Servos & Simulation's Blog

Why can't you build a 6DOF motion platform for $5k?

Posted by Rachel Baker on Feb 2, 2017 5:31:30 PM

710-2-500.jpgBy Rachel Baker and Rick MacLean

If you have ever surfed the net looking for motion base platforms you will have witnessed Do It Yourselfers using everything from drill motors to wooden platforms. These systems are ingenious, but beyond testing the feasibility of a design, they are non-functional in the long run. Let’s explore why…


What does it take to build a motion base? At the least, one would require something to move the axis with. This is typically a motor of some kind. This can either be hydraulic, electric, pneumatic or magnetic. Hydraulic would be a little out of reach of a DIY’er due to requiring a pump, oil container and the like (hazmat anyone?). Magnetic doesn’t hold a load very well. Pneumatic is hard to control with a servo loop (you either go or not) and the pump is loud. The obvious solution? - electric motors.

So which type of motor? AC or DC? DC motors are the simplest to control but can be expensive. AC motors require knowledge on how to obtain full torque at zero RPM. This seems easy (hey! Tesla Motor Cars is doing it, so why not me?) but, it isn’t. Many DIY’er don’t even consider AC motors for their systems due to not being able to make a stable servo. If they even consider making a servo loop at all. We will get to why a servo loop is required in a moment… The motor also determines how much payload the MB can pickup and hold at zero RPM (also known as full torque at zero).

 

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So you have picked a motor… now what about the other parts. Is the actual driving mechanism going to be linear or non-linear? Linear means that the assembly moves up and down. Non-linear means that the motor uses some sort of crank and pushrod assembly. Linear assemblies are nice because there are less parts to design for the user and easier to do the math on, but they have their flaws. One major flaw in a screw type linear mechanism is that the bearings that make the system move induce chatter into the system and the bearings have a shorter MTBF. Eventually, they will need to be replaced. Non-linear requires more parts to be designed, but the movement of the system, if designed properly, will be nice a smooth and should last basically forever. A gear box of some sort can be used with the motor to facilitate lifting the payload which has to be matched to the crank.

So now you have designed a working mechanism… so how are you going to drive it and know where it is? DC and AC motors require some sort of signal to know where to go and how fast. Most motors have a matching drive amplifier. This has to be wired to the motor, to some sort of control device and to the feedback device. This is what is called a servo loop. Why is this important? In order to control the axis, you have to know where the motor is and how fast it is going. In this way, you can control the position of the driving mechanism and how fast or slow it reacts. This is important. All mechanisms do not react the same. Different aircraft react differently to each other. Same with cars, boats and cranes (oh my!).

So now, what do we use to actually control the motor? This depends on the drive amplifier for the motor… typically, this is either an analog or digital signal.. There are many types available from a computer with a digital to analog conversion board to a PLC to a simple waveform from a signal generator. The better the resolution of the control signal, the better the system will respond.


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Many S/W programs, like X-Plane and Dirt Rally, actually send physics data or make the data available through third party s/w such as MS FSim which can be captured and interpreted for MB use. But, is this information correct for the system that you have designed?

710-6-2000 - Raytheon Motion Base.jpg
Do you have to design and write software to interface into the simulation software? Most of the time, yes. The complexity of the software is dependent on how realistic you want the motion base to be. For example, an airplane’s center of gravity (where it actually rotates about) and a helicopter's center of gravity are different. They require different math models for the motion base geometry to get the motion base to react properly. Is the system linear or non-linear for the driving mechanism? Does the user of the motion base platform require the non-linearity of the motion platform to be removed from the system? If so, the math that the software has to accomplish is very difficult which adds to the complexity of the software to control the motion base.

So all of that has to be engineered… now what about something to mount the motors and the driving assembly plus whatever is required to mount the items to the whole assembly (like a top for example). The geometry for all of this has to be designed so that you get the angularity that is required by what you want to simulate. Sometimes this is quite large but in the case of aircraft sims, very little is needed to fool the inner ear into believing what the player is feeling is real. 


Overhung load and the center of gravity of the payload has to be calculated to make sure that the geometry to the motor and the like can actually move the system. 

Now, get all the parts designed and manufactured, engineer all the electrical, wire, built and tested (not forgetting about safety stops) is a considerable task. So how much does all this cost? Depending on your design, the biggest cost that DIY’er forget to take into account is the engineering, build and testing time. Those costs are high… very high. Most DIY’er do not take their actual time into account because it’s a hobby! But, those of us that actually design these systems, it is not a hobby, it is our business, our livelihood. It is our business to make sure that the system does exactly what it is designed to do… safely. If we didn’t, our business would suffer and could possibly injure someone.

So is building a motion base for $5,000 USD feasible? The simple answer is no. 

So is $5,000 USD per axis feasible? possibly… as long as it is a basic design. 

So a basic standard 6DOF motion base (just the motion base and the electronics, not the controller and software) would cost about $30,000 for an average for a single rider system (around 500 pounds) with about ±15˚ in roll and pitch. This does not, however, reflect the cost for the controller, possible software and customization of the system. 

So there it is… this is why motion base platforms are so expensive. There is extensive engineering that is involved with designing everything from the mechanical to electrical to software to make it all work. 

So now that you know a few of the reasons behind designing a motion base platform system, you can understand why they are not inexpensive and should never be. 

Topics: motion control, control loader, flight simulator, simulation, motion platform software