Simplify Electromechanical Testing with a USB-Based Data Acquisition System

Electromechanical systems combine electrical and mechanical components for devices such as motors, compressors, pumps, sensors, actuators, and control electronics in manufacturing, aerospace, automotive, medical, and robotics applications. These devices must be tested and monitored electrically and mechanically to ensure proper operation.

To provide accurate and reliable data, the necessary equipment must be compatible with the device under test and the test method or procedures. The testing equipment must handle multiple analog and digital input/output (I/O) channels to measure and control these devices, along with basic measurement instruments like counter/timers and power supplies. The test instruments must work with integrated software to provide measurements, real-time displays, and detailed reports.

Selecting and integrating the necessary hardware and software to perform these tests can be time-consuming and costly. To assist designers, modular USB data acquisition instruments have been developed that combine the most up-to-date technology with a broad range of software testing tools to validate the most complex electromechanical systems.

This article describes the challenges facing designers testing electromechanical devices. It then introduces the mioDAQ instruments from NI and shows how they can be applied to simplify standard electromechanical tests to accelerate development and deployment.

Electromechanical testing

Consider a simple motor test rig comprising a motor mounted on a test fixture connected to a load suspended between two bearing blocks (Figure 1). The rig is controlled via a motor controller, which controls the motor’s speed based on an electrical voltage. The arrangement uses an optical tachometer to measure the rotational speed of the motor and three accelerometers to measure the mechanical vibration in the X, Y, and Z directions on the inner bearing block.

Figure 1: Shown is a motor vibration test rig that uses an optical tachometer to measure the motor's rotational speed and accelerometers to measure motor-related vibration along three orthogonal axes of the inner bearing block. (Image source: NI)

The goal of the test rig is to determine the peak vibration levels and the rotational speed at which they occur. The procedure involves varying the motor speed linearly while monitoring the vibration levels and recording both.

Various instruments are needed to run this test. First, analog measurement channels are required to monitor and record the three accelerometer outputs. Another analog channel must monitor the tachometer to measure the motor’s rotational speed. An analog output voltage is required to control the motor’s speed. A digital signal output alerts the motor controller to turn the motor on and off. Another digital signal output can be used to select the motor’s direction of rotation.

So, at its simplest, this motor test requires a minimum of four analog inputs, one analog output, and two digital outputs. More complex tests might add additional vibration sensors, temperature sensors such as thermocouples, and pressure transducers, among others.

The data acquisition system

For electromechanical testing, a data acquisition system (DAQ) comprising a DAQ device for measurement and control, a computer, and supporting software is needed. The NI mioDAQ USB data acquisition hardware fills this need with the NI USB-6400 series, which provides four USB DAQ devices to choose from (Figure 2).

Figure 2: This table summarizes the characteristics of the four devices in the mioDAQ USB-6400 series. (Image source: NI)

The mioDAQ series offers test engineers four choices in the configuration of a DAQ device:

• 16- or 20-bit amplitude resolution with ±10 volt maximum full-scale inputs
• 250 kilo samples per second (kS/s) multiplexed or 1 mega sample per second (MS/s) sampling rates
• Input channels arranged as 16 or 32 single-ended (SE) or 8 or 16 differential (DI) channels
• Two or four output channels with a ±10 volt range for control, simulation, or signal generation

All the models are controlled by and powered via a USB-C port and include 16 digital I/O lines and four 32-bit counter/timers. They also use a 100 megahertz (MHz) onboard timebase that drives all digital circuitry, including the sample clocks, trigger lines, and counter/timers. Each channel type has a separate timing engine based on the onboard timebase. Timing for the analog input and output channels and the digital I/O lines can be set to different rates. The NI mioDAQ USB devices also include self-calibration via the controlling software, which initiates self-calibration and compensates for environmental and systematic variations using a multivariate calibration equation for rapid calibration without noticeable processing delay. It stores the resulting data in an onboard EEPROM.

Another feature of the mioDAQ device is the Smart ID Pin, which adds intelligence to the test bench. The Smart ID Pin communicates with a user-supplied, 1-wire EEPROM to read the device under test (DUT) information and ensure the cables are plugged into the correct ports. The pin offers time savings and error reduction to the test bench.

Four models of specific data acquisition devices are available. The USB-6421 (789887-01) is the most economical device. It provides 16 SE or 8 DI channels using a single multiplexed analog-to-digital converter (ADC) sampled at up to 250 kS/s and includes dual analog output channels.

