Gas station without pumps

2017 November 16

Another day fighting SolidWorks

Filed under: Robotics — gasstationwithoutpumps @ 22:14
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I spent another day working on the SolidWorks model for my robot.  I’m getting better at it, but the user interface is still a struggle to deal with.  Here is my model so far:

I’ve got the first layer mostly done, but the second layer is barely started.

On the first layer, I still have to put in small starter holes for screwing down the optical reflection sensors, which I plan to mount on pieces of perfboard. Having the sensors on perfboard will make wiring them easier (I hope), and make them easier to replace if one fails. Also, recessing them into the perfboard should improve their directionality.

I’m planning to clamp the layers of the robot together with ¼” threaded rod—the acorn nuts I’ve put on the ends are just decorative, and there isn’t room underneath for an acorn nut.  My clearance to the ground is just 8mm, which is less that the ⅜” minimum height for an acorn nut.  There is room for one 7/32″-high hex nut or two 5/32″-high thin hex nuts, but that’s it.

[Update 17 Nov 2017: I’m now going to use 8-32 threaded rod, not ¼”, so there will be room on the bottom for the acorn nuts. See Hardware obtained today.]

The view here is mostly from the back, showing the power panel (with a voltmeter/ammeter and a hole for a barrel jack) behind the LiFe battery.  I should remodel the battery to curve the wires up to the jack and to tuck the charging wires back along the battery.

I have to decide on how the AT-M6 ball launcher interacts with the rest of the components—if I use an accelerator wheel  underneath (which I was planning), then there needs to be clearance for the wheel between the first and second layers.  But the tops of the drive wheels look like they’ll interfere with such an accelerator wheel.  I could mount the accelerator wheel in front of the drive wheels, but then I’ll have the track-wire sensor further from the edge—will it be sensitive enough? I guess I’ll to make the sensor and test it. I should also prototype the accelerator-wheel design, to make sure that it will throw ping-pong balls.  I still have a few inches of uncollapsed 1-½” PVC pipe that I could use for making (or at least prototyping a launcher.

The track wires will be running a current of 180mA—the design used in the targets is a 12V power-supply with a 1.2V drop from using a darlington transistor (why not an nFET?), and two 120Ω 1W current-limiting resistors in parallel.  The typical VCE for the TIP122 darlington at 180mA is more like 0.75V, so the current is going to be more like 188mA.

To imitate that with my track-wire that has 2 47-ohm resistors in parallel, I’d need a voltage of 4.4V.  The power dissipation would be (4.4V)^2/(23.5Ω) (0.5) = 412mW, which is just below the ½W limit from using two ¼W resistors. Rather than wire up a special circuit (using an LM555, as they did), I’ll just use the Analog Discovery 2 power supply for the 4.4V and the function generator for the pulse train to the gate of an nFET.  It’ll be a little inconvenient, since the function generator, power supply, and oscilloscope leads are so short, but I can make it work with some “extension cables”.

I think that tomorrow I’ll work on sensors, both electronics and programming, and leave the mechanical design for later—I think I’ll make more progress on the more familiar tasks, and I’m so far behind most of the groups that I’m not sure I’ll ever catch up.  I don’t even have a name for my robot!


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