Target stand V1

This first target stand is clearly very basic. It's actually powered by a car door lock, which will pull the stand into position and then it's returned by a spring.

I went with a car door lock as the actuator because it is strong (10lb pull) and car door locks are common enough to be very cheap (~$15).

The problem is that in order to show the target, the actuator has to be pulling constantly. After about 1/2hr of turning the actuator on and off I noticed it was getting sluggish. On closer inspection that actuator is very hot. On measurement, the actuator was pulling 2 amps while it was on. This is not acceptable, far too much heat. After significant work, the machine is going to overheat. I need a different system.

Target Stands

Question: What is an active target?

Answer: An active target moves on it's own. Re-active targets are already quite common in the world. The simplest version being a single plate of steel that, of course, makes a *ping* noise and moves when you shoot them. By contrast the active target will move into and out of sight to make shooting that target harder

Question: Why would you build one?

Answer: I'm guessing that if the targets at the range also move it will make me a more reactive shooter and be a better game to play. The problem is that the only active target I've seen for sale is LaRue's ~$1500 popup targets.

Question: How can you build it better?

Answer: The 'professional' versions here use a big steel plate that you shoot at. This means you can't use large weapons like a 50BMG. This also means you need a big engine to move it up and down quickly. And then you need a big steel plate to protect all the machinery that moves it up and down. And a big battery to power that motor. Etc. Etc.

Instead, I'm going to have a simple paper or wood target that isn't going to try and resist bullets. Similarly, instead of having the target lying down and pop-up, it'll be cheaper from a power perspective to have the target turn on an axis.

Mech: Understanding strength and deflection of truss structures

Have you ever wondered whether a truss with a single cross bar is better than a truss with many of them? How many cross bars exactly do we need?

Sure, we see a lot of things with truss bars at 45-deg angles so we can assume smart engineers determined this was best. But was it best for reducing deflection or was it best for strength to weight? If they were doing it for total strength to weight but not for deflection to weight, I might want to do it differently.

Let's start with strength calculations of the single cross-brace truss. If we assume the weight of the steel in each beam is 1 lb/ft and the truss is 3' then we end up with 11.16lbs for the truss.

What's the strength of that truss? Let's assume the steel will not buckle or compress, and it has a pull strength of 10,000 lb/beam. So if a beam was hung vertically and we put 9,999lbs on it, it'd be fine. If we put 10,001lbs on it, it'd break. If we put a weight on the end of this truss we see that center beam will break at a weight of 3,160 pounds on the end.

What about if we had that same 3' truss with 3 spans? Now the weight is 14.24, a lot heavier. What's the strength? This is harder to calculate. The way to do it is one truss section at a time. Let's start with the last truss section, it will take 7,070 pounds to break that center beam. What about the one before it? Well the last truss section only adds a few pounds of weight so this one will also break at around 7,070 lbs. Same with the section before this. So this truss is actually the strength of any one section, assuming the weight of the truss is negligible.

If we divide the weight a truss can hold by it's own weight, the multi section truss gets a ratio of 496 while the single section truss gets only 283. So the multi section truss is 75% stronger for it's weight.

What about deflection? Does the single truss deflect less? Could that be an advantage?

I made a model of this on my nifty new structure simulating I've been using for the mech. I allowed for 20% stretch in the sections that would be experiencing it; both the top piece and the cross bar of each section. I modeled trusses with 1 section, 3 sections, and 6 sections. The 3-section truss puts each cross bar at exactly 45 degree angles.

It's easy to see from the model the 3 section truss is deflecting less. It deflects 1.8' while the 1 section truss deflects 2.2', 22% more. Then again, the 3 section truss weighs 28% more. Here though I don't think we can just take the weight ratio. The weight ratio was appropriate before because we could just thicken the truss bars to increase weight and strength linearly. The bar thickness will not have a linear relationship with deflection. For starters we haven't defined what the stretch algorithm is, we just set it to 20% arbitrarily.

How are we going to get a handle on this? For starters we know that stretch is usually linear with strain in steel. And that mild steel will top out around 0.2% stretch before it breaks. So if we made our bars in the 1 section truss 28% thicker, we'd reduce the stretch about 28%; to 0.1448% stretch. Now our two sections are of equal weight but the 1 section truss deflects 20% less.

Unfortunately, to get here I had to make an assumption about the stretch on the top of the truss, that we're going to have all truss sections of equal thickness, and that buckling never happens. But in the end it's clear the many-section truss is shit, the truss with 45-deg cross bracing is strongest by a good margin, and the 1 cross brace truss probably has the least deflection but only by about 20% of minimum possible deflection.

Mech: More detail on the feet and legs

The real goal here is to get the models to fully represent the mech. Now the act of modeling everything in detail will force me to consider the entire design before I start building and serve to make the simulations work just like the final product will.

Mech: Material Science

I had some problems with the mech's feet and their colossal weight. I've made some reductions in the main spans to get the margin of error to about 8X it's standing strength. I also made the assumption I'd be using an alloy steel that's about 2x as strong (125k psi instead of 60k psi).

This has brought the mech feet down to 298lbs each, which is exactly under my target weight.

I'll need to do more calculations about the torsional forces on the foot that I've not yet figured out how to do. I also bought a book on material science to better understand what I'm doing now that I'm relying on more complex material properties for strength.

Also, while I was mulling over my changes to the mech I spent some time working on the table. I finished the H-bridge controlled that will drive the motors. It needed some debugging but then when it 'worked' it blew up, fracturing one of the transistors with the heat. It's clear there was a moment when both the high and low on one side were both conducting, allowing a high amperage of 120V through it.

Investigating some non-home-made H-bridge drivers I see that they often tout their abilities to prevent this. I bought some. I hope that their bragging about this feature does not imply that some of them still fail at it...

In other news, I left the soldering iron on when I got distracted by an attractive girl in my bed and need to buy a new tip for it.

Mech feet: It looks like they're too heavy

I just added the density settings to the mech feed that I designed and it looks like they're already super heavy. Just one foot is 852lbs.

I'm using 3"x3"x0.25" steel for the main spans, 2"x2"x0.25" for the long (1") cross bracing, and 1"x1"0.25" for the short cross bracing.

Mech feet

The goal at this point is making the mech match what I'll actually build. That way the simulations will be accurate enough to determine the exact walking sequence. Similarly, I'll be able to get the collision detection and weight measurements correct.

Got the basic supports done for the mech foot. I'm using a helix or spring-like pattern to prevent the truss that connects to the leg from bending in the face of the torque from the feet. I'm not yet sure how to measure the stresses on it.