Instrumentation - Dynamic Resoluiton

When sensor signals are being converted to data the capabilities of ADC (analog to digital converter) should be considered. The dynamic resolution of an ADC is related to a number of steps the original signal is approximated with. If the ADC has 10 bit resolution, that means that the full input range will be digitized using 2^10 (two to the power of ten) or 1024 steps. For many applications it is too crude, especially if the signal is not amplified. Typically, for vibration measurements ADCs with 16 or even 24 bits of resolution are used. For industrial vibration monitoring a 16 bit ADC is quite enough. A sensor with the sensitivity of 100 mV/g and the signal range of -10/+10V will have the smallest step of 3mg at 16 bit ADC. Dynamic resolution is sometimes listed in dB in device specifications.

If more dynamic resolution is needed, the signal can be amplified, or an ADC with higher resolution can be used. There are trade-offs with either method. An analog amplifier can introduce noise while a higher resolution ADC results in a more expensive device. There is a signal processing technique that can boost dynamic resolution through oversampling.

Instrumentation - Data Acquisition

Sensor signals have to be converted to data (digitized) before they can be processed and stored. This process is called data acquisition. For vibration data the best solution is to acquire a dynamic signal, the function of acceleration over time. This function can later be processed with several algorithms to yield valuable machine condition and diagnostic data. The data acquisition process assigns a number to every level of acceleration at equal time intervals. It takes place in a data acquisition device that contains a micro processor, an ADC (analog to digital converter), necessary filters, and other components. In addition, the data acquisition device has to have means for sending the data out for further processing and storage.

There are many decisions that have to be made about the data acquisition. Here are some of the questions that are typically asked about data acquisition:

How much data has to be collected?

What is the required sampling rate?

What is the proper ADC resolution?

How the signal has to be filtered?

From the practical standpoint these selections must be made behind the scene in software and not by the user. However, many data acquisition systems assume technically savvy users who know how to use these parameters. A more user friendly system should ask a different set of questions:

What is required frequency resolution?

What is required analysis bandwidth?

What is required dynamic resolution?

Answers to these questions help set up proper data acquisition parameters and they borderline with our next topic - Data Processing and Storage.

Low Power Sensors

One important point was not covered by my previous post. As almost any sensor needs power to operate, it is becoming more important how much power each sensor uses. It could be surprising since we are talking of very low amounts of power in any case. Why would we care about this? Because power wiring of machine monitoring systems adds costs in installation and in some locations simply cannot be done.

As wireless data communications are becoming a standard, the last remaining tether is the power connection. In the field of remote machine monitoring we don't deal with a mobile application because heavy equipment does not normally travel around the plant. Even with that, there is a need for fully autonomous monitoring solutions that will not need an external power hookup. These systems will have to harvest energy from the environment or the machines they monitor.

Suddenly, the power consumption by the sensors has become important. Here again we can find that MEMS sensors require significantly lower power to operate in comparison with traditional analog sensors. The MEMS technology will be evolving to produce sensors with more features and lower power requirements in the future, but even today their benefits are obvious.

Instrumentation -- Sensors

Since we are talking specifically about machine condition monitoring, our primary interest will be in vibration sensors. Over many years the standard vibration sensor has been an accelerometer. The sensing technology in a standard accelerometer is pretty much the same today as it was thirty years ago.

In spite of its wide use the standard accelerometer is not an ideal vibration sensor. It requires a charge amplifier that is usually built into the sensor body and an analog signal conditioner, which adds to the cost. Low frequency applications are challenging because the sensor is not sensitive to static acceleration while the dynamic acceleration is very low at low frequencies. These sensors require regular recalibration in condition monitoring applications. However the main issue with traditional accelerometers is cost. An accurate, temperature stable accelerometer with a wide frequency range can cost a few hundred dollars.

The good news is that soon this is going to change. New technology is already here and is waiting to be widely adopted. I am talking about MEMS accelerometers. The MEMS technology has been widely used for sensors and MEMS accelerometers are commonly used in electronic devices as motion sensors. Industrial applications could not benefit from the MEMS technology until last year when Analog Devices has released the ADXL001 accelerometer. Designed specifically for industrial applications, this accelerometer features a wide frequency range, high temperature stability, excellent low frequency characteristics, and does not need recalibration.

Sounds like a perfect vibration sensor? Well, not so fast. There is a gap between the sensor's microchip form factor and the industry's requirement for robustness. This sensor needs an appropriate packaging, in which the sensor will be sealed from the environment and have means to be mounted to a machine. InCheck Technologies is working on a version that a user will be able to permanently mount into a small hole in a bearing housing. The package will also include a temperature sensor microchip, forming a dual sensor.

Speaking about temperature sensors, MEMS sensors can compete with traditional thermocouples and RTD's in the -40°C to +125°C temperature range. Unlike RTD's MEMS temperature sensors produce linear output and unlike thermocouples they do not require external electronics.

Did I mention the main advantage of the MEMS sensors? Yes, they are pretty inexpensive. The ADXL001 is still a little pricey, but it already beats the traditional technology hands down. And since the MEMS output is compatible with standard ADC input, no addition analog electronic components are necessary. With machine monitoring applications the low cost will be the key to wide adoption. The future apparently is with the MEMS technology.

Machine Health Monitoring

Assuming now that we are all on board for predictive maintenance, what do we do next? Our goal is to prevent failures, so, obviously, we need to monitor our machines. This could be done just by inspection (listening to noise, touch for temperature and vibration, visual inspection) or by collecting data. Since the inspection by perception is biased and hard to document, I personally prefer data. Specifically, if we are talking about rotating machines, we need vibration data. With proper processing the vibration data can help diagnose almost any mechanical problem, especially when the data is tracked over time. Other parameters can be monitored as necessary. Temperature, pressure, electrical parameters, etc. - all can yield valuable machine health data.

We will talk about manual "walk around" data collection on a separate occasion. For now we assume a permanent installation. Whether we collect data manually or install permanent sensors, the monitoring system has to have a few layers between Machines and Users that look like this:

Machines --> Instrumentation --> Communication --> Processing and Storage --> Presentation --> Users

Instrumentation layer consists of sensors and data acquisition hardware. To minimize cost of installation the data acquisition hardware should be installed close to the machines. Long sensor wires are not only expensive and hard to install, they sometimes cause deterioration of the analog signal and loss of accuracy. The instrumentation layer outputs data in digital format.

Communication layer transmits the data to a server where it is processed and stored. Today it would be a shame if we ignored standard network infrastructure that makes this transmission easy. Costs are drastically reduced by using standard off the shelf networking components. Standard Ethernet (wired or wireless) and the Internet are the best choice for the communication layer.

Processing and Storage layer is typically a dedicated server computer that runs a few key pieces of software - communication server, digital signal processor, and database. It also contains an alert generator that informs users if something goes wrong.

Presentation layer is a web portal where user can login to monitor the data and control the system. The presentation layer sends to user's browser a program that shows the data in graphic format, gives interactive access to stored data and to all machines regardless of their location. The web portal can be accessed from anywhere with a secure login.

Clearly, the above system architecture is somewhat simplified, but for the purposes of this discussion it is fine. The point we are trying to make here is that today's technology provides tools to make a very inexpensive system that can monitor machine health condition with high accuracy.

Next time we will talk about sensors.