Since the daily inspection of the large number of refrigerated containers carried on board a vessel takes a significant time, a number of shipping companies have started to use systems which enable remote monitoring of the containers. Data is exchanged between the ship’s computer and the containers. As a result, the crew is in a position to react to problems more quickly. When relying only upon daily rounds, it is possible that a container has had an alarm for 24 hours earlier, and which gets noticed at the time of the rounds. Using a remote monitoring system can drastically cut down the time and cost of inspecting the containers _ and enables the crew to react more rapidly to potential problems in the event of a refrigeration unit failure.
These systems could be wired systems or wireless systems. The information may be about current temperatures, any alarms that have occurred etc. Printed logs generated as a result of this exchange can effectively replace manually recorded temperature data.
Types of Remote Monitoring Systems
There are two basic types of Remote Monitoring Systems:
Four Wire System
Here, a separate monitoring cable with four wires is used to record the status messages “Compressor running”, “Defrost” and “Temperature in range”. Around 80-90 % of all refrigerated containers have a socket to connect them to this type of monitoring system.
Four-wire (4-pole) monitoring system consists of a cable with three individually insulated wires which are used to transmit three signals as active 24 V signals. A fourth common return wire is connected to the chassis of the container. It is possible to determine whether a container is connected to the four-wire monitoring system by checking for a connection between this return wire and the ground wire of the container.
The main disadvantage with this system is contact problems. The signal sockets on the containers and on the ships are equipped with protective flaps, but the sockets still regularly suffer from corrosion damage due to the rough environment or carelessness in not replacing the caps after disconnecting the cables. The cables are also prone to damage as they are often subjected to rough treatment when twist locks and lashing rods are dropped. Sometimes the cables are also simply torn off, because no-one bothered to remove them before unloading the containers. The signals are transmitted as voltage signals with the status 0V and 24V, so it becomes impossible to determine whether or not there is a reliable electrical connection.
Three signals are provided by four-wire technology, the most important signal is undoubtedly “Temperature in range”. If this is not issued, an alert is triggered. Very simple monitoring systems can therefore only evaluate this signal. The other two signals are “Defrost” and “Compressor running”, which are status signals which are required to provide further information. For example, the “Defrost” signal, can be used to suppress a temperature alarm as during defrosting, the temperature is expected to deviate from the nominal value. A cooling compressor which is constantly running (Compressor running) in low temperature mode can indicate a fault in the container. During normal cooling operation, the cooling compressor runs in on/off mode. During defrost, the compressor is switched off and switched on again after defrosting. The “Temperature in range” signal will not be issued during defrosting, since the air in the cooling unit is being heated, but must be issued again, later.
Power Cable Transmission System (PCT)
In this system, data is transmitted via the three-phase power cable of the containers. This enables an unlimited amount of data to be transmitted between the container and the receiver on board or on land. Since data can be exchanged in both directions, it is possible, for instance, to change the set point value of the temperature of a container fom the remote location. There is a tremendous cost-saving potential offered by the option of making remote Pre-Trip Inspections (PTI) of the containers on board or in the terminal, as well as to read out data logger information after a loaded passage.
PCT system is currently available in two types: Narrowband transmission, which operates at a fixed frequency to modulate data on to the power supply system, and Wideband transmission, in which data is transmitted over a frequency spectrum. These systems are not compatible with each other. So, depending on what modems are fitted to the containers, both systems have to be installed on board to be able to communicate with all containers. By 1997, about 6.6% and 5.3% of reefer containers were equipped with narrowband modems and wideband modems, respectively, depending upon the choice of the shipping line and the route. All the problems that arise from using an extra cable in the four-wire monitoring system are eliminated by use of PCT. Since it merely depends upon the transfer protocol used, PCT also has a significantly larger range of functions, since any data can be transmitted. The data is modulated onto the three-phase power supply system of the ship or the terminal as a high-frequency signal and received by one or more master modems. It is then transmitted from there via a bus system to the control computer.
Long Distance Systems
The following different types of technology are used by different providers to cover longer distances.
Single Master Modem System
A system with a capacitive network only needs one master modem which, is connected to the three-phase power supply system over a kind of signal line and one or more Capacitive Bridge Units (CBUs). Transformers are bridged by use of Transformer Bypass Units (TBUs).
Multiple Master Modem System
This system uses a number of master modems, which are connected to each other via a field bus. Depending upon the network configuration and the distances to be covered, the number of master modems required has to be decided. However, this system is not without its disadvantages. The first of them is that no more than one master modem may be running at a time, which significantly reduces the effective average data transfer rate in the event of several master modems. Additionally, it is possible that some containers are positioned in the catchment area of a number of master modems, meaning that, the same data is transferred several times unnecessarily, needing to be filtered out.
Data Transmission via Narrowband
This is the older of the two methods. Data is modulated onto the power supply network at a fixed carrier frequency of approximately 55 kHz. It is transmitted at a rate of 1200 baud, which is why this system is often also referred to as a “low data rate system”.
Sealand were already carrying out their first trials with PCT at the end of the 1970s. At the beginning of the 1980s, Thermoking collaborated with Sealand to develop the first marketable system, known under the name of ThermoNet. Sealand and Matson were the first to use this system on a large scale, principally in the relatively closed refrigerated container trade routes in the Pacific.
