By Corey Glassman
Although oscilloscopes have been used for decades, they are still one of the most valuable tools an engineer or electronics technician can have. Modern "lab scopes" combine the sophistication of computer-based technology with age-old capabilities, and the net result is a compact device capable of recreating an accurate visual signature of an electronic signal. The interpretation of this signature opens up the circuit for scrutiny.
Automotive techs have been sharing waveforms via the Internet for some time now - on the International Automotive Technicians' Network, for instance - or through a variety of computer database programs. Many trainers use scope technology to diagnose fuel pumps and injectors, clearly identifying defective commutator bars, seized bushings or defective coils. A scope opens a window into the electronic world and helps you in your diagnosis.
You may already own a graphing device similar to the Snap-on Vantage. This is a great tool, one that combines a database of connections, schematics and sample waveforms to help you diagnose more than 80 percent of the problems you'll encounter. But many signals are simply too fast for these devices, and electrical noise in the nano second time domain can stump them. They coexist very well with today's lab scope and should be a part of your diagnostic arsenal.
Most of the true lab scopes on the market today are designed for general-purpose use. NASA uses them for satellite construction, the medical industry uses them to repair CAT Scan or MRI systems and the military uses them to fix some of the most sophisticated defense systems of our lifetime. They are highly versatile and easier to use than ever before. Some years ago, operating a lab scope was similar to programming in Basic or FORTRAN, or running a DOS-based computer program. The scopes of today are similar to running the latest Windows 98 or 2000 program, and contain pop-up menus for navigation and built-in help screens for assistance. One of the most apparent advancements is in display technology, hosting high definition and full color. They are no longer small and hard to see with a dim green backlight; they are laid out in landscape format, which displays a greater number of segments for you to analyze and have bright white electro luminescent displays.
New vehicle technology such as telematics, satellite navigation, hybrid vehicles and fuel cells are driving the need for high-speed test equipment. When we refer to high speed, we are referencing the instrument's ability to capture a signal having high-speed characteristics. Some of the newest lab scopes, for example, offer bandwidths up to 200 MHz, or 200 million cycles per second. The latest research shows that a scope with a 60 MHz bandwidth is fast enough to work well into the future for the automotive systems under consideration. In fact, you may have to incorporate an external low-pass filter to cut down the bandwidth under 5 MHz, but many scopes anticipate this need and have built-in filter circuits. At 60 MHz, the scope will sample the signal at 500 mega samples per second. What does this mean to you? You won't miss a thing!
When configuring the scope, think about the signal you're looking for and set the time and volts per division accordingly. Most mechanical devices operate in the millisecond time domain (thousandths of a second - .001). Fuel injectors, switches and relays operate in this time domain. The PCM, on the other hand, communicates in the microsecond time domain (millionths of a second - .000001), as well as transistor switching and buss communication. Electronic noise is found in the billionths of a second time domain (nanosecond). Most of the new scopes will automatically set the volts per division, time per division, and even the triggering levels and slope for you. In the manual modes, you can adjust the scope to include time delay, video and pulse width triggering. It's like setting a mousetrap and then just waiting for the right (or wrong) signal to come along.
Most lab scopes have dual inputs, which allow you to compare cause-and-effect relationships between signals. On some of the newest scopes, both inputs have their own digitizer, so you can simultaneously acquire two waveforms and analyze them with the highest resolution and detail. If an anomaly flashes by on the screen, just press the “replay” button to see it again. Some scopes even have deep memory that stores 27,500 points per input. This is similar, and yet more powerful, than most automotive “flight recorders.” This deep memory function will record events up to 30 hours. They might also have multiple recording modes, such as an automatic capture and replay of 100 screens, where the instrument will continuously memorize the last 100 screens. Each time a new screen is acquired, the oldest is discarded. At any moment you can “freeze” the last 100 screens and scroll through them, picture by picture, or replay them as a “live” animation.
Cursors have come a long way too, and are designed as measurement pointers that automatically analyze various parts of the waveform for you. They can digitally identify peak voltages or time differences at various points on the waveform. One example is the Fluke 190 Series ScopeMeter, which features 26 automatic measurements, cursors, zoom and a real-time clock. The analysis can be made directly or later when linking to the computer. Up to two recordings and 15 scope screens (including their related setup string) can be stored for analysis, printout or download to a PC.
Even the batteries have changed. The newest equipment contains NiMH battery packs, which don't have as large a memory effect as the older NiCad. The equipment now has a feature to cycle the batteries over 12 hours to restore their effectiveness, in essence renewing the pack. They also have a longer active cycle, more than four hours before a recharge is required.
Accessories
There are other useful tools that spark some interest. A wide variety of accessories are now available to round out the capabilities of your equipment. Test leads and connectors are now fairly long, enabling you to reach into the engine compartment or under the vehicle, and are well shielded to minimize noise corruption.
Current clamps come in numerous configurations, allowing you to monitor current inductively, without interrupting the circuit. Some allow your scope or meter to display and record high currents and some provide a means of displaying and recording low currents as well, down to 10mA. An 80i-110s low current clamp can connect directly to your scope's BNC connector and is very well shielded against external electrical noise. This clamp also has a high frequency response, so the pattern displayed will closely represent the actual characteristics of the signal, even if it's high speed in nature. Temperature products can also connect to your test equipment. This allows you to record and display temperature variations over time.
If you test coil packs or plan to work with hybrid vehicles such as the Toyota Prius, you need some state-of-the-art tools. One such instrument (Fluke 1520 MegOhmMeter) is an insulation resistance tester that also measures voltage and checks connections with a low-Ohms function. It actually generates high voltage, sends it to the item under test through the test leads, and then calculates the resistance - accurately! It uses Ohms Law, E=IxR (remember the triangle?). It knows the voltage being applied - 250V, 500V or 1000V - and it knows the current. It then uses Ohms Law to calculate the resistance, under an actual load.
Electric and hybrid vehicles use batteries that source hundreds of volts, so the smallest short to ground can be disastrous. To test for the short to ground, simply disconnect the vehicle's source batteries, follow the manufacturer's recommendations for discharging capacitors and then connect the 1520 between the motor winds and ground, the cables and ground, and other isolated points and ground. You can also use this tool to check coils; ones with isolated primary and secondary windings.
The bottom line, do the proper research to select the tools that best fit your needs.
There is a high degree of synergy between the automotive tools required to fix today's vehicles and the tools designed for the electronic engineers. I would recommend a visit to your local industrial or electronic supply house. They have a wealth of test equipment and tools on display for your review.
| Corey Glassman is the automotive program manager for Fluke Corp. He is past president of the Automotive Training Managers Council; ASE triple master certified; a member of the Society of Automotive Engineers (SAE); and co-author of Advanced Engine Performance Diagnosis, published by Prentice Hall. Glassman has “wrenched” for a living and taught at Denver Institute of Technology in Colorado.
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