The data collection stations must provide power and communications for the train sensor and real-time feeds to a traffic management center. To increase versatility in the early stages of the rail data collection process, the decision was made to construct mobile data collection stations. This flexibility allows for easy repositioning of detectors along the test corridor as it is unclear just where sensors are to be deployed for best results. The mobility aspect requires a data collection station to be self-supporting in power and communication. The use of low energy radar (approximately 7 watts measured power consumption) enables each site to be powered from a single solar panel and storage batteries. A 60-watt solar panel from Kyocera Corporation was chosen for the job. The panel measures 26 x 30 inches (780 sq. inches), supplies ample energy and is very lightweight. A charge controller to regulate energy transfer to the batteries completes the power generation design. Each site employs two large deep cycle marine batteries storing enough energy to supply the site’s equipment for approximately 6 days without any recharging current. Real-time communication from the field sites to the management center is the task of a corridor-wide wireless data network discussed later. For permanent installations, the solar / wireless option eliminates the need for costly trenching and hookup fees to acquire commercial power and communication (telephone service). Additionally, solar and wireless have no recurring costs.
The final piece of the design is a field control computer for each station. The field control computer manages all the different electronic equipment at the site, and performs the following tasks:
Performs all packetizing and protocol conversion for information moving on the wireless network
Controls the operation of the radar sensor including rebooting and direction sense
Controls heartbeat generation
Monitors the battery voltage and electronics compartment temperature
The field control computer is programmed to pole the radar sensor twice per second with each pole returning a speed between 0 and 99. Speeds of zero are filtered out while any other value is immediately packaged and routed to the data radio for transmission. The field control computer also monitors temperature and battery voltage and reports this information in periodic heartbeats. The heartbeats are used to monitor the quality of the communication link to each site. Missing heartbeats is an indicator of system failure at the site or severe interference on the RF channel.
Permanent installation - Radar on left, solar panel above
The field control computer chosen for the task is the Basic Tiger from Wilke Technology GmbH. The Tiger is essentially a single board computer / controller demonstrating the state of the art in microcontroller technology. It is inexpensive yet computationally powerful, physically small (not much larger than a traditional CPU chip) and consumes little energy. The Tiger offers a programming package utilizing the BASIC computer language, a common choice for low to medium duty controllers. A very powerful feature of the controller is its built-in multitasking operating system and use of software drivers to interface peripheral devices. This concept, standard in larger computers, is very helpful for rapid code development. An application rarely consists of a single monolithic task with linear processing in a large loop. Finally, the controller provides numerous channels for analog and digital input / output as well as support for several RS232 data ports.
Wireless Data Network
Example field cabinet with Tiger controller and wireless modem
A data radio network provides communication for the mobile remote and future permanent collection stations along the Wellborn Corridor. The current thrust in RF devices is to provide a robust service while being spectrum and power conservative. Spread spectrum technologies are method to achieve the goal. Several vendors offer products in this category using either direct sequence or frequency hopping techniques to produce the spread spectrum output. Spread spectrum equipment operating in the popular unlicensed 900 MHz ISM (Industrial, Scientific and Medical) band was selected as the communication hardware. The new spread spectrum radios offer several important advantages:
Increased interference immunity
High data throughput (33.4k bits per second, higher now)
10 mile maximum range for line of sight systems
Low power consumption / solar power capable
Small compact size, easy integration
No FCC licensing required
Smaller, lighter weight antennas
Choosing the unlicensed radio band enabled quick deployment but there are drawbacks. Unfortunately, the unlicensed units are power limited to 1 watt and antenna gain limited to 6dB, making for a overall maximum RF effective radiated power (ERP) of 36dBm by law. With such a low ERP and high operating frequency, all radios on the network must be in direct line of sight for reliable communication. Since the remote stations can be scattered anywhere along the corridor, a radio repeater visible to all stations was required. Height is the principal property for good long range repeater visibility and coverage. With the many campus buildings located along the corridor, a rooftop installation appeared to be the optimal choice. A structure assessment was made and several campus buildings were considered useful for the purpose. The initial location of the radio repeater was on the roof of Rudder Tower on the Texas A&M University campus. The location was excellent yielding widespread coverage over the entire corridor. The arrival of AT&T’s powerful cellular telephone transmitters signaled the end of this location for our low power repeater. The higher power cellular signal rendered our equipment deaf. An alternate location was selected on the top of the Kyle Field (football stadium) press box, also on the university campus. This has become the permanent location for the repeater, its solar panel and storage battery. The repeater has been in continuous operation for approximately 9 months with no significant problems. All of the field data collection stations communicate back to a base or master station through the repeater. The master station is housed in TTI's new Gilchrist Building located approximately 1 mile from the Kyle Field repeater. The master station is currently a PC running the wireless network communication software and an application to serve raw data to any client. The software was designed and coded in house specifically for the rail integration project. Several desktop PC’s and notebook computers are used to support the field systems. All software development and preliminary laboratory testing of new systems utilize these machines in the laboratory.
