Embedded Sensor Specifications

Each of our enDAQ senors has within it a primary accelerometer and then 7 additional embedded sensors. This article will provide specific details on these embedded sensors, many of which are common to every one of our products.  The main accelerometer is the primary difference between each enDAQ sensor after the form factor.

In this Article


Digital Capacitive Accelerometer

The triaxial capacitive MEMS accelerometer is great for general purpose testing and in some of our sensors it is the primary accelerometer.  In models that have the piezoelectric or piezoresistive accelerometer, this serves as a secondary accelerometer to extend the measurable range (provide a DC response or better lower amplitude information for shock sensors). See our blog on accelerometer selection for more detail on different accelerometer types.

Triaxial Digital Capacitive Accelerometer Performance
Products Comes standard on all our sensors
Sampling Rate Per Channel 1 User Selectable from 12.5 Hz to 3.2 kHz Selectable with Provided Software
Frequency Response Within
±5% Accuracy
X and Y Axis: 0 Hz to 1,000 Hz
Z Axis: 0 Hz to 500 Hz
See Frequency Response Plot
Transverse Sensitivity <10 %  
Low-Pass Filter 2nd Order (Filter Frequency is 1/2 Sample Frequency)  
Sensor Part Number ADXL345
ADXL375
 
Measurement Range ±16 g ±200 g  
Broadband Noise < 0.01 g RMS < 0.14 g RMS Reduces with slower sample rates
Resolution 2 0.004 g 0.05 g 13-bit
Nonlinearity ± 0.5% ± 0.25% Percentage of full scale
Temperature Response ± 0.01 %/°C ± 0.02 %/°C  

1The sample rate varies over temperature by -0.1% per degree C.  The enDAQ sensor's clock accurately accounts for this drift; but just note that long recording files may not have perfectly equal spacing between points throughout the entire recording (important to note for FFTs).  Also, the sample rate is typically -2.2% lower than specified by the user at room temperature (for example a 100 Hz sample rate configuration will result in a 97.8 Hz sample rate).

2The resolution at a sample rate of 1,600 Hz or greater is reduced to 12-bit

Note that the MEMS accelerometer has a DC response (can measure down to 0 hertz). The plot only goes to 1 Hz because it is on a logarithmic scale. The accelerometer data was generated with the sample rate at 3.2 kHz, and the unit taped to the mounting fixture. 

Piezoelectric Accelerometer

The triaxial piezoelectric accelerometer, when compared to the DC MEMS accelerometer, offers a higher data quality. Piezoelectric accelerometers are the most popular because of their versatility but they have two disadvantages: an AC coupling and their charge amplifier can become saturated during high frequency and/or amplitude shock events. See our blog on accelerometer selection for more detail on different accelerometer types.
Triaxial Piezoelectric Accelerometer Performance
Products Comes standard in the Slam Stick X (LOG-0002)
Sampling Rate Per Channel: User Selectable from 100 Hz to 20 kHz Selectable with Provided Software
Frequency Response Within
±5% Accuracy (X, Y & Z Axis)
Aluminum Enclosure: 5 Hz to 2,000 Hz
Polycarbonate Enclosure: 5 Hz to 1,000 Hz
See Frequency Response Plot
Transverse Sensitivity <10 %  
Nonlinearity ± 2%  
Temperature Sensitivity 1 - 0.3 %/°C  
Low-Pass Filter 5th Order Hardware Butterworth (Linear Phase & Software Tunable)  
Sensor Part Number 832M1- 0025 832M1- 0100 832M1- 0500 834M1-2000
 
Measurement Range ±25 g ±100 g ±500 g ±2,000 g  
Broadband Noise 2 < 0.01 g RMS < 0.04 g RMS < 0.20 g RMS < 0.80g RMS  
Resolution 3 0.0008 g 0.003 g 0.015 g 0.06 g 16-bit
Sensitivity (mV/g) 50.0 12.5 2.5 0.62 ±30%, @ 3.3 Vdc Excitation

1Temperature compensated with software and internal temperature sensor. This linear compensation is only valid between -20C and 60C (non-linear at extremes).

2Tested with a 20 kHz sampling rate and a 5 kHz filter frequency. Noise levels will be lower with slower sampling rates and/or lower filter frequency.

