Components of safe motion control
The purpose of safe motion control is to avoid dangerous movements. Traditional servo drives consist of non-safety-related components, and achieving safe motion requires additional safety elements to ensure motor power is cut off or to monitor motor movement safely. Today's market is moving towards integrating safety functions internally into the drive, primarily consisting of safety isolation, safety detection, and safety control (ensuring no unsafe movements).
Components of safe motion control
Secure isolation
A servo driver consists of two parts: a power circuit and a control circuit. The power circuit mainly comprises rectification, capacitor filtering, and inversion; the control circuit mainly comprises commands or reference variables, closed-loop control, and PWM modulation pulses. Furthermore, the power circuit and the control circuit are isolated by optocouplers.
Servo driver schematic diagram
The servo driver's structure diagram is converted into a signal path diagram, as shown in the figure below. In this diagram, the optocoupler safety disable (Figure 3), the setpoint or reference value switch (Figure 4), and the control enable (Figure 5) are used for safety isolation in motion control. Due to implementation difficulties and driver technology, the motor contactor (Figure 2) is usually not used as part of the safety isolation. Because the discharge time of the large capacitor in the power circuit is very long, usually more than 5-10 minutes, disconnecting the main power supply contactor (Figure 1) cannot guarantee that the servo motor is de-energized.
Normally, safety isolation is only triggered when safety detection and control functions malfunction.
servo driver signal path diagram
1. The main contactor for power supply of the power circuit is used for the input power supply of the isolation drive;
2. Motor contactor, used to isolate the input power supply to the motor;
3. Optocoupler safety interruptor; optocouplers are used to isolate power circuits and control circuits.
4. Setpoint or reference value switch; the setpoint will be set to 0.
5. Control enable: The signal controlling the optocoupler will be disabled;
Safe motion detection and control
Safety detection and control cannot be understood in the same way as traditional detection and control. Their biggest difference lies in whether they can effectively address the risk of failure. In practice, traditional detection and control do not consider detection mechanisms and diagnostic scope, common causes, high component reliability, and safety system design.
The safety functions listed in standard EN/IEC 61800-5-2 require safety monitoring of kinematic acceleration, velocity, and position. Since non-safety-related components are typically used in actuators, implementation requires the integration of sensor technologies, such as safety encoders, in addition to monitoring the actuator's operation and status.
Safe motion control requires more than just safety detection; it necessitates the safe control of motion itself. The most common safety control is safe motion limitation, which can be implemented in three ways: fixed-value, setpoint selection, and dynamic modification. Fixed-value limitation involves pre-setting limits via configuration software, and these limits cannot be changed during motion. Selective limitation involves multiple fixed values that can be switched during operation via safety I/O or a safety bus. Dynamic modification allows for modification during operation. For example, in the previously mentioned example of a robotic arm and human safety, the safety function reduces the robotic arm's speed based on the distance between the person and the robotic arm; the closer the person is to the danger zone, the slower the robotic arm's speed.
Human and robotic arm
To meet safety control needs, Pilz offers the PMCprotego D series of drives, which integrate the Protego S1/S2 safety card and provide safety functions such as SS1, SS2, SOS, and SLS, helping customers improve production efficiency while ensuring safe production.
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