In automation applications, servos are used as motion control actuators. In addition to performance requirements, safety protection is also very important. This article summarizes all the emergency stop and safety protection functions of Delta Servo, and explains the use timing, trigger conditions, and corresponding deceleration time.
Emergency stop DI.EMGS (0x21) trigger AL.013
Triggered by DI.EMGS (0x21), emergency stop is used for unexpected dangerous situations, such as the safety door is opened, foreign objects are sensed, the emergency switch is pressed, etc., in order to ensure safety, the wiring must use the B contact (Normal Close) to avoid loss of protection when disconnected. In order to prevent people from being pinched, the servo motor will be disabled when this function is activated, that is, the motor current is cut off to avoid torque output. However, when an emergency stop occurs, in case the motor is rotating at high speed, if the servo is directly disabled, the motor will not be able to decelerate and stop immediately. It can only be stopped slowly by friction, and it will still slide for a long distance or even crash the machine, which is still dangerous.
Use dynamic brakes, not mechanical brakes
In order to solve the aforementioned uncontrolled coasting problem, the drive provides “dynamic braking” as a deceleration function in the Servo disabled state. Dynamic brake is not a mechanical brake installed behind the motor [Note 1], do not confuse it. Dynamic braking means that when the servo is disabled, the driver short-circuits the three-phase winding U/V/W of the motor, so that the rotating motor cuts the magnetic field and generates current, thereby generating decelerating torque. Due to the conservation of energy, the electric energy produced by the coil will consume the kinetic energy of the motor, so the motor will decelerate and stop. This function can stop the uncontrolled motor as soon as possible, which can be set by parameter P1-32.Y, The default (P1-32.Y=0) has enabled dynamic braking, so when the servo is disabled, pushing the motor will obviously feel heavier and not easy to push. It can also be set to (P1-32.Y=2) that the dynamic brake is turned on when the motor has speed, and it is automatically released when it stops. However, when the inertia of the mechanism is large, the motor cannot be stopped immediately even when the dynamic brake is used in an emergency situation, and the protection function is still somewhat incomplete.。
Emergency stop (DI.PFQS) with programmable deceleration time
Based on the aforementioned considerations, Delta Servo later added DI.PFQS (0x47) to provide an emergency stop function with programmable deceleration time (P5-03.C), that is, the servo is still enabled during deceleration. After stopping, the servo is disabled. Since the motor is still controlled during deceleration, the maximum torque can be used to decelerate, so the motor can be stopped as soon as possible to avoid rushing out of the stroke and damaging the mechanism. As shown in the figure below, AL.35F will be displayed during deceleration, and AL3CF will be displayed after stopping.
STO （Safe Torque Off）Redundant hardware circuit protection
STO is based on the safety standard defined by IEC regulations. It is currently only available for some Delta A3 and B3 models. It uses an independent wiring to detect errors and cut off the torque output of the servo. The characteristic is that it has dual circuits. Any circuit break will trigger the protection and disable the servo and generate the corresponding AL500 ~ AL.503. Because the motor is not controlled, the deceleration time cannot be specified.
Hold and resume of PR commands
This is a stop function unique to PR mode, which is triggered by DI.STOP (0x46) or writing P5-07=1000, which can stop the executing PR positioning or speed command, and specify the deceleration time by P5-03.D. This command is not used for abnormal protection, so keep the servo enabled during operation. The stopped command can be started again with a “increment=0” positioning command to complete the remaining distance.
Abnormal protection in controlled state
The servo motor remains enabled during this type of protection, and the deceleration time can be specified by P5-03, and it remains enabled after deceleration and stop. For example, PL (positive limit), NL (negative limit), SPL (software positive limit), SNL (software negative limit), etc.
The detailed characteristics of the above various protection functions are summarized in => Emergency stop and safety protection-summary table (under writing…)
[Note 1] The mechanical brake is located at the back of the motor. It is clamped when the power is off or when the servo is disabled and the speed is lower than P1-38, and it is released when Servo is enabled. It is usually used to clamp the position of the mechanism when the power is cut off, especially for the mechanism used for vertical movement to avoid damage due to gravity sliding after the power is cut off. It is not used for deceleration after the motor is disabled, and is different from dynamic braking.