Non-contacting guards

Transcription

1 General information Non-contacting guards Dr. Peter Kocher Definition In the introduction of the standard EN [1], which is relevant for non-contacting guards, such guards, as protective equipment, are defined as follows: "Electro-sensitive protective equipment (ESPE) is fitted to machines which present a risk of personal injury. It offers protection by causing the machine to enter a safe state before a person can enter a potentially hazardous situation." Neither this definition nor the English title of EN 61496, "Safety of machinery Electro-sensitive protective equipment" leads to the conclusion that this guard is based on a non-contacting operating principle. Additionally, "Electro-sensitive personnel guards, e.g. light barriers, contact mats, electromagnetic detectors" are mentioned in Annex IV, Section B of the Machinery Directive [2]. In the English version this is described with "Electro-sensitive devices...". This might suggest that the EN series also describes contact mats, electromagnetic detectors, etc. From the content of the standard however, it becomes apparent that the German title of the standard "Sicherheit von Maschinen Berührungslos wirkende Schutzeinrichtungen" (Safety of machinery Non-contacting guards) is more precise, because this deals exclusively with non-contacting devices, whereas other safety components are dealt with in other standards e.g. contact mats, contact strips and other pressure sensitive guards in EN 1760, electromagnetic detectors in EN , etc. Consequently, non-contacting guards are taken to include light barriers, light grilles, light curtains, scanners, ultrasonic systems, etc. Translator's note: In order to minimise confusion in this English version, the German term "Berührungslos wirkende Schutzeinrichtung (BWS)" is translated as "Electro-sensitive protective equipment (ESPE)" in the following since this reflects the language used in the English standards. Standards In order to elaborate the harmonised standards required in the EC Machinery Directive, the CEN- ELEC/TC 44X started work on EN From the first two parts of this standard, EN and EN , drafts were elaborated ready for voting and these are still frequently quoted as pren and pren The work of the CENELEC/TC 44X was commonly continued by the IEC/TC 44 which is why the standard received the new designation IEC The following parts of EN exist or are planned: EN : Safety of machinery - Electro-sensitive protective equipment - Part 1: General requirements and tests. In line with its title, this part of the standard describes aspects common to all ESPEs. In particular, four types of ESPE are defined (ESPE Type 1 to ESPE Type 4), which differentiate according to their requirements on fault behaviour. This part of the standard was transferred unchanged from the IEC. EN : Safety of machinery Electro-sensitive protective equipment Part 2: Particular requirements for equipment using active optoelectronic devices (AOPDs). 1/6

