Dec 31

Before the electrical design can start some items need to be considered.

I want to stack more add-on boards on top of the FOXBoard, this allows me to re-use some of the boards (or designs) when I start on another robot. The FOXBoard uses 2 headers of 2×20 pins, these are available as female sockets with variable pin length ranging from 3 to 17mm. Examples are the 962-60206-12 from EPT that offers a body height of 11.45mm and a pin length of 12.2mm. Using a PCB thickness of 1.5mm and an inserting of the pin in another female socket of 3 to 6mm this offers a maximum component height of 16.15 to 19.15mm.

If one board is stacked on top of another one connectors, LED’s, buttons and so on are not longer accessible, for this reason all these components should be placed on the side of the board and be a 90 degrees bended version.

For the CAN interface to the actuators, 2 signal wires are needed that must be twisted. Based on the basic electrical architecture of SnakeBot, there are 3 additional wires needed: power, ground and charge. It might be an option to use normal UTP cable, this consists of 4 pairs of 2 twisted wires. In this case one pair can be used for the ground, one for the power, one for the charge and one for the CAN interface.

A wire in an UTP cable is a 24 gauge copper wire, this limits the current per wire to 0.577A or 1.15A per pair. This is not so much, when using 10 or 15 segments for SnakeBot each charging with 250mA as a minimum but with more advanced charging up to 2.5A the total current through the charge wire can be in the range to 10 to 15A. This is indeed not very realistic since the charger must be capable of supplying this current, but nevertheless the 1A is by far to low.

I guess the best solution here is to use normal household appliance power cord cable for the three power lines and use a separate two wire cable (shielded and/or twisted) for the CAN interface. This also “feels” better since drawing high current spikes trough a conductor that is a millimeter away from a communication bus running at 1Mbps is probably begging for problems.

In order to connect the power cables, best option is to use terminal blocks with a screw. Reason for this is that there will be a rather thick core that needs to be inserted. When wiring SnakeBot the (for example) charge line need to go into a segment and leave the segment again to go to the next segment. You do not want to place a connector on the front of the PCB and route the charge line over the board to the other side and place a connector here as well to go to the next segment. If this was done than all the charge current of all segments combined will run though the PCB of the first segment causing serious layout problems. Instead, the charge line will be cut, both ends stripped, twisted together, soldered and than inserted in a single connector that only supplies the current to that segment.

A power cord that can handle 16A uses a core of 2.5mm2, twisting both of them together results in a 5mm2 surface that makes a circle of SQRT(5/3.14)*2=2.6mm in diameter. That would require a terminal block that can handle AWG 10 or less, these are available but the height is 21.5mm which is too high and they are not 90 degrees bended.

Next best solution might be automotive crimp terminals, these can handle high current, are available in many shapes and have PCB terminals. For example this female connector and this pcb connector.

In principle this is the same for the CAN interface except that the cores are much thinner. Remains the problem that the screw can not be fastened since there might be another board on top. There are terminal blocks available that offer both a male and female part. The cable entry part is 2.5mm2, so inserting two wires will not work. As an alternative the power cable cut be cut, stripped and third small piece of about 5cm could be added to make a T-junction using shrinktube for isolation. Than the 5cm long wire could be inserted in the terminal block. For the CAN-wires this is not needed, two wires can be twisted, soldered and inserted without any problem.

For now, I will use the terminal connector using the male-female combination. For some reason this feels the best. I will split the connector in 2 times 3 pins connectors. For one that the PMC will only use the CAN signals and not the power lines but also giving me the option to buy a bit more of them and keep them on stock for other projects (2 and 3 pins connectors are used the most, 6 pins much less).

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