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I have the Mianne power lift bridge, but one thing bothers me, and it's already bit me.  While the upper limit switch adjustment is not that critical, a 1/4" one way or the other on the upward travel isn't a big deal, the same can not be said for the lower limit switch!  The adjustment of the lower limit is very touchy as you want the bridge to fully seat and be in alignment.  However, the difference between it fully seating and turning off the motor and not quite seating when the motor stops is a tiny adjustment!  Worse yet, a tiny adjustment in the wrong way and the motor happily unspools the cable and tangles it in the pulleys!  That's what happened to me, and I don't really know why.  I know the bridge was working fine and then one day it wasn't.  Nobody was anywhere near the limit switches, but obviously something changed.

OK, time for a design enhancement.  One "constant" is that the cables that lift the bridge are under constant tension all the time until the bridge is fully down, then if the motor keeps running, the cable will first lose tension, and then bad things happen right after that!  What if I could simply sense when the cable is going slack?  I could shut off the motor after the tension was lowered, but before the cable was fully loose and falls off the pulleys.

Here's my solution.  Tim at Mianne graciously gave me a sample spring loaded pulley just like I needed at York for one piece of the puzzle.  The other piece is a long arm microswitch and a mount to position it to sense when the slack starts to happen in the line.  I got a bag of the microswitches on eBay for peanuts, I had the aluminum sheet stock.

MIanne Lift Bridge Limit Switch Upgrade N1

The mounting bracked it just some 1/16" aluminum sheet that I hacked a mount out of.  These two pieces mount under the lift bridge in the center where the cable slack will first be noticed.  Here they are mounted in position, the switch senses the slack just as I intended.

MIanne Lift Bridge Limit Switch Upgrade N2

All that's left is to get the proper wire to run the limit switch over to the lift bridge and crimp a couple of terminals on it.

I think this will enhance the reliability of the bridge, and will also allow it to positively fully seat in the down position.


Images (2)
  • MIanne Lift Bridge Limit Switch Upgrade N1
  • MIanne Lift Bridge Limit Switch Upgrade N2
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I think this should be more positive than the switch on the leg.  The adjustment is not at all critical, if I need a bit more slack before the switch trips, no problem, there's lots of travel in the spring pulley.  Also important to me is having the bridge totally seated.  I was having issues trying to get it fully seated before the motor cut off.  This should eliminate that issue entirely as once the line starts to go slack, the bridge isn't moving any more.

Just waiting on the wire I need to run from the controller to the new limit switch.

rockstars1989 posted:

John, shouldn't there be a brake on the motor so it stops instant when power is shut off by the limit switch?  Nick

Nick the motor stops very quickly.  The problem is the adjustment of the lower limit switch is very critical.  The difference between a fraction of an inch is the bridge doesn't seat properly (with the obvious implications) or the limit switch doesn't trip and the pulley unwinds and creates a tangled mess!

With this rig, after the limit switch trips and the bridge is seated, there's still tension on the cable, so it stays on it's pulleys and all is well.  Since I didn't have the actual flexible rubber coated wire I'm going to use, I sky-wired it to test, worked like a champ.

Next comes the dual switch mod so I have a switch on either side of the bridge to activate it.  Having one control is yet another PITA, my bridge is four feet wide!

Alentown posted:

Thanks for the info, John. I (actually my future son-in-law) am in the final stages of installing the liftgate. Mostly going well, but we are having some 'slack' issues with the wire. I look forward to following this topic and your future contributions.

Chuck, this should be the silver bullet for that issue.  I'm looking forward to not having to be careful as the bridge seats in case the limit switch got knocked out of adjustment.

John, Actually, I see nothing wrong with your solution. Have at it!

But having said that, I see 3 ways to handle the situation:

1. Install and program a stepper motor. (Expensive) It always "knows" where it is in the cycle. And you can program it to stop or start  anywhere you want.

2. Install a sensing system to detect slack in the cables. This is what you have done. Down side: Not doing it properly will allow too much slack and take you back to the original problem. I think you have that covered.

3. Maintain tension on the cable, and use something else to detect the lowered position. This is what I had in mind. You add a moving pulley with a weight on it. The weight can move up and down in a trough or other guide. There is a stop at the top of the guide to prevent the pulley from getting out of position, but still allowing the cable to move. The cutoff switch is somewhere near the bottom of the trough. When the bridge goes up, the pulley goes up, and hits the stop. Your upper limit switch determines how high the bridge goes. When the bridge goes down, the pulley stays up b/c the cable is under tension from the bridge. One the bridge is down and seated, the weight keeps the cable under tension and pulls the pulley down until the limit switch in the trough is activated. End sequence.



Chris, I thought about optical sensors, etc., but the slack detection was so easy and had a lot of room for "slop" while still accomplishing the goals. 

#1 was a non-starter, that would be VERY expensive for the size stepper motor needed and the control circuitry needed to manage it.   We're taking at a minimum hundreds of dollars and a ton of development work!  That sounds like killing flies with a sledgehammer!

