Autopilot drive unit questions and answers
|Question: Which parts exactly makes up a good functioning autopilot system.....|
|Answer: You need more parts than one would think to make a good functioning autopilot system|
The above schematic indicates the minimal parts needed for a proper working autopilot system. The autopilot junction box (also called course computer) is the core in the system:
|Question: It doesn't matter which autopilot one chooses, they are all the same and working identically....|
|Answer: No, it really does matter which autopilot you choose as there are big differences!|
There are many important features in autopilot electronics which will contribute in a good working autopilot setup.
Full speed control and dynamic braking are the two most important features of a modern autopilot course computer.
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). The
autopilot has trouble to steer the boat as no real subtle
movements are possible and the pilot would often overshoot the required
rudder position. Itís like driving a car through a city with only full
throttle or full brake. You will get there, but it is a bumpy drive.
Often these on-off pilots are developed as power boat autopilot and
meant to drive a hydraulic cylinder (which is nearly always controlled
with an on-off signal).
Dynamic braking: "Why do I need
any braking? I want to power the rudder, not brake!". Modern sailing yachts
require rudder and steering systems and autopilot drives that are very
light running so the maximum amount of feedback from the rudder is
transmitted to the helmsman for the most optimal "tiller feel". On top
of that the tendency is to use bigger and bigger steering wheels with
larger moments of inertia (flywheel effect) or twin wheel systems that
also double the wheel inertia.
|Question: I don't need an electro-mechanical drive unit, a hydraulic cylinder works as well....|
|Answer: No, it really does matter which autopilot drive unit you choose as there are big differences!|
There are big
differences between various autopilot drive units. The
biggest differences can be found between electro-mechanical and hydraulic
drive units. Electro-mechanical JEFA drive units are much better suited
for sailboats than
hydraulic drive units:
Drag/friction: Much less drag while not in use. Especially on small boats itís very important to have the minimum force required to back drive the autopilot. Some of these boats use a tiller and the others a very sensible mechanical steering system. Lots of money is invested nowadays by the yards in roller bearings for the rudder shaft to achieve the maximum in feedback and the minimum in drag. Adding a hydraulic cylinder to the rudder/steering system is like adding a big damper. Hydraulic autopilot rams tend to back drive very difficultly as the cylinder has high pressure seals that brake the piston heavily and the oil has to be pressed from one side to the other through a small bypass valve causing a high friction.
Efficiency: Much higher efficiency. On a power boat (where hydraulic autopilots were designed for) the autopilot efficiency is not important as you have a continuous source of energy. On sailboats you donít have this source. Autopilots are continuous power users as often they work round the clock. In an electro-mechanical drive you have one energy transformation. You have electrical energy and it's transformed to mechanical energy. In a hydraulic drive you have three energy transformations. First you have electrical energy and you transform it to mechanical energy in the electro motor. The electro motor drives a hydraulic pump and transforms the energy into hydraulic energy. The hydraulic energy is transformed to mechanical energy in the cylinder. Every energy transformation has itís specific efficiency and loss. Due to three energy transformations the losses in the hydraulic unit are much bigger than in the electro mechanical unit. Even when you would have 75% efficiency per transformation (which is not realistic) you get 0.75*0.75*0.75=0.42 = 42% efficiency.
Another big difference between the hydraulic and electro-mechanical unit is the type of electro motor used. The hydraulic unit uses a standard brushed DC electro motor (small diameter, long shape) which are cheap but have a limited efficiency (45-55% efficiency). The electro-mechanical unit uses a flat wound (pancake) motor with more than 80-90 % efficiency. This results in a total efficiency of hydraulic units to be between 25 and 35 % and electro-mechanical units between 70 and 80 %. In practice this will mean you need up to 3 times more power to get the same output out of the hydraulic unit. The differences are even bigger on larger (>55 foot) boats. You can't use a reversible type hydraulic drive any more (not available as the motor currents would be too big to control). The hydraulic option on bigger boats is a "continuously running power pack". This power pack keeps a hydraulic reservoir continuously under pressure and the autopilot uses solenoid valves to pressurise each side of the cylinder. Due to the continuous running power pack motors, this system consumes even more electrical power and could use up to 10 times more energy than an electro mechanical solution.
Speed Control: Much better speed control. Modern autopilots use pulse width modulation to vary the power sent to the drive unit from zero to hundred percent. This way they can accurately control the speed of the drive unit. JEFA electro-mechanical drive units use flat wound (pancake) electric motors, which have a thin but very large diameter rotor, have a very high start torque (due to the large rotor diameter) and a very small inertia as there is no iron in the rotor, just windings. Due to these features, they follow exactly the speed curve sent by the autopilot.
Normal standard brushed DC electro motors, used in hydraulic drive units, are not made to start and stop and reverse all the times. They are made for continuous and constant rpm rotation. They have a low starting torque (due to the small diameter rotor) and a high inertia (due to the heavy rotor). The result of this is a cut off of the ideal autopilot speed curve below 50-60% speed as these motors tend to need at least 50% of the maximum voltage to even start rotating. Flat wound motors start rotating at 5-10% of the maximum voltage.
The result of this all is a near binary (on-off) reaction of the hydraulic unit to the speed control of the autopilot and a smooth full speed control with soft start and soft stop by the electro-mechanical unit with a fraction of the power consumption.
|Question: I don't need an rudder feedback unit, the autopilot system can work nicely without....|
|Answer: No, we don't approve a system without a rudder feedback unit.|
|The rudder feedback unit is connected to the rudder shaft via a lever, so the autopilot can track the rudder angle. Some autopilot producers try to safe money and deliver autopilots without rudder feedback unit. We advise not to use these autopilots as not knowing where the rudder is located can be very dangerous as the drive can be sent through the end point and cause damage to the hull and drive unit. It's also very handy to see the actual rudder angle on the display as feedback info for the helmsman. Failing to install a rudder feedback unit will invalid the Jefa warranty terms as serious damage will happen at a point in time.|