Introduction to Industrial Oscilloscopes/Scopemeters

Welcome to an introduction to Industrial Oscilloscopes and Scopemeters

This overview will help you understand oscilloscopes and a wide variety of waveform capturing and recording test instruments.

Use the concepts to evaluate, to choose, and then to use industrial oscilloscopes and scopemeters, in order to enhance your electrical and electronic troubleshooting skills.

This overview will be followed with another section on advanced features and troubleshooting techniques.

Oscilloscopes and Scopemeters save time on the installation, commissioning, and troubleshooting of industrial processes, automation and machinery.

Seeing the actual signals, waveforms, and timing relationships helps you find what is working and what is not working.

Some problems show up the minute you see the signal. Others are identified by comparing two or more signals to each other on the screen. Timing measurements help synchronize steps in a process that are not working together properly.

Digital multimeters can only you take you so far.

Many times the issue is more complicated. Being able to capture and display a waveform identifies the problem faster, allows you to see the unseen, and, many times, is the only way to find it.

Meters take one slow measurement and display it.

Oscilloscopes take many rapid measurements sequentially, connect the measurements graphically on the display, and display a reproduction of the waveform present at the test point. Numeric measurements can then be made on different characteristics of the waveform.

Adding Time expands one-dimensional measurement into two dimensions, opening up a whole new realm of measurements and troubleshooting.

We can take measurements and see events more rapidly, visualize how things are happening at the speed of electricity and electronics. Time scales of milliseconds, microseconds, nanoseconds and even picoseconds are used, impossible to measure with slower intruments.

In industrial settings, most of the work is in the millisecond and microsecond world.

Resolution and accuracy of the measurements is traded off for speed. Faster measurements require faster analog-to-digital converters that have less bits of resolution.

Meters resolve measurements across 2000 to 10,000 counts, where an oscilloscope resolves measurements over 256 to 1024 counts.

To take measurements a million times a second , faster a/d converters with less bits and bytes are used. Where a meter might use a 16-24 bit A/D, an average oscilloscope will use an 8 bit to 12 bit converter.

Oscilloscopes compensate for the loss of resolution and accuracy by expanding the types of measurements they can make, and showing timing relationships..

Pretty much all industrial oscilloscopes have at least two channels, where two test point signals can be compared to each other, in relation to the timing as well as the amplitude.

Simple math can be applied between the signals, such as multiplying voltage times current to get power, and then seeing when the most power is used (time).

Relationships between signals can track down more difficult to find intermittent, “once in a while” trip-outs and failures.

• Does motor drive A trip out whenever machine B cycles on?
• Does the spike or noise happen once on a while or all the time?
• Is the box position sensor signal matched up at the right time with the conveyor “on” signal?

More measurement possibilities and the additional dimension of time opens up a wider range of capabilities not found in other testers.

Industrial oscilloscopes and Scopemeters allow you to see what is actually happening in the circuit, system, and controls, by capturing and displaying waveforms instead of just numbers.

Here is a case where a UPS was not charging its batteries.

The power would go down, the UPS would try to turn on, then it too would go down, shutting down all the computers in the room.

Imagine if this happened in an automated factory, or a trading floor at the stock market?

The problem was caused by “flat-topping” of the power signal suppying the UPS, which charged the batteries off the peak of the waveform. No peak, no charging.

The problem was solved by simply moving the UPS closer to the main power connections, shortening the cable between the two.

Electricity needs to flow to get work done.

Waveforms are electricity in motion, displayed by tracing amplitude changes on a vertical scale over time on a horizontal scale.

Every part of a circuit or system has the potential to change the waveform as it flows through it. Sometimes this is good. Sometimes this is bad and causes problems and failures.

Checking out different characteristics of the waveforms can help you decide if these changes are good or bad.

Frequency is the speed at which a cyclic (repeating) AC signal completes one cycle, or repetition. An analogy in the mechanical world is RPM, with the change being that Frequenty is in “rotations per second” instead of “rotations per minute”.

Hertz, or Hz , is the unit of measurement, formerly referred to as cycles per second. Common measurements are in KHz (1000Hz), MHz (1,000,000Hz), and GHz (1 billion Hz) increments.

