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When it is not safe, the process must stop to prevent major catastrophe! Safety Monitoring System, while operating independent of main PLC and controls, provides the vigilance needed to avoid that catastrophe. It overrides the main controls and stops the process if it detects any safety breach.
Manufacturing of wire and cable products involves various types of equipment that requires some sort of safety measures to guard against injuries or losses. Safety precautions need to be taken against rotating parts, heat, chemicals, steam etc. Various signs and bright colors have been used along with electro-mechanical devices such as door locks.
Rotating equipment designed and produced by OM Lesmo Group of Italy; incorporate a unique Safety Monitoring System (Fig 1) in most of their equipment. Such equipment includes double and single twist bunchers, rigid and planetary cablers, drum twisters, bow cablers (skip stranders) and tubular stranders. This system is stand alone, separate from the main PLC and other electronics that monitor and control the equipment and process. The function of this system is to continuously monitor each and every safety related component and their status. In an event that any one of these safety components is not meeting the safety criteria it is designed to guard against, the Safety Monitoring System overrides the main control system and stops the process.
The three most important safety control systems adopted by OM Lesmo are –
- Safety guard and sound proof housing control system
- Zero speed control system
- Emergency stop control system
1. Safety Guard and Sound Proof Housing Control System:
All the rotating equipment from OM Lesmo has a sound proof housing and guard to protect the operator from potential hazards generated by moving parts rotating at high speeds. The safety guard control system must -
The risks involved in today’s high speed machines require a redundant safety control system illustrated in Fig 2 and 3.
Switches (1) and (2) detect if the door is closed. They are controlled by the Safety Monitoring System (3), which sends two stop commands to motor drive (4) in case the door is not closed. Motor drive has a safe stop function integrated in order to safely disconnect the energy feed to the motor.
This redundant configuration (two switches, two stop commands) combined with the Safety Monitoring System and safety integrated motor drive, assures three fundamental characteristics:
1) Safety: machine can run only if guard is closed and locked.
2) Reliability: in case of a component failure the safety guard control system is still operating. For example, if there's a failure of switch (2) (mechanical breaking or electrical short-circuit), guard monitoring is still guaranteed by switch (1).
3) Failure detection: in case of a component failure, the system is able to detect the fault before machine restarts, and doesn't enable motor running until the fault-cause has been removed. This is an important feature in order to prevent faults accumulation. In above example, a failure of switch (1) and later the failure of switch (2) could compromise the system’s capability to detect the door status and, consequently, the safety control would be no longer be operating. But OM Lesmo Safety Monitoring System detects the first fault that is switch (1) failure, and doesn't allow machine restart until switch (1) has been repaired. It ensures safety control system is always operating. Furthermore, switch (1) locks the closed guard in order to prevent accidental door opening while machine is running. Door is unlocked only when machine is at a standstill condition.
2. Zero Speed Control System:
Safety guard housing on rotating equipment has door lock mechanisms that will prevent the machine from rotating if any of the door lock is open. The zero speed control system will keep the doors locked until the machine comes to a full stop. The door is unlocked when machine is at standstill condition. An incorrect zero speed detection could cause guard opening while machine is still running and, consequently, creating a dangerous situation for an operator.
In order to reduce the risks related to an incorrect zero speed detection, OM Lesmo adopts a redundant zero speed control system structured as shown in fig.4. Similar to the Safety Guard and Sound Proof Housing Control system, the Zero Speed Control system assures three fundamental characteristics:
1) Safety: safety guard opening is enabled only if machine is at a standstill condition.
2) Reliability: in case of a component failure the zero speed control system continues to operate. For example, if there's a failure of motor encoder (1) (mechanical damage or electrical short-circuit), speed detection is still guaranteed by the sub-system composed of the inductive sensor and the PLC.
3) Failure detection: in case of a component failure, the system is able to detect the fault before machine stopping, and doesn't enable guard unlocking until the fault-cause has been removed. As described previously this feature is very important in order to avoid faults accumulation.
Fig 4 illustrates how the Safety Monitoring System is incorporated with a Zero Speed Control system.
3. Emergency Stop Control System:
All machines are equipped with an emergency stop circuit. The purpose of this circuit is to stop the machine in a very short time and prevent restarting until safety conditions have been restored. Emergency stop is actuated by red-mushroom type pushbuttons with a yellow background shown in Fig 5, which are located on a control panel as well as more strategic places where potential danger to the operator is present.
Emergency push-buttons, E-Stops are controlled by Safety Monitoring System (2). When an E-stop is actuated, Safety Monitoring System activates an emergency pneumatic brake (3), which stops the machine in a short-time, and sends two stop commands to motor drive (4). Motor drive has a safe stop function integrated in order to safely disconnect the energy feed to the motor and avoid unexpected restart.
E-stops are double contact type with direct opening action and mechanical latching. Once an emergency stop has been activated and machine stopped, next restart is not allowed until a manual reset of the emergency stop device has been performed at each location at which the emergency stop was activated.
An illustration of the three major safety control systems described above, can be seen in Fig 7, where two machines are running in tandem and the Safety Monitoring System is controlling all the respective safety features in both machines.
The sound proof housing and guards are controlled by two switches, one of which locks the closed door. Each machine is provided with zero speed control system. An emergency pushbutton (e-stop 5) is located on control panel. Internal safety guard zone is a potentially dangerous area, for example related to operator entrapment risk with rotating components at high speeds. There are four emergency pushbuttons located inside each guard enclosure (e-stop 1 to 4 and 6 to 9). All safety components are monitored and controlled by The Safety Monitoring System.
As it is evident, SAFTY cannot be taken for granted. Today’s technology allows equipment manufacturers to produce machines that can rotate at very high rotating speeds. This creates potentially dangerous situations unless every possible safety precaution is addressed right at the design stage. Use of appropriate hardware will prevent some of the safety hazards, but use of Safety Monitoring System that operates independent of the main controls, will assure safe and reliable operation. Don’t take safety for granted. Make sure it is continuously monitored, controlled and is effective.