Using a proximity switch to detect speed

[Brijm]

Beej,

Sounds like you have a slow changing system, that you need to compensate for deviation. Depending on how important accuracy is (or if the speed only needs to be in a certain range) I think your prox will work. At these slow speeds, with slow devieation. You might be able to calculate the number of pulses over say 30 seconds, to determine your speed. (The longer your average, the more accurate it will be, but the slower the control) Compare this to your setpoint, and either increase or decrease your reference accordingly. I recenlty did something similar on a fan system. I didn't want to control for fast fluctuations, but needed to increase the fan speed, as the filter gets dirty to maintain a set flow. It's a simple / slow control, that gets the job done.

The only thing that I'm not sure about is if this is a normal reaction of the process, or a component failure. If it's a normal reaction, then it is controlable. If you are simply covering up symptoms, this ussually comes back to bite you.

brian

 

[rsdoran]

You do realize this is a 4 year old topic?

 

[Jimmie_Ohio]

You do realize this is a 4 year old topic?

The original poster did say that his motor shaft was turning slow... Maybe not at .25 RPY (revolutions per year)

 

[10BaseT.]

Hi,

First time posting a question so bear with my ignorance.

I'm trying to figure out an easy way to use the pulsed output of a proximity switch to provide 2 basic functions:

1) A zero speed switch which will shut down the system if the proximity switch does not pulse at least once in 4 seconds. (OK, I think I know how to do this. ..The second one is the harder one..)

2) Measure the period between the leading edge of the pulses from the proximity switch. Details: Reading off of a 48 tooth sprocket at between 0.40 to 1.50 RPM. Very slow system.

I would like to use the calculated speed to vary the output of a variable speed drive to increase the rated speed regulation to a more acceptable level. (Approximately 0.25%) The VSD is controlled by a 4-20mA signal from a PLC. The PLC will integrate the prox speed feedback to slightly alter the 4-20mA signal from the PLC to match the commanded speed more accurately.

The VSD is a Mitsubishi FR-S510-0.4K which has a rated speed accuracy of 1% of full speed. Full speed at 75Hz is 1.5RPM. Lowest speed is about 20Hz (0.40RPM).

Any ideas?

Thanks.

Nothing wrong at all - I use a little block I wrote in S7 where the drive enable signal starts to back to back timers (after a user specified ramp delay) one TOD , one TOF , if either one time out , then a fault is generated -
Just using the fault signal from the VFD is sometimes not enough when you can tear the heart out of equipment if it jams - though you can compare SP with PV as well -

Is this really an old topic that has resurrected itself ?

" have a great weekend " - I can only conclude that the OP went on one hell of a bender ...

 

[rsdoran]

Is this really an old topic that has resurrected itself ?

Above names is the date and time each post is made, the original here was posted Jan. 16, 2003 at 3:15 PM.

What is said is I remember this post and a few others made long ago but have problems remembering what I did 5 minutes ago or What were we talking about?

 

[renatus]

Sorry guys am a student from UDSM, am seeking for technical assistance about detecting the speed of motor. am using proximity switch with PLC, what else should I need? and how this is done? please help me in that, I need Ideas so that I can work out. Thank you for your concern.

 

[Mickey]

Sorry guys am a student from UDSM, am seeking for technical assistance about detecting the speed of motor. am using proximity switch with PLC, what else should I need? and how this is done? please help me in that, I need Ideas so that I can work out. Thank you for your concern.

see link below.

http://www.mod-tronic.com/Dynalco_Speed_Switches.html

 

[acbalan]

Sorry guys am a student from UDSM, am seeking for technical assistance about detecting the speed of motor. am using proximity switch with PLC, what else should I need? and how this is done? please help me in that, I need Ideas so that I can work out. Thank you for your concern.

can use direct input to calculate speed in PLC ; or you can use some like this http://www.pepperl-fuchs.us/usa/en/classid_1830.htm?view=productdetails&prodid=28900