Robotic kinematics
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This post is deleted! -
how's the work going? I had read the post you deleted, I am waiting to see your robot, you say that it is very stable I could take it slow and build it if you release the construction schemes and reduction ratios. I have not tried to build anything even if I would like to try to see how the firmware behaves, I prefer to wait for your answers!
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@tony73 I did delete the post, because I didn't want to tell you what to do. You should decide yourself. (and as I said, you'll get all the information I have. I learned a lot about manufacturing the last days also)
I have new design problems, the arms were too heavy for the steppers. But I have solved it with counterweights, so it's proceeding. The printer is in fact very stiff, I expect good print results. Current status is, I have to redesign actuator 2 (it was in the way of the arm 2 movement ;-( ).
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ok! I try to contact you in a few days!
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@tony73 I have a prototype now, not ideal, but working to validate the firmware. (The M114 was not wrong)
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@JoergS5
did you make a prototype with 5 actuators plus an extruder for 3d printing? -
@tony73 5 actuators yes, extruder not yet*). I have a weakness in specifications: when arm 5 rotates to the left (180 degree eg), arm 3 can be in the way for the hotend+extruder if arm 3 is "flat".
*) my robot has a very low payload. I still have to learn a lot about how to optimize gear ratios, counterweight and belt tension, to get a good solution. This is quite different from developing the firmware!
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I have some images of my first pre-Prototype, before I will reconstruct it:
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The first idea was to place Axis 1 behind the construction. This seemed to be a good idea because the axis is fixed at two points top and down and can be calibrated easily. But the disadvantages are, I have no good place to install actuator 2 and the counterweights have not much room also.
Arm 2 and 3 are 27 cm and 20 cm long. For production the same length will be better, but I differed them to test firmware.
The gears are insufficient. One stepper has a 1:27 gear, but the problem is the little 60 teeth pulley which is not sufficient. I'll produce 200 teeth pulleys as a next test now. The belt tensioning is very important also, because otherwise the belts slip very often.
Arms 2 and 3 have 4 arm elements 20x10x2, connected horizontally by 100x20x2. The placement of the connectors are important, they define the possible axis angles.
The upper and lower arm construction is 18 cm distance and makes sure the arm 4 is vertical at all times, so axis 5 is vertical also. I will remain this construction, but axis 5 supported at two hinges and with a gear.
The actuator 4 on the third image is a dummy to check the firmware code for R0.
I will try to avoid counterweights, because they support wavering of the construction.
The hinges are constructed by 8 mm aluminium or steel shafts, F688 ball bearings with 16 mm holes and the 8 mm holes for the arms which are fixed to the shafts. Distances will be implemented by 8x12x0.5 washers.
The arm connected pulleys are connected with screws to the arms, so they cannot slip when only connected to the shafts.
One thing to be aware is, if turning off or installing new firmware, the steppers loose current and the robot arms fall down...
The rebuild to the prototype will have axis 1 below the robot on a plate, bigger pulleys and a better belt tensioning system.
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Hello ! I'm sorry but yesterday I couldn't write to you, I saw the photos, to understand better, are you using this robot with some modifications, or do you have another one? however you have already worked so hard to do this!
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@tony73 I constantly change this one. I am currently redesigning axis 1 and 2 and making the axis pulleys bigger. Then I will be able to install the extruder and print.
The most work are the constant iterations, especially assembling and disassembling. I wished I have a robot who assembles it for me.... I could spare some effort with a good design program, but to understand some of the principles, I have to experiment.
Currently important aspects to build the robot are:
- hinges and arms without play and wobble
- maximizing possible angles
- find optimal gear ratio and avoid belt slippage
- balancing the arms, avoiding counterweights by replacing the steppers (the steppers function as counterweights)
- a procedure to protect against power off and arms falling down
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from the photos it seems to me that you have steppers with planetary gear motor 1:27 primary pulley how many teeth? secondary 60 teeth, the steppers are cut to what, full pitch, 1/2, 1/4, 1/8, 1/16, 1/32, etc .... the gearmotor how much size at birth? I would like to understand how much reduction is needed! since you would like to put 200 tooth pulleys instead of 60 tooth ones.
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@tony73 the gears were only for testing (and at axis 2 I had space problems), I want to avoid them because they have backlash. I will use a two or three step belt gear or a self constructed harmonic drive (axis 5). A 20/60 and 20:200 will result in 1:30.
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@JoergS5
60 ÷ 20 × 200 ÷ 20 = 30 reduction 1: 30 right? do you want to eliminate cycloidal motor reducers, are they the ones that have play? have you already solved the problems of movement on x coordinates? -
@tony73 yes this is my calculation, I hope 1:10 at the last stage is enough, I will have to test it*). I had to stop firmware test, but the X values of M114 were ok, they were the world coordinates (mixing: xyz = world coordinates and UV = angles). There may be the bug: comparing X world coordinates with M208 angle of X.
*) I calculated the Nm of the stepper, this was enough torque for a 60 tooth. But the belt slipped and I had to use a lot of tension to avoid it. It has an effect on wobble also, depending on acceleration/speed, weight and gear ratios.
I am currently working on Axis 1 and will publish images when finished. I like cross roller bearings, but they are very expensive at the 300 mm size (hundreds of bucks), so I am building a DIY-able substitute.
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@JoergS5
can you tell me in the config.g in M350 which microstepping you put? -
@tony73 I use 16 I1 in most cases, because for Duet 2 it was the only mode to support 256 microsteps by the controller. Duet 3 supports more modes for I1, but I stayed with 16.
You have to correct M92 accordingly: as example, a 200 step stepper, 16 microsteps and gear ratio 1:30:
200*16 steps/360 degree * 30 = 266.66667
In documentation it's called steps/mm, but in case of robots it is steps/degree. -
@JoergS5
M92 200 * 16/360 * 30 the result and 266,6666666667 this would be steps / mm! and how would the result be for robots? same ? but with the difference that they are steps / degree, and not steps / mm? or does the result change? -
@tony73 the M92 means steps/degree in case of robot kinematics. There is no mm movement of the actuators, because they rotate
(with the exception of the rail, which is in mm) .This is also true in the firmware, where the parameters stepsPerMm in functions CartesianToMotorSteps and MotorStepsToCartesian are correctly stepsPerDegree.
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@JoergS5
Hello! I have never understood the difference between interpolated and non-interpolated stepper motors! I don't understand the concept well, could you explain me with simple words, or if you have a link where they explain the function?
