Hi All,
My first posting and it's a doozy.
My skill level is basic. I actually had some EE courses but that was back when dinosaurs ruled the earth and you did your calculations with slide rules.
I came up with a schematic for this project but I have my doubts about it.
I'm building a sofa-bed for a RV. The bed uses two electric actuators to go from the bed to the sofa position and back again. Since this is an electronics forum let's just assume the mechanics are what they are and can't be changed. That's a whole other discussion.
I'm using actuators from Firgelli Automation. I'd provide a link but I don't want anyone thinking this is a stealth ad. I tried to get this to work from just one actuator but failed so I went to two. That can also be a whole other discussion. These actuators use non-adjustable internal limit switches and have just two wires for input. You apply current and the actuator fully extends then stops on it's own. Reverse the current and it closes all the way then stops. One actuator extends 6" and the other 12". The 12" is already installed and the 6" is in the mail.

My images show up in the message editing window but not in the preview window. I've included a link for each.
Here is the sofa-bed under construction.
http://larry.wvnet.edu/~van/pics/100314-32-j.jpg

Here is the mount of the 12" actuator. The board is mounted vertically under the middle of the sofa. You're looking at the end that will be pointed at the back of the sofa-bed.
http://larry.wvnet.edu/~van/pics/100407-030-j.jpg

This is looking at the middle front of the sofa-bed. The "seat platform" rests on and is bolted to the angled metal you see here which is driven by the 12" actuator. The actuator is attached to the right side of this board which I'll refer to as the "drive board".
http://larry.wvnet.edu/~van/pics/100407-088-j.jpg

On the left side of the drive board is mounted this lifting arm. The 6" actuator will be attached to this when it arrives.
http://larry.wvnet.edu/~van/pics/100412-29-j.jpg

Here is a rough sketch of how things are laid out. Not all elements are shown and not to scale. The sofa-bed is in the bed position. In the view from the right end of the sofa-bed you can see that the 12" actuator is closed. It is attached to what I call the "seat platform". The seat platform is attached to the "back platform" via hinges where they meet.

http://larry.wvnet.edu/~van/pics/Start-0-right.jpg

Here is the view looking from the left end. The 6" actuator is attached to the "lifting arm".

http://larry.wvnet.edu/~van/pics/Start-0-left.jpg

Here is the schematic I've come up with. It uses relays, limit switches and diodes. I have plenty of each and I'm comfortable working with with these. It might be nice to make this more solid state at some time but I've been working on this thing for almost two months now and I'd just like to get it finished.

http://larry.wvnet.edu/~van/pics/electrical_layout-500.jpg

In the schematic actuator "A" represent the 6" and "B" is the 12" one. The inputs on the left for each actuator are tied together at a DPDT (on) off (on) switch. I'll add the limit switches to the drawings as I walk through the steps I'm trying to accomplish.

Step 1. 12v Power is applied to both actuators. Positive power to the top most of each pair. Assume correct polarity at the actuators. Actuator "B" doesn't move since neither #3 or #4 limit switches are closed. Actuator "A" extends to 6" causing the lifting arm to pivot upward raising the "back platform" which is hinged to the "seat platform". During this pivot the lifting arm momentarily trips limit switch #2 but because of the diode in parallel with the switch it has no effect on Actuator "A". At actuator "A"'s full extension it closes limit switch #3 and stops.

http://larry.wvnet.edu/~van/pics/sofa_positions-1.jpg

Step 2. With limit switch #3 closed power now goes to actuator "B". This causes the seat platform to move to the left, in this view, which also causes the back platform to tilt further up. As the seat platform moves forward it first trips limit switch #4 which has no effect since it is already receiving power from via limit switch #3. After the seat platform has travelled far enough forward it trips limit switch #1

http://larry.wvnet.edu/~van/pics/sofa_positions-2.jpg

Step 3. This is the step where I have my doubts about my schematic. With limit switch #1 tripped it powers the two relays which reverse the polarity of the power going to actuator "A". This causes actuator "A" to start to close and lowers the lifting arm. It comes off of limit switch #3 but Actuator "B" continues to extend because of limit switch #4. Actuator "B" will stop when it's internal limit switch is tripped at full extension. Actuator "A" stops closing when it trips limit switch #2. This represents the sofa-bed in the sofa position. The back platform is supported by the "back rest" at the top and by the lifting arm at the bottom.

http://larry.wvnet.edu/~van/pics/sofa_positions-3.jpg

Here's my two doubts about this step. Is it safe to assume the two relays will activate simultaneously? If not you have a short. After the two relays trip will the path through both relay coils provide a path of least resistance so no power goes to actuator "A"?

Step 4. To go from the sofa position to the bed position isn't as big a deal since Actuator "A" doesn't have to change directions. Reverse power activates just Actuator "B" the seat platform travels far enough to the right to allow limit switch #1 to open. Then Actuator "A" goes to the closed position which lowers the lifting arm. Both actuators then stop at internal limits.
http://larry.wvnet.edu/~van/pics/sofa_positions-4.jpg

OK, for those of you who I haven't either bored or confused to death, Thanks for your time. I hope I was able to make sense of it all. If you have any questions just let me know. After two months this thing is giving me headaches.

