Category: Dropship

Found a deal on eBay here in Independence, MO.  Much bigger than I need but too good of a deal to pass up.

Picked it up 6/28, bolted it to the trailer that weekend. It uses 3/8 NPT connectors (yes, two of them) so I had to buy a 3/8 plug and 3/8 NPT to 3/8 ID barb (at a 90 degree angle to boot) which they don’t carry at local hardware stores. Got those in the mail this week and fired up the unit tonight for the first time. It started on the first pull and purrs like a kitten although my wife said she thought it was loud when it was filling from our bedroom. We are waiting on a plastic piece called the “air conveyor” that was broken on the top of the unit (didn’t see that when I inspected on pickup). Cargo Largo (the seller) refunded me $15 for the part.

It has been so hot lately that is really my reason for not getting much progress outside. I have been working in the basement though. I moved the overhead furring strips 2″ out either side and fixed the wall on the left side. Starting to finish the wall out on the right side now. I had a lot of trouble getting the monitors on the left side off – one bolt broke on the 55″ and two broke on the 42″ it is so frustrating. I have the 55″ apart now and see that there is a better way – rather than trying to run a bolt through wood into a hole in the TV I can’t see I will attach bolts from the TV which go in the holes and get nuts on the other side (while they conveniently hold the TV in place). I can’t believe I spent $20 or more on their stupid M6 and M8 size friggen’ bolts!

Design weight of 1800 pounds loaded up!

So the good news is that the weight can be moved around and main actuator is holding the weight.  The bad news is that, although the weight is evenly distributed about the pivot point, the CG is moving around (something I should have accounted for).  Basically as the platform tilts forward more of the weight is ahead of the pivot point and it continues to get worse until it hits the bottom of the piston.  And the weight ahead of the CG exceeded the 200 pound capacity of the pistons so we were dead in the water.

I came up with a solution using springs to counteract the effect of the CG moving forward – basically the springs will keep it in a neutrally stable situation where the actuators are responsible strictly for positioning.  This should make the calculations simpler as well.  In an effort to get a spring load I took our bathroom scale out.  The bathroom scale is destroyed but best I can gather is we are looking at 160-190 pounds. The CG will be higher in the completed product so I doubled that capacity with the spring.

Finished the other roll actuator, added a sheet of plywood to the avionics bay to support the weight, and loaded up 1000 pounds of the topsoil this weekend.  Haven’t had time to do any motion tests – it sure is scary!

Removed the spacer block from the top of the roll actuator to make up the difference in size of the bottom attachment point.  Currently filling the hole with liquid nails held in place with masking tape so that the pilot bit of the bore has something to bite into.

I cut the U-joint and widened the holes slightly where it is now attached to the base of the actuator.  Then I used the existing eye bracket by trimming about a quarter inch from the front, back, and top so it fit inside where the spline shaft goes.  Only thing now is to find two half-height nuts to reattach the bottom flange but it isn’t going anywhere.

I tested it out and immediately notice a reduction in friction (no force on the linear bearing anymore).  There is just enough room for the actuator to move around without hitting the deck of the trailer.

 Saw it at 12:01 this morning with my dad in IMAX 3D.  Quite a ride!  The sets and special effects were everything I expected.  The acting was great.  The music start out great but ended on a good note (pun intended). The story was good… it has a couple of holes that could probably be explained away but for the masses it will be a head-scratcher.  Definitely a prequel to Alien, no doubt about it… but we are on LV-223 (if you missed it) not LV-426.

Stopped at the salvage yard on the way home.  They wanted $35 on average for a spring/strut assembly and wouldn’t sell the spring separately even if I had the right tool (which I didn’t). They also only had one of the U-joints that I liked so I ended up having to buy something different.

A few notable changes in the software between day 5 and today:

Dynamic control limit for joystick (0-20 degrees)

Calculating and logging acceleration rate

Clamping rate desired and rate differential

Heave is now enabled again

Configurable frame time

Two additional event buttons

Fixed counter for 10 second min/max calculations

Fixed min/max calculation (not resetting pit/rol rates)

pitlimit was NAN

The latency can be visualized as a phase shift in the attached Excel chart.

I have a couple of theories to try for day 7 tests.  First, the static friction value should only be used initially if the current voltage above trim is less than the dynamic friction value.  Therefore we must also declare a dynamic friction value (in degrees per second).

Second, I think we should try effecting a change on the system and then waiting 0.3 seconds for the results (EDIT: verified this was already the case).

Finally I wanted to investigate the desired rate calculation… is 60°/s really necessary to adjust 20°?

To speed things up I am also initializing the values for all the sliders to the most recent successful numbers.

Home Depot dropped the price from $1.25 to $0.98 per 40 pound bag.  I picked up 50 yesterday (1 ton) to eventually load-test the platform.

Actually ran this test series yesterday and analyzed the data today.  It is becoming more apparent that acceleration is a factor we need to take into account.  We also will need intensify adjustments based on how well the last adjustment responded (i.e. if we call for 300°/s² and get 150, we need to double whatever we did in the last frame).

It appears the transducers are more responsive than the 250ms latency I have seen published.  I am bumping up the master timer to 125ms to see if that fixes some of our weird occilations.

There may be an fundamental design problem with the actuator attachment points.  In the design I thought that since the center of rotation of the double-cardan main joint was 5 inches below the bottom sheathing that the centers of rotation of the pitch/roll actuators needed to also be at 5 inches.  What I am seeing now is pressure on the linear bearings (especially roll actuators) as they try to “straighten” with the now translated extended rotation point.

Is this the reason other 3DOF designs use 3 actuators instead of 4?  I thought it was to save coin and that the calculations for 4 actuators is actually easier since you have 2 dedicated to pitch and 2 dedicated to roll.

Yes, I know I didn’t document any findings from day 3 and that is because it was a complete regression.  We had some luck today and added a new feature – data recording!

We are definitely getting somewhere… with the goal to put weight on it soon.  Now I just have to make sense of the motion tests 5-26-2012 data.  I will need to add a separate “weight” slider for roll (lateral) than pitch (longitudinal) because the roll actuators are much more responsive.  I also need to figure up a minimum “overvoltage” to break static friction and get the platform moving on command.  The aggression should probably be divided by 10 so that we can select a more fine tuned value.

On another note, testing these old Planar Toshiba LTM10C942 (640×480 10″) LCD monitors that run at 75Hz with the AV-620 works great (they don’t advertise the 640×480 because the Windows 7 UI doesn’t show it as an option but if you drill into Advanced Settings -> List All Modes you will see it in there.