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• Designing the Body

  agp.cooper • 07/27/2026 at 09:58 • 0 comments

  Designing The Body

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  I have started designing the body. First thoughts is a bumper bar:

  It serves several functions:

  • A floor to the battery and micro-controller board.
  • Protection for the legs (they look rather fragile).
  • A sensor platform.
  • A structure to increase rigdity.

  I think I will need to extent the motor plate forward and aft, to reinforce the bumper bar floor.

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  I have selected 2 mm thick PTFE M3 washers.

  Bought the rods and pins.

  Bought the Nema 14 round steppers.

  A couple for the motor shaft (5 mm) to the drive shaft (3 mm).

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  I looked at buying M3 collars but after a long consideration, stayed with gluing the "fixed" rods/collar to the shafts.

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  While there are commercially  available stepper drivers, a long long time ago I built discrete drivers based on TTL logic:

  I seem to remember that I added protection diodes between ground and the output, and the output to the power supply.

  The input transistor was not used but looks like a good idea.

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  For the micro-processor I feel rather retro. As in the vein of "How to build your own working robot pet." by Frank DaCosta. A book I repurchased after more than 40 years. So I am looking at the Intel 8085.
  

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  When I look at the bumper bar, I could add a nose and it would look like a dog from above!

  That is the bumper bar looks like a head and a set of ears, all it needs is a snout and a nose,

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  I started this project on the 28th of June so tomorrow is one month.

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  Its been a few days with some success and failures to report:

  • Good progress on refactoring the code, getting slot and tab working:

  If you wondering, the big holes in the carriage torsion box,  are used to get access to the stepper motors.

  - 

  Balancing the Rotating Mechanisms

  I looked at a balancing the crank:

  The double thickness counter balance matches weight and centre of mass so should work okay.

  -

  Here is my first pass counter balance design:

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  Recalculated the counter balance weight (as best I could):

  Made the counter weight balance small as practical, but no allowance for reciprocating parts. 

  Usually an allowance of 50% to 90% of the moment of these parts is made (for internal combustion engines).

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  Bugs in OpenSCAD

  Bug in OpenSCAD are silent (but Syntax errors are noisy).

  -

  To implement Slot and Tabs, I wrote Function Slots(). Easy enough, create some slots, translate them to the edge in question and take the difference:

      // Add Slots for Bulkhead
      mirrorCopy([1,0,0])
      translate([cgap/2-5,0,0])
      rotate([0,0,90])
      slots(wgap,plateThick,3);

  -

  Worked fine until it does not!

  Spent a day working through this, the answer was a vertical version of slots to avoid the rotation: 

  -

  Slotting

  The slotting code work with odd and even matching slots or tabs:

  // Make Slots for edge joins
  module xslots(l,d,n) {
    let(m=n%2)
    let(w=l/(2*n+2*m-1))
    for(i=[1-n:2:n-1+0.001])
    translate([i*w,0,0])
    cube([w+0.01,d+0.02,3*d],center=true);
  }
  
  module yslots(l,d,n) {
    let(m=n%2)
    let(w=l/(2*n+2*m-1))
    for(j=[1-n:2:n-1+0.001])
    translate([0,j*w,0])
    cube([d+0.02,w+0.01,3*d],center=true);
  }
  

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  For example, adding slots and tabs using  tab = 3 and slots  = 4, to bulkheads:

    // Add Bulkheads
    color("Red") mirrorCopy([1,0,0]) { 
      translate([cgap/2-5,0,0])
      rotate([0,90,0])
      difference() { 
        cube([wgap,mgap,plateThick],center=true);
  
        // Add Tabs on Sides
        mirrorCopy([1,0,0]) 
        translate([wgap/2-plateThick/2,0,0])
        yslots(mgap,plateThick,4); // 4 slots and 3 Tabs
  
        // Add Tabs on Top and Bottom
        mirrorCopy([0,1,0]) 
        translate([0,mgap/2-plateThick/2,0])
        xslots(wgap,plateThick,4); // 4 slots 3 and Tabs
      }
    }

   -

  First the bulkheads with edges oversized:

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  First set of slots cutout:

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  Second set of cutouts:

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  Other cutouts:

  -

  So the trick here was to use n=3 for tabs and n=4 for slots, keeping d=depth and l=length the same.

  Note: (1,2) and (3,4) and (5,6) etc, are matching sets. 

  Now some matching slots in the Top:

    color("Magenta") translate([...

  Read more »

• Building an OpenSCAD 3D Model and Animation

  agp.cooper • 07/22/2026 at 08:48 • 0 comments

  Learning OpenSCAD (work in progess)

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  Using the WikiBooks OpenSCAD User Manual Strandbeest (https://en_wikibooks.org/wiki/OpenSCAD_User_Manual/Example/Strandbeest), as a base, I reworked the code (it has lots of bugs) for a 4 Bar and add

  3D offsets.

  It is still a work in progress:

  -

  Here is a 3D GIF of a three leg walker (one side):

  -

  Removed the cogs and replaced with a side crank. Only one of the axles need to be driven:

  -

  Next is to add spacers or washer between moving parts:

  -

  Notes:

  • The two colours Red and Yellow are inference fits to the rods. The other as sliding fits.
  • The spaces are 25% of the plate thickness but this is not that important.
  • The design has been changed to a "side-rod" drive. The two driven shafts are now centrally located.
  • Now I need to design the motor mounts and the internal cargo space.

  -

  Update:

  • Refactored the code to make creation more systematic and easier to debug.
  • Replaced most of the individual spacers and shim code with a function and a list.
  • Added cross-rod reinforcements and motor mounts (had to make the body a little longer).
  • To coupling the motors to the drive rod, I will use a flexible tube.

