Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in

Photoelectric Sensor (or similar) for measuring velocity of moving object

Status
Not open for further replies.
sirbrandon

sirbrandon

Mechanical
Jan 4, 2009
19
0
0
SG
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 travelling along the shaft.

I'm trying to figure out if I can use a pair of photoelectric sensors mounted at a pre-determined distance apart, and each will be "tripped" as the object moves past them. The time takaen will be recorded, and the velocity calculated accordingly. Can anyone enlighten me if this setup is possible?

Another option I thought is to use a distance sensing sensor, but I'm not sure how to go about doing it.

Can anyone help?
 
Replies continue below

Recommended for you

Sort by date Sort by votes
The speed is < 2 m/s. Accuracy 5% for a start?

It's not so much as whether the sensors can do the job, but reading through the catalog, there does not appear to be a ready-made off the shelf system. From my research, some programming or logic control is required. I'm totally new to this PLC stuffs, so I'm really starting from ground zero.
 
Upvote 0 Downvote
Upvote 0 Downvote
Short of some Off The Shelf speed meter which I image is available, a PLC would be a typical go-to solution.

There are also a lot of radar speed thingies out there where you you just point them at the oncoming product. The product probably needs a weee bit of metal in or on it.



Keith Cress
kcress -
 
Upvote 0 Downvote
As long as the time period between sensors is something reasonable (2 m/s speed should be easy) it's really easy to use the first sensor to initiate a timer in the PLC logic and the second one to end the timer. The resolution, and therefore the accuracy of the result, will be dependent on the scan time for the program logic, but in this day and age, that's usually pretty small. Simple logic like this will get confused if a second object comes down the track before the first one has passed the second sensor. If the object has an irregular shape and is tumbling or spinning rather than sliding straight down the tracks, it isn't going to work well, either.
 
Upvote 0 Downvote
Thanks for the replies.

I've met some vendors peddling their laser distance measuring equipment. Nice, but comes with a nicer price tag too. I thought I can do with something that I Can cobbled together.

Hey itsmoked and BrianPetersen, I'm not an automation engineer by training, but would you be able to share some references or keywords for me to look up on? I've been toying with the speed gate concept, but I can't find anything conclusive.
 
Upvote 0 Downvote
I would go to Automation Direct and peruse their sensors.
AD photo sensors

You probably can not beat the value of most these sensors. The 8mm ones are crazy cheap and utterly waterproof and egg proof(don't ask me how I know).
Back out a page if you want to see the rest of their sensors like inductive and capacitive.

While I haven't actually looked you can probably use one of their timers
AD timers
that you could calibrate to spit out a speed value garnered from a time period.


Keith Cress
kcress -
 
Upvote 0 Downvote
Thanks again guys.

Baby steps here for me... itsmoked, so these sensors will then go to some kind of a timer? Direct or via some kind of controller?
 
Upvote 0 Downvote
Those sensors just light up a LED and change their signal output when they "see" something. That's all they do.

You want the speed? You need to 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)

Find an off-the-shelf timer that 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 ans shows the time period that occurred.

Divide that number by the physical distance separating the two sensors and you have the speed.

There are hundreds pf timers out there with probably half them able to do that function. Look at Factorymation, Automationdirect, and Newark.

Keith Cress
kcress -
 
Upvote 0 Downvote
Yes, something like that! I read the confusing manual and saw:

"Count Start with INP A edge, count Stop with
INP B edge (InA.Inb)"
Click to expand...

That is the type function you're looking for. If you were to place the sensors 1 meter apart and the timer allows 1/x to display the number shown would be meters/second for the speed. Seems like that timer could do that but you'd have to dig out which of the four model they mash-up in the user's manual covered all that.

If the item moves 1 meter in a 1/2 second then 1/(1/2) = 2m/s


And those P&K optical sensors would work too. Those are nice. Maybe a little pricey but nice.

What item are you measuring the speed of?

Keith Cress
kcress -
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

JayWell
  • Locked
  • Question
Career path question for Automation Industry
Replies
1
Views
42
IRstuff
IRstuff
BenDavis007
  • Locked
  • Question
How do you feel about the engineering tools for data collection and analysis in manufacturing
Replies
6
Views
49
BenDavis007
BenDavis007
orfefx
  • Locked
  • Question
Market viability of 3D displacement sensor
Replies
5
Views
6
GregLocock
GregLocock
Fossy99
  • Locked
  • Question
Journal for a brief paper
Replies
2
Views
53
Fossy99
Fossy99
Ivan Silva
  • Locked
  • Question
Good references for automation machines and mechanisms
Replies
1
Views
36
FACS
FACS
Back
Top