Safety Contact Strips: Taking Machinery Guarding to the Edge

By Joe Lazzara, President & CEO, Scientific Technologies, Inc.

Whether safety light curtain, safety mat, or hard guard, active machinery safe guarding typically involves a stationary detection system. However, mounting a safety contact presence-sensing device on a moving gate or door adds another dimension to improve the safety equation.

Safety contact strips are pressure sensitive devices of an elastomeric substance, such as rubber, which internally contains two conducting surfaces, or contacts, acting as a switch to signal the presence of a force applied to the strip. The external housing also performs as an insulating cover to protect the two contact strips, which form the switching element. Safety contact strips are typically mounted to the edges or ends of guards, doors and safety gates using a small aluminum rail. They are positioned at potential crushing or shear points to detect an object and signal a stop to the hazardous machine motion.

The theory of operation of a safety contact strip is very similar to a safety floor mat, and indeed, both are considered pressure sensitive safety devices and both interface to a safety controller as the connection to the guarded machine. Because of their small size and flexibility, safety contact strips offer another solution to machine guarding. For example, strips have the ability to move with the object they are protecting, such as on the edge of a sliding gate.

For a variation on the design, safety contact strips are available with a wide selection of elastomeric covers, often called profiles, which expand the capabilities of this sensing methodology. For example, larger profiles increase the detection surface of the safety contact strip and help to provide a cushion effect. These larger profiles can also serve as a weather-stripping or gasket, such as for the bottom of rollup doors and on the edges of gates. The actual sensing contact element is contained within the profile.

Safety bumpers are a variation of "edge" type guarding, and perhaps should be thought of as safety contact strips on steroids! The bumper is a length of foam rubber covered in polyurethane, which is mounted over the safety contact. Applications for safety bumpers include automated guided vehicles (AGV), transportation and material handling installations. Because of the extra thick cushioning afforded by the foam rubber, these devices may be suitable on applications where the moving hazard is unable to stop immediately, such as an AGV.

Applications
The following sample applications provide examples of how the convenience and flexibility of safety contact strips can solve your tough guarding applications.

A window manufacturer has an assembly machine that is used to hold and cycle the window through its various positions. A pinch point was accessible between the window and the retaining fixture while the window was rotated. A safety contract strip and controller were installed on the moving fixture and used to detect an object, such as a hand, caught between to window and the fixture. A signal sent by the safety contact strip controller is used to halt and momentarily reverse the window rotation.

An automated silkscreen machine manufacturer need a way to protect against injury while the machine clamped the screen assembly in place. The screen would be held in this position during the actual imaging process. The machine operator needed access to the clamping area to assist in proper registration of the screen, thus a safety light curtain was not suitable for the application. A small safety contact strip held in place by an aluminum rail, was mounted adjacent to the area exposed during the clamping process, detecting access to the pinch point.

An automated guided vehicle manufacturer need to protect vehicles while under motion from injury to personnel and prevent damage to fixed objects. The solution was to use a safety bumper, mounted along the bottom edge of the AGV. The foam rubber provided protection while the AGV would come to a complete stop. The customized bumper provided cutouts for the wheels of the AGV.

Safety contact strip construction details
The construction of a safety contact strip consists of a aluminum mounting rail, a safety contact, and a rubber profile which houses the safety contact. The safety contact strip has two wires. Applications require only a single circuit of two wires from one end connected to the safety contact strip controller, while the second set of wires, located at the opposite end of the contact strip is terminated to an 8.2k ohm resistor installed in a protective end cap.

The rubber profile material performs several functions. First it protects users from making contact with the low voltage electrical circuitry, as well as protecting the contacts from damage. The design of the profile enables the contact strips to be actuated at an angle that may be less than directly perpendicular to the strip surface.

The profiles are available in two choices of elastomers, either EPDM (ethylene propylene diene monomer) or NBR (nitrile butadiene rubber). The EPDM material offers excellent chemical resistance to most acids and alkaline solutions. The NBR elastomer is better suited to applications where oils and many hydrocarbons may be present.

The safety contract strip controller monitors the sensing circuit. When sufficient force, measured in PSI or Newtons, causes the two conducting surfaces to contact, the controller senses the change in resistance and generates an "on" or "off" signal to the guarded machine controller or a safety circuit designed to stop or retract the guard or gate on which the safety strip is mounted. Likewise, a broken or shorted wire from the controller to the safety strip would result in a fault (also called interlock) condition. The controller is approved as a Category 3 device to EN 954-1.

The safety contact strip controller has four LED indicators — providing status and diagnostic information. An illuminated green LED indicates that the power has been applied. The yellow LED indicates that the safety contact strip has been activated, causing the controller's safety outputs to open. The red LED indicates that a fault condition has been detected in the safety circuit. A second yellow LED indicate the status of the auxiliary relay – a normally open contact will close for three seconds whenever the safety contact strip is activated. This is useful for reversing a door or gate, provided doing so does not cause a hazard. The controller also has a provision to monitor the machine primary control elements (MPCE), usually the motor relays, to help detect whether the redundant MPCEs are operating normally.

The safety contact strip controller has two operating modes. In the automatic start mode, the safety outputs cycle as the pressure to the contact strip is applied or removed. In the start/restart interlock mode, a reset pushbutton is needed to restart from pressure on the contact strip or a power up condition. In effect, the safety outputs are latched open, until reset.

Safety strip considerations
The use of any piece of machinery safe guarding equipment requires common sense considerations. The device must be capable of stopping its motion anywhere in its active stroke or cycle. It must do so in a consistent, minimum time requirement. A machine with an inconsistent stopping or reaction time, or application in an environment where chemicals may degrade the safety equipment performance, are inappropriate conditions for the use of safety contact strips.

Users must also remember to consider all areas that require guarding. Other machinery safety equipment may be required for a complete machinery guarding solution. If mechanical brakes or other stopping equipment is required for the guarding solution it must be tested regularly to ensure that its performance does not change. Operators must be trained in the use of the equipment, and users must follow proper procedures for the safety equipment's operation.

Most importantly, safety contact strips offer another means of machine and hazard guarding and are an important addition to the safety engineer's toolbox of application solutions. My thanks to Lou Schubert for his assistance with this article.

Be safe out there!

Designed for flexibility

The rubber safety contact strip has a multifunction construction. It has a flexible and hollow construction to prevent injury or damage when a safety guard or gate comes in contact with a person or object. It also houses two double-wire (for redundancy) cables that run to the connecting cables. Safety contact strips are assembled from modular components and can be as long as 6100 mm (almost 20 ft) in 10 mm (0.4 in.) increments.

Inserting the safety contact in the switching chamber in the middle of the safety strip permits proper use as a safety device. The ends are sealed with an adhesive formulated to have permanent elasticity. The safety strip is then pressed in the aluminum rail mounted to the desired edge of a gate, guard or machine.

The flexible design enables the switch to be closed in a range from 28 to 72 psi depending on design. Since the safety strip may encounter a person or object at different distances and orientation, the safety strips with selected profiles have actuating angles of 30, 45 and 60 degrees from perpendicular.

Safety contact strips come with safety bumpers — foam rubber covered in polyurethane mounted on a rubber base. The strips are sized to fit the required length and incorporate a co-extruded contact strip with a low resistance to act as a long presence-sensing switch. This is connected to a safety controller that interlocks with the operator of the gate or guard to send a stop signal in less than 30 msec.