Some Q on Stabilizers.

[fcfc]

I was reading the thread on roll stabilization.

Why stabilizers are hydraulic actuated ?
There are now some efficient brushless DC servo motors. Automotive guys now have 750W 12V servo motors for Electric Power Steering. If you go to 24V or even 48V, you can get several hp servo motors.
Coupled with a ballscrew, these brushless servo motors have a better control, better accuracy, better cost than hydraulic. That s why automotive guys are replacing hydraulic power steering by EPS in newer designs.

Why no foldable/retractable stabilizers (in small size) ?
A long narrow fin will have a better efficiency than a large chord small span one. It will also move farther the thrust point from the roll center, yet a better efficiency. The only caveat is that they will catch anything in the sea or the harbour. So make them rotating like a dagger. It will then also be possible to extend them beyond the boat max beam.

Thanks for educating me.

[fcfc]

Are my questions too silly ?

[Dutch Peter]

No, but I have no real experience with stabilizers.
But my thoughts are:

Ofcourse depending on the size of craft I think the servo motors will be very large to take the loads induced, much larger than the hydraulic unit on the stabilizer (not talking about the powerpack).
What about maintenance/lifespan of servo's? also better than hydraulics?

Retractible stabilizers do excist!

That's all I can come up with, for now.

[FAST FRED]

Motors of the electric type are not suited to making a very small corection , say a 1/2 turn.

They also may suffer burnout if stalled , or given low voltage.

Hudraulic suffers from none of these hasslesand is VERY powerfull in a small size.

A 2 hp hyd motor is a one hand lift , 2hp DC in 24V would weigh a hundred pounds!

FAST FRED

[Dutch Peter]

Originally Posted by FAST FRED

Motors of the electric type are not suited to making a very small corection , say a 1/2 turn.

Fred,

That's not true, as memory serves me right, servo's are specially designed for that. Among others, servo's are also used in CNC machines and are able to make very turns!

Please correct me when I'm wrong.

[fcfc]

I do not know what are the induced rotating forces on a stabilizer fin, but I am not sure they are heavy. Other forces may be stronger (side, rear) , but are handled by the fin bearings, not the rotating device.

First, all stabilizater fins are balanced.

Second, most small boats have a tiller, or a mechancaly linked helm. No additionnal power than the helmsman arm to move the rudder. (And E. Tabarly even had a tiller on Pen Duick VI).

And last : just look at old wind autopilot. The aerial moved a small flettner that rotates verticaly a blade, which then rotate along an horizontal shaft, and then that rotation drives a rope attached to the tiller. You move your rudder from a very small force.

[Dutch Peter]

Originally Posted by fcfc

Second, most small boats have a tiller, or a mechancaly linked helm. No additionnal power than the helmsman arm to move the rudder. (And E. Tabarly even had a tiller on Pen Duick VI).

And last : just look at old wind autopilot. The aerial moved a small flettner that rotates verticaly a blade, which then rotate along an horizontal shaft, and then that rotation drives a rope attached to the tiller. You move your rudder from a very small force.

Yes, on small boats, try that on a 60 m yacht. Rudder forces can be considerable. To streamline the discussion it's maybe better to determine the size we're talking about.

[fcfc]

I was implicitly speaking of 12-18m displacement powerboats (40 -60ft).

In fact, boats which may not have otherwise an hydraulic system. Size range for electric bow thruster, electric windlass etc ...

Thanks.

[CDBarry]

A motor would also have to have a generator with a pretty serious peak inrush capability, but a stabilizer could be run off of a Buick power steering pump on the main engine and be operated with a 2 x 8 Prince cylinder, for a cost of about $400 a side (less out of Northern Hydraulics).

[D'ARTOIS]

In all cases I have been faced with the use of active stabilisers, they were hydraulic driven and not electric for the heavy loads distributed on the system. The hydraulic cylinders, however, are electronically monitored.

[tspeer]

While control surfaces may be largely balanced, the hydrodynamic center moves with angle of attack so it's not possible to be completely balanced over the whole range of motion. Plus, once the torque to overcome the static hinge moments drops well below the torque required for the acceleration necessary to meet the bandwidth requirements, you may not be saving much with regard to sizing the actuator. And it's desirable for the surface to be naturally stable in various failure scenarios.

