Electrosensitive protective devices for safe machines — Part 1

The optoelectronic technologies available for machine safety protection are nowadays very diverse and provide advanced functions to not only protect workers, but also to improve productivity.

The measures and products for implementation of machine safety requirements have become more diverse over the years. The goal is ever better integration of the functional safety in machines and systems for safeguarding. Various technologies for implementation of protection measures are now available.

With electrosensitive protective equipment (ESPE) — in contrast to physical guards — protection is not based on the physical separation of persons at risk from the risk itself. Protection is achieved through temporal separation. As long as there is somebody in a defined area, no hazardous machine functions are initiated and such functions are stopped if already underway. A certain amount of time — the so-called stopping/run-down time — is required to stop these functions. An electrosensitive protective device (ESPD) must detect the approach of a person to the hazardous area in a timely manner and, depending on the application, the presence of the person in the hazardous area. The safety requirements for ESPDs independent of their technology, or principle of operations, are stated in the international standard EN 61496-1¹.

Benefits of electrosensitive protective devices

If an operator frequently has to access a machine and therefore is exposed to a hazard, use of an ESPD instead of (mechanical) physical guards (covers, safety fencing, etc) is advantageous due to:

• Reduced access time (the operator does not have to wait for the protective device to open)
• Increased productivity (time savings when loading the machine)
• Improved workplace ergonomics (the operator does not have to operate a physical guard).

What hazards are not protected?

Since ESPDs do not provide any physical barrier, they are not able to protect persons against emissions such as ejected machine parts, work pieces or metal shavings, ionising radiation, heat (thermal radiation), noise, sprayed coolants, cutting oils, lubricants, etc. The use of an ESPD is also not possible on machines with lengthy stopping or run-down times, which require unrealisable minimum distances. In such cases, physical guards must be used.

Technologies for ESPDs

Electrosensitive protective devices can implement the detection of persons through various principles: optical, capacitive, ultrasonic, microwave and passive infrared detection. Due to poor accuracy, capacitive and ultrasonic systems have proven inadequate. Passive infrared detection offers no certainty of distinction and microwave systems have not yet been adequately tested. In practice, optoelectronic protective devices have been proven in use over many years and in large numbers.

Optoelectronic protective devices

The most common ESPDs are optoelectronic devices such as:

• Safety light curtains and photoelectric switches (AOPDs: active optoelectronic protective devices)
• Safety laser scanners (AOPDDRs: active optoelectronic protective devices responsive to diffuse reflection)
• Camera-based protective devices (VBPDs: vision-based protective devices).

Safety light curtains and photoelectric switches (AOPDs)

AOPDs are protective devices that use optoelectronic emitting and receiving elements to detect persons in a defined two-dimensional area. A series of parallel light beams (normally infrared), transmitted from the sender to the receiver, form a protective field that safeguards the hazardous area. Detection occurs when an opaque object fully interrupts one or more beams. The receiver signals the beam interruption by a signal change (OFF state), which is used to stop hazardous machine functions. The international standard IEC 61496-2² includes the safety requirements for AOPDs.

Typical AOPDs include single-beam photoelectric safety switches, multiple light beam safety devices and safety light curtains. Multiple light beam safety devices offer a detection capability of more than 40 mm and are commonly used to protect access to hazardous areas. AOPDs with a detection capability of 40 mm or less are called safety light grids or safety light curtains and are used to protect hazardous points directly (see Figure 1).

Figure 1: Access protection and hazardous point protection. For a larger image, click here.

On multiple light beam safety devices and safety light curtains, not all light beams are generally activated at the same time, but are switched on and off, one after the other, in rapid succession. This improves resistance to interference from other sources of light and increases the reliability accordingly. For state-of-the-art AOPDs, sender and receiver automatically synchronise through an optical link.

By using microprocessors, the beams can be evaluated individually, allowing additional functionality beside the pure protective function.

Safety laser scanners (AOPDDRs)

AOPDDRs are protective devices that use optoelectronic transmission and reception elements to detect the reflection of the optical radiation generated by the protective device. The reflection is generated by an object in a defined two-dimensional area. Detection is signalled by a signal change (OFF state) in its output signal. Safety laser scanners are mainly used for stationary and mobile hazardous area protection.

Figure 2: Stationary and mobile hazard area protection with a safety light scanner. For a larger image, click here.

Safety laser scanners scans the surroundings with infrared laser beams in two dimensions and monitor a hazardous area near a machine or vehicle. They operate on the principle of time-of-flight measurement. The scanner transmits very short light pulses and, if the light strikes an object, it is reflected and received by the scanner. The scanner calculates the distance to the object based on the time difference between the sender and receiver. A uniformly rotating mirror in the scanner deflects the light pulses so that a section of a circle is covered. The scanner determines the exact position of the object from the measured distance and the angle of rotation of the mirror. The user can program the area in which object detection trips the ESPE (protective field). State-of-the-art devices allow simultaneous monitoring of several areas or switching of these areas during operation. For example, this feature can be used for adjustment of the monitored area to the speed of the vehicle or a graduated response (warning field, protective field) to prevent unnecessary interruptions in operations.

