Hello everybody.

I am an engineer on Acoustics in Mexico, and I am not very related to Automation and Control. I work as a consultant but now I have a Customer who wants to use a microphone with his PLC. Microphones we use are ICP (or IEPE) type.

The only info I could get is that microphone must provide 4-20mA, BUT the response of our microphones is in mV!!

I need your advice and support, can we use this kind of microphone?

Any comment is welcomed.

Thanks and regards.
Norma B.

 

I am having a hard time imagining what your customer intends to accomplish by connecting a microphone directly to a PLC. I'm not aware of any PLC which has the necessary hardware to sample process audio signals. They just don't have the acquisition hardware, memory buffers, or processing power to do anything with an audio signal.

Either your customer doesn't know what they want, or else their requirements have not been adequately communicated to you. I imagine that they need to connect the microphone to be connected to a system that can actually do something with the audio signal and then provide some sort of test result to the PLC. This will be much more complicated and expensive than hooking a microphone into an analogue input.

 

What does he want it to do with the PLC? Noise measurement, voice control? I would expect that you will need some sort of an intermediary to turn the audio into a representative quantity to pass to the PLC. Bear in mind that the analog inputs to a PLC have almost no bandwidth as they are sampled typically at 100Hz at best. This according to Nyquist means you could deal with frequencies up to 50 Hz. And that's fairly optimistic.

Regards
cww

 

I should mention that the PLC I am working on would handle this. It includes a DSP.

Regards
cww

 

> I have a Customer who wants to use a microphone with his PLC.

The task (I think it connected with sound spectrum analysis) could be better solved with so called softPLC and LabVIEW (e.g. have a look at Avalue's products - industrial PC ~$500, IP 65, fanless etc. (or google "IPC")). (Really I think old fashion PLCs have no abilities to fulfill the task)

I think you need no 4-20ma output in the microphone. Just use the microphone input. Or you can use an additional preamplifier for microphone and use the line input.

Best wishes, Vladimir E. Zyubin

 

Another possibility if you need to do spectrum analysis is to use one of the very inexpensive Digital Storage Oscilloscopes to do the FFT and parametrics and pass the results to the PLC or PC host. Many can test to standards and compare with a stored set. That's a lot of capability for $400. I've used one for vibration analysis for a tiny fraction of the cost of dedicated systems. Google Rigol or Instek.

Regards
cww Because that's the kind of stuff I do :^)

 

Hello, Thanks a lot for your kind replies.

This is going to be used for Outdoor Noise measurements.

They just need to obtain and read the noise levels.

Today I found that there is a thing called "transmitter" that can be used in order to convert the output from an ICP accelerometer to 4-20mA... ready to the PLC!
Check: http://www.documentation.emersonpro...s/data_sheets/9330_allds0508i_09330031008.pdf

Both mic and accelerometer are ICP, so I have the crazy idea that a "vibration transmitter" will work for a ICP microphone. But transmitters have their specs very related to vibration, velocity or displacement.

I would like to know if there is a "generic" transmitter, for mV input and mA loop output.

What do you think? Please let me know your comments.

Norma B.

 

There are some problems here too. If the noise levels are for safety compliance, they generally have to have a particular weighting curve. And you still have the issue of the very low bandwidth on the receiving end. And the processing would take many scans. One thing you could do is find a noise meter with a DC analog output and let it give you an overall noise level as a single weighted and averaged qty. That the PLC could handle and alarm on, etc. I'll bet it would be cheaper than the Emerson gadgets too. I looked and there were a lot of meters that would fill the bill.

Regards
cww

 

It is possible to do what you want with a 4-20mA transmitter and an IEPE microphone (ICP). If the overall noise level is what they want, you just configure the transmitter with a lowpass and highpass frequency bandwidth, and calibrate the unit with the microphone sensitivity. The transmitter will then output the DC value of the overall noise on a scale of say 0-100dbA = 4-20mA