Sensors & transducers
Cost-effective monitoring and protection for smaller plant
important as the focus to improve efficiencies, reduce energy consumption and eliminate downtime has moved beyond protecting more expensive assets such as turbines and compressors. However, achieving the right balance of cost versus benefit in using more sophisticated and expensive systems for smaller machinery operating in less critical applications can be challenging, especially given the exceptionally tough circumstances facing many plant operators today. Sensonics has been working with
P
industry for nearly 40 years, developing effective and affordable monitoring and protection devices including those for smaller items of plant where the need for low-cost sensors which offer direct connection via 4-20mA current loop, is more important than ever. However, matching this type of product to the application is not always straightforward so it is important to outline the key points which need to be considered along with the problems that can occur. For example, the majority of absolute
vibration transmitters available today utilise a standard piezoelectric compression technique coupled with electronics performing a current sink in the circuit loop to the PLC / DCS. The advantage of this solution is that no other power is required for the sensor and a single cable pair can be used from the connecting system to the machine. However, whilst the sensor is simply mounted on the machine bearing case and connected to the current loop, there are pitfalls to be avoided. These include the earthing regime, low frequency vibration and high acceleration noise, which can lead to measurement error and spurious machine shutdown in protection applications, resulting in expensive downtime.
OvercOming earthing PrOblems Earthing and grounding issues are where the majority of problems in new installations occur, particularly in heavy industry where power usage is high and effective earthing regimes can be challenging. The internal sensor arrangement relies
on a very high impedance circuit to extract the charge from the ceramic sensor and with limited isolation to the external case pick-up from a noisy earth is unavoidable,
40 February 2021 Instrumentation Monthly
rotecting smaller, yet critical items of rotating plant such as industrial fans and pumps has become increasingly
appearing on the output as an unstable current reading. This is because at the connecting system end the transducer low point is usually connected through a terminating resistor to a different, 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 PLCs) 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
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 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74
