moeller easy plc and proxy freq connection 3

[1hagar]

I need to connect a proxy reading freq. between 2000 and 5000hz which is on a diesel loco engine fly wheel. all this does is it converted to rpm but I need to shut dowm the engine when it over revs.I have a moeller 512 rc dc plc.(12vdc unit)
will i need to convert the proxy signal to an 4-20 ma signal or a voltage signal?I have never done this before and need to know what to do and buy.Someone said i could use a high speed counter if the plc is fitted with it.How do i do this?

 

[Skogsgurra]

Do you really get between 2000 and 5000 Hz from a proximity switch on a flywheel?

The speed of the engine is usually in the 2000 - 5000 RPM range and that means that your frequency is more likely to be in the 33 - 83 Hz range. There is a fair chance to count that low frequency in the PLC program itself. Some PLCs have count instructions that makes the task even simpler.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[niallnz]

Yes you will have to convert your frequency signal to a voltage as the Easy PLC doesn't have a high speed counter input. Your frequency range is correct as typically the engine fly wheel will have about 155 teeth, which at 1500rpm is 3875Hz.

Cheers Niallnz

 

[niallnz]

The Easy 800 PLC does have high speed counter inputs so you could interface your prox probe directly if you upgrade from the Easy 500 that you currently have.

 

[Skogsgurra]

OK, I see. Didn't think of the teeth. Sorry.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[1hagar]

Thanks guys very helpfull. sorry did not tell about the teeth.I work out it was just over 3 hz per rev.full speed is +- 1360rpm and idle 600. Not at work now but next week will do tests.I want to use a stand alone plc so I will go for the 800 I must just check if it is a 24 dc unit as Loco has a 72 to24 converter in.

 

[controlsdude]

if your not using high speed counter just look at a regular input see what the change of state is. Then look at your scan time see if it is 2-3x the prox input, then you should be ok and not have to use a high speed counter.

so how do you determine this above?
guide to go by in calculating this is

calculate the diameter of the flywheel
calculate circumference
calculate inches/pulse = circumference/dia

convert to your range of speeds you are testing for
then do a table on inches/milliseconds to MPH in same table
What is the known scan time? 10 milliseconds for example
calculate
inches/process scan = (inches/millisecond)*scantime

now compare this to your calculation on inches/pulse

inches/process scan < inches/pulse

if true then your plc will be able to handle that speed.

remember if you keep adding more stuff in your program this will change your scan time if the scan is continuous. Thats why people invented high speed counters, event driven scans, periodic scans, etc... as stated above