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  1. #11
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    Re: New 3doF platform construction

    Hi Roland,

    This is great. You are definitely the expert on moving platforms.


    Regards Wendy

  2. #12
    150+ Forum Groupie Roland's Avatar
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    Re: New 3doF platform construction

    Regarding controllers, supply voltage and matching motors:

    There are a number of parameters that are important:

    1. Motor DC resistance (in stalled condition)
    2. Motor inductance value
    3. H-bridge (controller) PWM frequency
    4. H-bridge supply voltage

    Motors with low DC resistance (normally low voltage 12V types) will draw considerable current during platform sudden motion steps. When applying more supply voltage than rated, the current will increase. (i.e. a 0.5 ohm motor will draw 12V/0.5=24Amp max at 12V supply with H-bridge fully on. At 24V the current becomes 24V/0.5 = 48A! the controller H-bridge MOSFETS may not be able to handle that kind of current.

    The motor inductance value together with controller PWM frequency determine the current ripple through motor and H-bridge, even during steady state. Low inductance means more current ripple, and low frequency also means more current ripple. Too high current ripple heats up the motor and bridge during operation.

    I normally use 50V rated DC motors, DC resistance about 1 ~ 1.5 Ohm. Inductance about 1mH. These run fine from 24V - 36V battery supply, (peak currents about 36A). I set my PWM frequency to 20kHz, so you won't hear any noise. Ripple in this case is rather low, about 1A pp.
    Note that power loss in these systems is mostly coming from conduction losses, so selecting higher voltage/lower current application is better than selecting lower voltage/higher current (even though power is the same: P=V*I)

    Note on controllers:
    They sometimes specify 2 voltages: one voltage for the small signal section (i.e. 5V or 12V) and one voltage that is the supply for the H-bridge. It is this voltage that gets chopped and drives the motors. This H-bridge voltage can indeed be varied from smaller to larger values, to get smaller or larger max motor current.
    But if the controller has only one supply (small signal supply is derived internally from the main supply), there may be a minimum and/or maximum limit to the supply.

    Check the controller spec carefully! Motor specs are sometimes hard to get, so measuring is a better way to know if your type is suitable.
    RR

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  4. #13
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    Re: New 3doF platform construction

    Hi Roland,
    Thats what I mean, I`m a complete noob about currents , resistance and Inductance.
    I have more understanding in hydraulic or airdriven machines
    I can connect a motor to a controlcabinet ,
    If I have a drawing but dont ask why you need this type of motor for that kind of machine
    offcourse I know by experience that a certain type of machine needs a least a certain type of electric motor but the story behind that is a questionmark
    I can however get all the electric hardware I need, but havent got a clue about electric parts.
    I can mechanical build you an actuator if I have all the data ,
    For that reason I`m looking for someone who can tell me what I need electric wise.

    The bottomline is I want to use the 40SPU-1 card with the MD03 controllers, It looks great and seems easy to install.
    I building a platform size 1600x1200mm from 50x50mm steeltube gross weight incl Cessna cockpitsection and person aproxx 400 kilo.
    The platform will be connected with a cross coupling with the central gravity zero out(balanced) on coupling aproxx 60/70 cm above ground.

    As I understand the MD03 uses 24/36 volts to feed the motors for moving the platform.
    My question is :

    Can I use this current as a( in Dutch stuurspanning ) control current and use 220/400 volt for feeding the motor itself with out blowing the MD03 out .
    I can get motors for building dicated collumn actuators up to 2 KW 220/400 V 12/24 control current 50 HZ.
    However will it work without a meltdown?
    This is my question in a nuttshell, my wife says ̀`m nutts
    but enough said bla bla bla
    greetz Henk

  5. #14
    150+ Forum Groupie Roland's Avatar
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    Re: New 3doF platform construction

    Hi Henk,

    The main reason that there are not so many homebuild platforms out there is because building one requires a number of skills that are are not often found in one and the same person: Mechanical, Electrical and Software. (and an understanding partner)

    As far as I know, there are no ready for assembly kits avaliable that can be put together without above mentioned knowledge. Ian's plans go a long way, but still require some skills.

    Difficulty increases when building bigger and heavier platforms. That's why I have kept the size and payload of my platforms small, as I would run in some serious issues when trying to build bigger.

    Like you say, you are good in hydrolics, but not in electronics. The only thing I can advice you is to learn the skils you don't have yet. If you are trying to build a big size rig without electrical knowledge you're going to run into some serious issues.

    I have tried to put most info on my site as clear as possible, but it is defintely not a plug and play guidline, so needs lots of experimenting.

    I hope I did not put you off, but I'd say start simple, and after you gain experience, go for bigger projects.
    RR

  6. #15
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    Re: New 3doF platform construction

    Hi Roland ,
    thank you for your reply,

    Thats why I place my questions on the board, maybe someone can shine a light on the issues I`m struggling with.
    like:
    Can I use this current as a( in Dutch stuurspanning ) control current and use 220/400 volt for feeding the motor itself with out blowing the MD03 out .
    I can get motors for building dicated collumn actuators up to 2 KW 220/400 V 12/24 control current 50 HZ.
    However will it work without a meltdown?

