The PED System is an ultra low frequency through-the-earth communications system used for paging, control, and remote blasting. PED is the only commercially available, and proven, through-the-earth (TTE) mine communication system in use at mines. It has been deployed in over 150 mines since the first installation in 1990.
PED is an acronym for Personal Emergency Device. The system was originally developed to provide a fast and reliable method of informing underground miners in emergency situations. Due to the system enhancements and the inherent ability to readily contact personnel, wherever they are underground, PED has also come to stand for Productivity Enhancement Device, as it complements other systems such as leaky feeder and digital networks.
The use of low frequency (ULF) electromagnetic signals enables the PED signal to propagate through several hundreds of metres of rock strata. The signal can therefore be received at any location throughout the mine with an antenna on the surface only, or a small underground antenna (please refer to Operation Schematic in “Transmission System” section).
For this reason, PED is an extremely effective emergency communication system. The ability to transmit actual messages is vitally important in allowing, not only a warning to be issued, but specific information regarding the situation to be sent (such as where a fire is, which evacuation route to take, etc).
The PED system can communicate with any, or all, of the following receivers:
- ICCL – personal receiver worn by miners which is integrated to the ICCL cap lamp.
- AutoPED — vehicle mounted receiving units.
- ControlPED — remote control of underground equipment.
- BlastPED — remote, centralised blasting system.
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.
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.
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.
PED Receivers – Personal Paging, Remote Control & Remote Blast Initiation
The PED system, as a paging device, has the ability to contact individuals, groups of personnel or provide a general broadcast to all PED receivers. This could be:
- to make contact with another person
- to give an instruction to evacuate
- a request to go to a different location
- an instruction to attend to a breakdown
- a request to bring equipment or supplies to a particular location
The PED will enhance the total effectiveness of the underground communications network. Specifically, the ability to immediately contact individuals, regardless of their location, will increase the efficiency and safety of underground operations.
The same PED Transmission System used for personal texting utilises other receiver types to:
- remotely initiate blasts (see BlastPED)
- switch equipment, such as fans and pumps, on & off (see ControlPED)
- send a text message to a vehicle regardless of who is driving (see AutoPED)
To expand, these receiver types are:
The Personal PED Pagers (BeltPEDs and ICCLs) are fitted to the miners cap lamp battery. The Personal PEDs will display any message up to 32 characters in length. On reception of a valid message the cap lamp will flash (full to half brightness) and a buzzer will sound. The message is displayed on a backlit Liquid Crystal Display (LCD). The units will store two messages. The Personal Pagers can be paged individually, or paged in groups, or all paged simultaneously.More Information on BeltPED Receivers
AutoPED Vehicle Mounted Receivers
These units are installed in mine vehicles. Due to the shielding effect of a solid steel cabin, and the electrical noise from the alternator, the AutoPED is separated into an external antenna unit and an internal display/receiver unit. The external antenna removes any limitations imposed by the surrounding cabin. The AutoPED, has similar features to the personal PED, additionally it can be connected to the horn or lights to indicate message reception.More Information on AutoPED Receivers
These units allow the remote control of devices, such as fans and pumps, underground. More precise time management of these devices will reduce energy consumption.More Information on ControlPED Receivers
BlastPED Receiver Initiators
These units are utilised for remote, centralised blasting in hard rock mines. Though the underground version is not generally used in coal mines, this application for blasting demonstrates the high reliability and signal integrity of the PED Transmision System. The BlastPED unit uses capacitor discharge into the firing line, thereby initiating electric detonators. The BlastPED system has several levels of security, both hardware and software, to ensure safe operation.More Information on BlastPED Receivers