3The resolution depends on sampling rate: 16-bit < 8.5 kHz, 15-bit < 16 kHz, 14-bit > 16 kHz

The piezoelectric accelerometer sampled at 20kHz with a 5 kHz filter. Adequate compression of the tape was achieved by first bolting the unit to allow the tape to set, then removing the bolts. The mounting torque of these bolts was 70 in-oz.


Piezoresistive Accelerometer

The piezoresistive accelerometer offers similar advantages as the piezoelectric in terms of data quality. But they have the added benefit of being capable of measuring static accelerations and low-frequency vibrations. They also have internal gas damping to widen the dynamic frequency range of the accelerometer. These accelerometers are increasingly becoming more popular for shock and vibration testing applications. See our blog on accelerometer selection for more detail on different accelerometer types.

Triaxial Piezoresistive Accelerometer Performance
Products Comes standard in the Slam Stick S (LOG-0004)
Sampling Rate Per Channel: User Selectable from 100 Hz to 20 kHz Selectable with Provided Software
Frequency Response Within
±5% Accuracy (X, Y & Z Axis)
Aluminum Enclosure: 0 Hz to 2,000 Hz See Frequency Response Plot
Transverse Sensitivity <10 %  
Nonlinearity ± 0.5%  
Temperature Sensitivity ± 0.02 %/°C  
Low-Pass Filter 5th Order Hardware Bessel (Linear Phase & Software Tunable)  
Sensor Part Number 3255A-100
3255A-500  
Measurement Range ±100 g ±500 g  
Broadband Noise 1 < 0.08 g RMS < 0.40 g RMS  
Resolution 2 0.003 g 0.015 g 16-bit
Sensitivity (mV/g) 20.0 4.0 ±10%, @ 5 Vdc Excitation
Damping Ratio 0.7 0.5 Typical

1Tested with a 20 kHz sampling rate and a 5 kHz filter frequency. Noise levels will be lower with slower sampling rates and/or lower filter frequency.

2The resolution depends on sampling rate: 16-bit < 8.5 kHz, 15-bit < 16 kHz, 14-bit > 16 kHz

Note that the Piezoresistive accelerometer has a DC response (can measure down to 0 hertz). The plot only goes to 1 Hz because it is on a logarithmic scale. The piezoresistive accelerometer sampled at 20kHz with a 5 kHz filter. The unit was bolted down with a mounting torque of 100 in-oz.

Orientation Sensor (Gyroscope & IMU)

As of November 2017, all of our enDAQ sensors now include an embedded "smart sensor hub" that has an integrated triaxial gyroscope, triaxial accelerometer, triaxial geomagnetic sensor and a microcontroller for intelligently fusing raw data from the 9 sensors.  The output this sensor hub include the following:

  • Rotation (gyroscope only)
  • Relative orientation (output in quaternions relative to starting location)
  • Absolute orientation (output in quaternions relative to earth's magnetic field - involves a startup time for the geomagnetic sensor to self-calibrate)

We published a  blog that goes into more detail; but here is an excerpt that explains the difference between relative and absolute orientation mode.

There are two options for the orientation sensor: absolute and relative. The absolute measurement provides a rotation relative to magnetic north, and the relative measurement is the orientation relative to the orientation the sensor had when the acquisition started. Both sensors work by integrating the rotation and applying a correction factor from the accelerometer, including a detection of the gravity vector. The absolute orientation sensor also uses the magnetometer to orient itself to the magnetic field.

Since the absolute orientation sensor uses a magnetometer, it can be better at detecting slow turns, which result in a low rate of degrees per second applied over a number of seconds. Similarly, it should be less likely to drift over long periods. But, since the absolute measurement uses a magnetometer, it increases the power draw (3 mA vs 1.5 mA; but this equates to roughly a 10% increase on the overall power consumption depending on configuration). The absolute sensor also requires some movement before the sensors all lock on for high accuracy data. How to get high accuracy data is detailed here

Orientation Sensor Performance
Products Comes standard on all of our enDAQ sensors
Sampling Rate Per Channel  User Selectable from 0 Hz (off) to 200 Hz Selectable with Provided Software
Sensor Part Number BHI160
BMM150
 
