How crucial is absolute performance of a sensor in the given application - Page 1
Forum section: electronics / projects

author: krish2487
author_rank: Frequent Contributor
timestamp: September 14, 2018, 05:28:35 pm
content: Hello everyone,I have had an interesting project at hand that I would like some more informed and knowledgeable opinions on..I am a project where I need to measure the vibration of a motor for a given purpose. Suffice it to say, the motor behaves differently under normal running conditions and abnormal conditions.I am assuming a 3 axis accelerometer and/or an IMU of the requisite ratings, mounted to the motor mount should be good enough to measure the change in the vibration pattern when running normally and under fault conditions.It is going to be mounted on a vehicle. Hence I am assuming that any vibrations generated will not be dampened entirely but will me transmitted from the motor to the point of contact of the vehicle to the ground.It is given for us to understand that the noise and vibration of the vehicle is perceptibly different by ear and touch during normal and fault conditions by the users.So far, no issues. A colleague and I differ on the need for absolute performance of the accelerometer/IMU in this application. I feel that once the motor has reached a steady state operation while running i.e left running for a certain time to allow for temperature, humidity, pressure to stabilise along with the requisite load the absolute readings of the acceleration are not as crucial as the difference between the readings when normal and fault conditions.My colleague is of the opinion that having an absolute characteristic of the sensor like noise, drift and temperature would be required in order to get an more correct picture. He thinks that the system noise itself and /or gaussian noise if any would be a more influential factor on the readings returned from the IMU/accelerometer (whatever the term gaussian noise is.. and how pertinent it is to this application) under normal running conditions and consequently fault conditions.I am of the opinion that the differential measurements returned will be necessary and sufficient for us to establish that a fault has occurred, instead of trying to model the system with changes in temperature, pressure, humidity and then arriving at the absolute numbers.I would grateful if someone with more experience in this area can pitch in and educate me and help me.Thank you

author: SeanB
author_rank: Super Contributor
timestamp: September 14, 2018, 07:21:17 pm
content: As it is a vehicle, how about the most common vibration sensor used in a vehicle, the common engine knock sensor, a piezo element with a tuned mass attached, bolted to the motor. Cheap, rugged and with quite good long term stability, and a high output voltage into the right load resistance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 14, 2018, 07:22:18 pm
content: 1. Do you /have/ to do it with an accelerometer? There are an awful lot of sources of interference on an automotive chassis...2. You need enough noise floor not to mask the symptoms, and enough dynamic range / headroom to be able to extract the symptoms from large excursions in that interference.3. Can this really not be done electrically, or with another sensor like a torque sensor, temperature sensor, or something less agnostic of what the load is? (That is, what is the load, is it known?)Tim

author: krish2487
author_rank: Frequent Contributor
timestamp: September 14, 2018, 07:48:55 pm
content: @SeanB and @teslacoilWe have to identify the failure of a pump mounted on a trailer. Any sensors we use will be eventually mounted on the pump itself.I am assuming that rules out any sensors integral to the vehicle itself.We did try some preliminary tests with a piezo sensor albeit without a tuned mass. It was not sensitive enough to pick up smaller vibrations. There is a possibility we conducted the tests wrong. We just manually vibrated the sensor. nothing particularly scientific.@teslacoil1. Not at all, we did consider using flex sensors, piezo sensors and accelerometers. It is still possible to adapt to other sensors.2. I agree, hence my question. Is it necessary and sufficient to only measure the difference in any sensor output between normal and fault conditions even if the normal running conditions are masked by the noise floor of the system, as long as the fault condition has an output large enough to be picked up by the sensor?3. We do not have sufficient data as yet to identify what the failure mode is. I suspect there should be a more straight forward way to observing the failure of the pump.As I mentioned, it is a pump mounted on a vehicle.Thank you for the help so far.. its. did shed a lot of light on what I need to understand the system better. I, clearly, need more information about the failure causes and normal running conditions before trying to identify a failure in the pump..I am still open to suggestion as to how to establish that the pump has failed. and I use the term "failed" in a very loose sense.. :-)Thank you again!!

author: Benta
author_rank: Super Contributor
timestamp: September 14, 2018, 09:41:22 pm
content: I'm with your colleague here.If you want to analyze fault conditions, you first need to quantify normal operation. This only works with absolute measurements. You need to record figures, eg, acceleration, frequency, amplitude and, and, and.When that's documented, you can go into analyzing fault behavior. Having an absolute baseline, you can consider differential vs. absolute behavior. But as you already have the absolute test setup, why not continue using it?

author: DaJMasta
author_rank: Super Contributor
timestamp: September 14, 2018, 10:00:19 pm
content: I'm not 100% sure on what your absolute performance requirement is. Are you talking about specification of the sensor itself, perhaps being calibrated against a standard when installed, or are you talking about a baseline normal operating condition's data as a control to test against?I think the relative differences versus normal operation are going to be the key to identifying a fault, but fully characterizing the system under normal operation (and installed in the chassis) will be imperative for avoiding false positives and missing symptoms of failure that are dampened or otherwise modified by having the rest of the vehicle present and operating.It honestly sounds like a data analysis problem, where you just want to capture as much high sensitivity data as possible from the fully operating system, and then use that to characterize the vibration characteristics of the failure, which in turn gives you the required sensitivity for the sensor and any sort of mechanical damping or electronic filtering that is helpful in eliminating some of the massive background noise. The characteristic of the fault may also vary a lot between different sensor locations, so it could be useful to pick several places on the motor to measure from and see whether fault signals appear more clearly at certain locations on the body of the motor.It's a tricky problem, but I'm not sure you and your colleague are disagreeing all that much. To identify the problem initially, absolute measurement is imperative, but when you know the characteristics of a fault, relative measurement is all that the system needs to be designed for.