2 This part of the standard is arranged such that only the deviations or supplements to Part 1 are listed. It describes aspects inherent in the AOPDs (Active Optoelectronic Protective Devices). Here, an AOPD is "a device which detects the interruption of the optical beam, which is produced in the device itself by measurement with optoelectronic transmission and receiving components, caused by an opaque object in the defined protective field", i.e. light barriers, light grilles and light curtains. This part exists only as IEC [3] and has not yet been accepted by CENELEC. EN : Safety of machinery - Electro-sensitive protective equipment - Part 3: Particular requirements for Active Optoelectronic Protective Devices responsive to Diffuse Reflection (AOPDDRs). This part of the standard is also arranged such that only the deviations or supplements to Part 1 are given. It describes aspects particular to the AOPDDRs (Active Optoelectronic Protective Devices responsive to Diffuse Reflection). Here, an AOPDDR is "a device which detects the diffuse reflection caused by an opaque object in the defined two-dimensional protective field from the optical radiation produced by the device itself through measurement with optoelectronic transmitter and receiver elements." This part of the standard also includes scanners. However, it is not just restricted to devices which establish the two-dimensional protective field by scanning with a light beam, but also includes optoelectronic buttons. This part only exists as IEC [4] and has not yet been accepted by CENELEC. Other parts of the standard are currently still being elaborated. In order to simplify international understanding, the English abbreviations are used in the German version, e.g. AOPD or AOPDDR. The usual and more precise abbreviation BWS is used for "Berührungslos wirkende Schutzeinrichtung" instead of the English abbreviation ESPE (Electro- Sensitive Protective Equipment). Types of ESPE In EN four types of ESPE are defined (ESPE Type 1 to ESPE Type 4). These types are primarily differentiated by their requirements on the error detection. Mainly it can be said that an ESPE of Type n must fulfil the requirements of Category n according to EN [5]. Because EN was transferred from the IEC without modification, the text in the standard does not however refer to EN since it is not an IEC standard, but explicitly describes these requirements. Summarised briefly the following applies roughly to the types of ESPE: ESPE Type1: Requirements are still under discussion, so they are not defined in the standard. ESPE Type 2: An ESPE Type 2 must have a device for a periodic test in order to cover a potentially hazardous failure. ESPE Type 3: Requirements are still under discussion, so they are not defined in the standard. ESPE Type 4: With a ESPE Type 4 a single fault must generally lead to the ESPE entering the interlocked state. This enables the fault to be detected. If the fault cannot be detected, then it must not cause any potentially hazardous failure. The occurrence of further faults must not lead to a potentially hazardous failure. Note: In the German-speaking region the non-standardised abbreviations BWS-T for ESPE Type 2 and BWS-S for ESPE Type 4 are sometimes used. According to EN 954-1, with a control system of Category 2, a single fault may lead to the loss of the safety function if it is not promptly found by the periodic test. In contrast, with a control system of Category 4, a single fault does not by definition lead to the loss of the safety function. Therefore it can be said without doubt that the conclusion is allowed that the fault tolerance with an ESPE Type 4, which corresponds to Category 4 according to EN 954-1, is higher that with an ESPE Type 2 which corresponds to Category 2 according to EN The (normative) Annex B to EN describes faults which may be excluded during the fault analysis. For example, with an ESPE Type 2 a short circuit between any two conductors can be excluded if they are permanently connected and protected by a cable duct or armouring against external damage. In contrast, with an ESPE Type 4 this type of fault may not be excluded and must therefore either remain without influencing the safety function or it must put the ESPE into the safe state. Apart from the behaviour in the case of a fault, the types also differ in their requirements on EMC immunity. In EN general requirements are made for different sorts of EMC effects and for ESPE Type 4 additional requirements are quoted which leads, of course, to a higher interference immunity. 2/6

3 Fields of application, advantages and disadvantages Here, the definition is quoted once more: "Electro-sensitive protective equipment (ESPE) is fitted to machines which present a risk of personal injury. It offers protection by causing the machine to enter a safe state before a person can enter a potentially hazardous situation." In this respect ESPEs can be applied anywhere where the machine can assume a safe state so quickly that injuries can be prevented. Example 1: Manually loaded press A separating guard (protective cover or grille) which must be manually or automatically brought into its protective position after the workpiece is inserted each time would significantly reduce the speed of production and would be awkward to operate. There would then be the risk that the guard would be defeated. tion. A practicable protective field can be specified in this case with a scanner. Example 3: Grinding machine. The protection objectives for a grinding machine must also protect persons from forcibly ejected parts. An ESPE presents an effective method of protection against access to the danger zone, but cannot protect against ejected parts, no matter whether this may be from the workpiece or parts breaking off the tool. Consequently, a separating guard should be used instead of an ESPE in this example (Fig. 3). Consequently, ESPEs are often used here, primarily in the form of light grilles or light curtains (Fig. 1). If the operator interrupts one or more light beams with a body part, the machine is brought into the safe state. Fig. 3 Unsuitable application of an ESPE on a grinding machine Interface to control systems Based on the results of the hazard analysis required in the Machinery Directive [2], Annex 1 or due to expressed requirements in harmonised C- standards, the manufacturer of a machine will employ an ESPE Type 4 in cases in which a fault in the ESPE represents a high risk (Fig. 4). Fig. 1 Application of an ESPE (light grille) to a manually loaded press Example 2: Pilotless transport system The guard on a pilotless transport system should detect persons or objects standing in its path (Fig. 2). Since the guard must be carried on the moving vehicle, ESPEs which are based on the interruption of one or more light beams cannot be used. Therefore, it is practicable here to employ an ESPE which reacts to the reflected radia- This alone is not sufficient to fulfil the requirement for higher safety. Rather, the interface of the ESPE to the safety-related parts of the control system must also be included in the hazard analysis and the measures which result from it. ESPE Type 4 OSSD 1 OSSD 2 L1 L2 L3 KM1 FTS KM1 KM2 KM2 M Fig. 2 Application of an ESPE (scanner) on a pilotless transport system Fig. 4 Interface of an ESPE Type 4 to the machine control system 3/6

