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INTRODUCTION

Conveyor belts are a critical component of many mining and processing operations. The loss of a conveyor belt on a critical conveyor line will result in signifi cant downtime and cost the operation signifi cantly due to lost productivity. There are a number of methods that can be utilized by operations to monitor their belt in order to maximize the belts productivity. Operation monitoring sensors continually monitor the interaction between the material being conveyed, the conveyor belt and the conveyor system in order to detect situations that are considered to be out of the normal operating conditions. Included in these sensors are belt slip sensors, belt tracking or misalignment sensors, plugged chute sensors and metal detectors, to name a few. By monitoring the operation for potentially catastrophic events it is possible to minimize or avoid damage to the conveyor belt that would result in prolong downtime of the mining operation.

13:1 Operation-Based Sensors Slip Sensors

Slip Sensors monitor for belts running on a frozen pulley or a pulley driving a belt that is not moving. In either case, this type of event will result is a large amount of heat from frictional forces between the pulley and the belt that can result in fi re and potentially a broken belt. The slip sensors monitor the rotation of two different pulleys on the

system and compare the speed differential between the two pulleys. This is typically accomplished using encoders or proximity sensors mounted on two different pulleys, normally a drive and a separate non-drive pulley. Assuming the system is functioning properly, the speed of the belt should be the same in both locations.

Alignment Switches

Alignment Switches are used to measure when a belt tracks off of the conveyor structure. These are used to trigger a belt stop when the belt pushes a bar attached to a limit switch beyond the limit setting for that switch. Belt tracking could also be tracked by an ultrasonic sensor; however, the fail safe characteristics of the alignment switches are the most common methodology implemented in mines.

Figure 13:1: Misalignment Switch Mounted Near Edge of Conveyor Belt

Plugged Chute Switches

Plugged Chute switches are used to shut the belt down if the chute becomes blocked and the load is not being carried through the process as expected. Chute switches operate under a number of different sensor types including

microwave, radio-frequency type capacitance probes, ultrasonic, radar, nuclear, and laser technologies. All of these sensors are interacting in some way with the material in order to detect its presence in the chute structure when the chute is fi lling up due to a chute blockage.

Infrared Technology

Infrared Technology in the form of spot, line and camera sensors are often used in the coal industry to monitor the temperature of the material being carried; however, in some cases, IR cameras are also being used as a means to detect heat build in pulleys or idler rolls.

Counter Weight Limit Switches

Counter Weight Switches are set up on the system to monitor the position of the counter weight on the take-up ystem. If the counter weight goes to the low or high tension ends of the take-up, it is an indication that the

tensions are too low or are exceeding the upper limit of the design. A limit switch on the counter weight will be set such that if the take-up pulley exceeds its displacement range trying to fully tension the belt, it is an indication that there is not enough tension on the system. In some cases this is one of the fi rst indicators of a transverse belt tear having occurred on the system. Alternatively, if the belt tension forces the take-up pulley to the higher tension end of the displacement range, a limit switch on the upper limit of the displacement range will be activated resulting in a system alarm.

Rip Detection

Rip Detection can be achieved by several methodologies from those that monitor material spillage or belt tracking to those that involve interaction with embedded components within the conveyor belt.

Material Spillage Detection

Material Spillage Detection is utilized by a number of sensor systems to detect longitudinal belt rips. As the event progresses, material will spill from the belt at the event. There are several manufacturers that offer pull cord devices that detect material spillage as it falls on the conveyor or is pulled along the return conveyor. These pull cord systems require wires to be strategically placed transverse to the conveyor belts direction of travel. When material falling through the ripped conveyor belt strikes the pull cord, a relay is tripped in the device that indicates an issue has been detected in that area. Ultrasonic and laser based sensors have also been utilized in a similar fashion to detect material dropping from the belt indicating a rip is taking place. In this case, an ultrasonic or laser beam fi eld is disturbed and the sensor alarms.

