Page 24 - Industrial Technology magazine February 2021 issue

SYSTEMS INTEGRATION

PLANT MONITORING

SENSORS AND SYSTEMS

THE EXPERTS AT SENSONICS EXPLORE COST-EFFECTIVE
MONITORING AND PROTECTION FOR SMALLER PLANT

rotecting smaller, yet critical items of rotating Earthing problems
plant such as industrial fans and pumps has Earthing and grounding issues
become increasingly important as the focus to are where the majority of
Pimprove efficiencies, reduce energy consumption problems in new installations
and eliminate downtime has moved beyond protecting occur, particularly in heavy
more expensive assets such as turbines and compressors. industry where power usage is
However, achieving the right balance of cost versus benefit high and effective earthing
in using more sophisticated and expensive systems for regimes can be challenging.
smaller machinery operating in less critical applications The internal sensor
can be challenging, especially given the exceptionally arrangement relies on a very high
tough circumstances facing many plant operators today. impedance circuit to extract the
Sensonics has been working with industry for nearly charge from the ceramic sensor
40 years, developing effective and affordable monitoring and with limited isolation to the
and protection devices including those for smaller items of external case pick-up from a
plant where the need for low-cost sensors which offer noisy earth is unavoidable,
direct connection via 4-20mA current loop, is more appearing on the output as an
important than ever. However, matching this type of unstable current reading. This is
product to the application is not always straightforward so because at the connecting
it’s important to outline the key points which need to be system end the transducer low point is usually connected Fundamentally the sensor element is an accelerometer
considered along with the problems that can occur. through a terminating resistor to a different, normally with a wide bandwidth of measurement (beyond 10kHz)
For example, the majority of absolute vibration and auxiliary systems running at higher speeds can
transmitters available today utilise a standard generate high acceleration vibration at high frequencies
piezoelectric compression technique coupled with (20g at 1kHz for example). These represent a very small
electronics performing a current sink in the circuit loop vibration displacement (a few µm) but have a large impact
to the PLC/DCS. The advantage of this solution is that on the transducer as the high charge output from the
no other power is required piezo-ceramic material causes saturation in the
for the sensor and a single integration circuitry and subsequently an erroneous
cable pair can be used from reading.
the connecting system to the The sensor’s primary purpose is to measure
machine. However, whilst the vibration at the machine running speed,
sensor is simply mounted on the typically in the range of 10Hz to 50Hz and to
machine bearing case and connected avoid spurious readings its important there are no
to the current loop, there are pitfalls to be low frequency structural movements (<3Hz) present on
avoided. These include the earthing regime, low cleaner, ground point which the machine. Since the sensor has limited filtering
frequency vibration and high acceleration noise, which offers no common mode advantage. capabilities, a low frequency event can have a significantly
can lead to measurement error and spurious machine If the transducer low could be connected to ground at the large impact on the measured reading due to the inherent
shutdown in protection applications, resulting in expensive system end (rarely available in PLC’s) there is the option integration in the device (higher gain at lower
downtime. to ground to the machine which effectively provides the frequencies).
required common mode rejection. However, this is This type of issue is rare, but is a reminder to ensure
against best practice and could result in large the transducer is well mounted and best practice applied
current flows in the sensor cables due to the to the routing of the connecting cable.
potential differences identified with the Sensonics has recognised the need for a sensor which
initial problem. can meet the demands of protecting smaller machinery
One practical method utilised on such as fans and pumps which can be utilised on a wide
sites to reduce this effect has been the range of plant with reduced sensitivity to auxiliary
installation of a galvanic isolator/current systems. The VEL/GDC is an electro dynamic sensor
repeater between the sensor on the machine providing a 4-20mA current sink output proportional to
and the connecting system. This has the velocity vibration and offers the advantage of offering
effect of terminating the sensor 4-20mA double isolation in conjunction with a low impedance
circuit with a common earth and repeating circuit making it suitable for high noise environments.
the current onto the connecting system earth Due to the electro dynamic nature of the sensor assembly
regime with no continued interference. both high and low frequency events are filtered
Whilst the transducer is normally utilised mechanically and since no integration is required the
on the main drive train bearings, other arrangement is immune to the saturation seen in
mechanical systems on the machine (oil pumps, piezoelectric devices.
fans, etc) can interfere with the measurement. MORE INFORMATION: www.sensonics.co.uk

24 INDUSTRIAL TECHNOLOGY • January/February 2021