Period is the time it takes to complete one cycle of a cyclic AC signal, analogous to “minutes per rotation” in the mechanical terms. Because most AC signals are labeled by their frequency, Period is calculated by dividing 1 by the frequency, inverting the “cycles per second” into “seconds per cycle”.

Common measurements on scopes have a time unit of measurement in milliseconds (msec, or .oo1 second) , microseconds (usec, or .000001 sec), nanoseconds (nsec, or .000000001 sec), and picoseconds (psec, or .000000000001 sec).

Knowing the approximate period of a signal can help you set the timebase on the scope.

Electrical signals are just like physical mechanical parts.

Electricity cannot make instant changes from one level to another. It takes time to change from one voltage level to another, one amperage level to another, and so on.

Rise Time is defined as how long it takes a waveform to go from a lower level to a higher level, mostly used on square waves and pulses.

Fall Time is how long it takes a waveform to go from a higher state to a lower state.

Electronic signals have very fast rise and fall times. It is important to maintain straight vertical edges , especially in networking cables like Ethernet, where the frequencies are in the billions of cycles per second (GHz) and the rise times are many times faster than the periods of the data signals.

Something as simple as using the wrong cable can slow down the rise and fall times, causing data errors during transmission and slowing down the network. The same is true for controls. variable speed motor drives, and many types of equipment.

RMS, or Root-Mean-Square, is a common AC level measurement displayed on multimeters. This is the integrated amplitude value of one cycle, or “the amount of work”, equal to the heating value of the same DC voltage level (See AC Measurement Basics in Section 4 of DMM basics for more explaination)

The AC RMS value is less than the peak value. The peak value can be calculated from the RMS value through multiplication.

For example, the 120Vrms AC sine wave from an outlet has a Peak value of (120) x (1.414) = 169 Vpeak.

The negative peak is the same, only negative at -169 Vp, and a Peak-to-Peak swing of +169V to -169V, or 338Vp-p. ( I have a video using a meter to show this)

“Calculated” is assuming a theoretically “clean” sine wave- a rare occurrence in the real world, especially when something is going wrong.

Oscilloscopes can measure the Peak and Peak-to-Peak values off the waveform, regardless of how close the signal is to its ideal state. Measure in place of math, since the math may be wrong anyway.

(see the “flat topping on a UPS” example a few slides back)

Noise is caused when unwanted signals add themselves into the voltage or current waveforms.

A little noise might go unnoticed – a lot of noise can wreak havoc on signals and the operation of whatever they are driving.

Noise levels can be seen and measured with scopes.

Glitches, Spikes and Transients are unwanted fast events that can be regular (every cycle or so) or random (every once-in-a-while).

Capturing, displaying and measuring these events are where digital oscilloscopes excel. These transients cause problems on equipment and systems if they are large enough or happen at a bad time.

For example, a circuit breaker on a machine might trip out randomly, caused once in a while by someone pressing the start button on a microwave in the breakroom. Both are on the same electrical circuit.

What are the chances of this happening? One in a million?

60 Hz is 60 cycles per second, 3600 cycles per minute..512,000 cycles per day; “one in a million” can happen every two days.

A Pulse is a square wave signal used often in control systems from sensors and to turn things on and off. It is also the shape of computer data signals flowing through wires and wirelessly ( digital “0” or “1”)

Measuring the Pulse Width (“On” time) is important to see if the electrical and electronic cricuits and devices are working properly. This is also where Rise and Fall times are very important.

Measuring the timing relationship between two or more pulses is often needed to check the operation of circuits and machinery.

Timing is everything on a process line.

One sensor’s pulse feeds into another sensor or actuator, telling machines what to do when.

Here is a case where moving a sensor changed the timing between two sensors in a process.

By lining up both sensors pulses on the scope screen, the timing between the two was seen to be off, and then adjusted back into alignment.

The Phase of a signal is how many degrees it is shifted from zero (“0”) degrees on a circle.

• In-Phase is “0” (zero ) degrees phase difference between two or more signals-they line up perfectly and overlap on the screen.
• Out of Phase, or Phase-Shifted, is having greater or less than Zero phase difference between two or more signals. They do not overlap or line up with each other perfectly in time.

