Digitally controlled friction and PCB ground planes

After replacing the servo D-axis to fix the backlash in the left throttle handle I’ve been testing a power transistor to drive the clutches as an alternative to the relays. This worked fine, but I noticed that there were still synchronization problems between the left and right throttle handles, mainly near the middle throttle positions. Which sounds logical, as near the ends the mechanical friction between the plates can be controlled quite accurately with the screws, but near the middle position there’s less control.

So the next test was to see how well the handles are synchronized if there is no mechanical friction from the plates.
That means however that in manual mode some other mechanism is needed to keep the handles in position, especially near the ends where gravity pulls them to the end positions.
The new idea is to use the clutches for this: drive them full current in autothrottle mode and drive them with a small current in manual mode for just enough friction to keep the handles in position.
This could be done by varying the base current of the power transistor. By using a digital potentiometer it might even be possible to let the Arduino control the amount of friction of the individual throttle handles in manual mode.

In manual mode the digital potentiometer (MCP41010 PA0/PW0 -> PB0) and 5KΩ resistor are in series, in autothrottle mode the 5KΩ resistor is shorted for a higher clutch current

 

 

Because the PCB for the throttle quadrant is quite large it sounds like a good idea to use ground planes in the design. I was a bit puzzled how to do this in KiCAD when using the external FreeRoute autorouter: normally the ground connections would be routed as wires by FreeRoute and the ground planes are created afterwards in KiCAD, but this would result in double ground connections both via wires and ground planes and more fragmented ground planes because of the ground wires.

While searching for a solution on internet I found this interesting tip.
The trick is to manually remove the GND connections from the netlist that goes into FreeRoute. By increasing the cost for connections at the back side of the PCB FreeRoute will try to make more connections at the front side so that a larger unfragmented area remains at the back side for the ground plane. At the cost of more vias, so some experimenting with the cost parameters is required.

When drawing the ground planes after autorouting the majority of the GND pins are connected to the ground planes.
To find any pins that could not be connected to the ground planes the KiCAD DRC tool must be used. Usually the unconnected pins are in ground plane fragments that are completely isolated by surrounding wires. Because ground planes were created on both sides of the PCB however most of the time the isolated fragments can be connected by manually adding some vias.

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