GB2256057A

GB2256057A - Testing electrical systems for electromagnetic compatibility

Info

Publication number: GB2256057A
Application number: GB9111264A
Authority: GB
Inventors: Robert James Ball; Paul Anthony Jennings
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1991-05-24
Filing date: 1991-05-24
Publication date: 1992-11-25
Anticipated expiration: 2011-05-24
Also published as: GB9111264D0; GB2256057B

Abstract

The electromagnetic compatibility (EMC) of a component in a complex electrical system such as a wiring harness in a vehicle is predicted using the results of a low level swept frequency test. Transfer functions of induced current and field strength are established (Figure 1) at various locations in the system over a range of frequencies and the results are analysed and multiplied to establish a probability distribution (Figure 4) of the current at any point in the system under high-field EMC test conditions. By comparing (Figure 5) the probability distribution with the known current actually required to cause failure of a component (as determined in a separate test), the probability of failure of the component at high fields can be established. <IMAGE>

Description

ELECTROMAGNETIC COMPATIBILITY TESTING AND APPARATUS THEREFOR The invention relates to the testing of electrical systems for electromagnetic compatibility.

A radiated susceptibility test for the electromagnetic compatibility (EMC) of an electrical system can be validly performed when the system is installed in the particular vehicle, appliance or building where it is to operate and by transmitting a field of such strength as might be experienced in use - eg in close proximity to a ratio transmitter. However, it is impracticable to use a high strength field for such testing without the danger of interfering with other signals unless the test takes place in a shielded anechoic chamber.

Such chambers are in use, eg for EMC testing of vehicles, ana, whilst they are essential for meeting legal obligations, it is possible to carry out only a relatIvely limited amount of testing of prototypes.

This has led to the development of low level swept frequency (LLSF) testing followed by bulk current injection (BCI) susceptibility testing of individual components. This technique has been described in several papers, including one entitled "Development of a System Level Bench Test for the Automotive Industry" by P H Lever presented at an IEE Conference on EMC at York in August 1990.

Whilst the combination of LLSF and BCI testing has already enjoyed some commercial success in the aircraft industry, the combination requires a large number of readings if the results are to be used with confidence.

The present invention has as an object the development of LLSF testing to allow the design of electrical circuits and components for EMC with a higher degree of confidence than hitherto.

according to one aspect of the invention there is provided a method of testing an electrical system for electromagnetic compatibility, the method comprising the steps of:submitting the system as installed to a low level swept frequency test and establishing a transfer function between an emitted electric field strength of low predetermined magnitude and the induced current at various locations in the electrical system over a range of frequencies and orientations to the emitted field; multiplying the transfer functions by a factor dependent on the maximum ambient field strength for electromagnetic compatibility (the EMC field strength) to establish an EMC test current for each location and frequency; establishing the mean and the standard deviation of the EMC test currents at each frequency and establishing a probability distribution; performing an electromagnetic susceptibility test on a particular electrical component of the system to determine the current required for malfunction (the component susceptibility current) over a range of frequencies; and establishing a probability of failure of the component at the EMC field strength by comparing the probability distribution ana the component susceptibility current.

Preferably the electromagnetic susceptibility test is a BCI test.

A further object of the invention is to provide apparatus for performing at least part of the method according to said one aspect of the invention.

Thus according to a secona aspect of the invention there is provided apparatus for performing an electromagnetic compatibility test on an electrical component, the apparatus comprising: means for receiving data from a low level swept frequency test on an electrical system of which the component will form a part, said data comprising the results of establishing a transfer function between an emitted electric field strength of low predetermined magnitude and the induced current at various locations in the electrical system as installed and over a range of frequencies and orientations to the emitted field, multiplying the transfer functions by a factor dependent on the maximum ambient field strength for electromagnetic compatibility (the EMC field strength) to establish an EMC test current for each location and frequency and establishing the mean and the standard deviation of the EMC test currents at each frequency and estaolishing a probability distribution; means for determining the current required for malfunction of the component (the component susceptibility current) over a range of frequencies; and means for comparing the probability distribution and the component susceptibility current and establishing the probability of failure of the component at the EMC field strength.

One example of the invention will now be described by way of example and with reference to the accompanying drawings, of which: - Figure 1 is a graph showing the transfer function of induced current against frequency at a point in a vehicle wiring harness during an LLSF test at 1 V/m; Figure 2 is a graph showing the predicted EMC test currents at various points in a vehicle wiring harness; Figure 3 is a histogram used in the analysis of the currents shown in Figure 2; Figure 4 is a graph showing a probability distribution based on currents shown in Figure 2; and Figure 5 is a graph showing the mean of the currents shown in Figure 2, the BCI susceptibility profile of a particular component and the probability distribution of Figure 4.

An electrical system as installed in a Rover 827 saloon was subjected to a LLSF test on an open field test site. The vehicle was placed on a turntable and exposed to an electric field of 1 V/m over a frequency range of 20 to 400 MHz for four orientations of vehicle to transmitting antenna (nose-on, tail-on and both sides), and for vertical (20 to 200 MHz) , horizontal (20 to 200 MHz) and circular (200-400 MHz) wave polarisations. In each case a current pick-up probe linked to a spectrum analyser via a fibre-optic link was used to measure the level of current induced in the vehicle wiring harness at various points adjacent to electrical components or systems whose EMC was important for vehicle performance ana/or safety.

