Advances in servo technology mean customers expect their servo-controlled machines to operate with ever-increasing performance. One performance metric is machine positioning accuracy. Better machine accuracy ensures higher quality manufactured parts and products. Therefore, precise positioning is a critical requirement when selecting or developing servo systems.
Servotronix has developed many methods to overcome positioning errors and improve machine performance, especially for linear motion systems.
During runtime, the system's accuracy may be affected by multiple conditions or factors, leading to unacceptable performance. Example:
Encoders: During the manufacturing process of these devices, mechanical, electronic, or optical defects introduced into the encoder may lead to positioning errors. Environmental conditions and electronic noise may also affect the quality of the encoder signal.
Load: Bending of components in a mechanical system can cause positioning errors.
Orthogonality: This applies to precise positioning using an XY stage, where the travel of the X and Y axes must be perfectly perpendicular (orthogonal). If the two travels are not orthogonal, the Y-axis travel will produce a positioning error in the X direction, and vice versa.
Backlash: Backlash is a function of the clearance between the meshing teeth of gears in a transmission. Normal backlash allows gears to mesh without jamming, providing lubrication space. For example, excessive backlash can occur when the lead screw nut frequently rotates in the opposite direction, leading to positioning errors.
Hysteresis: Hysteresis error refers to the difference between the actual position and the commanded position caused by the inconsistent response of the system to increased and decreased input signals.
To apply the most effective method to correct positioning errors, the first step is to determine whether the error is repeatable. When the deviation from the target position is measurable and repeatable, certain 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, optimal correction can be achieved through external devices.
Error repeatability
Repeatability refers to the ability of a motion system to return to a specific position repeatedly. Accuracy refers to the range of measurements taken when the system returns to a specific position. Precision refers to how close the system is to a measured or true position.
Generally, the repeatability of positioning errors can be determined by moving and measuring the defined position. This process can utilize external precision feedback devices, such as laser interferometers.
Suppose the motion controller instructs a linear segment to move to a specific position. Once the movement is complete, the device measures the actual position of that segment. The commanded motion measurement cycle is repeated until it can be determined whether positioning errors have occurred, and if so, whether they are consistently equal. Positioning errors can vary throughout the stroke; therefore, it is necessary to perform repeatability tests on a series of points within the linear motion system.
When the errors are repeatable, their occurrence is predictable, and the servo drive firmware can provide the necessary corrections while achieving and maintaining accuracy without the need for auxiliary or external feedback devices.
Linear platform motion system
Harmonic compensation
To determine whether harmonic compensation should be applied to the servo control loop, the disturbances within the motor cycle must exhibit a fixed pattern. This indicates the presence of harmonic errors in the system. For example, motor cogging torque is caused by the motor's mechanical structure. Cogging torque is commonly found in iron-core linear motors and can therefore be corrected through harmonic compensation.
The CDHD2 servo drive from Hikvision incorporates a harmonic compensation algorithm to correct for torque and feedback disturbances that may be caused by mechanical defects in the motor and/or defects in the feedback. The harmonic correction algorithm can handle disturbances with repeatable patterns at a single motor pitch in a linear motor or a single mechanical speed in a rotary motor.
Before applying the algorithm, it is also important to correctly identify the source of interference and use the correct type of harmonic compensation. If a system uses resolver feedback and detects two interference patterns per cycle, it will likely require feedback-based harmonic compensation.
Harmonic compensation control loop – based on torque
Error mapping correction
Some repeatable positioning errors cannot be corrected by analytical expressions. The motion system may lose accuracy, and only a few points along the travel may require compensation. For such errors, an external measuring device can be used to generate an error mapping table, which the driver can then use to compensate for errors at specific points.
For example, the load position on a linear axis can be measured using a laser interferometer. For simplicity, we assume the axis travels one meter. The drive software sends a command to move the motor at 100mm intervals, allowing the motor to move within 10 positions. As the motor moves the load, the interferometer measures the distance traveled by the load, comparing this distance value at each point with the motor encoder position. The difference between the two values is the positioning error.
Once an error mapping is generated, it will be stored in the non-volatile memory of the driver, and error compensation can be activated in the driver.
An algorithm is inserted between each point. In this example, to move the stage to a position 275 mm from the origin, the controller retrieves the two nearest data points from a lookup table (200 and 300 mm) and calculates the correction value at 275 mm.
The advantage of the positioning error correction method, which can be performed by the CDHD2 servo drive, is that the drive can retrieve the correction value in real time based on the actual position and apply the correction in real time. Once the correction is implemented, the error is negligible and no additional position feedback device is required.
Laser interferometer for measuring travel distance
Dual-loop control
To compensate for random, non-repeatable errors, linear motion systems require a method to detect and warn the driver of errors occurring during operation. An effective and relatively inexpensive method for overcoming non-repeatable errors is to install a second encoder on the load of the motion system. This second encoder can provide accurate feedback in real time, thereby compensating for deviations in the motion system.
The firmware in the Servotronix CDHD2 servo drive features a dual feedback control loop. In dual-loop applications, motor feedback is used for the speed control loop and rectification, while secondary feedback is used for the position loop.
The CDHD2 drive supports a variety of secondary feedback devices, such as incremental encoders and serial encoders, as well as analog position feedback devices.
The dual-loop configuration requires adjusting the ratio of secondary feedback to motor feedback, as well as a specific tuning method, as shown in the figure.
Dual feedback control loop
Servotronix's dual feedback control loop has been implemented in a range of GE Healthcare PET/CT scanners for clinical imaging, in which the patient's pedestal support axis is mechanically driven by a ball screw.
To counteract the backlash in the GE scanner system, two encoders can be connected to this axis. A position feedback encoder is mounted on the motor, while a secondary feedback encoder monitors the load. This dual-loop control solution improves the smoothness of the imaging system's operation and positioning accuracy. It also features safety functions to detect load slippage or collisions.
Every linear motion device application presents unique challenges and solutions. The versatility of the CDHD2 drive allows customers to implement certain error correction methods – such as dual-loop control, harmonic compensation, or error mapping – to achieve the highest accuracy and machine performance.