#3 I'm having trouble visualizing exactly how that works or solves my problem.  The issue is the table settles into place, but not necessarily all corners at exactly the same time.  There is a certain amount of "wobble" as it goes up and down.  Not much, but if you are looking, it's noticeable.  It sounds like this sensing would share the issues of the current system, I wouldn't be positive that the table was fully seated. 

The upward stop was never a problem, that's a totally non-critical adjustment.  It's the lower stop that's the tricky one.  When the table is coming down, it's the motion of the table that trips the limit switch.  However, once the table hits the lower stops, it stops moving.  If the switch is not precisely adjusted, the table stops just before it's totally down.  However, once the table finally is totally down, if the switch doesn't trip simultaneously with that happening, then the motor keeps running as the switch will never trip, no movement after the table is all the way down!  My mod makes sure the table is fully down, and only after that there is a positive action that opens the limit switch and stops the motor.  I can't imagine a simpler method of doing this, and this should be dead reliable.

Steamfan77 posted:

John, can you post a video of the new set up in action?

Once I get my flexible rubber cable and run it, I suppose I can.  However, it'll look exactly like it runs now, the only difference is how the stop is triggered.  Actually getting a picture of that action would probably be somewhat difficult.


That's what I like about #3: The position of the table is NOT critical, so long as it seats in position. It's the weight that trips the limit switch. You can build as much or as little cable travel in the system and still not worry about the slack. I'll give you a drawing on Saturday.

Question for you: Is there one motor or 2 to drive the lift gate? If one, what are you doing to be sure the table moves without binding? If you are lifting from only one side, I would expect trouble.

BTW, I only threw the stepper motor in there because I had one to play with a while back. But that was well before Mianne came on the scene...





That is so strange.  My Mianne lift gate has never had an issue not seating.  It does have a lot of weight on it with the 3/4inch plywood top, the track and scenery so maybe that helps.  My cable never loses tension.  Even down it has some tension on it, so it can't come off the pulleys.  I think my adjustment for the lower limit is just a hair past the stop so it keeps tension on the cable.  So its kind of like it tugs a little on the cable after the top fully seats down.  I didn't have to go through all the stuff you described.  Perhaps you installation is not perfectly square and it is racked a little.  I could see where that would cause it not to seat all the way.  I tested the gate without the cable first, manually pushing it up and then dropping it down to verify it seated even before I installed the motor or cable.

If you go to time spot 18:26 in this video you can see it.  Also this shows how to wire up an inside, outside control switch.



Last edited by Sean's Train Depot

Chris, since I think I have it licked, I'm not going to put any more development time into it.

Sean, as long as the lower limit switch is adjusted just so, mine works the same way.  However, for reasons that I don't fully understand, it's gotten mis-adjusted twice.  As for square and level, Tom and I spent quite a bit of time making sure the table was 100% true, so I don't think that's the issue.  It seats fine and true if the limit switch is adjusted perfectly, it's just that seems to be harder to maintain that adjustment than it should be.

I have the inside/outside switch figured out, just have to get the switches mounted.  I looked your video, and I did basically the same thing.  The only difference is, since the switch has two terminals wired together, I only used 5 wires from the two remote switches.


Finally picked up some jacketed two-conductor wire for the limit switch mod, it works perfectly.  The bridge comes down and firmly seats, and the spring pulley keeps plenty of tension on the cable to keep it in place, couldn't have worked any better.  I used the push-on connectors so I didn't have to do anything but put the cable in place, plug it onto the switch and the existing limit switch wires from the winch, and we're in business.

My "final" mod to the lift bridge controls.  I added the two switches in parallel, they work great.  I used cable grommets and drilled a 1 1/2" hole in the fascia and bonded my fiberglass mounting plate behind the fascia.  Neat, and since it's a friction fit, I can even remove the grommet to seal the fascia, a step I need to do after any other possible mods to it are made.

Remaining was the remote possibility that some pair of characters would try to activate both switches in opposition and short things out, probably not likely, but then not impossible either.  So, I addressed that eventuality with this wiring mod, an 3.7A PTC is in series with one of the hot wires.  Excess current will trip the PTC until whatever yahoo that's holding the switch lets it go.

Mianne Lift Gate Dual Control Addition


Images (2)
  • mceclip0
  • Mianne Lift Gate Dual Control Addition
@RJT posted:

GRJ are you imply some folks don't pay attention/listen to instructions or just following the KISS Principle in order to make your life easier?

Perish the thought!   Actually, the control enhancement seemed to be the easiest way to get it done, and the PTC was just stuck in to insure no fireworks.

@Alex M posted:

Nice work John

Thanks Alex, one little step at a time.

I just discovered another major plus to the enhanced lower limit switch on the lift-bridge!  I was working on the layout and I needed to put the bridge down, on the way down suddenly it clicked a couple of times and went dead!  OH CRAP!!! I thought!  I looked and a light was blocking the way down!  Oh, I can fix that, I raised it a bit, moved the light, and all was well.

Rewind to the original lower limit switch technique.  The bridge would have stopped, no doubt about that.  However, since the lower limit is a hard limit on the travel of the bridge, the motor would have kept running and unspooled the cable creating the mess that you are familiar with if you've ever had the lower limit switch mis-adjusted or something blocked the bridge coming down!

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