What to consider when deciding between battery operated vibration sensors and continuous vibration monitoring.

Battery-operated wireless vibration sensor or single wire daisy chain TriVibe (semi-wireless solution)?

Machine Saver is a Houston Texas USA based machine condition monitoring company based in Houston Texas USA that specializes in advanced remote monitoring of machine condition.

Machine Saver has completed extensive research on battery technologies available today and this paper discusses the pros and cons of a battery-operated wireless vibration solution versus a semi-wired continuously powered vibration solution and the technology advances therein for each.

I wish my cell phone could last two or three years like the wireless battery-operated vibration sensor manufacturers say their sensors will last before it is time to swap out or charge the batteries.

The same is true for all battery-operated vibration sensor devices; the more you use them the quicker the battery fails.

Battery compromises:

For battery-operated wireless vibration sensors to operate for years there are sacrifices that must be made. (1) The battery will be on briefly and most of the time the sensor is in sleep mode. (2) The samples per second must be a consideration for battery life. More samples per second = less battery life. Less samples per second means less resolution in the vibration data. Less resolution of the data or poor data translates into missed diagnosis and machine damage or catastrophic failures. (3) The length of the sample time is typically shortened to save the battery. This means less resolution for slower speed machines which require longer sample times to get more revolutions of the machine in the sample taken. (4) Battery-operated sensors typically have a ‘one size fits all’ solution in the sensing element (5) Connectivity is often a major issue.

We have come across many first-time users that were excited by batteries due to the simple installation only to end up disappointed when the battery maintenance becomes an issue or more importantly, they sacrifice performance and resolution for up front installation simplicity.

One type of wireless protocol is the ZigBee format the wireless vibration sensors may communicate through each other to reach the receiver or data collector.

The problem with ZigBee format is the sensors (blue) closest to the receiver or data collector are on and active more than the sensors (yellow) farther away from the receiver (red) so they have the batteries fail first. The blue node’s battery fails first as they work harder and are ON longer.

Labor and manual effort:

Another type of wireless solution is Bluetooth. For Bluetooth wireless vibration sensors, we still put personnel in haps way going on routes to periodically collect the vibration data. If you are sending personnel on routes you are missing the purpose of an automated system.

Extreme temperatures:

Another problem are extreme cold or warm temperatures are very hard on battery life.

A typical motor bearing operating temperature is +60C to +71C (Per SKF Evolution article Feb. 15, 1998)

Motor bearing temperature may vary based on model, load, bearing type and speed of course but the fact remains that machines and processes tend to run hot or warm and +70C is very common in many machine and process applications. Most battery-operated vibration sensors are limited on the high (+70C) and low end (-20C) of the temperature use specification because the batteries fail quickly under hot and cold environments. Buyers should be aware that when they advertise 2 or three years of life, they are not saying it will last two or three years at +70C operating temperature.

The Machine Saver OPERATIONAL temperature is +105C continuous.

As you use these wireless battery-operated vibration sensors in hotter machine areas such as the bearings the batteries will fail quicker. If you do not mount the sensor on or close to the bearing you will not get the best vibration data.

Number of axis monitored:

Many battery-operated vibration sensors are single axis. Many modes of failure may occur on the axial plane of the machine first. Vibration does not occur in a single plane. Vibration is orbital as all machines move in the X, Y and Z axis. If you use a single axis vibration sensor you only see 1/3 of the machine vibration data. With the TriVibe we see all 360 degrees of machine movement. Nothing is missed.

Many problems may start in the axial plane such as pump cavitation and then progress to the radial X and Y planes as the impeller is damaged. Another example could be an overhung fan blade issue may often start in the axial (Z) plane.

All battery-operated systems must make compromises to make the battery last for years.

(1)   Periodic versus continuous - They do not continuously monitor. All battery-operated vibration sensor systems wake up take a dynamic vibration snapshot and then go back to sleep mode to save the battery. This sleep time is when you get the “bump in the night.” When the wireless vibration sensor is in sleep mode many damaging things can happen in the off time while the battery is sleeping on the job.. A pump could run dry, a pump could run in cavitation due to over or under fluid pressure. A compressor could be ruined by a slug of liquid, ice could build up outside on fan blades, a metal cover could come loose or a machine footing could break loose or fan blades could rub a fan shroud.

Machine Saver TriVibe sensors never sleep. They monitor your machines 24 hours a day.

Why purchase a vibration monitoring system that sleeps on the job?

(2)   Sample time - When battery-operated sensors collect the dynamic data in order to get more resolution, they must take a longer sample time example 1 second vs. 250 milliseconds. This shorter sample time will decrease the ability to get excellent resolution of the vibration data on the machine.

Machine Saver Trivibe default is 1 second of data capture but we can adjust that snapshot dynamic data capture to get 10 seconds or more of dynamic data for slower speed machines. This is important to get good accurate data on slower rotating machines. You want to get many rotations of the machine in each sample to get the best data.

Frequency resolution - Battery-operated sensors typically have poor bin frequency resolution.

For Machine Saver we can see the resolution of ? Hz but many battery-operated sensors have a resolution of 1Hz or 5Hz minimum. This resolution is important on bearing analysis for example.

Often the sampling rate (samples per second) of the battery-operated vibration sensor is reduced to save on the battery. In the case of the TRIVIBE our dynamic data may be sampled at over 50,000 samples per second. No sacrificed data!

