Today Helen and me finished the schematics for the USB charger for the battery. We used the LTC4088 chip. This chip has a good current load capability and also can drive an external transistor to allow the power to be drawn directly from the battery. Hopefully this is the final design. We hope to do the routing over the weekend now.
First of all, I would like to apologize for not having given you news about Ball-E for such a long time.
Let me make a short summary of where we are now.
Before the Athens week, we decided several components of our robot and we received most of them. Actually, we have our 3 stepping motors (Trinamics QSH5718), our two-rows 4″ omniwheels (Kornylak). With Pierre-Hugues, we made the pin assignment of our STM32405RG-based card and finished the schematic of our PCB. Routing will be done as soon as Alexis will have corrected our schematic. Scott and Otilia are working hard in order to implement a Kalman filter and to understand a huge part of the mechanics.
Alexis lent us a card with an accelerometer, a gyroscope and a magnetometer so that Otilia and Scott can test their Kalman filter. Pierre-Hugues is going to code tomorrow a bootloader which will make the card dump the values of the sensors through the usb port.
My objective of tomorrow is to build the whole robot so that it will be ready for real tests when we will receive the PCB.
Now I hope that this is done, for now. I chose the LTC4055 from Linear, which, besides having a Li-ion charger from USB power, it can also seamlessly switchover the source power from battery and the USB line, thus allowing the robot to be used also while recharging. The LTC4055 datasheet says that the output current is only 900µA, but, according to some feedback I got from asking to the professors and to stackexchange, I’m almost sure that this figure must be wrong (probably they meant 900mA). A design note from linear proposes a circuit that can balance the load from the USB power and the battery, which means that we can not only use the robot while it is recharging, but also to draw some power from the battery if the USB power is not enough to power the robot.
Most of the last week, including the weekend I spent searching for an architecture for the USB charger + a buck-boost converter for the board components, which would require a source for 3.3V and 5V, plus another separate regulator of 3V exclusively for the motor.
The USB charger schematics was mostly done by this Sunday (only the schematics of the CI was missing in the library, thus I left it to place in a second time) until we meet with the professor Monday. We found some downsides:
- The battery would provide from 2.75V to 4.2V, thus the buck-boost CI that I chose could not provide the right regulation at 3.3V (since the CI could do buck or boost, but not both at same time).
- There was an issue for using the robot while recharging.
- The motor had to be changed to another one that needs 9V of input.
- The components that required 5V were removed.
So all the time spent making the schematics was written-off. At least I could study how to build a (more or less) proper USB charger. Now I hope that the new power source architecture is ready and can be written on stone, or I risk to increase even more the time of the development.