Discussion & brainstorm topic: Reducing motor load

last night it finally sort of dawned on me

I was assuming that a spring/piston or.... would need to help the motors pull the platform down and push it up, but value1's remark about the center of gravity being above the pivot shed some light on the case.

If I'm correct this is what happens in this case:

While the platform is horizontal and correctly balanced the center of gravity (c.o.g. for now) is centered above the pivot and no force is applied to the motors.

As soon as the motors start to move down the platform is tilted and the c.o.g. shifts next to the pivot which results in the platform falling on the motor side and the motors having to catch the falling platform and bring it down to the wanted position in the correct timespan..

As soon as the motors start to move up the platform is tilted and the c.o.g. shifts next to the pivot which results in the platform falling on the other side, thus rising on the motor side and the motors having to catch the rising platform and bring it up to the wanted position in the correct timespan.

So instead of needing springs/cylinders or ... to help the motors pull down or push up the platform we need them to help the motors catching it and bringing it back in a level position.

Hope this makes sense and sheds some light on a possible solution for people with more pneumatics knowledge than me.

To me it sounds logical that a cylinder pressurized on both sides would do that, but still not sure.

You should probably consider a centered counter weight on the bottom side of the platform, altho designing would need a little extra thought.
How much are you trying to shift weight wise?
Are you trying to make the weight neutrally balance throughout the axis rotational range? Which will make a consistent force neccessary to push the platform at all times in any direction.
I think the principles of a glide or steady cam may help you http://www.youtube.com/watch?v=X0i7ViEwlyI

However I think reducing motor load can be more complicated and costly rather than just getting more powerful motors, you may even find the weight an advantage with a PID controlled system and motors that are too fast but not as torquey.

Total weight including driver will be approx 125kg.

Small cylinders come fairly cheap ($ 30,- for a 32x150mm cylinder) and something that can be used as an expansion tank doesn't have to be expensive either (e.g. a small 2kg fire extinguisher: $ 7,50).

So I think stronger motors will be more expensive since they need a stronger driver and psu as well.

I'm still curious about the right setup though. One says a double action cylinder is needed, pressurized on both sides so it acts like a spring, the other says to apply pressure on one side so it acts linear.

Exactly! I think if the cylinder is pressurized separately on each end when it's centered, it will cause it to try to hold the center position. Don't use an expansion tank in this scenario, though, because the cylinder will cancel itself out then. You want the force to increase towards the ends, and drop to zero towards the center, but expansion tanks wil cause the force to be 100% the whole way. 100% force pushing both ways results in zero.


The difference between a double and single action cylinder is that the double action has air connections on both ends. A single action cylinder has only one air connection, and an internal metal spring which pushes it back when air pressure is removed.

But a linear one (double action with single tank) keeps it in the wanted position which reduces motor load as well (e.g. in long corners) so it's still all rather confusing

It won't hold any position if you apply a linear force pushing both ways. It'll be the same as not having any pressure applied at all.

I meant linear one way

Edit: e.g. apply pressure so it only just can keep the platform in the top position, the motor will only have to apply a little force to center it or pull it down.

Or am I missing something AGAIN?

I made a drawing to illustrate how it works without an expansion tank.

In the top figure the platform is level and balanced, and the pressure in each end of the cylinder is the same.

In the center figure the platform tilted back, moving the piston 50% up. The air in the top chamber is compressed to twice the normal pressure since the chamber volume is halved. The pressure in the bottom chamber is reduced by 50% since the chamber volume is increased by 50%.

The bottom figure shows the same as the middle figure in the opposite direction.

As you can see, the pressure differential in the cylinder will try to push the platform back to center. You can calibrate the force by changing the pressure in the chambers. Make a hose from each chamber with a schrader valve from an old bicycle tube on the end and fill the chambers with a bicycle pump, for example. Road bicycle pumps can pump up to about 10 kg/cm².

And here's what would happen with expansion tanks. No pressure differential between the two chambers, means no force. Expansion chambers are great for something like Aldoz's sim, because they are under a contant lifting force. One cylinder for each actuator would be great in a lifting sim.

And the bottom sketch is with an expansion tank only on the bottom connection of the cylinder and no connection at the top one?

Wouldn't pressurizing the bottom part of the cylinder create a constant upward force (since atmospheric pressure is 1 kg/cm2 and e.g. we add 3kg/cm2 to the bottom part). Resulting the platform being tilted upwards when idle and due to the expansion tanks the motors only having to aply a little force to pull it down?

Sorry for being a pain in the #$$ but I want to try and understand it

You're not a pain. I have another project going that needs something similar so it's a useful thought experiment for me too.

With only the bottom chamber pressurized, and with an expansion tank, the platform in my drawing will want to tilt back all the time. The motor will have to oppose both the force from the cylinder, plus the weight of the sim when it has to move the platform forward after going back. The only time the motor will be relieved is when the platform is fully forward.

that makes a lot of sense indeed! I (once again) didn't think of the weight wanting to tilt the plaform backwards in the top position.

Conlusion for now: 2 cylinders with a minimal stroke equal to my maximum travel and 4 valves is all I need to get started (does the bore make any difference? 25mm bore is about 2/3 of 32m bore)

Make sure you pick cylinders that are big enough. For example: A 16 mm bore cylinder has an area of 2 cm². With a max pressure of 7 kg/cm² the highest force it can produce is 14 kg. Multiply the area in cm² by the pressure in kg/cm² to get the force. Also remember that the rod end of the piston in the cylinder has a slightly smaller area than the other end due to the thickness of the rod. But with a little size margin you can just adjust the pressure to compensate.

Been doing some research on affordable cylinders on ebay.

One thing I noticed is that they are 14cm longer than their stroke. In case I would place them next to my motors I would need a100mm stroke, resulting in a 240mm long cylinder. Adding half the stroke and mounting brackets, rod ends etc. that would result in a platform height of approx 45cm which is 20 cm higher than I intended

I could place the cylinders half way between pivot and motors resulting in a needed stroke of 50mm which results in a 190mm long cylinder and approx. 350mm platform height.

Would this cause any problems due to more load on the cylinders and smaller air chambers?

Am looking at 32mm bore 7kg/cm² cylinders btw, here's a 75mm sample:

http://www.ebay.com/itm/MAL-32x75-Doubl ... 7771749073

Have you thought of mounting them upside down? That will shorten the installation since the body can stick up beyond the mounting point.

Have thought about that, but that means they would have to be fixed on one side and thus would make them tilt outwards (the other end moving out- or inwards) when the platform tilts and I have no clue on how to mount them in that case because the platform tilts in two directions resulting in the other side making sort of a circular movement.

Like the bottom drawing here.

Since this still is a brainstorm topic and regarding springs like bsft uses: Just found these on ebay

http://www.ebay.com/itm/Hot-2Pcs-Adjust ... es&vxp=mtr

Would be quite easy to make something similar a bit more upscaled I reckon (don't think these would be strong enough)

I know, but when the top platform tilts sideways the end of the piston rod will move to the left or right (looking at your sketch) and if it tilts forward or backward it will move towards or away from you, so how to mount that end in such way that the rod can move and won't bend?

I would either make a basic gimbal or use a spherical bearing. The simplest would probably be a metal angle with a rubber mount working as a flexible joint.

You can probably make those coil springs work for you too. They looks pretty strong.