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Prototype SCARA Mark II

My second SCARA project.
This time it will be twice as big and geared.

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The first SCARA prototype was direct drive, that is relying on the stepper motor bearing for the elbow. Okay, it works but the resolution is low and the arms oscillate. This time around I will be using a DIY slew ring and a 2mm GT2 belt and pulley. The slew ring was the subject of another project (https://hackaday.io/project/165560-a-25d-slew-ring).

The First SCARA Prototype

Here is my first prototype:

The arms are 100 mm and 60 mm resulting in a 2 to 1 working area of 160mm wide by 80 mm high.

And an example of the work (1/8 micro-stepping):

Note the "clipping" on the left of the image, this was deliberate to test the clipping routine.

Also the wavy lines are mix of stepper resolution and arm oscillation. Mostly arm oscillation because they die down towards the end of the "stroke".

The Second SCARA Prototye

The second prototype will be bigger (arms 200 mm and 150 mm) and geared (more torque and higher resolution).

I will be using 2 mm GT2 pulley and belts. I initially started with T5 pulleys and belts but they have too much backlash.

The key design problem for the SCARA is the elbow. With the prototype I relied the stepper motor for the elbow axle. For the geared version, I have designed an integrated pulley and slew ring (https://hackaday.io/project/165560-a-25d-slew-ring):

The slew ring uses a 6808 roller bearing. This bearing is 52 mm outside diameter, 40 mm inside diameter and 7 mm thick.

The SCARA Design

I have iterated through many SCARA designs.

Here a section of the current version for this project:

  • The base board and the first three layers are 12 mm MDF.

The following layers are likely to be 6 mm acrylic:

  • A double slew ring.
  • The the first arm (200 mm between centres). The arm has been stiffened with a vertical element (making a "T" section).
  • The single slew ring attaches the second arm (150 mm between centres).
  • A laser engraver will be attached to the end of the second arm.
  • The second arm is also stiffened with two vertical elements (making a "U" section).

Here are some plans:

The first arm (bottom of image) shows the stepper motor mounts and the belt tension adjustment system. The stiffening elements are slot and tab design, and will glued.

I am contemplating a "U" section design for the first arm. The rearrangement may result in a smaller arm with better balance.

Laser Cutter Layout

I have laid out the design for a laser cutter. The bearing pockets can be cut later with my 2.5D CNC machine:

The design is not quite ready, however for the laser cutter. I still have to design the base board.

AlanX

ezx_graphics.pdf

The ezx graphics package

Adobe Portable Document Format - 92.72 kB - 07/12/2020 at 06:56

Preview
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ezxdisp.h

The ezx graphics package

x-chdr - 4.33 kB - 07/12/2020 at 06:56

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libezx.a

The ezx graphics package

x-archive - 28.44 kB - 07/12/2020 at 06:56

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Test.c

The Kinematic Test Program

x-csrc - 7.92 kB - 07/12/2020 at 06:55

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3mmLaserBoardSmall5BarSCARA.dxf

The design is ready for laser cutting

AutoCAD DXF - 1.50 MB - 07/12/2020 at 06:52

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  • Design Sent for Laser Cutting

    agp.cooper3 days ago 0 comments

    Laser Cut Design

    Sent off the design to get cut in 3 mm Acrylic. I have some acrylic glue (have not used this stuff before). The fumes are known to be carcinogenic!

    Here is the final work area:

    For homing, I have put 3 mm diameter holes in the far arm and in the base board to lock the home position.

    I have use an "I" section for the arms. Not that worried about twist with this design. The arms are offset vertically so that the I beams can "nest". I have include 1.2 mm diameter, alignment holes for the GT2 pulley:

    Note: The green outline is just to show the acrylic outline.

    Hopefully I have made no serious mistakes.

    Ah, I just thought of something, I have not check if the laser module will fit in-between the arm webs!

    Alan

  • Adding an Offset Laser Engraver

    agp.cooper06/29/2020 at 03:35 0 comments

    An Offset Laser Engraver

    Mathematically it is just a polar adjustment (i.e. a radius from the right elbow with a rotation. For my design:

    • LL=207.020 mm
    • LA=-4.671 degrees (i.e. on the right)

    Mathematical Instability

    Before the transformation:

    After the transformation:

    Note the mathematical instability around the right arm pivot. It is due attempts to solve near zero angles with the Cosine Rule. It is not an issue as i is not inside the design operational envelop.

    Other Transformation Problems

    Here there are some transformations into the minus Y area:

    If I lift the minimum Y to ~36 then "minus Y area" transformation problems are gone (other than the bottom right "tick"):

    So now I have a stable mathematical model (if restricted to the working area above Y=~36).

