System Examination: Safe Logic

Safety relays or programmable safety systems can perform the following tasks in systems with safe drive functions, depending on the application:

• Evaluation of input devices on protection equipment
• Activation of safety functions 
• Drive shutdown
• Evaluating the status of safely monitored drive axes in a multi-axis system
• Establishing the plant’s overall safety
• Specifying new limit values during operation 
• Interface between the drive controller and the safety functions

The safe logic can be implemented either as separate, external components or as drive integrated components. Safe logic is the interface between the sensors on the protection equipment and the safe monitoring unit. Drive-integrated solutions enable simple functions in single axis systems to be implemented economically. Sensors are connected directly on the drive and are evaluated. The limited number of safe interfaces makes cross-communication between the drives and complex logic links impossible. The scan time of the programmable safety system must be included in the assessment of the overall reaction time.
Depending on the size of the user program, this will range between 50 to 200 ms and therefore dominates over the delay in the shutdown path. It’s also necessary to consider a delay time on safe, digital inputs, which arises due to the input filters.