The PED Transmission System
The PED Transmission System (refer to Figure 1 following), in its simplest form, consists of the following:
- A Windows PC
- A proprietary Modulator Card
- PEDCALL Software package
- A PED ULF Transmission Headend
- A Loop Antenna
PEDCALL PC & Software - The Operator Interface
The computer, Windows based, which is running PEDCALL controls the Transmission System. The PEDCALL software provides the interface from the operator to the system in a simple and efficient manner.
The operator inputs information, such as the destination and the message content, then the PEDCALL software will encode this information. Encoding of the destination and message uses advanced encryption methods to eliminate any chance of invalid information being transmitted. These encryption methods also ensure the receivers can decode the information precisely in adverse signal conditions.
Modulator Board
The Modulator Unit is a stand alone unit that connects to the PC through a standard RS-232 Com Port.
The output signal is a 0-20 mA Current Loop. The output is usually connected to the Transmitter by a twisted pair, though optic fibre or radio links can be used if necessary.
PED ULF Transmission Headend
The PED Headend is mounted in a 19" Rack Cabinet and is connected to the incoming 0-20 mA Current Loop. The main role of the Headend is to boost the signal into a high power output capable of driving up to 250 volts at 5 amps into a large loop antenna.
The Transmitter within the Headend utilises an efficient technique to achieve this role continually, it is also protected from thermal overload and short circuit conditions. The Headend also contains Antenna Safety Unit and Loop Matching Transformer (see below) to optimise signal quality and safety.
Each Headend can drive one loop antenna, therefore if a large mine requires two loop antennas, then two transmitters will be connected. The Transmitter has a maximum output of 1.2 kVA, hence requires locating in a cool ambient environment, it should also be relatively dust free. The output from the PED Headend feeds into the Loop Antenna.
Loop Antenna
The Loop Antenna layout is critical to system performance. The layout will determine the range of signal transmission. Generally, the larger the loop (up to a maximum of 12 km) the better the coverage will be.
The Loop Antenna carries 5 amps of current (nominally) and, due to this current flow an electromagnetic field is created around the Loop Antenna. This field, appears as concentric bands radiating off the cable. The concentric pattern ensures signal is present inside, above, below and off the edge of the loop. The antenna is typically 8 gauge or 10 mm2 single core cable.
Due to the signal radiation pattern, one centrally located loop can cover an average size mine.
Surface loops are most desirable, due to the infrastructure being on the surface rather than underground. Underground loops are used where surface access is difficult. Underground loops work as effectively as a surface loop and are usually smaller, as the signal does not have to travel as far.
Antenna Safety Unit
The Antenna Safety Unit (ASU) is one of the modules within the PED Headend and provides protection to personnel from the potentially lethal voltages present in the PED Antenna Loop. The ASU offers Earth Leakage and Open Circuit protection.
Other components that can be used with the transmission system are:
- PED Loop Matching Transformer - The PED Loop Matching Transformer (PLMT) is located within the Headend and is designed to optimise transmitter output into the antenna. The PLMT is inserted between the transmitter and loop antenna. To ensure the best results are achieved the PLMT is customised for each installation.
- PED Protector - The PED Protector is located within the Headend and is a surge protection device. The unit is installed immediately before the loop antenna, hence any surges (e.g. those induced by lightning) on the antenna will be diverted and therefore minimise the risk of damage to the other transmission system components.
- Uninterruptable Power Supply - An Uninterruptable Power Supply (UPS) protects the transmission system from power fluctuations on the incoming mains supply. The UPS will ensure a clean and constant AC power supply to the computer and Transmitter. An UPS is highly recommended when the PED System is installed as a mine wide warning system.