Gas station without pumps

2017 November 11

Bending PVC failed

Filed under: Robotics — gasstationwithoutpumps @ 11:01
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In Thursday’s post Preliminary design review in mechatronics, I mentioned the idea of bending PVC tubing to make a ball ramp for storing the ping-pong ball ammunition. Yesterday I went to the hardware store (San Lorenzo Lumber, who may be calling themselves “ProBuild” these days—I can’t keep track of their name changes, and everyone locally still calls them San Lorenzo Lumber) and bought a length of 1 ½” Schedule 40 PVC pipe and a small bag of “play sand”.  I just noticed today that they charged me a lot for the PVC pipe—I think that they had stuck an incorrect barcode label on the 2′ length of pipe and charged me $4.49 for it (the correct price should have been around $2, as 1 ½” PVC pipe usually runs around 60¢ a foot).  It might have been cheaper for me to buy a 10′ piece and cut it up myself—that would have given me about 5 tries for the only 50% more cost.

I took a ping-pong ball with me and confirmed that a ping-pong ball does fit through the pipe, though it is a narrow fit—dropping a ping-pong ball through the 2′ section takes about 3 seconds, because of the air drag.

I made a form to bend the pipe around, by cutting a redwood scrap of 2×12 into a rough circle and turning it down to 7″ diameter on my wood lathe.  That’s the first time I’ve ever done faceplate turning, and it seems to have worked ok, given the soft, fibrous redwood and the knot in the wood.

The bending form is just a 1-7/8″ thick chunk of redwood turned to a 7″ diameter circle.

I tried the technique of heating sand in the oven to 450°F and using the residual heat of the sand to heat the pipe to bending temperature (170°F–220°F, or 75°C–105°C).  It didn’t work—the pipe never got hotter than about 130°F, nowhere near soft enough to bend.  I see two possible reasons for the failure:

  • The sand was not hot enough. An oven is an inefficient way to heat things, and the sand in the center of the can may not have gotten anywhere near the temperature at the surface of the sand.  I could try stirring the sand occasionally while heating it to get a more uniform temperature and raising the oven temperature.
  • The sand transferred the heat too slowly to the pipe.  The outside surface of the pipe is radiating heat as the heat is transferred from the sand.  If the transfer of heat from the sand to the pipe is slow, then the radiation from the pipe will keep the temperature from rising too high.  I don’t see any way to increase the flow of heat from the sand, other than raising the temperature of the sand.

It may be that using the residual heat of the sand only works for smaller diameter pipes, which are floppier to begin with.

I tried again today, heating the sand in a skillet instead of the oven, so that I can heat it faster and hotter.  I heated the sand to 250°C (about 480°F), as measured with an infrared thermometer. This got the sand hot enough to make the pipe flexible and I could bend it around the form.  The inner side of the pipe buckled a bit at the tight turning radius (3.5″ on the inside).

I had not made anything to hold the pipe in place as it cooled, and I did not want to hold it by hand (actually I was using my feet to hold the pipe in place) for the time it would take to cool, so I poured out the sand too soon and the pipe collapsed.

The collapsed pipe will be of no use for making a trough for ping-pong balls.

If I make a form with holes to insert pegs around the outside of the pipe after bending, I can probably avoid the collapse, as I can leave the sand in the pipe as it cools. But I don’t think that this will eliminate the ripples on the inner part of the curve, as they formed while the pipe was still filled with sand.  A better support mechanism (like a metal hose might help reduce the kinking and ovaling, but at some point the amount of stretch needed on the outside will limit the bending radius.

I looked for information about minimum bending radius for PVC pipe, and the best source I found was a video from BendIt, where they managed to bend 1 ½” PVC to a radius of about 8″ using internal metal hose support, with the pipe getting a bit oval. That is still a lot wider curve than what I need for my design, so I don’t think that this track design is going to be one I can make.

I’m going to have to rethink my ammunition storage for the robot, which will likely affect the delivery of the balls to the targets also.

I’ve been putting in so much time on the mechatronics lately that I’ve not made any progress on my book, so I think I’ll have to put the mechatronics to one side for a day or two, and work on my book instead.  Maybe when I come back to the mechatronics I’ll have a more doable idea on how to deliver the balls.

If I can’t put in the time that the mechatronics course takes, my fallback plan is just to make the base platform and sensing and do the coding, without the ball-delivery mechanisms (perhaps lighting LEDs when it thinks things are lined up well enough to fire).  It would not meet the requirements for passing the course, but I’d still have something to build on as a hobbyist.

2017 November 9

Preliminary design review in mechatronics

Filed under: Robotics — gasstationwithoutpumps @ 21:26
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Today in the mechatronics class, 28 groups (29 if you count me) presented their designs in about 90 minutes—3 minutes per group.  The first few groups were slower, as the professor and TAs provided more detailed feedback, and the later ones were shorter, as much of the critique had already been delivered on similar designs.

