Need advice from a mechanical engineer re solenids/linkages

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Need advice from a mechanical engineer re solenids/linkages

Postby andy » Sun Feb 12, 2012 6:31 pm

I am working on an art project which inclues a 12-20' arm (somewhat similar to a RR crossing gate) which needs to move up and down, maybe a little faster than a RR crossing gate. My idea was to counterbalance it to make it balanced around the axis of rotation and use solenoids to give it a push as needed - however there are places along its arc of travel that it needs to stop and bounce back from, and at other times pass through unimpeded. I put together an idea involving 4 solenoids and 6 rotating metal pieces (not sure what they are called "cams?") but to be frank I am just guessing and need the advice of someone who actually knows what they are doing. I am trying not to use a servo motor for power/dust reasons.

If you are an expert on solenoids and/or mechanical contraptions, please post or PM - thanks!
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Re: Need advice from a mechanical engineer re solenids/linka

Postby ezdolphin » Fri Feb 17, 2012 4:22 pm

Hi Andy,

To give better advice I would need to know more. But here are some thoughts on solenoids and motors...

By using solenoids you are injecting a short pulse of energy into the arm to move it compared to a longer pulse from something like a motor. Solenoids can use large amounts of power for short periods of time whereas a motor should use less power over a longer period of time. The net result may be the same amount of energy used. Theoretically this means no power saving. Either way the counterbalance is a good idea.

The solenoid may be less prone to dust but not necessarily. That depends on the part. A reasonably sealed gearhead motor will give you precise control over the position of the arm and prevent it from blowing around. But depending on what you are trying to make the natural motion of solenoid drive may be appealing since it is like a human giving something a shove and then the system dynamics take over. IE it would be less rigid and more flowing.

Im not sure how you plan to couple the solenoids. One thing to think about is how the arm will react to the wind. IE if your solenoids are not energized and the wind kicks up will it blow the arm around? You may need a way to lock it into position if this is an issue.

Using a solenoid to throw a stop (put something in the path of the arm) into the arc will work to stop it from passing through the arc under your control.
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Re: Need advice from a mechanical engineer re solenids/linka

Postby Major Krash » Fri Feb 17, 2012 5:21 pm

the cam idea has merit, if the motion will be the same each time (otherwise you will have to switch out cams for each different motion, or somehow transfer to each cam which can be tricky depending on the forces involved). The trick is to determine the beginning and ending positions for each part of the stroke, then determine the "push-rod" change in distance at each position, then connect the dots (in a circular way). You can get tricky by changing the profile and speed up or slow down the motion. And a fair amount of power can be directly transferred from the cam (depending on it's size and material - several sheets of plywood bolted together 2-3 feet in diameter would work and give you a good mechanical advantage for your motor with a small drive pulley)

edit; it is also possible with a cam to have different motions, if the different motions happen the same each time around. For instance, divide the cam into 4 parts if you have 4 different motions - each motion is cut into it's part of the cam. Off course, you cannot change the order, and generally cams only work in one direction....
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Re: Need advice from a mechanical engineer re solenids/linka

Postby EspressoDude » Fri Feb 17, 2012 7:19 pm

solenoids draw as much as 10 times their rated power/current when the plunger or moving part is not in it's ultimate destination. This end position completes the designed magnetic path which lowers the current draw. I have seen solenoid valves burn out when operated at pressures higher than their rating, because the valve won't open allowing the plunger to move. I have also seen motor starters and contactors burn out because of dirt getting into the moving parts and preventing the designed magnetic path to be completed. If the motion cannot be completed in a few seconds, solenoids are likely a bad choice.
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Re: Need advice from a mechanical engineer re solenids/linka

Postby Elliot » Fri Feb 17, 2012 7:21 pm

Screw jack?
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Re: Need advice from a mechanical engineer re solenids/linka

Postby EspressoDude » Sat Feb 18, 2012 10:34 am

Elliot wrote:Screw jack?


I didn't think Jack was your flavor Elliot....

screw jacks / linear actuators will handle lots of load but some are very slow. They are a good choice because they are motorized and are often available used, for the old style satellite TV antennas.
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Re: Need advice from a mechanical engineer re solenids/linka

Postby Elliot » Sat Feb 18, 2012 12:33 pm

EspressoDude wrote:
Elliot wrote:Screw jack?


I didn't think Jack was your flavor Elliot....

:lol: :lol: :lol: You are correct. Strictly Jill for me! :!:
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Re: Need advice from a mechanical engineer re solenids/linka

Postby ezdolphin » Sun Feb 19, 2012 2:12 pm

A cordless drill may make a good off the shelf actuator. They are pretty efficient and energy dense. Plus you can screw jack and/or jill with it.

Replace the trigger and reversing switch with your drive signals and relay.
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Re: Need advice from a mechanical engineer re solenids/linka

Postby EspressoDude » Sun Feb 19, 2012 3:00 pm

wheel chair motors are very good for this also. They usually have good mounting surfaces, lots of torque. Usually 400 - 700 watt 24vdc motor works will on 12v. Often avail at wheelchair service centers. If one on a chair goes bad, both are replaced, so there is one on the shelf 'used'. Only possible issue, most newer ones have 17mm? shafts and finding adapters can be a challenge.
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Re: Need advice from a mechanical engineer re solenids/linka

Postby Tiahaar » Fri Feb 24, 2012 12:29 am

If you couple the solenoids to the swing arm through a spring with a shock absorber/damper (to control oscillations and prevent damage to the swing arm or person blocking its movement heh) you can set it up so the solenoids fully latch without waiting for the swing arm to move...should work nicely.
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Re: Need advice from a mechanical engineer re solenids/linka

Postby EspressoDude » Fri Feb 24, 2012 7:27 am

coil compliance coordinator
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