Idec plc too slow

2stroke

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Sep 2010
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I have a idec pentra fc5a 24volt and I am trying to use the frequency option to count pulses on a three toothed rotor from a snowmobile engine, then convert to a 0-10 volt signal to send to a vfd which is running a water pump. I need the water pump to increase and decrease in speed in relation to the snowmobile engine. The pump is running into a water brake dyno. Right now there is about a one second lag in the system. I have gone through all the wiring and tried different programming but no use. So I phoned idec tech and they said there is a one second calculation period for this option. Why would they make according to them, the worlds fastest micro plc and then slow it down. Does anyone know of a way to get around this or am I looking for another plc.
 
Where is the 1 second delay? In the "frequency function"? or in the pid?

I use the fc5a all the time for pid loop functions, water presssure control mostly.... but i don't use the built in pid function, I write my own. It's not that hard and it lets me customize.

I am not that familiar with the "frequency function" but I would imagine you could use the "high speed counter", and count for less time, less than a second that is.

Are the pulses being registered correctly?
 
Yes I can use the counter and send it to a data register before it resets but there has to be an amount of time to sample some counts and that time will be the lag. I am just not that impressed with the Idec, it's like taking a motor with lots of horse power and then putting it in a heavy chassis, still not fast. Can anyone recommend something better, it would be much appreciated.
 
It wouldn't matter if you had the fastest PLC on the planet, it would still need time to take a reading. The problem isn't the PLC, it is your application of the PLC. The delay between a change in a system variable and a controller registering that change is called latency, all system have latency, you cannot get rid of latency, but you can often reduce it to an acceptable level.

Frequency measures how many pulses received in a second (hence your 1 second delay). On your system you may be able to reduce latency by measuring the time between pulses (or group of pulses) rather than frequency. I haven't used IDEC and so can't comment on how you achieve this.

Bryan
 
Thanks for clearing that up, makes sense. I got in contact with a company and they won't use plc's as they are too slow. He is going to build me something that should work.
 
The frequency to analogue converter has the same problem as the PLC it counts the number of pulses over a second. So you end up with the same delay, at least the one I choose to try to solve a similar problem did!!!

Bryan
 
How about a dc generator and somehow limit the current, then I could just gear it accordingly to give the 0-10v however I want it.
 
Also known as a tacho generator, if you Google you will find loads of information. That was how the speedometer in your car used to work, and the engine RPM speed, now it is probably all digital.

Out of interest, when you were measuring frequency what was the maximum values you were getting?

Bryan
 
The flywheel had three targets on it and most engines rev 8500rpm max so 425hz. Here is some pics of the snowmobile engine I build and my homemade engine dynamometer.

TWO LITRE.jpg 2000CC.jpg IM000006.JPG IM000001b.JPG
 
You are in an awkward band with that frequency. It is too high to be able to time between pulses because your timer has a 1 millisecond resolution and only between 2 and 3 milliseconds per pulse (1000/425). And you don't have enough pulses that you can use a 10ms time based interrupt because you would only capture 4 or five pulses per sample. If you could find another place to measure the speed where you can get 100 pulses in 10ms or 10000HZ (10kHZ) you could use you existing PLC. Then 100 pulses would be 100% of speed, 0 pulses would be 0% of speed. You would have a latency of 10ms (1/100 of a second) and about a 1% accuracy at full speed.

edit: looking at your pictures there has to be somewhere you could pick up, or make more pulses, maybe attach an encoder to the end of the water brake dyno.

Bryan
 
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Thanks,that may be hard to do because of the physical size needed for that many targets. The reason for three targets now is because my prox switch wasn't fast enough to pick up the 86 teeth off of the ring gear. I am definately going to mention that to my programmer, who should already know that, Thanks again.
 
You could make your own coil directed towards the gear with a cap. The gear would product spikes of power and the cap would sum it. Taking a differential voltage over the cap and some other simple conditioning circuits could give you your 0-10V that you were after.

Don't always think digital!
 
Tap off the points on distributor. Assuming two stroke then pulse rate per minute X 3 should equal RPM???? Think I got math right. Granted you have now gone to the opposite extreme.

On second thought divide by 3 - now you are back to where you started. Oh well it sounded good until I thought on it for a second or two (four ??).

Dan Bentler
 
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I am definately going to mention that to my programmer, who should already know that, Thanks again.

I noticed no one has mentioned using a VFD that uses a pulse train input as a speed reference. A quick scan of the Lenze SMV manual indicates that a pulse train input is not available on that model. By using a pulse train input directly to a VFD a PLC is not required for the speed reference.
 

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