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Unread 05-02-2008, 03:32
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Limit Sensors for Lift

We just got our lift moving today, and it seems to work very nicely. The only issue we have is that if we keep the lift motors on during extension, we will eventually break our cable, so we need some limit switches of some kind.

Microswitches may or may not work, because the arm will not stop immediately after hitting the switch and may break the switch. We also thought about a quadrature encoder, but that's not absolute, so we'd have to calibrate it to a known position every time the robot is turned on.

What do other teams use to limit the movement of a lift?
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Unread 05-02-2008, 03:53
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Re: Limit Sensors for Lift

Ive been thinking limit switches for our earlier design, which seems to be scrapped now. Second idea, you said you have a cable, I assume a motor? Ive been toying with the idea of putting something on the shaft that would trigger the gear tooth sensor, extra gear, custom thing, etc. and using that to be able to set position, since you know the starting height, and then its just a matter of measuring how high its lifted for a certain period of use. Timers, maybe? so that it always runs the same amount of time?
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Unread 05-02-2008, 07:22
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Re: Limit Sensors for Lift

Limit switches do not have to live in the path. You can place them next to a moving object so that a piece of the your moving part, swipes across the actuator. You can also add something to your cable (like a washer or crimp on sleeve) so that when the cable gets to a particular point, the switch is pressed.
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Unread 05-02-2008, 10:55
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Re: Limit Sensors for Lift

We have used encoders in the past to find where our arm in, for a number of reasons, and have had great success. We attach a second rope to the arm, and as it goes up it pulls the rope around the encoder. The way that we have reset the encoder is with a magnetic proximity switch. When the are comes all of the way down, it pulls up the switch and triggers the reset command in the programming.

hopes that helps

Anthony
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Unread 05-02-2008, 12:15
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Re: Limit Sensors for Lift

You can place limit switches at the extents of your life travel (i.e. top and bottom), but make sure you pair them with a hard stop that will prevent switch damage.

Also, since the lift motor won't stop exactly when you hit your limit switch, it's always a good idea to build in a mechanical fuse/damper somewhere along the lift chain. A simple solution would be to terminate the lift chain on something compliant, like a spring, so that if the motor keeps rotating a short time after you've hit your limit, the spring will stretch instead trashing the motor or bending something on the frame.
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Unread 05-02-2008, 13:06
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Re: Limit Sensors for Lift

If your lift motor turns less than ten turns to go from
one end of the lift to the other, try a ten turn pot on
the shaft. It gives you a lot of position flexibility in
the software.
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Unread 05-02-2008, 23:23
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Re: Limit Sensors for Lift

I don't know where the "the arm doesn't stop when the switch is pressed" comes from, but we plan to use the "microswitch" limit switches so the trigger runs parallel to the switch so it doesn't push the switch off, not on the top of the trigger. We are using speed controllers even though it's only going full on for the braking feature.
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Unread 05-02-2008, 23:47
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Re: Limit Sensors for Lift

i would suggest that you use a combination of hard stops top and bottom.
and some type of sensor.

You might try a string potentiometer. This is what we are using on our lift this year. They are also called Cable position transducers.
They are wonderfully adaptable sensors. They have been used in the aviation industry for a number of years. (We are Aviation High School after all...)

Simply described, a string potentiometer is a spring-loaded potentiometer that has a cable attached that will pull out and provide a position based on how much has been pulled out. It can work quite well on a straight line lift and can be used in many other situations too.

Please take a look at the URL below to find out about them:

http://zone.ni.com/devzone/cda/tut/p/id/4185

Take a look at this site...

You can use either digital or analog versions and get pretty good reproducibility...,,

A good lift requires a good system of control.

Encoders can also work well but are a bit harder to program...
You would have to use quadrature encoders to figure out whether you were going up or down....

Single encoders will get you rotational speed.

Good luck!!!
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Last edited by Bob Steele : 05-02-2008 at 23:51.
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Unread 06-02-2008, 00:55
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Re: Limit Sensors for Lift

In my personal opinion your best bet would be use a MA3 absolute analog encoder. We had and elevator arm last year at St Louis and thats what we used

You can purchase them here if you would line one....
MA3 ENCODER
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