How Does An Autopilot Controller Work?

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How Does An Autopilot Controller Work?

An Autopilot is one of the more useful instruments you can fit to your vessel.  It contains the intelligence needed to automatically steer once you have instructed the autopilot where you want it to go.  Once installed it is easy to use, but it is important to understand how it functions and why it behaves in the way it does.  This ensures you get the maximum benefit from the instrument and knowing your equipment is important for safety at sea.  All autopilots work in much the same way, they compare the actual course to the required course and generate an error signal which is applied to the steering to correct the course.  Clearly you need to read your manual carefully and familiarise yourself with the controls so that in the event of an emergency adjusting the autopilot does not need you to get the manual out!!

When a vessel turns off course or the reference course is changed, the autopilot applies helm in a way that brings the vessel quickly back onto course without overshooting the reference course.  The correct rudder angle depends on the amount of error, the speed of the boat, it's size and the effectiveness of the rudder.  In order to customise the autopilot performance and adapt it to a specific vessel in specific sea states there are two control settings that need to be tuned.  Normally once set up there is no need to change these settings.  If you need to optimise the settings it is important to know what they do.  I want to explain these two important settings - the Sensitivity and Rudder Factor.

Sensitivity (also known as Yaw, Weather etc)

The main task of the autopilot is to lock the heading of the vessel to a predefined reference course.  When in the Auto steering mode, if there is deviation from this reference course the autopilot adusts the steering to allow the rudder to correct the course.  There is inevitably a trade off between the accuracy holding the reference course and the activity of the rudder movements.  The more accurately you want to hold the reference course the more the steering drive unit has to work.  Changing the Sensitivity parameter lets the user fine tune the balance between these two factors.  It gives the user the ability to adapt the Autopilot to changing weather/sea conditions.  The Sensitivity control governs the amount of "wander" in the vessels steering.  In TMQ autopilots, a high value for sensitivity will allow the vessel to drift further off course before correction is applied.  Whereas the low value will attempt to keep the vessel more precisely on course.   You may want to get the lowest value without the steering continuously "hunting" from side to side.  This minimum setting is affected by several factors including the amount of slack in the steering system, vibration around the rudder feedback mounting position, characteristics of the steering system and the weather conditions.  If you allow the steering to continuously hunt then you will cause unneccessary wear on the steering system.

Rudder Factor

The rudder factor is a tool to adapt the autopilot to the steering system on your vessel.  Depending on the rudder size, shape and design the amount of force needed to turn the boat can be adjusted using this parameter.  This function makes it possible to "tune" the autopilot electronics to the rudder angle, so that the a given number of degrees of helm are applied for a given course error.  In large or slow boats this would be more and in small or fast boats less.  Setting the rudder factor too high causes over-steer.  Too slow a setting causes under-steer and a slow response.  In the diagram above the top course shows what happens when the rudder factor is too high and the lower course when it is too low.  The adjustment of this parameter should be done in a calm sea.

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