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  1. #1
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    Yet another cheap and easy to build motion platform!

    Hello everyone,

    I'm new here (I just joined last week). When I introduced myself to the forum I mentioned that I've built a motion platform. AndyT reply to my post immediately and requested that I post pictures, diagrams, etc. Well here it is. I don't pretend to know all the answer far from it. Actually, it's my first time doing this and I'm sure some of you guys are much better at this than me. Just before I begin I want to say that I took many great ideas from Jim at http://www.jimspage.co.nz/intro.htm and from Roland van Roy at http://www.simprojects.nl/. I'm sure you have visited these sites already. They are simply superb sites that give lots of information about building motion platforms.

    Ok the specs are: (refer to the pictures)

    - 2 axis platform.
    - The top of the platform is 4 feet on the roll axis by 6 feet on the pitch axis. I figure I will have enough space for 2 passengers (One can dream no ?). The bottom part of the platform is 5 feet on the pitch axis by 3 feet on the roll axis.
    - I needed a platform that could move around about 500 pounds.
    - I use 2 Ford Crown Victoria 1997/1998 wiper motors. One for each axis. They cost about 50 Canadian dollars each if you buy them used.
    - I used a pulley system. The more pulleys you use the less torque it creates on the motor shaft. The drawback of this is of course a slower movement.
    - I use a high quality ansi standard #25 (1/4 inch between the links) chain and 2 sprockets (one for each motor) of 23 teeth. I needed about 25 feet of chain. The cost was about 100 dollars for the 2 sprockets and the chain.
    - I use a modified car driving shaft in the middle of the platform (Thanks Jim for this one!) as a support. The universal joint moves about 35 degrees on both axis so that's more than enough. The driving shaft was cut and welded between 2 steel plates of 1 foot by 1 foot. I did not do this myself. I went to a welding shop and they charge me 50 dollars.
    - The platform is made of wood.
    - The power supplies for the motors are: One 24 volts 10 Amps and one 500 watts ATX computer power supply for supplying 12 volts. More on this below.
    - It had to be cheap and easy to build (It cost me about 350 Canadian dollars and it was really easy to build. Trust me, if I can do this anybody can.

    Some notes about the platform (again, refer to the pictures):

    - I made sure that the chain for the pitch axis is exactly in the middle, perpendicular to the roll axis. The same goes for the roll chain. This is important because when you roll and pitch at the same time you want to keep the chain on the sprocket and in the pulleys! Since the driving shaft was right in the middle, thus blocking the passage of the chain, I had 2 choices. Make a hole in the middle of the shaft to pass the chains through or raise the driving shaft and pass the chains under it. I chose to raise the shaft.

    - The pulleys are wide enough for the chain and they have a nice ridge so the chain does not come off. The pulleys and motors are held in position by metal corners (buy strong ones especially for the ones holding the motor as it generate a lot of torque). The pulleys are able to rotate on their own axis thus minimizing the friction of the chain against the pulleys. You can buy all this at any hardware store.

    - The motors have 2 speeds (slow and fast) and they are usually ran at 12 volts (voltage of a battery in a car). I choose to run the motors at 24 Volts for fast speed and 12 volts for the slower speed. I need the slow speed for the wash out algorithm. Why 24 volts ? Well, they have a lot more torque and they turn faster. I know I'm probably running them over spec but I can't seem to kill these guys. They are just very tough. I've been doing a lot of stress tests and they are still in great shape. Besides, they are not that expensive anyway if I need to replace one.

    - During my load tests I would put some weight say in the front until the motor could not lift the platform anymore. It's normal and I suggest you don't fight this unless you have a lot of money to spend on more powerful motors or you want to get into hydraulics). The key is to use the physics to your advantage. When I'll build my enclosure and I'll put panels and screens, etc. I will take great care at making sure that the weight of the front is equal (well as much as I can) to the weight in the back taking passengers into consideration. The same applies for left and right. If you balance everything properly, it will take very little effort to lift the platform. That being said, I will obviously not construct the enclosure with 2x4! I will keep the weight to a minimum.

