Outside view is a wooden box. Balancing bungee via 10 strings in folded
The drive shaft is fastened to double timing belt via a wooden block. The 10
strings balancing bungee are also fixed to this wooden block to relieve the
belts from the platform static load. The shaft is positioned such that there
is no force offset between belt and shaft, which keeps the forces mainly
axial, so the bronze slide bearings at the shaft top are not stressed in
sideways force too much.
The two timing belts are driven via a 72/20 teeth timing belt transmission.
It gives an outgoing force of 50N/A motor current.
At 12A motor current (24V supply) I get 600N outgoing shaft force.
With 24V drive, the motor does 11 turns per second. The belt drive does
3 turns per second, which gives 0.5m/sec drive speed.
With total 180kg platform weight, I calculated I need 50kg of balance force
per actuator at platform lowest position, which requires 10 strings of 8mm
The actuator length ranges from 1.3m to 1.65m. This could possibly be
increased to ~ 1.8m for max 50cm travel, by reducing the distance between
the two shaft bearings to 15cm.
Total platform footprint will be 2.3mx2.3m, and max pitch / roll angles
For the software I plan to use Ian's 6DoF driver, and the hardware will be
based on hardware servo loop, similar to Platform drive III and IV.
January 16, 2011: 6DoF platform ideas
Above drawing shows a simple cable suspended 6DoF. Basic concept: 3 posts
in triangle, with a another triangle platform in the center that is
suspended via 6 cables in an inverted Stewart configuration. 6 winches (2 at
each post) wind and unwind 6 cables. They must be driven from 6DoF software
that correctly adjusts cable length according inverted Stewart configuration.
I'm assuming in this design that platform weight will keep the cables tight
during all movements, but there is a limitation on platform position where
cable slack will occur.
The zero tension condition of one or more cables depends on load CG,
cable angles and platform angles. Some offset cable
tension force could be added via bungees from each platform attach point to
the floor. Weight balance could be done similar to my rig #3 : Each cable
loops through the winch and then attaches to a bungee underneath the
Footprint of this setup is pretty big, but the inverted Stewart keeps height
low, so it does fit in my attic.
I guess I'll build a small mock-up first.
October 31, 2009: Finished 1:2 scale model of alternative 3doF
Finally hooked up everything to the servo drive: 12V powered tests with
30kg ballast went well, no real issues discovered yet.
Also interfaced to FS2004 and X-plane show good performance. See YouTube
Note that one motor is a bit different from the other two: therefore the
step response between the legs is a bit different. Final model will use 3
Shown on FSweekend 2009, now started
DIY motion platform IV pages
October 28, 2009: 1:2 Scale model almost finished.
Second larger scale model, (scale 1:2) more close to final construction.
The construction now includes the balancing system, via bungee cords
that are wound on the wooden drum during platform lifting.
Triple Sarrus stability mainly determined by the bearing construction in the
September 2, 2009: Alternative 3doF motion platform
A completely different construction, using 3 partial Sarrus linkages
construction placed in triangle.
I build a scale model to check the construction on stability and travel. It
can be seen on this
August 25, 2009: New actuator design for improvement on push-rod
To reduce noise, a different push-rod support could be used which makes
use of rubber wheels. See drawing below.
Actuator design that makes use of 30mm rubber wheels. The push-rod can be a
square steel profile. The front wheels are fixed to the actuator case, and
support the push-rod in 4 points. The other end of the push-rod has 4 wheels
mounted to it, and they slide along the actuator case sides and an elevated
board underneath the V-belt.
Some pictures of a prototype I build: I chose to make two guiding points
for the square push-rod: One at each V-belt pulley. The wheels guide the rod
pretty well. No more noise. But V-belt tension is too weak, and the bungee
cords make a tight fit inside.
The overall length of the actuator is at least 3x its actuator travel.
pushrod support via 3 wheels and the V-belt pulley. Sturdy clamp that
holds the V-belt , pushrod and bungee.
Aug 24, 2009: Quick prototype of new actuator design.
Before ordering new materials for the new actuator, I first made a quick
prototype from some leftovers to check performance. Below pictures show this
prototype. Bungee not yet installed.
The measured torque and speed performance with the AMETEK E56617 is pretty
close to the calculations.
Belt tension is critical. Needs to be really tensioned for good friction on the small pulley for
operation without slip.
There is considerable noise from the linear ball bearings. My previous
actuators were very quiet. This new design is much worse.
The pushing rod in the prototype is 12mm, but it needs to be much thicker to
be able to withstand the sideways forces that will happen with the top
driven platform. Next available linear ball bearings for rod diameter size
is 16mm, then 20mm.
August 17, 2009: Modified compact actuator design.
For the top-driven platform below, the actuator height can be about 1
meter, with 45cm or actuator travel. Therefore the actuator design can be
slightly simplified regarding the push-rod support guidance.
Below design has about 35cm spacing between the linear bearings, that
should be sufficient to support the pushing rod regarding sideways forces.
