Simpler Sensors, Switches, More-Flexible Safety

Aug. 29, 2012
For the Future, Laser Scanning, Safety Cameras, or Even Non-Optical Systems Could Make Their Way Into the Safety Sensor Arsenal

[sidebar id="2"]For machine guarding, simpler safety sensors and switches are better, but flexibility and performance are important, too.

Consider light curtains from Panasonic Electric Works. Its SF4B comes in three standard sizes that offer finger, hand and arm/foot protection, with beam pitches of 10, 20 and 40 mm, respectively. It has a 14 ms response time, which includes a scan through 192 channels.

That speed is important to ensure the light curtain shuts down machines for real safety issues and not false ones. In industrial settings, such safety sensors can trigger when a welder or another light-generating activity fires up. Older light curtains fault in such situations, but not the SF4B. "It will restart the scan again and see if the problem followed it," says Charlie Strobel, product manager. "Then if it did, it would error out. But most of the time it corrects the issue."

By incorporating a controller, Panasonic light curtains are easier and less expensive to set up. They also are more resistant to industrial environments, having been upgraded to an IP67 water and dust ingress protection rating within the last year or so. Similarly, the company beefed up its light curtain frames to better take occasional and accidental blows.

Another example of simplicity, flexibility and performance can be found in Rockwell Automation's  just-launched SC300 Hand Detection Safety Sensor. It complements inexpensive light curtains, says Edward Tillinghast, global product manager.

Rated for PLd or SIL 2 machine guarding applications, the product consists of a safety sensor and reflective tape. The safety sensor is installed in the corner of an opening and the reflective strips are applied on a frame to form the boundary being guarded. After being taught where the strips are, the safety sensor will then detect appropriately sized objects and switch its outputs off, causing machine shutdown.

"It's not really a camera, in terms of image processing," Tillinghast says. "It's more pixel evaluation. In the most common sense, it's a kind of retroreflective sensor."

The device has a 20 ms response time. It requires a risk assessment be done because its rating is not such that it can be used in settings requiring a higher level of safety. It could, however, be used in labeling, filing, dispensing, measuring and weighing types of machines, Tillinghast says.

For the future, he sees other technologies — laser scanning, safety cameras, or even non-optical systems such as radar — finding their way into the safety sensor arsenal.

One such invisible connectivity method forms the basis for another instance of simpler, yet flexible safety products. Siemens Industry recently launched a line of non-contact door monitoring switches that employ RFID transponder technology. Doing so cuts in half the number of devices needed to achieve the highest safety levels, says John Burns, control products division product manager.

Contacting safety switches suffer wear and tear when machine doors open and close. Non-contacting switches eliminate this problem, but traditionally require two pairs of switches to achieve a PLe or SIL 3 safety rating. Siemens' Sirius 3SE63 switches can provide that level with a single set of devices, accomplishing this because RFID technology enables the switches to identify themselves and authenticate each other.

"By reading the code information coming off of the RFID chip, we can actually match up specifically receiver and transmitter with each other," Burns says. "You have a matched pair; only that matched pair will work with each other."

This coding eliminates the possibility of device substitution and override. The RFID safety switches can be family-coded so that they recognize a group, or they can be individually coded, with the capability for either single or multiple teach-in. The switches can also be connected jointly in the same circuit, thereby, for example, providing safety for all four panels accessing a robotic cell.

In the future, RFID could well be used in a wider variety of non-contact safety applications. If so, it'll be part of a larger trend toward reducing the number of components, Burns says.