    - The platform can move about 15 degrees on both axis. It's more than enough as I'm building this for an airliner not a F-16! A note on this. When I started this project, I was tempted to build the platform so it could move say 30 degrees on both axis. I realized quickly that it's much more complex to build and when I finally hooked that platform to MS flight sim I noticed that 8 degrees is of movement is plenty. You will be tempted to do the same but please resist. You simply don't need that amount of movement. What we are interested in is the "start" of motion in a direction not the total amount of degrees the platform can move.

    - One last thing. Be patient. I've built and rebuilt and rebuilt.... many times before I got it right. It's just the nature of this hobby. When I thought I got it right, I would perform some load tests (I would put lots of weight on the platform) and I'd realized that a pulley was misplaced or something else was wrong. Whatever it was, I would take it apart and fix it. It took me about 80 hours to build this. One very last thing. Don't be a hero. If you cannot do something than pay somebody to do it. You will save a lot of time. In my case I did not know how to weld and I did not want to know either! So I paid 50 dollars to a guy to cut and weld the driving shaft to the steel plates.

    I hope that this example will help you with your project.

    cheers

    Louis
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  2. #2
    1000+ Poster - Fantastic Contributor AndyT's Avatar
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    Louis,

    Very nice. Thanks for the pictures.

    How much weight have you placed on the weld on the driveshaft? I'm wondering how strong it is. It looks like it was spot welded and not a complete seam weld.

    I never heard of driving the wiper motors at 24v. That makes them more versatile but might burn them out faster. I'm not sure on that.

    I like how you have everything nice and neat under the platform. Will you be putting a skirt around it to keep it clean and keep pets and children out? Safety issues are very important when building motion platforms.

    If you had raised the platform a few inches higher by making the driveshaft longer, you would have added more degrees of movement without adding complexity, but since you are building an airliner, this should be no problem.

    The only thing I don't like about the U-joint on a drive shaft is that it has a sine-wave shaped motion because of the way its built. Have you taken that into account? I suspect it will not be a factor for you since you are only having a few degrees of motion.

    It is looking really good! Do you have any videos of it moving? That would be really good to see. I keep wondering why the guys on here with these really nice boeing and airbus pits don't add motion like this. It would really increase the realisim of their pits. And the weight should not be a factor if it is built and balanced correctly.

    Where did you get the software to run the motors? Did you write your own? Would you like to share it with us if you did?

    Great job so far! Keep us informed.
    God's in command, I'm just the Pilot.
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  3. #3
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    Hi AndyT,

    I've put 300 pounds+ on the shaft. It is not spot welded. There is a pipe inside that is welded to the plates. You just cannot see it on the pictures unfortunately. You only see the cover.

    Sine wave ? Hmm, I'm not so sure I'm following you. Can you elaborate on this ?

    As for the motors running at 24V, I agree with you that I might burn them faster but IMO the extra torque is well worth it.

    Putting a skirt around the platform is a very good idea. Thanks! No pets but I have curious children...

    Yes I did develop the software myself and Yes I will share it for sure. I'm just working on it right now. The platform went "wild" while I was taxing with FSX last night!! So it needs some more work for sure.

    I added a very small video of the platform moving at slow speed. Because of upload limits I could not upload a longer one. Rename the file to .mov as I could only upload big files with the pdf extension. It should give you an idea anyway. I drive the platform with the control software I made. I included a screen shot (done with my digital camera).

    cheers

    Louis
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  4. #4
    300+ Forum Addict RobiD's Avatar
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    Hi Louis

    Great work. What interface board and motor controllers did you use.

    Any more info you can give us is greatly appreciated.

    David

  5. #5
    1000+ Poster - Fantastic Contributor AndyT's Avatar
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    Red face

    Maybe later I will have time to sketch a picture, but in the meantime, get a universal for a socket wrench and slowly turn it to its fullest flex. Now slowly spin it keeping it in its fullest flexed position and you will see that everytime it comes to the 'ears' of the joint it moves away from being flexed by a few degrees.

    Good for having the pipe inside to add strength to the weld.

    You can upload videos for free to YouTube and provide a link here. That way you can post a decent size video that shows all directions of movement.

