Archive for December, 2009

Limit Switches Arrived

Saturday, December 26th, 2009

The limit switches came in today.  They are even smaller than I expected.  I knew what the measurements were, but it didn’t really connect until I had one in my hand.  These would fit in a tight space fairly easily.

I pulled off a strip of ribbon cable that is three conductors wide.  I colored one edge black and the other edge red with permanent markers.  I separated about half an inch of conductors at one end and pressed them into the connector using a small screwdriver.  I think they are in far enough to cut through the insulation and make contact with the wires.  If they don’t make good contact when I test them in a week or so, I will have to find something else to press them in further.  The holes are pretty small.  Perhaps a straightened out paper clip would work well.

Limit Switches

Wednesday, December 23rd, 2009

While searching the digikey website for photo detectors for another purpose, I came across some optical sensors that should work very well as limit switches.  They reflect infrared light off a diffusing surface with a range of 2 to 22 mm.  I assume the optimum distance is about 11 mm.  Right now I plan on positioning them so they reflect off the bottom surface of the bearing block.  They are small and flat and should fit nicely under the bearing block.  I ordered two sensors (425-2042-ND), three connectors (one spare) (A98801-ND), and ten feet of ribbon cable (AE09G-10-ND) for about $13 plus shipping.  The sensors use three wires: GND, 5V, and signal.  I plan on pulling off three wire strips of ribbon cable and coloring one edge black (GND) and the other edge red (5V) with permanent markers.  I can separate the wires at each end and keep a nice clean cable in between.  My original thought was to buy the individual components and design my own circuit, but it would have cost more than buying the prebuilt sensor.

Calculations

Sunday, December 20th, 2009

When I picked out all the hardware during the design phase, I didn’t really explain any of my calculations.  Here are a few numbers of interest:

Encoder calculations

I decided I wanted encoders with a minimum resolution of 1000 pulses/rev.  With quadrature  counting, that gives me 4000 counts/rev.  For the pendulum, this makes the resolution 0.09 degrees/count (or 0.00157 radians/count for those that care).  For position measurement, the 1.25″ diameter gear on the encoder gives a position resolution of 0.00098 inches/count or 0.0249 mm/count.  These resolutions should give adequate feedback even when the speeds are fairly slow.

Motor calculations

IG32P Motor Performance Chart

IG32P Motor Performance Chart

I started selecting a motor by figuring out what the power limitations were.  From the beginning I planned on using the NI 9505 module, which has a limit of 30V and 5A.  I decided to use a 24V motor since that is a standard voltage and there are a large number of robot motors available that use 24V.  The motor I picked is 24V with a stall current of 5A.  The motor (without gearbox) has a maximum speed of 6000 RPM and a stall torque of about 1900 g-cm (0.186 N-m) according to the chart above.  There were several gearbox combinations available, so I picked the 19:1 ratio that will give me fairly high stall torque of 3.54 N-m.  With an approximate total cart and pendulum mass of 1.4 kg and a moment arm of 1″ (gear radius), the max acceleration of the cart will be 99 m/s^2.  However, this drops off quickly with speed.  Other factors (cable carrier, support platform) will probably limit the acceleration to 50 m/s^2 or so.  With the 2″ gear, the maximum speed is 33 inches/sec (84 cm/sec) and will be fast enough to travel the length of the track in about 1.5 seconds.  If the max speed isn’t high enough, I will look into getting the 14:1 gearbox, which would reduce the torque a little bit and increase the speed.

Modules arrived

Thursday, December 17th, 2009

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.

Finishing construction

Monday, December 14th, 2009

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.

Cable connectors

Sunday, December 13th, 2009

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.

Getting closer

Friday, December 11th, 2009

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.