Machinery safety becomes increasingly necessary as complex plant floor operations become more automated. Accidents can injure people and damage machines. In addition to a multitude of costs associated with worker accidents, companies have an added incentive to use safety equipment to minimize downtime and improve operating efficiency.
Incorporating automated machinery and equipment into plant operations adds another dimension to operations, making worker safety and production reliability more important. Increasing the amount of automated equipment on the plant floor adds to cost and also multiplies the number of opportunities for something to fail. To compensate for this added "burden," automation and its supporting technology must deliver a benefit greater than its cost.
Although a return-on-investment concept amounts to basic engineering economics, automation only supplies economic benefits when its associated machinery is operating. Protecting the automation, the machinery it controls and the people working with automated equipment makes sense in terms of worker safety, machine guarding, improved productivity and cost avoidance.
Any approach to machinery safety and its accompanying benefits involves a careful engineering analysis and a measured use of each safety technology that is applied in combination. This includes hard guards, safety switches, emergency or e-stop switches, safety light curtains and safety mats. The application of machinery safety equipment and its suitability can be determined by a thorough and consistent safety review (risk assessment).
Even with automated equipment, hard guards, cages and mechanical locks make sense for locations where moving equipment (pulleys, gears and belts) or errant debris, gases or even radiation or heat may be released during a machine's operation. In general, these areas mean limited or no access for people during a machine's operation. They may only enter these areas when the machinery has reached a safe state after shutdown.
Hard guard applications typically are used in areas that do not impede productive operation. Users can combine hard guards with safety switches that act as sensors for these areas to ensure equipment reaches a safe state before someone can enter or shut down equipment when the hard guard opens.
E-stop switches take many forms and are typified by the large, red mushroom button on an operator console. They enable workers to activate a safe shutdown procedure in case of an accident or a machine malfunction that is not otherwise detected by the system or other safety guards. A material jam in a machine is one example of when an e-stop switch may be necessary.
Key to Automation
One area of growing importance is that of safety equipment integrated into the automated control of equipment to protect workers and machines when and where they must interact. These devices work without requiring operators or repair personnel to directly engage a safety interlock or remove a guard to work with or on a machine.
In cases where an operator must perform a repetitive operation with the machine, safety light curtains and pressure-sensitive safety mats can minimize operator activity and fatigue by eliminating the need to manually open a guard or activate a safety switch. The elimination of these activities also helps to improve machine productivity by integrating safety devices into the machine's control circuitry.
The light curtain typically responds to momentary blockage of an infrared light beam and must be reset from an outside location. When someone steps onto a safety mat, it continues to open the safety circuit until that person steps off the mat and again activates a reset circuit. This makes safety mats more suitable for use where anyone has to spend an extended time for manual setup, repair or maintenance. Typical applications include robotic work cells, welding robots, punches and presses, as well as many assembly or material handling machines.
With light curtains and mats, the safety controller can only be re-energized by the use of a manual reset switch, which must be located outside of the hazardous area. The location of the reset switch must enable the operator to observe the entire hazardous area. This prevents the possibility of the machinery restarting when the circuits are cleared.
In some cases, safety mats and safety light curtains may work together guarding different sections of a work cell. In other cases, safety mats may be required in environments where a light curtain is unsuitable. Smoky, dusty or other environmental constraints block beam reception or degrade light curtain sensitivity, making them unusable. While safety mats still have their own environmental considerations, they will extend the principles of machinery safeguarding into more applications.
Safety Mat Principles
As with any device, knowledge of the technology and design enables users to better understand its advantages and shortcomings. A safety mat may be compared to a large, normally open switch that is integrated into a safety circuit.
When the pressure or weight that exceeds a specified minimum amount is placed on top of the mat, the top conductive plate makes contact with the conductive plate on the bottom of the mat. When this contact closes, the safety circuit sends a stop signal to the machine's primary controller to put it into a "safe," or stopped, state before anyone can get close enough to the hazardous area to be injured.
Users can combine safety mats and wire them in a series to protect a total area. As with any "safety-rated" device, its design must comply with international standards. This includes redundant circuits and a design that will not allow any single point of failure to render the device inactive, resulting in an unsafe condition.
The design of the safety mat will result in a safe condition if a wire breaks, is separated from one of the plates or otherwise is disconnected from the safety controller. In any of these situations, the safety controller will de-energize its output relays and send a stop signal to the guarded machinery controller. In a similar situation, if a safety mat is punctured and causes a short circuit similar to closing the circuit under pressure, the controller will not restart until the punctured mat has been replaced.
Scientific Technologies Inc.'s (STI) safety mats incorporate a four-wire system to connect the safety mat to the safety mat controller. The conductors are encased in a molded insulating material, typically polyvinyl chloride (PVC). The STI safety mat has a dimpled surface that enhances traction and minimizes the possibility of slipping.
Sizing Safety Mats
Safety mats do not have any way of physically restraining people from hazardous areas. As a result, the layout and size of the area a safety mat covers and protects becomes a function of the time a machine takes to reach a safe state and the time a person takes to get to that point, referred to as the "safe minimum distance."
This distance (S) is calculated using the formula:
S = (63 inches/second x T) + (47.2 inches - 0.4 H).
In this case:
- H represents the distance above the horizontal plane or the thickness of the safety mat.
- T is the sum of times t1 and t2, or overall stopping time.
- t1 is the sum of times from when a safety mat is actuated to the time the system achieves a safe state.
- t2 is the maximum time that the guarded machine requires to reach a safe state or stop.
This area must be guarded by safety mats or by other means. Alternatives include fencing to control access points. This calculation typically results in an area much larger than most people would estimate based on a visual inspection.
In cases where the safety mat is mounted on the floor, H has a value of about 0.5 inch (the height of the safety mat), making the last term negligible. This reduces the equation to:
S = (63 inches/second x T) + 47.2 inches.
The 63 inches-per-second figure represents an estimated walking speed of just over 3.5 miles per hour. A person's stride length affects the necessary length of a safety mat. This dimension has been derived from EN 999, which has declared the stride is equal to 750 millimeters, or 29.5 inches. This must be included in calculating the minimum safe distance.
Installation and Use
Once the user has calculated the minimum safe distance from each potential approach to the machinery hazards, the mats can be installed. Because most areas will not conform to a standard size, mats of various sizes can be mounted together and wired in a series to cover the entire area.
The surface for mounting the mats must be flat and free of debris. Anything left under the mats after installation can cause problems by shorting the circuit between the mat plates or by causing a hole or tear, which can allow moisture to enter between the plates. Both cases may lead to mat failure.
Once the mats have been installed, users need to make sure the cables are installed and protected. Cables should be routed to protect them from damage. STI safety mats can be installed and secured in place with trim covers and corners. Several designs and styles are available.
A two-part ramp trim provides secure mounting and ease of installation. The tapered top surface minimizes tripping, and the integrated wireway located in the aluminum base provides a raceway for routing the mat wires to the controller. Where multiple mats are installed, the STI-patented, two-part "active" joining trim provides the same ease of wiring, allowing up to two mat cables to be routed in the wireway. Blunt trim for placing mats against walls and molded corners fill out the safety mat trim offering. This helps users protect wiring and route it away from the active surface of the mat.
Safety mats will work in many areas and are an alternative to light curtains in areas that may limit optical sensing such as mists, dust and smoke. Users need to determine that the mat will not be subjected to an environment that will cause problems. Check the safety mat's resistance to chemicals with which it will come in contact to make sure it will resist their effects.
About the author: Lou Schubert is product manager, Pressure Sensitive Safety Products, for Scientific Technologies Inc., Fremont, Calif.