    Can you give information on your software? I'm guessing you are using Simconnect? Or are you using FSUIPC? What do your washout algorythms look like? How have you calculated the actual angle of flight to the platform angle? 1:1 ratio or 2:1 or some other ratio...?
    God's in command, I'm just the Pilot.
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  6. #6
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    Hi Louis really nice work you have done there , always great to see another option on the motion platform front one of the lessons i learnt along the way was Over-engineer everything to start with and then trim back, cos when you have over 300 lbs moving around ( and you'll be supprised how l little that is when you start to add bits ) the transmission and motor couplings take a pounding especially at extremes of travel ( moving there is easy getting back 0000 major strain) i also built my base out of wood and so far so good she seems to be able to cope without too much Flexing . yo may wanna have a look at the back driving effect when the platform is loaded , i have my motors driving through steering racks and Steering boxes ( a Bit of Jim and a bit of Craigs Concepts ) and i still find that there are situations where the wiper motors can get back driven .. Not taking anything away from you work cos what you have doen is ggod stuff just passing on some of the things i have picked up on in my attempts . Keep at it you are now well and trully Gonna be hooked and this is the best place for a fix ( if you dont Mind the pun LOL )
    Last edited by wannabeaflyer; 10-10-2007 at 08:12 AM.

  7. #7
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    Wannabeaflyer, what are the specific situations where the motors are back driven ?

    RobiD, I did my own control hardware. I use the parallel port of a PC to output 8 bits (up, dn, left, right, slow and fast). This goes into a NOOP buffer 74LS244 as a security measure (I don't want to blow my parallel port!). The output of the buffer go to 8 5/12 Volts relays that control 8 12/30 Volts relays. I need the stronger relays cause there is quite a bit of current drawn by the motor. Instead of the 5Volts relays, I could have used switch transistors with pull-up resistors but I already had these relays so I used them. It adds to the propagation time of the signal though. I've included a picture of the circuit. Yes I know it's messy and Yes it's on a breadboard. I appologize to the hardcore electronics guys here. As you can see on the picture, there is a part of the circuit that is not used. At the time, I thought I would get the feedback (pitch and roll angles) from the platform through the parallel port as well but I decided to go with a 2 axes phidget accelerometer. It is a USB device ultra easy to use.

    AndyT, yes I'm using SimConnect. I'm planning to do a netpipe interface as well as I don't have the PC to run FSX properly. This way I'll have both an interface to FS 2004 and FSX without having to change the software doing the cues itself. Basically, the flow goes like this. I made a C++ SimConnect application that broadcast a few parameters (pitch, roll, speed, onground, etc.) on my IP network every 100 ms using UDP. Then I have a Java application running on a PC (the one that has the parallel port to the motor circuit) that picks-up the UDP packets then activate the proper relay(s) via the paralle port. The key is is the "activate" obviously. That's where all kind of magic needs to take place. I'm really not satisfied with the way I coded the ground cues. I need to rework the whole thing. I don't use a ratio angle (from sim to platform). Actually, I'm more interested in the angle derivative (radian/sec) rather than angle displacement although I find this an interesting concept. Did you coded your cues using this technique ? can you elaborate ?

    cheers
    Louis
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  8. #8
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    Hi Louis with my platform mechanical setup i found when there was no power applied to the motors and with the weight of the platform seats .screens etc that i could in fact ( with a lot of effort) move the platform to a new position , i guess what im saying is that with the method of control that i was/am using at the moment ( Double pole Change over relays ect) that if the motor is not powered and the platform was say near the max of travel it could be backdriven to the limitswitches inadvertantly .. Have to say that since using a 24V power supply to drive the motors the recovery is a lot quicker so i dont notice it as much Like i said its not a critisism of what you have done its a piece of advice that was passed onto me and turned out to be a small factor ..How are you getting on with your C# Programming ?

  9. #9
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    Thanks for the info wannabeaflyer. Since I don't have the enclosure yet (with screens, seats, etc.) I obviously don't see this problem. I'm wondering if I could put some springs to help the platform recover from one side to help this situation. I remember Jim did this on his platform though I can't recall if it was to fix this specific situation.

    I unfortunately don't know C#. I use both C++ and Java for this project.

    thanks again

    Louis

  10. #10
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    Hi Louis Some guys have used springs and Bungees to assist in the return to center operation and to take the load off the motor a little bit , as we have all found out its always trial and error ,because we Build with DIY limited facilites but the good news is once you see any of your creations in motion sorry to say there's no Going back LOL i have had to start to learn C# for my attempts at programming but talk about headache hoping and promising to post my code as soon as it's out of my brain . Hang in there its all good stuff from this forum

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