Sampling Frequency Problem

Hello

I am having a problem with OB35 I am calling OB35 with 10Hz frequency each 10ms and sometimes PLC gives SF. What is the real limit for OB35?

Also I have a bigger problem. I have to produce a 6Hz sine wave with 315-2dp but as far as calculated by Nyquist criterion I need at least 12 Hz for sampling but I cant even have 10Hz with OB35. Have anyone seen this kind of problem before? Is there any way to overcome this problem. Thanks.

Use the diagnostic buffer to show you what SF is occurring. 10Hz = 100ms period yet you mention 10ms - is this a typo

Sorry I corrected it. I wrote it wrong

Hello

I am having a problem with OB35 I am calling OB35 with 100Hz frequency each 10ms and sometimes PLC gives SF.
What is the real limit for OB35?

Also I have a bigger problem.
I have to produce a 6Hz sine wave with 315-2dp but I cant manage to create a proper sinewave with 100Hz. I only have 16 samples per period which is not enough.
Have anyone seen this kind of problem before?
Is there any way to overcome this problem. Thanks.

erdemsvri said:

I have to produce a 6Hz sine wave

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Why? To do what?

with 315-2dp but I cant manage to create a proper sinewave with 100Hz. I only have 16 samples per period which is not enough.
Have anyone seen this kind of problem before?

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Yes, all the time.

Is there any way to overcome this problem. Thanks.

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Not with a PLC. Buy something that can sample every millisecond or half millisecond.

Right now we are working on a test machine which we need 6Hz sinewave and triangle wave for input to a propotional valve.

Just before I read something that tells me I can call cyclic interrupt each 1ms but doesnt tell me how to call.

Is that true?

Use something more appropriate.
http://www.intersil.com/data/an/an013.pdf

for example.

Thanks for your attention but I need low frequency sine oscilator. I can produce upto 0.5Hz or something like that what I need is to use cylic interrupt faster so I can reach upto 6 Hz.

The chip you linked me is 20Hz-20kHz It is too fast for me

How about post #2 - have you examined the diagnostic buffer contents ?

You may struggle to get the job done with a 315-2dp it really depends on how much else is going on in the processor. If you call a new interrupt before the last one has completed then it will cause and error. As previously suggested - check the diagnostic buffer.

The other problem you may face is analogue convertion time (assuming that you use an analogue output). Vipa (www.vipa.de) do processors that are interoperable with S7 but much faster - they also have a high speed interface bus and a range of high speed cards; they tell me it's fast enough to modulate an MP3 file via an analogue output but I haven't tried it.

Nick

I know something about hydraulic test machines.

erdemsvri said:

Right now we are working on a test machine which we need 6Hz sinewave and triangle wave for input to a propotional valve.

Click to expand...

You won't be able to do this easily if at all on a PLC. There are hydraulic motion controllers that are designed to be the main control of a test system and generate sine waves for position or force or any other wave form you may want. The sine wave generator can do linear or logarithmic sweeps. Frequencies, amplitudes and offsets can be changed smoothly on-the-fly.

http://www.deltamotion.com/applications/index.php#
http://www.deltamotion.com/products/motion/rmc70/index.php

These controllers have 64MB on ram on board so you can generate any test pattern you wish and record up to 16 channels of information at the control loop rate. In addition the 100 MB Ethernet will allow you to stream the data back to a PC. There is a VI to interface directly with National Instruments Labview.

Warning, if you are making a hydraulic test stand you should know that more often than not the hydraulics are not designed well enough to do what is required. Even with the best systems your controller must be able to generate feed forwards to compensate for the lags in response of the system.

L D[AR2 said:

How about post #2 - have you examined the diagnostic buffer contents ?

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I dont have the 315-2dp Cpu right now. I am working with 313C Cpu?Will this have an effect? and if it affect how much?I havent tried at 315 but I made lots of trial with 313C always gives SF.

I am not at the company right now so I cant check it but tomorow first thing i will try again and write the diagnostic buffer. and if 315 cpu arrives tomorow I will give her a shot too.

But the real problem as I mentioned even if I achive to work with ob35 by 100Hz it is not enough for the sine wave so I am thinking if I can create sine wave by PWM with 313C cpu which has PWM output. Do anyone have idea about this?

Thanks very much for your interest and help pals

Manglemender said:

The other problem you may face is analogue convertion time (assuming that you use an analogue output). Vipa (www.vipa.de) do processors that are interoperable with S7 but much faster - they also have a high speed interface bus and a range of high speed cards; they tell me it's fast enough to modulate an MP3 file via an analogue output but I haven't tried it.

Nick

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Yes we do have an analogue output.It will cause us problem too

The other problem is I have to apply PID control to the sine wave after I created it so I should use the very same ob35 and it is going to be very hard for PLC to handle.

Maybe we can check VIPA but I think the customer wont accept it.

Module MP3 file int the PLC output?Interesting...

Peter Nachtwey said:

You won't be able to do this easily if at all on a PLC.

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You are right

Peter Nachtwey said:

There are hydraulic motion controllers that are designed to be the main control of a test system and generate sine waves for position or force or any other wave form you may want. The sine wave generator can do linear or logarithmic sweeps. Frequencies, amplitudes and offsets can be changed smoothly on-the-fly.

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I checked the link you sent. This motion controlers are the real answer for us but this company doesnt have a distributer in Turkey . I will check out the other companies in Turkey for this controller do some research about them.

Peter Nachtwey said:

Warning, if you are making a hydraulic test stand you should know that more often than not the hydraulics are not designed well enough to do what is required. Even with the best systems your controller must be able to generate feed forwards to compensate for the lags in response of the system.

Click to expand...

We designed a feed forward PID pattern for lag compensation.

The most interesting part is if I dont get online in OB35 CPU works without a problem but when I tried to be online in OB35 I get SF error.I can get online in OB1 or other functions no problem but not with OB35.

I am posting the Diagnostic Buffer of the PLC. Thanks very much for your interest.

erdemsvri said:

This motion controlers are the real answer for us but this company doesnt have a distributer in Turkey . I will check out the other companies in Turkey for this controller do some research about them.

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Delta Motion Distributor in Turkey

I have not had the need to use any yet but i would suggest you wont go far wrong in using one of peters delta products if it is chosen correctly.

And note his warning about hydraulics.

Thanks for the link and your advice.

I will contact the delta distributer in Turkey and ask them if they have experience about this subject.

Thanks.