I have built a small network of seismic sensors using the SM-24 geophones. Although I have collected data and am processing it, I am still working to understand the readings coming off of these things.
I see, from the datasheet, that the sensitivity value reads 28.8 V/m/s (Volts per meter per second). Additionally, with the other values such as the natural frequency and the coil resistance, I am having a difficult time making sense of what these values mean and how they relate to the final readings I have acquired. Can someone walk me through these values? thank you!
So from studying this tutorial, I have begun to make more sense of the geophone’s specs with respect to the output. Now, after playing with the basic setup from this tutorial, I am seeing that the voltage readings are clipped after a certain voltage generated. Are there considerations I am not seeing as to why this may be happening (short of the geophone experiencing an impact that maxes out how much the inner coil can travel)??
is there a chance the model of ADC is maxed out? should I consider how much resistance I need to apply on the calibration bridge connected to the SM-24? thank you!
What is the voltage at which the clipping occurs, and how does it compare to the voltage provided to the ADC? Are you referring to the actual voltage from the geophone, the reading returned by the ADC, or the voltage shown in the data written to the file (the clipping could be occurring in any of the three devices involved)?
I’ve measured the voltage coming off the geophone itself, and I am seeing the voltages up to 25-30 volts (average voltages between 7 and 15 volts) with some pretty decently violent vibration. when I measure outputs off of the ADC, 1 can see readings up to 2 volts. I might be answering my own question here. I seem to understand the ADC less than I do the geophone itself.
That’s still not quite clear. You mention ‘outputs off of the ADC’. Is the 2V reading the value calculated in the Pi from the ADC data, or is it the voltage across the ADC differential analog inputs?
If it’s the voltage across the ADC inputs then that sounds about right. You can check the current by consulting table 2 in the datasheet and using the input impedance values for your selected amplification factor. If the current is in the vicinity of the maximum allowed then the voltage might be getting clipped. But 1 to 2V seems about right. In that case the issue is not clipping but scaling, and that needs to be allowed for in the calculation in the Pi that converts the ADC reading to m/s.
If the 1 to 2V is the calculated value of the ADC reading then this is also a scaling issue, but it would be worth checking the current at the differential inputs to the ADC. Your reading of 25-30V at the sensor is well above what was anticipated for that circuit, so increasing the protective resistors might be worthwhile.
I hope this clears things up. The current coming off of the geophone is reading between 5 and 6 mA with the same vibration I applied in the first place. I am measuring with an oscilloscope. From the analog inputs, I am measuring across analog 1 and analog 0 and I that’s where I am seeing the steady 2V max and I reading a maximum of 0.990 mA as well.
My end goal here, (after some testing using the tutorial setup) is I now need to read voltages up to 120V (absolute last resort max) so if there’s any suggestions for an ADC that can handle those currents and voltages, that would be helpful. thank you!
120V is well outside of ELV and I’d recommend that you get in touch with an electrical engineer to find an alternative or a safe solution to measure this.
Also wanted to confirm that you are using the same ADC, from the symptoms above the ADC’s range is being hit and might be damaged.
I think you are saying that you need to read velocities of up to ~4m/s. This is way outside any reference I have seen for using this device - it is typically used for measures in the mm/s range. It is well outside the usage contemplated for the circuit you are using. You need to consult table 2 (or figure 7.5, depending on your version) as advised above, note the ADC differential input impedance for the amplification (FSR) you are using, and then implement a circuit that will create a voltage across the differential inputs that is within the maximum allowed range of the ADC (0-Vdd). More effective isolation than a pair of resistors will be required. I suspect that the fixed 2V that you are seeing across the inputs is the failure mode of the ADC. Do you get the same result if you use the other pair of differential inputs (with much reduced vibration!)?
You probably are but be aware that to measure here you will need an oscilloscope with isolated (from ground) differential inputs or remove the mains earth from your instrument. This prevents accidental grounds (and possible shorts) via this connection.
Another way if you don’t have isolated inputs may be to measure with the A and B inputs WRT ground and use the math function to subtract one from the other. This might work, don’t know have never tried it. My oscilloscope has the mains ground disconnected via a short length of mains cable with the green wire hanging out of the connector very visible so it is obviously not connected. I normally use it this way at all times unless doing measurements on mains operated equipment.
I have a second instrument 5VDC operated so this one has no mains ground connection.
Cheers Bob