Measurement Range 2000 °/s ± 1300 µT  
Resolution 0.06 °/s 0.3 µT
Transverse Sensitivity ± 2% Not Specified
Nonlinearity ± 0.1%
± 1% Percentage of full scale

External Temperature, Pressure & Humidity Sensor

As of February 2017, all of our enDAQ sensors now include temperature, pressure, and humidity sensors in the external "control pad" (has LEDs and button).  This addition was largely due to a request from the Navy.  They were using a Slam Stick to record cabin pressure on every F-18 flight because of the cabin pressure issues they've been having in F/A-18s, but they wanted better pressure data.  This new sensor has improved accuracy and response on its own; but with its external location, the data will be a better representation of ambient conditions.  This sensor also has an integrated humidity sensor that was enabled as of firmware revision 20.

External Temperature, Pressure, and Humidity Sensor Performance
Products Comes standard on all of our enDAQ sensors
Sampling Rate Per Channel  User Selectable from 0 Hz (off) to 10 Hz Selectable with Provided Software
Sensor Part Number MS8607

Sensor Temperature
Pressure
Humidity
 
Measurement Range -40 to 85 °C  1 to 200 kPa 0 to 100 %RH  
Accuracy ± 2 °C ± 0.4 kPa ± 5% RH -25°C to 80°C
Resolution 0.01 °C  1.6 Pa 0.04% RH 24-bit

Internal Temperature & Pressure Sensor

This sensor is still used to compensate for change in sensitivity of the piezoelectric accelerometer over temperature which is why it is still enabled. 

Internal Temperature and Pressure Sensor Performance
Products Comes standard on all of our enDAQ sensors
Sampling Rate Per Channel  Always on at 1 Hz
Sensor Part Number MPL3115A2


External Visible and UV Light Sensor

As of Firmware version 20 the light sensor on the control pad is enabled. This sensor is capable of capturing the intensity of visible and ultraviolet light at a rate of 4 Hz. This sensor is not calibrated and is intended for rough, relative measurements only. We wrote a blog on light sensors in general too that can help provide some background on how this sensor can be utilized. The sensor is placed as shown below:

External Light Sensor Performance
Products Comes standard on all of our enDAQ sensors
Sampling Rate Per Channel  About 4 Hz

Sensor Part Number Si1133


Commonly Asked Questions

How accurate is the enDAQ sensor?

This is a little more difficult to answer than one may expect.  An accelerometer will have different specifications that will affect "accuracy."  These include noise, resolution, non-linearity, temperature sensitivity, and frequency response.  But frequency response has the highest impact; so as long as the frequency of the vibration is within the rated range for the specific sensor the device should be within 5%.  On the piezoelectric accelerometer there will be a low-frequency negative error.  And on all accelerometers as the frequency approaches resonances (either within the sensor, or due to case construction).  All internal accuracies get trumped however by mounting.  If the device (or any accelerometer) is not mounted correctly you can experience grossly wrong results.  We wrote a blog on mounting that provides more insight on this impact on accuracy.

What are the noise characteristics of the accelerometers in the enDAQ sensor?

The previous tables provide the broadband noise levels of the accelerometers; but some users are also interested in the noise vs frequency level.  The following three plots are FFTs of each of the main accelerometer types when recording in our sensor.  There are some frequencies that will appear in the FFT because of the noise from the accelerometer and/or sampling electronics and are not characteristics of your vibration/shock environment.

ADXL

An FFT of the ADXL345 (digital capacitive accelerometer) shows relatively good/low broadband noise levels.  There is a noticeable spike around 25 Hz that appears inherent to the digital component. This data is with a sample rate of 200 Hz, a higher sample rate will see harmonics of this 25 Hz frequency component repeating at a similar level of ~0.001g.

3255A

Data from the 3255A accelerometer (piezoresistive accelerometer) is shown.  There is known frequency around 25 Hz, and then the harmonics, in every component/axis that is a result of the on-board programmable gate array doing the temperature compensation. It’s a characteristic of this accelerometer.

832M1

The piezoelectric accelerometer has no inherent noise issues; but the enDAQ sensor's electronics introduces a small amount of noise when saving to disk.  This frequency is about 1/4000 th of the sampling frequency.  Midé is hoping to improve this in future releases.

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