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Tensioning system to improve performance in bottom corners

chain
tension

#175
  1. So my question from an earlier post remains…could you build a gantry style system (von Neumann-like using the Maslow) and then install the Maslow hardware to operate that.

  2. Could you increase the weight on the sled, but use a pulley like system (block and tackle) to reduce the forces at the motor. According to my memories of 8th grade physics classes (some 30 years ago) the increased forces used to tension the chain more could then be halved at each pulley. No?


#176

We have talked a lot about a gantry system. It would be possible, and the weight wouldn’t really matter. You can use counter weights or spring to lighten the system. You would have better accuracy, and better z axis control. Enough z axis control to start engraving. However, the challenge is cost. To keep it under 500 dollars, this is the way to go. Search the forums and you can find where we even found plans for an upright gantry style cnc. I’ll help design it with on shape if that is something you want to try.


#177

my problem is not the mechanical hardware but the software. Also, I think that, if you make the gantry out of plywood, the additional cost for the rollers etc should not add much more than $200 to $300…but then again I have not even designed a single edge yet of his idea :wink:


#178
  1. So my question from an earlier post remains…could you build a gantry
    style system (von Neumann-like using the Maslow) and then install the Maslow
    hardware to operate that.

not without re-writing the software to work with that style hardware

  1. Could you increase the weight on the sled, but use a pulley like system
    (block and tackle) to reduce the forces at the motor. According to my memories
    of 8th grade physics classes (some 30 years ago) the increased forces used to
    tension the chain more could then be halved at each pulley. No?

you could, and it would run even slower. diagram out how the chains and pulley’s
would be arranged, I think you will find that you are adding a lot of complexity
without solving any fundamental problems.

you cannot eliminate chain sag, no matter how much weight you put on the sled.

David Lang


#179

David,

Thank you for being patient with me. As to:

I was pretty sure that would be the case…just not sure how much effort that would be. I just had a rep from IGUS (www.igus.com) here at work talking about their polymer based, self lubricating linear guides,etc…some pretty low cost neat stuff.

  1. I will just have to wait until I get my Maslow and then see how it works out for me. And if the are of cutting is not the whole 8’ by 4’…oh well…I will just move my stock to the sweet spots.

Thanks for all the hard work

Aymeric


#180

close to a complete re-write, you would probably be better of working to modify
grbl to handle DC motors and encoders.

David Lang


#181

soo…
Am I beating a dead horse if I’m trying to make a linear sled system that can go on the backside on the lower third of the waste-board and move opposite the main cutting sled?
So far I’m still designing in onshape and my design can only move about 1m, which will be centered, so half a meter in the opposite direction of the cutting sled.

I was thinking to attach a elastic cord to the main sled, wrapping around the wasteboard using a small guide that can also pivot around the mounting point on the back side of the wasteboard.

I think my question is if people was at an agreement that tensioning up the sled was a good or a bad move?
My idea should improve horisontal tension in the lower parts of the cutting area so maybe helping out in two problematic areas.


#182

There is no agreement on this.

someone needs to try it and see if they can make it work.

David Lang


#183

I have an idea that I’d like some feedback on and I apologize if this has already been suggested. It’s my experience that the sled always tilts to CCW in the lower left and CW in the lower right. My hypothesis is that if the left and right bricks are replaced with motorized spools of wire/cable that are tied to the lower left and lower right corners of the frame, respectively, and the spools are controlled to either let-out/pull-in wire/cable such that the sled stays vertical, then the sled would be pulled into the bottom corners and accuracy improved.

I think if we could attach an Inertial Measurement Unit (IMU) to the controller and then feed out PWM signals to the two spool motors, we could use the IMU as the feedback mechanism to adjust the PWM signals. It shouldn’t negatively affect the positioning of the sled by the main control system since all it should be doing is rotating the sled. I don’t think the tensioning system needs to be precise in its operation or even precisely located on the sled. It just needs to be good enough to keep the sled vertical.

I know we are pin-starved and I’m not sure if we could actually connect an IMU to the current controller, but I believe this actually could be done by a completely separate system if needed… that is, it doesn’t even have to communicate with the controller since all its doing is checking to see if the sled (via an IMU) is vertical and making the necessary adjustments to the motorized spools (though it could because there are open serial port pins on the current controller).


#184

Hi All,

I have been reading many posts etc. I am not yet a Maslow owner.

2 suggestions that may be worth considering

1- Chain slack- can be mitigated by a bicycle spring tensioner- cost normally around circa 15 gbp.

2- further stabilisation could be possible in using a brick shape container filled with sand (5/8 full)

If there is comments or individuals who would like to try this, I would be happy to help sketch some designs.

Here is a link showing an example of a cycle tensioner that I was thinking about.

https://www.cyclestore.co.uk/shimano_cts500_alfine_dual_pulley_chain_tensioner-ID_58629?sv_source=google&sv_medium=cpc&sv_campaign=[campaign_name]&sv_term=[keyword]&gclid=Cj0KCQiA1NbhBRCBARIsAKOTmUsE5vIas7SrhF8gDbbiI9Vn0Y_FJ6wjfQNc9yw9oyv_kFlbKuiiT6EaAtmcEALw_wcB&gclsrc=aw.ds


#185

Hi @HTENS, and welcome to the thread! Would that tensioner be used on the sled side of the chain assembly or the chain slack side?