4 Often it is found in practice that a high quality ESPE Type 4 is used, but that it only switches off the machine via a single component (relay, contactor, valve or similar). Due to its characteristics on the occurrence of a fault and when considered on its own, the ESPE does not lose its safety function and the fault is detected in that it enters the safe state. However, if the sole switch-off component should become faulty due to, say, welded contacts or a jammed valve, then the ESPE is completely ineffective. The characteristics of fail-safety and the fault detection as they are described in EN must therefore be implemented by the sensors of the ESPE whose evaluation unit with its components actually switch off the potentially hazardous energy. It must be ensured that for the whole safety function, the required safety-related performance is available appropriate to the risk. Reaching over Reaching underneath Again, it is shown with this example, that a correct safety concept can only arise in that all persons working on the development of a machine co-operate entirely. Classical faults, foreseeable misuse As with each protective device, it is also important with ESPEs that they are installed and used properly. Passing behind, reaching around an ESPE: For example, a certain distance to the hazard zone is needed due to the reaction speed of the ESPE and due to the approach speed of body parts. Often the ESPE is installed taking into account the required distance, but what is not considered is that access to the danger zone is not just possible by the path protected by the ESPE, but is often also possible via other routes, e.g. between the ESPE and the machine (passing behind it, reaching around it) via the back of the machine. Such routes must also be protected, preferably with separating guards (Fig. 5). Rendering an ESPE ineffective due to reflection: With ESPEs which react to the interruption of one or more light beams there is the risk that the reflection at an object guides the light beam via a detour from the transmitter to the receiver, although the light beam would be broken along the direct path by an obstacle, e.g. a body part (Fig. 6). The ESPE is therefore rendered completely ineffective. Irrespective of whether this occurs unintentionally due to an object (workpiece, metallic parts of the equipment, etc.) or intentionally (foreseeable misuse), the manufacturer of the Passing behind Fig. 5 Reaching over a guard, under guard and passing behind a guard ESPE must expect it and take appropriate countermeasures. EN therefore places special requirements on the effective aperture angle (EAA) of this type of ESPE. Also here, different requirements are made depending on the type of ESPE, these being higher for ESPE Type 4 (smaller permissible aperture angle) than with ESPE Type 2. S A1 S A1 Reflective surface E A1 Despite access into the danger zone, the light beam is not broken Reflective surface E A1 With a small aperture angle the light beam is reliably interrupted Fig. 6 Rendering an ESPE ineffective by reflection 4/6

