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Musings on a new design


#1

So I’m really trying my hardest to take a proper break to rest and recuperate, but I can’t stop thinking about Maslow stuff so I want to write out what I’ve been thinking so that I can 1) not have to remember it all 2) get community feedback 3) put my ideas in the public domain 4) maybe someone else wants to make them so I can just but the kit :stuck_out_tongue:

There are four main weakness with the current design that I would like to improve. They are:

Ease of calibration - this is a big stumbling block for a lot of folks and needs to be made easier to get widespread adoption
Accuracy in the extremities - Especially in the lower corners we loose accuracy
Cutting speed - To compete head to head with gantry machienes we need to be faster
Productizable - I would love to see companies take the idea and make a product out of it, but for that to happen it would need to be small enough to ship and fit on a store shelf

What I am thinking to meet these goals is a design which uses four kevlar (or some other low stretch material) cables instead if the chains and which has all of the electronics and motors mounted to the sled. Cables need to be used instead of chains because if the motors are on the sled we need to store the extra in a compact area. Also the chain has proved to be a hassle.

This type of design would simplify calibration by making it possible to calibrate with a single button press. By pulling each of the pairs of cables taught one after the other we could measure the distance between each of the four attachment points. This would give us enough information to fully calibrate. It would be nice to have current readings from the motors rather than stalling the motor as we do now.

This type of design would improve the accuracy in the extremities because in the lower corners we could provide the extra tensioning needed to pull the long cable tight and the sled would be lighter without the bricks allowing us to go higher between the motors.

This type of design could cut faster because we are currently limited by the force provided by the weight of the bricks and gravity. If we control all of the forces we can apply more force in the direction we need to cut.

This type of design would also be more productizable because all of the parts come attached to the sled and could be purchased as a built unit. The only external thing which would be needed would be four solid attachment points. This design would also work in fully vertical or horizontal modes.

The first big challenge to building this design would be how to accurately measure the length of the cables. Chain on a spocket moves a fixed distance per rotation while cable going onto a spool winds in more or less cable with each rotation depending on how the cable is stacking. This rules out our current method of using an encoder on the back side of the gearbox which is a significant loss.

To make up for that we would need a very high resolution encoder on the other side (8000+ steps/revolution) and we need a way to relate it’s rotations to the cable movements. I would like to explore using pinch rollers which roll as the cable moves in and out. We would need to verify that they won’t slip; ie if the cable moves in and out a lot of times a mark made on the roller and the cable will still line up.

If that looks ok we need to find a way to read the position of the roller. A larger roller will be less likely to slip and more reliable in general, but a larger roller would require a higher resolution encoder. I’ve played around with the magnetic AMS encoders and they aren’t linear enough for this type of work, the other ones I’ve seen are too expensive.

What I would like to test first is a system to use the sensor out of an optical mouse to read the movement of the roller. Optical mouse sensors are cheap and easily available and can have very high resolution. Unfortunately they have a problem with drifting over time. To solve that I would like to color code the roller with black and white striped to give us a concrete reference for position. Reading the black and white stripes gives us a low resolution position (~10 steps/inch) which won’t drift while the optical sensor gives a very high resolution position (~8000+ steps/inch) which drifts, but which can be corrected by the measurements from the stripes.

Other than that some more thoughts I have are the possibility of using an angled track to have the bearings run on like this:

And standardizing on the DeWalt palm router which is cheap and available in 120v and 240v versions.

With this type of z-axis:

I think that the zaxis motor could potentially be mounted to the same ring that the bearings run along.

There would also need to be stops on the cables such that when the cables are pulled all the way in for storage they can be “zeroed”.

What does everyone think? Glaring flaws? Things we should test early to see if the idea is viable? Potential materials for the cables? Thanks for reading all the way to the bottom!


#2

This is the kind of cable I’m think of:

I would need to test how little it truly stretches

Here is a stronger version:


#3

Something like this is what I am thinking for the pinch roller

If it seems like the idea was going to work we would have to have them made with a pattern on them but that could be done


#4

This is the type of chip I am thinking we read the roller with


#5

This thread is relevant:


#6

You don’t need a very big spool to spool up 9 feet of cable/rope on top of itself (it’s ~9 feet from one corner to the other). I was/am working on a design very similar to what you are talking about, but it had a spool that had a width equal to the cable/rope so that the cable/rope stacked on top of itself. I was going to use metal wire rope and assumed there would be no crush when it was wrapped. The spool would start at about 3-inch diameter and max at around 5 inches. I was trying to find a different geared motor to handle the larger diameter spool but also thought a simple chain drive arrangement of say 4:1 could work and the original motors could be reused. I’d plan to then use some math to figure out how much cable is fed out as the motor turns (I’m sure this part could be worked out). I was planning on fixing the location of the spools, but allowing them to swivel along the midpoint of the spool.