    Update

    I was not that satisfied with the above solution. This solution looks better:

    I suppose it is time to look at the base board. Okay here is the base board:

    Too hard to solve the equations analytically so I just exported the "edge" points and fitted by eye the arcs. Then I fitted the largest possible 2:1 rectangle.

    Small Board Format

    Rather than use the full board, I am now looking at a small board format. The top edge of the board is the bottom edge of the work area:

    The small board allow 100mm behind the arm pivots.

    Homing

    The last SCARA I built used micro-switches to find the home position. It was not effective just to drive the arms into a stop position, as the SCARA was direct drive and used micro-stepping. It would just "bounce" off the stop position depending on the micro-step position. This time, it could be more effective if the stepper motor pulley is loosened and re-tightened,  then the home position is a power-on detent position.

    Stepper Motor Mounts

    Time to work on the stepper motor mounts.

    Added the work so far to the small board:

    The design is far from done but it looking okay (note, the arms do not show any stiffening flanges).

    I have to check how far the near arms need to fold back:

    Yep, pretty extreme, the left arm: 251.60 (i.e. 71.60) degrees and the right arm:  -52.25 (i.e. 52.11) degrees. I have only made an allowance of 58.11 degrees.

    Actually, if I constrain the SCARA to the potential work area above Y=188.0 mm then the there is no problem (i.e. UL=38.02 degrees and UR=26.67 degrees):

    Alan

  • Bad 5 Bar!

    agp.cooper06/27/2020 at 02:10 0 comments

    Work Area

    I was looking at work area issues and like other SCARA types the optimum work area occurs when the arms are of equal length. No that important however to be exactly equal.

    Force Transmission

    For efficient force transmission I selected a minimum far arm angle of 45 to 135 degrees.

    This however is pretty inefficient with regard to work area. To improve the work area I adopted a square area rather than a 2:1 rectangle. Here is the result:

    Note: The arm lengths allow proper folding for "storage".

    The small work area pretty well explains why the this SCARA configuration is not in "common" use for router applications.

    Alternatives

    I have found a variant with a powered far arm:

    But this does not offer any real advantage over a convention (single arm) SCARA except it is more rigid (particularly for twist) and subject to less vibration due to the second arm.

    Aside, the above SCARA would benefit from increased work area if the spacing between the fixed arms was reduced. The bed lift mechanism should be replaced with a vertical bed lift mechanism. But I understand that the current lift mechanism can be fixed in software.

    The 30 degree 5 Bar SCARA

    Here is the 30 degree SCARA:

    Okay, I will have to proceed on the 30 degree SCARA basis.

    It all in software anyway, a laser could run at 15 degrees while a router could run at 45 degrees:

    And if I really have to I could power both far arms!

    Compromise

    I can use 45 degrees when out reached (need more force in this case) and 150 degrees (or more) when reaching inwards:

    Alan

  • The Mathematical Model

    agp.cooper06/26/2020 at 09:24 0 comments

    The Mathematical Model

    I am not going to go into the mathematics. It is high school maths (Cosine rule and solutions to quadratic equations). You just need to send time (i.e. a week) working through it.

    What is important is what it does:

    What you can see here is a test of the model:

    • The white dots are the provided (test) coordinates, and the blue dots are the coordinates that were plottable by the machine (given the machine specifications).
    • The red dots that outline of the blue dots are the machine physical limits.These red dots are a one to one transformations of the white dots.
    • The large red rectangle is the largest 2:1 work area available for the given machine specification.
    • The other red dots are for the soft constraint (i.e. the 30 to 150 degree far arm angle limit). These enclose work areas that have a degree of rigidity. Well at least if the 30 degree angle constraint is a good number to use.
    • The yellow line shows the movement of the extreme top right point to the soft constraint boundary, and the arm positions (green). Demonstrating that the mathematics appears to be working.
    • The only thing missing from this model test are the left and right elbow positions (next time!).

    What Next?

    How much force can be transmitted through the arms using a 30-150 degree far arm limit? Should I use a 45 to 135 degree far arm limit?

    What arm lengths should I use to optimise the soft constraint work area?

    What driver motor spacing is best? The closer the spacing, the more work area, but what about force transmission?

    Why Truncate Unplottable Points?

    Basically, he motion planner needs the transformed constrained points to navigate around these infeasible areas:

    Consider the above drawing, although I don't have to plot the constrained points at the top of the page, but I do need to move the machine head via those points towards to first plottable point.

    Alan

  • Abandoned and Restarted

    agp.cooper06/24/2020 at 09:36 0 comments

    Why I Stopped

    I stopped because I was waiting for 1.2mm diameter end mills for the gear wheel. As they take about six weeks from China fro delivery I went on to other things.

    I had moved on by the time the 1.2mm end mills arrived!

    I did have a look a the project a month or so ago, made some edits to the design, nothing exciting. The problem remained, the project was complicated and expensive.

    So I put it on hold.

