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.

2017 October 26

MockRobot

Filed under: Robotics — gasstationwithoutpumps @ 22:32
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I can’t share most of what I’m doing for the Mechatronics course, because I don’t want to put up things that students will blindly copy in future years, but I feel that I can safely put up pictures of MockRobot, the prototype we build in Lab 2 for learning SolidWorks, laser cutting, and foamcore prototyping.  The MockRobot is not very different from many other pictures students have access to, and pictures of other prototypes is not going to save them much (if any) effort.

A perspective view showing the overall shape.

The foamcore tower is properly bent with smooth paper on the outside. The lap joint was too weak with the glue I had available, so I used painter’s blue tape to hold the cylinder together. The box also has smooth bends on the bottom edge and mitered lap joints at the corners. The crushed foam was too springy, and one of the lap joints tore the paper, so again I used painter’s tape to hope things together.

The front view, showing the motor mounts on the edges of the platform, the foamcore tower, and the front skeg.

The top view with the tower in place does not show much.

Without the tower, the mounting holes for the tower and for a perfboard are visible. The tops of the two skegs are visible front and back.

The slots for the tower were a little too thin—fitted perfectly for aligned foamcore, but with slight misalignment of the tower, it was impossible for me to get the tabs in without crushing or buckling the tabs. More clearance is needed for foamcore.

The side view should show the screw holes for mounting the motors, but they are hidden behind the 3″ diameter wheels.

A detail of the motor mounting, showing the problems I had with having to reglue the parts several times—the glue kept the pieces from fitting properly after a while.

The motor mount was constructed with tab-and-slot construction on the very edge of the board, so that the wheels did not have to pass through wells. This turned out to be a poor choice, as the hot glue joints were not very strong, and repeated regluing ended up with thick layers of glue that spoiled the alignment.

In future designs, any parts that need to support weight (especially cantilevered weight like the motors here) will have slots that surround their tabs on all 4 sides (a full mortise and tenon, rather than a finger joint) and have bracing on both sides, probably using a crosslap joint. I might also have the tenon go all the way through so that it can be secured by a pin on the other side. I won’t count on hot glue to withstand any tensile forces, and only small shear forces.

I might also design wheel wells, so that when the robot bumps into things, it won’t be with the wheels. I already had trouble with the roach robots rubbing off their wheels if they had too much side force (while sliding along a wall at an angle), so I should have been more more protective of the wheels in this design.

I spent way too much time on this lab fighting the steep learning curve for SolidWorks.  It has a very arcane interface, where buttons do very different things depending where on the screen you are and what mode you are in.  Undoing things is difficult, and I frequently had to scrap several hours work and start over, because I could not figure out how to  reverse some poor early choice I  had made.  I’ve used a lot of software with bad interfaces, but SolidWorks has one of the hardest-to-learn interfaces I’ve ever been stuck with.  Despite the numerous features that SolidWorks has, I would never pay money for it—it is an experts-only tool, and I don’t want to spend the years it would take for me to become expert enough for it to be of value to me.

2012 February 21

Science fair poster boards

For the past several years, my son has used the same black foam-core poster board for science fair each year. This year, he decided that it was getting too beat up, and he wanted a new one, so we designed and built a different one for this year. The constraints were the same as before:

  • When set up, the board has to fit within the science fair limits of 48″ wide, 30″ deep, and 72″ high (the county limit—state allows 78″ high).
  • When folded up and in a protective carrier, the board has to checkable as luggage without extra fees (at least from Southwest—other airlines charge fees for everything).
  • When in its carrier, the board has to be easily carried by hand for a mile and easily transported by taxi or airport shuttle.
  • The board has to stand freely on a table and not be easily knocked over.
  • The board needs to have a lot of room for text, diagrams, plots, and pictures.
  • The board had to be fairly cheap (less than $50 in materials).

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