In the rotating machinery field, vibration measurement plays a crucial role in machine condition monitoring. The level and frequency of machine vibration can help operators recognize potential problems and safely shut down the rotating equipment before major damage occurs. Typically, vibration is measured using a system of integrated circuit piezoelectric (ICP®) accelerometers, an accelerometer interface module, and a rack mounted analyzer. Although these types of systems have been successfully used for quite some time and produce very accurate measurements, ICP accelerometers are susceptible to producing overload signals when the natural frequency of their piezoelectric crystal is excited (approx. 18 kHz). This can occur during transient operation or other non-optimal operating conditions (such as a pump running in cavitation). The result is an unreadable vibration signal, which may force the operator to shut the machine down prematurely.
To overcome this problem, Cryodynamics and PCB have jointly developed a new generation of ICP accelerometer that is resistant to excitation of its natural frequency. This was done by decreasing the overall sensitivity of the accelerometer as well as adding an internal filter between the accelerometer crystal and its internal amplifier circuit. The result of this design is accurate, stable vibration measurements at all possible running conditions.
Previously, Cryodynamics supplied its customers with eddy current probes as the sensors used to monitor rotational speed of all hydraulic liquid expanders. These probes operate by generating a magnetic field through an active coil in the probe tip, which in turn causes an eddy current and induced magnetic field in the probe target. As the gap between the probe tip and target changes, the impedance of the active circuit also changes, which can then be measured and used to determine the distance between the two objects. This active sensor is good for both static measurements and dynamic changes in distance.
When used to measure rotational speed, a cylindrical target with a specified number of notches is mounted to the rotating shaft. As each notch passes the probe, a change in impedance is measured. The frequency of these changes is directly proportional to the rotational speed of the shaft and the number of notches used.
Eddy current probes have been successfully used in some speed measurement systems; however, the current system has proven to be somewhat problematic in field installations. The sensitive nature of the impedance measurement, coupled with shielding problems from the instrumentation feedthrus, make this system susceptible to radio-frequency (RF) interference. Erroneous readings and faulty probes have often caused disruptions in the proper operation of expanders at various customer sites.
The frequency of problems arising from different sites because of instrumentation failures prompted the investigation into new technologies for measuring rotational speed.
The instrument chosen to replace the eddy current probe type sensors is the variable reluctance proximity sensor. Unlike the eddy current probes, the variable reluctance sensor is a passive device that does not require an external power source. The sensor is composed of a winding wrapped around a ferromagnetic substance, referred to as a “pole piece.” A natural magnet is placed behind the pole piece, generating a magnetic field that extends beyond the tip of the sensor. Changes to this external magnetic field, caused by introducing and removing another ferromagnetic material, result in the generation of an electric current within the winding which can be measured as an AC voltage. When placed near a notched target for rotational speed measurements, the frequency of signal pulses is proportional to the speed of rotation.
The theory behind these magnetic sensors precludes them from being used as static position monitors since they require changes in the relative gap between sensor and target to produce a signal. However, the alternating current pulse generated is much easier to measure and is much less susceptible to outside interference. The measurement gap for the magnetic probes is also much larger than that of the eddy current probes. The variable reluctance sensors can operate with a gap up to 0.100 inch, depending on the speed measured, compared to the eddy current’s 0.050 inch, making the installation requirements much less painstaking. Since the application of a speed sensor does not require the probes to be used for static measurements, and becasue of the benefits of improved signal integrity inherent in these passive devices, the variable reluctance proximity sensors were chosen for further investigation and testing in simulated field conditions.
To further simplify field installations of the new probe, a signal preamplifier was developed. This device is placed in the junction box and converts the probe output signal to both 4-20 mA and five-volt pulse output. These signals can then be connected directly to the DCS system for speed monitoring.