Electric Steer By Wire / Rudder Torque

Discussion in 'OnBoard Electronics & Controls' started by jim_restoration_project, May 10, 2014.

  1. jim_restoration_project
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    jim_restoration_project Junior Member

    I'm looking into the numbers involved in converting an old steering solution to be a new shiny Steer By Wire.

    I'd appreciate fresh eyes, both on the specific numbers and thinking, and general feedback/ideas please!

    Current steering arrangement

    Twin Screw, Single 'tear drop' shaped unbalanced rudder. 70° Lock to Lock.

    Height = 0.852, Width = 0.734M, Area = 0.535 Sq. M

    Hull speed on this displacement hull is circa 8.5 knots.

    Steering shaft connects to the Rudder via a 20.5:1 gearbox (calculated by the fact it takes 4 revolutions of the steering shaft to turn the rudder 70°, which is 1440° : 70° ... 20.5:1)

    Note (for interest): The Ships Wheels used to be connected indirectly to the shaft via other shafts + chain and gears, giving two steering positions.

    Torque Calculations (@ 8.5 knots )

    Vetus formula = 813 N.m
    Ultraflex formula = 891 N.m

    I'll be going with the worst figure of course, 891 N.m for deciding on the minimum required motor torque.

    Electric Steer By Wire

    So my thoughts are to put on the steering shaft, just outside the gearbox, a 24VDC 200W motor with integrated wormgear reduction (internally 50:1 I believe). This gives a no-load speed of 34 RPM and 49 N.m Torque.

    This would give a lock to lock speed of ~8 seconds, which sounds fine.

    Given the Rudder Gearbox is 20.5:1 that gives a total of 49 N.M * 20.5 = 1005 N.m which is plenty over the worst 891 N.M needed at max hull speed. Of course most of the time, cruising speed, the torque would be ~ 1/3rd of that.

    A Worm Gear reduction gearbox on the motor means the rudder ought to stay in the position it is left in (subject to holding power of course).

    (I'd leave enough left on the shaft that I could remove a cover in the Wheelhouse, insert a 'windlass handle' to provide emergency manual rudder capabilities. Just in case of electric/electronics issues)

    Control

    The Ships Wheel would be turning an 8 turn potentiometer read by an embedded microcontroller. This in turn controls both the rudder and the twin screws (due to PM DC motors providing electric propulsion).

    I do need to think of a nice elegant solution for determining rudder position still. Two fold, one is to know where the rudder is on power on (and reset it to rudder amidships) and also to ensure that the rudder is obeying the steering input and holding it (in case the motor/gearbox can't hold it without further input).

    Future possibilities

    This opens up the future possibility of the equivalent of Variable Speed Power Steering, Autopilot and Remote Steering (such as when forward when mooring, as visibility isn't great due to hull shape/length/width etc).

    I'm unsure of the merits of the first and the other two have their own set of ancillary problems, which is a separate discussion. Best to get the fundamentals in and settled in before moving onto anything advanced!

    Thanks!
     
  2. CDK
    Joined: Aug 2007
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    CDK retired engineer

    This is OK if the wheel is replaced by a joystick, else the lack of feeling makes this a poor solution.
    I am an electronics man, but in this case I'd hydraulics are a better choice.
     
  3. jim_restoration_project
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    jim_restoration_project Junior Member

    Yes, a good point. I'd thought of that but left it out above as the post was getting rather long as it was.

    Aesthetically the old Ships Wheel stays. It's a beautiful Wood & Bronze fella, 75 years old and quite heavy.

    I'd put that on a short run of heavy chain in the form of a closed loop, and putting some artificial resistance in. This ought to give it some 'weight' and I could adjust that till it felt right on a trial and error basis. This chain would also provide the 'stops' to ensure the Wheel only does a maximum number of turns lock to lock (probably 4 like the original setup).

    In the future perhaps even add in some electronically controlled variable resistance to that system which could increase/decrease with speed/rudder position, but I'll worry about that further down the road.

    I'd thought of hydraulics and have discarded that based on cost and other factors.
     
  4. jim_restoration_project
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    jim_restoration_project Junior Member

    I think I've had a better thought for the choice of motor.

    Hybrid Stepper, still with a right angle hollow shaft gearbox (but no need for worm gear which is inefficient but does have the merit of holding a load). Until today I didn't think I'd be able to find one powerful enough at a decent price but it seems there is quite a choice available.

