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RX3S Gyro Stabiliser

The RX3S V2 stabiliser for RC models, as it is first experienced by a new owner, is subject to a number of shortcomings. These can be overcome by a process of re-flashing an internal chip with new firmware which significantly improves the stabiliser’s performance.

The first part of this guide will give a brief outline of the unmodified RX3S including a description of how the unit may be used in its original form and the features that need improvement. The second part of the guide will refer users to Internet links that show how to modify the unit, followed by a guide to its subsequent use.

The ORX3s as supplied

The original RX3S V2 is contained in a small orange case, 44mm x 26mm x 13mm, with a block of 8 connectors at one end, 6 dip switches, and 3 adjustment pots marked YAW, PITCH and. ROLL

The dip switches have the following functions.

1 –          Selects between having the gyros on all the time and having them switchable ON/OFF from the transmitter.

2 –          Reverses the direction of the rudder gain direction including the servo output.

3 -            Reverses the direction of the elevator gain direction including the servo output.

4 -            Reverses the direction of the aileron gain direction including the servo output.

5 –          Mixes the aileron and elevator outputs for V-Tail operation.

6 -            Mixes the aileron and elevator outputs for Delta operation.

When the switches are set to the right side of the unit (towards the connector blocks) they are in the normal, or off position. Set to the left they are in reverse position.

The adjustment pots control the output gains of the functions, as marked, from 0% fully anticlockwise to 100% fully clockwise. (Most of this adjustment range is obtained from only a small part of the full movement.)

The inputs and outputs are labelled, the AUX channel being connected to a switched channel on the transmitter in order to switch the output gains between the selected values and OFF. As supplied this device does not provide proportional gain control, it is strictly ON/OFF. Also, it does not provide any form of auto levelling of an aircraft. It corrects only for momentary diversions from the flight path that are not the result of the pilot’s commands, such as turbulence and wind effects.

Weaknesses in the original functions

The first noticeable feature is a continual jittering of most types of servos, regardless of how the gain is adjusted or which channels are being used. (This fault is reported as having been corrected in Version 2.1).

The dual aileron outputs appear to have the effect of a built-in Y-lead. This is not the case because where twin aileron servos are fitted in the manner adopted by most models the dual outputs move the ailerons in the same direction, both up or down together. It is simplest to ignore one output and to use a Y-lead on the other. A flaperon function is definitely not available. There are mechanical ways of re-fitting servos, or servo arms, or control horns, to overcome this but for the most part the second aileron output has no value.

Fine adjustment of the gain control pots is difficult. Most of the control effect occurs over a small part of the pot movement so that getting exactly the gain desired becomes a matter of trial and error until the pot finally hits the right spot.

Reversal of a gain control output is managed by use of the dip switches. Unfortunately this reverses the servo movement at the same time, so whenever the sense of the correction output is reversed the servo direction has also to be returned to its previous direction by reversing it at the transmitter.

Some examples of the unit have occasionally been found to lock a control surface intermittently at full deflection. (This fault is reported as having been corrected in Version 2.1).

The modified version

Much work has been done with excellent results to alter the performance of the RX3S. This is extensively documented on the Internet. To begin finding the method for flashing the firmware use this link: -

http://www.rcgroups.com/forums/showthread.php?t=1794672

The topic is titled Open Flight Stabiliser. It contains much useful information, full details of how to obtain the materials to connect to the circuit board, and as usual on a forum some unhelpful waffle.

The end result of following the procedure is a version of the RX3S with considerably enhanced performance and the old problems resolved.

Improvements

The servos are no longer subject to jittering.

The dual aileron outputs now function as would be expected, moving ailerons in opposite directions when worked by twin servos. The gain control pots now use a large part of their movement for easy adjustment.

The direction of the gain outputs is no longer controlled by the dip-switches; it is controlled by the pots. The 0% position is the centre of the pots’ travel. From this point anticlockwise movement increases gain in one direction, clockwise movement increases gain in the other.

Gain control from the transmitter is now proportional and is best controlled by a slider or a pot, though a 3-position switch can be used to give two pre-set gains and ‘off’. The top dip-switch does not now select gyro on or off, the gyro is at al times under the control of the remote gain slider. OFF is obtained by centring the slider or switch that controls overall gain.

The locking of control surfaces has not been reported with flashed units.

Operation of the flashed stabiliser

The unit should be placed in a model in a convenient position near the receiver so that there is access to the gain control pots for making adjustments. It should be level in the fuselage, preferably with the connector block towards the tail. Connections to the receiver are made by short M-to-M leads. The servos plug directly into the unit. Closeness of the unit to the centre of gravity, contrary to some opinions, is not necessary as the gyros will detect rotations at any point within the model.