Sine waves are just circles stretched out over a wire, so realtionships between signals are often expressed and measured in turns of angles, or degrees on a circle (think cosine theta in the electrical world).

This concept illustrated in the “Introduction to Power and Power Quality ” tutorial.

Phase Shift is a measurement of how many degrees one signal is out of phase with another reference signal assigned as the zero phase. For example , in a three phase electrical system, Phase B is 120 degrees out of phase with Phase A (zero phase), and Phase C is another 120 degrees phase shifted from B, a total of 240 degrees shifted from Phase A.

Power Factor is a measure of the phase shift between the voltage and current signals. For the maximum true power, the current and voltage need to be in-phase.

When a generator is switched on to replace utility power, it is important for the phase of the generator voltage signal to line up with the phase of the utility voltage signal before switching over.

This is taken care of automatically during normal operation, but may need to be checked during installation or after maintenance.

Scopes can show this phase relationship between the generator and utility power, and help adjust the phase controls if the phases are not matched up.

Taking Digital Measurements in an Analog World

Physical analog parameters have infinite resolution, in that there is a smooth line of values without steps between these values.

“Digitizing” analog parameters slices them into steps, with the resolution limited by the size of the steps.

A digital oscilloscope takes measurement samples at a high speed, stores each into a screen memory location, then connects the amplitude numbers in these memory locations into a waveform displayed on the screen.

The Sampling Rate is the speed at which the measurements are taken sequentially on the waveform connected to the probes.

The Sampling Rate Specification is the highest, or fastest sampling rate at which the instrument can take measurements. This only applies at the fastest time base setting, and is also important for the shortest (fastest) glitch that the glitch detection function can capture.

The Bandwidth is the fastest , or highest frequency signal that the instrument can pass through its input circuits without attenuating (decreasing the level) past a 50% (-3 decibels) point.

Bandwidth limitations are a characteristic of electronic connections, cables, and input jack electronics. Any slower signals pass into the instrument with minimal or no attenuation. For comparison, digital multimeters have very low bandwidths, sometimes limited on purpose to measure signals like variable speed drive outputs to motors.

Something called the Nyquist-Shannon sampling theorem describes a minimal sampling rate to waveform frequency ratio, in order to have enough sample points to reliably reproduce a waveform on one pass. The theorem lists this to be 4 samples per waveform period, not enough to recreate waveforms for troubleshooting. A minimum of 10 samples per waveform period is a ” rule of thumb” for oscillscopes-more is better. This translates into a sampling rate specification that is at least 10 times the bandwidth for single-shot waveform capture. This is where a sampling rate higher than the frequency of the signal being captured is important

For repetitive signals, a scope can take samples over multiple cycles, then combine these samples to display a waveform with a frequency higher than the sampling rate. This is where you see a bandwidth higher than the sampling rate.

As mentioned above, the Sampling Rate Specification is the highest, or fastest sampling rate at which the instrument can take measurements. This only applies at the fastest time base setting, and is also important for the shortest (fastest) glitch that the glitch detection function can capture. This is set internally according to the Time Base setting and the number of locations in the screen memory for each channel.

The Time-Base Setting is the time per division you see on the screen, with the total time across the length of the screen being the time-base setting multiplied by the number of horizontal divisions.

The actual sampling rate changes with each time-base setting because there are a limited number of memory locations for samples to be stored on each channel. This is taken care of internally by the instrument, and is generally not a concern. Often the scope samples at a high rate, and only stores the samples matching the times for memory locations.

Why? If you were looking at a 60Hz power signal on a 1GS/s scope running at 1GS/s (1 billion samples per second) you would probably need 1 billion memory locations per channel, instead of the 10,000 or so memory locations on a typical industrial oscilloscope. This is way more detail than is needed for everyday measurements.

The Glitch Detection specification is an exception to this. A scope with glitch detection samples at a higher rate of speed, captures and saves events outside of the expected amplitude values, saving samples into screen memory at normal speed.

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With more dimensions (2 on the scope verses 1 on the meter), there are additional settings to take into account when making waveform measurements on a scope display.

These settings are displayed alongside the waveform to help interpret what is being displayed.

