Reducing robot risk: how to design a safe industrial environment
factory. Be aware, however, that as an electro-mechanical technology, safety mats are subject to physical, environmental, and operational abuse, and the contacts can wear out with long- term repeated use. Still, safety mats are widely used in modern highly automated factories. A light curtain prevents access to hazardous areas A safety light curtain is a suitable choice if the designer needs to isolate a hazardous machine or cordon off an area behind an “invisible wall” (Figure 4). A light curtain consists of a set of photoelectric transmitters that project an array of synchronized, parallel infrared light beams to a matching receiver unit. When an opaque object interrupts one or more beams in the sensing field, the receiver detects the event, and
the light curtain sends a stop signal to the guarded machine. The transmitter module contains LEDs that emit sequenced pulses of infrared (IR) light. The pulses are modulated – pulsed at a specific frequency – and the corresponding phototransistors in the receiver are designed to detect only the designated pulse and frequency. This technique improves the rejection of external light sources. In an application such as a conveyer belt, the light curtain incorporates a muting function that causes the line to stop only when an unexpected object passes through it, but not when a product passes through. If the muting transmitter/receiver pair detects the presence of a product on the conveyer belt (Figure 4a), it can disable a defined number of channels to prevent a
Figure 1: A hazard control hierarchy ranks protective measures by their impact
and implementation. (Image source: SICK)
switch closure results in a signal to the mat controller that initiates a stop signal to the machine being guarded. Omron Automation’s UMY5S- 48X37.4 is a good example: it employs two conductive plates made of 24 gauge steel that together form the switch (Figure 3). An activation force of 30 kilograms (66 lbs) closes the switch. The mat includes an integrated four-wire cable that allows a controller to monitor for fault conditions such as a permanently open connection or physical damage. Multiple mats can be wired in series to form a safety zone around a machine. The safety zone combines with a mat safety controller such as Omron’s MC4 . Dating back to the 1950s, safety mats have long been the standard form of area protection in the Figure 3: the Omron UM5 series safety mat system includes the mat, controller, and a variety of trim pieces to secure the mat to the floor. (Image source: Omron)
For retrofitting existing work areas, active safeguarding devices have another advantage: speed of installation. They can be added without redesigning the work flow or training personnel in new safety procedures, both of which can take months to implement. Design a multi-layer approach to work-cell safety The modern industrial work cell uses a multilayer approach to safety: designers must combine active measures such as safety mats, light curtains, and lasers with visual and audible warning indicators (Figure 2). The desired result is that an operator receives warning of a potentially unsafe situation; if they then venture into
harm’s way, several independent devices shut down operations before disaster can strike. Increasingly, an industrial network links these devices; the data is used to track and log incidents, as well as identify areas for future improvement. A discussion of some of the main safety devices and their application follows.
Safety mats: a traditional, but still effective solution A safety mat is a cost-effective means of preventing a human or wheeled machine from entering an unsafe area. Typically, a safety mat uses an open switch that closes when a specified minimum weight presses on the mat. The
Figure 4: A light curtain can discriminate between a workpiece and a human operator (a) by performing muting control for each beam axis. It can also include a blanking action (b), so permanent obstacles are ignored. (Image source: Panasonic Industrial Automation)
Figure 2: A variety of safety devices is used in an automated work cell so that if someone ventures into harm's way, several independent devices can shut down operations. (Image source: DigiKey)
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