Belt Width Monitoring

In some systems, the width of the belt can be utilized as a means of detecting longitudinal rips. If an edge strip is taken from the edge of the belt or the belt overlaps in a center rip situation, the measurable width of the belt will be eeduced and the system will alarm. Similarly, if there is an expansion of the width of the belt, the belt width monitoring systems can detect that change and stop the belt

Metal Detectors

Metal Detectors are often utilized to detect metal debris on the belt that may result in belt damage or a longitudinal rip. If a large enough piece of metallic debris is detected, the unit will shut the conveyor down to enable operations to remove the metal. In high risk areas, mines will implement electromagnets to remove as much of this metallic debris as possible out of the material being conveyed before it causes damage.

Speed Monitors

Speed Monitors are used to monitor the belt speed in order to ensure the belt drives are controlling the belt speed properly. Speed sensors can either be encoders that make physical contact with pulley or a non-contact proximity sensor that detects a target system mounted to the pulley (Figure 13:2). In either case, the revolution of the pulley is converted to a belt speed and this speed is used to verify that the conveyor is functioning properly.

Inductive Rip Detection

Induction Rip Detection has been around since the 1970’s. The principle of an inductive system is to place metallic loop antennas across the width of the belt at regular intervals along the belts length. A transmitter is used to induce electrical currents in these antennas that are then detected with a receiver as the loop passes. If the loop is good, the induced current is detected by the Receiver and a signal is sent to a control unit indicating the loop is good. A damage loop will not carry an induced current and will not be detected by the receiver. These systems normally utilizes the time between loops or the distance between loops to monitor the loops in the passing conveyor belt. If the expected time or distance between loops is exceeded due to a loop being damaged by a longitudinal rip, the system will alarm by opening a relay to stop the belt.

Figure 13:3 Transmit and Receive Detector Monitor Loop Integrity

Magnetic-based Rip Protection

The most recent advance in longitudinal rip detection is the magnetic based rip detection technology. Like the inductive systems, the magnetic systems utilize sensors that are installed at regular intervals along the length of the belt. The magnetic insert are composed of transverse or biased wires that cover the width of the belt. After magnetizing these inserts, the integrity of the inserts are monitored using magnetic sensor arrays and the previous record of the rip insert. Unlike the inductive loops, the magnetic insert can sustain some damage without signifi cant effect to its magnetic signature. As a result it is considered to be more durable.

Periodic and Continuous Conveyor Belt Health Measurements Conveyor Cover Wear

One of the measures of the remaining life of a conveyor belt is how much of the belts top or “carry” cover is remaining. Knowing the wear rate and the amount of cover left, a mine can determine how much more life to expect out of the conveyor belt that is otherwise in good shape. Based on the loss of material between measurements at two different times, one can calculate the wear rate to see if there has been a change in wear rate and/or to predict the expected life remain- ing for that conveyor belt.

Ultrasonic Wear Measurements:

Periodic measurements using ultrasonic gauges are commonly used by site surveyors to assess how much cover wear has taken place on textile conveyor belts and steel cord belting. The ultrasonic measurement determines the gauge of rubber above the carcass by transmitting the ultrasonic wave through the material to the carcass and measuring the refl ected wave to determine the gauge of the rubber cover. In fabric belts, the ultrasonic wave refl ection is mainly from the fi rst layer of fabric and these refl ected waves are then detected by the ultrasonic sensor to determine the cover gauge of the belt. Similarly for steel cord belts, the ultrasonic waves will refl ect off of the steel cables. It should be noted that for the best results, it is important to know the properties of the rubber you are measuring as the transmission of the ultrasonic waves through different rubber compounds will yield slightly different results.

Eddy Current Wear Measurements:

Similarly, periodic measurements using eddy current-based sensors can also be used on steel cord belts to mea- sure the cover gauge. The eddy current sensor emits a high-frequency alternating-current magnetic signal. When the sensor is moved towards a conductive surface, eddy currents are generated on the surface of that conductor. The magnitude of the eddy current signal detected is dependent on the conductive properties of the steel cord and the separation of the steel cord and the sensor surface. Given that the conductive properties of the steel cord are known, the gauge of the rubber can be determined in order to give a measure of the remaining rubber gauge on the conveyor belt.

