by Andrew Waugh, Product Manager for Sensors and Safety at AutomationDirect.com
Operator safety is a top priority for machine and robot installations, and safety switches help implement the desired level of protection.
Protecting an operator from safety hazards should be at the top of the list for all designers of automated equipment such as machines and robots. Once the appropriate safety guarding level is defined, access to the automated equipment is only permitted through properly interlocked doors and other access points. To control access, safety switches and other related components must be specified to meet specification and installation requirements.
Safety switches are available in a variety of mechanical options such as tongue interlock, hinge interlock, cable pull and limit. These contact switches work well in many applications, as do non-contact safety switches in other instances.
But before we get into a detailed discussion of safety switches, let’s look at some of the applicable standards.
Standards and specs
A variety of personnel operate automated production equipment, from experienced manufacturing engineers to the newly hired production workers. All of these employees need protection from machine operating hazards, which can be provided by following safety specifications defined by several national and international specifications (Table 1).
Specifications such as 29 CFR 1910.212 identify a variety of machines that usually require point-of-operation guarding. ANSI/NFPA 79-1991 and ANSI/RIA R15.06-1986 also discuss guarding requirements, as do ISO 13849-1 and EN 62061.
These specifications make it clear that machine guarding shall be provided to protect the operator and other employees in the machine area from hazards. These hazards include those that are created by point of operation, pinch points, rotating machinery, flying debris and sparks.
Barrier guards are commonly used to protect machine operators and maintenance technicians. These barrier guards are designed and constructed to prevent plant personnel from having any part of their body in hazardous areas during machine cycles.
These barrier guards typically include a gate or guard door to protect the operator from machine hazards. These gates must be in the closed position before the machine can cycle. To confirm the gate or guard door is closed, safety switches are used; there are many choices to satisfy regulatory requirements and provide a safe working environment.
As these safety switches are designed into the automated equipment and control system, machinery safety standards such as ISO 13849-1 and EN 62061 must be taken into account to ensure compliance. It is important to understand the performance levels and risk factors, such as severity of injury, frequency of exposure and possibility of avoidance.
These machinery safety standards help define the reliability, redundancy and fault detection of the safety system. When specifying safety switches, understand the needed category, for example PLe or SIL3. The category depends on the application, and some judgment must be used, so a good rule-of-thumb is to go to a higher category when in doubt.
Safety Integrity Level (SIL) is a measure of safety system performance in terms of probability of failure on demand. There are four levels: SIL 1, SIL 2, SIL 3 and SIL 4. Performance Level (PL) is a similar measure to SIL, with levels of PL a, b, c, d or e. Higher SIL numbers or PL letters indicate safer and more expensive protection regimes. Specifically, the higher the SIL number or the PL letter, the safer the system, as there is a lower probability the system will fail to perform properly. As the SIL number or PL letter increases, the installation and maintenance costs and complexity of the system also usually increase.
Mechanically actuated safety switches
It is always the designer’s responsibility to ensure compliance with the safety requirements of an application and to select the correct protection level. Here are some suggestions for guard door safety switches that provide point-of-access protection for operators and meet protection level requirements.
A variety of safety switches are available in the market (Figure 1). Single or dual contacts from each of these switches are connected to a safety controller (safety relay, safety programmable relay or safety-rated PLC) as inputs. When the switch is activated, the safety controller’s outputs remove energy, such as power or air, from motion-causing devices including motors, solenoids and pneumatic actuators.
Tongue interlock safety switches are often used to monitor door position. With these devices, a key is mounted on a door, and it mechanically engages and actuates the switch when the door is shut.
Locking tongue guard switches are similar to tongue interlock safety switches, but use a spring and solenoid. These locking tongue guard switches can be configured to unlock/release the tongue when energized or de-energized.
Hinge interlock switches can also be used to guard doors, and are available in different styles. Some mount to a pin on the top of the door, while others mount to the hinge pin and monitor the position of the door.
A commonly used safety device on a machine or production line is a cable pull switch. These switches monitor the tension on a cable stretched along an open machine or conveyor. If the cable is pulled or cut, tension is respectively increased or released from the cable and the switch breaks (opens) the safety circuit connected to the safety controller.
