Making Light Work of Machine Safety

Increasing global competitiveness drives the continuing efforts to reduce the production cost of manufactured goods through techniques such as ‘Just in Time’ and ‘Lean Manufacturing’.  But, however plants and factories are organized and however often they are re-organized, safety cannot be compromised. 

Protecting personnel from machine hazards need not be a costly affair, but it does require highly reliable equipment.  With the advent of powerful and low cost LEDs, together with easily programmable control electronics, safety light curtains can be conveniently configured to match a wide variety of machines, and efficiently reconfigured when necessary. 

This article will outline recent technological advances in safety light curtains.  Higher light intensity, smaller packaging and easy control are important aspects, but variable range/resolution, long life, high reliability, and especially low cost, are making safety light curtains a more compelling choice in many applications.

Specific reference will be made to models in the F3SG range from Omron Automation & Safety and the SF4B range from Panasonic Industrial Automation Sales.

Invisible light

On its website, Omron has provided informative and technical data on the operation and application of safety light curtain products¹.  In its simplest form, the industrial safety light curtain consists of optical transmitters and receivers used to detect swiftly any object that breaks the light barrier.  A signal is sent to stop any hazardous machine movement instantly within the protected area. 

The light curtains typically incorporate photoelectric transmitters in the form of infrared LEDs, which project an array of synchronized, parallel invisible (infrared) light beams to the receiver unit.  The light beams are sequenced and modulated to pulse at a specific frequency.  The receivers are programmed to detect only the pulses emitted at the designated frequency, thereby avoiding interference from other light sources.

Safety light curtains can be configured to meet a wide variety of applications, providing different levels of protection across different distances and resolutions. There are three broad categories: point of operation or pinch point (guarding a specific machine or operating area of a machine); perimeter guards (protecting a robot work cell, for example); and area guards (effectively an optical fence). 

Figure 1: Safety light curtains control access to potentially hazardous areas in an industrial environment.  The highest resolution (center) will detect the presence of a finger.  Medium resolution (right) will sense a hand, while still lower resolution can be set to sense an arm or a whole person.  (Photo credit: Omron STI)

Safety light screens for industrial machinery are typically constructed to meet IEC61496-1 or -2 international standards requirements, and described as Type 2 or Type 4.  Type 2 is for lower risk applications where an accident would result in a slight injury.  These include the guarding of small assembly equipment, automated production equipment such as pick and place machines, table-top robotic workstations and small packaging machines. 

Type 4 light curtains are required for higher risk applications that could result in serious injury or death.  These light screens have high levels of fault tolerance through redundancy and constant monitoring, plus many other safety features, and are generally constructed in more robust housings.  Typical applications include mechanical and hydraulic power presses, molding presses, stamping, forming, riveting, and other automated assembly machinery.

Once the appropriate standard has been determined, a number of other factors must be considered.  For example, the minimum separation distance from the sensing field to the hazard is determined by the hand/body speed, machine response time, safety light curtain detection capability and response time.  

The size of the object to be detected will determine the resolution of the optical system, which is a function of the beam diameter and spacing of adjacent beams (Figure 1).  High-resolution systems can be specified to reliably detect a finger, for example, while medium resolution screens can be set to signal an arm or foot/ankle.  Low-resolution systems would be used for perimeter and access guarding applications, detecting when someone has stepped right through the curtain.  While high and medium resolution curtains can be oriented in any direction, low-resolution systems are usually only configured in a vertical orientation.

Features and flexibility

Early light curtain designs had drawbacks.  They could be difficult to install effectively for certain types of equipment.  Today’s designs are more flexible in terms of mounting options.  Smaller curtains, configured vertically and horizontally, can be made to fit into tighter spaces within a machine framework.  This not only helps avoid blind spots and dead zones, but can also allow operators easier access to the equipment while still being protected from hazards. 

Some machines require intermittent access by the operator, either in certain areas, or at certain times.  A muting function is available on many systems that allow the light curtain, or sections of the screen, to be bypassed or the sensor unit disabled while the machine is in a non-hazardous part of the cycle, such as an upstroke. 

Figure 2: With a muting control function, the Panasonic Industrial light curtain will allow specific size objects through, but will detect a hand, for example, and the machine will stop instantly. 

In certain medium resolution applications, material or tooling may need to be fed into the machine through the light curtain without stopping the machine.  A blanking feature can allow the light curtain to be programmed to ignore objects of a predetermined size, or permanent stationary objects such as tooling or conveyors that obstruct a part of the field. 

