The functional testing of automotive electronic control modules (ECMs) presents many challenges, mainly due to the physical loads connected to ECMs: solenoids, motors, spark plugs, electromechanical transducers and so on. These represent an array of inductive and resistive loads that must be simulated in the production test environment to ensure proper testing of the ECM.

To accomplish these tests, a test engineer faces a “make vs. buy” decision: Design a load solution in-house or purchase a commercially available solution. Each approach has strengths and weaknesses. This article uses two key criteria—cost-of-test and time-to-production—to evaluate the load strategies that can be applied to functional testing in a production environment. It also presents key strategies to consider when planning for production testing.

Self-designed load solutions

For a typical test engineer, the creation of a load solution or “load box” doesn’t pose much of a technical challenge. The physical design is relatively straightforward. Most test engineers allocate ample physical space to house the various loads, the relays that switch them and a control interface to the relays. This may be implemented in a card cage large enough for the load cards or by building everything on a single plane such as a tray.

Independent of the physical package, other factors can complicate the design:

• Is the load box safe and reliable?
• Does it provide the required accuracy?
• How many load channels should be supported per card or tray?
• How much current is needed to support each channel?
• Is flyback protection required? (This is especially true for inductive loads.)
• Is access needed to measure the current flowing through a load?
• Must the load box mount on an instrument system?
• Is everything properly shielded from EMI?

Commercial load solutions

Many of the issues mentioned above are fully documented and specified in commercially available load solutions. That level of certainty is enhanced by the principal benefits of commercial solutions: lower cost-of-test (COT) and faster time-to-production (TTP). Let’s take a closer look at these benefits.

Cost-of-test

This evaluation criterion focuses on production and the test strategy. COT can be summarized with a simple formula.

The goal is to minimize COT. Inside the brackets, fixed cost and equipment lifetime are two important factors to consider when selecting a commercially available solution. The lowest COT is achieved through the lowest fixed cost and the longest lifetime (within practical limits). One consideration: The lifetime of a load solution can be extended if it is easily reusable in other projects.

Comparing fixed costs

Fixed cost is the initial investment in the load solution. The cost of fabricating a design will vary according to factors such as complexity and the number of load channels. As an example, it would cost approximately $1,500 to create a simple design with up to eight load channels, switching of 5-A currents and overvoltage protection. Comparing this to a commercial solution, a card alone may cost about $1,500; if a card cage must be purchased the total cost increases.

Figure 1. As the number of channels grows, the costs become comparable

Figure 1 compares the costs of the in-house and commercial approaches. As shown, the in-house approach costs less, but these solutions tend to be purpose-built for a single test. In comparison, a commercial design is more costly but it can cover a wider range of uses at that price point. One note: The chart suggests a crossover near 200 channels, which exceeds the typical need in ECM testing.

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