Underground mining has unique and significant safety hazards compared to surface mining, due to high-stress concentrations, weak rock masses, and limited access and air quality.
Traditional monitoring equipment for stope monitoring is ineffective and impractical in most cases, due to the difficulties in placing the monitoring equipment inside unsafe areas of the stopes.
Drones come handy here for engineers to perform various mines missions, which are too “dull, dirty or dangerous” for humans. Equipped with sensors, motors, propellers, cameras, and GPS systems, they can complex mining tasks in a matter of hours with fewer man-hours, boosting productivity and efficiency.
Notably, fast technological advancements in passive and active sensors have empowered drones’ capability in various types of missions. Sensors come in different specifications depending on drone size, investigation goal, and the lighting condition.
This post will explore some of the commonly used sensors on mining drones, including infrared sensors (IR), ultrasonic sensors, RGB sensors, stereo cameras, laser range finders (LRFs), Ultra-Wideband Radar (UWB), and hyperspectral sensors.
1. Infrared Sensors (IR)
Infrared Sensors (IR), considered heat sensors, are low-cost obstacle detector sensors that can detect objects’ energy radiation. Generally, in the infrared spectrum, all materials above absolute zero emit waves.
Despite the limited resolution, infrared sensors can easily detect humans. It also has the advantage of sensing through fog, smoke, day, and night. However, the sensor images can be distorted by flame and any other high-temperature sources. Moreover, it does not work well through the thick dust.
2. Ultrasonic Sensors (US)
Ultrasonic Sensors (US) are also inexpensive and uncomplicated sensors viable for various applications. The only standard sensors in the drone technology that is not based on electromagnetic waves (EM), they detect the obstacles by radiating high-frequency sound waves and collecting reflected waves. The distance to the obstacles is determined by calculating the time-of-flight. One disadvantage is that they have a shorter range compared to other sensors.
3. Red-Green-Blue (RGB) Sensors
RGB camera is commonly used in surveying and mapping, road traffic monitoring, stockpile volume calculation, security monitoring, inspection, etc. It captures RGB (Red Green Blue) images and has two active stereos or time-of-flight sensing for depth evaluation.
The selection of RGD camera selection needs to be done carefully, considering the drone’s energy consumption. A compact camera is preferred for fixed-wing drones in a normal situation because they cannot carry heavy devices.
4. Stereo Cameras
The stereo camera is equipped with two or more lenses to create high-resolution 3D images, similar to the human visual system. Using separate image sensors, it can develop three-dimensional images with high accuracy in a clean environment. However, it has poor performance in fog, smoke, and dust because the light waves are distorted in such conditions.
5. Laser Range Finders (LRFs)
Laser Range Finders (LRFs) are expensive sensors widely used for obstacle detection in drones. In LRFs, a laser beam is radiated to an obstacle to measure the distance to an object by receiving a reflected wave and considering time-of-flight. Since LRFs use optical wavelengths of light, it is not suitable for fog, smoke, dust, or similar adverse conditions.
6. Ultra-Wideband Radar (UWB)
Ultra-Wideband Radar (UWB) carries out obstacles detection by emitting electromagnetic waves in the radio spectrum. Similar to the US and LRFs, target distance is measured by calculating the reflected wave and times-of-flight. The radio waves have a longer wavelength than visible light and infrared, providing better penetration than visible light in the dust, smoke, fog, and other adverse conditions.
UWB also has other features that make it most suitable in mines. It has a precise and higher image resolution than the ultrasonic sensors in harsh conditions. Second, UWB uses low energy ie, less than 1 Watt. This saves considerable drone battery power. Third, UWB has minimum interference with other wireless uses like flight controller and telemetry link regarding low spectral density. Finally, UWB can detect objects with different characteristics like edges, corners, etc. It can also identify the three-dimensional coordinates of the nearest object.
7. Hyperspectral Sensors
Most of the multispectral imagers like Landsat, SPOT, and AVHRR detect reflectance of Earth’s surface material at several wide wavelength bands, separated by spectral segments. Lightweight hyperspectral imaging (HSI) sensors, on the other hand, assess reflected radiation as a series of narrow and contiguous wavelength bands.
Typically, these bands are measured at 10 to 20 nm intervals by hyperspectral sensors. They provide information that is not accessible by traditional methods. In general, these sensors are widely used in geology, mineral mapping, and exploration.
8. Magnetic Sensors
The magnetic sensors produce accurate measurements of the magnetic field. They assess disturbances, and changes in the magnetic field include flux, strength, and direction. The normal weight of a Cesium magnetometer is about 0.82 kg. Four magnetometers are required to derive three-dimensional magnetic field gradients, producing a combined weight of 3.28 kg. These sensors are mostly used for mineral exploration.
9. Visible and Near-Infrared Spectral Range (VNIR)
The visible and near-infrared (VNIR) portion of the electromagnetic spectrum has a wavelength at intervals of about 400 and 1400 nanometers (nm). This range consists of a complete visible spectrum with an adjacent part of the infrared spectrum up to the water absorption band at intervals 1400 and 1500 nm.
VNIR sensors, used on drones due to their small size and low weight, can measure surface moisture of open pits, tailing dams, underground spaces wall, and surfaces. Besides, each particulate mineral has a special signature in VNIR spectra, which is an advantage in mineral exploration by drones equipped with a VNIR sensor.
10. Air Quality Sensors
Apart from all the sensors mentioned above, specific sensors can be installed on a drone for a specific mission, for instance, to monitor air quality, gas sensing, dust monitoring, etc. Typically, air quality sensors are based on optical, ultrasound, and electrochemical sensing elements. They can be installed on a drone depending on the type of contamination, release time, and measurement requirements.