How to make a sensor selection

Sensor Selection

When selecting sensors scan movable guards, the question arises as to whether such sensors can be connected in series to an evaluation device, and if so, how many? The answer to this question depends on the faults that can be anticipated (refer to the fault lists in EN 13849-2). The following example of safety gates connected in series is intended to illustrate this point: 

1. The example shows three safety gates connected in series to an evaluation device. Initially all the safety gates are closed and the relay’s outputs are “on”, i.e. the machine can be operated.
2. On the left-hand safety gate, a short circuit occurs in the line to the switch with the N/C contact: At first the fault is not detected and the machine can continue operating.
3. The left-hand safety gate is opened, an event which the left switch signals to the relay. During a feasibility comparison of the two switches the relay discovers an inconsistency and switches to a fault condition, i.e. once the safety gate is closed the machine cannot be restarted.
4. Now the right-hand gate is also opened. Via these signals the relay once again detects a normal condition. The fault condition is reset, the safety gates can once again be closed from left to right and the machine is ready to start up again.

This example illustrated an undetected fault in the safety circuit. An additional fault could cause the whole safety gate guard to fall to danger. As a result, this series connection may not be used in applications which require Category 4.

However, switches with integrated fault detection are available to solve this problem. It is possible to connect several of these in series without causing the above error. 

In this case the question relates to the need for mechanical redundancy and the number of switches on a safety gate. Assuming that the circuit is intended to provide safety in the event of an anticipated fault, redundancy is normally necessary. However, the anticipated faults depend partly on the application. It’s conceivable that an actuator subjected to particularly heavy vibration could break off from the switch at some point. So if there were only a single switch in this case, the safety function would be rendered inoperable by a single fault on the mechanical side, despite having redundancy on the electrical side. The same applies to roller lever limit switches, should the lever break off.

The recommendation is to perform a brief risk assessment to establish the need for one or two switches, based individually on the application.