Thanks again,
Dave

PS. In case you want to kill some time:
Actuator "B" in operation - 6MB avi file.

My FAIL of a lifter mechanism - 6Mb avi file
As a added attraction you can hear my Quality Control Engineer giving her opinion of the project at 34 secs.

PPS. In case you are REALLY BORED:
My build thread at RV.net.

 

Well, you have a few problems going on there.

Limit switches, unless you pay lots of money for them, really aren't made to carry much current. You're better off to use limit switches to control current through the coils of relays. The coils don't take much current, the relay contacts can take plenty of current, and automotive 12v relays are cheap and plentiful.

You want to design the circuit so that there is no possible way that a short could occur. And no, it's not safe to assume that both relays will energize simultaneously. What if one of the coils burned out, or contacts got welded together? Zap.

I'm running out of time here because the board goes down in a few minutes for maintenance.

However, here's a basic circuit I've posted a number of times on here for reversing a motor direction.

Pressing S1 causes the motor to run forwards; pressing S2 runs it in reverse.

S3 and S4 are limit switches.

There are no combination of switch openings/closings that can cause a problem. The motor either runs or it's stopped.

 

Well, that is quite an opening post. As a carpenter and electronics hobbyist with many a trip, into the great wide open (And the coldwater streams) with my grandfather. This is very interesting to me. I have also read 6 of the 12 pages of your build thread.. You do good work.

I hate painting, but unfortunately I do it well. Therefore, I am always called into action by family and friends who need the task done.

SgtWookies' advice in using the limit switches on the coil side of the relay should be heeded. His circuit will give you what you need to operate the actuators from button-press up or down. You will have to use two of his circuits. Connecting them to work with both actuators should be an easy task.

 

After sleeping on it, the easiest way to control the angle of the portion of the bed/sofa that acts as the back would be to mount the 6" actuator on the underside of the movable platform itself.

Otherwise, you're going to have to make up some kind of crazy linkage, and have the short actuator running in and out to get the alignment right. Having the actuator mounted to the platform itself keeps the relationship between the actuator and at least one of the two surfaces constant.

The basic sequence would then go something like this:

1) The unit is in the "sofa" position. A button is pressed (S1) that engages a relay (RLY1).

2) The 12" actuator (M1) operates in the forward (extend) direction until it is fully extended, and the front edge of the "sofa" overhangs by about a foot. When the limit is reached, a limit switch (for now, S3) is engaged.

3) When the limit switch is engaged, a relay is toggled which removes power from the 12" actuator and applies it to the 6" actuator.

4) The 6" actuator runs in the reverse (retract) direction until the rear platform is lowered all the way flat.

5) When the rear platform is flat, a limit switch is contacted which removes power from the relay coil.

Converting from bed to sofa is exactly opposite:

1) A button (S2) is pressed which causes RLY2 to apply power to the actuators in the other direction. If the platform back is not angled to the "up" position, the 6" actuator runs in the forward (extend) direction until it is.

2) A limit switch is contacted which toggles another relay that removes power from the 6" actuator and applies it to the 12" actuator.

3) The 12" actuator runs in the reverse (retract) direction until it is fully retracted.

The use of either two pushbuttons or an (ON) OFF (ON) type toggle or rocker switch is suggested. (ON) = momentary; spring-loaded to the center position. The switch should be in a location where the entire sofa/bed can be viewed during operation, and encourage the operator to not be in the way of moving parts. It should also be in a location where a restless sleeper could not accidentally begin operation of the actuators.

 

Hey guys,
Thanks for the quick reply and the kind words.

I agree about the activation switch placement. I'll be mounting it at the drivers seat so it can't be reached from the sofa-bed.

I'd already considered mounting the 6" actuator under the seat platform to raise the back platform. If the platforms had a metal framework that's probably the design I would have used. But since I can't weld I made the platforms from 3/4" pine. When you sketch out the force diagram needed to get the back platform to swing upward when it's laying flat and the actuator is laying along that same plane you're putting a great deal of stress on the board where it's attached to the hinge. I wasn't comfortable with stressing the wood that much so I went with the design I'm using with the short actuator running in and out. If it was all metal, no problem.

The limit switches I have on hand are rated at 15A (they aren't small) and the actuator only draws 4A at full load. Wouldn't these be OK to us as I've drawn?

Using the two relays to reverse polarity is just a bad idea. I thought of it because I have a bunch of 12v automotive relays on hand. If I replace the two spst relays with one single dpdt relay that solves the possible sync problem.

That still leaves me with the path of least resistance question concerning the power going parallel paths to both the relay coil and the actuator.

Thanks for the help,
Dave

 

I don't know why you came here to ask for advice if you were going to simply reject it outright.

I don't know how you consider using a pair of SPDT relays to reverse the polarity on the actuators "a bad idea". Actually, it's a very good idea; they even sell automotive relays made specifically for that purpose.

Without using a pair of SPDT relays, I don't know how else you're planning on reversing the direction of current to your actuators, as you don't have a complete wiring diagram.

