autopilot dynamic braking and speed control

[OliverFdeV]

I am looking into the 711c autopilot and would like to know whether it has speed control and dynamic braking.
I found the following on the jefa.com website:

Speed control: Some autopilots control the autopilot drive unit with an on-off signal. This can result in a bumpy autopilot steering and high peak forces in the system as all movements are abrupt (no soft start and soft stop).

More modern autopilots are able to control the speed of the autopilot drive unit from 0 to 100% without any steps via so called pulse width modulation. Autopilots without speed control often start "hunting" at low boat speeds. Hunting is an oscillating movement around the mid position where the steering wheel continuously swings around the mid position.

Dynamic braking:
...due to the inertia of the wheel and the fact that there is no damping, the rudder overshoots the targeted end position and the autopilot starts to correct this and reverses the power and this goes on and on and on. This is called “hunting of the autopilot” an oscillation around the amidships position. This causes high stresses in the construction, could cause damage to the steering and rudder system, but above all will empty your batteries very quickly! Modern autopilots have a feature called dynamic braking to prevent this.
This holding power is the second advantage of a dynamic braking feature. While sailing on autopilot, the rudder needs a constant cut angle to the water flow to keep the ship on course. This requires a constant force of the autopilot drive driving the rudder. On systems without dynamic braking, the autopilot commands the drive unit to a certain rudder angle and takes the power off. But within seconds, the drive unit is pushed back by the rudder forces to the neutral position. The autopilot will have to drive the rudder to the required rudder angle again and when it is reached, the rudder slowly turns back neutral again. This not only gives a non constant course, but also uses a lot of power, causes high wear in the drive unit and heats up the drive unit to immense temperatures.
With dynamic braking, the autopilot can "grab and hold" the rudder fixed when it is a certain position and hold it without any movement and without using any energy. As shown in these points, speed control and dynamic braking are very important and make or brake an autopilot system.

So here are the questions:
- does the navpilot 711c have speed control?
- does it have a damping feature so it prevents overshooting and hunting?
- can this autopilot grab and hold without power?

Thanks for a reply.

 

[Melville]

The answer to your first question is yes though I have never heard it called speed control, It is proportional meaning it varies the speed of the rudder depending on correction needed. This is accomplished using variable speed pump sets. The speed is controlled by applying more or less voltage. The 711C is also speed adaptive and learns to apply different amounts of rudder for different vessel speeds. The autotrim circuit will determine where to leave the rudder to so as to reduce rudder movement while still maintainability a set heading. I believe this is what is meant by "grab and hold" though I never heard it described that way.
Overshooting is prevented via the counter rudder algorithms.

 

[OliverFdeV]

@Melville, thank you that sounds good then.

Can you tell me what kind of power comsumption I would be looking at? How many amps would it take from the battery over a 24 hour period?

I am trying to figure out a complete furuno set for a 33 ft sailboat, hence power frugality is a big thing.

 

[Acer]

Power consumption is hard to guess. There are a lot of factors like boat trim, sea state snd kind of hydraulic pump. Wild guess, about 80 AH

 

[OliverFdeV]

Thanks for your insight, much appreciated

 

[Taniwani]

I think the 80Ah per day is a good guess. Fixed is what the course computer and the control heads draw when powered but in standby. On my boat (49ft sloop) the clutch current is the biggest consumer. It is a Whitlock drive and clutch draws 1.4A at 24V, that’s over 30W. In modest sea the Motor draws a fraction of that. I see it run about 15% of the time at low power. So I get to less than 50W. Your Jefa drive needs only half the power on the clutch, so subtract 15W. And that would come out at an average of 3A at 12V or 72Ah a day.