KAP Rigs
As an amateur woodworker with a well stocked shop, wood seemed like a natural KAP rig material. My rig is crafted from 1/4" and 1/8" birch plywood (available at many hobby stores) and scraps of spruce and poplar. The rig is in a constant state of flux, having progressed from carrying a Kodak DC-20 digital camera to an Olympus Stylus Epic 35mm point-and-shoot film camera. A video downlink was added recently for real-time view finding. Interference between the video transmitter and the RC receiver forced me to graduate from my two channel Futaba Attack AM RC system to a four channel JR FM unit. A recently purchased Canon Rebel X sits in a box awaiting a rig of its own.

My rig started out looking like this...
In it's first incarnation, the rig could accommodate either the DC-20 or Epic cameras. The DC-20 was modified for external shutter control (not for the faint of heart) and the Epic was controlled by an Olympus RC-100 IR remote, powered by a servo circuit board running on the same RC servo signal as the pan motor. The "coffin" design doesn't require a 1/4-20 mount on the camera (the DC20 has none) and provides a flat surface for mounting filters. I have discovered that filters confuse the Epic's auto focus mechanism unless tilted approximately 15 degrees from the film plane. My success rate was also too low, so I decided to add a video viewfinder.
...and now looks like this.
The coffin has been replaced by an open carrier for the Epic (the DC20 is temporarily grounded). A nylon 1/4 bolt/wing nut combination threads through the mount and into the Epic. The RC-100 remote has been replaced by a small infrared LED on the end of a short wire that runs into the right arm of the rig. A microcontroller in the right arm mimics the combined operation of the servo circuit board and RC-100 remote control. A little red paint makes a difference.

The rig currently uses four RC channels as follows:
  1. Directly controls the tilt servo.
  2. Controls the pan servo through a microcontroller which expands the servo deadband to avoid stutter (especially in the presence of video transmitter interference).
  3. Controls the video transmitter and camera shutter. Moving the stick left toggles the video transmitter on and off. Moving the stick right triggers the camera shutter.
  4. Is unused, but will directly control the third axis of camera rotation in the next rig. With the video downlink, this should make it easy to achieve level horizons.
Details...
As the Epic can be controlled by an IR remote, it is not necessary to break into the camera to construct a "servo-less" shutter release. A microprocessor controlled infrared light emitting diode (IRLED) operates directly off an RC servo signal and recreates the sequence of infrared pulses produced by the Olympus RC-100 remote control. The microprocessor automatically "learns" the neutral stick position when the rig is powered up and sets the shutter trip threshold about 1/2 stick to the right (for a L/R stick). This learning technique makes the shutter release insensitive to RC neutral stick pulse width and trim position, so the remote can be used with virtually any RC system without adjustment. The same microcontroller intercepts the pan servo signal and modifies it to allow continuous pan without any electrical modifications to the pan servo. The normal servo deadband is so narrow that the servo seldom rests. This wastes battery power and makes it difficult to get off a clean shot. The microcontroller increases the deadband by disabling the servo pulse stream entirely when the servo is close to the neutral position. Servos disable their drive circuits in the absence of a servo signal, so disabling the servo pulse also disables the motor. When the stick is moved sufficiently far from neutral, the microcontroller again enables the servo line and the pan servo turns left or right. To achieve endless rotation, the servo output shaft stops are removed, the position feedback pot is disconnected from the output gear and set to mid-travel.

The Rig on the kite line.
I'll get a better image someday. The rig is suspended from the kiteline by a bridle called a Picavet (named after its French inventor). The Picavet lines attach to the kite line at two points several feet apart and proceed down to an "X" shaped mount on the top of the rig. The two attachment points on the kite line prevent the rig from twisting back and forth. The four attachment points on the rig prevent the rig from wiggling around. It's claimed that this suspension technique is more stable than a pendulum. I've never built a pendulum, but this seems like a reasonable claim.

Simple Line Hanger
Coat hanger hangup. I recently purchased some vinyl coated aluminum "clothes line" wire. It's lighter than the steel hanger and the vinyl coating should cause less wear on the kite line.


Camera Rig Ideas
Below are some ideas I've incorporated into my "Rig".

Extension of Servo Travel to 90 Degrees and Beyond
A standard RC servo is designed to produce approximately 90 degrees of rotation, often less, for full travel of the control stick. This isn't quite enough for the tilt axis of a KAP camera which should reach from the horizon to straight down with a little extra range to accommodate tilt in the camera platform. While it is possible to construct mechanical linkages to extend the range, the easiest and most efficient way is to simply increase the servo's inherent travel.

A servo's position is a function of the pulse width transmitted to it and the feedback potential from the position potentiometer (pot). We can extend servo travel by increasing the range of transmitted pulse widths or by decreasing the feedback from the position pot. Increasing the range of pulse widths sent by the transmitter requires intimate knowledge of the transmitter electronics and would cause the transmitter to produce a wider range of servo motion in any servo it controls. This may be undesirable if the transmitter is also used for purposes other than KAP. Alternatively, we can reduce the amount of feedback produced by the position pot in the servo itself. By placing 2 fixed resistors in parallel with the servo pot, we can reduce the amplitude of the position feedback signal. The servo will compensate for this reduced feedback by increasing shaft travel.

