Data acquisition
PHASE MATCHING IN BETWEEN ISOLATED CONDITION MONITORING CHANNELS
conditions, ensuring the safety, reliability, and performance of assets and machinery. One of the main parameters being monitored is vibration, where the signal’s amplitude, frequency, and phase contain an abundant amount of information on an asset’s condition.
C ARCHITECTURE
CM systems consist of multiple sensors or transducers. Many systems use the centralised system architecture, each connected to a separate channel or input on a DAQ solution through analogue cables.
The sensors can be of various types, measuring parameters such as vibration, sound, and current. They can gather data across multiple points and axes on a single asset, or even simultaneously across separate assets. The data from these channels are then processed to provide valuable insights into the system’s behavior, such as predicting machine failures before they happen or anticipate maintenance needs before they become urgent.
USE CASES
Using a Multichannel Simultaneous Sampling ADC
The first use case is when a CM system uses phase analysis from two or more orthogonal sensors to monitor machine operation and anomalies, such as imbalance, misalignment, and loose footing. Instead of using a tachometer, the phase of one of the sensors can be used as the reference in determining the fault location. In multi-axis sensing, postprocessing of time and frequency information relies on preserving a relatively constant time delay between signal capture. This translates into requirements of synchronised simultaneous sampling and channel-to-channel phase matching to preserve magnitude and phase (time) domain information for high performance. Otherwise, this would lead to less precise measurement of the phase angle between the sensors. Vendors of CM systems have phase matching specifications that go as low as 1° at 20 kHz, even including the delay and jitter from isolation circuitry. For achieving this, using multichannel
simultaneous sampling sigma-delta (Σ-Δ) ADCs, such as ADI’s AD7768-4 or AD4134 can be convenient. See Table 1. Sigma-delta ADCs are preferred in CM applications over successive approximation register (SAR) ADCs due to their
20 Figure 1. A centralised DAQ architecture.
higher DC-to-100 kHz resolution and advanced filtering suitable for time and frequency domain analysis of vibration signals. For more information on this, refer to the article “Condition Monitoring System Design Choices and Their Impact on Signal Chain Implementation.” However, phase mismatch errors can come from the signal chain when using discrete-time sigma-delta (DTSD) ADCs like the AD7768-4. Due to the inherent lack of alias rejection of DTSD ADCs at multiples of its sampling
frequency, the system can be vulnerable to out- of-band interferences, which may corrupt the CM signals of interest. To improve rejection at these frequencies, its ADC driver stage is designed with an analogue antialiasing filter (Figure 3), typically of third order or higher, while preserving minimal in-band magnitude error. For example, a second-order Butterworth filter designed for –80 dB rejection at 16 MHz (sampling frequency) and f3dB of 160 kHz (input bandwidth) could have a phase mismatch of
TABLE 1. PHASE MATCHING PERFORMANCE AND PHASE CALIBRATION RESOLUTION ACROSS VARIOUS ADCS
AD7768/AD7768-4
Channel-to-channel phase matching at 20 kHz (max)
Phase calibration resolution at 20 kHz
Not measured 0.88° AD4134 0.024° 0.3° September 2024 Instrumentation Monthly
ondition monitoring (CM) systems play a crucial role in various industries, from manufacturing and aerospace to healthcare and infrastructure. These systems help detect and analyse various
Condition monitoring systems that are capable of simultaneously capturing data across multiple sensors typically use a channel-to-channel isolated solution to eliminate ground loops. While board-level discrete signal chains are faced with high channel-to-channel phase mismatch errors due to component tolerances,
Analog Devices’ precision signal chain µModule solutions achieve minimal phase mismatch with the use of ADI’s integrated passives (iPassives) technology. This article from Malcolm Leeland Kwok, product applications engineer at Analog Devices, examines the importance of accurate phase measurement in a CM system and the challenges it faces in data acquisition (DAQ) involving multiple simultaneous sampling channels. A variety of traditional solutions are discussed, while introducing an innovative approach to set a new level of phase matching performance.
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