Cam operated actuation usually takes the form of a positive mode limit (or position) switch and a linear or rotary cam (as shown in Figure 75). It is generally used on sliding guards and when the guard is opened the cam forces the plunger down to open the control circuit contacts. The simplicity of the system allows the switch to be both small and reliable.
Guard Shown Closed Positive Mode
Limit Switch
Figure 75: Positive Mode Limit Switch
Position (limit) interlocks must not be used on lift-off or hinged guards.
It is extremely important that the switch plunger can only extend when the guard is fully closed. This means that it may be necessary to install additional stops to limit the guard movement in both directions.
It is necessary to fabricate a suitably profiled cam that will operate within defined tolerances. The guard-mounted cam must never become separated from the switch as this will cause the switch contacts to close. Such a system can be prone to failures due to wear, especially when badly profiled cams or the presence of abrasive materials is a factor.
It is often advisable to use two switches as shown in Figure 76. One operates in positive mode (direct action to open contact), and one operates in negative mode (spring return).
Trapped Key Interlocks
Trapped keys can perform control interlocking as well as power interlocking.
The movement of the guard is interlocked with the direct switching of the power to the hazard. For equipment using low voltage and power most types of interlock switch can be used for power interlocking. But because most industrial machinery uses a relatively high power three phase supply we need purpose designed power interlocking systems with the power interrupting switch capable of handling and breaking the load reliably.
A
A A
Figure 77: Trapped Key System
The most practical method of power interlocking is a trapped key system (see Figure 77). The power isolation switch is operated by a key that is trapped in position while the switch is in the ON position. When the key is turned the isolation switch contacts are locked open (isolating the power supply) and the key can be withdrawn. The guard door is locked closed and the only way to unlock it is by using the key from the isolator. When this key is turned to release the guard locking unit it is trapped in position and cannot be removed until the guard is closed and locked again.
Therefore it is impossible to open the guard without first isolating the power source and it is also impossible to switch on the power without closing and locking the guard.
This type of system is extremely reliable and has the advantage of not requiring electrical wiring to the guard. The main disadvantage is that because it requires the transfer of the key every time, it is not suitable if guard access is required frequently.
Whenever whole body access is required, the use of a personnel key is recommended as shown in Figure 78. The trapped key range is available in double, triple, and quad key versions for such a requirement.
A A
A
Figure 78: Full Body Access⎯Operator Takes "B" Key
The use of a personnel key ensures that the operator cannot be locked in the guarded area. The key can also be used for robot teach mode switches, inch mode controls, etc.
In another example shown in Figure 79, rotate and remove Key "A" from the power isolator. Power is then OFF. To gain access through guard doors Key "A" is inserted and rotated in the Key Exchange Unit. Both "B" Keys are then released for guard locks. Key "A" is trapped preventing power from being switched on. Two "C" Keys are released from the guard door locks for use in the next sequence step or as personnel keys.
Positive Mode Limit Switch Guard Shown Closed
Negative Mode Limit Switch
Protective Measures and Complementary Equipment
Figure 79: Multiple Doors Are Accessible
Figures 80 shows another example of trapped key interlock applications by using both single and double key locking units and keys with different codes together with a key exchange unit, complex systems can be formed. Besides ensuring that the power is isolated before access can be gained it is also possible to use the system to enforce a pre-defined sequence of operation.
Figure 80: Defined Sequence of Events
Because the entire safety of this type of system depends on its mechanical operation it is critical that the principles and materials used are suitable for the expected demand made on them. If an isolation switch is part of the system it should have positive mode operation and it should satisfy the requirements of the relevant parts of IEC 60947.
The integrity and security of the system revolves around the fact that under certain conditions the keys are trapped in place, therefore two basic features need to be ensured:
1. THE LOCK CAN ONLY BE OPERATED BY THE DEDICATED KEY. This means that it should not be possible to "cheat" the lock by using screwdrivers, etc., or defeat the mechanism by mistreating it in any straightforward manner. Where there is more than one lock on the same site it also means that the specifying of key codes must in itself prevent any possibility of spurious operation.
2. IT IS NOT POSSIBLE TO OBTAIN THE KEY IN ANY WAY OTHER THAN THE INTENDED MANNER.
This means that, for example, once the key is trapped, any excessive force applied to it will result in a broken key as opposed to a broken lock.
Operator Interface Devices Stop Function
In the U.S., Canada, Europe and at the international level, harmonization of standards exist with regard to the descriptions of stop categories for machines or manufacturing systems.
NOTE: these categories are different to the categories from EN 954- 1 (ISO 13849-1). See standards NFPA79 and IEC/EN60204-1 for further details. Stops fall into three categories:
Category 0 is stopping by immediate removal of power to the machine actuators. This is considered an uncontrolled stop. With power removed, braking action requiring power will not be effective. This will allow motors to free spin and coast to a stop over an extended period of time. In other cases, material may be dropped by machine holding fixtures, which require power to hold the material. Mechanical stopping means, not requiring power, may also be a used with a category 0 stop. The category 0 stop takes priority over category 1 or category 2 stops.
Category 1 is a controlled stop with power available to the machine actuators to achieve the stop. Power is then removed from the actuators when the stop is achieved. This category of stop allows powered braking to quickly stop hazardous motion, and then power can be removed from the actuators.
Category 2 is a controlled stop with power left available to the machine actuators. A normal production stop is considered a category 2 stop.
These stop categories must be applied to each stop function, where the stop function is the action taken by the safety related parts of the control system in response to an input, category 0 or 1 should be used. Stop functions must override related start functions. The selection of the stop category for each stop function must be determined by a risk assessment.
Emergency Stop Function
The emergency stop function must operate as either a category 0 or category 1 stop, as determined by a risk assessment. It must be initiated by a single human action. When executed, it must override all other functions and machine operating modes. The objective is to remove power as quickly as possible without creating additional hazards.
Until recently, hardwired electro-mechanical components were required for e-stop circuits. Recent changes to standards such as IEC 60204-1 and NFPA 79 mean that safety PLCs and other forms of electronic logic meeting the requirements of standards like IEC61508, can be used in the e-stop circuit.
Emergency Stop Devices
Wherever there is a danger of an operator getting into trouble on a machine there must be a facility for fast access to an emergency stop device. The e-stop device must be continuously operable and readily available. Each operator panel must contain at least one e- stop device. Additional e-stop devices may be used at other locations as needed. E-Stop devices come in various forms. Pushbutton switches and cable pull switches are examples of the more popular type devices. When the e-stop device is actuated, it must latch in and it must not be possible to generate the stop command without latching in. The resetting of the emergency stop device must not cause a hazardous situation. A separate and deliberate action must be used to re-start the machine.
For further information on e-stop devices, read EN418 (ISO13850), IEC 60947-5-5, NFPA79 and IEC60204-1, AS4024.1, Z432-94.