Test Your Engine Control Units for EMI Immunity

An ECU controls many of a vehicle’s basic functions. It senses a vehicle’s operating parameters and controls several critical systems. The ECU may monitor vehicle speed, exhaust oxygen, engine rpm, throttle position, intake-manifold vacuum, and intake airflow. It can control spark timing, air/fuel ratio, fuel-injector timing and duration, and vehicle speed (cruise control).

Vehicles may generate several types of EMI. Spark plugs generate an arc, which produces radiated and conducted EMI. Motors, solenoids, switches, and relays constantly turn on and off, creating transient signals on the vehicle’s power distribution lines. Horns can generate large repetitive transients (Fig. 1) on the vehicle’s power supply. People entering and exiting the vehicle can accumulate static charge that can result in ESD if they touch the vehicle. These types of interference can couple into an ECU’s power lines and I/O lines and cause the ECU to malfunction unless it’s properly designed and tested. (Other types of EMI are described in “Additional Sources of EMI”)

Figure 1. Horns create transient signals as high as 6150 V on vehicle power distribution lines.

The Society of Automotive Engineers (Warrendale, PA, www.sae.org)  publishes SAE J1113, which along with subparts, describes the tests that ECU manufacturers can perform on their products.¹ Table 1 outlines the tests described in J1113 and lists the test equipment you need. The standard and its subparts also specify how to set up and perform the tests.

These ECU immunity tests aren’t mandatory. For ECUs, automakers select which tests they want their ECU suppliers to perform, so each ECU design requires its own EMI immunity test plan. Table 2 highlights some items you should include in such a plan.

Three Basic Tests

ECUs need testing for radiated immunity, conducted immunity, and electrostatic discharge (ESD). Radiated immunity tests relate to the ECU being exposed to radar, land mobile, cell phone, and broadcast signals. You can perform radiated immunity tests using antennas (J1113/27), a TEM cell (J1113/24), a strip line (J1113/23), or a tri-plate (J1113/25).

You also can test an ECU for immunity to magnetic fields (J1113/22) by simulating its exposure to magnetic fields from electric power lines and electric trains.

For conducted immunity tests, you need to inject dips, surges, spikes, and other transient signals—such as the 6150-V horn transient signal in Figure 1—into an ECU’s power wires and I/O cables. These tests simulate the effects of disconnecting inductive loads, interrupting a series current, switching off power to a DC motor, operating switches and horns, disengaging a large motor, and disconnecting a battery.

Get an Injection

Document J1113/2 describes a method for injecting RF energy into an ECU’s power wires. J1113/3 specifies how to directly inject RF energy into the I/O cables between an ECU under test and its sensors and loads. To inject the signal, you must build a broadband artificial network (BAN), which simulates the inductance of a vehicle’s wiring harnesses.

If you include J1113/4 in your test plan, you’ll perform bulk-current injection (BCI) to send RF signals up to 400 MHz into an ECU’s cable harness. Use a current probe around the entire harness to excite the signal wires and power wires with RF energy.

If you’ve ever stepped out of a vehicle on a cold winter day and then touched the vehicle’s door to close it, you’ve probably received a shock. For ESD immunity tests, you must inject ESD into power cables, I/O cables, connectors, and the ECU’s enclosure. ESD tests, defined in J1113/13, let you test an ECU’s immunity to ESD under controlled conditions. Current from the ESD event can travel into an ECU through cables or its enclosure.

To perform EMI immunity tests, you must simulate the operating environment of the vehicle, for the ECU under test must operate as if it were in a vehicle. During EMI immunity tests, select those operating states that you and the ECU’s design engineer feel will make the ECU most susceptible to EMI.

You need a complete functional test system that simulates the signals, loads, and operations of the vehicle. You need analog signals that simulate signals such as exhaust oxygen, intake airflow, throttle position, and manifold pressure. You’ll also need digital I/O signals to simulate sensors and control signals including vehicle speed, wheel speeds, and spark timing.

Your functional test equipment also must be immune to the simulated EMI. Otherwise, you won’t be able to trust the measurements you take during the tests. Test your test equipment using dummy loads and dummy input signals, and then subject it to the same EMI interference that you’ll use on the ECU under test. If you find that the test equipment isn’t immune to the interference, review the test setup. You may have to add filters or shields to the setup or use fiber-optic cables.

You also must make sure that the test setup doesn’t affect the EMI performance of the ECU under test. Be careful to terminate all cables with the same impedances they’ll see in a vehicle.² Perform tests exactly as described in the appropriate test standard and the test plan. If you automate the tests, then review any automation software with the vehicle manufacturer to ensure you perform the tests in accordance with the test plan. Consult the vehicle manufacturer if any questions arise during testing. Take photographs of the test setup should any questions come up later, or in case repeat testing is required. A digital camera is ideal for documenting the test setup.

Following the tests, report data in the format requested by the vehicle manufacturer so the recipients can easily interpret the results. If you report any tests as having failed, you should refer to the test plan, which should define a “failure” as a result of EMI immunity testing. J1113/1 defines four failure regions. These regions range from the test EMI having no effect on the ECU under test to the ECU simply not sustaining any damage following the tests. But the ECU may have failed to operate during and after the tests. In the consumer’s eye, however, the vehicle should operate flawlessly under all EMI conditions.

If the ECU failed to meet test requirements, perform a design review (hire a consultant if necessary) to determine the most effective modifications that will permit the ECU to meet requirements. Repeat the component tests to verify that the fix is effective. T&MW

FOOTNOTES

1. SAE J1113, Electromagnetic Compatibility Measurement and Limits for Vehicle Components (Except Aircraft) (60 Hz to 18 GHz), July 1995, Society of Automotive Engineers, Warrendale, PA, 724-776-4970, www.sae.org. 

2. Smith, Doug, “Investigate System-Level ESD Problems,” Test & Measurement World, November 1999, p. 28.   

ACKNOWLEDGEMENT
Thanks to Terry Ryback, EMC engineer at General Motors, for compiling Table 1.