The wiring diagram you DO show does not correlate which actuator is which. You need to be more consistent in use of reference designators, or nobody will have any idea of what you are talking about.

 

Wookie, I read it that way also.. If you re-read it, you will see that he is saying his original idea was a bad one.. Not yours.

a bad idea. I thought of it because I have a bunch of 12v automotive relays on hand.

 

Hey Guys,
I wasn't rejecting your ideas outright. I wanted to explain the reasons I used to come up with my ideas. And retched is right. I didn't say your idea was bad I was trying to say MY idea of using two spst was bad from the start.
Sorry for the confusion.
I'll fix the diagram when I get to my home computer.
Thanks,
Dave

 

Sorry for misinterpreting what you were trying to say, and sounding a bit miffed.

I had to read your prior post over another couple of times after Retched's post to understand that you were talking about your implementation, rather than my suggestion. I didn't see that the first several times I read it.

Anyway, take a look at my admittedly really awful "rough concept" drawing, attached.

The blue round bump on the top side of the platform is your hinge point (I assume you're using a piano hinge).

The green circles on the projections are the attachment points for the 6" actuator.

The idea here is that the green pivot point on the seat bottom bracket is at an angle from the hinge that will minimize the stress on the board that is the seat. The lower the pivot point is, the less stress on the seat board.

I don't know what kind of pine board you used. The pine available nowadays isn't like the pine you could get 40 years ago; it's more like "white wood" that has very low density. Gluing on a square of oak would give good reinforcement. Oak is far more dense than pine, and will have much higher strength.

If you're using wood screws in pine and applying sideways force, the screws will quickly wear the hole oblong and strip out.

 

Hey Sgt,
I think you might have something there. I'm going to have to take some measurements do some more pondering. I'm good at pondering. After I stop pondering is when I run into trouble.

 

This is a bit closer to what I was thinking about:

Except I'm just using MS Paint instead of a "real" cad program, and I don't know the dimensions of your project, so it's difficult to make anything even close to scale.

Basically, similar types of reinforcing & standoff blocks with iron or aluminum angle linking to the actuators. That will spread the stress over a wide area, and the offset from the bottom of the seat area will give the actuator more of a mechanical advantage.

The disadvantage of having the actuator on the movable part of the platform means that you will need a flexible wiring harness to connect the actuator and limit switches to the base. This really isn't that hard to do. You might even use a section of flexible air hose anchored at each end, and run stranded wire through it - that might be a bit tight though.

 

Hey Sgt,
You've cracked it! I'd award you another stripe if I could.
I hadn't thought of mounting the 6" actuator so low. I was always thinking of plans that put it in the same plain as the platform it was mounted to not nearly 90 degrees off it. Like you said it's all about the mechanical advantage.
Even thought I'm not going to be using the 6" actuator like I originally planned it's a good thing that that's the one I ordered. It's 10.5" long when closed and there's a little less than 12" clearance under the platforms so this should just fit with a little to spare.
I'll have to figure out the best place/way to mount but I already have a couple good ideas for that. The actuator should be here tomorrow so I can get right to testing.
I'll post a photo when I get it all done.
Thanks again. You're the best.
Dave

 

Note to Sgt Wookie: It's getting there.

 

Watching and waiting. I have been bouncing over to rv.net to watch your progress. You are doing quite good work for designing on the fly.

 

Note to Sgt Wookie: It's getting there.

I sort of see...
Is that a piece of flat stock at the bottom of the actuator? I meant for it to be angle iron/angle aluminum. I'm afraid that flat stock won't be rigid enough to avoid flexing, which would eventually result in metal fatigue/failure.

Is the piece of ply attached to the bottom of the seat panel area using angle or a 2x2 or the like - so that it can move with it? That's how I meant for it to be.

 

Hey Retched,
Thanks for the kind words. I appreciate it.

Hey Sgt,
I'm using two 1" x 3/16" steel bar stock to hold the base of the actuator.

I'm confident that will do the job.

Is the piece of ply attached to the bottom of the seat panel area using angle or a 2x2 or the like

Of course. Just like you described.

Dave

 

UPDATE: Video of the Sofa-bed in operation.

http://www.youtube.com/watch?v=RioZNVmrtQM

Thanks for the help guys.
Dave

P.S. This isn't the last idea I have for the RV conversion. I'll probably be back asking for help again before too long.

 

Good! Job!. You should be awful proud. I was watching your progress over on rv.net, and I was impressed by your DIY from start to problem to solving to seeing it through.

Congratulations.

 

Hey, it works!

Tad noisy though. You might consider using some rubber bushings between the motors and the mounts to avoid "telegraphing" the motors' vibrations through the entire structure. Something like automotive heater hose or surgical tubing perhaps. Use some silicone lube on the rubber to keep it from wearing out too fast.

 

Thanks Retched,
For as much time and effort as I put into it I'll admit to being pretty proud of how it turned out.

Hey Sgt,
The sofa-bed isn't really that loud. I used a regular digital camera, not a video camera, and it seems to have magnified the noise level.