TruTrak Digiflight Install Manual - Manual (Page 13)

TruTrak Flight Systems DigiFlight Autopilot Installation Manual
11 March 2002 Printing
control system lockup at the extremes of servo travel. Insure that any “lost motion” in the linkages is eliminated or minimized,
in order to maximize the performance of the autopilot. Lost motion (dead zone) will result in wandering or slow “hunting”
behavior in flight.
The next step in the check-out procedure is to verify that the serial input from the GPS receiver is being properly received and
interpreted. With the aircraft outside of any building, power up the GPS panel-mount receiver and the autopilot. After the GPS
receiver acquires its position, the autopilot will begin to flash the “*” character once per message from the GPS unit showing
that valid position data is available. The display will still show MAG followed by a flashing “*” character, followed by the
present approximate magnetic heading. If no “*” is displayed even after it is known that the GPS unit has a position fix, the
problem must be diagnosed. Possible reasons for such a problem are,
" Pin 25 on the connector is not wired to a source of RS-232 serial data
" The GPS receiver’s baud rate disagrees with that selected within the autopilot
" The GPS receiver’s serial output port has not been properly configured to provide the information
The remaining adjustments relate to the dynamics of flight and compensation of the magnetic backup system in the autopilot.
DigiFlight 200 / 200VS First Flight
The first flight should be done after having completed all the setup and testing on the ground. For the first flight, it is important
that the GPS unit is properly functioning with the autopilot, so that the dynamics of flight can be set without consideration of
the calibration of the magnetic backup system. As discussed earlier, when there is proper connection to the serial input of the
autopilot, the display will show a flashing asterisk “*” in the display to the right of the word MAG; once taxi speed exceeds 10
knots, the display will change from MAG to TRK if the GPS unit has achieved a position fix and sufficient groundspeed. If this
does not occur on fast taxi speeds, it is best to diagnose the problem prior to first flight of the autopilot.
The two activity adjustments (LAT ACTIVITY and VRT ACTIVITY) determine how briskly the autopilot responds to roll and
pitch disturbances. They can be adjusted, in flight, over a wide range; thus the autopilot can be tailored to adapt to any aircraft
installation.
Each of the two activity adjustments covers a numeric range of 0 to 12. Unless the value for a particular aircraft is provided by
TruTrak, it is advisable to start with a setting of zero and work up from there. Most installations would ultimately require
somewhat higher settings.
On the first flight, manually fly the aircraft to a suitable area for testing. Press ENTER to engage the autopilot. Observe that the
SEL field in the lower-left now shows the captured present ground track (shown after TRK in the upper-left of the display).
DigiFlight 200 DigiFlight 200 VS
Press ENTER to engage Altitude Hold The SVS field will be showing the approximate rate of
climb or descent in feet per minute.
Press ENTER to move the cursor to the SVS field,
select the value 0 ( which will engage altitude hold).
Press and hold the knob for about eight seconds until LAT ACTIVITY is shown on the display, along with an underlined
value. Rotate the knob to select the value zero (0), and observe the resulting control movement. Increase the value one setting
at a time, taking time to observe an increasing level of control response. At some point, if too high a setting is chosen, the
autopilot will be jittery and over-active. Back the setting down until the autopilot is responsive but not over-active. It is best if
these adjustments are made in conditions of moderate turbulence (the TruTrak loves turbulence) so as to make it easy to
observe the response of the autopilot to disturbances. It will be noted that a fairly limited range of activity setting will be
acceptable; too low a value will result in sluggish response, while too high a value will result in nervous, inappropriate
response. Within this acceptable range there is room for individual preference; some people prefer a more aggressive autopilot
than others. It should be noted that any builder can accomplish this adjustment procedure and no professional is required.
Once the desired LAT ACTIVITY level is established, press ENTER to store the value.
Next, the LAT TORQUE field is adjusted. Again, it is best that this be done in light to moderate turbulence. The reason is that
more torque is required of the autopilot in turbulence than is the case in still air, because the velocity of the servo is greater as
turbulence requires more rapid servo movement. This means that when activity is set to the high end of the acceptable range, a
higher torque setting will be required.