Rotating Shaft Monitor

normdeg

Member
Join Date
Apr 2006
Location
Ripley
Posts
3
We just built a prcessing facility. We are using a Siemens S7-226, TP070 touch screen and EM241 modem.
We are using the PLC to monitor augers, fill hoppers etc...
I am in the final stages of programming the PLC but am having trouble with timers for the proxes on the flywheels of the augers.
The flywheels are turning at about 450RPM but I can't get the software to acknowledge that they are turning. The lights on the PLC flash on and off whenever the flywheels are rotating. I have setup a network on another rotating shaft which runs about 12RPM and it works fine. Is it possible that i'm having aliasing problems?
Any help is much appreciated.

-norm
 
450 RPM, how many poles to snse?
How fast is your input time? Is the Semiens variable on the input time, or is it a fixed scan type?
When I do this, I try to have 2 input checks for every lo-hi transition of a sensor.
If I can get 2:1, I know I won't miss.
 
There is one flag on each flywheel. I'm not sure if the Siemens input time is variable. What would a second flag accomplish? The PLC is seeing the flag everytime; what I'm wondering is if the flag is too small, if the PLC doesn't "see" it fora long enough period of time and because of this doesn't make a properly squared sine curve. Maybe I'm totally wrong, I'm just going out on a limb.

-norm
 
A popular application is sensing the teeth of a sprocket as it rotates, and other applications where there is more then one pole to sense.

With 450 RPM, you have 7.5 Revolutions Per Second, which means you have a signal once every .133333 seconds. That's plenty slow for any PLC to pick up the input. How big is the flag? How big is the sensor?

Can you put a bigger flag on? If I had to guess, I'd say the flag is too small.
 
The flag is a bolt with a 1/2 head, the prox is about 1/4 inch.
I could put on a bigger flag on with a simple washer. The prox should pick it up for aobut 2.4ms, is that enough for the PLC?

-norm
 
I had exactly the same issue recently on another job. The two areas to look at are frequency and "on" period. Theoretically, to effectively pick up the running frequency, you need a scan time of at least twice the frequency you are trying to measure. The second is the "on" period of your measuring tag. This is proportional to the distance covered by the tag compared to the total distance being measured. Example, You are measuing a 314mm circumferance disc and your tag is 20mm wide, you on input will only register for around 1/16 of the total time period. To accurately pick this up, your scan time would have to be 1/16 of the period of rotation.

Solutions:
If you want to monitor the speed of rotation, a high speed counter card would need to be used.
If you just want to see if it is actually rotating, you can buy rotation sensors that are wired between your prox and PLC input. These will detect the input changes and send a signal back to the PLC that it is rotating.
 
Add up the losses.
1) The prox won't be on until most of the bolthead is covering the face. Assuming the prox won't be on until 50% coverage, then calculate the bolt diameter. If it needs more than 50% coverage, then the true diameter is reduced by twice the amount.
2) Prox switching time.
3) PLC input switching time.
4) PLC scan time.

Your on time might be a lot less than 2.4 ms. A 6mm prox has 1-2mm sensing distance, and probably needs almost full coverage to see the flag. Rotation means it's seeing the flag at an angle. The only 'flat' time is when the 2 are lined up.
Try mounting the prox 90 degrees to the bolt. Sensing the flat of a hex head bolt will cure the angle issue.
If you can't do that, then try a magnet. They make magnets for Hall sensors. Years ago, I found proxes saw a magnet much better than steel. That may not be true today. Should be easy enough to test.
Alternatives are a larger diameter prox (better sensitivity) or Capacitive prox (excellent sensitivity to metal).
A reflective type sensor would allow a large flag without the mass, but needs to be clean.
 
Last edited:

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