EMC Awareness
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Radiocommunications Agency EMC Awareness |
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Marine
Radar interferes with ship steering gear, nearly causing collision
Vessel collisions, sinkings, and unintended movement of an offshore platform
Commercial computers interfere with radiocomm’s on a small vessel
| Radar interferes with ship steering gear, nearly causing collision |  |
Description
Two navy warships nearly collided when the radar beams of one disabled the steering of another. The minehunter HMAS Huon went out of control and veered across the bow of the frigate HMAS Anzac.
Huon – the first of six state-of-the-art coastal minehunters – lost its steering as a result of electromagnetic interference (EMI) from Anzac and passed ahead of the frigate “at close range” . The incident occurred in June 2000 while the warships were sailing to Singapore.
An official report on the incident rejected Defence Department claims that EMI testing was expensive and not necessarily cost-effective and said that T&E (testing and evaluation) should be conducted as early as possible in order that risks could be reduced before they became dangerous. “In extreme cases, inadequate T&E could have tragic consequences,” the report said.
Commentary
Since the HMAS Huon’s steering control gear was not a radio system, the problem is one of RF demodulation (sometimes called audio rectification). Any non-linearity, such as a corroded metal connection, semiconductor or IC, will demodulate RF and produce a baseband signal which is a d.c. offset which varies according to the amplitude modulation of the RF field. It is not unusual for the d.c. offset to be so high, or be amplified by later circuits, that the circuits are driven into saturation.
Warships have powerful radars which operate at above 300MHz and can create pulsating field strengths of 1000V/m or more nearby. In a normal ship the electromagnetic environment below-decks is quite well shielded by the metal hull and decking and all-welded construction. But the parts of the steering control system which are in the Bridge are exposed to the full external environment, due to the Bridge’s windows.
Also, minesweepers are usually made of composites, not metal, to help prevent triggering magnetically fused mines. Such construction provides little inherent shielding and as a result the below-decks electromagnetic environment can be as bad as the external. As a result, EMC mitigation techniques need to be applied with great care.
References and links
“Loose radar blips nearly sink ships” by Wayne Smith, The Courier Mail (Brisbane, Queensland, Australia), Friday February 1st 2002, http://www.couriermail.news.com.au.
Links to Mitigation Techniques
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| Circuit design to reduce RF demodulation |  |
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| PCB layout | |
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| Filtering with CM cable-mounted chokes |  |
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| Filtering | |
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| Enclosure shielding | |
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| Cable and connector shielding | |
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| Vessel collisions, sinkings, and unintended movement of an offshore platform | |
Description
The incident occurred outside UK waters and was reported in a safety notice issued by an offshore operator. The safety notice was seen by an HSE inspector on a bulletin board on an offshore installation, dated 30 September 1999, which referred to the incident as having happened 'recently'.
The case in the Rotterdam harbour is an 'old case' of about 15 years ago: we have installed X-band (1 kW) radars for Vessel Traffic Control and due to one of these transmitters the steering machine of a small towing ship was influenced in such a way that the ship hit the quay.
Electromagnetic interference (EMI) problems are endemic and range from being mere irritants to presenting life-threatening malfunctions. There is some evidence that EMI may have contributed to two boat capsizes, via autopilot malfunctions. One was the 16 metre fishing vessel the “Dalewood Provider” which capsized on August 17 1989, the other was the 64 tons “Martin N” which sank on April 25th 1987. In the latter case three lives were lost. In both cases the concern is that the on-board VHF radiotelephone system interfered with the autopilot sufficiently to turn the rudder hard over.
Staff at the Centre report that erratic alterations in a boat’s course when autopilot is engaged and VHF radio used is commonplace, generally due to insufficient EMI suppression at the autopilot’s interface and control cables. This interference problem is apparently so common that it is routine on some fishing vessels for the crew to turn off the autopilot when operating the vessel’s radio equipment.
When a technician tried to use his ‘com’ radio in the engine room, the connection was continually bad. By letting the radio antenna touch a cable harness, the connection became much better: the radio got a much improved “antenna”. Unfortunately, the energy in the cable screens also went elsewhere. It went via the cable screens to the thruster control equipment, which interpreted the energy as a signal for adjusting the position of the platform.
Commentary
All non-linear electrical circuits will demodulate radio waves, creating a d.c. signal modulated by the intensity of the wave. This is often called ‘audio rectification’ of an RF signal. All transistors, integrated circuits and other semiconductors are non-linear, so suffer from this problem. Analogue electronics is generally more susceptible than digital, because correctly-designed digital electronics allows a margin for noise which analogue circuits do not have.
In the case of a vessel’s or platform’s steering or positioning controls, the cables which connect the control input (e.g. a joystick control) to the electronic control unit act are quite efficient ‘accidental antennas’ for VHF radio signals, so they ‘pick-up’ the VHF signals and present them to the electronic unit where semiconductors demodulate them causing errors in steering or positioning control.
Printed circuit copper traces inside enclosures which are insufficiently shielded can also act as ‘accidental antennas’, but typical trace lengths are not as efficient as an antenna as the joystick cables, at VHF frequencies (but note that they can be just as efficient as cables at UHF, microwave or radar frequencies).
