Gas station without pumps

2019 August 19

Shakespeare cookies v5

On Saturday, my son and I baked shortbread cookies using version 5 of the Shakespeare cookie cutter:

The difference between version 4 and version 5 is mainly around the left eye (on the right in this photo). Version 4 had a lot of trouble with the dough getting stuck in the small regions there. (See prior post for cookies made with the V4 cutter.)

Despite the simplifications, Shakespeare’s head is still quite recognizable.

We used the classic recipe (2 cups flour, 1 cup butter, and ½ cup confectioner’s sugar), but this time I used pastry flour instead of a mixture of all-purpose flour and sweet rice flour.  The dough works about equally well either way.

The cookies came out good, but the cookie cutters are still having problems with dough sticking to the cutters. Chilling the dough after rolling helped a little, but stickiness was still a problem. We also had problems rolling the dough out to a uniform 6mm thickness—sometimes we had the dough too thin, and the interior lines were not clear, and sometimes we had it too thick and couldn’t get the cookie out of the cutter without destroying the cookie.

My son had two suggestions, both of which I’ll follow up on:

  • Go back to having separate cutter and stamp (as in Version 3), but don’t try to connect the two.  Make the stamp just have a few alignment marks so that it can be hand-aligned to the cookie outline.  The stamp can have a lot of open space, so that the visual alignment is relatively easy, and so that the cookie dough can be easily separated from the stamp.  The stamping can even be done after the cookie has been transferred to the baking sheet, to make distortion from moving the cookie less of a problem.
  • Make a set of 6mm thick sticks that can be put down around the dough, that the rolling pin can rest on.

Version 6 of the stamp failed, because I made the alignment markers too thin and they did not survive even gentle handling.  I’m now printing Version 7, which has more robust alignment markers.

 

2019 August 14

Beginning design of a cat drinking fountain

Filed under: Uncategorized — gasstationwithoutpumps @ 22:11
Tags: , , ,

One of our cats likes to drinking from running water (a bathroom sink on a trickle setting), so my wife challenged me to make a drinking fountain for the cat that recirculates water in a water dish.  This project will be mainly physical design (3D printing, gluing things together) with a little electronics to control the pump.

I started by buying a very cheap pump from American Science and Surplus: an ET 23 series pump that they are getting rid of for only $2.50.  Somewhat surprisingly, there is a data sheet available for this pump from the manufacturer: http://www.et-pump.com/brushless_23.html, but (not so surprisingly) the specs are different from what American Science and Surplus claims. The manufacturer says that the pump is submersible and can be run at 5V to 12V, while American Science and Surplus says it is not submersible and runs on 4V to 6V.  Because the pump electronics are fully potted, I tend to believe the manufacturer on this one.

The pump uses a brushless motor with two sets of windings (and only 2 transistors to power them) and seems to start pumping at about 4V.  To characterize the pump, I used my Analog Discovery 2 to sweep the power-supply voltage from 0V to 10V, measuring the current through a 0.5Ω resistor.  The results were interesting:

At low voltage, the current seems to be exponential with voltage, as would be expected from having a diode in the circuit—the nonideality of 5.6 is consistent with about 3 silicon diode drops. Above 4V, the motor behaves about like a 26Ω resistor, though with a lot of noise. The turn-on and turn-off behavior between 2V and 4V is interesting—the pump takes a lot of power at these voltages. All these measurements were taken with the pump running dry—it likely behaves differently when pumping water.

The “noise” in the I-vs-V curve is not random noise—it is fluctuation in the amount of current taken as the circuitry for the brushless motor switches between the two sets of coils. If we set the power supply to a constant 5V across the motor in series with the 0.5Ω resistor, we can observe the voltage and the current for the motor:

The two coils seem to take slightly different peak currents when the switch for them is turned on, but both spikes are about 2.8 times the average current. The frequency is around 643 Hz, which implies a speed of around 19300RPM.

I tried controlling the pump with one of the PWM LED controllers that I made for the desk lamps. With a 6V power supply, I need about 60% duty cycle to start the motor, but then can turn it down to about 15–20% duty cycle.  With an 8V power supply, I need about 40% to start and with a 10V supply about 30% to start. All these were crude measurements by turning a potentiometer until the motor started, but they are consistent with about a 3.3V average starting voltage and ability to keep running down to almost 1V.  If the pump stalls at low voltage, one has to bring it up to about 3.5V to turn it back on.

The motor runs even with fairly slow, low-duty cycle PWM. The current spikes at the beginning of each cycle are large.

The PWM control seems to work even with a PWM frequency as low as 270Hz, which is somewhat surprising.  There does not seem to be much in the way of voltage spiking, even with no capacitor or flyback diode added.  There is a short-lived initial current spike of about 3.5A (staying above 2A for about 4µs), which probably comes from charging capacitor C3 in the motor, which is across the power lines after the diode D1 (which seems to be there to prevent reversed power supply).  The 11µC spike is consistent with C3 being about a 1 µF capacitor (or maybe 2.2µF), which seems plausible.  I’m not sure why the current drops to 0 before the motor voltage drops more than about a volt.

