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*2-Motor Walker Balance Calculator

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Eric's 2-motor Walker Balance Calculator

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I built this tool (originally) to help me salvage a walker I built -- namely, to figure out why this particular walker drug its back feet, rather than walking correctly. You can use it for two things:

Your experience may differ, but since I developed this tool, I use it more for the second purpose than the first... Note that the tool is currently set up for a walker with a single geometry (front motor shaft horizontal, rear motor shaft vertical); I'm working on an updated version of this tool that does not have this limitation. Note also that the coordinate frame I use has its origin directly below the front-leg attach point on the front motor shaft.

In order to run this tool, you just need (at least a rough guess for) a few parameters describing your walker. Note that this tool doesn't care what units of length you use (inches, cm, meters, furlongs...), as long as you are consistent and use a single unit throughout.

  • Front and rear motor angles -- how far you want your front / rear motor to rotate each way from "zero" (i.e., if a motor's total range of motion will be 60 degrees, you'll input that motor's angle here as 30 degrees). This value is a function of the motor you use, the voltage you're running it at, and your driving circuit timing (i.e., how long you drive the motor for in each "spurt"). I wouldn't recommend you go above 45 degrees for either of these angles.

  • Foot touchdown points -- coordinates of your walker's feet, if all 4 were on the ground at the same time; refer to the diagram below. Note that if your geometry looked like the "stick-figure" in my diagram, "Xf" would be a negative number.

  • Walker frame length -- distance between the front-leg attach point on the front motor shaft, and the rear-leg attach point on the rear motor shaft. If you're just tinkering around, I'd suggest you use "1" for this value (then all the other dimensions are just percentages of the frame length).

  • Front motor axis height -- most 2-motor walkers are pretty "low-slung," so this parameter will generally only have a small effect on balance. Still, for completeness....

As for outputs, there are three important ones, and a couple that are available to satisfy your curiousity:

  • Foremost & rearmost extent of allowable CG location range -- as is mentioned elsewhere, 2-motor / 4-legged walkers rely on correct location of walker CG in order to achieve a useful walking motion. This allowable CG range is shown in the diagram below as a green diamond (though, strictly speaking, you should strive to keep your CG as close to the walker center-line as possible).

  • Percentage of frame within allowable CG location range. As I've described in my walker balance discussion, the larger this number is, the less-constrained your walker design will be. But don't get too carried away with maximizing this, since walkers with a very high number for this value tend to tip over sideways more-easily.

  • Rotated foot touchdown points -- you won't normally care about these values, I only output them to make it easier for me to check this tool's results.

 

Maximum motor rotation angle (+/-)

Front motor angle

degrees

Rear motor angle

degrees

Allowable walker CG location range

Foremost extent
CGf
Rearmost extent
CGr

% of your walker's frame is within the allowable CG range

"Rotated" foot touchdown points
Front feet
Xcf
Ycf
Rear foot (cw)
Xcr
Ycr
Rear foot (ccw)
Xccr
Yccr

 

Foot touchdown points (with all 4 feet on a plane)

Front

Xf
Yf

Rear

Xr
Yr

Walker frame length (motor axis to motor axis)

Front motor axis height

Zc

 

 


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