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+--- Forum: Autopilots (/forumdisplay.php?fid=23)
+--- Thread: Autopilot drive constantly hunts (/showthread.php?tid=4389)
Autopilot drive constantly hunts - Derek - Raymarine - Moderator - 12-08-17 10:58 AM
There are a number of situations in which the autopilot drive continually hunts to and fro.
This has a number of adverse effects:
- Poor course holding
- Constant movement to and fro of the steering wheel in mechanical steering systems
- Excessive current consumption
- Excessive heat generated in both the computer/ ACU and the drive unit
- Accelerated wear and tear on the steering system, drive unit, rudder bearings
- excessive noise as the steering is constantly operated
Most of these issues can be easily solved through correct installation, correct commissioning and correct calibration.
The installation causes are best separated by the two main types of steering system, Mechanical steering and Hydraulic steering.
Mechanical causes can be due to flexing of the mounting, poor installation or loose fastenings of the rudder reference unit - which will then feed poor quality position information back to the course computer/ ACU
Excessive slack in the mechanical steering system - loose cables, worn sheaves or Rudder stock bearings, worn shafts or bearings, worn gears/ gearboxes or worn worm drive, loose quadrant on rudder stock
Inertial effects in the steering system caused by the steering wheel (or wheels) acting as large flywheels, or the weight of the rudder - larger boats tend to have more steering inertia which causes the system or quadrant to 'flex' when reversing direction leading to positional inaccuracy of the rudder reference unit.
Also such things as water flow over the rudder, (or rather lack of it) can cause the steering to become very light when operated quickly and can trigger oscillation.
Clearly the loose and worn fixtures, bearings and gears must be replaced, cables and mountings tightened to the correct torques. The mounting for the rudder reference unit must be secure and the correct installation guidelines for the rudder reference unit followed. Provided all this is done first and if the drive still oscillates, then check that the commissioning process has been fully followed, noting that RUDDER DAMPING in the drive calibration menu may need to be increased, especially on larger boats to 4, 5 or even higher to compensate for the 'flexing' of the steering system and system inertia, (this is the correct purpose for rudder damping). Note that RUDDER DAMPING should be kept as low as possible, but should be increased to the appropriate level to improve the drive motor stability.
With Hydraulic steering systems, the possible causes are:-
Air in the hydraulic system
Poor installation or loose fastenings of the rudder reference unit - which will then feed poor quality position information back to the course computer/ ACU
Excessive slack in the mounting of the hydraulic cylinder
Incorrect size of pump fitted for the autopilot causing incorrect Hardover to Hardover time.
Once The ram capacity has been checked to ensure the Autopilot pump fitted is the correct size and the Hardover time when the autopilot is operated is confirmed as between 8 to 12 seconds, click here for explanation and the Rudder reference unit installation checked.
The steering system should also be fully bled of any air.
Note that all hydraulic oil contains microscopic air bubbles which coalesce over time and can lead to larger trapped air bubbles, which can then cause the ram to move in a jerky unstable motion leading to positional instability.
Finally, as with Mechanical steering systems, you may find that you need to increase rudder damping as described above, due to the size and nature of the installation, (for example, hydraulic hoses all flex slightly under pressure, hydraulic fluid has inertia of its own etc . . .)
Reducing the 'hunting' of the drive will have a massive benefit for the system in terms of improving efficiency and reducing wear and tear.
RAYFAQ#