Safety switches are not just designed to protect operators, but machines as well. Over travel, or drift on machine travel stops, can cause catastrophic damage to equipment, even when these conditions don’t directly jeopardize operator safety. Safety limit switches are used to prevent these types of problems. They can have the same form as common limit switches with levers, rockers, plungers and rollers—and are designed with safety standards in mind to ensure that possible machine failures are detected early.
Non-contact safety switches are another option. Although they perform the same function as tongue interlock switches, they provide several advantages since the switch and actuator do not come in contact.
Non-contact switch advantages
Non-contact safety switches offer several advantages over standard mechanically interlocked safety switches (Table 2). They work well in applications with poor guard alignment and high maintenance requirements, and where long mechanical life is required.
Non-contact safety switches allow easy and unobtrusive installation because they are small and have a variety of mounting footprints and actuation paths. Because the actuator and the switch of non-contact devices are not mechanically engaged, these units have zero mechanical wear and, therefore, a long service life. These switches typically also have increased environmental protection, making them less susceptible to water and dirt ingress.
Another advantage of non-contact safety switches is their flexibility with respect to alignment of the actuator and switch. Unlike conventional interlocks where one must carefully align the key (tongue) path to enter and actuate the switch, non-contact switches can have upward of ½ in. of misalignment—so a sagging guard door, tight swing radiuses and awkward angles won’t pose a problem.
The main challenge with non-contact switches is their inability to mechanically lock a door or other moving part. However, there are options available that use electromagnets that provide up to 1,500 N of force to hold a door closed or a moving part in a designated position. Although the metal actuator and electromagnet do come in verifiable contact, there is no engaging key as with contact switches.
Most non-contact switches can be purchased as non-coded or coded, depending on the level of required safety, and can use a variety of different methods to provide protection.
Non-contact type switch technologies
Non-contact safety switches are available in a number of styles and configurations. Non-coded switches are simple magnets, while coded switches can be magnetic- or RFID-coded to provide a higher level of safety. Non-coded switches use a redundant reed switch for safety. When the magnet actuator is present, it pulls in, or closes, the switch contacts. These units do not require separate power and act like dry contacts in an electrical circuit when used as inputs to safety controllers.
Coded, non-contact switches require additional power, but in turn provide a digital output, LED indication and increased levels of detection if plant personnel try and defeat the switch. Personnel defeat safety devices for a variety of reasons, but doing so is never a good idea, and coded switches can be used to help deter these detrimental actions.
The switch actuator in a coded non-contact switch is composed of a series of magnets whose poles are placed in a particular pattern (for example N-S-N). The switch is set to only see this particular magnetic pattern; therefore, it cannot be defeated by a generic magnet.
RFID-coded switches provide an additional layer of defeat prevention. The actuator is composed of an RFID tag that the switch is programmed to detect. These switches can be master coded (detecting all RFID tags) or uniquely coded (seeing only one tag). With the unique coded RFID tags only seeing the actuator that comes with it, the possibility of defeating them is reduced even further.
Here’s an example of how these safety switches can be used in machine and robot protection schemes:
A common application involving a locking safety switch is a guarded robotic work cell (Figure 2). The maintenance access door is held closed while the robot is in cycle by a solenoid-operated locking tongue interlock safety switch. An operator would request access to the guarded area by pressing a push button. The push button would activate an input on the safety controller, which would issue a cycle stop request. This request would signal the robot to finish its cycle and stop.
Once the robot’s cycle completes and it’s stopped, the safety controller is notified by a discrete signal input. One of the safety controller’s outputs then energizes the safety switch solenoid, releasing the door lock. When the guard door unlocks, the robot’s guard safety circuit is disabled. This inhibits robot motion unless the robot is placed in manual mode and the dead man’s switch, on the handheld pendant, is actuated. The operator can then safely open the door for maintenance and can operate the robot manually if need be.
Using the locking safety switch in this application allows the robot to stop in a controlled and predictable manner, which prevents possible machine damage or harm to personnel. It also makes it easier to restart the system as it was automatically stopped in a known state by the safety controller.
There are many machine and robot applications, and each must be examined to determine the required protection level. Once this level is determined, the right type of safety switches, safety devices and safety controllers must be selected to meet the protection level requirements, and to provide protection to personnel and machinery.