On guards: Machine-guarding basics

In this installment of Automation Zone, learn to protect your people by choosing the right machine guards for your needs.

By Steve Weighart, president, Gordon Engineering Corp.

Machine guarding is a vital part of many plant operations. Enhancing the safety of workers who operate and support the operation of production machinery is of paramount concern to companies. Secondarily important is maximizing manufacturing efficiency and keeping costs within affordable margins.

Machine safety concern and government action, at least in the U.S., is documented at least as early as 1915 with the formation of the National Safety Council and the American Society of Safety Engineers. There are three basic categories of technology for machine guarding. When implemented properly, each of these machine guarding technologies enhances safety by directly guarding against someone putting part of his or her body or an object into a machine area that is dangerous while the machine is operating. How the guarding is achieved is different for each technology.

Machine-guarding technologies

Hard guarding, the first machine-guarding technology, is still in use today. This technology makes use of a physical barrier between machine operators and potentially hazardous moving machine parts. These machine parts can include point of operation, power transmission, functional components, pinch and shear points. The physical barrier is usually movable so that operators can access the machine when it is stopped to service it for maintenance or repair, reload stock or address a machine stoppage resulting from stock being stuck in the machine. Today, moving barriers are typically mounted on a hinge and include electronic interlock circuitry that detects that the guard is being opened and sends a signal to shut down the machine, if it’s not shut down already, before attempting to move the physical barrier.

Next is what is referred to as radio frequency (capacitance) presence-sensing, or RF guarding. It’s an interesting technology that combines radio frequency (RF) signal transmission with capacitive presence sensing. A system using this technology has three basic components: a control unit, a coupler, and an antenna. RF guarding technology uses a coupler that generates a low-level RF field around the antenna. Any part of a person or object that intrudes into the area around which the antenna is placed attenuates the signal. The control unit senses the attenuated field and triggers a machine stop.

Here’s where things get even more interesting and flexible. There are two ways RF guarding technology is flexible to the benefit of users. First, the antenna (made from readily available copper tubing or conduit) can be fashioned in any shape necessary to guard complex areas. Second, the sensitivity of the alarm threshold can be adjusted on the control unit to change the intrusion detection distance.

An industrial light curtain is presence-sensing machine safety technology that uses opto-electronics to send and detect a series of LED light beams across a 2D plane. The units are placed in pairs of transmitter and receiver, with each laser pair needing to be aligned across an opening. Any object, hand, etc., crossing the beams interrupts some or all of the beams, and a control is sent to a device that shuts down the machine being guarded. The number of light beam pairs needed can be anywhere from several to more than 100, depending on the length of the plane being guarded, covering about 0.5 inch to 60 feet. These systems typically are able to detect object presence within 0.5 inch to 1 inch, depending on the product and its length. Single-beam light curtains for detecting a person walking by a large perimeter area are another application of this technology.

Matching applications with guarding technologies

So which technology is best for your application? While each application is unique, some general guidelines can be considered. Hard guarding may be best for a large area, e.g. an area the size of a room. In this case, hard guarding can make the most sense because of its lower cost. Another possible application for hard guarding technology is for an area that is accessed infrequently.

RF guarding technology makes sense if flexibility of the guarding area is needed or if areas that need to be guarded are more complex than a single plane. One example is around corners (two planes). This allows customization in 3D for guarding one or more areas as the application needs.

Another example is in situations where vibrations from machinery are present (which could cause misalignment of light curtain arrays); vibrations do not affect RF guarding installations. In addition, sensing distance is adjustable from having to nearly touch an antenna to being up to a foot away to trigger a machine shutdown. This degree of flexibility can provide a greater safety margin by shutting down the machine upon an intrusion when the person or object is at the furthest distance possible from the hazardous moving machine components, while balancing this need with sensitivity to eliminate false stops. Complex 3D antennas of up to 75 ft in length can be implemented using this technology.

RF guarding technology is usually substantially less expensive to implement than light curtains, so cost can be another consideration for choosing this technology.

Light curtains offer a relatively straightforward setup and make sense in applications where there is a plane or “flat” open area that needs protecting or where there is more than one 2D plane. Users simply select a length that fits their opening within the increments offered, mount them across the opening to be guarded, and then align them. Networking of multiple guard controllers is an option with some manufacturers’ products. A stamping machine with jaws opening and closing is an ideal light-curtain application. While the transmitter-receiver pairs are enclosed in rectangular housings that must face each other across the opening to be guarded, newer products have a thinner profile to provide the most access through the opening they guard.

Both RF guarding and light curtain technology products are available that perform continuous diagnostics to ensure proper operating performance of the machine guard presence detection equipment. All of these technologies improve plant safety, and the manufacturers who produce them and their representatives can be helpful in choosing and implementing the best system for their specific application.

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