The dip switches now have the following functions.

1 –      Not used.

2 –      Used only in setting dual aileron without gain (see diagram below).

3 -        Used only in setting dual aileron with gain (see diagram below).

4 -        Not used.

5 –      Mixes the aileron and elevator outputs for V-Tail operation.

6 -        Mixes the aileron and elevator outputs for Delta operation.

The adjustment pots control the output gains of the functions, as marked, from 0% in the centre to -100% fully anticlockwise and +100 % fully clockwise.

Any mixers in the transmitter that have been set for V-tail or Delta aircraft should be cancelled, as the mixing is carried out in the RX3S.

Switching on is no longer as simple as it used to be. The modified circuit needs to establish the neutral positions of the three main control sticks, so the aircraft must be kept still for the first 15 seconds after power-on. It will probably give a slight twitch of some of the control surfaces at the moment of powering on, but in about 5 to 8 seconds the surfaces should give another three distinct twitches, (i.e. left-right-centre). After 15 seconds the circuit has all the information it needs for flight. From then onwards proceed as usual.

Modes

There are two modes of use, RATE mode and HOLD mode. These are selected by the gain control.

RATE mode gives normal gyro correction of external disturbances. It also reduces the effect of the gyro as the transmitter stick is moved away from the centre position so that the gyro does not attempt to counter the pilot’s commands during aerobatics. Personal experience shows that it is difficult to estimate the amount of proportional control by only the feel of the position of a pot or slider. A 3-posotion switch works well with one end programmed for 0% gain and the other positions programmed for slow flight (landing) and high speed — the latter limited to a maximum value that will prevent the onset of oscillations which can damage an aircraft.

As an example a Multiplex TwinStar is comfortable with settings on aa 3-position switch of 0%, 40% for general flying, and 80% for landing.

HOLD mode allows the gyros to hold the control positions as they are when the pilot centres the sticks. The aircraft can be flown as usual but if the sticks are released (centred) the controls will be adjusted to hold the aircraft in the attitude it was in at the moment of release. This mode is best tested at height because flying becomes different, since any moment-to-moment control input is locked as soon as the stick is centred. The model’s responses are not the same as usual. For initial sessions in HOLD mode it is advisable to use travel-adjust to restrict the maximum amount of gain available to as little as 15%, and increase the value later in the light of experience. Be sure of being able to revert instantly to RATE mode, or OFF, if the model shows any signs of becoming difficult to control.

 Gain control

There are two levels of gain control, individual control of each channel by the pots on the unit, and overall gain from the transmitter. The pots adjust the gains of each of the three channels separately to balance them to the needs of the aircraft, then the transmitter control raises or lowers them together proportionally. A slider, a pot or a three position switch may be used as the gain control, the first two giving proportional control.

All of these by default have a centre position which gives zero gain and can be used to switch the gyro effect off. From the centre in either direction the control progresses to full gain at the end of the travel. In one direction this gain is in RATE Mode, in the opposite direction it is in HOLD mode. On programmable transmitters it is possible to set limits to the amount of overall gain by using the travel adjust function.

Switch settings for model layouts

For a conventional model with wing and tailplane layout, all the switches should be to the right. The top switch does not now select gyro on or off; this is at all times controlled by the proportional overall gain.

For a V-tail or Delta model the appropriate dip-switch should be moved to the left. Delta and V-tail mixing are managed inside the unit, so any mixing to produce the same effects should be cancelled in the transmitter.

The modified unit is now able to provide dual aileron function using two separate inputs from the receiver. The two aileron outputs then become independent, making possible the use of flaperons. This is at the cost of losing the ability to control gain from the transmitter.

There is one other possible arrangement for dual aileron use and gain control but it requires additional alterations to the circuit board.

The connections mentioned above are at the bottom edge of the circuit board below.

Alternative orientations of the unit in a fuselage, such as attached to the side, or to a former, pointing backwards, or inverted are possible but they call for a re-arrangement of some channel connections. As an example, if the unit is mounted to the side of a fuselage the gyro component that normally senses yaw now senses pitch, and the gyro that normally senses pitch now senses yaw. The sensing of roll in not affected.  Plugged connections between $$$$$$$$$$$$$$$$$$$$$$$$ and the direction of gyro correction have to be altered accordingly.

The unit must not be re-oriented in this way for V-tail and Delta aircraft. It must be placed level in the airframe. This is because of the way in which the internal mixers work, making it impossible for alternative channels to be used.

Three of the dip-switches, those previously used for reversal of the three main channels, are no longer needed as the pots have taken over that function, so the dip-switches are available for other purposes. What those purposes may be is not very clear at the moment, but this is not the end of the story.