Power board, control board and isolation module PCB design

 Hi!

Lately, I've been working on a few PCBs located in Wolfie's torso. In theory, there should be at least two PCB's - one responsible for power supply distribution (in my case called the "power board") and the other serving as SPI to CANFD converter equipped with an IMU, magnetometer, and wireless communication (in my case called the "control board"). 

the internals of the torso 

Having talked to my friends from MABrobotics I decided to galvanically isolate the main computer and the control board from the high-power actuators to minimize the chances of damaging the control unit in case any bad things happen on the side of the actuators. At the same time, I didn't have any chance to play with isolated circuits before, so it seemed to be a great opportunity to learn some new stuff. 

As there isn't much free space inside the torso and I didn't want to place the power board near RPI I had to go for a modular design. I decided to split it into the non-isolated base part and the optional isolation module. I think it is quite a reasonable solution because in case there's something wrong with the isolation part, I can always use the basic, non-isolated variant. After a short introduction let's have a look at the torso electronics diagram:

Starting from the left side, there is a 4S2P 16.8V Li-ion battery with a BMS integrated into the battery case (I plan on doing another write-up about the battery itself) and the main supply switch. Next, there's the power board PCB. It has a main shunt resistor that is used to measure the current going in for the whole system (ie. the actuators and control electronics). The small voltage drop is amplified using a current sense op-amp and the battery voltage is monitored using a voltage divider. Both voltage signals are fed to a connector that goes to the control board. In the basic variant (without isolation), RPI and control board power supply comes from a non-isolated DC/DC converter. The actuators are turned on and off using a HotSwap controller that is equipped with its own shunt resistor just for the actuators and controls three external MOSFETs. 

Power board

In case the isolated part works properly in the first iteration the current amplifier and DC/DC converter are not going to be populated on the powerboard and the isolation module is going to be placed on top of it. The isolation module is equipped with an isolated DC/DC converter supplying the 5V bus (3A max), an isolated ADC, and a photocoupler for isolating the digital on/off signal for actuators. There are also two LDO's for the primary and secondary side of the ADC and actually, that's it. The main difference is that no analog voltage signals are connected to the control board, but rather an SPI bus that is used to read the ADC's readings. 

Isolation module

There are only six connectors on the power board for powering the actuators, but these get multiplied on the front and back side using two small distribution boards like this:

        

CANFD bus and supply distribution board

I'm not particularly happy with them - I think they are really packed and kind of ruin the slick design on the inside of the robot, but trust me there's no other way. The robot is simply too small to fit that many connectors in any different area. 

The blue rectangle on the main diagram is illustrating the components placed on the control board. The board is designed to fit as a "hat" on the Raspberry Pi. It has two G4 microcontrollers - one solely for CANFD communication with the actuators (the communication is isolated as well), the other working as orientation estimator (IMU+mag), taking care of wireless communication and SPI readings coming from the isolation module. It's also partially responsible (together with RPI) for controlling the actuators supply bus. In case the RPI or the navigation algorithm detects any faults they can, independently of each other, turn the actuators off.

Control board

I guess the explanation combined with the diagram will help to grasp the overall idea. The PCBs are going to be sent for manufacturing soon, but I'm mostly worried about the global IC shortages. We'll see how it goes - hopefully not messing up my plans :) 

The next post is going to be about the custom battery I'm preparing right now ;)