Connecting Accelerometers to a DAQ Device

Included in the Section

Before You Begin

This document provides step-by-step instructions for wiring and configuring your NI DAQ device for accelerometer measurements. Before you begin using your NI DAQ hardware, you must install your application development environment and NI-DAQmx driver software. Refer to the Installing LabVIEW and NI-DAQmx document for more information.

Acceleration Measurement Fundamentals

You can use an accelerometer, a sensor that represents acceleration as a voltage, to measure acceleration with an NI dynamic signal acquisition (DSA) device. Accelerometers are specified by the range, frequency response, and the sensitivity of the sensor. Accelerometers come in two axial types. The most common accelerometer measures acceleration along only a single axis. This type is often used to measure mechanical vibration levels. The second type is a triaxial accelerometer. This accelerometer can create a 3D vector of acceleration in the form of orthogonal components. Use this type when you need to determine the type of vibration—lateral, transverse, rotational, and so on—that a component is undergoing or the direction of acceleration of the component. Both types of accelerometer can be either passive or active sensors.

Both types of accelerometers also come with either both leads insulated, or isolated, from the case or with one lead grounded to the case. Some accelerometers rely on the piezoelectric effect to generate voltage. To measure acceleration with this type of sensor, the sensor must be connected to a charge-sensitive amplifier.

Other accelerometers have a charge-sensitive amplifier built inside them. This amplifier accepts a constant current source and varies its impedance with respect to a varying charge on the piezoelectric crystal. You can see this change in impedance as a change in voltage across the inputs of the accelerometer. Thus, the accelerometer uses only two wires per axis for both sensor excitation, or current, and signal output, or voltage. The instrumentation for this type of accelerometer consists of a constant current source and an instrumentation, or differential, amplifier. The current source provides the excitation for the built-in amplifier of the sensor, and the instrumentation amplifier measures the voltage potential across the leads of the sensor.

Analog Input Channel Configurations

The NI DSA devices support one or both of two terminal configurations for analog input: differential and pseudodifferential. The term pseudodifferential refers to the 50 Ω and 1 kΩ resistance between the outer connector shell and system ground.

If the signal source or device under test (DUT) inputs are floating, use the pseudodifferential configuration. You must provide a ground reference for a floating signal. If you do not provide a ground-reference point — for example, selecting differential mode with a floating shaker table input amplifier with differential inputs — the outputs or inputs can drift outside the device common-mode range.

Figure 1. Connecting a Floating Signal Source

If the signal source is grounded, use either the differential or pseudodifferential configurations. Provide only one ground-reference point for each channel by properly selecting differential or pseudodifferential configuration. If you provide two ground-reference points— for example, if you select pseudodifferential mode with a grounded accelerometer or amplifier— the difference in ground potential results in currents in the ground system that can cause measurement errors. The 50 Ω and 1 kΩ resistor on the signal ground is usually sufficient to reduce this current to negligible levels, but results can vary depending on the system setup.

Figure 2. Connecting a Grounded Signal Source

Configure the channels based on the signal source reference or DUT configuration. Refer to Table 1 to determine how to configure the channel.

Source Reference

Channel Configuration

Floating

Pseudodifferential

Grounded

Differential or pseudodifferential

Table 1. Analog Input Channel Configurations

Some devices are automatically configured for differential mode when powered on or powered off. This configuration protects the 50 Ω resistor on the negative pin.

Input Coupling

You can configure each NI DSA device analog input (AI) channel for either AC or DC coupling. If you select DC coupling, any DC offset in the source signal is passed to the analog-to-digital converter. The DC-coupling configuration is usually best if the signal source has only small amounts of offset voltage or if the DC content of the acquired signal is important. If the source has a significant amount of unwanted offset, select AC coupling to take full advantage of the input dynamic range.

Selecting AC coupling enables a highpass resistor-capacitor (RC) filter into the positive and negative signal paths. Note that NI-DAQmx does not compensate for the settling time introduced by the RC filter when switching from DC to AC coupling. To compensate for the filter settling time, you can discard the samples taken during the settling time or force a delay before you restart the measurement.

Using AC coupling results in an attenuation of the low-frequency response of the AI circuitry. Refer to you device specifications for information about the cut-off frequency for each device.

IEPE

If you attach an IEPE accelerometer to an AI channel that requires excitation from the DSA device, you must enable the IEPE excitation circuitry for that channel to generate the required excitation current. You can independently configure IEPE signal conditioning on a per channel basis on all DSA devices.

A DC voltage offset is generated equal to the product of the excitation current and sensor impedance when IEPE signal conditioning is enabled. To remove the unwanted offset, enable AC coupling. Use DC coupling with IEPE excitation enabled only if the offset does not exceed the voltage range of the channel.

Common IEPE excitation values are 2.1 mA, 4 mA and 10 mA. Refer to you device specifications for a list of supported IEPE current values for each device.