The result for a nose-on orientation using vertically polarised radiation at one point in the vehicle wiring harness is shown in Figure 1 as a graph of current level (in dB micro-amps) against frequency (MHz) and represents the transfer function for 1 V/m.

It has been established that the induced current is directly proportional to the applied electric field (see Jennings, P; "System Level Test & Chamber Correlation", ISATA Conference, May 1990). Using this principle, the transfer function of Figure 1 was multiplied by a factor of 50 to obtain the current level that would be induced if the vehicle was exposed to an electric field of 50 V/m. This is a field strength which represents ambient conditions for electromagnetic compatibility (the EMC field strength) and the calculated current is conveniently referred to as the EMC test current.

The MO test current not only varies with frequency and orientation of the vehicle to the field emitted by the transritting antenna but also varies according to the location in the vehicle wiring harness, as is illustrated by Figure 2 which shows in dashed lines the induced current (at 50 V/m) against frequency at various locations in the vehicle wiring harness, with the worst case (ie maximum induced current) being shown in full lines.

The present invention follows the surprising discovery that at any given frequency the probability distribution of the induced currents is substantially the same.

At each frequency, the mean and standard deviation of the various EMC test currents were calculated and the currents split into bands. The number of EMC test currents following into each band was counted and analysed as a histogram as shown in Figure 3.

This procedure was repeated over a range of frequencies and a probability distribution evaluated as shown in Figure 4.

This represents the probability of an EMC test current lying at a particular number of standard deviations away from the mean rMC current.

For each component a BCI test establish the current required for malfunction over a range of frequencies (the component susceptibility current) . This is illustrated in Figure 5 as a plot of current against frequency. Plotted against the same axes is the mean EMC test current. Onto this the probability distribution is superimposed (in a third dimension).

At any given frequency the probability that a component would fail at the EMC field strength was derived from the intersection of the probability distribution with the BCI susceptability current. Figure 4 shows this graphically, the probability being the ratio of the area A to the total area under the distribution curve. The probability of failure at various frequencies was calculated and the maximum value used as a final result.

Whilst a BCI test (or some other susceptibility test) is needea on each component to establish the component susceptibility current, it is not essential that the current readings in the LLSF are taken at the points in the wiring harness appropriate for the connections to the particular component, always provide that sufficient readings are taken at enough locations in the wiring harness for the proper establishment of the probability distribution. Hence the vehicle transfer functions, taken from a LLSF test, can be used as input data for a controller of a BCI test apparatus which can then generate a direct reading of the probability of passing or failing an EMC test if a particular component is fitted to a particular vehicle.

T'nus electrical components and systems can be developed and tested for EMC before they are fitted to a particular vehicle. This is expected to reduce development time considerably and reduce the cost of components where these would otherwise be over-designed for EMC.

Whilst LLSF and BCI tests usually use the same type of probe to measure induced currents, these do have certain frequency limitations and where appropriate other means of current measurement may be employed. Similarly, the electromagnetic susceptibility test on a particular component may be carried out using any valid means of current injection.

Whilst the invention has been described with particular reference to motor vehicles, it is applicable to any complex electrical system.

Claims

1. A method of testing an electrical system for electromagnetic compatibility (EMC), the method comprising the steps of: submitting the system as installed to a low level swept frequency (LLSF) test and establishing a transfer function between an emitted electric field strength of low predetermined magnitude and the induced current at rious locations in the electrical system over a range of frequencies and orientations to the emitted field; multiplying the transfer functions by a factor dependent on the maximum ambient field strength for electromagnetic compatibility (the EMC field strength) to establish an EMC test current for each location ana 'frequency; establishing the mean and the standard deviation of the EMC test currents at each frequency and establishing a probability distribution;; performing an electromagnetic susceptibility test on a particular electrical component of the system to determine the current required for malfunction (the component susceptibility current) over a range of frequencies; and establishing a probability of failure of the component at the EMC field strength by comparing the probability distribution and the component susceptibility current.

2. A method according to Claim 1 wherein the electromagnetic susceptibility test is a bulk current injection (BCI) test.

3. Apparatus for performing an electromagnetic compatibility test on an electrical component, the apparatus comprising: means for receiving data from a low level swept frequency (LLSF) test on an electrical system of which the component will form a part, said data comprising the results of establishing a transfer function between an emitted electric field strength of low predetermined magnitude and the induced current at various locations in the electrical system as installed and over a range of frequencies and orientations to the emitted field, multiplying the transfer functions by a factor dependent on the maximum ambient field strength for electromagnetic compatibility (the EMC field strength) to establish an EMC test current for each location and frequency and establishing the mean and the standard deviation of the EMC test currents at each frequency and establishing a probability distribution; means for determining the current required for malfunction of the component (the component susceptibility current) over a range of frequencies; and means for comparing the probability distribution and the component susceptibility current and establishing the probability of failure of the component at the EMC field strength.

4. Apparatus as claimed in Claim 3 when incorporated in bulk current injection (BCI) equipment.

5. A method of testing an electrical system for electromagnetic compatibility substantially as described herein with reference to the accompanying drawings.