FPGA-Based Tachometer Signal Acquisition for Vibration Monitoring Applications

Author(s):
Daniel Hooks - Cal-Bay Systems, Inc.

Industry:
Research, Oil and Gas/ Refining/ Chemicals

Products:
Data Acquisition, Digital I/O, LabVIEW, PXI/CompactPCI, FPGA Module

The Challenge:
Developing a tachometer signal acquisition module to seamlessly integrate into a PXI-based vibration monitoring system. Provide the flexibility to connect a variety of sensor types in which signal frequency, amplitude, and DC offset may vary substantially. Avoid the use of expensive signal conditioning hardware while also minimizing processor loading during intensive vibration analysis and display.

The Solution:
Utilizing the National Instrument R series FPGA hardware alongside National Instruments LabVIEW software and the National Instruments LabVIEW FPGA Module, a solution was rapidly developed to address the requirements. The self-configuring algorithm automatically identifies input signal characteristics and provides reliable tachometer speed readings. Customized hardware is not necessary, and no additional load is introduced to the processor performing vibration analysis calculations.

Tachometer signal acquisition requires converting pulse trains into a series of timestamps and speeds.  Many tachometer sensors create pulses whose frequency varies with speed, but their amplitude, DC offset, and noise level may vary with changes in location, temperature, speed or other operating conditions. For example, proximity sensors are often used to measure rotational speed by detecting the location of a key cut into a rotating shaft. Small changes in the location of the rotating shaft with respect to the fixed proximity sensor have a significant impact on the DC value of the proximity sensor’s signal. If a simple comparator is used to detect the pulses, the signal can be lost as the DC value changes.  

One of the world’s largest suppliers of natural gas compressors came to Cal-Bay Systems for help with the tachometer signal acquisition portion of their NI LabVIEW vibration monitoring and analysis system. This system continuously monitored vibration signals using NI PXI-4472 cards. Tachometer signals were acquired using a PXI-6602 counter card with comparators for front end signal conditioning. An MXI-4 interface was used to transfer vibration data to a dual-processor PC for analysis, display, and logging utilizing the LabVIEW, Sound and Vibration Toolkit  and the Order Analysis Toolkit. Since the software analyzed numerous vibration channels, heavy processor loading was already a concern.

Incoming tachometer signals could originate from a variety of sensor types, with varying waveform shapes and properties. Existing solutions required custom hardware for tachometer signal processing. Costly, cumbersome, and inflexible, the hardware required manual configuration based upon signal properties. It could not adapt to different input signal types or to variations in DC offset and signal profile at runtime. Greater flexibility and accuracy were required.

FPGA-Based Solution

The National Instruments R Series Intelligent DAQ Devices were used to address the problem at hand. FPGA hardware, in conjunction with LabVIEW and the LabVIEW FPGA Module, rapidly and intelligently processed the incoming signals while introducing zero additional processing load to the PC. The PXI-7833R platform was chosen for this application.  It seamlessly integrated with the existing PXI hardware, while providing analog signal acquisitions of up to eight independent tachometer signals at 200 kHz per channel. A 3 million gate FPGA chip provided plenty of parallel processing power to meet the goals of our application.

Algorithm

The output signal of most common tachometer sensors is a distorted AC waveform. Each of the PXI-7833 analog input channels were connected to individual sensors, allowing the acquisition of eight separate speeds. For each channel, high and low values of the input signal were tracked over time. Based upon these measurements, a threshold value was calculated and used to detect pulses. Each pulse indicated that some portion of a rotation had been completed. Since the signal may have included additional noise, a hysteresis value was also determined and used for filtering. Threshold and hysteresis values were periodically updated to track changes in the DC value and offset of the tachometer signal. Incoming pulses were counted and used to determine the speed of the rotating shaft, which was correlated to other data using timestamps for vibration and order analysis.

 

Using National Instruments FPGA hardware in conjunction with LabVIEW and the LabVIEW FPGA Module allowed for the rapid design, simulation, development, and testing of a tachometer signal acquisition and processing system. The hardware interfaced seamlessly with a PXI-based high-channel-count data acquisition system, adding no external hardware for processing. Performing all calculations on a PXI-7833R card meant that a PC used for vibration analysis was not additionally taxed with software-based tachometer signal processing. The number of available vibration input channels was not reduced, and sampling rates were not altered to meet the needs of tachometer signal acquisition. The system was highly scalable and re-usable. Processing algorithms were flexible enough to accept input from most common tachometer sensor types, and if more than eight channels were required additional FPGA cards could be added to the system. Customer requirements for highly flexible and accurate detection of tachometer pulses for determining the speeds of rotating equipment were met and exceeded.

For more information, contact:

Daniel Hooks

Cal-Bay Systems, Inc.

3070 Kerner Blvd, Suite B

San Rafael, CA 94901

Tel: (415) 258-9400

Fax: (415) 258-9288

E-mail: dan@calbay.com

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