Building an Inverted Pendulum System The Ups and Downs of Defying Gravity…

The 9411 and 9505 modules came in today.  As I expected, there were not any D-sub connectors since they don’t normally come with them.  What surprised me is that the power connectors were missing also.  Those connectors are not quite as common, so I panicked for a minute.  I ended up measuring the connector socket and finding a matching connector at Digikey.  It will be green instead of black, but otherwise it looks like a perfect match.  If anybody else needs to buy some, the two wire connector is part A98283-ND and the four wire connector is part A98451-ND.  I also ordered the D-sub connectors (L717SDE09P-ND, L77SDA15S-ND) and backshells (909GME-ND, 915GME-ND).  Total cost is about $9 plus shipping.  That’s a really good price for four connectors.  I am glad I waited until the modules came in to order the connectors so I didn’t have to place a second order.

I met with the guys at Shaltz Automation today, and they are going to help me finish up the construction of the pendulum hardware.  I feel kind of guilty, because they took all the parts home with them to finish them up with equipment that I don’t have.

One of the tasks is cutting off the extra shaft length for the linear bearing.  It comes 55″ long and needs to be trimmed to just under 48″.  The problem is that it is hardened steel with a chrome coating, and hardly anything can cut it.  Bill is going to try to cut it with a rubber saw blade, which I had never heard of.  Apparently the friction heats up the metal, softens it and gradually removes it.  As long as it works, I will be happy.

Dana is going to drill the holes in the pendulum.  With the machine tools he has at home, he will be able to get the holes perpendicular to the rod and centered.  He is also going to mount the bearing shaft on the shelf.  After some discussion, we decided the best way to mount it is to use nylon spacers under the shaft mounting brackets, which will raise up the bearing shaft and make the cart level.  With the equipment he has, he should be able to trim the spacers pretty accurately.  This should be fairly easy to do, and the height can be adjusted later if necessary by cutting new spacers.

Therefore, by the end of the week I should have fully functional hardware, and I will just need to wait for the cRIO chassis to arrive so I can start controlling some motion.

I don’t think the NI modules are going to come with connectors, so I did a little shopping on Digikey’s web site.  I need a 9 pin connector for the 9505 and a 15 pin connector for the 9411.  I can get the connectors and backshells for about $6 plus shipping.  Digikey is a great place to get electronic components!!  I will probably wait until I get the modules before I place the order, just in case.

I found out that it is going to take a while to fix the sbRIO board, so I didn’t think anything would happen on the project in December.  Yesterday I found out that I will be using a cRIO chassis for another project and can borrow it for my pendulum until the project is completed.  That should give me a cRIO chassis for a month or two.  I called NI and asked them to loan me the 9505 and 9411 modules.  Today I was told they are in the mail, so as soon as the cRIO chassis comes in I can start controlling the pendulum.  Now I just need to wrap up the construction.  If I can get this running before Christmas, that would give me a fun toy to play with during the break.

Today I finished cutting and drilling all the parts of the cart.  I started with a new piece for the arm that connects the front section to the bearing shaft.  The old one was pretty rough on one side since we cut it from an angled piece.  Since I had to attach stuff to the top and bottom of the arm, I decided to cut a new arm from the pieces I bought from Online Metals, which gives me nice flush mounting on both sides.  Drilled a bunch more holes with the drill press and discovered a few things:

• It is much easier to see the spot punched in the metal if you remove the paper template you used for punching.
• A small drill bit will usually “pop” into the punched spot, making things line up very nicely.
• A large drill bit has a mind of its own and tends to ignore the punched spots, giving me terrible hole alignment.
• Next time, for any hole about 1/4” or larger, I will drill a smaller hole first, then make it larger.
• When drilling a really large hole, start small and step up slowly.  Large steps make the drill chatter.

After drilling several holes that didn’t quite line up, I had to fix them.  My first effort is to just drill a slightly larger hole.  If that doesn’t work, I use a rotary file to enlarge a hole in a selected direction, making it a little more like a slot.

The final cart assembly looks pretty good.  I mounted all the different components onto the cart.  I put the broken encoder in the position for the pendulum so I can test the pendulum without breaking another encoder.  After I am comfortable with the pendulum behavior I will switch to the working encoder.  I used a couple of cable ties to keep the wires from bending and breaking.  Eventually they will be replaced with a cable carrier.  Funny detail – every set of screws in the cart is a different size and type.  It has metric (M3, M6), english (8-32, 10-32, 4-40), pan head, button head, flat head, slotted and phillips.

Completed cart - top view

Complete cart - bottom view

The next step is to attach the bearing shaft to the shelf at the right height so that the cart is horizontal and the pendulum will swing vertically.  Getting the height right could be tricky.  I also have to figure out how to make the pendulum so the hole is centered and straight.  Then I will be all set for a control system.  I hope that sbRIO board gets repaired soon.

Drilled holes in shelf and attached rack gear.  It looks pretty good.  Now I just need to find time to finish building the cart and get it all mounted on the shelf.

The first picture is the entire shelf with the rack mounted flush to the front edge.  The second picture shows a closeup where the rack gear is attached to the aluminum bar using bolts, and the aluminum bar is attached to the shelf using wood screws.

Shelf with rack attached

Rack Closeup

This picture gives you an idea of what the whole system will look like once the cart is completely assembled and the linear bearing is mounted to the shelf.  Of course, the second encoder will be moved to the new bracket on top of the cart once I make it.

Whole system layout

I did a few calculations, and the bearing loads on the encoder from the swinging pendulum are going to exceed the maximum loads specified for the encoder.  I don’t want to destroy the encoder, so I decided to redesign the cart.  I added a bracket on top of the cart that extends out beyond the gear.  The encoder will be mounted on this bracket, and the pendulum will be almost flush to the bracket.  This will minimize the loads on the encoder shaft.  Time for some more cutting and drilling, I guess.

Inverted Pendulum Design v2 (pdf)

Replacement encoder arrived today.  Checked it out to make sure everything works.  A and B signals are fine.  Found out there is no Z signal, but I really don’t need it anyway.  It is interesting to look at the guts of the encoder – obviously older technology than the other encoders.  Replaced the short cable with the last piece of my ethernet cable.  This time I was a little smarter and left about two inches of the old wires and just soldered new wires to old wires, then put a heat shrink tubes over the joints.  I coiled up the extra wire inside the encoder case – it was a tight fit.  One amazing thing – the mounting holes in the new encoder are the exact same size and location as the old encoder.  I don’t have to drill any new holes in the brackets to mount it.

Here’s a couple of pictures of what the cart looks like so far.  Haven’t drilled all the holes for the bracket yet, but you can get a pretty good idea of what the final version will look like.

Cart - Top View

Cart - Bottom View