The View Up Here

Random scribblings about kites, photography, machining, and anything else

KAP Upgrades

Posted by Tom Benedict on 21/12/2012

I’m on vacation! W00t!! For the record, this is my first real (meaning more than one day) vacation since Rydra was hospitalized last year, about 17 months ago. And for the record, this was way way too long. I’m a little crazy right now. I’m sure my co-workers would agree.

Yesterday, my first day of vacation, I pulled apart my KAP bag and took stock of where I am. I also dumped a couple hundred grams of sand from my bag, took out bits of gear I’d accumulated but didn’t necessarily use, and overall made my bag a good bit lighter. But the real purpose of the exercise was to catalog all the minor annoyances that have been bugging me over the past few years, and address them. Aside from the KAP bag inventory, here’s some of the stuff I did:

KAP Improvements Montage

The shock cord in my BBKK rig legs has been fraying on a zip tie I installed backward. So thing #1 was to replace the shock cord. While I had the legs off I also sanded over some of the rough spots on them to make them a little more smooth. Thing #2 (moving clockwise) was to cut off the offending zip tie and replace it with one that’s facing the proper direction. No more rough edges biting into the leg shock cord!

Thing #3 was completely serendipitous, but I’m glad I did it. Some while back on the KAP Forum, Dave Mitchell posted to a thread on line work about replacing the typical knot at the end of your kite line with a splice. Dave uses hollow-cored Dacron line similar to the stuff that Brooks Leffler sells. So of course I had to give it a try!

The main reason for using a splice, aside from the neater aesthetics it offers, is that it’s stronger than a knot. Every line, rope, and string has a breaking strength. Every knot will weaken the line to some degree, usually expressed as a percentage of line strength. Even a good knot, like a bowline, will appreciably weaken the line at that point. In the case of the bowline it’s something around 80% line strength. Other knots present far lower numbers. Of course there are knots with exceptionally high percentages. But in this game the splice always wins.

Dave posted a nice set of pictures showing how to splice hollow-core braided Dacron line. I used his procedure to splice mine, and love the results. Yesterday afternoon I took my gear to Hapuna Beach and flew my Nighthawk in full-blown offshore tradewinds. When I brought the kite down the pull on the line was more than I could wind, so Rydra and I walked it down. The splice hadn’t shifted or stretched at all. I’m impressed!

The last of the little things I did, to complete the circle, was to add a sticker to the bottom of my BBKK rig showing what function each channel on the radio handles. I had one of these in the past, but somehow lost it. It’s good to have it back, especially since I’ve been trading out between the Canon A650IS and the Canon T2i. Each takes a different shutter cable, so it helps to know where the thing needs to plug in.

But that’s the small list. The annoyances. Now for the big list.

I’ve had a couple of KAP projects hanging around for far too long. One big one is to gut my video downlink and re-vamp the power system to get rid of some persistent noise I’ve been dealing with. That’s going to happen this weekend. I’ve also been planning to re-visit my panoramic KAP rig to add a rotary encoder to the pan axis. That’s what I did today.

The first step was to remove the pan gear and drill out twelve holes to take twelve neodymium magnets. This went relatively smoothly using my Foredom as a live spindle on my Taig lathe. (Sorry, no pictures of the setup. My shop is still a mess.)

Pan Gear Drilled

The magnets were epoxied into the holes using West Systems epoxy. Any epoxy would have done. But I’ve been using the West Systems epoxy on my boat, so it’s what I’ve got on-hand. I also love their pump system for measuring. I haven’t had a single batch fail to kick.

Pan Gear Installed

Years ago I picked up a bunch of Honeywell SS495A1 ratiometric Hall-effect sensors. These are analog devices that can be used to sense a magnetic field. They’re pretty flexible about input voltage, but I’m driving mine using +5V/0V with the output varying between 0.2V for -670 gauss to 4.8V for +670 gauss. No magnetic field is 2.5V.

All of this is being sensed by the Pololu Robotics Micro Maestro servo controller. I picked one of these up a couple of years ago, after a fellow KAPer posted to the KAP forums about how easy it was to press them into service as autoKAP controllers. It’s been the controller for my panoramic KAP rig ever since I built it.

The Micro Maestro can use its six I/O pins to drive servos, for analog or digital input, or for digital output. I set up Pin0 on mine to read the Hall-effect sensor. The magnets I’m using are only 1/8″ diameter x 1/16″ thick, so they’re not all that strong. From playing around with the sensor and a magnet, I found a 3mm sense distance to be just about right. (My apologies for mixing units like this… I drive everyone nuts, no matter which system they prefer to use.)

All that was left was to make a bracket for the sensor, hot-glue everything into place, and put it all together.

Pan Sensor

The control scheme is pretty simple: Rotate until there’s a rising edge on the Hall-effect, and decelerate the pan servo to a stop. Once it has stopped moving, trip the camera’s shutter for half a second, then accelerate into the next move. No real smarts at all. The very essence of autoKAP. The advantage this system has over a strictly time-based setup, however, is that the position changes are quite accurate, there’s no chance of a missed shot, and the distance moved is completely independent of motor speed, battery level, etc. Here’s what it looks like in motion:


Unfortunately it’s raining right now. Cats and dogs. Welcome to winter weather in the tropics. So the first flight of this rig will have to wait. Meanwhile I’ve got more KAP upgrades to do. Video transmitter, here I come!

– Tom


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