health & SAFETY is generated. �e type of alarm varies with the type of display. �e most common type of radar used in collision warning systems employs Doppler shi� detection. �ese systems only detect relative movement of objects by sensing a frequency shi� in the re ected signal. For the system to activate an alarm, either the object within the transmitted beam pattern must move toward the stationary vehicle, or the vehicle must move toward the object. If both object and vehicle are not moving, no alarm is activated. �is has the advantage of providing alarms only when a vehicle is moving and a collision is imminent. However, it has the disadvantage of not providing an alarm that allows sufficient time to stop if a stationary object is very close to a vehicle that starts moving. Other types of radar, such as FMCW, can be con gured to operate in this manner also. Pulsed radar is also used for collision warning systems. Because of the pulsed nature of the output signal, the average power output by the radar is extremely low( microwatts). Also, time of ight for the re ected signal is easily measured, and accurate range information can be provided as long as the detected object is not immediately next to the antennas. Systems using this type of radar can sense the presence of an object whether there is relative motion or not.
Radio-frequency Identi cation Systems Several companies have applied RFID systems to the collision avoidance problem. �e systems typically consist of a tag reader, tags, and an operator interface or alarm display. �e tag reader detects radio transmissions from a tag if the tag is within its reading range. �e tag reader is mounted on the mine equipment, while tags are mounted on any item that is to be avoided. �ese items can include other vehicles, pedestrian workers, power and utility poles, or even a building. �e small cost and small size of most tags allows them to be mounted on hard hats, personnel belts, or anywhere on the exterior of a vehicle. �e nature of radio-frequency signals makes this technology ideal for harsh environments because radio transmissions are not adversely affected by harsh weather, dust, or moderate amounts of mud buildup on the antennas. However, radio signals can be affected by interference from multipath effects and other radio signal sources. �ere are two types of tags: passive and active. For passive tags, the tag reader constantly transmits a signal that activates a tag if it is within range. �e tag then answers with a unique signal that is detected by the tag reader. �e advantages of passive tags are that there is no requirement for external power, they need little maintenance, and they are very inexpensive. �e disadvantage is that they o�en have a more limited reading range than active tags. With active tags, the reader can be passive and just listen for the unique transmissions from the tag. �e disadvantages include the need for batteries and increased cost and size when compared to passive tags. If a reader detects a tag within its reading range, signal processing so�ware analyses the signal to determine signal strength and integrity. If the acquired signal meets alarm criteria, then an audible and / or visual alarm is indicated at the alarm display. �e alarm thus warns an operator that a tag is in proximity to the equipment.
FMDZ | Jan- Feb 2017 | Page25