Designing the Electronics
Lately I’ve been busy designing a shield for the Intel Galileo which will interface all of our actuators. This includes the linear actuator and three servos. They are all controlled in different ways, all of which is handled by my board.
The 2 larger servos, the RMCS-2203’s, are powered from a 12V source (not routed through the board, since there is high current draw) and controlled via I2C. It is also often necessary to communicate with these servos over a serial connection (which basically opens up a command line interface for setting certain servo settings). It’s pretty cool and advanced for a simple motor. Anyway, I want to enable all of these communication channels between the Galileo and each motor. I reserve pins on the Galileo for software serial, 2 for each motor (RX and TX). These are pins 2, 3, 4, and 5. Then I link the SDA and SCL pins out of the Galileo to each motor. I use JST headers to as the motor plugs.
The smaller servo is controller via a simple PWM signal (like most servos) and is powered from a 9V regulator onboard which receives 12V from the 12V source as input. Pin 9 is reserved as the small servo control pin.
Lastly, the linear actuator is controlled by dropping 12V across it in either direction. Reversing the polarity will make it change directions. For this reason, we need a setup which uses two SPDT relays. This will allow the Galileo to reverse the polarity of the linear actuator while also remaining isolated from its power supply. I looked up a schematic for a single SPDT relay from Sparkfun for inspiration: https://www.sparkfun.com/tutorial/Outlet/RelayBoard-Large-v11.pdf
Powering the linear actuator through the board is tricky, since it can draw up to 7 amps. I used this calculator: http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
and found that 7A with 1 oz/ft^2 of copper thickness requires a trace width of 451 mil, which is enormous. I can’t quite make that, so I’m hoping 200 will do. I only expect bursts of 7A so it should be okay.
I’m continuing to work on this, but here is my schematic so far:
And here’s the current layout of the board. Notice the thickness of the traces around the relays:
Also the thick blue traces on the right appear to be connected, but that is merely due to the small resolution of the picture. They’re isolated from each other in the actual design.
Hoping to get this shipped to us by next week!