I bought some cheap plastic bowls from a thrift store, and my next task is going to be to design a 3D-printed base to hold the pump and the electronics under the bowl and a clip to hold a ¼” ID vinyl tube over the bowl.  The pump is not self-priming, so I need to drill a hole in the bottom of the bowl and glue on the pump to make a gravity feed to do the priming.

2019 August 11

Star-of-stars, another large pendant

I’ve previously posted about my 3D-printed stage jewelry: the 3D slugs , the diamond, the chain of office, and large pendants printed on my Monoprice Delta Mini printer using CC3D Silk Gold PLA filament.

I designed another pendant yesterday, and printed it today—this one using stars instead of spheres as the main design element.

Once again, I had to clean up the stringing and blobbing using a riffler.

// Star of stars
// by Kevin Karplus
//  Creative Commons Attribution-ShareAlike  (CC BY-SA 3.0)
// 2019 Aug 10

use <BOSL2/std.scad>
// BOSL2 from https://github.com/revarbat/BOSL2/
// used for offset

function inner_radius(r_outer, n, k) =
    assert(k<n/2) assert(k>0)
    let(straight_ratio = cos(180/n) + sin(180/n)*tan(180*k/n))
    r_outer/ straight_ratio;
    
function star_points(r_outer=5, n=5, k=2)=
   // Points on circle centered at (0,0) with radius r_outer.
   // First point on positive x axis.
   // k determines how far out the inner points of the star are, 
   //   with k<1 making a convex polygon with 2n sides,
   //   k=1 making a regular n-gon
   //   k=2 making a star that connects alternate points
   //   k=3 making a star that connects every third point, ...
   // k need not be integer
   // You can get a nice, fat star with k=(n-2)/2
   let(r_inner = inner_radius(r_outer, n, k))
    [for (i=[0:2*n-1]) 
        (i%2==0? r_outer: r_inner)*[cos(i*180/n), sin(i*180/n)]];
    
    
module star(r_outer=5, n=5, k=2)
   // Make a polyhedral star with n points.
{   points = star_points(r_outer=r_outer,n=n,k=k);
    polygon(points=points, convexity=n);
}


module star_outline(n=5, r=50, line=2,k=undef)
{
    k_star = k==undef? (n-2)/2: k;
    points = star_points(r_outer=r,n=n,k=k_star);
    echo(points=points);
    inner = offset(points, delta=-line, closed=true);
    echo(inner=inner);
    difference()
    {   polygon(points);
        polygon(inner);
    }
    
}

module star_of_stars(n=5, r=50, line=2, k=undef)
{
    k_star = k==undef? (n-1)/2: k;
    r_sub = inner_radius(r, n, k_star);
    star_outline(n=n, r= 2*r_sub, line=line, k=k_star);
    for (i=[0:n-1])
    {
        rotate((2*i+1)*180/n)
            translate([2*cos(180/n)*r_sub,0])
                rotate(((n+1)%2)*180/n)
                    star_outline(n=n,r=r_sub+0.001, line=line, k=k_star);
    }
}



module solid_star(n=5, r=50, k=undef, height=undef)
// Make a solid star with n points and outer radius r
//    k is a skinniness parameter (0 to n/2), as defined in star
//      default value is (n-2)/2, which makes a slightly fat star
//      (try n/2 for a skinny star)
//    height is the height of the star, default is r/3
{
    k_star = k==undef? (n-2)/2: k;
    h = height==undef? r/3: height;

    linear_extrude(height=h, scale=0)
       star(n=n,k=k_star, r_outer=r);
}


module solid_star_of_stars(n=5, line=2, r=50)
{   
    small_r = 3*line;
    r_sub = inner_radius(r, n, (n-1)/2);
    outer_center= [(2*cos(180/n)+1)*r_sub-small_r,0];
    
    difference()
    {   union()
        {
            linear_extrude(line)
               star_of_stars(r=r, n=n, line=line);
            intersection()
            {   translate([0,0,0.0015]) cylinder(r=1.2*r, h=2*line, $fn=20);
                
                for (i=[0:n-1])
                {    rotate([0,0,i*360/n])
                        translate([r_sub,0,0])
                        {   linear_extrude(line) star(r_outer=3*line,n=n, k=(n-2)/2);
                            color("blue") translate([0,0,line])
                                solid_star(r=small_r, height=2*line, n=n, k=(n-2)/2);
                        }
                }
            }
            intersection()
            {   translate([0,0,0.001]) cylinder(r=1.2*r, h=2*line, $fn=20);
                
                for (i=[0:n-1])
                {   
                    rotate((2*i+1)*180/n)  translate(outer_center)
                     {  rotate(((n+1)%2)*180/n)
                        {   linear_extrude(line) star(r_outer=3*line,n=n, k=(n-2)/2);
                            color("red") translate([0,0,line])
                                solid_star(r=3*line, height=2*line, n=n, k=(n-2)/2);
                        }
                    }
                }
            }
        }
        
        for (i=[0:n-1])
        {   
            rotate((2*i+1)*180/n)  translate(outer_center)
               cylinder(d=line, h=5*line, center=true, $fn=30);
        }
    }
}

solid_star_of_stars(n=5);