  -

  Refactored the code and removed hacked code. Now able to rebuild with parameters except for the stepper motor. Cool to be able to shop online for some Teflon washers and adjust the walker for the washer thickness that was available. 

  To do:

  • Controller board mounts.
  • Battery mounts.
  • Sensor mounts.
  • And more.

  AlanX

• Optimising the 4 Bar Linkage

  agp.cooper • 07/22/2026 at 05:44 • 0 comments

  Optimising What Exactly?

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  It is not easy to optimise most problems have:

  • multiple local optima
  • multiple objectives
  • objective functions may result in degenerative (useless) solutions
  • constraint functions may result in degenerative (useless) solutions
  • the need to start with a feasible solution

  The process involves mapping out the feasibility areas and then focusing in on the most important objectives that result in the best solution.

  -

  Finding an initial feasible solution

  Fortunately we can start with DIY Walkers solution (but at half scale):

  • Axle: (0.0 mm,0.0 mm)
  • Hip:  (-48.0 mm,33.0 mm)
  • Crank L1: 30.0 mm
  • Thigh L2: 60.0 mm
  • Idle    L3: 40.0 mm
  • Shin   L4: 70.0 mm
  • Knee Angle: 90.0 degrees

  -

  Here the foot trace down range is 120 degrees between the green markers and 180 degrees inclusing the green markers:  

  Note how the foot trace slow down near each left/right extremes, and is very fast when the foot up.

  -

  Extending the Flat Bottom Range

  The first major objective identified was the need to have a flat bottom (as much as practical):

  While this objective has been meet for 120 degree range (suitable for a three leg walker). How ever foot lift has been lost.

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  A Symmetry Constraint

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  For symmetry, the difference between each Y point at each horizontal extreme is minimise, using:

  • DeltaY = ABS(Yfwd-Ybwd)
  • Minimum mid-section upper and lower range of 10.0 mm

  Here is the result:

  --

  Using this as an initial feasible solution, we can impose a flat bottom:  

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  The final walker parameter are:

  • Axle: (0.0 mm,0.0 mm)
  • Hip:  (-53.1 mm,52.5 mm)
  • Crank L1: 30.0 mm
  • Thigh L2: 60.0 mm
  • Idle    L3: 59.3 mm
  • Shin   L4: 60.0 mm
  • Knee Angle: 90.0 degrees

  Reducing the minimum mid-section upper and lower range to 5 mm for the final result:

  Note the flat bottom has been optimised for 120 degrees
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  The same design as imported into a CAD package:

  Note the cyan bottom range is for 180 degrees.

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  This is what I see after my optimiser runs:

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  Clearly the 4 bar linkage can be used for a three foot (120 degree) walker, but its use would be best for relatively smooth floors as the minimum clearance is about 5.0 mm (for a 94 mm step).

  The cyan vertical range (for 180 degrees) is about 3.5 mm, suggesting a two foot (rather than three foot) walker is possible, but perhaps some what bumpy ride.

  AlanX

• Modelling the 4 Bar Linkage

  agp.cooper • 07/22/2026 at 05:06 • 0 comments

  Modelling the 4 Bar Linkage

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  It is convenient to build the initial model in a spreadsheet. The results can be displayed as a graph:

  -

  First steps would be to start at the Axle (0,0) on the black base plate and calculate the red crank position:

  X1 = L1*COS(RADIANS(A0))+X0

  Y1 = L1*SIN(RADIANS(A0))+Y0

  Where:

  • (X0,Y0) is the Axle position
  • A0 is the rotation angle (clockwise from the X axis)
  • L1 is the Thigh length
  • (X1,Y1) is the Crank position

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  Next is the find the intersection of two circles from the Crank and the Hip:

  -

  There are generally two solutions and the correct one needs to be selected. If the order of the parameters are consistent, then the selected solution will be consistent.

  -

  One method to solve this problem follows:

  Source: https://math stackexchange.com/questions/256100/how-can-i-find-the-points-at-which-two-circles-intersect

  -

  Here are the Circle Intersection Calculations:

  CX1 = Crank X

  CY1 = Crank Y

  R1 = Thigh length (L2)

  CX2 = Hip X

  CY2 = Hip Y

  R2 = Idler Length (L3)

  D = SQRT((CX1-CX2)^2+(CY1-CY2)^2)

  L = (R1^2-R2^2+D^2)/2/D

  H = SQRT(R1^2-L^2)

  IX1 = L/D*(CX2-CX1)+H/D*(CY2-CY1)+CX1

  IY1 = L/D*(CY2-CY1)-H/D*(CX2-CX1)+CY1

  IX2 = L/D*(CX2-CX1)-H/D*(CY2-CY1)+CX1

  IY2 = L/D*(CY2-CY1)+H/D*(CX2-CX1)+CY1

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  The two solutions are:

  1. (IX1,IY1)
  2. (IX2,IY2)

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  For the way I ordered my circles, the solution 2 (IX2,IY2) was chosen for the Knee position:

  X2 = IX2

  Y2 = IX2

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  Finally the Foot has to be extended (L4) from the Knee at 90 degrees toward the Floor:

  Thigh Angle: A = DEGREES(ATAN2(X2-X1,Y2-Y1))

  Foot X: X3 = L4*COS(RADIANS(A+90))+X2

  Foot Y: Y3 = L4*SIN(RADIANS(A+90))+Y2

  Note: Spreadsheets use atan2(Cos,Sin) while C code uses atan2(Sin,Cos).

  -

  Having satisfied myself that I the equations are working, I can code the model in C code for optimisation.

  AlanX

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