Fin / Digifin¶

The tail fin is the Slocum's steering assembly — and on modern gliders it is a self-contained, self-calibrating digital fin ("digifin"). The fin houses far more than the rudder: it integrates the vehicle's antennas (Argos 401 MHz, Freewave 900 MHz, and the combined GPS 1575 MHz / Iridium 1626 MHz quad- helix), the recovery strobe, the rudder and its motor, and a leak detect circuit. The whole assembly is rugged enough to be used as a handle to manipulate the glider.

What's in the fin¶

The G3 fin is built around a titanium boom, a motor housing, an antenna feed-through, and a plate bolted to a PEEK mast with a barrel-nut fastener system. The rudder rides on a magnetic coupling (an external magnet drives an internal one — there is no shaft seal at the rudder), and the strobe is integrated into the leading edge of the mast in the orange Noryl housing shared with the GPS/Iridium antenna.

Four independent subsystems live in the fin and should each be tested (per the FCP):

Steering: how it works and how to test it¶

• c_fin is the commanded rudder angle; m_fin is the measured angle (both in radians). The autopilot drives c_fin to steer toward c_heading, and m_heading / m_roll report the result.
• Sign convention: positive fin deflects the rudder to starboard; negative deflects to port. With a properly ballasted glider, positive fin → increasing heading.
• Range is roughly ±25° (about ±0.45 rad).

Bench / FCP test (from LabMode):

wiggle on
report ++ m_fin c_fin      # verify c_fin drives m_fin, no warnings/oddities
                            # m_fin > 0 → rudder to starboard; m_fin < 0 → port
wiggle off

A normal rudder sweeps only ~5–10° either side of center and stops a few millimetres short of the rudder keeper at the top of the fin.

m_digifin_status — reading the fin's health¶

m_digifin_status is updated every glider cycle and is a bitwise OR of event flags. Information bits (≤ 2^20) clear themselves when read or when the condition clears; error bits (> 2^20) stay set until the device is taken out of service.

For example, m_digifin_status = 2097216 = 2097152 + 64 → a stalled motor that tried (and failed) to recapture its position, i.e. a hard-over, faulted fin.

Leak detect¶

The fin's leak-detect A/D is read via m_digifin_leakdetect_reading:

• It holds near 1025 (the manual cites ~1025; in practice readings sit in the low 1000s) when dry. Lower values mean water.
• A reading below 1018 is considered a leak and should be reported to glidersupport@teledyne.com. Release 7.20+ aborts on a digifin leak; on older firmware, add m_digifin_leakdetect_reading to your config.srf so it's reported. An abort requires digifin_leakdetect_count = 5 consecutive indications (it won't trip on a single dip).
• Occasional single out-of-range samples (~885) appear in many datasets and are not believed to be a concern; sustained low readings across multiple CPU cycles are.

Maintenance and field calibration¶

Replacing the rudder¶

The rudder is a field-serviceable item (a common breakage — tangled lines snap them, so carry spares):

1. Remove the two Phillips-head screws securing the mast top to the mast.
2. Hold the rudder and lift the mast top away from the fin.
3. Lift the rudder off the magnetic coupling.
4. Fit a new rudder and reassemble in reverse.

Replacing the internal moving assembly (beyond the rudder/magnet) is not a documented field operation — contact glider support.

Field calibration (set/check center)¶

From GliderDOS, manually command the fin to a centered position, then:

digifin wr 119 2     # disable start-up calibration
digifin rr 112       # read previous centered cal value
digifin rr 100       # read current fin position
digifin wr 112 <value>   # write the read-back position as the new center

Field troubleshooting¶

A digifin that throws warnings or takes itself out of service mid-mission is one of the more common at-sea problems. The general strategy is to limit the fin's range to what still works, relax the abort criteria, and keep navigating — then fix it on recovery.

Useful commands / sensors for limping a fin home:

put f_fin_safety_max 0.437       # clamp the usable range to measured limits
setdevlimit digifin -1 -1 -1     # set the error count that aborts to "infinite"
put f_digifin_movement_retry_max -1   # retry forever, never warn on a stuck fin

Reducing rudder noise (and saving power)¶

Excess fin movement wastes energy and generates acoustic noise. A WHOI/TWR experiment found that keeping u_hd_fin_ap_deadband_reset = 1 is important for navigation — with it at 0, heading variability rose and the glider was blown off course; at 1, navigation and speed-over-water were normal while rudder activity dropped. Reported settings that cut fin movement while still navigating:

put u_hd_fin_ap_deadband_reset 1
put u_heading_deadband 0.261        # ~15°
put u_heading_rate_deadband 0.0261
put u_hd_fin_ap_run_time 120

In low power mode, the fin autopilot uses separate gains (u_low_power_hd_fin_ap_gain / _igain / _dgain); in strong currents the low-power gains can be too gentle to hold heading even though the fin reaches its commanded position. See the Power Saving page for the low-power context.