Released on Thingiverse as https://www.thingiverse.com/thing:3805111

2019 August 10

More large pendants

I’ve previously posted about my 3D-printed stage jewelry: the 3D slugs , the diamond, and the chain of office, printed on my Monoprice Delta Mini printer using CC3D Silk Gold PLA filament.

I’ve done a couple more designs since then: two more large pendants that could be used with a chain of office.  These were designed for fairly fast printing, being fairly thin:

Flower pendant 1 has 12-fold symmetry (including mirror symmetries).

Flower pendant 2 has 16-fold symmetry, including mirror symmetries.

Both pendants were simple OpenSCAD code, as they consist of unions and intersections of spheres (cut to just the positive-z half-space, to get a flat back).

// Flower pendant 1
// 12-fold symmetry
// bumps in center
//
// License: Attribution-NonCommercial-ShareAlike (CC BY-NC-SA)

// Kevin Karplus
// 2019 Aug 1

module round_facet(r=15, h=5)
{
    $fa=2; $fn=60;
    intersection()
    {   cylinder(r=1.3*r, h=h);
        union()
        {
            difference()
            {   sphere(r=r);
                carve_r=1.8*r;
                rim_h = 0.4*h;
                raise = sqrt(carve_r*carve_r + rim_h*rim_h -r*r)+rim_h;
                translate([0,0,raise]) sphere(r=carve_r); 
            }
            inner_r=0.35*r;
            translate([0,0,h-inner_r]) sphere(r=inner_r);
        }
    }
}

n=6;
r=40;
for(i=[1:n])
{   tran=0.3*r;
    color(c=[i/n,0.1,(n-i)/n])
        translate(tran*[cos(360*i/n), sin(360*i/n),0])  
            round_facet(r=r-tran,h=0.3*(r-tran));
}
// Flower pendant 2
// 16-fold symmetry
//
// License: Attribution-NonCommercial-ShareAlike (CC BY-NC-SA)

// Kevin Karplus
// 2019 Aug 2

module round_facet(r=15, rim_h=2, carve_ratio=1.7)
{
    $fa=2; $fn=60;
    intersection()
    {   cylinder(r=1.3*r, h=rim_h*2);
        difference()
        {   sphere(r=r);
            carve_r=carve_ratio*r;
            raise = sqrt(carve_r*carve_r + rim_h*rim_h -r*r)+rim_h;
            translate([0,0,raise]) sphere(r=carve_r); 
        }
    }
}

module flower(petals=6, r=40, height_ratio=0.07, translate_ratio=0.4, carve_ratio=1.7)
{
    for(i=[1:petals])
    {   tran=translate_ratio*r;
        color(c=[i/petals,0.1,(petals-i)/petals])
            translate(tran*[cos(360*i/petals), sin(360*i/petals),0])  
                round_facet(r=r-tran,
                    rim_h=height_ratio*r, 
                    carve_ratio=carve_ratio);
    }
}

flower(petals=8, height_ratio=0.08);

I have not released these designs on Thingiverse, because the site keeps being unresponsive when I try to upload new designs. I realize that I shouldn’t complain about a free service, but I’m about ready to give up on Thingiverse. Is there a better 3d-printing sharing site?

Update 2019 Aug 10: Thingiverse finally let me upload as https://www.thingiverse.com/thing:3802142 and https://www.thingiverse.com/thing:3802138.

2019 July 31

Thirty-eighth weight progress report

Filed under: Uncategorized — gasstationwithoutpumps @ 16:54
Tags: , , , , , , , ,

This post is yet another weight progress report, continuing the previous one, part of a long series since I started in January 2015.

My weight has been about the same as a year ago, but I hope that I don’t put on as much weight as I did last summer and fall.

My exercise level has been a bit low, averaging 3.4 miles a day of cycling in July.  This is a little misleading, as I have started doing some weight training, treadmill running, and elliptical machines at the “Wellness Center” on campus, though not as frequently as I should.  It also does not include the walking I have been doing, which is hard to keep track of.

When I was young and skinny, I did not realize how difficult it was to lose weight once one put it on—in those days my problem was trying to increase my weight, to keep from looking like a skeleton.

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