Data Transmission via Wideband
In the middle of the 1980s, the wideband system arrived, transmitting data in a frequency spectrum of approximately 140-400 KHz. Transmission over a number of frequencies was intended to ensure reliable transmission even with interference frequencies such as those generated by frequency converters. By distributing the signal over a frequency spectrum, the strength of a signal on any frequency is lowered than with narrowband laying claim to a greater range. Since the data transmission rate on wideband systems is theoretically 19,200 baud, it is also known as a “high data rate system”, though in practice this speed benefit is barely discernible.
ISO Protocols
An ISO sub-committee was set up between 1987 and 1990 to define a standard as it was discerned that a monitoring system is useful only when there is a standard for data exchange – and genuine saving effects can only be achieved if all refrigerated containers can be monitored by PCT as far as possible. The set of Standards was published as ISO 10368. Due to the non-commonality of interests among the various participating companies, no consensus was reached regarding hardware (i.e., the transmission frequency), and consequently there are still two systems available in the market. The frequency ranges for each system were defined, so that they could both be operated simultaneously.
Additionally, the standard primarily regulates the data transmission protocol (i.e. software) and defines the minimum range of functions for Remote Communication Devices (RCDs) comprising of the following:
Standard Prescribed Functions
- Identification number of the container or clip-on unit
- Porthole container number (for porthole containers refrigerated with a clip-on unit)
- Date and time of any change to the porthole container number
- Current return air temperature
- Current supply air temperature
- Manufacturer/type
Important Optional Functions
- Operating mode (Full Cool, Partial/Lower capacity cool, Modulated Cool, Fans only, Defrost, Heat) • Nominal temperature
- Alarms (status query)
- Current alarms (in the order in which they occurred)
- Product temperatures
- Data logger interval
- Power consumption
- Port of destination of the container
- Port of discharge
- Origin
- Results of the self-check (PTI)
- Commands for controlling & programming (in so far as the controller of the refrigerated unit supports them):
- Change the nominal temperature
- Start self-check (PTI)
- Change the identification number
- Change the data logger interval
- Set the date and time of the data logger
- Change the operating mode
- Download data logger information
- Change the porthole container number
- Change the destination
As the standard had not defined precise data protocols for all commands, room was left for subsequent extensions in the form of “private sessions”, which can be used by individual manufacturers to transmit proprietary data, which was used extensively by certain manufacturers. The stage has reached that many functions available today are transmitted within these non-standardized protocol sections. Disagreement still persists on which of the protocols should be put in the public domain and therefore be made available to competition for this type of transmission, and under what conditions of usage. The ISO Standard has only documented the two existing systems and prescribed some very basic queries. Even if all transmission protocols were put in the public domain it would mean today, that a separate software driver would have to be available for every modem type. Because the controllers of the refrigeration units and the data loggers which are used also have different ranges of functions and data formats, there is a need for a large number of drivers to support all potential configurations.
Data Protection
Data Protection was another issue which was not dealt with by ISO. Practically, and also in accordance with ISO, all data on all containers equipped with modems is available on the power supply network. Therefore, it is theoretically possible for third parties with access to the power line network via a master modem to read out and even change information on the containers (e.g., the nominal values). This was never a problem while shipping companies were only using PCT on board their own ships and terminals. However, once it began to be used on multi user terminals, the network operators (terminal operators) have had to ensure that only authorised persons have access to information on containers which pertains to them.
Limitations of Remote Data Transmission Systems
Power cable transmission usage has generally been restricted to shipping companies with a high proportion of refrigerated containers. There is no evidence of all refrigerated containers being generally equipped with modems. Two different systems will still be deployed, which means that the evaluation installations used on ships and terminals must be able to cope with both systems for the foreseeable future in order to effectively exploit the savings potential. Though it appears that recent investments are generally being made in wideband, there are still too many narrowband containers to expect them all to be converted to wideband. However, as a first step in the long term, the modems of the first generation will have to be upgraded, since the impedance values are too low and this interferes with dual band line transmission.
Already four-wire monitoring technology has become extinct and we do not see much of its application on board. It has been replaced by power line transmission systems. On a ship or at a terminal, every refrigerated container slot must have the relevant sockets available to connect the signal cable. These sockets are often integrated directly into the refrigerated container power outlets. The signals from the individual containers are transmitted from there either via a field bus system or via the available power networks to the evaluation computer. The signal cables themselves must also be available.
Looking at the Future of Recording Systems
It is now possible to transmit data by radio frequency, e.g., using “Wireless LAN” technology, as this promises higher transmission rates and lower interference.
Originally, it was thought that using satellites to monitor refrigerated containers generally fails, because when the containers are stacked they cover the antennas of the containers below them, thus making data transmission impossible. The same applies when containers are stowed below deck. However technology has now made available this facility at a reasonable cost. Orbcomm has at present made commercially available Low Earth Orbiting Satellites (LEO) and Tri-mex has designed cargo tracker service for temperatureregulated cargo. It has integrated its Windowsbased tracking and monitoring technology with Orbcomm LEO communicators and has generated a service where a cargo can be tracked and monitored anywhere in the world – whether on land or at sea. For the perishables transportation industry a new age has dawned. Low-cost technology is allowing tracking, monitoring and intervention on all classes of cargo. This increases the information available to the client and improves the delivered quality of the goods. For the first time, conditions being experienced by cargoes in reefers can be monitored remotely _ and responded to in minutes in many cases electronically. This will improve the service record of movements of perishables and will bring down operational costs – to the benefit of shipper and customer alike. It is also likely to bring down the insurance costs significantly.