How to do it...
If you don't know the value of the position feedback pot in your servo, you may be able to measure it with an ohmmeter (measure across the outside terminals with the servo unpowered). Every Futaba servo I've seen uses a 5K ohm pot. Purchase 2 miniature fixed resistors of approximately half the resistance value of the pot (2.7K for a 5K pot). Carefully solder one end of each resistor to the outside terminals of the pot. Then connect the free ends of the resistors together and to the central pot terminal (the wiper) with a short piece of wire or a resistor lead.

With the receiver turned off, gently turn the servo clockwise until it hits the internal mechanical stop. Mark the position. Turn the servo counterclockwise until you hit the other stop. Mark the position.

Turn on the receiver and transmitter. Move the stick and trim tab together to their full travel in each direction and verify, using the marked positions of the mechanical stops, that the servo does not hit the stops in either direction.

With any luck you have increased the travel of your servo beyond 90 degrees. If full stick travel on the transmitter causes the servo to hit a mechanical stop a third resistor (same value as the first two) may be required. With the additional resistor there are three possible adjustments that may solve this problem.

  1. Insert the resistor in parallel with either of the other two resistors. This will offset the zero position of the servo, increase servo travel and introduce some nonlinearity between stick and servo position. The direction of the offset depends on which of the two resistors you parallel with the third. If the servo still hits the stop, parallel the other resistor. If the servo then hits the other mechanical stop, try method 2.
  2. Insert the resistor in series with either of the other two resistors. This will offset the zero position of the servo, decrease servo travel and introduce some nonlinearity between stick and servo positions. If the servo still hits the mechanical stop, install the resistor in series with the other of the first two. If the servo then hits the other mechanical stop, try method 3.
  3. Insert the resistor between the connected ends of the first two resistors and the center terminal of the pot. This will reduce the servo travel, possibly to less than 90 degrees.
If the servo hits a mechanical stop at either end of its travel it may also be possible to use the trim tab to shift the range away from that stop. Take care to maintain the trim setting. The servo draws considerable power and may damage itself when run up against the stops.


Continuous Pan / Shutter Release on a Single Channel
This topic has been covered by other KAPers over the years. The most popular approach seems to be to modify a servo by disconnecting the position feedback pot from the servo shaft, removing the shaft's mechanical stops and inserting a diode in series with the motor. This has the effect of opening the feedback loop and allowing the motor to spin endlessly in only one direction. The unused polarity of motor drive is used to activate a relay for shutter release. In effect, the servo electronics are being coerced into producing a three state control (Pan / Neutral / Shutter). The problem with this approach is that the width of the neutral position is a function of the servo's deadband. In my Futaba servos that deadband is so small that it's impossible to hold the servo still for any length of time. Eventually either the pan motor stutters a bit (a minor nuisance) or the shutter release trips (ouch).

By using the electronics from another servo (the cheapest you can find) it is possible to construct a pan/shutter system with a very large neutral range. Inadvertent shutter release is eliminated.

How to do it...

First the Pan servo
Disassemble the Pan servo and cut off the output gear's mechanical stops. Unsolder one motor terminal and insert a small diode between the circuit board trace and the unconnected terminal (don't worry about diode polarity just yet). Adjust the transmitter trim control for the Pan/Shutter stick to dead center and set the stick to the position desired to initiate panning (I use 1/2 of full left travel). Turn on the transmitter and receiver. Adjust the servo pot until the motor just stops. Center the Pan/Shutter stick. If the motor starts spinning again flip the transmitter channel reversing switch or reverse the diode. Mark the position of the pot shaft. The servo adjustment is now complete. Cut off the end of the pot shaft or unsolder and lower the pot so it no longer engages the output gear. If you can't cut the shaft or lower the pot you may replace the pot with a trim pot or a pair of fixed resistors selected to match the resistance measured from each terminal of the pot to the wiper. Make sure the pot is in the marked position and reassemble the servo.

Then the Shutter servo
If you intend to push the shutter button with a servo, you need only attach a second servo in parallel with the first. That servo will operate normally and may be used to operate the shutter via some mechanical linkage. If you need an electronic shutter release you must disassemble the second servo to get at the electronics.

Remove the circuit board from the servo housing and unsolder the motor from the board. Connect one side of a low voltage DC relay coil to one of the circuit board's motor contacts. Connect the other end of the coil to the other motor contact through a diode. Set the Pan/Shutter stick to the position desired to actuate the shutter (I use 1/2 of full right travel). Turn on the transmitter and receiver. Adjust the position pot until the relay contacts click. Center the Pan/Shutter stick. If the relay contacts remain closed, reverse the diode. The shutter servo adjustment is now complete. You may shorten the legs on the pot or cut the shaft if desired. Removal of the motor may leave enough roon inside the servo case for installation of the relay. Save the motor and gears as spare parts for your other servos.