The amplitude settings of the vertical attenuators indicate the volts (or other function, like “amps” ) per vertical division for each channel being displayed. If only channel A is being used, only channel A is shown.

The time-base setting displayed is common to all channels.

The Trigger setting is displayed as to which channel is trigger off of, and the type of trigger (rising edge, falling edge, pulse-width, N-cycles, ….). Often the trigger level is displayed on the triggering waveform.

If the waveform is running through the display and the trigger level is shown above the signal, bringing the trigger down into the waveform usually displays a clearer waveform.

The controls on a digital oscilloscope match up to these settings displayed. For portable, battery operated “meter-like” industrial oscilloscopes, the controls are push buttons similar to the ones illustrated in the slide. Line-powered, more bench-top oriented oscilloscopes have turning dials to adjust these settings.

What settings to use? The easiest way is to have an approximate feel for the time-base to use, hook up the signal, and adjust the controls until you see a good signal displayed.

An even easier way is to use an instrument with AutoSet, which is more and more common in newer scopes. Similar to autoranging in a multimeter, the scope samples the waveform and automatically adjusts the vertical attenuator and time-base to display a stable signal. A caveat is this only works with repetitive signals that are stable.

The Trigger settings stabilize the display of a waveform. Simple trigger settings are at a specific amplitude level on the signal, and whether it is rising or falling. By lowering the trigger indicator on the display using the trigger control, the waveform can be stopped from running over itself on the screen

More complex trigger may or may not be available-check the specifications of the tester you are using, or look at the trigger menu on the display.

With single-shot capture, the trigger is set at the level to capture and freeze a waveform. The trigger is armed like a mouse trap, and captures the first signal to meet the criteria. Some scopes allow you to capture multiple single-shots, without having to re-arm for each. This is handy for looking for problems caused buy transients, or any out-of-tolerance condition that happens once in a while.

More complex triggering schemes include triggering on a pulse-width, or on N-cycles of a trigger point. “N-cycles” means triggering on the Nth occurance, like the 10th pulse, or the 24th pulse. this can be handy for process control, say where a box should be closed and moved off the line when 24 items are placed into it, but it is being thrown on the floor instead. Or where full glass bottles of a liquid are being thrown off the conveyor against the wall instead of moving on to packaging. Hey, it happens.

Trigger settings as listed above are useful when two or more channels, with two or more separate waveforms, are being measured.

Being able to see and measure how each signal relates to the others in time is an important use of industrial oscilloscopes.

What I hope you get out of this set of slides is that industrial oscilloscopes and ScopeMeters are just as easy to use as digital multimeters. The idea is to dispel some of the “fear factor” that these testers are too complicated for everyday use.

Their usefulness far outweighs the additional controls, which become second-nature quickly, just like your meter dial.

Reading the display takes some getting used to, and can be deciphered easily by breaking it down through focusing on the key indicators. It is an extension of the meter you use every day.

If you are not comfortable with your meter, please see the tutorials on digital multimeters.

If you feel you and your team can benefit from a hands-on session, maybe over donuts or pizza for an hour or so, please contact me-I’ve done this hundreds of times over the years.

Digital Oscilloscopes can act as paperless recorders by slowing down the time base to seconds, minutes or hours.

Combining screen memories into one long memory enables the recording to run over many screens, in some instruments over a hundred. For example, a scope with ten divisions across the screen , at 2 minutes per division = 20 minutes across one screen. over a hundred screens, this is 2000 minutes, or a little over 33 hours.

Monitoring power, controls, and timing sequences over hours and days can find intermittent issues without having to have someone sit there and watch it, waiting to catch a failure.

Oscilloscopes and ScopeMeters typically have two or 4 channels.

A color screen helps separate one channel from the other visually

The channels can either share a common (like a DMM) or have isolation between them.

Isolated channels are an important safety feature, especially working with 3 phase power and variable speed drives. If the channels share a common, the phases will be shorted out inside the instrument between these non-isolated channels when measuring across the phases, such as A-B, B-C, and C-A. At 480V with 100’s of amps behind it, this can cause serious injuries in an unprotected instrument.

Power and Power Quality Analyzers are specialized forms of industrial oscilloscopes.