The USB-6423 (789882-01) doubles the number of multiplexed channels to 32 SE or 16 DI and increases the analog output capability to four channels.

The USB-6451 (789888-01) increases the number of ADCs to eight. It also increases the AC resolution to 20 bits and the maximum sampling rate to 1 MS/s. It offers eight channels with simultaneous sampling and up to 16 channels in multiplexed mode.

The USB-6453 (789884-01) offers the most significant capability; it doubles the number of 20-bit, 1 MS/s ADCs to 16 and increases the maximum channel count to 16 with simultaneous sampling and 32 in multiplexed sampling mode.

All four models are housed in an enclosure measuring 177 millimeters (mm) wide by 30.4 mm high by 116.7 mm deep (Figure 3).

Figure 3: Shown is the full view of the USB-6453 (left) member of the USB-6400 series, along with its front (right, top) and rear panels (right, bottom). (Image source: NI)

The front panel provides access to all the analog and digital signals. Connections are made using two 36-position front mount spring terminal connectors, which accept #26 AWG to #16 AWG wires. Back shells for the spring terminal connectors are supplied for strain relief. Cold junction compensation (CJC) is built in for thermocouple measurements.

The mioDAQ device package includes zip-tie mounting holes on the back and sides and a USB locking screw on the back to quickly secure cables and integrate the instrument. Optional mounting kits are available to secure the device to a 19 inch (in.) rack or DIN rails with horizontal or vertical orientation.

The mioDAQ’s use of a QR code means lost documentation is a thing of the past. Users scan the QR code on the back of the module to quickly access the user manual, specifications, pinout, and links to download control and analysis software and drivers.

Channel specifications

Up to 32 analog input channels are available, with a maximum full-scale range of -10 V to +10 V, 16-bit or 20-bit resolution, and a maximum sampling rate of 250 kS/s or 1 MS/s (model-dependent). Lower ranges of -0.2 V to +0.2 V, -1 V to +1 V, and -5 V to +5 V can match the input signal to the input range to optimize dynamic range.

The analog outputs have a voltage range of -10 V to +10 V and are clocked at 200 kS/s per channel. They can create non-periodic or periodic waveforms to generate analog control signals or simulate sensors.

The digital I/O lines can be independently set to be either input or output. They are programmable with logic voltage thresholds of 5, 3.3, or 2.5 volts and can route external clocks or trigger signals into the device or drive the internal counters/timers.

DAQ software

The mioDAQ devices can be controlled with multiple software packages, including NI’s LabVIEW, LabVIEW+, Python, and NI’s FlexLogger logging software. NI’s NI-DAQmx driver supports custom programming in C/C++, C#, VB 6.0, and VB.NET and includes programming examples and library functions for DAQ operations.

FlexLogger is a no-code software package that enables test engineers to control, view, and save test data from DAQ devices. It allows limits to be set on measured values while alarms warn of out-of-range conditions and permit the detailed analysis of test data with built-in processing tools. FlexLogger Lite, which is free, is intended for manual data logging and basic operations of NI DAQ hardware. An example of a channel setup for the USB-6421 is shown (Figure 4).

Figure 4: Shown is a FlexLogger Lite view of the channel setup for the USB-6421, including the analog input, analog output, and digital I/O settings. (Image source: Art Pini)

The analog input channels are configured to read three axes of vibration data and measurements of pressure, temperature, and sound level. Each input is scaled to read the signals in units appropriate to the measurement. The analog outputs produce 5 and 3.3 volt power levels, while the digital I/O is set up to read two digital inputs.

FlexLogger is a fuller-featured program intended for automated testing and extended data analysis. It allows customization of the user interface visualization tools by adding graphs, numeric indicators, and meters. Figure 5 shows the data from a motor (inset) test.

Figure 5: Shown is the FlexLogger view of a motor’s test results. (Image source: NI)

The waveforms from three accelerometers and a tachometer appear in the upper grid. The acceleration data is the scaled vibration level in g’s versus time. The tachometer readout, measuring rotational speed in revolutions per minute (RPM), is shown as a dial gauge in the lower right corner. Applying a fast Fourier transform (one of the available signal processing tools) to the vibration data shows vibration level (amplitude) versus frequency in the lower graph.

Conclusion

The NI mioDAQ devices merge modern measurement technology with an easy user experience. Test engineers can build sophisticated electromechanical test systems using mioDAQ components matched with no-programming software like NI’s FlexLogger or award-winning systems software like NI’s LabVIEW for more sophisticated testing requirements.