Factory Calibration & Recalibration
We perform a calibration on the accelerometer sensors internal to the Slam Stick recorders. Re-calibrations are also offered. This article details that procedure, some data on the calibrations we've applied and the process of sending the unit in for recalibration. Users can also program their own calibration which is detailed in the User Calibration Configuration article.
*Note Slam Stick is now enDAQ
In this Article
To complete a recalibration with us, you will need to order a recalibration through the website. You will receive an automated email with instructions to mail the unit to the following address:
ATTN: enDAQ Recal - Order Number 475 Wildwood Ave Woburn MA 01801
- It's important to include the order number
- To help ensure there is no confusion, we also ask to print the receipt/invoice you received after completing the order and include it in the shipping box.
- The customer is responsible for shipping the sensor to our facility. Return shipping is included in the price of re-calibration. Once we receive the unit(s) we will typically complete the recalibrations and ship back within 3 business days.
Factory Calibration Procedure
All enDAQ sensors are put through a calibration procedure that vibrates the device at 10g peak, 100 Hz in all three axes. A closed-loop shaker control system with a calibrated reference accelerometer is used to ensure the profile is as expected. Below is an image of one of these recordings/events, and here is the recording file.
As can be seen from the data, the recorder did not accurately measure the acceleration at 10g, instead, it was approximately 8g. A software script calculates this gain needed (1.25 = 10/8) and applies it to the device. In a similar calculation for DC response accelerometers, it calculates the DC offset to account for gravity. A calibration certificate - download certificate example (pdf) - is generated and saved to the device, an example is shown below.
he calibration applied to the recorder is also saved internally to the device, and these parameters can be accessed in the ‘Factory Calibration’ tab located next to the ‘Measurements’ tab in the configuration menu. This menu displays the specific gain and offset terms (and any bivariate terms if/when temperature compensation is needed) and it also displays the bivariate polynomials created from the initial calibration process of the enDAQ sensor.
These equations are how the enDAQ sensor converts raw acceleration data into a calibrated form when displayed in the enDAQ Lab software. There are either one or two levels of calibration. One converts from raw integer data to engineering units and double format. The second level of calibration/conversion is applied by Mide to compensate for part-to-part variance. In the example above we have a 25g accelerometer. This means it can measure a 50g range and has 16-bit precision. The raw data is saved in 16-bit integer so in order to be converted to engineering units it needs to scale the 16-bit range to 50g. This is calculated by multiplying by 50 and dividing by 2^16 (65,536) which equals 0.00076 g. Then this range needs to be decreased to be centered around 0, so subtracting 1/2 the measurement range.
Then the per channel and part-to-part variance is calibrated with more simple terms that should be either close to 1 for gain, and close to 0 for compensating DC offset inaccuracies.
Many customers request that the calibration is compliant with a specific calibration standard such as ISO 17025, ANSI Z-540, ISO 10012, NAVAIR 17-35QAC-01 and many others. Unfortunately, there are no standards for calibration of embedded data loggers. Because we are logging data and have no means to synchronize to the data point an acceleration measurement to a reference one we needed to develop our own procedure. The calibration we apply isn't necessarily NIST traceable itself, instead, it was completed with a reference accelerometer that has a NIST traceable calibration.
We had a lot of back and forth with NAVAIR instrumentation groups as part of our initial development where we had to address and overcome this obstacle. Below are two documents that were approved by this group for use in F/A-18s:
Are Calibrations Needed?
In the simplest terms, accelerometers are mechanical systems and specifically springs. It is this spring stiffness that relates to the sensitivity of the accelerometer. And the stiffness (and corresponding sensitivity) is dependent upon manufacturing tolerancing of the sensors and usage over time. The initial calibration is important to not only ensure the sensor performs as specified, but also to compensate for any manufacturing variance. A recalibration is important to ensure that the sensor is "healthy" and potentially compensate for any slight change in the sensitivity over time and use.
Mide Calibration Data
We've performed thousands of calibrations on our recorders now and have some interesting data to share. The piezoelectric accelerometer used in the device has the greatest variation from unit-to-unit with gain terms. These gain or trim terms reflect how much the sensitivity needs to shift from the nominal value. For example, a value of 1.2 for a ±100g accelerometer will mean the range will actually be ±120g after calibration. As you can see from the data below, the piezoelectric accelerometer has a wide variation and that initial calibration is incredibly important to achieve accurate results.
The piezoelectric accelerometer clearly needs a calibration; but the variable capacitance and piezoresistive accelerometers are made with MEMS manufacturing technology. This manufacturing approach is able to produce parts with very small unit-to-unit variation. Over 85% of our calibrations of this accelerometer are within ±2%.
The same is true for the piezoresistive data, very little variation unit-to-unit. But the average gain term is 1.25 which means that the ±500g accelerometer typically can measure ±625g. We believe these units have a higher gain term to compensate for the temperature dependency the unit has to still allow for ±500g at colder temperatures.
The offset terms, however, are quite variable with both the variable capacitance and piezoresistive accelerometers that are DC-coupled (remember the piezoelectric accelerometer can't measure gravity and this DC offset is filtered out). But this DC offset can be determined by anyone in the field, a shaker table is not required, and it is actually advised to self-calibrate for the DC offset after mounting to your system to account for any mounting issues.
Mide Recalibration Data
The recalibration data tells a similar story to the calibrations. Recalibrations for piezoresistive and variable capacitance accelerometers seems unnecessary. All the units besides 1 that we recalibrated these accelerometers for had gain adjustments of less than ± 2%. The piezoelectric accelerometers do however shown some variation over time and it is advised to get a recalibration after roughly one year of use.