author: SeanB
author_rank: Super Contributor
timestamp: September 15, 2018, 06:25:35 am
content: Industry standard for pumps and motors is vibration analysis, using a piezo sensor directly mounted on the motor or pump. One on the bearing housings gets bearing failure data, and one on the pump on the inlet flange will give you data as to cavitation in the pump, which I assume is your failure mode, insufficient flow or entrained air being sucked in with your fluid, or running with excessive outlet pressure. Very simple sensors, available off the shelf with single bolt fixing and just needing a small diameter machined surface and a correct threaded hole to get good mechanical coupling to the castings.The sensors give a very high output when appropriately mounted, try to mount it on a steel plate ( about 5kg, flat steel in a rectangle, about 10mm thick) then measure the output when you drop a marble onto the plate from various heights. Tap it lightly with a hammer and you probably will get over 100v into a high impedance ( 10M 10x scope probe) load, plenty enough voltage to measure.I used one with a PC sound card to record bearing vibration on an old hard drive, not too loud but did pick up the bearing noise when mounted to the drive case with the bolt, which fitted the one hole in the mounting area. There are industry standard ones, but the cheap G77 VAG sensor is probably good enough to try, and it is plenty rugged enough to use as a try. Single hole mounting and a specified torque to get repeatable performance out of them, with a 30V or so signal when used as engine knock sensor.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 15, 2018, 04:27:02 pm
content: You may wish to consider a second sensor, mounted to the vehicle close to where your motor is mounted to the vehicle. Capturing and correlating these two sources may allow you to differentiate the data that is unique to the motor you're trying to monitor. I would definitely do this at first, during development... in production the second sensor may not be necessary but you may learn a lot about how to handle the production solution by having both sets of data. Maybe the second sensor is needed in production, maybe not, but I'd start there and see what you learn.

author: krish2487
author_rank: Frequent Contributor
timestamp: September 15, 2018, 06:25:23 pm
content: @bentaIt does make sense, but it does take a lot more effort. We do have the absolute test setup. We were merely having a discussion as to how to strike a balance between speed of deployment and complexity of the task at hand.@dajmastaThe absolute specification of the sensor itself. My colleague is of the opinion that even to establish a baseline normal operating condition we cannot take the reported sensor readings. And it does make sense since the sensor itself has certain drift and noise and non linearity etc. But to spend more time characterising the sensor rather than focus on the task at hand, I feel, is diminishing returns.I am of the opinion we take the sensor readings at face value and use them to establish the baseline normal operating conditions rather than characterise and quantify the sensor in near laboratory conditions first.To put it simply, I suggested we use whatever data the sensor returns as good data. take the readings under normal operating conditions and under fault conditions. end of story.My colleague differs from me in the sense, we factor in t he sensors characteristics and tolerances, noise, accuracy etc also into the mix. then redo the steps above.@seanBThank you, that sounds a lot more like the way we have also redo our analysis and experiments. To be honest, I am not sure what is the failure mode we are trying to capture. I ll have to approach the end client on this first.I tried looking up for the G77 sensor and I found some products that look like the ones you are referring to.@IDEngineerThat makes logical sense. To have two sources of data rather than one. And yes, we are wanting to place two such sensors at two places.

author: max_torque
author_rank: Super Contributor
timestamp: September 15, 2018, 06:36:53 pm
content: If your pump runs at a fixed speed in the application, you may be able to get away with a simple amplitude based monitoring with a suitable band pass filter. if the pump runs a multiple speeds, then the usual approach would be to FFT the signal out into the frequency domain. If you know (or measure) the pump speed, then you can only look at frequency bands that you know will contain the vibration information

author: DaJMasta
author_rank: Super Contributor
timestamp: September 15, 2018, 09:45:32 pm
content: In that case, I'm with you, then. So long as the response of the sensor is linear in the range that's of concern and it is measuring the right stuff, the absolute specification wouldn't matter to me much. Can be useful if you're trying to adhere to a code or test specification, could be useful as a sanity check on your readings, could be useful when comparing your data to data from other systems, and it's probably good practice because it makes your work portable to other setups using different hardware, but it's certainly not a requirement of the task as I understand it, and I can't think of any case where the absolute spec would help you identify the fault any better unless a specification is provided about the performance of the motor under nominal conditions or for the external vibration noise in a normal situation.It's nice to have a reference so the work is portable, but depending on the scope it may be a fair amount more work and expense for an extremely unlikely use case (where you have to use a different measurement system with your current system's baseline data). If the expense and time isn't too much, or if you can do a coarse calibration of your equipment of some kind to at least get close, I'd probably elect to do it for the sake of posterity and portability, but the people doing the work usually also aren't in charge of the budget...

author: krish2487
author_rank: Frequent Contributor
timestamp: September 17, 2018, 07:20:45 am
content: @max_torqueThats an interesting idea. It definitely sounds doable. thank you! :-)@dajmastaThat is what I am thinking as well. We still have to come up with a more economical way (in terms of time and effort) to characterise the system if we decide to do an absolute measurement system.Thank you everyone for the inputs and suggestions. It definitely provided a lot more insight into the problem and how to address it. :-)