Cost-effective monitoring and protection for smaller plant

SENSONICS HAS developed effective and affordable monitoring and protection devices for smaller items of plant where there is a need for low-cost sensors which offer direct connection via 4-20mA current loop. But, matching this type of product to the application is not always straightforward.

For example, most absolute vibration transmitters currently available utilise a standard piezoelectric compression technique coupled with electronics performing a current sink in the circuit loop to the PLC / DCS. 

Nevertheless, there are pitfalls to be avoided with this solution. 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.

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. 

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, 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. 

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.                                             

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). 

Sensonics recognised this and developed a sensor which can meet the demands of protecting smaller machinery such as fans and pumps. 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.     

Russell King is managing director of Sensonics 

For more information: 

www.sensonics.co.uk

Tel: 01442 876833