(3)    Monitor or protection - Machine Saver TriVibe is a monitoring system AND a machine protection system. Because we never sleep any alarm instantly notifies the operator of a change in the overall vibration. The Trivibe system can be wired back to the PLC or the DCS for machine protection. This is an important differentiation. Battery-operated vibration sensors are only periodic monitoring device. The machine Saver TriVibe is a continuous machine monitor as well as a machine protection system. Protection of the machine refers to the option to automatically alarm and control the shutdown of the machine.

(4)   Wired versus semi-wired - In reality every machine has wires going to it to power it anyway so the allure of a true wireless system is limited. Machines always have power. Machine are also typically grouped. You have a group of fans in an area such as a group of cooling tower cells or a group of motors or perhaps several compressors in an area.

Machine Saver wires locally on a group of machines and then wireless via WI-FI or a cellular connection. In this way we never have battery maintenance issues, never sleep and no lost data because even if we temporarily loose a connection due to a cell tower loss for a bit or something blocks the WI-FI in the plant our MachineGate will buffer the data from all the TriVibe vibration sensors connected to it and then upload all the vibration data when the connection comes back.

The battery-operated vibration device loses the data in the event of a loss of connection

(5)   Maintenance - If you only had 100 vibration sensors in your plant you would end up replacing two batteries a week in short order. The blue nodes in the ZigBee mesh, near the ZigBee controller, may be replaced more often. Do you not already have enough daily task to do? The Trivibe system is maintenance free

(6)   Where does the system intelligence reside? Machine Saver alerts you instantly by phone text or e-mail. Battery-operated systems smarts are usually in the software and not the sensors. Going to look at software daily to see what happened last is not an efficient use of your time. Machine Saver monitors by exception. We watch the vibration trends for you and alert you when there is an issue. The sensor itself is intelligent with alarms, high pass or low pass filter capabilities and memory built into the sensor.

(7)   Connectivity - Notice the spec of battery-operated wi-Fi vibration sensors often refers to “line of site.”

Many times, the best place to mount the vibration sensor is not a good place to receive a WI-FI signal. The bearings and rotating parts often have a cowling or a metal cover to protect people for safety issues. Personnel safety - Machine Saver TriVibe keeps users safe as our customers are not out in harm’s way replacing batteries. Another example of a difficult place for WI-FI is on the gearbox inside of a cooling tower between the motor and the gearbox. Good luck changing that battery!

Best sensor mounting location - Machine Saver Trivibe best sensor location on the machine bearing is never compromised by “line of site” issues.

(8)   Functionality - The Trivibe sensor also has unique functions because we are not limited in scope due to battery preservation. One such unique analysis tool is mechanical looseness or impact monitoring for reciprocating applications.

Wi-Fi may also be susceptible to interference from other sources. Battery-operated vibration sensors often have connectivity issues because of changes in a plant. Some erects a scaffold, or a crane comes in and blocks the “line of site”

(9)   Value - If you purchase a Machine Saver system each sensor is on and is working 8,760 hours per year (every minute of every day)

If you purchase a battery-operated wi-fi sensor it is on and working less than <66 hours per year.

In year one you are paying about $5 per hour of data for a battery-operated Wi-Fi sensor versus 5 cents for an hour of data from a Trivibe sensor.

(10) Smarts - The Machine Saver Trivibe sensor has intelligence. For most battery-operated WI-FI vibration sensors the intelligence is back end at the software. Having the intelligence in the sensor allows us to have many special features in our sensing capability that are impossible in a battery-operated sensor such as true machine protection and the ability to be integrated into the PLC for comparison with process.

Because the Trivibe is a MODBUS RS485 system we may also integrate other MODBUS type process sensors on our data bus such as pressure or temperature.

(11) Consider different machine speeds - Another VERY IMPORTANT point is the Trivibe sensor has many different low frequency and high frequency internal sensors integrated into a single TriVibe unit. For slower speed machines you need a high output vibration sensor with a very low g range to get great resolution on slow machines. For the higher speed machines, you will need a sensor with a higher g range and lower output. All machines are not equal and it is not one size fits all applications (except for the Trivibe).

IMPORTANT – Most battery-operated Wi-Fi sensors have a high G range (>20g) so it will work OK for the faster machines like a gearbox but on slower speed machines such as a fan there is no way it will provide an acceptable resolution.

EXAMPLE a fan at 300 RPM has a ROUGH vibration level at 0.03g. This is because on slower speed machines there is very little acceleration energy present (acceleration energy increases with speed).

A ROUGH vibration level is 0.03 g on the 300 RPM fan at the point you want to turn it off and inspect before a machine failure.

The Machine Saver Trivibe robust signal to noise ratio on the low frequency internal vibration sensor will easily detect early machine issues on low speed fan machines.

(12) Calibration – periodically most vibration sensors must have the calibration verified. Usually it is on an annual basis but that depends on the criticality of the machines or the criticality of the process. Traditionally a portable shaker with a reference “gold standard” vibration sensor is used to verify the accuracy of the installed vibration sensors. Sine the Trivibe has multiple independent sensors mounted on the same axis the TriVibe accuracy is available 24/7 and no manual effort is required to periodically verify the calibration of the sensors. The battery-operated WI-FI vibration sensors should be verified for accuracy periodically.

“You would not hire people that sleep on the job so do not hire a machine monitoring system that is usually sleeping on the job.”

Machine Saver is USA design and built 100% in Houston Texas. Find out more at [email protected] 1(832)581-9908