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