Critical Aspects of Manned Vehicle Communication Systems

Critical Aspects of Manned Vehicle Communication Systems

Peck, Jerry M

Plan the Communication System Early During Vehicle Design for Successful Results; Here are Some Guidelines

As man presses deeper into the world’s lakes and oceans, he must turn to different technologies that allow longer stays at these depths. To meet this challenge, the obvious choice is one-atmosphere diving systems such as hard diving suits or submersibles.

Many such systems have been developed over the years, but recent technological advances have made these vehicles more reliable and practical. Because the vehicles are proving themselves dependable for both research and industrial applications, more and more designers are going to the drawing board. Unfortunately, some designers of one-atmosphere vehicles do not even consider one of the most important components: the underwater communication system. Instead, the communication system is ordered as an off-the-shelf item at the end of the construction phase.

Problems

To fully understand the problem of inadequate attention to communication system design, one must consider the following real-life scenario. In this case, the submarine was fully designed and about to be tried at sea when the call came in to order the underwater communication system. The first problem encountered was the extremely limited space available for the electronics package. Because of contract and timeline constraints, an off-the-shelf system was ordered. The housing had to be stripped away and, in some way, the remaining internal board had to be mounted so it would not interfere with the pilot.

All was well until the computer was powered up. There was little space between the onboard computer and the communication system circuit boards. Computers produce considerable radio frequency interference, so the noise through the headset was very loud- and that was not the only source. The submarine had DC-to-DC converters and motor variators (pulse width modulation controllers), which create extensive electromagnetic interference (EMI).

While internally mounted instruments can cause noise interference, there can also be external sources. For instance, a system may be noise-free until the transducer cable is fitted with a connector. Usually, a bulkhead connector is fitted to the pressure hull and soldered to a coaxial cable routed from the underwater telephone. The cable leading from the transducer mounted outside the pressure hull is fitted with the bulkhead connector’s mating connector. Most connectors do not shield the transducer signal as it passes through the hull so at the hull interface, noise can be coupled and mix with the transducer’s weak receive signal.

Since the submersible’s hull is not part of the ground system, any electromagnetic interference radiating from electrical cables can couple to the hull. Indeed, these electrical lines often assume a parallel path along the submersible’s skin. In some cases, an EMI field can couple directly into the ceramic transducer and mix with the intended received signal.

This scenario often plays out when the attention given to the major construction aspects of the submersible (such as pressure rating, speed, etc.) are at the expense of ignoring a critical component needed for successful and safe submersible operations: the communication system. The underwater communication system should be included early in the design to address important issues such as where the system is to be installed and where electrical conduits and cable bundles should be routed to cause minimal communication system interference.

System Selection

Which system to install depends on several factors. In larger submersibles, such as large tourist craft, there is usually more than enough room for a rack-mounted system. However, on the other extreme are the one-atmosphere diving suits (such as the Newt Suit) and the Deepworker 2000-class of one-man submersibles. With these systems there are clearly space problems, and the communication system must either be rather small or be mounted on the outside in its own pressure housing. A small control module for volume, squelch, etc. is all that is needed inside. Since an externally mounted system is usually located away from potential electrical noise sources, there is likely to be less chance of interference. However, there is still the possibility that EMI may pass into the ceramic transducer.

Recently, two Deepworker 2000 one-man submersibles, plagued with interference problems, were fitted with a long-range 70-watt diver transceiver as a solution to the space constraints of a small craft. Instead of the usual diver’s earphone and mask microphone, the unit was fitted with a headset with a boom microphone, the transducer was removed and replaced with a connector and cable, and the end of the cable was soldered to the hull connector. The mating connector was fitted to an external transducer. This setup provided a very small beltworn transceiver for the pilot to wear, and enough power for transmission in and around offshore oil platforms at a depth greater than 1,500 feet. Although there was minor interference because the bulkhead connector was not shielded, communications were more than satisfactory.

Frequency Selection

Frequency selection is usually made in response to the mission’s demands. This includes compatibility with other surface or submarine craft. For example, if the mission involves military communication with North Atlantic Treaty Organization (NATO) craft of a different member nation, the NATO frequency of 8.0875 kilohertz upper single sideband must be selected. On the other hand, if the submarine is used for commercial or research purposes, a frequency like 25 kilohertz upper sideband is usually used.

Transducers

The range of frequencies that can be used is limited by the transducer’s bandwidth. Usually, the transducer’s peak output and sensitivity are selected to match a particular frequency. With most research submersibles, the reference frequency is 25 kilohertz. If this transducer is operated at 20 kilohertz, transmit and receive responses are decreased. For systems requiring a large frequency spread, a single transducer will not be effective. Typically, a transceiver will have dual-frequency capability and employ two transducers, one for 8.0875 kilohertz (military) and the other for 25 kilohertz (research or commercial).

Some systems must have two transducers for each channel. These transducers are mounted in different locations on the submersible, usually on the top and bottom to communicate with vessels both above and below. The critical aspects here are the transducer cable, the location of the transducer with respect to other electrical cables and the entire mounting structure.