I don't have experience with ship roll control systems, but I do have experience with aircraft fly-by-wire systems that are basically similar. Since there don't seem to be any control system designers contributing, I'll chime in. There are a lot of different competing engineering considerations when it comes to selecting an actuator. Not to mention the experience of the designers and operators themselves wth one type or the other.

When it comes to actuators in a feedback control system you actually have three choices: hydraulic, electromechanical, and electrohydrostatic. Your choice of an actuation system depends on being able to hold the required torques about the surface's shaft, have a bandwidth that is adequate for stabilizing the roll in dynamic conditions, is compatible with the sources of power from the ship's subsystems, and has acceptable failure characteristics. There may also be issues of the mechanical envelope required, weight, and maintenance.

Hydraulic actuators have a pump, usually mechanically driven, and a system of lines that deliver the power to the actuator with fluid flow and pressure. The actuator itself consists of a servo valve that is driven by mechanical, electrohydraulic, or a direct-drive electric motor to meter the flow of hydraulic fluid to either a piston and cylinder driving a linear ram or rotary vanes that apply a torque about the shaft of the surface.

Hydraulic actuators are capable of holding large static forces, have adequate bandwidth, and have good failure characteristics. For example, if something in the system fails, a shutoff valve can cut off the supply and return lines and shunt fluid from one side of the piston to the other, allowing the surface to float in a damped trail position instead of being stuck hard-over or at the position it was in when the failure occurred. Hydraulic actuators also lend themselves to redundant systems for increased safety and availability, such as the dual-tandem ram which has two pistons, each driven by a separate hydraulic system so the actuator can still function with complete loss of one of the hydraulic systems.

The position feedback in a hydraulic actuator is typically provided by a linear variable-differential transformer (LVDT) (possibly redundant) mounted inside the ram. There probably also be feedback sensors on the servovalve itself and solenoid to operate the shutoff valve. The hydraulic ram is compact for the amount of force it delivers, and much of the weight and bulk of the actuator is often in the servovalve and all its internal plumbing. The hydraulic fluid is also very efficient at carrying away heat generated in the actuator and can be routed through heat exchangers to dissipate the heat. Hydraulic actuators can be operated in environments with explosive vapors, such as fuel fumes.

The main drawbacks to hydraulic actuators are the need for a source of hydaulic power, having to run high pressure hydraulic lines throughout the vessel, and the maintenance associated with leaks. I don't know what the pressures are for shipboard hydraulic systems, but in aerospace we are moving away from 3,000 psi systems toward 5,000 psi system or higher. There are obvious safety issues with that kind of pressure running around. Hydraulic fluid can also be very flammable or chemically hazardous. And there's a continuous energy loss, especially if the pump is of the constant displacement type. Although hydraulic actuators can be operated in explosive environments, they can also create such an environment, depending on the hydraulic fluid.

Electromechanical actuators have elecric motors driving a gear train and typically have a ball screw that acts like the ram of a hydraulic actuator. For redundancy, two motors can drive the output through either a torque-sum gearing or a speed-sum (like the differential on your car) transmission. The power distribution is electrical and no fluid is required. Position measurement is typically by LVDT, and there will probably current feedback and velocity feedback in the controller. If the actuator has to operate in an explosive environment, the commutation of the motor may be done digitally in the controller and each coil of the motor powered individually by the controller.

The main advantage of electromechanical actuators is the electrical power distribution, since electrical power is increasingly abundant to meet other demands. There is no danger of high-pressure fluid leaks, no need to stock replacement fluid, and no chemical contamination. Electromechanical actuators are also easier to make since they don't require the extremely high tolerance machining required for something like a hydraulic servo valve.

However, electromechanical actuators have difficulty holding sustained loads. If you size the motors to be able to hold the maximum surface hinge moment on a continuous basis, the thermal requirements drive you to very large, heavy motors. These large motors have no problem meeting the bandwidth needed from the actuator, but the controller and the power system may not be capable of handling the large fluctuation in power demands. This may require adding what amounts to huge capacitors to the system to supply peak currents and to absorb the electricity generated by the actuator when it moves in response to an aiding load. Otherwise these power fluctuations can degrade the electrical quality for the rest of the vessel.