    If that is the case I know what to purchase or if not what then.
    Experiment costing a lot of time and money and I don`t want to invent the wheel again.

    I see a lot of platforms all over the internet , but when I ask to explain in plain English or German/Dutch for that matter.
    I get so much technical info , after a few lines I`m lost,
    You have to have an academic degree in electronics just to understand what they talking about.

    I`m taking a lot of effort to make a platform, but electronics is not my cup of tea.
    I will talk to you on the fair, tomorrow.
    greetz
    Henk

  7. #16
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    Red face Re: New 3doF platform construction

    Hi Roland can i ask that age old question again regarding backdriving, i have looked at your saruss design but was not quite sure if this was a factor as i could not see a worm and pinon setup .. i appreciate the design will lift the calculated load but wondered about the non powered state or park position of the platform when no feedback signal is applied .. dont mean to be awkward its just that over the years of my dreaming waiting and still tinkering with platforms ( None finished yet LOL backdriving of the lift actuator always seemed to be a factor, in the ideas i have looked into or seen elsewhere .. does the feedback circuit always hunt for positon and in so doing keep the motor energised and hold the platform in a steady state ?? love the stuff you do and as always no critiscism intended just trying to learn a bit more and get another perspective on how things work .. oh by the way did i say i love your ideas Ps Nice explanation in laymans terms about motors now thats the stuff i like .... it eventually seeps through the dark matter thats my brain and i get that eureka moment ..Cheers Roland

  8. #17
    150+ Forum Groupie Roland's Avatar
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    Re: New 3doF platform construction

    Just returned from FSweekend show, which was really great! Very nice to talk to all fellow builders and many other interested folks. Saturday evening we had a wonderful dinner and lively discussions. Thanks Matt!

    The prototype has proved the concept, as it is still in one piece after 2 days of continuous operation in heavy turbulence settings and some shakey X-plane sessions.
    Thanks everyone for the great comments and some really good suggestions for the bigger version.

    To come back to the back-drive question:
    I have always used straigt spur-gears for my drive train, so there is no worm-gear involved. This means that the motor will back driven when the payload is not properly weight-balanced. The bungee cords will need to produce sufficient up-force to cancel the platform weight down-pushing force back to the motor.

    In my servo system, the position feedback signal will be continuously compared to the input signal, and the motor will be driven in such a way to make the position feedback signal equal to the input signal. If you would try to push the platform down in this equilibrium situation, the motor drive would immediately react and apply power to the motor to keep the current position. So weight unbalance will create considerable steady state power dissipation to the motor, heating it up and draining the battery.

    Some worm gears cannot be back-driven, in that case you could do without weight-balance. But it means that the motor will have to work much harder to push the platform upwards than when going downwards. Most likely, there will be an unequal up/down speed, and also considerable more wear in the worm-gear without some weight-balance scheme.

    The reason I'm not using worm-gear is because I'm not familiar in specifying this type of gear and construction. (I'm an electrical guy). I also have the feeling that backlash is much more difficult to control in worm-gears than in my straight spur gear drive train.

    BUT worm gears seem to be the choice of many commercial platform makers, so there must be an advantage to it. I welcome anyone to present some info on making succesful actuators with worm-gear that don't need weight-balance and still can achieve fast response, equal driving speeds and minimal backlash.
    Last edited by Roland; 11-11-2009 at 03:52 PM. Reason: correct some worm-gear info
    RR

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  10. #18
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    Re: New 3doF platform construction

    Hi Roland,
    Nice talking to you on the fair,
    You put me on the right track as I was going round and round,
    Now I know, what I have to ask the electric guys at my work to fill in the gaps and details.
    If you want to know more about actuators and wormgears take a look on this site.
    http://www.elsto.nl/index.html
    This is a part of the Holding I`m working in.
    greetz

  11. #19
    150+ Forum Groupie Roland's Avatar
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    Re: New 3doF platform construction

    Thanks for that link Henk,

    And indeed it was nice talking to you. Looks like the colleagues in your company should be able to give you some expert advice on transmission system drive. If they are interested they may even join the team!

    The PDF files in the link you gave do show some servo type worm-drive gears. Seems that backlash is solved by preload pinion shaft, that can adjust backlash to zero. But it sounds like this is a solution a bit outside the scope of the DIY-er.

    Still not sure how these worm-gear driven units would operate under a-symmetrical load. (like an unbalanced motion platform)
    RR

  12. #20
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    Re: New 3doF platform construction

    Hi Roland ,
    In the winches and capstansindustry I`m working in ,we`re using frequentie-inverters and sensors, But as you say a bit outside the scope of the DIY-er.
    I`m only making attachments for the sensors and drill countingholes for the sensors.
    We have a specialist who`s main job is to install and ajust the parameters within certain bounderies.
    But daar I have no kaas van gegeten.
    Also the backlash is partial depending on the inertia of the wormwheelgear itself,
    At a certain ratio you can have some kind of backwinding up to a absolute stop for tonnes of weight.
    greetz Henk

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