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

Couldn’t find anywhere to order a longer cable with a shielded single pair of wires heavy enough to power the motor.  Sent an email to Super Droid Robots asking if I could get a longer cable from them.

Also having trouble finding cable for the encoders.  The original cables have four pairs of twisted wires and the whole thing is wrapped in foil.  Found a place online to order cables, but the price was fairly high for the number of feet I need.  Finally found the best solution – it turns out that ethernet cable also has four pairs of twisted wires.  If you get CAT6 ethernet cable, it is shielded.  Now I just have to find some cheap, shielded ethernet cable.

The encoders arrived today.  I was really surprised when there were three encoders in the box and I only paid for two.  One of them turns pretty roughly though, so it may not work.  I will definitely need to replace the cables – the existing cables are about 6 inches long, and I need about 4 feet.

The power supply arrived also.  It is a nice and shiny metal box with terminals on one end.  Looks like it should do the job.

Just got a message that the encoders were shipped today.  I assumed they had been shipped a week or two ago.

The Nordex gears showed up today.  They are incredible!!  They are so light they feel like they are made from plastic.  The set screws will make it really easy to attach them to the shafts, also.  Still have one problem to resolve, though.  The gear holes are 1/4 inch, and the motor shaft is 6 mm.  Just slightly different, but enough that the gear would wobble as it turns.

It looks like NI will be able to loan me a sbRIO to control the system.  The form factor doesn’t fit into my design quite as well, but internally it is identical to a cRIO.  I really can’t complain if they loan it to me for free…

Bearing kit arrived today.  I put it all together but the slide units have a lot more friction than I expected.  After looking closely at them, I realized there is a rubber seal on each end of the bearing that rubs against the shaft.  I decided that for my use case I don’t need the seals.  I tried to remove the seals, when I discovered the seal has a metal washer inside it for strength.  I ended up just scraping off the sealing edge with a razor knife.  The slide unit moves much, much easier now.  If removing the seal was a bad idea, I always have a spare slide unit that still has the seals intact.

Motor arrived today.  It looks pretty good.  The only problem I see is that the motor wiring kit came with a one foot cable, and I need about 4 feet.  I will have to find a new cable somewhere.

Decided the steel gears from McMaster are just too big and heavy and would be too difficult to mount.  Ordered some aluminum gears from Nordex and returned the McMaster gears.  The prices are almost equal, so shipping is the only thing I have to pay to switch the gears.

Placed a bid on a 24 V, 12.5 A power supply this morning and won the bid in the afternoon on ebay.  This is going to be way more power than I need, but it was a good deal.  Besides, it never hurts to have more power!!  (I can hear Tim Taylor grunting on Tooltime…)

NI appears to be interested in helping me with the project.  They are going to try to find a cRIO system that I can use to control the system.  If this happens, it will be totally cool.  The FPGA side of the RIO will run the motor control loop really, really fast and keep track of the cart and pendulum positions.  The RT side of the RIO will run the logic for inverting the pendulum and send the setpoints over to the FPGA side.

Checked around for cable carriers and prices to keep the cables from flopping around.  It is a difficult problem to solve, because the space between the cart and the shelf where the cable carrier has to fit is about 0.7 inches, and the cable carrier has to support two feet of cable.  Right now the best solution is from Kabel Schlepp.  They are the only ones that can support the full two feet of cable.  Their price is in the middle (Gortrac lowest, Igus highest).  The only problem is the minimum order of $50, which I am hoping they will waive.  Interestingly, Igus does not have a minimum order, and Gortrac has a minimum of $100.  The cheaper the carrier, the higher the minimum order.  I guess they have to make their money somehow.  This is the last thing I will need, so I will wait until everything else is done to see what will work.

The gears from McMaster came in today.  I love ordering stuff and getting it the next day!!  The rack gear is pretty big and kind of heavy, but it looks like it will work well.  The spur gears are really thick and very heavy!  They are going to add way too much mass to the cart.  Maybe they can be machined down or something.  There is way more metal than I need there.

Spent a good chunk of money today on the project.  I ordered a 20 mm linear ball bearing assembly from VXB for about $88 including shipping, which includes a 55 inch shaft, two slide units and two shaft supports.  I ordered a IG32P 265 RPM motor and a wiring kit from Super Droid Robots for about $42 including shipping.  I ordered a 4 ft rack gear and two pinion gears (2”, 1.25” diameter) from McMaster Carr for about $82 including shipping.

Still need a power supply, aluminum pieces to build the cart, and a control system.  Also need a way to keep the cables from flopping around when the cart moves.