FEATURE Sensors
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MEMS-based solution for vibration
detection in condition monitoring By Thomas Brand, Field Applications Engineer, Analog Devices
C
ondition monitoring is one of today’s core challenges in systems that include motors, generators and gears. Planned
maintenance is becoming increasingly important for minimising risk of production downtime, not only in the industrial sector but anywhere machines are found. Among other tasks, machine vibration patterns are analysed to achieve this.
Gearbox vibrations are usually analysed in the frequency domain as multiples of shaft speeds. Irregularities in these frequencies point to wear, imbalance or loose parts. MEMS (micro-electro- mechanical system) type accelerometers are often used for measuring frequency. Compared with piezoelectric sensors, they feature higher resolution, excellent drift and sensitivity characteristics, and better signal-to-noise ratio (SNR). They also enable detection of low-frequency vibrations, close to DC. Here we discuss a highly-linear, low- noise, wideband vibration measurement solution based on the ADXL1002 MEMS accelerometer. The solution can be used to analyse bearings or for engine monitoring and in applications where a large dynamic range (to ±50g) and a frequency response from DC to 11kHz are required. Figure 1 shows an example circuit. The analogue output signal from the ADXL1002 is fed via a 2-pole RC fi lter to the successive approximation register (SAR) analogue-to-digital converter (ADC) AD4000, which converts this analogue signal to a digital value for further signal processing. The ADXL1002 is a high-frequency, single-axis MEMS accelerometer that provides an output signal passband extending beyond the resonant frequency range of the sensor. This is desired so that frequencies outside the 3dB bandwidth can be also observed. To accommodate this, the output amplifi er of the ADXL1002 supports a small signal bandwidth of 70kHz. Capacitive loads to 100pF can also be directly driven with the output amplifi er of the ADXL1002. For loads above 100pF, a
18 June 2023 | Automation
Figure 1: Example circuit for the ADXL1002
series resistor ≥ 8kΩ should be used. The external fi lter at the output of
the ADXL1002 is required to eliminate aliasing noise from the output amplifi er and other internal noise components of the ADXL1002 that arise, for example, through coupling of the internal 200kHz clock signal.
In the setup of Figure 1, where R1 = 16kΩ, C1 = 300pF, R2 = 32kΩ and C2 = 300pF, attenuation of about 84dB is achieved at 200kHz. Also, the selected ADC sampling rate should be higher than the amplifi er bandwidth (for example, 32kHz). For the ADC, the ADXL1002 supply voltage should be selected for its reference because the output amplifi er has a ratiometric relationship with the supply voltage. In this case, the voltage supply tolerance and the voltage temperature coeffi cient (which are usually connected to external regulators) run between the accelerometer and the ADC so that the implicit error associated with the supply and reference voltages is cancelled out.
Frequency response The frequency response of the accelerometer is the system’s most important characteristic. The gain increases at frequencies above 2-3kHz. The resonant frequency (11kHz), yields a peak value for the gain of about 12dB (factor of 4). To display measuring range overshoots (over-range), the ADXL1002 has a
Figure 2: Frequency response of the ADXL1002
corresponding output (OR pin). The integrated monitor signals a warning when a signifi cant over-range event occurs.
Mechanical considerations It’s important to correctly place the accelerometer. It should be mounted close to a rigid mounting point on the board to avoid any vibrations on the circuit board itself and any measurement errors due to undamped circuit board vibrations. The placement ensures that every circuit board vibration on the accelerometer lies above the mechanical sensor resonant frequency, making it practically invisible to the accelerometer. Multiple mounting points close to the sensor and a thicker board also contribute to lowering the impact of system resonance on sensor performance.
CONTACT:
Analog Devices
www.analog.com
automationmagazine.co.uk
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