These last few years have seen tremendous changes in the level of technology being employed by everyday modelers, but in all fairness, anyone who is involved with helicopters is probably at a higher level of commitment. Helicopter flyers have witnessed cutting edge in developments in the engineering of their machines and radios. Most recently, developments have occurred that revolutionized our concept of rate gyros.
First we had the mechanical gyro, and if it was a good one, it would be mounted in ball bearings with and external mixer and amplifier. Then came a range of piezo electrical gyros that utilized the characteristics of a resonating ceramic element. Both gyros functioning to dampen movement about the sensed axis, whether it was initiated by an external disturbance, or commanded by the pilot. This worked against the pilot in some instances when the pilot required high displacement rates which would be dampened to a much reduced rate by the feedback of the gyro. Additionally, in order to dampen an uncommanded displacement, an initial displacement must have first occurred; which by definition means that the heading has already wandered. Once motion was sensed, the gyro's electronics reduced the rate at which the displacement was occurring. The end result was that there would be a definite movement in favor of the external force.
Just recently, 2 gyros have been introduced that stand the traditional expectation of rate gyros on their head. Arcamax and CSM Technology have introduced rate gyros that employ processing to sense unwanted displacement and actively correct for it. Early next year, I will be producing an article (in layman's terms) on the methodology used to generate this sophisticated (to the hobby industry) processing. One thing to note; this technology has been in use for 40 years in the aerospace industry. Any aircraft that uses a stabilized platform as an inertial reference uses exactly the same processing of rate information. More later on this subject.
The Arcamax PI System gyro comes packaged (a loose term in this instance) in a ziploc bag with a card label stapled to it. For such an incredible piece of technology to come packaged like this is somewhat surprising; I expected it to come packaged in similar fashion to the JR or Futaba equipment. Still, the gyro only cost me $197.00, a full $150.00 less than the Japanese products.
What you get for your money, is an extremely small package. That is a penny in the picture above, but don't let the size of the box put you off purchasing one of these jewels. Inside is a PIC which is basically a microcomputer on a chip, plus and external analog to digital converter (12 bit resolution), hardware pulse width modulation and 10 bit servo I/O.
The PIC used in the Arcamax gyro is slightly more capable than the PIC in the CSM gyro. The CSM gyro uses software to produce its PWM and a lower resolution ADC. However, the CSM gyro uses an addition connection to allow alteration of its software configurable parameters. Arcamax have told me that any advancements in their software will be available as an upgrade for a nominal charge ($30 was quoted to me).
Included in the package are instructions on the initial setup and installation of the gyro. These instructions are poorly reproduced, and have amateur directions on how to set up your transmitter. What follows are a set of replacement directions which may give you a better understanding of this process.
The most confusing part of setting the switches is caused by the misleading screen printing on the front of the gyro. There are two switches on the face of the gyro. Look at the photo above, then at my diagram below. The diagram shows the proper arrangement of the switches which appears to be opposite to the screen printing on the gyro case.
Set the switches to match your equipment (either 2700G super servo or any other) and the sense direction.
There are 3 connections. Simply connect OUT to your servo, IN to your rudder channel, and GAIN to your gear channel. If you understand your radio well, then use the gain connection to set your gyro gain from any of your other available channels; BUT, be aware that the gain channel is used to change the mode of the gyro.
There are 2 modes of operation. You must understand how the operating mode is determined prior to connecting to any channel other than the gain channel, or adding sophisticated gain and mode changing in your transmitter.
The modes are 1. Normal 2. Heading Hold. The operating mode is determined by the amplitude and polarity of your gain controller channel. For an initial setup, this will be your gear channel. For example, a value of +100% would give you 100% gain (at your transmitter) and command the gyro to its Heading Hold mode.
Conversely, a value of -100% would give you 100% gain but this time, the gyro would be in its Normal mode. These maximum values correspond with a 1.0 millisecond pulse width for -100% (Normal) and a 2.0 millisecond pulse width for +100% (Heading Hold).
Initially, set the ATVs (endpoint adjustment) to 50% in each direction. If you purchased the unit with the 2700G option, adjust the potentiometer on the gyro to mid travel. The potentiometer adjusts the maximum gain of the gyro, while the ATVs on the gear channel determine what percentage of the gyro gain will be used.
At this point, do not connect your tail rotor linkage to the servo. You must ensure that there is no tail rotor revolution mixing active, no rudder offset programmed, no throttle hold offset programmed, no subtrim, nothing, nada, zilch. The only thing coming out of your receiver in the rudder channel is the pure input from your stick. This is necessary when setting up the gyro for the heading hold mode. Any mixing you add later must only be used for the normal mode and switched out when entering the heading hold mode. Additionally, adjust your rudder ATVs to +100% and -100%.
Turn on the tx and rx. Ensure your rudder trim slider is physically centered on the tx. Switch your gear channel and observe the output arm of the servo. If the servo starts to drift immediately upon entering heading hold mode then use the subtrim adjustment to eliminate it. How do you know you are in Heading Hold? Easy, when in Normal, the servo will follow stick input returning to its start position when the stick is centered. In Heading Hold, the servo will not return to its initial position.
Experiment with the subtrim value so that you can switch between modes without the servo drifting off for several seconds. When this is accomplished, reposition your servo arm so that it is at 90 degrees to the tail rotor control rod when in Normal mode. The length of the servo arm may be calculated by holding the rudder stick hard over; the linkage should not bind at any stick position (it is not necessary to "overdrive the linkage" as you would with a conventional gyro). Any adjustment in tail trim when hovering in Normal mode must be accomplished using the linkage. This is also the recommended practice when adjusting a normal rate gyro.
Install the gyro securely using 1/8th inch servo tape. Ensure that it is mounted square in all axes or cross coupling will occur.
Start the engine and lift off into a hover while in Normal Mode. If the helicopter yaws, DO NOT use the rudder trim slider on your transmitter. Adjust this mechanically using your linkages. Remember, any command from the transmitter is interpreted as a rate command; therefore, using the trim slider to correct a drift problem in Normal Mode will only result in a yaw in Heading Hold.
Why? Well, the gyro uses a 1.5ms pulse width in Heading Hold to mean "Zero Rate Commanded." The software then drives this active closed loop control system so that the helicopter does not yaw, or corrects it if it does. Our radios may not be transmitting an exact 1.5ms pulse width when the trim slider is physically centered hence the process of adjusting the neutral (zero command) using subtrims.
All that remains is to find the maximum gain that can be set without the tail wagging on the model. On the 2700G optioned gyro, I started by increasing my gear ATVs from the 50% point up to 100%. The model would accept more gain so this was increased by adjusting the potentiometer on the gyro case. Right now, I am at 75% gain approximately on the gyro and 100% gear ATVs. The system will obviously take more gain but bad weather has precluded further testing.
Things to Remember
I have now flown 20 or more flights with this gyro. The only change that I have made was to increase the ATV. I can honestly say that this gyro is most impressive. Almost all flights have been made in Heading Hold mode as I just have had no need for the standard yaw rate device. The big advantage of this gyro over the CSM is simply size. Both gyros fly exactly alike with technical differences being irrelevant esoterica.
The Tsurugi Web Site
Copyright © 1998 Chris Berardi. All rights reserved.
Revised: January ,(, /),(.