# Implementation of Explosion Safety Regulations in Design of a Mobile Robot for Coal Mines
## 4 Implementation of IEC 60079 for TeleRescuer
### Automatic Safety Gas Detector  
Even with the presence of methane in the atmosphere around the robot, methane should not get through the flameproof enclosure into the robot body. If, however, the enclosure is damaged, the second level of protection--automatic safety gas detector--prevents an explosion by turning the power of the whole robot off (except for the gas detector itself, which is designed with intrinsic safety) [34].  
The design of the safety gas detector has a high safety level. It is purely hardware based (no microcontrollers and software), starting with an ATEX M1, SIL1, 0-5% _v/v_ CH4 sensor from Dynament. The output of the sensor is per the British Mining Standards, 0.4-2 V. Two independent under-voltage and over-voltage comparators are connected to sensor output through high-value resistors, to avoid crossed-comparator fails--see Figure 13. Each comparator energies a relay, and the contacts of these relays are connected in series. Under-voltage (V < 0.4 V) is interpreted as sensor failure. Over-voltage is interpreted as CH4 > 0.5%. In both cases, power is disconnected by relays. Even if one comparator fails, the other will open the circuit. Intrinsically safe power supplies with appropriate voltage levels are included in the design of the safety gas monitor. The design is intentionally non-self-resetting. If the relays trip, it will remain de-energized until the arming (or re-arming) switch is operated. This feature is also used for avoiding draining the battery during long-term storage.  
The detector also provides a logical signal for the main control system, which acts as a warning about an imminent power-down because of increased methane concentration. This allows for the control system to switch off in a controlled manner and primarily to disable DC motor drives to lower the currents for safer power-off switching. After a short delay, the logical signal is followed by power-down of the whole system.  
The same automatic safety gas detector is installed separately in the main robot body, in the sensory arm and the sensory head; and a similar system is implemented also in the 3D LIDAR. The'safety gas sensor is a part of the power management system, which distributes power from the batteries. The command for switching power off can come from several sources:  
Figure 13: Implementation of safety gas detector into the robot subsystems—simplified diagram.  
* manual control of the power of the whole system (the main power on/off button),
* manual activation of the safety central stop button,
* command from the operator control system (safety central stop button on the operator panel),
* the dangerous concentration of methane detected inside the subsystem, and
* activation of an independent watchdog monitoring the embedded control system.  
The methane detector was tested in atmosphere that contained methane and other gases in well-known amounts in a special gas chamber. A calibrated gas sensor Draeger X-am 5000 (reference) and the methane detector were closed inside the chamber with gas entry for methane mixture and a small hole for cable harness and to allow a small airflow. Figure 14 shows one of the graphs that were obtained during the testing after calibration, where the measured values closely match values from the reference sensor.