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Besides sound and vibration signals, most order analysis applications require an analog tachometer signal or a digital tachometer signal to provide a rotational reference. The most common tachometers are proximity probes and optical transducers. These transducers generate pulses at a rate proportional to the rotational speed, typically once per revolution.

A proximity probe can detect the presence of a keyway slot. The probe then generates a pulse at a certain fixed amplitude as the keyway slot passes. The following illustration shows a proximity probe working as a tachometer to generate pulses.

Optical transducers observe a piece of reflective tape attached to the shaft. The coincidence of the reflective tape and the optical transducer produces a pulse signal. The following illustration shows an optical transducer working as a tachometer to generate pulses.

Optical transducers are well-suited for machines that cannot tolerate drilled holes or milled slots in the exposed shaft surface. Optical transducers also are appropriate for detecting pulses from high-speed machines.

An encoder is another common tachometer transducer. An encoder usually generates multiple, even several hundred or more, pulses per revolution. Thus, encoders can generate more accurate speed results for low rotational speed measurements that are slower than 100 RPM.

Analog Tachometer Signals

Analog tachometer signals are tachometer signals obtained through the analog input channel of a data acquisition (DAQ) device. By synchronizing the tachometer and sound or vibration acquisition channels, you can acquire the analog tachometer signal with the same DAQ device that you use to acquire sound or vibration signals. Use the Tachometer Processing VIs to set thresholds and detect pulses in the analog tachometer signal.

Some DAQ devices might have difficulty acquiring very high rotational speed signals from tachometers or from encoders that generate hundreds of pulses per revolution. Even if you choose a DAQ device that can sustain high enough sampling rates to provide sufficient resolution for the tachometer signal, sampling the sound or vibration signals at the same high rate is not efficient because of the demands that synchronization places on the measurement and computational effort. In this case, you cannot acquire the tachometer signal with an analog measurement channel. Instead, you can avoid unnecessary computation and system resource expenditure by running the acquisition of the sound or vibration signals at a lower frequency or by using a counter device to acquire a digital tachometer signal.

Digital Tachometer Signals

A digital tachometer signal is a tachometer signal properly conditioned for acquisition from the input channel of a counter device. The counter device can detect pulses directly without using additional VIs to set a threshold for the tachometer signal. A counter device also typically operates at a much higher sampling rate than the analog input channels used to acquire sound and vibration signals. For these reasons, a counter device is ideal for acquiring tachometer signals at high speeds or tachometer signals that encoders generate. A digital tachometer signal is usually more accurate than an analog tachometer signal.

Digital tachometer signals must be transistor-transistor logic (TTL) compatible. Acquiring a digital tachometer signal requires additional devices, such as a counter or timer device and a signal conditioning device, to condition the tachometer signal for TTL compatibility.

You also can acquire a digital tachometer signal using a multi-function reconfigurable I/O device, such as the NI PXI-7831R. This kind of device allows you to use the LabVIEW FPGA Module to configure the digital lines as inputs, outputs, or counters. You also can perform tachometer signal conditioning with the device or configure a 64-bit counter.