FEATURE TEST & MEASUREMENT
OSCILLATING BEAM RIG HELPS TEST LOW FREQUENCY SENSORS
The calibration of vibration sensors at frequencies below 2Hz requires specialist equipment, as that frequency range is too low for most test shakers. Andy Anthony, managing director of Monitran, describes the development of a linear beam oscillator that can generate oscillations at frequencies as low as 0.2H
A
ccelerometers and velocity sensors are used to measure vibration at a
wide range of frequencies, including the low frequencies found in building maintenance. These can occur at less than 2Hz, and are useful in monitoring bearing wear in cooling tower fans and gearboxes for example. To characterise the performance of its
low frequency sensors in that range, Monitran has developed a new high displacement test rig that can generate oscillations accurately at frequencies as low as 0.2Hz. The system allows real- time back to back testing between different devices and its own in-built, calibrated MEMS sensors. The horizontal linear beam oscillator has a powerful motion control system that incorporates a precision AC servo with 17-bit encoder feedback. Running on two precision rails, it uses magnetic springs and magnetic damping to ensure smooth operation. Its software produces fast code, at 1.7 milliradians per step, which generates fine sinusoidal motion for the linear track driven by a rotary motor. The intelligent shuttle, which can carry a sensor payload of up to 0.5kg, incorporates seven MEMS accelerometers, signal processing and a microcontroller to monitor its motion. It also includes a mechanical low pass filter and has an operating distance of up to 1000mm, exerting up to 8g acceleration to the devices under test. That amounts to over 35km/h peak velocity, down to nearly 1/20th of walking speed. Some of Monitran’s ranges of
accelerometers that operate at these low frequencies are the MTN/7000 series. Their piezoresistive sensor elements operate from 0Hz to a few hundred Hz, depending upon the chosen operating range. Their low frequency response allows them to be used to measure very slow vibrations in massive structures, such as buildings adjacent to construction work that involves heavy duty machinery or tunnel boring machines for example.
22 SEPTEMBER 2017 | ELECTRONICS The units are sealed to IP65 and have
an integral stainless steel over-braided cable. They are suitable for laboratory and light industrial environments, such as research and development, test rigs, slow rotating machines and structural monitoring.
CALIBRATION AT OTHER FREQUENCIES Calibration is also vital for sensors operating at other frequencies, and the most commonly occurring vibration frequency of calibrators is 159.2Hz, which is equivalent to a radian frequency of 1000rad/s. The vibration displacement, velocity and acceleration of sinusoidal signals are connected with each other through the radian frequency. Advantageously, at 1000 rad/s the numerical values of the amplitudes for all three vibration quantities are the same. For example, a vibration acceleration of 10m/s² at 159.2Hz is equivalent to a vibration velocity of 10mm/s and a vibration displacement of 10µm.
Image 1:
Monitran has developed a test rig that can oscillate accurately at frequencies as low as 0.2Hz
All Monitran sensors are individually tested and shipped with a signed and dated test certificate. Calibration is conducted using a ‘back-to-back’ reference accelerometer excited by a precision frequency-controlled power amplifier, thus giving a traceable performance to UK National Standards. The sensitivity in mV/g for accelerometers and the 4-20mA range for velocity and acceleration transducers are confirmed and reported at 80Hz and 20ºC. Experience has shown that as long as the sensors are not misused their sensitivity will remain stable for many years. The result is that customers can rely on trending results from their equipment long after installation with little or no maintenance. This reliability is particularly relevant where the sensors are installed in remote areas or in harsh environmental conditions. However, for portable equipment everyday handling can adversely affect their performance, and users of the company’s portable vibration meters such as the MTN/VM220 are therefore recommended to recalibrate them annually to ensure the long-term reliability of results. Some applications, for example
scientific and R&D work, require sensors to be maintained in calibration. In such cases Monitran can offer a formal recalibration service. Alternatively, for users who have large numbers of accelerometers to maintain and certify, the company’s VC20 portable vibration calibrator can be an invaluable tool. The VC20 provides fast, easy
Figure 2:
Monitran’s VC20 provides fast, easy calibration of vibration transducers for acceleration, velocity and displacement
calibration of vibration transducers for acceleration, velocity and displacement. With a quartz controlled vibration frequency of 159.15Hz, the rugged battery-powered unit provides a load- independent vibration level for test objects up to 600 grams and has a vibration magnitude of 10m/s².
Monitran
www.monitran.com T: 01494 816569
/ ELECTRONICS
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