#186

Hi @madgrizzle and thanks for taking the time to do such a nice write up.

The main design target, as it stood, was to add as few parts and expense to the machine as possible so that it is more accessible to the community. However, I personally appreciate the mechatronic approaches to the problem. If one could execute your plan for say 25-50 (even $75 imho) dollars and hold the corner areas as tight as the rest of the tolerances on the machine, I think its a win for sure., and well worth any efforts.

I think the best one could do with the cable approach is compare chain let out to the tension using a load cell and potentiometer, and combine that with a geared DC motor to keep the tension as a function of let-out (closeness to corner) chain – doing that for both corners, and on a separate controller. We would then need to add a program function in GC that would re-calculate the chain sag variable so that it isn’t thrown off as tension is applied during the cut.

I personally believe that changing the physical characteristics/geometry of the machine is the best path forward, rather than trying to compensate for it’s current limitations, but I always want to assist in any exploration of other avenues!

-J


#187

What do you guys think about doing all of the slack compensation in software. If you had a software that could be taught manually by the user software location vs physical location. I am not saying this would be an easy algorithm to write but theoretically you should be able to have a compensation coefficient that is specific to the system that would incorporate organically such variables as drag, chain slack, weight of sled,…etc. (These variables should remain constant for each system…after a fashion at least)

You could have a set training set of shapes or positions that the router would have to hit. then you measure reality versus software and the delta should be a starting point for the software to understand the system.

Has this already been addressed? If so i apologize…


#188

I’m struggling 2 years that the software tells me where the centre of my sheet is in Y-axis.
It never got it right. On X is was always spot on.
I wish i could have centred the bit in a same sized hole in THE REAL CENTRE and was able to tell the software that this is my Y0 and nothing else.


#189

I like the concept, but there is an inherent difficulty in this that would have to be overcome (and it’s not isolated to what you are suggesting, I’ve experienced it in what I’ve tried to do).

What I’ve learned about all my work on the Maslow is that you can’t improve the accuracy of something unless you can measure to something better than what you are trying to achieve. For instance, if your goal is accuracy to < 0.5 mm, then you have to be able to measure better than that to maybe at least 2x times better based upon a gut feeling, but maybe much more is needed… I’m sure if I did some research and read some books on the topic, this has been figured out already.

So, with that said, if you want to put markers on the board that you move the router to and compare actual chain lengths vs. what you calculate, you have to place the markers extremely accurately. Across a 4x8 sheet of plywood, placing the center of a marker down to no greater than 0.25 mm accuracy will be incredibly challenging in my opinion. If you look at my work here: Optical Calibration Demo and Three Hours Working on a Bug you will ultimately see I used a 4x8 printed pattern on a banner. That pattern isn’t perfect in itself because the printer the company used isn’t perfect. So then what do you do?


#190

This isn’t necessarily true. There is a whole field of statistics which covers this topic. In summary, inaccuracies in your measurement system can partially be compensated by making multiple measurements. You measure the position of one point 12 times, each time ensuring that the measurement is not subject to the same bias. Statistically, the average of the 12 measurement points is more accurate than each single measurement.

My comments about statistics do not address this issue.

I think this is called a Measurement System Analysis. The purpose is to identify how accurate your system can measure something.


#191

I agree through statistical analysis of multiple samples you can improve your measurement results. I do that in the optical calibration by processing 10 different camera images and taking the mean value of the results that are within two standard deviations of the initial mean. I do that to make sure I don’t use a garbage measurement because the edge detection went wonky, but I also think it improves accuracy as you suggested.

However, I think the qualifier “each time ensuring that the measurement is not subject to the same bias” is one of many challenges to overcome. You’ve got the accuracy of the tape measure itself, the accuracy of how you place both ends of the tape measure, the accuracy of your eyes and how well you can read a tape measure and then finally, the bias of reading something and not just using the last value you read because you can’t be really sure its any different.

I’ll rephrase my original statement though :slight_smile:

“What I’ve learned about all my work on the Maslow is that you can’t improve the accuracy of something unless you have measurements that are better than what you are trying to achieve.”


#192

How about using laser pointers and a optical imaging system. you could even project a laser grid onto the work area and use a “laser bit” instead of a end mill in your router . Use a red laser for the grid and a green one to give you an x/y line that marks the center of the router and then write a learning algorithm to analyze the delta in the corner…NOT that I have even the faintest idea how to do this. You could have this run over night taking hundreds (or thousands) of automatic measurements with extreme detail. The teaching data set would give you a statistically meaningful data set that could overcome data errors.

Using a high MP camera and laser would give you much greater accuracy than you are trying to achieve…if you can calibrate it correctly

I feel that a one size fits all approach will not work. Since you probably even have to take into account the temperature of the chain to calculate slag (A colder chain will be stiffer than an warm chain…etc)

I am just trying to help with ideas/concepts…but I could be totally wrong…or this could be a monumental task…