How to convert sensor volts or counts to a physical value?

Data Acquisition Systems (DAS) convert volts to counts but what does that mean physically? Say you have a DAS connected to an accelerometer-type sensor. The spec sheet for the accelerometer is 1 g = 2.6 volts. The DAS spec is 420,000 counts = 1 volt. So if the counts is 420,000 that represents 1 volt or 1/2.6 g. More generally the physical value is:

Acceleration (g's) = counts X (volt/count ) X (g/volt)

So if the reading is 200,000 counts, the value in g's is:

200,000 X (1/420,000) X (1/2.6) = 0.183 g

The conversion a sensor does from a physical quantity to volts is termed the "sensitivity" of that sensor. The same with a DAS, from volts to counts is the sensitivity of the DAS. A pressure sensor may have a sensitivity of P volts/psi (English pressure unit, pounds/sq. inch) or Pascal (Pa), metric, (Newtons/sq. Meter). If you know the sensitivities of the sensor and DAS you can easily find the actual physical value.

What's this dynamic range stuff, anyhow?

Dynamic range refers to the ratio of the largest to the smallest signal recordable, expressed in dB (decibels). Each 20 dB is a factor of 10 and each 10 dB is a factor of 3. So if an accelerometer has a range of 4 g and a dynamic range (DR) of 70 dB, the noise floor is 4g/3000 = 1.3 mg. This means that signals below 1.3 mg will be below the noise floor and not visible. The cost of an accelerometer increases with its DR, so it is not economical to specify an accelerometer with a greater dynamic range than the smallest signal of interest. Calculated the other way, if the smallest signal of interest is 4 mg and the range needed is 4 g, the DR is 20log(4.0/0.004) = 60 dB. It is prudent to add to this number about 10 dB of "headroom", so a 70 dB accelerometer would suffice.

Note: Bob Pease who died in 2011 ran a column in
Electronics Design magazine where he addressed basic issues in electronics, especially linear. The headline in his columns always was, "What's this —— stuff, anyhow". Rest In Peace, Bob!

Analog Sensors

Any analog sensor can be digitized by NetDAS or the DAQ24USB. An analog sensor has a voltage or current output. Once digitized the data can be time-stamped by GPS. The Accel-90 is an analog accelerometer with a voltage output proportional to acceleration.

NetDAS Crash Recovery

The two main techniques for remote software crash recovery are Watchdog Timers (WDT) and Internet power switches. Every system will eventually crash so crash recovery is mandatory. One low-cost approach is a remote AC power switch controlled via a web page. When the system stops working the operator logs onto the web page and selects the power-cycle button. This cycles power, clears the fault and restores normal operation. The disadvantage is that the system may be down for a period of time before the non-operational status is detected.

A better approach is a WDT circuit. The system under
normal operation produces a periodic pulse which "kicks the dog", keeping the system powered up. If the system crashes, the WD pulse fails, the "dog is not kicked" and the WD circuitry power-cycles the system. In the case of NetDAS, the WD interval is about three minutes, so the longest outage will be about three minutes. No operator intervention is necessary, resulting in practically non-stop operation.

What causes system crashes? In new systems, the cause is often software bugs. In the case of mature systems like NetDAS, the cause is static discharge which corrupts the USB stack. Once the USB stack is corrupted, a software reboot is not sufficient to restore the USB stack and only a full power-cycle can clear this error. (This is a well known USB issue and has nothing to do with NetDAS). So why not just cut USB power? Unfortunately, at this time we are not aware of any Linux utility that can cycle USB power.

Original NetDAS at 60% Discount

NetDAS News: 24-bit NetDAS $1795!

September 2018
Precision instrumentation based on open industry standards

DAQ Systems is pleased to offer the last two of the original NetDAS-1 Data Acquisition System (DAS) at a 60% discount. NetDAS-1 systems offer 4 channels at 24 bits resolution, GPS time-stamped data, CF flash memory storage, embedded Linux and digital I/O and auxiliary 10-bit analog channels.


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NetDAS (25cm/10” X 20cm/8”X 18cm/7”)

NetDAS-1 Features

  • 4 channels @ 24-bit resolution

  • NetDAS Tools software bundle: multi-triggering, data concentration, multi-formatting, error logging, graphical viewer
  • Low latency, 30ms at 100 Hz sampling

  • MZ104 CPU
  • Military-Style (MS) connectors

  • NavSynch GPS

  • GPS NavSynch card, marine-grade Panasonic antenna (with base) and 15m coax

  • Ethernet port

  • Wide power input (10 – 16 VDC)

  • Linux 2.2 kernel

Options

  • Internal 65 dB low cost tri-axial accelerometer

  • Pre-made custom analog cable

  • 4 GB Compact Flash memory card

  • Nonstop Watchdog Timer

  • Digital I/O

  • Auxiliary 10-bit analog channels

  • Expandable to 8 channels @ 24-bit resolution


About NetDAS

NetDAS is a multi-platform 24-bit DAS based on open industry standards such as Linux, TCP/IP, USB, Secure Digital (SD) and CF flash memory, client/server software, as well as Microsoft Windows. NetDAS finds application in seismic systems and other sensory applications. Embedded versions of NetDAS run Linux on solid-state low-power ARM CPU cards. NetDAS also runs under Ubuntu Linux or Windows on industrial 19-inch rack-mount servers. Introduced in 2003, NetDAS was the first seismic DAS to run embedded Linux. NetDAS comes bundled with NetDAS Tools software clients including triggering (amplitude, STA/LTA, and external logic level), formatting, real-time display and data concentration. For more information, or to place an order, please contact us at the email address or phone number above.