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Sample rate of a 50hz signal

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engineer1000

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Hi
I am measuring the G force from a jack hammer using an accelerometer . This gives a voltage output proportional to the force. The Jack hammer runs at 50hz . Does this mean to collect all the info I need a sample rate of 10ms. ie 2x 50hz.
I've had a look at some data loggers but ones that go anywere near the 10ms sample rate are quite expensive. I just want to make sure I can't get away with a slower sample rate data logger
 

Perhaps you wish to catch the maximum readings? During every 1/50 sec? Or 1/10 sec? Or 1 sec? Then this might be a job for a sample-and-hold circuit.

It depends on what amount of detail you wish the data to contain (for instance, waveform as jackhammer makes single break through concrete).

Also depends on what timeframe of data you require (for instance, readings from a sharp new tool, versus a rounded worn tool).
 

Hi,

I don't think your calculation is correct.

Most probably the 50 Hz is the fundamental frequency...and most probably you are interested in the overtones.
You need to decide this.

You need to decide what max overtone frequency you need to identify. Then you shoud use at least twice this frequency.
"At least" means you should add some frequency headroom, because you need it for the anti aliasing filter to operate.
The more headroom, the simpler the filter.

Klaus
 

So the jackhammer has a cycle where the hammer accelerates out to its maximum position, then returns. That is one cycle. You are saying there are 50 cycles each second.

There are 3 scenarios here.

1. You sample at less than 50 Hz. The accelerometer will be sampled at a different point on the jackhammer's cycle each time. You will not be able to make any conclusion here, except that the G force is that much as some point on the jackhammers cycle.

2. You sample at 50Hz. If your sample rate and the jackahammer's are both exactly matched, then you will sample the acceleration at the same point each jackhammer cycle. You will be able to conclude what the G force is at only that point. But you could also determine the mean and standard deviation of that G force since you will have many samples at the same point.

3. You sample at something quite a bit greater than 50Hz, like 100 times greater. Then you will sample the accelerometer at 100 different points along the jackahmmer's cycle. You will be able to tell the point of greatest G force, and the point of least G force. You will be able to plot the G force against the displacement of the hammer in its cycle.

You probably need to do something like 3 to get any useful data. I don't know what kind of data logger you are looking at, but something running at kHz is still running relatively very slowly.
 

utztech said:
So the jackhammer has a cycle where the hammer accelerates out to its maximum position, then returns. That is one cycle. You are saying there are 50 cycles each second.

There are 3 scenarios here.

1. You sample at less than 50 Hz. The accelerometer will be sampled at a different point on the jackhammer's cycle each time. You will not be able to make any conclusion here, except that the G force is that much as some point on the jackhammers cycle.

2. You sample at 50Hz. If your sample rate and the jackahammer's are both exactly matched, then you will sample the acceleration at the same point each jackhammer cycle. You will be able to conclude what the G force is at only that point. But you could also determine the mean and standard deviation of that G force since you will have many samples at the same point.

3. You sample at something quite a bit greater than 50Hz, like 100 times greater. Then you will sample the accelerometer at 100 different points along the jackahmmer's cycle. You will be able to tell the point of greatest G force, and the point of least G force. You will be able to plot the G force against the displacement of the hammer in its cycle.

You probably need to do something like 3 to get any useful data. I don't know what kind of data logger you are looking at, but something running at kHz is still running relatively very slowly.
Click to expand...

Thank you for your advice, most data loggers seem to have a sample rate of around 720ms/sample (https://www.picotech.com/products/data-logger) which I'm guessing would be too slow?
 

Hi,

You still did not specify on what infornation you are intdrested.

Is it waveform, min, max, --> here you need a higher sample rate
Or average, RMS....--> here you need a lower dara rate, but additionally some analog circuitry.

Klaus
 

We only need to take a few readings say 5 minutes worth. The G force is proportional to voltage out of the accelerometer. we just need to capture a few spikes in the xy z direction
 

Hi,

Taking a few readings within 5 minutes will give meaningless information.
Still unclear what information you need. Please give complete and detailed information.

Giving just a piece of information makes the thread to be long in time and our posts are just assumptions.

Klaus
 

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