In the mining industry there are many conveyors that transport the raw material from underground facilities to the processing plant. The mineral bearing rock has to be moved over a long distance via robust conveyor systems. These conveyors can be longer than 2 km and have to be very flexible due to the different locations of the mining fields. ifm electronic has developed a conveyor control system using AS-i (Actuator-Sensor interface) which allows connection of all the stations of a conveyor system eg, safety rope switches, emergency stop switches, belt alignment switches, temperature sensors, speed sensors and start-up alarms.
System overview
The AS interface is a hot-pluggable alternative to conventional wiring technologies, it is easy to use and extend. Speed, reduction of installation, maintenance costs and high availability are features of this system. The interface is ideally suited for the robust transmission of small quantities of data under rugged industrial conditions. It has proven itself and has become accepted as the standard for the networking of sensors and actuators in many application areas. The two-core cable for data and energy transmission, the connection in the clamping technology, the mapping in the PLC as an I/O component assembly and the extensive diagnostic possibilities all lead to significant savings.
Designed for the lowest level of the automation hierarchy, the AS-interface offers an easy, reliable, and fast transfer with optimal price-performance ratio. Higher-level systems such as Profinet, Profibus DP, Ethernet/IP, Modbus, DeviceNet or CANopen are connected via gateways. Decentralised solutions relieve the control system, depending on the extent of the setup this permits cycle times of 5 to 10 ms for one AS-i branch. The system can be extended, may be commissioned in partial areas only, reduces the communication on higher-level field buses and offers an increased failure resistance through autonomous sub-areas.
Safety in the system
Based on the same technology and the same protocol, safety-oriented components such as safety rope switches, emergency stop devices, opto-electronic protective devices and safety guard interlocking devices can also be integrated. This requires only the installation of one safety monitor and some safe slaves on the branch. A mixed operation of both safe and non-safe AS-interface slaves is also easily achieved; the safety monitor monitors the data communication on the AS-interface line. For the safe slaves, dynamic code sequences (8x4-bit data sequence) that are stored in each slave are transmitted, the safety monitor ‘learns’ these during commissioning.
During operation, the safety monitor compares the expected with the actual sequence in each cycle and carries out a safe shut-down within 40 ms if there are any deviations, eg, as a result of device failure, communication problems or the like. The time for re-activation is 100 ms. Safe field and switch cabinet modules are available as slaves, including intelligent safety sensors and safety command devices with an AS-interface chip. The system can be used up to control system category 4 according to EN 954-1 or up to SIL 3 according to IEC 61508 [10] and may be used for stop category 0 and 1 according to EN60204-1.
The Safety Rope Emergency Stop switch detects both pull and slack rope conditions and allows integration of safety signals into the plant diagnostics. Visual indication is available to provide local indication of switch status. Steady green – machine running and flashing red – machine stopped.
The conveyor control system consists of an AS-i master, AS-i power supply, a safety monitor/relay unit, AS-i cable and various I/O modules (safe and non-safe). The main benefit is a safe control system that allows for fast and flexible expansion in a cost effective way. The status of safety rope switches and emergency stop switches is displayed locally by the AS-i master and also remotely in the PLC or on a scada system.
ifm electronic SA offers a two day AS-i training workshop including topics such as installation, maintenance, fault finding for mine system integrators and the maintenance team. The company has vast experience with applications in the mining industry and can offer assistance with AS-i network design, commissioning assistance, etc.
For more information contact Chris Cronjé, ifm electronic SA, 086 143 6772, [email protected], www.ifm.com/za
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