Is the Metrix Digital Proximity System, the MX2033 Driver or MX2034 Transmitter, as good as an analog driver or transmitter from our competitors?
This question is complex in that it has two answers, one regarding amplitude resolution and the other regarding frequency resolution. We will first discuss amplitude resolution.
Q. What is the amplitude resolution of the Metrix DPS MX2033/MX2034?
A. 0.012 mils (0.47 μm) The DPS driver / transmitter uses 16-bit resolution. Due to noise we remove 2 bits of resolution, leaving 14 bits between 1 volt and 19.0 volts. This gives us 0.012 mils/division of resolution (2^14 = 16384 divisions of resolution. 18.0 volts for 90 mils = 200 mV/mil. 18 V / 16384 divisions = 0.0011 V/div = 1.1 mV/div, which equates to a resolution of 0.012 mils/division (0.012 mils/division = 1.1 mV/div / 200 mV/mil).
Q. What is the resolution of an analog device?
A. Usually, most literature states "the proximity probe is highly accurate," however, what does that mean? One must understand that a proximity probe uses a high frequency radio carrier wave (~ 1MHz) that is modulated by the change in gap voltage. The modulated frequency is filtered from the carrier radio wave frequency, and this becomes the gap and vibration signal. This modulation and demodulation within the analog processing circuit has limits. This results in an amplitude resolution, or accuracy, of 0.1 mils for a typical 5 or 8mm analog proximity system (see Orbit Magazine Article "Bently Nevada™ dual Probe versus Shaft Rider" February 12th, 2015. The Metrix DPS can resolve 0.012 mils - it is almost 10X more accurate than an analog system.
Q. What is the frequency resolution of the DPS driver (MX2033), and buffered output of the DPS transmitter (MX2034)?
A. The DPS samples at a rate of 18kHz. That means that a 1 Hz (1 cycle per second) vibration signal will be sampled 18,000 times, during that second. Let's use a more practical example, let's say the machine operates at 3000 rpm, and it has one cycle of vibration for every rotation of the rotor; this is called 1X vibration. The 1X vibration is 3000 cycles per minute, or 50 Hz (50 cycles per second). With our DPS, the vibration signal is sampled 360 times for each vibration cycle (18,000 samples per second / 50 cycles per second = 360 samples per cycle). At this frequency, we get an excellent picture of the vibration waveform. Even with our high sample rate we have limits for the highest frequency of interest we can sample for, and that is called the Nyquist Frequency. The basic principle behind the Nyquist Frequency is that one has to sample at least two times during the vibration cycle to discretely determine the frequency of interest. The Nyquist Frequency for the DPS is 9 kHz (18,000 / 2 = 9000 Hz), which is quite excellent considering most machinery vibrations of interest are in the 1/4X to 3X vibration range. Also, note that the amplitude rolls off (decreases) at 16 kHz, so we can reliably detect frequencies up to 8 kHz (16 kHz / 2 = 8 kHz). For example; let's take an extreme case of a machine that operates at 100,000 rpm (1X = 100,000 cpm = 1667 cycles per second = 1667 Hz - Keep in mind most machines operate less than 10,000 rpm). The frequencies of interest for this 100,000 rpm (1667 Hz) machine are 416 Hz (0.25 x 1667 Hz) to 5001 Hz (3 x 1667 Hz), well within the DPS Nyquist Frequency range.
Q. What is the frequency response of an analog device?
A. The literature states 10 kHz at -3dB. If you look at the literature; another way to say this is that the amplitude resolution goes down after 2 kHz, and is down by 50% (-3dB) at 10 kHz. Due to frequency response limitations there is significant amplitude attenuation after 4 kHz (see 3300XL Datasheet page 27 of 35). In other words, the DPS has 2X the frequency resolution of an analog device. The amplitude resolution is very good to 8 kHz, which is the frequency found in our DPS datasheet (page 5 of 11).
Q. Why is the frequency response different for the MX2034 transmitter versus the MX2033 driver?
A. Note, when we discuss the MX2034 transmitter we are looking at overall vibration amplitudes including frequencies up to 5 kHz. When we discuss frequency response of the MX2034 we are talking about the buffered output found at the BNC on top of the device. The buffered output of the MX2034 is the same as the MX2033 output, although limited by the small amount of power provided by the 4 to 20 mA loop power. The low current (4 to 20 mA) being supplied is why the measurement device, or monitor, connected to the BNC has to be within 50 meters (150ft) and the frequency is limited to 5 kHz; which is still very good considering the analog device is limited to a reliable frequency resolution of 4 kHz.