Measuring Linear Displacement


Thread Starter


I have to determine the best type of sensor to use to find the time interval in which a metal rod drops about about 1 foot. The sensor can't have any physical contact to the free falling object. The sensor can't be directly above or below the object

I've looked at maybe putting a series of Photo Reflective Sensors on top of each other. These would be located to the side of the falling metal rod.

I have also looked into Laser Interferometer but found my knowledge is very limited to how these work. I haven't been able to find much useful information on the internet about them yet.

I wondering if any one had any information or resources i could look at for more in-depth look at a variety sensor. Or if any one has suggestions on type of sensors i should be looking at.

If anything is unclear, please ask!
Is it falling from rest? If so, I'm going to hazard a guess that the time for a dense small-footprint object like a metal rod to fall one foot will be 0.25 second. d=0.5(at^2) and all that jazz.

Steve Myres
Automation Solutions
If falling NOT from rest, how about two light curtains or ring sensors one foot apart?


Steve Myres
Automation Solutions
You can look at using two fiber optic type or photoelectric type photo sensors. If your mounting will allow it, use thru beam type of sensors since they [usually] don't depend much on the surface of your part you are detecting. I prefer the Keyence products for this because they are the best. I would recommend the units with the separate dual digital displays so you can set the threshold easily and tell if it is drifting due to dust, etc. Your local Keyence guy should help you out, they are usually willing to drive out to you and bring some hardware to test (at least in my area).

How big is the rod, and is it constrained (i.e. like a piston) or is it allowed to move around a lot (like a free falling rod in air with no guidance)? You want the detection object to be significantly larger than the range of motion that your rod is going to have so that it doesn't miss the beam path. Sometimes you can add a large flag to your sensed object to get around this, or use a wide beam device (this has limits).

Perhaps a more bullet proof solution if you have a very well constrained device (like a piston in a tight fitting bore) is to use one or more inductive proximity switches. These are not that effected by oil, dust, or surface finish and are also rugged and economical. Telemecanique (squareD now), turck, OMRON, and Automation Direct have proximity products. The reason these have to be well constrained is that the sensing distance is about 1-4mm and varies with material and shape of target and surroundings. If you go this way you will have to experiment.