    I guess I do need to put some limit sensors in still, to handle ungraceful shutdowns where I lose track of where the rudder has got to.

    Still, the purpose of the original post was twofold, the numbers and the implementation method so I'm still hoping to hear peoples thoughts.
     
  5. Poida
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    Poida Senior Member

    Sorry this is far too technical for me, but do you mind if I ask some questions so as to educate me.

    Why do you need to know where the rudder is? Don't you know that because of the direction the boat is going in?

    I assume you are not going to turn the wheel as such, merely move it either clockwise or anti clockwise to engage a linear pot, either to the port or starboard and the wheel would be spring loaded to keep it central when not required.

    I don't know how a stepper motor would be operated from a wheel but wouldn't it be difficult to manually override it?

    Thanks
    Poida
     
  6. jim_restoration_project
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    jim_restoration_project Junior Member

    No problem.

    When the Wheel is physically decoupled from the Rudder I do think it important to keep track of where each is.

    When the 'system' is first turned on how would you know where the rudder actually is unless you check?

    Whilst I'm not planning on doing 'speed sensitive steering' initially at least, it has occurred to me as an idea that could be tied into the twin electric propulsion props.

    Correct.

    Not necessarily, thinking about 'spring loading to return to amidships' but unsure as yet.

    Remote steering (2nd position, or phone/tablet) means it's worth thinking all the ramifications through.

    Not really. If you put a Isolator in the 'power steering' circuit then you can use the manual override any time you wish - but more likely you'd be overriding it if the electrics/electronics went down. The Isolator is there just in case/'belt and braces'.

    The whole steering system is still on the drawing board.

    Cheers.
     
  7. Poida
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    Poida Senior Member

    Thank you for your reply.

    Would it be an idea to have a radial segment on the wheel shaft which when turned engaged a proximity sensor. After the segment has gone past the first proximity sensor it engages another. There would be 3 proximity sensors each side of the vertical axis of the wheel shaft.

    Each of the proximity sensors would then engage a relay. Power going through the switch of the relay would then go through voltage regulators to control the speed of the motor for the first two and the third relay would provide full voltage.

    So, turning the wheel would engage slow speed, medium speed and then fast speed.

    One of the benefits in my limited view is you can have a thin gauge signal wire going to the coil of the relays rather than have heavy wires going to the motor from the wheel.

    You would also have 3 separate wiring systems on each side so a break on one circuit would not prevent the other two speeds from working.

    Could fix a plate to the steering mechanism which would engage either one proximity switch or another if the rudder was not in the central position. These in turn would energise either a Red light on the steering console if the rudder is turned to Port and conversely a Green for S/Board.

    Please note I do not or am I connected to anyone who sells proximity switches :p

    Poida
     
  8. jim_restoration_project
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    jim_restoration_project Junior Member

    Thanks for the ideas but you're overthinking it. A Hollow Shaft Potentiometer would do the job. It would need to be a Pot rather than an Encoder so the positional info doesn't get lost between sessions.

    Both the Shaft & Wheel pots would feed a microcontroller, which in turn feeds the motor controllers. So no need for Relays and Hall Effect sensors etc... what actually happens in translating those sensor readings into physical effects is defined in software.
     
  9. Yobarnacle
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    Yobarnacle Senior Member holding true course

    I plan on electric steering also.

    For rudder angle indicator, simple, foolproof, inexpensive, easy install...I'm thinking rearview camera intended for cars. Sells for 20$ on up. Could POV observe any part of the steering system, or maybe even underwater watching the actual rudder.
    I'll have to put an anchor or helm decal on the box myself. To make it official BOAT gear! :D

    Several here and many more come up in search.
    http://www.lightinthebox.com/c/car-rear-view-camera_2386
     
  10. jim_restoration_project
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    jim_restoration_project Junior Member

    I have bought and installed a few of those to cars & vans.

    That would work as a rough indicator but not good enough to know if/when you approach the hard mechanical limits of the rudder or not. IE visually could you tell from a 4-6" monitor screen whether you are close/on or past the rudder limit?

    If you are thinking of an electric steering system yourself I'd urge you not to rely on such a rough indicator otherwise you could easily continue to exert force on the rudder past the point you should - approx 1000 N.m in my scenario, a very considerable amount of force.

    So you either need to absolutely measure the rudder position and/or put in limit checks and then think of what happens if something fails etc.
     
