Safety components in industrial automation are defined in the broadest sense as per Article 1 of the Machinery Directive as follows.
(1) Components provided to ensure safety functions.
(2) A component that poses a threat to the health and safety of workers in the event of damage or malfunction.
The need for safety components in industrial automation and safety-related control systems arises when establishing basic principles to prevent mechanical accidents and achieve safety in machinery.
Specific items in the Machinery Directive
(1) Sensors that electronically detect workers.
(2) Logic units (checks safety functions in two-hand control devices.)
(3) Guard screens for presses
(4) Roll-over protective structures (ROPS)
(5) Falling object protective structures (FOPS)
Safety Components in Industrial Automation
Control systems that impact safety must be designed to minimize the likelihood of hazards even when there is a fault in an interlock device.
Safety devices in accordance with standards are equipped with functions such as direct opening mechanisms for switches and forced guide mechanisms for relays. These functions are designed to operate correctly in the control system in which they are used.
The following discusses the safety components in industrial automation that are commonly used to develop safety functions.
1. Emergency Stop Switches as Safety Components of Industrial Automation
Emergency stop switches
An emergency stop switch is a switch that connects to a machine to halt operation in emergency situations.
Emergency stop performance standards: ISO 13850 and IEC 60204-1
The most relevant standards for emergency stop applications are ISO 13850 and IEC 60204-1. ISO 13850 covers practical aspects and design principles. IEC 60204-1 pertains to safety of machines and equipment electrical systems.
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Emergency Stop Devices
Common types of emergency stop devices
- Button switch
- Wired switch
Requirements
- Electrical contacts must have a direct opening mechanism.
- Emergency stop devices must have a latching function that mechanically holds them in the stop position until the device is manually reset.
- Emergency stop actuators must be red and mushroom-shaped. The background immediately behind the actuator must be yellow.
- When using a wire as an actuator, the following must be considered.
(1) Required deflection to produce the emergency stop signal
(2) Maximum possible deflection
(3) Minimum distance between the wire and the nearest machine in proximity
(4) Amount of force required for operation
(5) Ease of locating the device for the operator using a flag marker or other methods
(6) Automatic generation of emergency stop signal in case of wire breakage or disconnection
2. Safety Switches
Safety switches come in the form of safety door switches and safety limit switches.
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A safety door switch can be a mechanical or electrical switch that connects to a direct opening mechanism and prevents the machine from operating unless specific conditions, such as the door being closed, are met. It also features functions that stop easily from being overridden.
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A safety limit switch connects to a direct opening mechanism and is used to monitor doors and ensure that objects are not driven excessively.
(1) Monitoring and Locking of Doors
Door monitoring and locking switches are one of the most important types of safety devices to prevent hazardous situations by cutting power to the machine.
When deciding to protect a machine with safety barriers, it must be ensured that the only access into the hazardous area is through the guard.
If the guard is opened, a mechanically activated position detector stops the machine. Each guard in the enclosure must have mechanical position detectors to ensure personnel safety.
A fundamental condition is that when the door is open, the machine must stop before anyone can access the hazardous moving parts of the machine.
The main selection criteria for an interlock device include:
- Conditions of use and intended application (ISO12100-1)
- Hazard present in the machine (ISO12100-1)
- Severity of potential injury
- Likelihood of failure of the interlocking device
- Downtime and access time considerations
- Frequency of access
- Duration of exposure to the hazard
- Performance considerations
The position switch must be activated in a positive manner. The contact break of the position switch should be of the “positive opening operation” type. (IEC60947-5-1)
The security of an interlock switch lies in its ability to resist attempts to “bypass” or disable the locking mechanism. An interlock switch should be designed such that it cannot be simply defeated.
This means that the intended operation can be easily accessible either manually or with an object. Accessible objects may include:
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Screws, pins, metal sheet pieces;
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Everyday objects such as keys, coins, tools necessary for the intended use of the device
(2) ISO14119
ISO14119 “Locking devices associated with guards” provides guidance on locking devices and is intended for use alongside EN60947-5-1 for electromechanical switches.
(3) Requirements for Door Monitoring
Door monitoring should ensure that the safety door protects from the hazardous area as defined in the risk assessment (ISO14121). Sensors and signal processing must comply with all necessary norms and directives.
- Switches should be designed to withstand all expected and foreseeable stresses.
- Switches should comply with safety standards, in particular, direct-opening contacts and safety door switches must be used.
- Redundancy and diversity principles should be considered in the mechanical design of the switches and signal processing.
- Signal processing should be designed according to ISO13849-1 categories as defined in risk assessments.
(4) Requirements for Locking the Door
When the stopping time exceeds the time it takes for a person to reach the hazardous area, a locking device with protective locking must be used.
This device is intended to lock the guard in the closed position and is connected to a control system so that:
- The device cannot operate until the guard is closed and locked.
- The guard remains locked until the hazard is removed.
For applications requiring frequent access, the locking device should be selected to create the least possible hindrance to the operation of the guard.
In this case, the intended use requirements, conditions of use, risk assessment, and downtime and access time should also be considered.
Devices that are mechanically activated:
1.Cam activated operation
When using a detector, it must be activated in a positive manner, because, alongside other features, this activation mode prevents failure of the detector in a simple manner. Higher-level protection against failure can be obtained, for example, by enclosing the cam and detector in a housing.
2. Key switch operated activation
The key operation switch is designed to prevent easy bypassing of the switch.
Each time a specific operation key is required.
These keys can be placed on sliding guards,...