Transducer Electronic Data Sheet (TEDS)

TEDS-capable sensors carry a built-in, self-identification EEPROM that stores a table of parameters and sensor information. A TEDS contains information about the sensor such as calibration, sensitivity, and manufacturer information. TEDS sensors have two modes of operation: an analog mode, which allows the sensors to operate as transducers measuring physical phenomena, and a digital mode, which allows the user to write and read information to and from the TEDS. Many DSA devices support modes for Class I TEDS sensors without any additional hardware. Some devices require an accessory such as the BNC-2096 to allow the user to digitally communicate with the EEPROM on Class I TEDS sensors.

Field Wiring and Noise Considerations

DSA devices typically have a dynamic range of more than 110 dB. Several factors can degrade the noise performance of input channels, such as noise picked up from nearby electronic devices. DSA devices work best when kept as far away as possible from other plug-in devices, power supplies, disk drives, and computer monitors. Cabling is also critical. Use well-shielded coaxial or floating cables for all connections. Route the cables away from sources of interference such as computer monitors, switching power supplies, and fluorescent lights. Physical motion or deformation can induce noise on sensitive analog cables. Use a transducer with a low output impedance to minimize system susceptibility to external noise sources and crosstalk.

You can reduce the effects of noise on your measurements by carefully choosing the sample rate to maximize the effectiveness of the anti-alias filtering. Computer monitor noise, for example, typically occurs at frequencies between 15 kHz and 65 kHz. If the signal of interest is restricted to below 10 kHz, for example, the anti-alias filters reject the monitor noise outside the frequency band of interest, and a sampling rate of at least 21.6 kS/s guarantees that any signal components in the 10 kHz bandwidth of interest are acquired without aliasing and without being attenuated by the digital filter.

When possible, use the differential configuration to minimize the effect of any noise produced by ground currents in the chassis and common-mode noise. If you have particularly noisy AC power, consider external filtering, such as a line conditioner or an uninterruptible power supply.

Refer to the white paper "Field Wiring and Noise Considerations for Analog Signals" for more information.

Locating Your DAQ Device Pinout

Before connecting any signals, locate your device pinout.

  1. Open Measurement & Automation Explorer (MAX) and expand Devices and Interfaces.
  2. Right-click on your device name, and select Device Pinouts.

Figure 3. Device Terminals Help

Configuring an Accelerometer Measurement

You can use MAX to quickly verify the accuracy of your measurement system setup. Using an NI-DAQmx Global Virtual Channel, you can configure an accelerometer measurement without any programming. A virtual channel is a concept of the NI-DAQmx driver architecture used to represent a collection of device property settings that can include a name, a physical channel, input terminal connections, the type of measurement or generation, and scaling information.

Follow these steps to begin:

  1. With MAX open, select Data Neighborhood and click Create New.
  2. Select NI-DAQmx Global Virtual Channel and click Next.
  3. Select Acquire Signals » Analog Input » Acceleration

Figure 4. Creating an NI-DAQmx Virtual Channel

  1. Select ai0 or whichever physical channel you intend to use to connect your accelerometer. A physical channel is a terminal or pin at which you can measure or generate an analog or digital signal.

Figure 5. Device Physical Channels

  1. Click Next and enter a name for the global virtual channel or leave the default.
  2. Click Finish and you should see the following screen in MAX:

Figure 6. Setting Up an Acceleration Channel in MAX

  1. On the Settings tab, type in the minimum and maximum acceleration values you expect to read from your accelerometer (-5g to 5g by default).
  2. Type in the sensitivity value of the sensor. This value is in the units you specify with the sensitivity units input. Refer to sensor documentation to determine this value.
  3. Select the source of excitation (Iex Source):
    1. External—Any excitation source other than the built-in excitation source of the device.
    2. Internal—Use the built-in excitation source of the device.
    3. None—Supply no excitation to the virtual channel.
  4. Specify the excitation value (Iex Value (A)).
  5. Select your terminal configuration. Terminal Configuration specifies the grounding mode used for the virtual channel. You can choose Differential or Pseudodifferential depending on your device (see above).
  6. Type in the dB Reference value. This is the reference value used when results are computed in decibels. dB Reference is expressed in the selected engineering units.
  7. Click the Device tab and choose your Coupling Mode (AC or DC).

Wiring an Accelerometer to Your Device

The next step is to physically connect the accelerometer to your DSA device. NI DSA devices use BNC or SMB connectors. The center pin of the connector, AI +, provides the DC excitation, when enabled, and the positive input signal connection. The shell of the connector, AI -, provides the excitation return path and the signal ground reference. To minimize ground noise, prevent the metal shells of the connectors from coming in contact with each other, the DSA device(s), or the chassis/computer.

Refer to figures 1 and 2 above for connection diagrams.

Testing the Signal

Use NI-DAQmx global virtual channels to preview your measurements.

  1. With MAX still open, click back on the NI-DAQmx Global Channel tab and click on the Run button to see the acceleration value of your measurement displayed at the top of the screen.

Figure 7. Previewing an Accelerometer Measurement in MAX

You can choose to view the signal in tabular form or as a graph by selecting Graph from the Display Type dropdown. You also have the option of saving your NI-DAQmx Global Virtual Channel should you wish to refer to this configuration screen again in the future.

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