  11. Yobarnacle
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    Yobarnacle Senior Member holding true course

    Limit switches remove the chance of jamming against the stops. when the tiller hits the limit switch, power is shutoff, no more travel in that direction. Power still available to turn the OTHER way.

    Rudder angle indicator is useful, more about knowing when you are amidship, or a degree or two either side.
    Every boat has a "Peg", a sweet spot on the helm, where she'll run straight a good while unattended. It's NEVER exactly amidship. Every ship I've handled in more than 40 years commercial sailing, carried a little rudder (one side or t_other) to run straight.

    Knowing where to 'Peg" the helm, takes a lot of the drudgery out of steering. If I was looking at the aft edge of the rudder on the monitor, camera on centerline aimed forward, then a simple scale in degrees, above or below the screen, would indicate where the rudder was. The trailing edge of the rudder is indicator, the movable line indicating the angle. And ACCURATE. What you SEE is what IS. Pots aren't very accurate, needing constant adjustment on weekly basis to keep them 'approximately' in synch with the rudder. IMHO.
     
  12. Poida
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    Poida Senior Member

    Hi Jim

    I am overthinking, that's because I'm an overthinker. My thinking (with the limited thinking apparatus I was blessed with) is on reliability.

    Potentiometers, the ones that I have met, have a wearing part, especially in the main area of operation and are not reliable, for boats except for a volume control on the radio.

    My system has automatic backup by using 3 proximity sensors in each direction and are non contact, except for the switch in them and don't rely on a wearing part. If one of the switches failed you still have two more to steer in that direction.

    If your controller fails it's gone.

    Why not go all the way, encoder to a controller to a servo motor with feed back to a monitor that will show you where the rudder is.

    I would have thought limit switches on the rudder mechanism would have been a "it goes without saying" inclusion.

    Yoba, don't put the camera facing the rudder, face it down so you see where the fish are.

    Poida
     
  13. jim_restoration_project
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    jim_restoration_project Junior Member

    Indeed. Alas my Rudder isn't a typical 'tiller' setup but is actuated via a shaft & gearbox arrangement. Physically installing limit switches is non-trivial.

    Made worse in Single Screws of course.

    You get what you pay for. It's unfair to compare $1 plastic pots with $300 industrial long long high precision pots.

    Synch issues probably occur due to slippage, perhaps a pot with a belt (like a pulley/belt arrangement)... not an issue with hollow shaft Pots if done right.
     
  14. jim_restoration_project
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    jim_restoration_project Junior Member

    :)

    Indeed, all Pots have a finite life but what in life doesn't?

    Cheap pots can't be compared to 4-10 million cycle life expectancy industrial heavy duty pots. That is many many years of usage, and they are used in all aspects of life, cars, trucks, trains etc.

    Subject to price, I'll probably put two on the shaft and compare readings from both.

    If the controller or one of the Pots fails, then I have manual steering as a backup.

    Encoders are relative rather than absolute meaning you'd have to go through a calibration routine each time and rely on limit switches to find the extent of travel.

    A monitor precludes the use of a secondary or remote steering position (such as stood on the Bow whilst mooring up). You can solve that but that involves extra cost and complexity (and the $30 analogue TV reversing camera then becomes a handicap in such a situation).

    All components will fail for some reason or another. I'd bet a quality Pot against a cheap Encoder any day of the week.

    I'd usually agree, alas due to the fact my steering is shaft via a sealed heavy duty gear box rather than a tiller arrangement means it is very challenging to install limit switches. (I'm still trying to get inside the gear box in a non-destructive way as I'm sure it needs a good clean out and renewal of grease etc).

    If I can put backup limit switches (or equivalent) in as well, I will do.
     

  15. Yobarnacle
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    Yobarnacle Senior Member holding true course

    jim_restoration_project

    I'm not arguing or discouraging toward your ideas. Just sharing mine, if that helps you. :)
    Maybe limit switches could be arranged along that chain drive you described.

    poida

    Actually I intend to aim down. Just offering possibilities.
    I changed my rudder from a balanced shaft and rudder tube, to a more favorable transom hung. I needed a bigger rudder, flips up if hits something or fouled, demountable when trailering, and I don't like underwater thru-hulls that can be avoided.
    So I'll place the camera in a waterproof box mounted under stern railing.
    An arc with degrees ticked on it, applied to screen with the picture of rudder head is the scale and pointer.
     
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