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Reliable application of vibration transmitters

Reliable application of vibration transmitters Matching vibration transmitters to applications is not always straightforward. The experts at Sensonics outline the key points to be considered and the problems that can be encountered

Vibration transmitters based on piezoelectric technology are commonly used throughout industry for the monitoring and protection of smaller critical rotating plant, such as fans and pumps. For more expensive assets - steam and gas turbines, centrifugal pumps and compressors - rack based systems are standard as they offer the required levels of flexibility in measurement.

The majority of absolute vibration transmitters available today utilise a standard piezoelectric compression technique coupled with electronics in the sensor which converts the measured acceleration to a vibration velocity signal. The sensor acts as a current sink in the circuit loop to the PLC/DCS providing a fixed range of vibration (mm/s or ips) with a 4-20mA current loop. 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 without any intermediate electronics (unless the machine is located in a hazardous zone where safety barriers would be mandatory). Typical requirements for vibration measurement are in the frequency range of 10Hz to 1kHz with a vibration range of 20mm/s to 50mm/s rms, covering machinery running at speeds between 500rpm and 3000rpm.

However, whilst the sensor is simply mounted on the machine bearing case and connected to the current loop, there are several important areas of consideration. 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, which can lead to expensive downtime.  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 are difficult to achieve.

Typical vibration transmitters are single case devices, therefore the sensitive piezoelectric element is effectively shielded at the same potential as the machine to which the transducer is mounted. The internal sensor arrangement relies on a very high impedance circuit to extract the charge from the ceramic sensor and therefore with limited isolation to the external case, pick-up from a noisy earth is unavoidable and can appear on the output as an unstable current reading.

This is because at the connecting system end the transducer low 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.  

It is possible to electrically isolate the transducer from the machine using a special mounting stud to prevent such electrical pick up, but care must be taken as any connection on the cable over sheath to the machine will negate this effect. Worse still, the device becomes case sensitive to human interference, producing a measurement spike when touched even though the resistance path to earth is relatively high. One practical method utilised on sites to reduce this effect has been the installation of a galvanic isolator or 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.

Whilst the transducer is normally utilised on the main drive train bearings, other mechanical systems on the machine (such as an oil pump) 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 µm) 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.

This can be overcome by specifically designing the transducer with additional filtering to remove the unwanted signal and to prevent the saturation. However, only minimal attenuation can be accomplished due to the size and power constraints of the sensor configuration. One other solution is to specify a mechanical mounting filter between the machine and the sensor which can provide large attenuation close to the wanted frequency band, but usually this solution is cost prohibitive.

The sensor's primary purpose is to measure vibration at the machine running speed, typically in the range of 10Hz to 50Hz. To avoid spurious readings it is important there are no low frequency structural movements (<3Hz) present on the machine. Since the sensor has limited filtering capabilities as described above, 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.  

Having described some of the application pitfalls with the absolute vibration transmitter, sometimes the only solution is a standard accelerometer connected to a monitoring system. This method has the flexibility to select an appropriate transducer (ie double screened), the measurement type, range and filtering to optimise the vibration channel to the application.

Sensonics has recognised the need for a sensor which can meet heavy industrial demands and 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.
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