|
Motion Control: Uncovering Signal Anomalies In Motor Drives |
|
|
|
|
MSO uncovers signal anomalies
in motor drives
By Johnnie Hancock
Author Johnnie Hancock is a signal integrity applications engineer with Agilent Technologiesí Electronic Products Group (agilent.com).
Engineers traditionally think of digital storage oscilloscopes (DSOs) as two-dimensional instruments that graphically display only voltage versus time. But there is actually a third dimension to a scope: the z-axis.
This third dimension shows continuous waveform intensity gradation as a function of the frequency of occurrence of signals at particular X-Y locations. In analog oscilloscope technology, intensity modulation is a natural phenomenon of the scopeís vector-type display, which is swept with an electron beam. Due to early limitations of digital display technology, this third dimension, intensity modulation, was missing when digital oscilloscopes began replacing their analog counterparts. Now, it is making a comeback with the latest mixed signal (analog and digital) oscilloscope (MSO).
   Display intensity gradation can be extremely important when you are looking for signal anomalies, especially when you are viewing complex-modulated analog signals such as digitally controlled motor drive signals. Intensity gradation is also helpful in a wide variety of mixed-signal applications found in embedded microprocessor and microcontroller technologies common in automotive, industrial, and consumer electronics applications. But even when you are viewing purely digital waveforms, intensity gradation can show statistical information about edge jitter, vertical noise, and the relative occurrence of anomalies.
Recently, all major digital oscilloscope vendors have begun to provide z-axis intensity gradation - with varying degrees of success - in order to emulate the display quality of an analog oscilloscope. The portable Agilent 6000 series DSO/MSO tested for this article provides up to 256 levels of color intensity gradation for each pixel based on deep memory acquisitions (up to 8 MB) mapped to a high-resolution display (XGA).
If you are working with complex-modulated signals, you need a scope with sufficient display quality to let you look at the big picture and then zoom in to see the details.
An example of a complex analog signal is a digitally controlled motor drive signal. A one-time start-up cycle of a motor would be classified as a single-shot phenomenon. Figure 1 shows how an oscilloscope with high-resolution display technology is able to reliably capture one phase of this single-shot start-up motor drive signal. You can also use this mixed signal oscilloscopeís digital/logic channels to synchronize and trigger the waveform capture based on the digital control signals of the motor. This capability can be extremely important when you attempt to synchronize acquisitions on not only power-up sequences, but also on particular motor positioning commands. Although not shown, this oscilloscope could just as easily capture all three phases of the motor drive signals simultaneously using its four channels of analog acquisition. Also shown in figure 1 are two zoomed-in images taken from the same single-shot acquisition. With this scopeís high-resolution display technology, we can see a bright vertical vector (near the centre of the display) in the middle image after zooming-in by a factor of 100. Further waveform expansion (20,000:1) on the pulse-width modulated (PWM) burst reveals a glitch shown in the image on the right. Again, other portable DSOs with shallow memory depths would be unable to show the level of detail shown in these screen images.
For a more in-depth discussion on oscilloscope display quality, refer to Agilentís application note 1552 titled, ìOscilloscope Display Quality Impacts Ability to Uncover Signal Anomalies,î which can be downloaded at agilent.com/find/scopes.
|
