Guard Locking IntroductionOverview
Guard locking switches are used to protect hazardous areas where a danger is not immediately removed after a stop request. On many machines removal of power of the motor or actuator will not necessarily cause a reliable and immediate stopping of the dangerous motion. Typical applications are: high inertia rotating machines, fast rotating machines, and machines where high pressure needs to be released from pneumatic valves.
Gates protected with guard locking switches are usually opened on exception basis. For example: to clear a jam or to regularly maintain the machine. This type of switch should not be used for frequent access during normal operation of the machine.
Guard locking switches use a solenoid to activate a lock which blocks or releases the tongue from the switch.
Rockwell Automation offers two different types of guard locking switches:
Power to Lock
When power is applied to the solenoid, the tongue is locked in the switch. When power is removed, the lock is released allowing the tongue to be extracted from the switch.
Power to Release
When power is applied to the solenoid the lock is released allowing the tongue to be extracted from the switch. When power is removed, the tongue is locked in the switch.
Why Use Power to Lock or Power to Release?
|Power to Lock||Power to Release|
|Advantage||When the power is removed from the cell after a controlled stop, the doors unlock allowing maintenance personnel to go in easily.||Power is not applied to the switch all the time, only when the door needs to be opened.
Sudden lose of power does not compromise safety of personnel, as the doors stay closed.
|Disadvantage||Sudden lose of power will unlock the door allowing personnel to go in the hazardous area and the machine may not be stopped.||Loss of power will not unlock the door and maintenance personnel will not be able to go inside the cell.|
Different methodologies can help decrease the risk that the danger is removed before the operator has access to the hazardous area:
The risk assessment process and stop time measurement will determine the maximum time for the machine to stop from its normal speed of operation. This time defines the delay between the request to open the gate and the authorization to access the zone by unlocking the gate by energizing (Power to Release) or de-energizing (Power to Lock) the solenoid.
This time delay can be implemented by using any of our time delay units such as the MSR178 or MSR138 safety relay or by software in one of our Safety PLC.
Another methodology is to measure when the motion is stopped. When the no-motion is detected, the lock is released to allow personnel to enter the hazardous zone.
The CU2, CU3, or MSR57 safety relay will be used to detect the motion is stopped.
Safe speed conditions
In some applications, the user may need access while the machine is running at a safe speed. The MSR57P used with encoder technology can handle this application. It will verify the speed of the motion and allow access only if the speed does not exceed a preconfigured limit or otherwise the machine will enter a stop condition.
Typical Sequence of Actions
|1.||The operator requests to enter the hazardous area|
|2.||A controlled or immediate stop of the machine is initiated|
|3.||The machine is stopped: time delay expired or stop motion detected|
|4.||The gate is unlocked by either energizing (Power to Release) or de-energizing (Power to Lock) the solenoid|
|5.||The operator opens the gate and works in the hazardous area|
|6.||The operator exits the hazardous area and closes the gate|
|7.||The operator restarts the machine|
|8.||The gate is locked by either de-energizing (Power to Release) or energizing (Power to Lock) the solenoid|
|9.||The machine returns to its normal speed|
In the situation where a person is still in the hazardous area, the door is locked and the machine restarts, the TLS guard locking switch product family provides two options for the person to escape the hazard (in addition of an Emergency Stop located outside of the hazardous area):
Option 1: Rear Escape (Not Latched)
A 40 mm push button is mounted on the back of the TLS and is accessible from the inside of the cell. Pushing the rear escape push button releases the lock mechanism inside the TLS guard locking switch allowing the door to be opened, the machine to stop and the person to escape the hazardous area.
Option 2: Flexible Release (Latched)
The flexible release push button accessory is designed to be installed inside the hazardous area to provide a means of escape for personnel who become trapped there. It provides remote access to the manual release mechanism within the TLS-GD2 switch in the event of an emergency situation. The flexible release can be retro-fitted to existing TLS1-GD2 and TLS3-GD2 switches or installed along with a new switch.
The unit is installed at an accessible height next to the guard door, inside the guarded area, while the TLS-GD2 can be mounted outside the guarded area. The flexible release is available with either a 1 m (3.28 ft) or a 3 m (9.84 ft) cable.
Pushing the black button on the flexible release, the movement of the cable activates the release mechanism within the switch, allowing the door to be opened, the machine to stop and the person to escape the hazardous area. The flexible release is then reset using the blue reset handle.
Typical Sequence of Actions and Contact Status
- The door is closed and locked with a 440G-MT safety switch.
- The robot is running.
- The GuardShield light curtain is muted when the robot is away from the assembly table.
- In order to clear the jam safely, the operator requests to unlock the door by activating the Open push button.
- The control system (MSR safety relay or SmartGuard 600) shuts down the robot and conveyor when the process conditions allow the robot and conveyor to be stopped without damaging the machine or the products (Controlled stop).
- When the robot and conveyor are stopped the control system allows the door to unlock by applying power to the solenoid in the 440G-MT safety switch.
- The maintenance person opens the door and clears the jam.
- When the task is done, the maintenance person exits the area, closes the door and activates the Restart push button.
- The control system restarts the robot and conveyor.
- The safety mats are in place to avoid the machine restarting when the door is closed and the maintenance person is still in the hazardous area. Without the safety mats a Flexible Release can be mounted inside the hazardous area to unlock the door if this situation was to happen.
- The push of any E-Stop push buttons will stop the robot and the conveyor immediately (Immediate stop).