supply and cutting it. This connector is soldered into the circuit above. The power supply then had a female XT90 connector soldered on in its place as shown below.
Any power supply that outputs between 7.2 and 21V and at least 3.4A will work. However, to ensure the system drains from it in preference to a battery it should output more than 16.8V.
Aka, all the things we have screwed up during assembly
- 3D printed PLA has a low melting point and being vaguely near a soldering iron is enough to cause it to warp.
- When soldering the XT90 connectors the plastic will become softer and the contact pins may move inside the housing. To prevent this plug it into a female while soldering them.
- The power box housing used is typically made as small as possible. You will need to plan the wire lengths and angles to ensure all of the parts fit.
- Power circuits are very good at conducting heat away from the solder, get a powerful soldering iron with a fat tip.
- There will be a lot of current going through this circuit and it is difficult to access. You should use plenty of solder and tape/heat shrink all the contacts so a bump can't break a connection or short anything.
- Do some sanity checks with a ohm-meter before plugging in a battery, if you short positive and negative with a big Lipo things will catch fire.