Mianne Lift Bridge Lower Limit Switch Enhancement

So now you're a "slacker?" 

If slack is the enemy you might as well sense that condition and take action once detected.  Should work!

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.

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.

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

he's using a spring loaded pulley so there is no need for a brake , the spring tension holds the cable on the pulley! 

Idler Pulley 101! Simple fix to an annoying problem.

Possible solution would be to use a chain instead of a cable.   

Jack

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.

Dr. Jack posted:

Possible solution would be to use a chain instead of a cable.   

Jack

How would that fix anything?

Use limit  switch as main shut-off and then physically  lock down the bridge with some type of mechanical lock you have to engage or release when the bridge is in place. Is there enough dead weight to seat the bridge correctly in the first place ?

Jim, plenty of dead weight to seat the bridge, the only issue is the limit switch operation.  I don't want to have to fiddle around with locks under the bridge to operate it, what's the point of having it if I have to crawl under it anyway?

John,

I have another idea that may work. Let's talk at the Ham Fam Saturday morning.

Chris

LVHR

Chris, I thought this was a pretty simple one, and it seems to work well.

John, if you have slack in the cable when lowering, the links in a chain will elimate any slack when raising or lowering the bridge. 

Jack 

I still don't get it Jack.  The issue is the limit switch adjustment, if it's not exact, the motor keeps running and unspools the cable.  I don't see how chain is going to solve this issue.

Obviously, you could redesign the whole lift mechanism, but I wanted something that works without a redesign, this is it.

Yes it would have to be redesigned.

Jack

Well, that's not going to happen!

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

Andy

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

LVHR