Q. What are the advantages and disadvantages of wired vibration monitoring?
A. Advantages
Data density: Vibration measurements require a high level of density in order to extract meaningful machine condition information. For most forms of spectral analysis, accelerometer data rate of 25k measurements per second is required. Wired systems easily manage the data-intense collection and processing necessary for advanced analytics.
Reliability: Most permanent vibration monitoring systems use Ethernet communication technology to transmit calculated information, which are already established and available for collection and distribution of vibration information to higher level PLC and SCADA systems. Wired systems are also less prone to lost data or data interruptions for more secure continuity of measurements.
Inherent security: Wired Ethernet offers the same full levels of security that is used in the primary industrial and business Ethernet systems. Your broadcast data is not vulnerable to interception or hacking.
Cost effective: Since wired systems use the same networking of existing systems, no secondary or redundant networks must be maintained or managed. Wired devices are based upon traditional industrial product designs intended to last the life of the machine. Once installed, there are no labor and material costs for maintaining batteries and sensors.
Disadvantages
Deployment costs: Wired systems require more upfront investment. Machine downtime can also be a factor. Routing of cables to prevent damage takes some engineering to ensure proper protection from damage and signal integrity.
Q. What are the advantages and disadvantages of wireless vibration monitoring?
A. Advantages
Convenience: The single biggest advantage of wireless systems is the ease of installation. Without wired power or communication, setup is fast and easy.
Service oriented: Most wireless systems are part of a complete hosted service. This makes getting started easy. Within minutes you can be collecting data.
Disadvantages
Battery life: Battery life is the arguably the biggest disadvantage to wireless systems. Vibration measurement calculations are power-intensive. To properly construct a RtM (Real time Monitoring) system, raw measurements should be collected and processed within seconds. Battery powered systems cannot do this. They are based on a basic average measurement of a small selection of frequency ranges with measurements made only a few times per day. This leaves big holes in any condition-based equipment evaluation.
Continuous labor and materials: Wireless systems require continuous maintenance. Batteries, and many times the complete sensor, may require replacement every 60 days for reliable data rates.
Slow response: Since typical wireless systems rely on just a few basic measurements per day, update rates of decision-making data and trends is often too late or difficult to access.
Signal integrity: Reliable signal integrity is often a challenge within manufacturing plants. Many processes make significant noise that interferes with wireless transmission. The dynamic nature of manufacturing often leads to intermittent connectivity of wireless transmission.
Q. What is crest factor?
A. The crest factor is the ratio of the peak and RMS value of vibration acceleration (a-Peak/a-RMS). The method is based on the knowledge that in the early stages of bearing damage, only small changes in the RMS of the vibration acceleration emerge. In contrast, the peak value has already risen significantly.
The crest factor is a characteristic value for the overall condition of the machine. It is a curve that has a low value at the beginning of damage and at end of life.
The following table represents the relative crest factor values.
| Condition | a-RMS | a-Peak | Crest factor a-Peak / a-RMS |
a-Peak x a-RMS |
|---|---|---|---|---|
| No damage | low | low | ~ 3 | low |
| Minimal single damage | low | increased | > 3 | slight increase |
| Several small single damages | increased | increased | > 3 | medium increase |
| Large single damage | increased | high | >> 3 | increased |
| Many large single damages | high | high | > 3 | high |
Q. What are the advantages of tracking crest factor?
A. There is no need for any speed information, bearing data, and calculation of limit values to get a statement about the overall condition of the machine. Because the crest factor is low at both the beginning and end stages, it is important that data collection is started on a known good application.
Q. How frequently are the measured values from a VV IO-Link enabled sensor updated?
A. The minimum cycle time for IO-Link of the VV is 11.6 ms. Internally, the values are updated every 2 ms based on a 500 ms window of raw vibration data. The minimum reaction time in SIO (Standard I/O) mode of the output is 4…7 ms.
Q. What is the resolution and sampling rate of the VSE inputs?
A. Analog inputs IN1 and IN2 (used as 4...20 mA)
Sensor inputs 1...4
Q. How many TCP connections to the VSE are possible?
A. A maximum of 3 clients can access to the VSE in parallel. If raw data or spectrum is requested from one of these clients, the other clients will not receive any values during this time.
Q. Why don't you offer calibration certificates for your VSA accelerometers?
A. ifm's VSA accelerometers use MEMS technology (MEMS technology explained) and unlike piezo sensors, they are stable long term. Piezo sensors have a preload force that is calibrated to match 0 acceleration. This preload can change over time and therefore, the sensors must be recalibrated, usually every one or two years. In contrast, the MEMS is an IC chip with no preload force. No calibration or recalibraion is required.