Safety laser scanners use individual pulses of light in precise directions and do not continuously cover the area to be monitored. Resolutions of between 30 and 150 mm are achieved through this operating principle. With the active scanning principle, safety laser scanners do not need external receivers or reflectors, but they need to be able to reliably detect objects with extremely low reflectivity (such as black work clothing). The international standard IEC 61496-3³ states the safety requirements for AOPDDRs.

Camera-based protective devices (VBPD)

VBPDs use image capture and processing technologies for the detection of persons. Special light transmitters are currently used as light sources. VBPDs that utilise ambient light are also possible.

Various principles can be used for detection, including:

• Interruption of the light reflected back from a retroreflector
• Time-of-flight measurement of the light reflected by an object
• Size and distance measurement of an object
• Monitoring of changes from background patterns
• Detection of persons based on human characteristics.
   

The international standard series IEC 61496-4-x⁴ includes the safety requirements for VBPDs.

Protective devices resolution

The detection capability is defined as the limit for the sensor parameter that causes the ESPD to trigger. In practice, this is the size of the smallest object detected by the ESPD within the defined monitored area (protective field). The detection capability is specified by the manufacturer and in general is determined by the sum of the beam separation and effective beam diameter. This ensures that an object of this size always interrupts a light beam and is therefore detected regardless of its position in the protective field. For safety laser scanners, the detection capability is dependent on the distance to the object, the angle between the individual beams of light (pulses) and the shape and size of the transmitted beam.

The reliability of detection is determined by the type classification in the EN 61496 standard. Type 3 is defined for AOPDDRs, while for AODPs there are Type 2 and Type 4 (Figure 3). Requirements regarding optical sources of interference (sunlight, different lamp types, devices of the same design, etc), reflective surfaces, misalignment during normal operation and the diffuse reflection of safety laser scanners play an important role.

Table 1: Main differences between AOPDs of Type 2 and Type 4. For a larger iage click here.

Figure 3: Calculating the minimum distance from a reflective surface.

Important factors that influence reliable ESPD protection

Minimum distance and stopping/run-down time

There is always a stopping or run-down time after the signal is given to cease a hazardous machine function. The delay for the entire system (the entire control chain) is included in the overall stopping time. This time determines the required minimum distance of the protective device from the hazardous area. The required minimum distance is calculated according to the EN ISO 13855⁵ standard.

The consideration of the minimum distance applies to ESPDs with two-dimensional protective fields, such as light curtains (AOPDs), laser scanners (AOPDDR) or two-dimensional camera systems.

The general formula for calculating the minimum distance (safety distance) is:

where:

S is the minimum distance in mm, measured at the closest hazardous point to the detection point, detection line or detection plane of the protective device
K is a parameter in mm/s, derived from the data for the approach speeds of the body or parts of the body
T is the overall stopping time of the system
C is an additional distance in mm.

The additional distance C is dependent on the detection capability (Figure 4) of an ESPD when approached at a right angle, and dependent on the height of the protective field above the reference level for a parallel approach.

Figure 4: Parameters for determining the required minimum distances when approaching at a right angle.

For the overall stopping time T, the following parameters must be taken into account:

• Stopping time of the machine
• Response time of the safety-related control
• Response time of the protective device (ESPD)
• Additions according to the detection capability of the ESPD, the protective field height and/or the type of approach.

In Part 2

In Part 2 of this article we will examine how interference can be prevented in complex environments with multiple ESPDs and more advanced functions of ESPDs, such as blanking and muting to allow for more flexible safety protection.

References

1. European Committee for Standardization 2004, EN 61496-1:2004: Safety of machinery — Electro-sensitive protective equipment — Part 1: General requirements and tests.
2. International Electrotechnical Commission 2013, IEC 61496-2:2013: Safety of machinery — Electro-sensitive protective equipment — Part 2: Particular requirements for equipment using active opto-electronic protective devices (AOPDs).
3. International Electrotechnical Commission 2008, IEC 61496-3:2008: Safety of machinery — Electro-sensitive protective equipment — Part 3: Particular requirements for active opto-electronic protective devices responsive to diffuse reflection (AOPDDR).
4. International Electrotechnical Commission, IEC 61496-4-x: Safety of machinery — Electro-sensitive protective equipment: Particular requirements for equipment using vision based protective devices (VBPD).
5. European Committee for Standardization 2010, EN ISO 13855:2010: Safety of machinery — Positioning of safeguards with respect to the approach speeds of parts of the human body.

Top image credit: ©stock.adobe.com/au/terex