Safety relay modules and safety controllers are devices used in the safety-related part of control systems to perform control so that the machine operation is permitted only when safety is confirmed.
Specifically, it checks signals from inputs such as emergency stop switches and interlock switches, determines that it is safe, and controls contactors and other devices connected to outputs based on the results of that determination. In other words, the safety relay module and safety controller play a central role as the logic unit in the safety-related part of the control system (Figure 1).

 

Safety Relay Modules

A typical safety relay module contains several force guided relays and electronic components in an enclosure. It is also designed to be installed inside the control panel.(e.g. on a DIN rail)

Function of Safety Relay Modules / Safety Controllers in the Safety-related Part of a Control System

To start, the overall function of the safety-related part of the control system will be explained. As an example, we use a system with a safety function where a contactor opens and closes the load (motor) circuit in response to operation of an emergency stop switch (Figure 2). If the emergency stop switch is pressed while the motor is running, the motor is stopped immediately.
 

Emergency stop switches use NC (normally closed) contacts, which are normally closed (turned on) and output a signal (safety signal) that indicates a safe state. On the other hand, if some problem occurs and an emergency stop switch is used (pressed), the NC contact opens (turns off), and the safety signal is no longer output.

The safety relay module / safety controller outputs a signal to the contactor that permits the motor to run when the safety signal from the emergency stop switch has been input and the start (activation) switch of the control system has been pressed. When the contactor contacts are closed (turned on) and power is supplied from the power supply, the motor will start running. Note that the contactor contacts are NO (normally open) and are open (off) when there is no signal from the safety relay module.

In this type of safety control system, if the emergency stop switch is pressed while the motor is running, the safety signal is no longer input to the safety relay module, and the safety relay module stops outputting the signal to the contactor (the signal that allows the motor to run). In response, the NO contact of the contactor shall also open (turn off), and the motor shall stop.

Furthermore, when the emergency stop switch is pressed, even if the start switch is pressed, the safety relay module shall not output a signal (to allow the motor to run) to the contactor, and so the motor shall not run. To restart the motor, the emergency stop switch shall be reset (released from the activated state), and then the start switch shall be pressed separately.

Safety Controllers

The safety controller is a safety device that, like the safety relay module, serves as the logic unit in the safety-related part of a control system.
As machines become more sophisticated and various safety devices are introduced, the safety-related part of control systems will continue to grow in complexity. The safety controller can be said to be a device that combines high safety performance with ease of use because the CPU functions can be used to easily set up the logic processing and combinations of various safety devices needed to ensure machine safety (Figure 3).