#7

This was my idea for the spool. The funky looking inner part is designed so the 1/8-inch cable terminates up against that jog and then wraps ccw. It’s all theory though.
image


#8

@bar I’m so excited

image

(edit) Gotta add @madgrizzle too!!


#9

Do you have an idea on how the rope would ‘exit’ the sled? Would it be safe to say that when the cable exists the pinch rollers, its on its way to the attachment point? If so, do the pinch rollers ‘swivel’ at all?


#10

Idea to solve the distance calculations.

Mount the thread spool on a threaded shaft so the thread is wrapped onto the spool as a spiral instead of overlapping. With that method a simple encoder on the spool shaft would be sufficient to calculate distance.


#11

I think that the cable might need to pass through an opening after the pinch rollers so that we can retract the cable fully to a hard stop to establish a zero length point. I am imagining that the rollers themselves are fixed but because they are attached to a carriage which can swivel around the router the cable can move to always point to the router bit


#12

I’ve read a little about the kevlar line and I am worried about the “low stretch” because “low” is not zero. I was once looking at heavy-test fishing line (e.g., spectra) as an option because it’s reported as “zero stretch” (though I know “zero stretch” is not zero stretch) but the memory of all the times as a kid I went fishing and the line snapping sort of scared me away from it and back to using metal rope. Metal wire rope isn’t zero stretch either, but my plan was to calibrate all the variability (cable stretch, quadrilateral kinematics, etc.) out using the optical calibration method. I think if you use a synthetic rope of any kind, you will have to deal with rope stretch somehow… hopefully it will be uniform and linear because, if so, just a simple multiplier would be needed to accommodate it.


#13


A simple way to have four motors rotate freely around a round sled would be to simply have two linkage kits. One for top pair or motors and one for bottom pair of motors.
the actual motors and encloders can be mounted directly to the center of the linkage arms
And since all the linkage arms would be the same,it would be realtaively cheap to get them laser cut.
crude sketch showing basic dimensions


#14

https://www.loosco.com/resource-library/technical-information/cable-wire-rope-stretch/
above website has wire stretch calculator, Using 1/8" dia wire cable the stretch is pretty small, not sure what diameter cable would be best. I would imagine the smaller the better 1/16 would probabaly be nicer to handle.


#15

re: synthetic rope, it is much easier to spool, doesn’t have a memory, is easier to ship, inspect, and terminate compared to wire rope

also, I would gather that 1/4" kevlar is 4-5x or more strength than is needed.


#16

For the cables (rope) I would recommend using a synthetic fiber rope similar to Amsteel Blue from Samson Rope:

https://samsonrope.com/commercial-fishing/amsteel--blue

This rope is available in different sizes including 1/8th-inch diameter, which has a breaking strength of 2500 lbs! AmSteel synthetic rope is at least as strong as steel cable of the same diameter. It is light weight, and has virtually no stretch.

I work on tug boats, and we use a 3-1/4" diameter version of the same rope that has a breaking strength of over 900,000 lbs! We also use smaller diameter versions of Dyneema fiber ropes for mooring lines, and for other jobs. Amsteel blue and similar synthetic fiber ropes from Samson (and others) are more expensive than the Kevlar rope you linked to, but will stand up to any abuse that a Maslow could throw at it. I realize that price does not equal quality, but I would be a little nervous trusting the safety of my machine to some cheap rope from Amazon.


#17

Does the cable need to stay stored on the roller where the distance let out/in is measured? If it came on enough to grip, and then was led off to a secondary storage spool, then the primary spool diameter could stay constant?


#18

Any idea what the minimum bend radius is on it (i.e., minimum recommended spool diameter)? I’m not a fisherman so I have no clue about what is used.


#19

If you are contemplating a modified ring kit (for full circumference), I’d love to see a full captivated carriage. I can’t imagine the current carriage with the two bearings on one side being easy to deal with if you have motors, spools, etc. also attached to it. I think you could easily build the ring with a series of standoffs coming up from the sled rather than using the L-brackets like the current ring does.


#20

That is what I think too. With a steel linkage kit, it would be easy too make the linkages thicker or wider to bolt on a motor and spool.