August 9, 2009: New motion platform design.
The new actuator design
could be used in a new type of 3doF motion platform that has the occupant
sitting below the rotation point instead of above it. A possible
construction is shown below.
In the above example, the pilot will receive less false motion cues
during acceleration and turning, as the frame initially moves forward during
pitch-up (acceleration) and leftward during left turn. It also more
accurately simulates the pitch up during rotation and roll in high winged
The rear actuators stabilize the platform in front-aft direction. The
front actuator has a lateral stabilizing function. Some gas spring cylinders
may be used for dampening the actuator's tilt motion.
Some sketches have shown that the new motion platform does not have to be
much bigger than the original 3doF platform: Actuator spacing can still be
1.6m, and total height about 1.5m for 40cm of actuator travel.
The construction is a bit more difficult. A welded frame is probably the
August 8, 2009: New compact actuator design.
I have been thinking of a construction for a new actuator design, that
can be used as push-rod construction or platform hanging construction. I
also wanted to integrate the counterforce bungee cord into the actuator
design, without the need of the long bungee strings running along the floor.
The basic idea is shown above. It still makes use of V-belt, which has
the advantage of strong, non-critical construction, with some flexibility.
Also very important: it hardly makes any noise.
The push-rod is fixed to the V-belt with two cable clamps, some distance
apart for more stability.
The V-belt moves over two pulleys, and the rod will move linear with the
V-belt. A front board guides the rod to the outside.
Two slider bars at the side give the pushing rod sufficient support for
Another rod is welded perpendicular onto the bottom of the rod. The
bungee cord is wound along the rod, goes over the top pulley, down to the
fixed rod. for equal L/R force, two lengths of bungee are used. For 50kg
pushing force, about 10 strings of bungee are needed.
Two slider bars at the side give the pushing rod sufficient support for
The motor drives the bottom pulley via a 3:1 gear-ratio. Pulley
diameters: 50mm for the small type and 150mm for the big type.
The total length of the actuator is about 1 meter, for about 60cm of
actuator travel. The actuator can also be shorter, by using shorter V-belt.
The motor type I plan to use : 2nd hand AMETEK E56617. They were used in
threadmills, and can be found on e-bay for DIY wind generators. The
(measured) motor spec:
Speed: 1300rpm @ 50V
Torque: 0.37Nm/A (a pretty good torque value!)
Coil DC resistance: 1 ohm
With these specs, the ideal actuator performance can be calculated:
Push-rod force: 47N/A. With 24V battery, the stall current will be about
20A. At 20A drive current you get 940N of force. The speed of the motor at
24V is 624 rpm. With the pulley diameter and gear ratio this gives 0.5m/s of
push-rod speed. Torque and speed can be increased by using higher battery
voltage, like 36V. This will give approximately 1410N of force, with
In practice, the torque / force values will be about 25% lower due to
The floor plate can be good grade 25mm plywood. Some V-belt tensioning
system may need to be added on the front pulley.
~to be tested and continued~
April 26, 2007: Adding Heave: trial #2
Not giving up, and now thinking of a completely different system, that
should work, but could fail due to my cable-pulley actuator power
restrictions. The basic idea was already described in
Motion Systems page.
The above system needs three actuators, and together they will need to carry
the weight of the platform.
To reduce the power in the servo motors, the actuator can be build in
such a way that most of the static platform lifting force is accomplished
via springs or bungee cords. A drawing of such an actuator is shown below.
The bungee cords need to be long to keep the pulling force equal over the
To determine the length and amount of bungee cords needed, I did some
A length of bungee cord, mounted to the ceiling. The other end has a basket,
that can be filled with lead weights. The stretching of the cord can be
measured as a function of weight added. I did this for several cord lengths.
The results are shown in the graphs.
As can be seen, bungee cord has a non-linear behavior in the beginning and
when fully stretched. Also there is a slight hysteresis between stretching
and un-stretching. (middle graphs) The shorter the cord, the smaller the linear range. The
force needed to reach the linear range is the same. Longer cord lengths
seem better, as less force is needed for a constant displacement in the
linear range. But a
compromise must be chosen to avoid excessive lengths.
Above graph shows some detailed characteristics of one particular cord
length (1 meter). From this length, total stretched length will become
1.75m, with 28cm of travel in both directions from 1.48m center value.
Center force is 48N, so one cord is good for 4.9kg platform lifting weight.
To check the actuator concept capability, one unit was build according
basic idea above.
As the pictures above show, a closed loop belt was tensioned between drive
pulley and bottom wheel. The bottom wheel has the gear transfer to position
potmeter. A rack-shelve metal beam was mounted to the belt for weight
mounting. A cable loop was fastened to the opposite side of the belt, and
goes down via the bottom wheel to the bungee cords. (See below)
Totally 6 meters of bungee cord were used, 6 strings in parallel, to
get a tension for balancing a weight of about 30kg.
Searched the whole house for something heavy, finally came up with a 25kg
sandbag and 10kg of lead weights. 35kg of weight , balanced pretty well by
the 6 bungee cords.
