As I was finishing my morning tea, a piercing alarm rang out from the workshop. Rushing over, I found that the new employee, Xiao Li, had failed to set safety limits while debugging the conveyor belt, causing the robotic arm to crash into a barrier. This scene reminded me of a similar mistake I made years ago. In the field of industrial automation, safety is not an option; it is a necessity. Today, let’s discuss how to utilize PLC function block diagrams to implement safety design in motion control.
We know that the PLC control system is the brain of factory automation, and function block diagrams are the language we use to communicate with this brain. Function Block Diagram (FBD) is a graphical programming language that implements control logic through pre-defined function blocks, making it particularly suitable for complex tasks such as motion control. Compared to traditional ladder diagrams, function blocks are more modular, making them as intuitive as building with blocks.
Imagine that a traditional ladder diagram is like building a house with single bricks, while a function block diagram is like assembling a house with pre-fabricated walls and roofs. This approach not only improves programming efficiency but, more importantly, enhances safety performance.
In motion control systems, the primary principle of safety design is “fail-safe”. This means that regardless of what fault occurs in the system, it should automatically transition to a safe state. To achieve this, we need to focus on three aspects: safety monitoring, exception handling, and redundancy design.
Safety monitoring is the eyes of the system. When implementing safety monitoring using PLC function block diagrams, dedicated monitoring function blocks can be created. For example, we can design a speed monitoring block that continuously checks the difference between the actual speed of the motor and the set speed. When the difference exceeds a threshold, a safety response is triggered immediately. This function block may internally contain basic elements such as comparators, timers, and triggers, but as users, we only need to focus on the input and output parameters.
SpeedMonitor(
ActualSpeed := Motor1.Speed,
SetSpeed := SetPoint.Speed,
Tolerance := 5.0,
TimeDelay := T#2s,
Alarm => AlarmBit,
Stop => StopCommand
);
This simple function block call can monitor the motor speed, triggering an alarm and issuing a stop command when the speed deviation exceeds 5% for 2 seconds.
Exception handling is the safety net of the system. In PLC function block programming, good exception handling should be hierarchical, forming progressive protection from minor warnings to emergency stops. We can create an error management function block that executes different levels of responses based on different types of faults.
I remember last year when I was debugging a conveyor belt at a food factory; relying solely on simple timeout detection caused the system to stop frequently. Later, I designed a three-level response function block: issuing a warning but continuing to run for minor deviations, slowing down and alerting the operator for moderate deviations, and only executing an emergency stop for severe deviations. This greatly improved production efficiency while ensuring safety.
Redundancy design is the safety pin of the system. For critical safety functions, a single monitoring point may not be reliable enough. When implementing redundancy design using PLC function block diagrams, the same parameter can be monitored from multiple angles. For example, when monitoring the position of a robotic arm, both encoder data and limit switches can be used as backup monitoring points. The two signals are processed through independent function blocks, and then a “voting” function block compares the results.
PositionVerify(
EncoderPos := Encoder1.Position,
LimitSwitch := Switch1.Status,
Tolerance := 2.0,
Valid => PositionValid,
Error => PositionError
);
The key to achieving safe motion control lies in “prevention is better than cure”. In function block programming, it is essential to develop the habit of adding condition checks before each control instruction. For instance, checking the system status before starting the motor and verifying the home signal before position control. Although these preventive checks may seem cumbersome, they can prevent potential risks before problems occur.
In practical applications, I recommend utilizing the safety-certified function block libraries provided by PLC manufacturers. These verified function blocks are designed and tested for specific safety levels, significantly reducing our workload. Siemens’ Safety Integrated, Schneider’s SafeMotion, and Omron’s Safety Network Controller all offer a rich set of safety function blocks.
For beginners, I suggest starting with simple safety boundary monitoring. Try writing a function block that monitors whether equipment is within a safe working area, automatically slowing down or stopping if it exceeds the boundary. Once you master this foundation, gradually add more complex safety functions.
Safety design is not a one-time task but a continuous improvement process. Every time the system encounters an unexpected situation, it is a good opportunity to enhance safety functions. Document these “lessons” and continuously optimize your function block library to ultimately create a reliable and efficient motion control system.
