Endevco :: Measurement Endeavors

Interfacing Accelerometers to Data Acquisition Systems
Part 1

Background:

Data acquisition systems (DAQ) have found widespread use for the collection and analysis of dynamic signals. Fast Fourier analyzers (FFT) have been essential tools for dynamic analysis for many years. More recently the manufacturers of these instruments have included the "plug-and-play" convenience of "built-in accelerometer signal conditioning" in the form of a constant current source.

The built-in signal conditioning provides the ultimate in user convenience, but it is essential that the potential user become familiar with the measurement systems current source specifications and the specifications of the transducer.

This series of articles will discuss the essentials of selecting the proper analog to digital input module for accelerometer applications. This first article will deal with signal conditioning basics and the criteria for selection for different accelerometer types. Subsequent articles will discuss resolution, sampling rate and filtering.

Available Signal Conditioning

The purpose of a signal conditioner is to modify transducer signals to match the DAQ inputs. The conditioner provides amplification, attenuation, normalization, filtering, excitation, etc.

Data acquisition systems generally offer a selection of Analog Input Modules. The module types of interest (for the purposes of this paper) include:

IEPE
Single-ended, +/- 10 Volt General Purpose
Strain Gauge Modules

Since piezoelectric and IEPE types are the most commonly used accelerometers, the first two options will be reviewed

IEPE Accelerometers

As a result of electronic research conducted by Endevco, high temperature limitations and sensitivity have been greatly improved. The maximum temperature of IEPE devices is generally limited to 257°F (125°C). Higher temperature versions have been available, but recently Endevco announced a a newly developed accelerometer with an upper temperature of 350°F (175°C) and sensitivities of 10mV/g and 100 mV/g.

IEPE accelerometers are marketed under different brand names (Endevco uses the name Isotron), but the interface follows a pseudo industry standard. IEPE devices all use a single coaxial lead and operate from a constant current source. The IEPE analog Input modules are probably useable in 80% to 90% of the cases.

The limitations and possible pitfalls are as follows:

Excitation Current Requirements
Many of the analog input modules provide an excitation current of 2 - 4 mA only. Some accelerometers require more current for proper operation thus a comparison between the accelerometer and input module current specifications is in order. If long leads between the data system and accelerometer are to be employed, a higher current may be required in order to compensate for high frequency loss caused by the cable loading the accelerometers internal impedance converter. As a general rule-of-thumb, the excitation current may need to be increased when cables exceed 100 feet in length. To calculate the current required, use the following equation:

I = 2πfCE

Where:

I = Current in Amperes
f = Frequency in Hz
C = Capacitance in Farads
E = Output in Volts, Peak

C is determined by the product of the cable capacitance (per foot) times the length in feet. Most coaxial cable is 30pF per foot.

Voltage Limitations:

IEPE accelerometers also have a voltage specification referred to as the "compliance voltage". If an inadequate voltage level is provided, the accelerometer will not be able to produce its full output voltage without distortion. In some cases, the accelerometer's internal electronics will not operate, or "turn-on" if the compliance voltage is inadequate.

While DAQ A/D input module compliance voltage (around 19 Volts) for accelerometer power is adequate for most accelerometers. Some accelerometers, especially specialized types including high temperature and seismic accelerometers will require a higher voltage in order to provide the full scale output voltage.

If the compliance voltage provided by the conditioner is lower than that required by the accelerometer, consult the manufacturer for information on the effects of the lower voltage.

Charge Mode Piezoelectric Devices

A charge-mode accelerometer is easily identified because the sensitivity is specified in pC/g (pico Coloumbs per g). While charge mode accelerometers are not directly compatible with the IEPE analog input modules, they can be easily accommodated using a remote charge converter (RCC).

Endevco model 2771C low noise charge converter

The charge converter performs the same task as the internal electronics found in IEPE accelerometers; it converts the high impedance charge signal into a low impedance voltage output. The RCC appears as an IEPE accelerometer to the analog input module or FFT analyzer. In some cases, a more sophisticated charge amplifier may be used and is discussed in the next section.

Overcoming Accelerometer/Measurement Instrument Incompatibilities
As discussed above, there are instances where the IEPE type analog input module will not do the job. There are also a lot of data acquisition systems that only have a single ended +/- 10 Volt input. For the instances mentioned above, the user can use an external signal conditioner. Many signal conditioners available are "dual mode" types and accommodate both IEPE and charge type accelerometers. These conditioner generally have a full scale outputs of +/- 5 Volts or +/-10 Volts.

For users of IEPE devices, these dual mode amplifiers often provide more flexibility in excitation voltage and current than is available in the data acquisition system's analog to digital module. Since the external conditioners usually feature an amplified output, they are compatible with the standard 10 Volt input modules. The external dual mode amplifiers most often include filtering.

The charge input (found in dual mode conditioners) will accommodate most charge mode piezoelectric accelerometers without the need for an external charge converter.

The Endevco model 133 provides a +/- 10 volt output and can be used with both IEPE or charge mode accelerometers

Interfacing to Piezoresistive Accelerometers

An option offered by many data acquisition manufacturers is a bridge conditioning input module. These are often referred to as strain gage input modules. Many of these modules are full featured with multiple inputs, filters, high gain and selectable excitation voltages. Since Piezoresistive accelerometers use a strain gage sensor, they are generally compatible with these plug-in modules. DAQs are available that are specifically designed for use with Piezoresistive modules and are truly "plug compatible". These "plug compatible" systems are widely used in automotive safety testing applications.

The user should verify the compatibility of the available conditioner voltages with that of the accelerometer requirements. If the input module's excitation voltage is different than that specified for the accelerometer, the accelerometer can usually be calibrated and compensated at the required excitation voltage level. For example, if the input module has a 5 volt excitation voltage available and the accelerometer requires 10 Volts, the accelerometer should be calibrated and compensated for 5 Volts. The manufacturers of these devices can usually provide this calibration service.

Some accelerometers may require bridge completion resistors. The selected module should have provisions for addition of these resistors using either hardware or software.

Variable Capacitance (VC) Accelerometers

VC accelerometers virtually include a built-in signal conditioner, thus simplifying interfacing. Most Endevco VC accelerometers feature a +/ 2 Volt full scale output. With the addition of a DC power supply, the device can be connected directly to the general purpose analog input module. Note that in this instance, the accelerometer will operate with a single-ended input. Please see Endevco Technical Paper 323 "Understanding the Endevco Accelerometer Electrical Characteristics" for more information.

Endevco VC accelerometers are designed for operation with a bridge conditioner. They can be connected directly to the strain gauge conditioner as described above. Since VC accelerometers have a built-in voltage regulator, any excitation voltage of 9.5 to 18 Volts may be used.

Other Considerations

This article covers some of the more common questions about DAQ interface concerns. As stated above, other items of importance include resolution, sampling rates and filters. The next article will discuss digitizer resolution as it relates to dynamic measurements.

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