    Another Approach

    While cruising the Internet, I came across the Maslow CNC project:

    Okay, it deserves a "WOW". Getting this technology to cut a 4ft x 8ft sheet of plywood is an achievement.

    Parallel or 5 Bar SCARA

    Okay, if they can get he Maslow CNC project working, I can get a parallel or 5 bar SCARA working (i.e. able to cut plywood). Here is an example of a % bar SCARA (http://fab.academany.org/2018/labs/fablabbottrop/group_week14_15.html):

    Why a 5 Bar SCARA?

    The main advantages of the 5 bar SCARA are:

    • Very simple construction.
    • The potential to use domestic hinges for bearings.
    • Heavy stepper motors are not on the arms.

    The main downside is:

    • Not a good design for the application of force.

    Possible solutions to the application of force:

    • Limit link angle to say 30 to 150 degrees (avoid near "straight" angles).
    • Use gearing.

    Other problems:

    • Need to use an offset laser or cutter if hinges are used.

    Here is a first drawing:

    And a close up of the motor gearing and hinges:

    Note: The gears are attached to the "near" arms.

    SCARA Mathematics

    Next is the Mathematics.


    Alan Cooper

  • Update

    agp.cooper06/07/2019 at 02:38 0 comments

    Update

    While I have been waiting for the end-mills, drill bits, 6mm acrylic sheets and acrylic glue to arrive, I have been working on other projects. The orders have arrived and I have finished the other projects.

    Where was I?

    Here is may last plan for 6 mm thick acrylic:

    My last Section:

    May last layout for my CNC mill:

    The layout shows two slew bearings while the section has three bearings. The prototype bearing was quite good so I don't think I need the second bearing on the tower.

    I also have not really considered the tower and the base board.

    CNC Mill Parameters

    • Restarts: Add origin and maximum extent realignment holes.
    • Job Size: One piece at a time.
    • Feed Speed: 1200 mm/min (up from 600 mm/min).
    • Depth Speed: 150 mm/min.
    • Drilling: Peck.
    • Over Depth: 0.5 mm (down from 1 mm)
    • Spindle RPM: As low at I feel comfortable with, about 3/4 on the dial.
    • End Mill: Flat two flute 2mm diameter (better if single flute).
    • Cut Depth: 0.25 mm (if all works well this can be increased to 0.5 mm).
    • Compensation: 1.8 mm cut width for nominal 2 mm bit diameter (may need to be reduced).
    • Use conventional and climb cutting directions where appropriate. 

    Base Board

    The working envelop needs to be 2:1, the key assumptions are:

    • Arm 1 length: 200 mm
    • Arm 2 length: 150 mm
    • Arm 2 rotation limits 10 to 140 degrees

    I calculated the following:

    Dy 173.735 Working Area Height
    Dx 347.469 Working Area Width
    L1 200.000 Arm 1 Length
    L2 150.000 Arm 2 Length
    A1Min -6.143 Calculated Motor 1 Range (Min)
    A1Max 115.600 Calculated Motor 1 Range (Max)
    A2Min 10.000 Motor 2 Range (Set Min)
    A2Max 140.000 Motor 2 Range (Set Max)
    Ox -173.735 Bottom Left Corner
    Oy 128.598 Bottom Left Corner

    Here is the base board layout including the tower location:

    It fits on a 400 mm x 400 mm board. This layout has a effectively wider feet spacing.

    The normal layout is still 400 mm x 400 mm:

    Tower Design

    TBC ...

    Limit Switches

    TBC ...

    AlanX

  • Working on the SCARA design

    agp.cooper05/25/2019 at 08:46 2 comments

    The SCARA Design

    I have spend a number of day on this but now the basic design seems "set":

    And:

    What is remaining?

    • The base board and tower.
    • The homing stops and switches.
    • And maybe a look at cabling.

    I am thinking of putting the controller on the first arm (to keep all the cables short, and only run the power and USB cables through tower and base board.

    The laser cutting service provider has published a recommended "kerf" of 0.21 mm for 5-8 mm acrylic. So the laser cut designs need to be adjusted for this (but not until I am ready to have the design made).

    At the moment the design fits on a 400 mm x 400 mm sheet:

    As I have said before, I will pocket the disks to fit the 6808 bearing with my 2.5D CNC mill.

    Another day and some more edits. Dropped the 1.25 mm alignment holes, I have some 3 mm pins so I can use the M3 bolt holes. Received the 2 mm end-mill bits and the nylon washers. Still waiting on the 1.3 mm drill bits. Moved the design less the pulleys to a 2.5D layout for my mill:


    Ordered two A3 6mm acrylic sheets (to suit the 2.5D mill) and a tube of acrylic cement.

    I will use the 400 mm x 400 mm x 8 mm Lexan sheet as a base board.

    AlanX

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