Gas station without pumps

2017 November 8

Half-scale mockup of chassis

Filed under: Robotics — gasstationwithoutpumps @ 15:38
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One of our first tasks for the project is to come up with a mechanical design for the robot.  This is my weakest area in the class, and I have not yet been able to come up with a full design.  Here is where working a group could really help, but I’m not sure I’ll have time to finish the course (I do have to get my book done, and mechatronics is taking all my time), so it would not be fair for me to pair up with someone actually taking the course.

I also have a hard time sketching ideas—something that mechanical engineers take for granted.  It takes me a long time and painstaking care to get even a crude sketch drawn.  Tools like SolidWorks don’t help much—they make the result neater, but still take me forever to get anything entered.

Rather than make a drawing for my chassis design, I decided it would be faster and more effective to make a half-scale mockup out of  foamcore.  I chose half-scale rather than full size for two reasons: I had very little foamcore on hand, and my foamcore circle cutter only goes up to 6″ diameter circles.  Because I was planning an 11″ circular robot, half scale let me cut 5.5″-diameter circles out of foamcore.

A view from the right front showing the three layers of the robot, the motors, and the wheels. The little board on top represents the all-digital beacon detector that I designed over the summer.

Right side view. The middle layer is for mounting the analog electronics and the AT-M6 killer. The skewers show the maximum allowed height.

Close up of the mockup from the right rear, showing the battery and (not clearly) the motors.

A detail of the front shows whiskers to use as bumpers—probably with a microswitch for each whisker. The layer needs to be at the right height for the whiskers to be at 3.5″ (the specified bumper height for detecting obstacles).

The bottom view, showing some ideas I have for placement of the optical sensors for the tape. The wheels are thinner than they should be for scale, but the wheel wells are the proper size (see Gearhead motors for mechatronics).

The bottom layer will be for the drive motors, the battery, the power electronics, and the optical sensors. The next layer will have the AT-M6 killer, the bumpers, and the analog and control electronics. The top layer will have ball storage and the Ren ship killer. At the very top of the whole robot will be the digital beacon detector, with a 6-wire cable down to the control electronics level.

For the power electronics, I plan to use one or two switching regulators to bring the battery voltage down to 6~V for the motors (see Review of cheap buck regulators, More on cheap buck regulators, and Correcting reasoning on buck regulators). These and the Polulu breakout boards for the MAX14870 H-bridges will be mounted (in female headers for easy replacement) on the power board on the bottom layer. I’ll put large capacitors on the battery side of the regulators, to keep the noise from propagating back through the battery connections to the rest of the electronics.

The Teensy boards I’ll use for control need a 5~V supply and provide 3.3~V on the board. I’ll probably route battery wires to the second layer and put the 5~V regulator there, to separate the noise from the motors as much as possible from the analog electronics.

The 3 layers of MDF will be separated by spacers of MDF, with the whole thing clamped together with threaded rods, so that it can be disassembled and reassembled. I’ve not finalized the locations and sizes of the MDF spacers—maybe just inboard of the threaded rods, about 3″ long, with two tabs on top and on bottom aligning to holes in the layers.  The skewers in this model are probably not in the locations of the threaded rods—I think that a 45° rotation to the “corners” of the robot would make them interfere less with necessary functions.

I’ve not designed either of the ball-delivery mechanisms yet.  I’ll do separate posts on the ideas I’ve had for them.

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