Laser Wear Measurements:

More recently, laser based scans have been used to accurately determine the thickness profi le of the conveyor belt by measuring its thickness across the width of the conveyor belt. In essence this is done by using lasers posi- tioned above and below the conveyor belt and doing a differential measurement to determine the overall gauge of the conveyor belt. This type of measurement is capable of giving very accurate information about the belt thickness belt; however, one must be careful that the belt is clean and that pulley cover wear is not contributing to the gauge variation of the belt.

Conveyor Belt Integrity

Conveyor belt integrity is often determined from conveyor belt scans. Many mine sites will have diagnostic scans of their conveyor belts completed on a regular basis in order to evaluate their potential risk of a transverse tear or splice failure. X-Ray Based Belt Scanning

For many years, X-Ray scanning of conveyor belting has been utilized to determine the integrity of the conveyor belt carcass in fabric or steel cord belting. These scans are typically done periodically and require radiation restrictions to be applied where the scans are performed. During an X-Ray scan, the X-Rays penetrate the belt and are measured on the opposite side of the conveyor belting. The integrity of the material reinforcement in the belt is measured as a function of the intensity of the X-Ray image. The X-Ray image intensity will vary with variations in density of the internal components of the conveyor belt. As a result, damage to the fabric

reinforcement or the steel cord reinforcements can be detected due to the density changes associated with these damage points.

Magnetic-Based Belt Scanning

Magnetic Scans of steel cord conveyor belts have become one of the most popular methods of scanning steel cord conveyor belts to determine the integrity of the steel cords running longitudinally. The majority of the systems rely on a permanent magnet mounting that magnetizes the cords in the conveyor belt as it passes over

or under the magnet. Once magnetized, a single steel cord will have a north polarity on one end and a south polarity on the opposite end. The magnetic fl ux lines that are emitted from these cord ends will be more per pendicular to the belt surface and hence magnetically distinct from the magnetic fi eld of the rest of the belt. These distinct magnetic fl ux lines can be detected using inductive coils or solid state sensor technologies, both of which generate voltages proportional to the magnetic fi eld strength in the region of the cord ends. With this technology, it is possible to map any cord damages that occur along any given length of belt. The fact that this sensor technology is not restrictive and can be used to monitor the conveyor belt continuously, these sensors are starting to fi nd their way into the mainline sensor arsenal of mines. The ability to detect and minimize the risk of a transverse tear event and the ability to detect longitudinal rips is expected to become a standard in steel cord conveyor belt monitoring.

Splice Monitoring

Splice monitoring in textile belts has always been a challenge. Historically, the two most common systems for splice monitoring in textile belting utilize optical or magnetic markers near the edges of the splice. By comparing subsequent images of the splice to a baseline image, the quality of the splice can be determined. Primarily, the analysis is looking at splice deformation compared to the baseline image in the form of de-linearization of the splice along designed

construction lines and/or the elongation of the overall splice length. X-Ray Based Splice Scanning

Periodic X-Ray scanning of textile and steel cord splices has also been a method of checking the integrity of the conveyor belt splices. Like in the conveyor belt, damage to the fabric or steel cords will be visible as density changes in the belt. Variation in splice lengths and deformations can also be determined by analyzing the X-ray images and comparing them to a benchmark image of the belt.

Magnetic Based Splice Scanning

Steel cord splice monitoring has historically been done as part of the belt analysis on a periodic basis. Once a magnetic scan was completed the magnetic signal is reviewed for evidence of change or degradation of the magnetic intensities. In some cases, once a magnetic anomaly has been identifi ed X-Ray images are

completed to verify the magnetic results. This type of analysis has been automated more recently in order to continually monitor changes to a splice over time and to alarm if the splice quality is degrading.

For the best combination of conveyor belt design and applicable sensor systems for a given application, it is recommended that the belt manufacturer is consulted.