5 Calculation, arrangement For the minimum distance of an ESPE with the application of active optoelectronic protective devices (AOPDs), the following general formula according to EN 999 [9] applies: S = (K x T) + C In this equation: S is the minimum distance (in millimetres), measured from the danger zone to the detection point, detection line or detection plane or to the protected field, depending which type of ESPE is being considered; K is a parameter (in millimetres per second), derived from the data about approach speeds of the body or body parts (see also Annex B of EN 999); T is the run-on period of the overall system (in seconds), i.e. the sum of the time between tripping the ESPE to the signal transfer to the protection equipment and the removal of the risk; C is an additional distance in millimetres which represents the intrusion into the danger zone before the guard device trips. With the application of the general formula, EN 999 differentiates between the following two cases: 1) "Electro-sensitive protective equipment with the application of active optoelectronic protective devices with a detection capacity of a maximum of 40 mm diameter": S = (K x T) + C Here: K = 2000 mm/s; C = 8 (d 14 mm), but not smaller than 0; d is the detection capacity of the device in millimetres. This then gives: S = (2000 mm/s x T) + 8 (d 14 mm) This equation is valid for minimum distances, S, of 100 mm to 500 mm. If the result, S, is greater than 500 mm when using this equation, then the following figure is used for the parameter K: K = 1600 mm/s; This then gives: S = (1600 mm/s x T) + 8 (d 14 mm) For applications in a non-industrial environment, e.g. in the presence of children, S must always be increased by 75 mm. 2) "Electro-sensitive protective equipment with the application of active optoelectronic protective devices with a detection capacity greater than 40 mm and less than or equal to 70 mm": Here: K = C = 1600 mm/s; 850 mm; S = (K x T) + C This then gives: S = (1600 mm/s x T) mm The following applies for the height of the beams: The height of the lowermost beam: less than or equal to 300 mm (in a non-industrial environment, less than or equal to 200 mm); height of the uppermost beam: greater or equal to 900 mm. For ESPEs which operate with a number of single beams, the following heights are recommended: Number of beams Height above reference level (in millimeteres) 4 300, 600, 900, , 700, , 900 SB2 SB2 SB1 EB1 SB1 EB1 LS LS SA2 SA2 SA1 EA1 SA1 EA1 Fig. 7 Muting (S = Transmitter; E = Receiver) 5/6

6 Muting Muting is understood to mean a chronologically limited and controlled deactivation of a protective device. According to EN muting must not lead to dangerous states, safety must be guaranteed by other means during muting, the safety functions must be established again after the termination of muting, the muting function must not reduce the required safety. A frequent application example of muting is the material and product transfer point. By evaluation of the chronological sequence of the tripping of the individual sensors, the ESPE recognises that the detected object involves the normal flow of material and temporarily bypasses the safety function so that the machine is not switched off. If a person attempts to obtain access through the material transfer point, the ESPE recognises that the object is not part of the permissible flow of material and switches the machine off. Hazard analysis Usually the application of an ESPE is the result of a hazard analysis and risk assessment. Here however, the manufacturer of a machine should not be satisfied with just the plain fact that an ESPE is used, but rather the ESPE principle (light barrier, light grille, light curtain, light button, scanner, ultrasonic system, etc.), the ESPE type (ESPE Type 1 to ESPE Type 4) and the arrangement of the ESPE (minimum distance to the danger zone, height of light beams, etc.) must all be properly considered. In each case the residual risks remaining after the introduction must be assessed. If they are not acceptable, additional measures must be taken. Bibliography [1] EN : Safety of machinery - Electro-sensitive protective equipment - Part 1: General requirements and tests. Published [2] 98/37/EC "Machinery Directive" Directive 98/37/EC of the European Parliament and of the Council of 22nd June 1998 on the approximation of the laws of the Member States relating to machinery (formerly 89/392/ECC). [3] IEC Safety of machinery Electro-sensitive protective equipment Part 2: Particular requirements for equipment using active optoelectronic devices (AOPDs). Published [4] IEC Safety of machinery Electro-sensitive protective equipment Part 2: Particular requirements for Active Optoelectronic Protective Devices responsive to Diffuse Reflection (AOPDDR). Published [5] EN 954-1: Safety of machinery - Safety-related parts of control systems - Part 1: General principles for design. Published [6] EN 999: Safety of machinery - The positioning of protective equipment in respect of approach speed of parts of the human body. Published Example: An ESPE Type 2 is used on a machine to make the danger zone safe. The remaining residual risk must take into account that such an ESPE is only tested periodically for a fault and that therefore the occurrence of a fault between the test intervals can lead to the loss of the safety function. If this is not acceptable, an ESPE Type 4 is usually selected. The residual risk that a number of faults occur simultaneously is then only very slight, so that the residual risk is in most cases acceptable. If the residual risk is still unacceptable even when using an ESPE Type 4, then in this case an ESPE is unsuitable as a protective device. 6/6