Importantly, today’s infrared LEDs are more reliable, and thermal issues are better understood, extending the lifetime of the components.  Microprocessor control and the ease by which LEDs can be programmed enable many more safety features to be incorporated as part of the curtain system.  These include built-in redundancy, self-test and restart interlock, thereby improving failsafe operation while providing greater flexibility and lower cost.

Omron Automation & Safety offers a wide range of models in its F3SG-R series of safety light curtains, meeting both Type 4 and Type 2 specifications.  Finger protection models range from 15 beams and a protective height of 160 mm up to 207 beams and a protective height of 2.08 m.  With a beam gap of 10 mm, object resolution is 14 mm diameter.  Operation range is from 30 cm to 10 m (long) and 30 cm to 3 m (short). 

The medium resolution hand/arm protection units range from 8 beams at 190 mm to 124 beams at 2.51 m protective height.  With a beam gap of 20 mm, object resolution is 30 mm diameter.  Operating range is from 30 cm to 20 m (long) and from 30 cm to 7 m (short).

Response time from on to off is 8 to 18 ms maximum normally, depending on the model, or it can be set to a slow mode at 16 to 36 ms.  Off to on response time is 40 to 90 ms. The effective aperture angle of Type 4 units is +/-1.5^o with emitter and receiver operating range of 3 m or greater, and +/-5^o for Type 2 units.

Compared to earlier versions, these units are installed in a more robust housing for long-term use in harsh conditions and withstanding shocks from impacts from tools or material.  The housing is more compact, yet torsion resistant.  Quick connect cables and connectors reduce the time required for wiring and set up.  Optical synchronization between emitters and receivers now requires only a single cable. The use of advanced electronic controls together with facilities such as a smart muting actuator, allows the system to be easily configured and reconfigured to suit different production lines and/or work pieces.

An advanced muting function detects the zone where work pieces pass on a conveyor, for example, and tolerating vibration, will disable only the relevant beams until the object has passed.  Where different sized objects pass on the same line, partial muting can be automatically performed.

Additional features aim to reduce the number of accidental machine stops while maintaining safety.  The reset switch, for example, can be programmed only to operate when certain conditions are met (i.e. all detected objects are removed).  A reduced resolution function allows the system to differentiate between objects, such as an automated transport vehicle, and the potentially dangerous ingress of a human foot.

Typical models include the F3SG-4RA0240-14, a finger protection unit with 23 beams and a protective height of 240 mm; and the F3SG-4RA0910-30 hand/arm protection system with 44 beams and a protective height of 910 mm.

Three resolutions

Panasonic Industrial Automation Sales has also recently upgraded its ranges of light curtains.  The Type 4 SF4B models, for example, feature new housings with a seamless structure and minimal joints, providing improved environmental resistance and easier installation and operation.  Improved self-checking circuitry enables the system to constantly check for correct operation and to identify fault conditions, such as cable wiring disconnects, short circuits and interference. 

Response time is 14 ms irrespective of the number of beam channels, axis pitches or the number of units connected in parallel.  Additional features include an easy to set up muting control function with handheld controller to control specific beam axes.  Thus, the unit can be programmed to allow objects through a line but not people.  Fixed and floating blanking is also achievable.

Safety mechanisms include an override for smooth restart and built-in external device monitoring.  Potential light interference is minimized with automatic scan timing shifts and a double scan/retry function.

The company offers a wide range of units in three resolutions for finger, hand and arm/foot detection.  The SF4B-F79(V2) high resolution model, features a sensing distance of 14 mm to detect fingers, with 79 beams on a 10 mm beam pitch and a protective height of 790 mm.  For hand protection, the medium resolution SF4B-H12-01(V2) is a compact unit with just 12 beam channels on a 20 mm beam pitch, providing a protective height of 230 mm.  Units up to 1.9 m protective height are available.

Finally, for hand/foot detection, the SF4B-A range provides from 16 to 48 beams on a 40 mm beam pitch, sensing objects of 45 mm diameter, for protective heights from 630 mm to 1.9 m.  The SF4B-A24(V2) is a typical example. 

Summary

Advances in microprocessor control, high reliability infrared LEDs and compact yet robust mounting design are giving industrial light curtains a new lease on life in the modern manufacturing plant.  Easier installation on complex machinery and better choice in terms of protective height and resolution, make this safety system more widely applicable.  Programmable functions such as muting and blanking ensure that the light curtain can be adapted to most production lines and machine guard applications, as well as to changing production requirements driven by the need for responsive, flexible automation systems in a more competitive market.

References

1. Theory of Operation and Terminology
2. Frequently Asked Questions