I. Concise Answer
If safety input units such as safety light curtains are used, but a regular PLC is used for logic control, the overall safety level of the safety system cannot reach a high level.
Because ordinary PLCs are much more likely to malfunction than safety controllers, if a safety system is required to achieve a high level of safety protection across its entire circuit, all components in the entire system circuit must also achieve a high level of safety, including input sensors , control units , and output actuators .
II. Detailed Explanation:
First, according to the international machine safety standard ISO 13849-1 "System Design Method for Safety-Related Controls (SRP/CS)" and the corresponding national standard GB/T 16855.1 2008, all risks to the machine are assessed to determine the PLr (Performance Level Requirement, the safety level required for the entire safety system) . Then, safety design is performed, and the final safety system PL level is determined by assessing the safety performance levels of the components used. Finally, it is checked whether the PLr requirements are met. The specific process is shown in the following diagram:
To elaborate:
Where 'a' represents low risk and 'e' represents high risk.
Based on the above three conditions, the required operational performance level (PLr) for safety protection at this workstation is finally determined. Each safety function must have its own PLr to protect against the corresponding hazard.
After assessing the PLr, safety circuits are designed for different hazards and their severity, including input components (emergency stop, two-hand buttons, safety light curtain, safety light grid, safety door lock, etc.), control units (controllers, etc.), and output components (relays, contactors, valves, etc.).
For different safety functions, calculate the total power PL of the entire safety loop and check if the final result is greater than or equal to PLr. If it meets the requirement, it is a suitable safety design. If it does not meet the requirement, it needs to be redesigned or some components need to be changed to make the total power PL of the system greater than or equal to PLr.
If PL-based components are combined with a series of control systems (SRC/PS), then the total PL can also be calculated and take into account the following:
a) The lowest performance level (PL low) of the components used
b) PL components and their quantity = N low
The table below provides a simple method to evaluate the PL that the entire system can achieve.
In other words, for the entire system to reach a certain PL value, the PL value of each component in the entire system must be at least greater than or equal to this value.
III. Simple Examples
Imagine a robot that picks up materials at one workstation and then moves to other workstations to perform other operations. People need to frequently enter and exit the area where the robot picks up materials to place the workpieces.
First, assess PLr. Since the robot can cause serious injury if it hits a person, select S2. Since the operator needs to frequently enter this danger zone, select F2. If the robot attacks, and the speed is relatively slow, the operator can escape the danger based on their skill and reaction time, so select P1. Finally, PLr = PLd.
Thus, according to the simplified estimation table of the overall system PL, the safety design requires the use of input and control elements that must be at least PLd or higher.
Since the PL level of a typical programmable controller will not exceed PLC , the safety level of the entire system cannot meet the requirement of PLr=PLd. Therefore, it is a design that does not meet safety standards and a safety controller of PLd or higher must be used.
Regarding the operational performance level (PL) of individual components, some can be found in the component's parameter table, such as safety light curtains, safety scanners, and safety PLCs. Other components, such as contactors and safety relay modules, require calculation. The PL value of a subsystem can be calculated and evaluated using SISTEMA software, which can then generate corresponding reports.
SICK China can provide specialized training courses on SISTEMA software and other security concepts and standards. It also offers security services such as security checks, downtime measurement, and system security inspections.
