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Photoelectric Sensor or Similar for Measuring Velocity of Moving Object
I am looking to build a system for measuring the velocity of an opaque object sliding up and down a chute, sort of like an elevator traveling along the shaft.

I am looking to build a system for measuring the velocity of an opaque object sliding up and down a chute, sort of like an elevator traveling along the shaft.

I'm trying to figure out if I can use a pair of photoelectric sensors mounted at a predetermined distance apart, and each will be "tripped" as the object moves past them. The time taken will be recorded, and the velocity calculated accordingly.

Another option I thought is to use a distance sensing sensor.

Can anyone help?

Sure you can calculate velocity by measuring the elapsed time over a known distance between the optical sensors.

There are commercial laser distance sensors that continuously update distance via an analog output. That's not a velocity value directly, but can be calculated if the sampling is at a known time interval.

https://www.ifm.com/ifmus/web/o1dl1.htm
https://cdn.automationdirect.com/static/specs/perectopttransit.pdf

Velocimeters seem to be Doppler units:
http://www.photonicsonline.com/doc/laser-doppler-velocity-sensor-0001

Photoelectric sensors just light up a LED and change their signal output when they "see" something. That's all they do.

https://www.pepperl-fuchs.us/usa/en/classid_47.htm?view=productdetails&prodid=43225

You want the speed? You must time the period between something being "seen" by the first one and subsequently being seen by the second one. The distance between the two positions and the time it takes the item to translate between the two is the definition of velocity.

v = (t1-t0)/(d0-d1)

t = time
d = position
v = velocity or speed(directionless)

http://www.clrwtr.com/PDF/Kubler/Kubler-523-524-52U-Timers.pdf

Above timer has three inputs. Reset, start, and stop. When the start input transitions the timer starts counting time. When the stop input transitions it stops timing and shows the time that occurred.

From your description is sounds like all you need is the velocity at one particular point in the travel. If that is the case, a pair of photo sensors is fine. But for good precision, each photo sensor should really be as photo interrupter. That is, a photo sensor paired with a photo emitter, forming a nice sharp point at which the light is blocked. The narrower the beam, the better defined the point of transition will be.

-Robert Scott
Real-Time Specialties
Hopkins, MN