INDUSTRY NEWS
normally cleaner, ground point which offers no common mode advantage. If the transducer low could be connected to ground at the system end (rarely available in PLC’s) there is the option to ground to the machine which effectively provides the required common mode rejection. However, this is against best practice and could result in large current flows in the sensor cables due to the potential differences identified with the initial problem.
One practical method utilised on sites to reduce this effect has been the installation of a galvanic isolator/ current repeater between the sensor on the machine and the connecting system. This has the effect of terminating the sensor 4-20mA circuit with a common earth and repeating the current onto the connecting system earth regime with no continued interference.
Spurious Frequency Issues Whilst the transducer is normally utilised on the main drive train bearings, other mechanical systems
on the machine (oil pumps, fans etc.) can interfere with the measurement. Fundamentally the sensor element is an accelerometer with a wide bandwidth of measurement (beyond 10kHz) and auxiliary systems running at higher speeds can generate high acceleration vibration at high frequencies (20g at 1kHz for example). These represent a very small vibration displacement (few um) but have a large impact on the transducer as the high charge output from the piezo-ceramic material causes saturation in the integration circuitry and subsequently an erroneous reading.
VSpringsAd 25/9/06 20:13 Page 1
The sensors primary purpose is to measure vibration at the machine running speed, typically in the range of 10Hz to 50Hz and to avoid spurious readings its important there are no low frequency structural movements (<3Hz) present on the machine. Since the sensor has limited filtering capabilities, a low frequency event can have a significantly large impact on the measured reading due to the inherent
integration in the device (higher gain at lower frequencies).
This type of issue is rare, but is a reminder to ensure the transducer is well mounted and best practice applied to the routing of the connecting cable.
Sensonics has recognised the need for a sensor which can meet the demands of protecting smaller machinery such as fans and pumps which can be utilised on a wide range of plant with reduced sensitivity to auxiliary systems. The VEL/GDC is an electro dynamic sensor providing a 4-20mA current sink output proportional to velocity vibration and offers the advantage of offering double isolation in conjunction with a low impedance circuit making it suitable for high noise environments. Due to the electro dynamic nature of the sensor assembly both high and low frequency events are filtered mechanically and since no integration is required the arrangement is immune to the saturation seen in piezoelectric devices.
Valley Spring is a member of the Springmaster®
Group
SPECIALIST PRODUCER OF MEDIUM AND LARGE DIAMETER COMPRESSION, EXPANSION AND TORSION SPRINGS.
Pottery Lane East, Chesterfield, Derbyshire S41 9BH Tel: +44 (0)1246 451981 Fax: +44 (0)1246 454327 E-mail:
sales@valleyspring.com www:
www.valleyspring.com
info@bellevillesprings.com
www.bellevillesprings.com March 2021
www.sosmagazine.biz 13
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
