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• Talk through design iterations

  David Brown • 10/13/2017 at 17:58 • 0 comments

  As I needed to put together a video, which ended being longer than required so I'm posting it here. This is a talk through of the different design iterations and how I ended up with the current design, including comment of limitations and areas that could do with further work.
  

  I do think the original concept could work but requires the right size stepper motor with integral lead screw to be available.
  

• Capability testing

  David Brown • 10/06/2017 at 16:37 • 0 comments

  I've now had a chance to run some coarse testing of the device. This was done by mounting a laser pointer directly to the moving platform, in the case of measuring the rotation angles. And then measuring the distance of deflection of the laser dot on a wall 3m away. In the case of measuring translation of the platform, the pointer was supported at one end so that, when the platform translated the pointer was rotated and the beam moved.

  Measuring the translation showed a good match when moving 0.05mm for both UZ and UX (UY assumed to be equal to UX). For the rotations there was more variability but this is likely to be a function of the weight and inertia of the laser pointer with its x2 AAA batteries.

  The test results PDF can be found in the files section. Overall I'm pleased with the capability, and i'm sure that the design can be taken further with a better choice of springs for the legs; slightly stiffer, with a varying diameter (wider in the mid-span) so that they are less likely to buckle in compression.
  

  Figure; Laser pointer mounted to the platform (test rotations)
  

  Figure; one end of the laser pointer supported (test translations)
  

• Prototype taking shape

  David Brown • 09/03/2017 at 21:27 • 0 comments

  So its been a while since I last posted, some of that was waiting on parts to arrive, such as the SLS printed nylon gears and lever arms. And some of it learning how to use a 3D printer and get it setup correctly, along with other sub-projects.
  

  The print of the frame took 15 hours and i was pleased with the quality, given that this was using the $150 printer with all stock nozzle, etc. I'll post up the models for the parts shortly. More after break below;
  

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• Iteration A4

  David Brown • 08/08/2017 at 20:23 • 3 comments

  So after a few iterations of re-design to work with NEMA-8 sized stepper motors which whilst not the smallest available are the smallest in the sub $20 bracket. In this case $12ea.

  With the increased size comes the headach of deciding on an arrangment to pack them as close together whislt having a viable mechanism to allow the small dislacement and high resolution with minimal backlash possible. More after the break below...
  

  units = mm
  

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• On to itteration A2

  David Brown • 08/02/2017 at 15:16 • 0 comments

  So, time for some more updates;  

  I recieved the most recent printed parts from shapeways and re-built the test actuator.
  

  The housing for the lead-screw nut had some dimensional issues as did one set of the actuator bodies. It appears that due to the dimensions of the parts when the print starts or finishes ( I can't tell which) there is significant shrinkage in the initial layer or after cooling that leads to warp/dishing of one of the faces as can be seen;
  

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• rev2 actuator body

  David Brown • 07/14/2017 at 15:47 • 0 comments

  So the initial build of an actuator has allowed me to get a better understanding of the behaviour of the SLS nylon 3D prints and the overall behaviour of the mechanism.

  • The HP multi jet fusion printed parts have much better resolution than the older 'strong and flexible' prints.
  • Need to minimise the moment generated by the stepper and the actuator pivot point, the motor needs moving as close to the pivot as possible. This also allows the nut to be better captured.
  • Slight change needed to allow the control board to be easily installed. The first revision peg has a fully enclosed hole for the resistor slider, as only the lateral force is need, the recess can be open vertically to allow the slider to be dropped into the recess.
  • Need to better control the moment about the peg to the body of the actuator, in the first revision a simple drop in cylinder was used as the pivot. Because of the print tolerances and the thickness of the body, there is a degree of movement that helps to bind the mechanism. The cyclinder has been replaced by using a brass insert in the peg and having a screw act as the pivot. This should provide better control of the movement perpendicular to the pivot axis.

  I've ordered the new geometry and it will be another few weeks wait for the parts to arrive. Meanwhile I need to sort out how and what the monofilament linkages will be attached and start looking at implementing the algorithm for controling a Gough Stewart mechanism. Images below the break

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• initial power-up

  David Brown • 07/12/2017 at 21:55 • 0 comments

  As you can see i've transfered the components to one of the HP fusion printed actuator bodies as these have better definition on the hinges, so require less force to move. The 'strong and flexible' (Shapeways SLS nylon) had too much load on the motor (hinge material printed out thicker). The video below is of the actuator moving back and forwards twice over. This required some adjustment (use of needle files), as the printed nylon has some variation from the design geometry.

  As you may be able to see the black 'peg' that couples the motor, resistor and actuator moves around a fair bit and there is obvious backlash from the motor nut to the peg. I need to investigate a new iteration for the peg geometry to take most of this out.

  Getting the motor aligned was also a tricky and required adjustment of the brass insert to get the angles close. This is fine for small numbers, but larger numbers will require a accurate jig for placement.
  

• Initial assembly

  David Brown • 07/11/2017 at 19:37 • 1 comment

  So the SLS nylon parts arrived from Shapeways and i've put together one actuator. unfortunatly the brass screw inserts which were supposed to be M1.2 turned out to be M1.5 so i managed to find x3 M1.5 screws in my bag of tiny screws from dismantled stuff. The others will have to wait another few weeks until the re-ordered inserts arrive, but at least i have one to run tests on, which I'm quite pleased about. More below the break....

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• Actuator components

  David Brown • 06/23/2017 at 15:20 • 0 comments

  So i've ordered the printed components from shapeways (SLS nylon) but the lead time is quite long, so will see how quick those parts come through.

  The linear variable resistors (ALPS RS08U11AZ001) have arrived - really small, see below;

  And i've ordered the boards that these mount to from OSH park. Kicad render below;

  This has been designed for both the LB1935FA and LB1848MC bipolar motor control ICs, as i've still got a few pieces of the now obselete LB1935FA. There is a single pin difference, so its a case of cutting one of the tracks, depending on which one is being used.
  

• Fatigue design in PA-12 / Nylon

  David Brown • 06/21/2017 at 01:05 • 1 comment

  The actuator body has now been designed for fatigue and to meet the requirements for Shapeways 'strong and flexible' sintered plastic (PA / polyamide / nylon 12).
  

  Figure: Final design for articulator body.

  The material data for this is available by searching for PA2200 material data sheet. The stress limits come from a paper I found [ref. High cycle fatigue properties of selective laser sintered parts in polyamide 12] the key findings of which are;
  

  • The building orientation of the SLS specimens has no influence on the fatigue properties, due to high percentage of fusion.
  • The fatigue life is significantly influenced by cyclic softening if the sample temperature reaches the glass transition region, leading to brittle fracture.
  • Where the print machine employs contour scanning the outer surface is of higher quality, damge to the surface by machining reduces fatigue life as cracks nucleate from unmolten powder particles and these are not present on the unmachined surface.

  The design followed an itterative process from the initial design, extending the length of the thin hinge material. Initially simply, but when I found the fatigue

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