Drive software can suppress machine vibration
Advanced vibration suppression technology achieves smoother motion, faster settling time, and more accurate positioning. Industrial robots and machine systems are often susceptible to vibrations that feed into the position control system and cause system instabilities. Vibrations are typically caused by resonant frequencies and nonlinear forces introduced by the robot joints or machine couplings.
The increasing clock speeds and lightweight construction of modern machines also lead to an increased tendency to oscillate. Gantry robot design, for example, may employ less rigid mechanical structures due to design limitations of weight or cost. In such applications with overhanging loads, the flexible links between the end effector and the gantry robot axes may cause the machine to vibrate excessively at every stop or point of movement reversal. To maintain accuracy, the machine must pause until the vibrations cease; to achieve maximum system capacity, the settling times must be as brief as possible.
Advanced vibration suppression control used in servo drives can quickly stabilize systems that develop vibrations at a constant frequency. Proprietary control algorithms reduce the following error and the settling time of the load.
The vibration suppression function runs in a closed loop, detecting oscillations as they occur and damping them immediately. By actively damping load oscillations, drives can significantly reduce the time it takes for a heavy load or an end effector to settle at the target position. Although the following error seen at the encoder level may be higher, the overall performance of the system, as evaluated at load position, is significantly improved.
Vibration suppression process
The vibration suppression process has four phases, shown in Figure 3.
Phase 1: Perturbations to the system are detected using control variables, such as position error and current, as input. A perturbation value is calculated.
Phase 2: The perturbation value is passed through a narrow band pass filter to select the perturbations that are induced by the system oscillations. The center frequency and the width of the band pass filter are set by two parameters.
Phase 3: Corrective outputs are calculated.
Phase 4: The corrective outputs are added to the control variables using a damping gain parameter. For vibrations that occur at high frequencies, the servo drives with vibration suppression rely on a second order low pass and notch filters. In addition, the proprietary vibration suppression function controls vibrations induced by low resonant frequencies, from 5 Hz to 400 Hz. The function also is capable of damping systems in which several distinct resonances occur.
Vibration suppression is applied automatically through the drive’s autotuning procedure. First, the oscillation frequency is measured, and a vibration suppression frequency is set. Then, a vibration suppression gain is raised progressively until damping is detected. Through the use of a graphic user interface software users easily can monitor and adjust the settings manually.
The method of vibration suppression described improves the performance of servo systems in which a load is carried by a flexible coupling, as shown in Figure 4.
This type of coupling has a significant amount of flexibility. If servo control of the motor is set for near-zero position error during movement, the load will oscillate strongly at each position. Moreover, every change in acceleration will produce a jerk, further increasing the oscillation of the load.
Figures 5 and 6 show the recorded traces of settling time before and after the application of the vibration suppression in this system. Without vibration suppression, setting time is more than 1.5 sec. With vibration suppression, the setting time is reduced to almost 0.25 sec.
Vibration suppression has proven extremely effective in robotic applications in which heavy loads cause deviations from the required path. Figure 7 shows the path following of a robot before and after the application of vibration suppression. Without the function, vibrations cause deviations from the intended path. Once the function is applied, path following becomes precise and smooth.
Whenever an automated application employs a suspended load, ball screws, belt-driven linear slides, or a nonrigid motor-load coupling, it runs the risk of vibration. With vibration suppression control as a standard feature, servo drives can effectively improve the cycle time and performance of machines in applications such as electronics assembly, semiconductors, machine tools, and laboratory automation.
– Dr. Markus Erlich is vice president marketing for Servotronix Motion Control. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, email@example.com.
Applications with overhanging loads can use vibration suppression software in servo drives to simplify machine design.
Vibration suppression takes measurements, uses logic to calculate corrective outputs, and applies the outputs via the servo motor.
Vibration suppression helps robotics, electronics assembly, semiconductors, machine tools, and laboratory automation.
Less vibration can improve performance and decrease wear.
Also see a Servotronix YouTube video demonstrating vibration suppression technology.
Also see the following article, linked below: Optimizing servo control with an adaptive nonlinear algorithm.