Electromechanical actuators can also have poor failure characteristics, tending to fail stuck at last position. When you backdrive an electromechanical actuator, the gear train amplifies the inertia of the motor and the friction in the transmission. So it's very difficult to get the kind of damped trail behavior the hydraulic actuator provides.

Heat dissipation is a huge problem with electromechanical actuators. They are usually cooled by conduction and natural convection, so they can heat up the whole compartment where they're installed. Making the motor compact for weight and efficiency just concentrates the heat production and makes the problem worse. Weight is also an issue, at least in aircraft. Although you do eliminate the weight of all the hydraulic plumbing. Electrics of all kinds are subject to corrosion in a marine environment, and since roll control systems are installed in the bilges, being wet or even immersed is an issue.

The electrohydrostatic actuator aims to capitalize on the good points of both electrical and hydraulic actuators. It has a hydraulic ram with its own hydraulic system, and an electrically driven pump built into the actuator plus a small hydraulic reservoir. All of the hydraulic fluid is contained within the actuator - there are no lines running anywhere. The power distribution is electric, so you eliminate all the overhead of the hydraulic system. But since the ram is hydraulic, it has all the nice failure characteristics and the ability to sustain a large static load of the hydraulic actuator. You still have to stock hydraulic fluid and can have leaks, but the quantity is far less and when you do have a leak it's impact is localized.

The choice of an actuator type is not an easy one, and moving from one tried-and-true system to a different type is expensive and risky. But the general trend, at least in aircraft, is away from hydraulics and toward all-electric systems. I suspect the trend in marine systems is similar.

[Navaldesign]

Hydraulic stabilizers do have their origin in larger ships, where hydraulic power lines exist anyway for other purposes as winches, doors ecc. so it came natural to use hydraulics. Reliability of hydraulics is far greater than that of electric actuators especially in marine enviroment. Maintainance also is very reduced and lifetime extremely long. I' ve been handling hydraulic actuators on prof. fishing vessels in Italy for 15 years and i have seen some running without any kind of maintainance for more than 30 years. On the other hand displacement yachts of a certain value even in the range of 15 to 25 mt LOA
are currently turning many of their equipment in hydraulic type since some of the onboard services such as hydraulic ladders, winches and emergency propulsion systems do require hydraylic power. So it comes natural, having a hydraulic powerpack onboard, to use it also for stabilizers. However the main reason for the use of hydraulic actuators is reliability and longlife.

[FAST FRED]

" but in aerospace we are moving away from 3,000 psi systems toward 5,000 psi system or higher."

On fighters and other military work the newest trend is for the hyd pump and motor to be electrically powered AT the controll surface.

The simple package has all the design & overload of any Hyd system , with out the weight of all that pipe and hyd fluid thru oit the aircraft.

Even better is the ability to power the hyd system with multiple electrical paths,
great for absorbing battle damage , and still getting home.

I would assume the more advanced windlass mfg will be first to offer this setup in a marine package.
The windlass would be lighter and more powerfull than is rational with just DC electric found on most smaller boats.

FAST FRED

[Navaldesign]

I think one of the most important factors that determinate the use of one or the other type of actuator is cost. Solutions like those on military fighters who have an incredibly high budjet aren't really suitable for civilian use.

[cyclops]

There is no budget in the military, if the improvement works. After a proving and use period, the " secret " is released to industry. If you have, had, the money, you could have a nuke powered yacht . Just have all the parts arrive in a 3 rd party country. 30 years ago I helped solve the erratic operation of a 2,200 ton metal forming press. It used a 5,500 psi variable swash plate pump. The technology is usually not so new. It is the very few open, creative and pit bull application people who spend days running down the bits and pieces of fact and parts, that make the improvements. And it is still the same, for today and tomorrow. 5,000 psi, metal press or fighter or roll fins. What is so new? The APPLICATION. As far as the roll fins go. 2 pumps. 1 is low pressure and very high volume to quickly empty or fill the low pressure portions of the cycle. 1 is high enough to add it's pressure on top of the other to finish the required cycle of the fins motions. Time of the swim cycle is so slow, a person could move a proportional hyd. valve by hand and keep the ship level. ANY LARGE crane operator is a living example of the roll fin requirements as he swings the load back and forth in high wind conditions. We are not going into deep space and back, here.