Positioning error correction in linear motion system
The progress of servo technology means that customers expect their servo control machines to run with higher and higher performance. One performance index is the positioning accuracy of the machine. Better machine accuracy can ensure that the manufactured parts and products have higher quality. Therefore, bearing development believes that accurate positioning is the key requirement when selecting or developing servo system.
Factors affecting accuracy
During operation, the accuracy of the system may be affected by multiple conditions or factors, resulting in unacceptable performance.
Encoder: during the manufacturing process of such equipment, the mechanical, electronic or optical performance defects introduced into the encoder may lead to positioning error. Environmental conditions and electronic noise may also affect the quality of encoder signal.
Load: bending of components in mechanical system may cause positioning error.
Orthogonality: it is applicable to realize accurate positioning through XY workbench. The strokes of X axis and Y axis must be at right angles to each other (orthogonal). If the two rows of strokes are not orthogonal, the Y axis stroke will produce positioning error in the X direction, and vice versa.
Backlash: the backlash is a function of the clearance between the meshing teeth in the transmission. Normal backlash allows the gears to mesh without getting stuck together to provide lubrication space. For example, when the lead screw nut often rotates in the opposite direction, excessive backlash may occur, resulting in positioning error.
Hysteresis: hysteresis error refers to the difference between the actual position and the command position caused by the inconsistent response of the system to the increased and decreased input signals.
Error correction method
To apply the most effective method to correct the positioning error, first determine whether the error is repeatable. When the deviation of the target position is measurable and repeatable, some functions or algorithms can be used in the servo drive to achieve and maintain the necessary accuracy. When the positioning error is random and irregular, the best correction can be achieved by external equipment. Next, take cdhd2 servo driver as an example.
Error repeatability
Repeatability refers to the ability of the motion system to return to a specific position again and again. Accuracy refers to the measurement range value when the system returns to a specific position. Accuracy refers to the proximity of the system to a measurement or real position.
In general, the repeatability of positioning error can be determined by moving and measuring the defined position. This process can use external precision feedback devices, such as laser interferometers.
Suppose the motion controller instructs a linear phase to move to a specific position. Once the movement is complete, the equipment will measure the actual position of the stage. Repeat the command motion measurement cycle until you can determine whether positioning errors occur and, if so, whether they are always equal. The positioning error can vary with the travel process. Therefore, it is necessary to test the repeatability of a series of points in the linear motion system.
When they are repeatability errors, their occurrence is predictable, and the servo driver firmware can provide the necessary correction while achieving and maintaining accuracy without auxiliary or external feedback devices.
Harmonic compensation
If it is necessary to consider whether the servo control loop should be compensated for harmonics, the disturbance in the motor cycle should have a fixed mode. This shows that there is harmonic error in the system. For example, the cogging torque of the motor is caused by the mechanical structure of the motor. Cogging torque usually occurs in iron core linear motor, so it can be corrected by harmonic compensation.
The cdhd2 servo driver contains a harmonic compensation algorithm to correct torque and feedback disturbances, which may be caused by mechanical defects in the motor and / or defects in feedback. The harmonic correction algorithm can deal with the disturbance with repeatable mode in a motor pitch in linear motor or a mechanical speed in rotating motor.
Before applying the algorithm, it is also important to correctly identify the interference source and use the correct harmonic compensation type. If a system adopts resolver feedback and two interference modes are detected in each cycle, it is likely to need feedback based harmonic compensation.
Error mapping correction
Some repeatable positioning errors cannot be corrected by analyzing expressions. The motion system may lose accuracy and only a few points along the stroke need to be compensated. For such errors, an external measuring device can be used to generate an error mapping table, which can then be used by the driver to compensate for errors at specific points.
For example, the load position on the linear axis can be measured by a laser interferometer. For simplicity, we assume that the travel distance of the shaft is one meter. The drive software sends a command to move the motor at an interval of 100mm to make the motor move within 10 positions. When the motor moves the load, the interferometer will measure the distance traveled by the load, and each point will compare the distance value with the motor encoder position. The difference between the two values is the positioning error.
Once an error map is generated, the map will be stored in the nonvolatile memory of the drive, and error compensation can be activated in the drive.
Insert an algorithm between the points. In this example, in order to move the stage to a position 275 mm from the origin, the controller takes the two nearest data points from the look-up table (200 and 300 mm) and calculates the correction value at 275 mm.
The advantage of the positioning error correction method that can be performed by the cdhd2 servo driver is that the driver can retrieve the correction value in real time according to the actual position and apply the correction in real time. Once the correction is implemented, the error can be ignored and no additional position feedback device is required.
Dual loop control
In order to compensate random and unrepeatable errors, linear motion system needs a method to detect and warn the driver of errors during operation. An effective and relatively inexpensive method for overcoming non repetitive errors is to install a second encoder on a load in the motion system. This second encoder can provide accurate feedback in real time to compensate the deviation of the motion system.
The firmware in the cdhd2 servo driver has a dual feedback control loop. In dual loop applications, motor feedback is used for speed control loop and rectifier, while secondary feedback is used for position loop.
Cdhd2 driver supports various secondary feedback devices, such as incremental encoder and serial encoder, as well as analog position feedback devices.
The dual loop configuration needs to adjust the proportion of secondary feedback to motor feedback and specific setting method.
This dual feedback control loop has been implemented in a series of GE Medical PET / CT scanners for clinical imaging, in which the patient's pedestal bracket shaft is mechanically driven by a ball screw.
To counteract the effect of backlash in the Ge scanner system, two encoders can be connected to the shaft. The position feedback encoder is installed on the motor, while the secondary feedback encoder monitors the load. The dual loop control solution improves the operation stability and positioning accuracy of the imaging system. It also has the safety function of detecting load detachment or collision.
Bearing exhibition learned that each application of linear motion equipment has unique challenges and solutions. The versatility of cdhd2 drives allows customers to implement some error correction methods – such as dual loop control, harmonic compensation or error mapping to achieve the highest accuracy and machine performance.