My decision not to have a scissor-jack or lead screw to raise part of the robot were confirmed by the comments on the designs that did include such features.  I was also pleased that I had not included a moving turret, as those were also critiqued as overly complicated.

I’m now thinking of really minimizing my mechanical hardware by having only one arm, hinged on the platform and aligned with fiducial sensors so that it can push a ping-pong ball through the small, high hole of the final target.  For the larger, lower holes, the arm could swing further and drop the ball through the hole.  One other group is trying that approach—most groups went for elaborate Rube-Goldberg contraptions instead.

I’ll have to look at the geometry to see whether this would work—if it pencils out mathematically, then I’ll build a crude prototype this weekend and see whether a servo motor can move the arm fast enough to keep the ping-pong ball in a cup as the arm swings past vertical.

One problem with the one-arm design is that the larger, lower  (AT-M6) targets are best hit by a side-mounted ball delivery, but the smaller, higher target (Ren ship) is probably best hit by a front-mounted ball delivery, so that the robot can drive ram the target with slightly different aims to try to align tabs with the fiducial marks.  It may be possible to use roller-ball microswitches to sense the fiducial marks for the Ren ship target, in which case driving along the wall and firing when fiducials are sensed may work for a side-mounted arm.

Note: I need to get some microswitches, both for this purpose and for the obstacle avoidance bumpers.

Assuming I can make the one-arm design work, then the biggest problem for me will be making a delivery ramp for loading the arm.  I’m thinking that 8 balls (2 for each target) should be enough ammunition to stock, and I’d like to have them be in a spiral ramp around the outside of the robot.  The ramp needs to be 4 cm long per ball, or 32 cm, which would be about 160° around the robot (assuming that the centers of the balls are on a 9″ diameter circle).  It is nice that it isn’t a full turn, as I should have room for the arm without interference from the ramp.  (I’ll also have to clear the central beacon detector, which means that the arm can’t be centered on the robot—or I’ll have to move the beacon detector to the edge of the bot.)

To make the helix, I’m thinking of getting some 1.5″ or 2″ PVC pipe, softening it in the oven, and bending it around a cylinder.  Bending PVC should be done between 170°F and 220°F (source: 10 Best Practices When Pending PVC Pipe and Conduit).  I’ll have to be careful to keep the temperature below 284°F, at which temperature PVC starts to decompose (source: Wikipedia PVC article). I’m not likely to follow the advice they give, as they are trying to sell $350 “PVC Bendit” tools.  Those might be nice for a makerspace or a place that bends a lot of PVC, but are too expensive a toy for a one-time project.

If I heat the PVC in an oven, it would be good to fill it with sand to reduce the likelihood of it collapsing when I bend it.  The technique suggested on the web is not to heat the PVC in the oven (probably because most people want to bend longer pieces than fit in an oven), but to heat the sand in the oven to much hotter than needed, then pour it into a capped length of PVC to heat the pipe.  That seems like a simpler and safer technique than others I found on the web, so I think I’ll try it tomorrow.

According to various web sites that give dimensions for PVC pipe, 1 ½” Schedule 40 PVC averages 1.61″ ID, which is just over 4cm.  That may be cutting it a bit too close for a closed tube, but if I cut the helix in half on my bandsaw to create a half-circular trough, it should be ok.  Alternatively, 2″ Schedule 40 PVC has an average ID of 2.047″, which would allow some collapse of the tubing without blocking the ping-pong balls.

If I bend a full turn of the helix (longer than I need), I’ll need about 72cm of pipe.  The interior volume of 1 ½” pipe would be about 946(cm)3 (just under a liter). Dry sand has a relative density of about 1.6, so I’d need 1.5kg of sand (3.4 lbs). Getting a 20 lb or 10kg bag of sand from a garden supply store would be good, if they sell such small quantities—I don’t want to get a cubic yard (1.2 metric tons) delivered. Even the small bags at Central Home Supply are 100 lbs. “Play sand” is sold at hardware stores like Home Depot for about $5 for 50-lb bag, which may be the cheapest route for the amount I need. (Aquarium stores sell decorative sand in smaller amounts at over $1 a pound, and craft stores charge even more.)

Now I need to find (or make) a 7″ diameter cylinder to bend the PVC around.

In other news, I spent the afternoon struggling with SolidWorks (it is a little less painful now, but still a far from intuitive interface). I managed to get a model built for the gear motor, mounting bracket, and wheel, and I put them together as an assembly.  I also started on the SolidWorks model for the base plate for the robot, getting the motors positioned and the wheel wells cut, but not getting the mounting bracket holes transferred yet.  Maybe by the end of the quarter I’ll be proficient enough with SolidWorks not to feel like cursing it continuously.


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