Pitch Vernier (battery movement)¶

The Slocum doesn't have elevators or a moving tail to set its dive angle — it slides its heaviest internal mass back and forth. A lead screw drives the forward ~10 kg battery pack fore or aft, shifting the centre of gravity to trim the vehicle's pitch. The manual calls this the Pitch Vernier: the ballast pump supplies the buoyancy that makes the glider dive or climb, and the moving battery is the fine adjustment that sets at what angle it does so.

• Moving the battery forward makes the nose heavy → the glider pitches down (dive).
• Moving the battery aft lifts the nose → the glider pitches up (climb).
• On the surface the battery is driven all the way forward to raise the tail (and its antennas) out of the water for comms.

The vehicle is designed to dive and climb at about 26°. This only works if the glider is properly trimmed — the H-moment must be 6 mm ±1 (see H Moment Calculation / Adjusting the H Moment in the Maintenance Manual) — otherwise the ballast pump's moment won't pitch the glider as expected and the battery vernier has to fight it.

Relevant sensors¶

Sign convention (with a properly ballasted glider): positive/forward battery movement brings the nose down; aft movement lifts the nose. As a practical example from the forum, an operator flying with fixed positions used ~0.45 in for dives and ~0.25 in for climbs.

Three ways to command pitch: c_use_pitch / d_use_pitch¶

Dive (d_…) and climb (c_…) behaviours in the yo*.ma file each choose a pitch mode plus a value:

• Servo flies the most accurate angle but works the motor the hardest. Fixed battpos (mode 1) and setonce (mode 2) move the motor far less, which is why they show up in power-saving and quiet-mission / acoustic setups.
• setonce can miss the target angle — it commits to one move based on current trim, so if trim is off it can settle well short of what you asked (operators have asked for 26° and gotten ~18°). Use servo if the exact angle matters.
• Firmware note: before release 7.9, servo mode hunts continually and restarts each dive/climb from whatever position it was last left at, so the battery is mis-placed at the start of every inflection. 7.9 and later remember the last good dive and climb battpos and start the servo from there.

Securing the pitch battery — always use blue Loctite¶

This is the single most common, most preventable pitch problem in the field: the pitch battery's mounting screw can vibrate loose during a mission, letting the battery detach from the lead screw. The fix is a build-step discipline.

During forward-section assembly the Maintenance Manual is explicit:

Place a drop of Loctite 243 (blue) on the pitch battery mounting screw, insert the pitch battery (feeding the cable harness through the centre opening), then secure it by tightening the mounting screw with the 5/32" × 12" red T-handle hex wrench.

How a loose pitch battery looks in the data (and why it fools you): the position pot reads the motor / lead screw, not the battery itself, so m_battpos and c_battpos can look completely normal even after the battery has come free. But once detached, the battery slides toward the tail under gravity — worst on the climb — so you see wild pitch, commonly pinned near ~60° nose-up (the pitch sensor's limit) and poor dives. If pitch is misbehaving but the battpos numbers look fine, suspect a loose battery and open the glider to check.

Testing the pitch motor (lab_mode)¶

From lab_mode on:

1. wiggle on
2. report ++ m_battpos
3. Confirm the battery completes a full extension and retraction without errors.
4. wiggle off, then report clearall.

The ballast command sets the pitch motor and ballast pump to 0 (and deflates the air bladder) for ballasting — never deploy a glider left in ballast or in lab_mode.

When the pitch battery won't move — or you must fly without it¶

Pitch battery not moving fast enough? The usual causes are mechanical — the pack is bound or its travel is obstructed so it can't slide freely — or the wiring to the position pot or motor is damaged/unplugged.

Fixed battpos not changing between dive and climb? Check the deadzone: f_battpos_deadzone_width (sets the x_ limit) and f_battpos_db_frac_dz (deadband as a fraction of the dead zone). If the deadzone (e.g. 0.2 in) is as large as the gap between your dive and climb positions, the battery never bothers to move. Narrow f_battpos_deadzone_width for that deployment so it always repositions.

Flying with a failing pot or a stuck pack: if the glider is intermittently aborting for "pitch battery not moving," or the pot is failing, you can fly without the pitch motor entirely:

1. If the battery still moves, put c_battpos to position it where you want (a good dive/climb compromise), then leave it.
2. use - pitch_motor to take it out of service.
3. Wait for callback, then in the mission add a nop_cmds behavior with nop_pitch(bool) 1 (commands pitch to _IGNORE to keep the behavior stack busy).
4. In the yo behavior, change start_when(enum) to 4 (start when the buoyancy engine is idle) instead of the usual pitch-idle trigger, so the yo still cycles without a pitch command.

Field troubleshooting¶