With the increase in the functional safety of machines and the use of more collaborative robots, and systems the defined areas between operators and machine have become more blurred. Were possible machine manufacturers are looking to remove fixed guarding and physical barriers and replace with alternative methods of protection such as a safety light curtain.
What is a Safety Light Curtain?
Before we look at the difference between type 2 and type 4, it is important to know what a safety light curtain is?
Safety light curtains have many different names in the industry such as safety light grid, safety light barriers, light guard or light curtains. Safety Light curtains have high safety standards applied to them as stated in BS EN 61496-1 Safety of machinery, Electro-sensitive protective equipment.
Despite their names, the principle and operation of these devices are the same requiring an optical transmitter and receiver units to send and receive infrared beams. The transmitter and receiver hold both the electronics and optic devices that are required to generate the infra-red detection zone. When these beams are broken or interrupted the light safety curtain will initiate a stop signal to the monitoring device such as a safety relay or safety PLC.
Safety light curtains are used for protecting operators of work equipment when constant access to the hazardous area is required. Light curtains are becoming a common feature in the modern factory, with machine builders and machine specifiers looking for production friendly safety components.
As light curtains present no physical barrier to the operator, the use of light curtains can increase the production of a machine and reduce operator fatigue whereby previously access doors and hatches would need to be opened and closed between operations.
Where a safety light curtain has several beams that are positioned together covering the danger area, a light grid only has one, two, three or four light beams. There are different light curtains for different applications; a light curtain offering finger protection has the beams positioned closely together compared to a light curtain that is protecting the body. The positioning of the beams is called the resolution, for finger protection a resolution of 14mm is common.
What is a Type 2 Light Curtain?
Type 2 safety light curtains are a simple but monitored safety function device; the safety function is monitored through periodic tests that break the output when a fault develops. For type 2 safety light curtains the internal tests occur periodically (every 500ms in some cases) which break the output if a fault occurs.
If a fault develops between the test sequence, it will only be detected during the next test sequence. The test function is often built into the device however other safety devices can initiate the tests such as the machines control system.
EN ISO 13849-1 states type 2 light curtains should be used in safety category 2 architecture performance level C, therefore type 2 devices should be used to protect a machine operator where the risk assessment shows only a minor risk injury for operators. Type 2 light curtains are normally used for hand and body detection applications.
With type 2 light curtains the maximum allowed scattering angle between the transmitter and receiver is ±5˚, the scattering angle is also referred to as the effective aperture angle and is the field of view for the device. A larger scattering angle means the device can be more prone to optical short circuits.
With the simplified nature of the device and the lower performance levels of EN ISO 13849-1, type 2 devices are more cost-effective.
To summarise type 2 light curtains;
- A simple optical safety device.
- Safety function is monitored periodically.
- Faults can develop between tests.
- Cost effective light curtain.
- Available for hand and body detection.
- Maximum allowed scattering angle is ±5˚.
- Used in Cat 2/PLc machines.
What is a Type 4 Light Curtain?
Type 4 light curtains offer the highest protection possible and according to EN ISO 13849-1 can be used on machines where CAT4/PL e is required. With type 4 devices faults are not allowed to develop to affect the safety function of the device and that any fault should is detected by the outputs closing immediately or do not reconnect after being disconnected.
Type 4 devices have a design architecture that offers redundant automatic cross-checking.
With the increased safety levels of EN ISO 13849-1 type 4 devices are used to protect a machine operator where the risk assessment shows a higher level risk of injury. Type 4 light curtains can be used in applications that require finger, hand and body detection.
Type 4 light curtains have a much smaller allowed scattering angle between the transmitter and receiver of ±2.5˚. The smaller scattering angle means the device is less prone to optical short circuits.
With the increased performance of a type 4 device and the more complex system architecture type 4 devices are more expensive than a type 2 device.
To summarise type 4 light curtains;
- Offer highest level of safety.
- Safety function is continuously monitored.
- Shorter response time
- Available for finger, hand and body detection.
- Maximum allowed scattering angle is ±2.5˚.
- Used in Cat 4/PLe machines.
Which Type of Light Curtain Should I Use?
When deciding the type of light curtain to use, firstly, you need to know the category and performance level that your machine requires. Once the required level of protection is known, you will then be able to select the type of device for your application.
As with all machine safety, the level of risk is the most important aspect when selecting components; it is vitally important to perform a thorough risk assessment of your machine. By only knowing the level of danger that the machine poses will the light curtain perform its function of keeping both operators and the machine safe and in good working order.
Not every application is the same and not every application will require a Type 4 device. Even though type 2 devices are a simpler system and sometimes seen as a lesser option, for certain machines, they are all that is required. Alternatively, some machines only require type 4 device as the added level of protection is required for the application.
Always remember that every machine is unique and only by assessing the risks will you know on the type of light curtain to use.