Transducer cables are adequately shielded against electrical interference; the hull bulkhead connector is another story. Unshielded hull bulkhead connectors mix interfering signals into the transducer’s signal. The source of this interference can be from DC-to-DC power supplies or motor speed controllers. The customer is advised to select a bulkhead connector that does not compromise the integrity of the transducer’s shielded cable. Even if the transducer’s signal is adequately shielded, there is still a possibility of electrical interference; the transducer itself is not 100 percent shielded.

There are two types of transducer connections, balanced and unbalanced. An unbalanced connection uses a simple coaxial cable to carry both transmit and receive signals. The balanced cable usually has two twisted center conductors surrounded by an overall shield. Interfering signals are conducted along the shield and are directed to the system ground. Because the two internal cables are twisted, they tend to be in a common phase and amplitude. Receiver electronics are designed to cancel these common signals and to amplify only signals that are in exactly opposite phases. Interfering signals can couple to the transducer electrodes differently, causing unequal signals to be sent down the twisted-pair transducer cable and, therefore, to be amplified by the receiver.

Electrical interference can be unpredictable and difficult to troubleshoot. One of the most unpredictable problems when working with audio transducers, including microphones and ultrasonic transducers, is the ground loop. The entire electrical environment becomes part of the electrical interference path, so there is no way to predict the flow of this interference. Interference problems are relatively severe on submersibles because, unlike a microphone amplifier that has an amplification factor of only 1,000 or so, the underwater transceiver’s factor is on the order of 1,000,000. With amplification factors that high, only a few microvolts of noise can render an underwater communication system’s receiver completely useless.

The transducer cable is connected via capacitors on both ends or only on one end of the cable. When both ends of the shield are connected, one capacitor is connected between the shield and the transducer’s metal mounting base, placing the transducer’s mounting plate at AC ground potential, and providing a lower resistance path for interfering signals to flow than the ceramic transducer element. Obviously, a DC connection to the hull must be avoided, since the hull must remain neutral. The transducer base plate should be mounted on a non-conducting plastic such as PVC.

Some extreme cases of interference through the transducer may require onsite trial-and-error testing to find the optimum configuration. The final solution may be a combination of the choice of transducer, the cable return connection, the transducer cable routing and modification/filtering of noise-producing equipment.

Power Supplies

Most underwater telephones require either a 12 or 24-volt power supply. The supply must be rated to handle peak currents when transmitting. If a DC-to-DC converter generates this power source, it should be very clean and free from noise spikes. These converters are known for producing generous amounts of EMI and should be carefully chosen for low EMI values. Most power supply manufacturers offer post filters that are placed between the converter and equipment.

If a DC-to-DC converter is not filtered properly, it can radiate EMI throughout the cabin. Cabin-radiated EMI can enter the transceiver directly through the circuit board. If the EMI is intense enough, and the frequency is low enough, it can directly influence components on the transceiver’s printed circuit board. The lower the frequency of the EMI, the harder it is to shield using metal enclosures around the electronics. It just so happens that most high-power DC-to-DC converters operate between 20 and 200 kilohertz and spew huge amounts of radiated noise. These frequencies are particularly troublesome, because underwater telephone frequencies lie within the lower portion of this range.

Another area of concern is the outboard-mounted power supply. Some submersible designers elect to place their power supplies inside pressure vessels, which is not a bad idea as far as submersible design goes. However, there can be intense EMI radiation from interconnecting cables. If the supply cables are not shielded (which can be costly), then they act as antennas radiating EMI around the proximity of the pressure hull. Since the hull is neutral, these lines of electrical force tend to travel along the surface and can enter the transceiver either through the transducer element or the bulkhead connector.

Other Sources of Interference

More and more onboard computers are being installed in modern submersibles. Computers and their peripheral equipment are known to be a potential source of interference. One of the worst offenders is the computer monitor. If equipped with a touch screen, interference levels can be quite high. If the interference is intense enough, it can penetrate the transceiver housing and generate noise current within circuit board components. This noise can mix with the desired receive signal and be amplified and sent to the pilot’s headset.

Careful thought must be exercised when routing cable bundles. Never route sensitive transducer or microphone cables in the same bundle as power or computer data cables. Doing so is just asking for trouble.

Conclusions

A well-thought out design, in which all systems and components work in harmony with each other, is absolutely necessary. Electronic interference can be very problematic, even after precautions are in place and guidelines such as those presented here are followed. Applying the information presented in this article can at least minimize interference problems. With this information and proper training, communications interference problems are no longer a mystery, and submersible designers can deal with them more effectively because they better understand the critical aspects of manned vehicle communication systems. /st/

For more information, visit our website at www.sea-technology.com.

By Jerry M. Peck

Technical Director/Chairman of the Board

Ocean Technology Systems

Santa Ana, California

Jerry Peck has over 35 years of experience in the field of electronics, specializing in underwater acoustics. In 1984, Peck, along with Mike Pelissier, founded Ocean Technology Systems. He is currently technical director and chairman of the board, with 50 percent ownership. Holder of several patents and author of several papers, Peck is dedicated to advancing the field of diver and submersible communication. His latest achievements are the development of a software-defined through-water transceiver and diver’s microphone.

Copyright Compass Publications, Inc. May 2005

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