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Anyone interested in building a PCB? - Patrick - 10-11-2019

Hello from a new member,

my build has recently started. While waiting for motors to arrive and print jobs to be finished, I was thinking about electronics. 

Most of you use PCA9685 boards and they work good so far. However, I've read that some people here fried their boards and also that some killed their servos during stall. This makes me think about a different approach I want to share with you. 
In my opinion, it would be neccessary for the safety of components and also for better operation of robot, if we can get any feedback of the servos. One way I see is measuring current consumption of each servo. If current raises above a given threshold, motor stalls and we can handle this by software (go into different pose or at least shut off everything). By adding a fuse to each stepper, we can hopefully save these expensive parts. 

Both ideas results in a schematics I've build in KiCad. Here is a first impression of the planned board. Lots of work still to do but I want to discuss the entire idea.The board provides power to stepper over a screw terminal. There is a car fuse (20A, or what ever size needed) behind followed by a diode as protection. A huge (2000uF electrolyte) capacitor acts as stabilization of power (sits on back side of PCB). All stepper VCC are connected to this main power rail by SMD fuseholders, so each stepper has its own fuse (2,5A or so). Also this board is equipped with 4 INA3221 current sensors. These nice I2C devices bring 3-individual channels each, so one INA for each leg. They can measure voltage, current and also raise events (over current, warning thresold etc.) Also, there is a PCA9685 to drive the servos. Because INA3221 and PCA use the same address range, it is only possible to place one PCA and four INA meaning you can attach 12 steppers and get 4 PWM without current measurement. 

This board fits to any MCU which supports I2C, so it doesn't matter if you use Arduinos, Jetson, RPI or what ever. The overall size is 59mm x 128mm, hopefully it is not too large?


What do you think? Should I go this way further? Any hints, ideas, drawbacks? Anyone interrested supporting me? This is my first PCB (before only prototyping boards), INA and PCA are simple components, but there might be some design error. Is someone able to check this? I'll share KiCad project later, might be something for the main git repository of SpotMicro?

RE: Anyone interested in building a PCB? - raincoat - 11-10-2019

Hi Patrick,

I am using a PCA9685 but I feed my MG966R directly so the PCA9685 has not to handle the high current the (12!) Servos draw, it just provides the signal. IMHO there is no need to develop a controller as the PCA9685 works fine.

How is your project going on?