Using a Linear Actuator to Automate the South Flap

[old_eyes]

I am setting this thread up separately to my main observatory build thread, because it asks specific question that other people might be interested in.

Due to my slightly seat of the pants design process, I did not consider automation of either roof or south flap for my observatory. However, I am now thinking that it might be rather cool and rather useful (remote operation potentially).

The roof should be fairly simple using the popular sliding gate opener with rack and pinion. That looks OK. The flap is slightly more difficult. Due to certain decisions already made, the super improved @ollypenricesiege engine mechanism doesn't look possible. I have seen @Gina's nautical pulley system, but that would get in the way a bit and involves careful setting to get the right mechanical advantage.

So I wondered whether I could use a linear actuator to operate the flap.

These are available from Amazon:

IP54 rated, 12V operation and with various throw distances. Another advantage is that they autostop at the end of travel in both directions, so you don't need additional limit switches. They pull 1000N (264 lbs) and push 1500N (330 lbs).

I thought that if I had a bar projecting from the bottom of the flap frame inside the observatory, I could push and pull on that to open and close the flap.

OK - could I make the geometry work? Desperately dredging up kinematics equations from A-level physics and maths I got myself into a real tangle before finding a free kinematics simulator. This enable me to check out whether it was at all viable.

I ended up with this configuration using realistic dimensions and a 200mm throw linear actuator.

The green line is the flap hinged at the base, the purple line allows for the thickness of the flap frame, the blue line the bar projecting down from the flap frame and the brown thingy is the actuator.

I have assumed the hinge and actuator anchor are in line.

Open it looks like this:

And a video of operation is shown below. There is a slight crank in the push bar to make sure the mechanism doesn't jam. I can take some of the pressure off the actuator by extending the bar and counterweighting it. The crank angle required is so slight that when open the bar and counterweights will not project significantly above the walls of the observatory.

I think that this would work, but my question to all you engineering types out there is "where is the mistake?"

[Davey-T]

May need some engineering calculations that are beyond me, last time was O level physics working out centre of gravity of a cane with a ball on he end 😂

Dave

[Gina]

It depends on the weight of your flap but it looks like you will have a lot of stress on those parts.  I suspect these will need to be steel, welded up but without all the information I can't say.  Otherwise, it looks viable to me.

Edited August 28, 2019 by Gina

[Davey-T]

More sound engineering wise might be to hinge the actuator and have it lift the flap from outside via a bearing rolling on a steel strip on the flap, would probably need counterbalancing anyway to reduce the load plus some sort of positive locking closed mechanism.

Dave

Edited August 28, 2019 by Davey-T

[ollypenrice]

1 hour ago, old_eyes said:

I am setting this thread up separately to my main observatory build thread, because it asks specific question that other people might be interested in.

Due to my slightly seat of the pants design process, I did not consider automation of either roof or south flap for my observatory. However, I am now thinking that it might be rather cool and rather useful (remote operation potentially).

The roof should be fairly simple using the popular sliding gate opener with rack and pinion. That looks OK. The flap is slightly more difficult. Due to certain decisions already made, the super improved @ollypenricesiege engine mechanism doesn't look possible. I have seen @Gina's nautical pulley system, but that would get in the way a bit and involves careful setting to get the right mechanical advantage.

So I wondered whether I could use a linear actuator to operate the flap.

These are available from Amazon:

IP54 rated, 12V operation and with various throw distances. Another advantage is that they autostop at the end of travel in both directions, so you don't need additional limit switches. They pull 1000N (264 lbs) and push 1500N (330 lbs).

I thought that if I had a bar projecting from the bottom of the flap frame inside the observatory, I could push and pull on that to open and close the flap.

OK - could I make the geometry work? Desperately dredging up kinematics equations from A-level physics and maths I got myself into a real tangle before finding a free kinematics simulator. This enable me to check out whether it was at all viable.

I ended up with this configuration using realistic dimensions and a 200mm throw linear actuator.

The green line is the flap hinged at the base, the purple line allows for the thickness of the flap frame, the blue line the bar projecting down from the flap frame and the brown thingy is the actuator.

I have assumed the hinge and actuator anchor are in line.

Open it looks like this:

And a video of operation is shown below. There is a slight crank in the push bar to make sure the mechanism doesn't jam. I can take some of the pressure off the actuator by extending the bar and counterweighting it. The crank angle required is so slight that when open the bar and counterweights will not project significantly above the walls of the observatory.

I think that this would work, but my question to all you engineering types out there is "where is the mistake?"

 

I'm obliged to use my present design because, if the village gets uppety, I use our south flaps to lob the odd decaying horse carcass in their direction to remind them that the plague is worse than turning out their lights... (This aspect of my foreign policy has been temporarily suspended during my application for dual nationality.)

Now I need to find out what a linear acuator is.

lly

[old_eyes]

16 minutes ago, ollypenrice said:

 

Now I need to find out what a linear acuator is.

lly

When I showed the Missis she said it appeared to be a very slow acting sex-toy 😮

[old_eyes]

30 minutes ago, Davey-T said:

More sound engineering wise might be to hinge the actuator and have it lift the flap from outside via a bearing rolling on a steel strip on the flap, would probably need counterbalancing anyway to reduce the load plus some sort of positive locking closed mechanism.

Dave

I thought about that, but see attractions in having the actuator inside obsy.

[old_eyes]

46 minutes ago, Gina said:

It depends on the weight of your flap but it looks like you will have a lot of stress on those parts.  I suspect these will need to be steel, welded up but without all the information I can't say.  Otherwise, it looks viable to me.

I’m going to make some estimates of weights and moments, but in principal the stresses would be no worse than in @ollypenrice‘s design. It is exactly the same thing except pull rather than push and coming at it from a different angle.

Anyway the good thing is that it is not essential and can be a later adaptation.

[ollypenrice]

13 minutes ago, old_eyes said:

When I showed the Missis she said it appeared to be a very slow acting sex-toy 😮

The best kind.

I'm told...

 

[Bukko]

Great idea to use linear actuators, plenty of torque for your application.

Just thinking about the plan to rotate the south flap. I looked at the actuator on Amazon and for not much more, you can get one with 450mm of travel. Would this be enough to simply raise and lower the end piece instead of rotating it? My thoughts are you would have less forces on the pivots and also nothing sticking out at (possibly) head height to catch someone during the night...

If you want more travel, then using a couple of pulleys would double the travel...

I have not developed the idea any more than this suggestion, but in my mind, it would be relatively simple..

Gordon,

[old_eyes]

Hmm! Interesting thought. My flap is deeper than 450mm, but you could use a simple lever to get the required travel. A portcullis instead of a drawbridge.

I’ll have to check. Some of the decisions about the building structure already made might get in the way.

Good idea though!

[old_eyes]

Linear actuator now installed and for a wonder it works!

Evidence here