Connected the servo drive, and moved the servo position by turning the
offset potmeter. Not bad at all! Test results are shown in the
Results with this type of actuator are good enough to start building a 3rd
type of platform, see DIY Motion Platform III
Jan 15, 2007: Ideas to add heave (vertical motion):
Another possibility for implementing simple "heave" is shown above. The
chair is mounted on springs, and can slide in vertical direction. Movement
up and down could be done via a simple pneumatic cylinder. Suitable for
simple up/down motion.
For one-direction heave (quick falling motion), the chair could be triggered
to fall down suddenly, and being returned to upward position slowly. This
method would not require a lot of power.
The same trick could also be done to the whole platform. Normally suspended
on cables, with some reserved slack in each cable, the platform could be
made to drop suddenly, giving you a falling sensation. The fall could be
stopped by the rubber inner tube (used in the DIY motion platform II) and a
rewind system on the cables could again bring the platform in previous
suspended position. Mechanic possibilities: pneumatic, threaded shaft with
release mechanism. I checked this: It takes a lot of force to pull
the cables to lift the platform.
Another method for adding platform heave, probably easier to implement,
could be done as shown below:
Feb 5, 2007: Adding "heave": trial #1
The easiest way for adding
small excursion heave is by partly depressing the rubber inner tube that
carries most of the platform weight. In this way, the upward force is
pressing evenly at the whole platform. The system is low profile, so it can
also be implemented underneath the platform without increasing platform
Front view drawing of the heave structure
Side view drawing of the heave structure
The inner tube depressing is done via an iron bar that is pulled down via
cables. It was possible to add this extra function without major
modifications to the existing DIY
Motion Platform II structure.
Details of the bar that will depress the tube.
The bar needs to be able to follow the roll angle. Therefore it is pulled
from the center.
There was no space for extra cables in the center, so the cable-pulley
system runs at the outside, with parallel cables for shared load. Fast heave
requires considerable force.
The cables are moved by winding them on a 20mm shaft, which is driven by a
DC motor via belt-pulley. Bungee cords will keep the cables tensioned.
Here you can see the bar depressing the inner tire. I needed to add some
wooden plates to increase the depressing surface.
Some tests with the platform mounted showed that the system works, but
unfortunately the heave effect is too little to add any real up/down feeling
to the platform. Total platform deflection is about 6 - 7mm. It also works
against the cable tensioning, and could impact the friction needed for
correct belt-drive. ~Experiment failed~
Weight driven motion cues:
From the experiments with the DIY motion platform I have
realized that getting realistic motion feeling is extremely difficult with
only pitch and roll.
For further experiments I'm planning to only add some small motion cues
to the simpit construction. Since hobby-room height is also limited, I have
been looking for ways to place the Simpit on a flexible base: Something
like foam rubber, or a large inner tire, or a mattress.
Small motion cues could be added by motor controlled moving weights, based on the
action = reaction principle.
The basic idea is shown above.
In order to see the effect of above solution, I build a simple trial unit
that could move a lead weight along a 1 meter rail. Then I mounted it on the
back of my platform while it was sitting on the rubber inner tire. Putting a
step response on the thing, (connect the motor to the battery for 0.5 sec)
would move the 6kg weight in one direction with good acceleration.
As always, only about one out of 10 ideas actually works: The sudden move of
the weight would indeed jolt the platform, but it would take at least 10kg
of weight per direction to get any real effect. All this extra mass on the
platform would make the thing extremely heavy, and all those weights would
have to be moved as well.
~ Idea abandoned ~
I bought a second hand tractor inner tire, that is big enough (1.2m outer
diameter) to place under my simpit. Some experiments with
balancing show that this could be a possible solution for the flexible base.
The tire needs to be only slightly inflated, but must fit completely under
the base board for equal balancing in all directions. If done like on the
picture above, the side (roll) balance is much more critical than pitch
balance. Since the center of gravity is higher than the rotation point, the
whole thing is somewhat unstable. Smaller tires (truck) also work, but the
whole thing becomes too unstable. Roll and pitch are possible, but the inner
tire does not expand, so heave is not possible.
Don't buy new inner tires, as they tend to smell terrible.
Other motion platform ideas:
Low profile flexible base drive using steering box actuators. Major
drawback: Platform will get heavier due to added weight of actuators.
Low profile hanging platform (folded cable-pulley actuator arrangement),
successfully used in my DIY motion
An idea to add sway (left-right motion) to the main pitch-roll platform. It
is intended for small sideways accelerations only. Although there is some
rotation in the motion, the pivot point is more than 1 meter in front, so
with small deflections it should feel mostly like sideways motion. The
actuator would need to push and pull a lot of kg's. Friction is going to be
critical, and would require low friction pivot point and wheels at rear end.
Note: It will probably not work on my cable suspended platform as the bottom
platform will move (in stead of the simpit).
New Interfacing ideas and Parallel Port experiments: see