Analogue signals, such as from a joystick, are more vulnerable than digital signals, but given sufficient field strength it is not impossible for this kind of interference to cause the apparent control signals to be hard ahead, hard astern, hard left or hard right, regardless of the manual position of the joystick or other control device. A portable radio transmitter, although not a very powerful device, would be quite able to create sufficiently high fields if placed close enough to a cable.
References and links
Personal communication from Simon Brown, HM Principal Specialist Inspector, Health and Safety Executive (HSE), 14th February 2000.
Personal communication from Dick Groot Boerle, EMC Laboratory Teamleader, Radar & Sensors Business Unit, Thales Nederland B.V., June 2002.
“Need for EMI/EMC standards and regulations on small boats: a Canadian perspective” by Byron R Dawe and Albert Senior of the Canadian Centre for Marine Communications, and Peter Ryan of Fisheries and Oceans Canada, EMC Technology magazine, Nov/Dec 1998, pages 17-19.
“Cable and pipe transits for EMC” booklet, Roxtec Ltd, December 2002, page 22, http://www.roxtec.co.uk.
Links to Mitigation Techniques
| Installation | Design & Development | Resources | |
| Circuit design to reduce demodulation | |
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| PCB layout for EMC | |
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| Filtering with CM cable-mounted chokes | |
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| Filtering | |
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| Shielding of areas and volumes | |
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| Shielding of cables | |
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| Commercial computers interfere with radiocomm’s on a small vessel | |
Description
The experiences of the crew of the research vessel (R/V) Deep Scan, a privately owned research and recovery ship, offer some insight into the complexities of integrating commercial off-the-shelf (COTS) computing equipment into a shipboard electromagnetic environment.
R/V Deep Scan is constructed as a commercial vessel with many of the electrical characteristics of military mine-clearing ships. Its hull and deck structures are constructed from wood, closed cell foam and fibreglass, and it shares EMI/EMC problems common to non-metallic ships.
Computing equipment on board is said to be compliant with FCC Part 15 for radiated emissions. A commercial workstation processes sonar and navigation track data from multiple transducers. A 386 PC processes both electromagnetic survey data from multiple detection transducers and data for navigation. Navigation data is provided by COTS GPS (Global Positioning System) and LORAN-C receiver systems. Depth information is provided by COTS depth sounding equipment. Heading data is provided by a COTS fluxgate compass.
Operating the marine VHF transmitter at more than 1W begins to corrupt collected data, and any use of HF SSB transmission causes the COTS computing equipment used for magnetic data collection and navigation to enter states that challenge rational explanation.
FCC rules limit the levels of unintentional electromagnetic radiation, but the close proximity of COTS computing equipment (the vessel is under 60 feet long) to the antennas used for data collection and communications is largely responsible for disruption of operations due to the EMI the COTS equipment generates.
EMI generated by the switching power supplies in the COTS equipment slightly degrades the LORAN-C signal-to-noise ratio through radiated coupling. COTS computing equipment generates sufficient radiated interference on the HF bands to render HF communications impractical. Broadband interference and harmonics from COTS computing equipment interfere with communications reception on selected VHF channels, in some cases enough to prevent useful communications.
Daily operations on board R/V Deep Scan are influenced by the EMI and susceptibility problems associated with the use of COTS computing equipment. Responding to a call on the VHF radio presently requires the crew to wait for a logical break in survey operations, or requires termination of survey operations. During survey operations, monitoring some VHF channels is not possible, HF transmission is impossible and HF reception is seriously degraded.
Commentary
Operations on board the ship were impaired by both the emissions and susceptibility of the COTS equipment. It is of course possible to modify the COTS equipment to overcome these problems, but this would negate the cost reasons for the choice of COTS instead of higher specification equipment.
The design of the vessel, specifically its non-metallic construction to permit the use of sensitive electromagnetic sensors, were a contributory factor, although non-metallic hulls are typical of smaller vessels.
The overall message from this article is that COTS (commercial/industrial) EMC standards and regulations are probably inadequate when used in close proximity to radio receivers, radio transmitters, especially those operating in the HF (short-wave) bands. To be fair, the EMC standards listed under the EMC Directive generally point out these shortcomings. Had the computing equipment met the standard for marine navigation equipment (IEC 60945) then most or all of these issues would have been addressed.
Commercial/industrial EMC standards have to strike a balance between cost and risk, and it is seen as inappropriate to make the majority of products and systems comply with EMC specifications that only a few of them will ever experience. This means that whenever one is considering buying COTS equipment for a special project it is not sufficient to look for a general statement of compliance.
Instead, assess the electromagnetic environment that the equipment will be required to operate in, including the sensitive equipment that it must not interfere with, and compare it with the EMC tests the equipment is claimed to have passed.
References and links
“Electromagnetic susceptibility and shipboard computing” by Bruce D Salati, ITEM 1994, pages 67, 70 and 71, http://www.interferencetechnology.com.
Links to Mitigation Techniques
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| All of the mitigation techniques could be applicable, depending on the equipment or system concerned | |
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