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.

Movable or fixed guarding? Which is right for you?

Movable Guarding

If access is required to the danger zone, a movable guard can be used, e.g. a safety gate.

The frequency with which access is required will determine whether the guard needs to be fixed or movable. The standards can help you make this decision.

EN 953

Where access is required only for machine setting, process correction or maintenance, the following types of guard should be used:

1. Movable guard if the foreseeable frequency of access is high (e.g. more than once per shift), or if removal or replacement of a fixed guard would be difficult. Movable guards shall be associated with an interlock or an interlock with guard locking (see EN 1088)
2. Fixed guard only if the foreseeable frequency of access is low, its replacement is easy, and its removal and replacement are carried out under a safe system of work.

Note: In this case, the term “interlock” means the electrical connection between the position of the safeguard and the drives to be shut down. In safety technology, the commonly understood mechanical “interlock”, meaning a lock is called a “guard locking device”.

EN 1088

7.5 Frequency f access (frequency of opening the guard for access to the danger zone)

7.5.1 For applications requiring frequent access, the interlocking device shall be chosen to provide the least possible hindrance to the operation of the guard.

A clear distinction should be made between the following:

o    The concept of frequent access required by the normal operation of the machine, as e.g. once per cycle to feed raw products to the machine and remove finished products

o    The concept of occasional access, e.g. to carry out adjustment or maintenance interventions, or for random corrective actions in danger zones

Each of these concepts is associated with an order of magnitude differing greatly as to the frequency of human intervention in the danger zone (e.g. One hundred times per hour in the case of one access per cycle, and several times per day in the case of occasional access for adjustment or maintenance during an automatic production process).

7.5.2 For applications using interlocking devices with automatic monitoring, a functional test can be carried out every time the device changes its state, i.e. at every access. If, in such a case, there is only infrequent access, the interlocking device should be used with additional measures such as conditional guard unlocking (e.g. separate approval), as between consecutive functional tests the probability of occurrence of an undetected fault is increased.

EN 62061

Frequency and duration of exposure

Consider the following aspects to determine the level of exposure:

o    Need for access to the danger zone based on all modes of use, for example normal operation, maintenance

o    Nature of access, for example, manual feed of material, setting

It should then be possible to estimate the average interval between exposures and therefore the average frequency of access.

Where the duration is shorter than 10 min, the value may be decreased to the next level. This does not apply to frequency of exposure ≤ 1 h, which should not be decreased at any time.

Select the appropriate row for frequency and duration of exposure (Fr) from the following table. 

 

Fixed Guarding

Fixed guards are permanently attached to the machine. This type of safeguard is suitable when it is unnecessary to remove the guard under normal operating conditions or when access is not required during the work process. Examples would be chain covers or grills in front of motor fans. 

Further aspects on the design of safeguards

Once the decision has been made to use a movable guard, the next step is to perform a risk assessment in accordance with EN 62061, EN ISO 13849-1 or, for a transitional period, even 954-1, to determine the safety level (category, safety integrity level SIL or performance level PL). The corresponding control system is then designed and validated.

These control systems will include sensors in the form of switches, when then detect the position of the guard. Via this detection feature, hazardous movements can be stopped as a result of the guard being opened. An additional safety function can prevent drives starting up unexpectedly when a safety gate is opened. The drive’s stopping time will need to be considered. When a safety gate is opened, if it can be assumed that a drive with a long stopping time will generate a hazardous movement, this gate will require a guard locking device. The guard locking device must be unlocked by actively operating a release. This is the only way to guarantee that the safety gate is not released unintentionally as the result of a power failure. In this case it’s also important to note that a person who is in the danger zone at the time of the power failure and has shut the safety gates behind him cannot be released by an unlock command on the machine control system. Such a case may be rare, but it is conceivable, so any guard locking devices that are considered will have a mechanical release function. However, operating staff must be sure to have the appropriate actuation tool available.