Avoiding the pitfalls of road testing

Automotive test engineers must design test systems that not only operate in the controlled environment of the test cell and test lab but that also work in vehicles during road tests. Many test-system design tasks, such as transducer selection, data acquisition, and data analysis, remain largely the same for both lab tests and road tests. The unpredictable nature of road testing, however, presents test engineers with additional challenges.

The five most common pitfalls of road test system design are: harsh environments, cabling problems, test system power, inflexible hardware configurations, and test system software.

A road test system needs to operate properly in the same harsh environments that the vehicle itself must endure. Most "lab" and "office" equipment is specified to operate from 0°C to 30°C. Road tests, however, are routinely performed outside of this comfort zone, because vehicles must be able to operate in extreme conditions, such as in the heat of the Mojave Desert where the temperatures can easily reach 50°C. Your road test system also must work at this temperature, and it must be able to withstand contaminants such as motor oil and salt spray.

It's clear that "rack and stack" lab equipment and desktop PCs are ill-suited for this type of application. You need to find a tester that can handle road test conditions.

Cabling is also more of a problem for road testing than it is for lab testing. Mechanically, you must consider how to route cables, how to provide strain relief, and whether or not you need drip loops. Electrically, you need to decide how to shield and ground the cables to prevent noise and crosstalk from compromising measurements.

One approach to reducing cable problems is to position small data-acquisition modules that include signal conditioning and analog-to-digital converters near the sensors (Figure 1). These modules can collect the data from several different sensors and transmit the results over a single cable to the system computer. This reduces the number of cables you need to run and also reduces the possibility that they will pick up unwanted noise.

Most test equipment, especially lab test equipment, is designed to work with a 125-VAC power supply. Needless to say, this is not available during road tests. You may be tempted to use an inverter, but inverters introduce problems of their own.

For one thing, the output of an inverter is often not a clean sine wave, so using it to power a data-acquisition system can introduce unwanted noise into your measurements. Another problem with using an inverter is that the test system will lose power during ignition cycles and at other times when the battery power is interrupted, such as when a battery cable becomes loose or disconnected or a battery dies during a test. When this happens, you lose test data.

To avoid these problems, look for test systems that operate on DC power, with a wide voltage input range that will allow you to power the system from a variety of sources. Ideally, the system will also have an internal battery backup that is tightly integrated with the system operation.

When designing a road test system, keep the system as flexible as possible. Generally, you will want to use the system in a number of different vehicles, and each will have its own configuration challenges. Even if you only use the system in a single vehicle, using a flexible configuration will keep costs down by allowing you to reconfigure a system quickly to run different tests.

To evaluate the flexibility of a commercial tester, ask yourself these questions:

• How much work is required to re-cable inputs to other channels? Don't forget about rerouting sensor excitation and sense wires.
• Is it easy to add channels? Don't forget about adding sensors and cables.
• Is it easy to change the function of a channel?
• Does the system connect to a vehicle network? Can it handle unknown protocols?
• Is it easy to add storage?
• Can the data-acquisition system run without the host PC? If so, it could capture data should the PC fail or should you want to use the PC for another function.

It's just as important to have flexible software as flexible hardware. The software should allow you to reconfigure channels quickly and allow you to display data in a variety of formats.

Also, your system software should let you monitor the data as you acquire it. With this capability, you can do a quick check as the test runs to verify that you're acquiring good data. This will help prevent wasted test runs and ultimately save test time and money.

Finally, your software should have real-time analysis functions, such as digital signal processing. With this capability, you don't have to wait to get results from your test system. This speeds up the test process and also saves time and money.