Caveat
In Futaba servos (and perhaps others) the circuit that drives the motor draws considerable current itself when active. A normal servo draws large currents only until the motor moves to the commanded position. The motor and drive circuit then turn off and the servo draws a small quiescent current. In a modified servo the target position is never reached because the feedback pot has been removed. The motor may not spin because of the blocking diode but the control circuit is still driving a voltage and consuming power. In Futaba 3003's the normal 10mA quiescent current increases to 44mA. For a system with two modified servos (pan and shutter) you will have an additional 66mA of current draw. Fortunately, servos do not drive their motors when the transmitter is off, so it is possible to reduce system power consumption by turning off the transmitter whenever possible. Some receivers unpower their servos when the transmitter is idle, further conserving power.


On the Addition of Video Viewfinding...
My foray into video viewfinding has been educational. I started with a CCD BW camera and 1Watt 440MHz crystal tuned ATV transmitter. Massive interference at the RC receiver made the rig virtually useless when the video transmitter was operating. A little snooping with a spectrum analyzer indicated that the transmitter was emitting substantial energy at 220MHz, 110MHz and 55MHz, all subharmonics of the intended transmission frequency. In particular, 55MHz and 110MHz are reasonably close to the 72MHz band used by the RC gear. I suppose I shouldn't have been too surprised. The transmitter was a kit and I was well aware of it's theory of operation. To reduce interference (and obtain additional RC channels) I switched from my basic 2-channel AM RC gear to a 4-channel FM set. Things improved, but not completely. I am now in the process of replacing the transmitter.

Quartz crystals are not available at frequencies above 100MHz or so. If a higher frequency of operation is desired, frequency doubling techniques must be used. Without getting heavily into RF electronics theory, suffice it to say that there are ways of doubling or trebling the frequency of a signal and that this technique has been used for decades to produce frequencies higher than available from crystals, but with the crystal's inherent accuracy and stability. Because a succession of frequencies from the base crystal all the way to the transmit frequency are present in the transmitter, care must be taken to ensure that only the final desired frequency is emitted. This generally involves tuning of numerous adjustable components in the design.

Fortunately, the development of surface acoustic wave technology (SAW) in the 1960's has made is possible to construct oscillators that operate directly at very high frequency. Since the only frequency present in the transmitter is the desired one, no tuning or filtering is required to achieve a pure transmit signal. Phase Lock Loop (PLL) oscillators are also capable of synthesizing very high frequencies from relative low reference frequencies with only a single multiplying stage. The large difference between reference frequency and transmit frequency offered by PLL tuned transmitters also allows easy generation of pure transmit frequencies. For the KAPer that is considering a video viewfinder, I suggest that only SAW or PLL tuned transmitters be considered.

Even with a well designed transmitter there may be some interference with the sensitive RC receiver. An antenna filter circuit may be required to block unwanted video signals from the RC receiver antenna. In a frequency doubler style transmitter, where subharmonics may approach the RC frequency, receiver antenna filtering may not work.

Which Transmitter Frequency?
Amateur TV transmitters (ATV) are available to operate on 440MHz, 900MHz and 2.4GHz bands. The choice of frequency depends on local interference conditions, government regulations, budget and convenience requirements. While the 900Mhz and 2.4GHz systems provide for the shortest antennae, they also provide the shortest range and are not directly compatible with television receivers. The 440MHz ATV band overlaps part of the cable TV spectrum (Ch 59, 60). This allows cable-ready televisions to directly receive ATV broadcasts. 440MHz is also just below UHF channel 14 and many inexpensive portable televisions with "slide-rule" tuners can tune down far enough to receive ATV transmissions. In particular, every hand held LCD television I have tried is capable of receiving 434-440MHz ATV. Frequency "downconverters" are required to adapt standard televisions for use with 900MHz and 2.4GHz transmitters. The downconverter receives the 900MHz or 2.4GHz signal from the ATV transmitter and re-transmits the video information at low power on VHF channel 3 or 4. A standard television receiver, tuned to the appropriate channel then receives the image. The extra cost, space and power requirements of a downcoverter must be considered when selecting an ATV system.

Supercircuits have a very small 434MHz ATV transmitter called the "PowerPlate". I have not used this transmitter, but it's SAW oscillator technology should minimze interference with RC gear. It's low transmit power (200mW) may not yield more than a few hundred feet of range, but this may be sufficient for many. It is quite small and light, and coupled with one of their tiny BW cameras should produce a very nice KAP video viewfinder.

Note that the use of any ATV system requires an amateur radio license. Just think of that as an excuse to get into yet another hobby.

I'll have more on this subject after this winter's tinkering.


Improved Futaba Unidirectional Servo Modification
The Futaba Servos I've seen (S148, S3003) can be easily modified for unidirectional operation without using a diode or unsoldering the motor. This simple modification also yields minimal operating current when the motor is not running. For clockwise rotation (as viewed from shaft), cut the trace marked "CW". For "CCW" operation, cut the trace marked "CCW". CW rotation corresponds to left stick travel, but this can be reversed at the transmitter. Similarly, a shutter release relay may be driven by a servo control board which has the "other" trace cut. Simply replace the motor with a relay, no diode is required.