Siemens FM450-1 Speed / Frequency Measurement

Hi, I have an application where I need to measure the speed of a rotating roll on a Siemens S7-400 system. I have an incremental encoder connected to that roll and this is connected to a FM450-1 counter module. I assumed that this module should be able to give me a speed (or frequency feedback). But I have found nothing that shows me how this is possible.

Of course the workaround would be to have a software timer and measure the speed based on a deltaCount/deltaT. But that would be quite inaccurate at low speeds and fast sample times. Any other ideas?

Hi there
Yes 450-1 will give the counts of the incremental encoder which you can then turn into your engineering unit.
450-1 module must have come with CD. Install the setup program in this CD, then you will be able to access the properties of 450-1 module in hardware configuration. Also this CD has the manual which gives the Sample program for counter card + how to wire the module.

My problem is that the frequency measurement would be inaccurate at low speeds. At a sample rate of 20ms and a speed of 10rpm, an encoder of 1024ppr (4096 edges) would give about 13.5 counts per sample (i.e. 13 counts in one sample and 14 in the next). This means that the speed measurement would fluctuate from 9.5rpm to 10.3rpm (10% variation) even at dead steady speed. This kind of variation is unacceptable and would get much worse if I try to reduce the sample time. Normally speed measurement requires a base clock that clocks the time between edges rather than count the number of edges per sample time. That is elementary. I don't seem to see any mention of this in any of the S7-400 pulse boards. I can't imagine Siemens does not have a accurate speed measurement board. I must be missing something.

I have a line that uses a peperl+fuchs frequency/current converter.
An inductive proximity picks up the rpm and the converter gives a 4-20mA output into the plc. I am measuring up to 2000rpm and the timing disc has 8 slots on - so it's pretty quick.
I do not know how accurate it is at slow speeds, but may be worth a look.http://www.pepperl-fuchs.co.uk/cps/..._list.kly?sterm=&tvpv=pa&listuserliststart=1#

Is it an option to use the FM350-1? (This has a rpm measurement mode) I think it's stupid this option is not available on the FM450-1...

http://support.automation.siemens.c...tandard&viewreg=WW&objid=28421837&treeLang=en

Or see this link:

https://www.automation.siemens.com/...ow.aspx?PageIndex=1&PostID=116233&Language=en

You should also check the full range of youre application. If always on low speeds it may be advisible to change the number of pulses per revolution. (Changing position of encoder from axel to motor? if possible with slip etc...)

Hope this helps...

Hi Werner, Thanks a ton. This was exactly what I was looking for.

It is sad that FM450-1 does not have this function. As always, assuming big is better, I had taken such a function for granted in the FM450-1 and procured it without even comparing it to FM350-1.I guess it is wise for me to switch to FM350-1.

Thanks Again.