A single-phase power analyzer is very similar to a two-channel scope, with the controls, screens, and measurements changed to simplify power and power quality measurements. The inputs are sometimes modified, where one channel is for voltage and the second is dedicated to a current clamp.

A Three-Phase Power Analyzer is different than a scope by having more channels and lower bandwidth. For three phases, you need to measure:

• 1 to 3 voltage channels
• 1 to 3 current channels
• 1 ground/common channel

This creates a need for 7 channels to connect probes to.

The math between 3 phases sum up totals between them can get complicated because of the difference phase angles between each phase. Three-phase power analyzers take care of this internally, displaying measurements such as kWH (kilowatt hours) THD (total harmonic distortion) and allowing you to drill down into each phase’s measurements to troubleshoot issues.

Power analyzers are tuned to detect, measure and create reports on power problems often found in industrial settings.

Finding these changes in power is simplified by measurement settings entitled as what is being measured, such as:

• “Power” screens include kW, kWh, Kvar, PF, and often K-factor
• “Sags/Swells” automatically sets up the analyzer to monitor for low or high voltage conditions
• “Harmonic” measures harmonics on each phase as well as the total across all phases
• Other measurements are labeled similarly in the measurement selection menus

The display is modified to show these parameters and more, in the selected language and also in numeric chart forms, in place of just showing waveforms like most oscilloscopes.

Safety ratings on industrial oscilloscopes are as important as they are on digital multimeters (DMMs). The scope and its probes are the last thing between you and the electricity.

Internationally standardized tester safety ratings are based on the largest amount of fault current possible, related to the location you are working in, as to how close to the power source you are. For example, if you short out an outlet in a room, chances are there is a 15 amp breaker to help protect you. If you short out the electrical cables feeding your building, chances are there is no disconnect until the current surges to 100’s of amps.

If you are unfamiliar with these standards, I recommend reviewing the “Electrical Measurement Safety” overview found on this site, and review the proper safety procedures with your safety manager at work.

Be aware that oscilloscopes designed for electronic work on test benches are mostly safety rated to CAT 1 ro CAT 2, insufficient for use in an industrial environment. Check the scope you are using for marks showing its safety rating before connecting it to higher energy circuits.

In industrial settings, the common, or reference, on machines and between phases on electrical systems is often not earth ground.

Line powered (plugged into wall) scopes common is connected internally to the electrical power ground for safety. This ground is transfered through the common, or ground clip on the scope probe, potentially shorting out a non-earth ground common on the machine or electrical system to the building ground. The result can be a damaged scope, a blown-up scope, or injury to the operator. Use isolation, often called differential probes , in these situations with line-powered instruments.

Battery-powered scopes eliminate this issue by being inherently isolated from the building ground by not being plugged into the electrical system. This is the same for your multimeter.

Ground isolation does not nessesarily include channel isolation. Channel isolation takes this concept further, by isolating the common for each channel from all the other channels. This is important when measurements are taking between test points with different references from each other. Check to see if your scope has isolated channels if you are making these types of measurements.

An example would be when measuring between all three phases in a 3-phase delta configuration simultaneously: Phase 1 referenced to Phase 2, Phase 2 referenced to Phase 3, Phase 3 referenced to Phase 1.

Don’t let your scope probes be a weak link. They are even closer to the electricity than the scope, and should be safety rated at an equal or higher level than the instrument they are connected to.

I hope you find this introduction to industrial oscilloscopes useful for the type of work you do. They are very useful testers in many lines of work, and can be as essential as a multimeter. I spent many years where all I carried into a site was a scope in one hand and my tool bag in the other.

Industrial oscilloscopes and ScopeMeters are just as easy to use as digital multimeters. Thegoal of this session is to dispel some of the “fear factor” that these testers are too complicated for everyday use.

I appreciate you reviewing this introduction, which will be followed-on with more advanced measurements and uses in a separate section. Having a good understanding of the content presented here enables you to evaluate different industrial oscilloscopes, as well as get the best use out of the one you may already have.

If you have a scope laying around, or sitting in a cabinet at work because someone else purchased it and no one knows how to use it, send me a message. There is a good chance I can help you and your team get started with what you have, to find problems either too hard to find or that take too long to investigate.

Thanks!