Mine roof subsidence sensor

Until the philosopher's stone is found, humanity continues to extract minerals from the depths. Although dreamers have promised us miraculous devices that will generate any kind of substance, science continues to solve the problem of increasing the efficiency and safety within working mines.

What's at stake

The collapse of the roof of a mine does not happen suddenly. Since some of the rocks are extracted from the mine working itself, the pressure of the rock masses arises in the roof. Rocks above the mine begin to delaminate, and the roof begins to sag. At some point, a collapse occurs.

The delamination rate, and, consequently, the moment of collapse, depends on the geological characteristics of the rocks:

• composition;
• rock fracturing;
• presence of impurities (sand, clay, etc.).

The roof drawdown is monitored so that the moment of collapse does not come as a surprise and does not lead to human casualties. It is not the absolute value of the roof subsidence itself that is important, but the dynamics – the rate of subsidence. The faster the roof begins to sink during a fixed period of time, the closer the moment of collapse. Therefore, it is necessary to record changes in the subsidence and take readings in a timely manner.

Deep benchmarks

To monitor the condition of the roof of the mine workings, deep benchmarks are used. These are mechanical sensors that allow one to determine the beginning of the separation of the roof of the mine workings.

The benchmark consists of tubes with scales and sling wire ropes fixed in a pre-drilled well. The benchmark itself, after installation, is placed at the mouth of the well, under the roof arch. The benchmark shows the presence of rock delamination at several depths at once, the number depends on the sensor design.

Different ropes are attached at different depths. It is customary to set benchmarks at distances from 30 to 250 meters from each other, depending on the type of rocks lying against the roof of the mine. The weaker the rocks, the greater the probability of collapse and the more often there is a need to install benchmarks.

The main disadvantage of traditional benchmarks is that there is a need to take constant visual readings. At any given frequency, the employee who is responsible bypasses all the benchmarks installed on the site of the mine entrusted to him and writes down the current readings in a paper log. In recent years, supervision services have been promoting the idea of creating Multifunctional Security Systems that represent a single control room equipped with servers and monitors in the form of wall panels, which display the parameters of mine systems that affect the operability and safety of mining.

Indicators can be as follows:

• airing speed;
• dustiness;
• the level of methane and other explosive gases in mine workings;
• current state of various equipment.

There are no automatic sensors for rock separation control. There are sensors for measuring linear displacement, LVDT-sensors, which cost from 20,000 to 50,000 roubles and are very accurate. The sensors are analogue and can't work with the Modbus protocol. Thus, they require improvement. Due to the high price, LVDT-sensors are impractical to install at 30 metre intervals; the mine workings stretch for many kilometres.

Development of an automatic sensor

To ensure automatic monitoring of the condition of the mine roof, we decided to develop a cheaper automatic sensor that independently measures the displacement of the ropes inside the benchmark and transmits the readings to a server. For control purposes, it is sufficient that the sensor can measure linear displacement in the range of 0 to 8–12 cm.

We chose the 2 mm measurement accuracy, since the benchmark allows for the measuring of the displacement with an accuracy of 1 mm. However, the more important indicator is not the absolute value of the displacement, but rather the dynamics or the rate of change in the readings. The faster the readings begin to change, the closer the moment of roof collapse.

Implementation

Controller peripherals

• USART is a universal synchronous-asynchronous transceiver. The data interface is used to send the measured data.
• ADC (analogue-to-digital converter) for measuring the voltage across the coils;
• TIMER – to generate a PWM-signal, which was smoothed to a sinusoid of a given frequency using an analogue RC-filter;
• External Peripherals: MAX485 chip for converting USARTs using TTL signal levels from 0 to 5 V to RS-485, using more noise-immune 0 to 12 V electrical signal levels and allowing signal transmission over long distances.

The displacement LVDT-sensor

• Three coils on one cylindrical magnetic core. This number of coils increases the sensitivity and allows you to measure movement in both directions from any given centre point.
• Coils are thin copper wire wrapped around a tube-like frame.
• Inside the frame, a magnetic core, that could be made of ferrite for example, can move freely, since it has high magnetic conductivity and low saturation.
• In our case, three coils allowed us to increase the measured travel length. For a minimum total length, three coils were stacked successively, since there was no need to measure displacement in both directions.

According to calculations, an accuracy of 2–5 mm was sufficient. The range was from 0 to 8–12 cm.

Connection and software

• At first, the system was connected to a computer using a cable representing a homemade USB-UART converter: USB – on the computer's end, UART – on the sensor's end.
• The converter is assembled on the FT232RL microcircuit. In the computer, it is defined as a COM port.
• For debugging, software was written in C# that received data from the COM port and drew the measured data as a sinusoid. The averaged signal amplitude is the very magnitude of the measured voltage. The displacement is determined from the measured voltage difference across the 2 sensor coils. Conversion of voltages to displacement is done by calibration.
• For the correct operation of the sensor, a calibration for each sensor is required. The calibration procedure must be included in the sensor's firmware.