Advances in servo technology mean customers expect their servo-controlled machines to operate at increasingly higher levels of performance. One performance indicator is machine positioning accuracy. Better machine accuracy ensures higher quality of manufactured parts and products. Therefore, precise positioning is a key requirement when selecting or developing a servo system.

Factors affecting accuracy

At runtime, the accuracy of a system can be affected by multiple conditions or factors, resulting in unacceptable performance. Example:

Encoders: Mechanical, electronic or optical performance defects introduced into the encoder during the manufacturing process of the device itself can lead to positioning errors. Environmental conditions and electronic noise may also affect the quality of the encoder signal.

Loading: Bending of components in mechanical systems can cause positioning errors.

Orthogonality: Suitable for precise positioning through the XY table, the travel of the X-axis and Y-axis must be completely at right angles (orthogonal) to each other. If the two rows of travel are not orthogonal, the Y-axis travel will produce positioning errors in the X direction, and vice versa.

Backlash: Backlash is a function of the clearance between the meshing teeth in a transmission. Normal backlash allows the gears to mesh without jamming together, providing space for lubrication. For example, when the screw nut is frequently rotated in the opposite direction, excessive backlash may occur, resulting in positioning errors.

Hysteresis: Hysteresis error is the difference between the actual position and the commanded position due to the inconsistent response of the system to increasing and decreasing input signals.

error correction method

To apply the most effective method of correcting positioning errors, you should first determine whether the errors are 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 positioning errors are random and irregular, correction is best achieved with external devices. The following uses the CDHD2 servo drive as an example to explain.

error repeatability

Repeatability refers to the ability of a motor 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 how close the system is to a measured or true position.

In general, the repeatability of positioning errors is determined through a process of moving and measuring defined positions. This process can use external precision feedback devices such as laser interferometers.

Suppose the motion controller directs a linear stage to move to a specific position. Once the movement is complete, the device measures the actual position of that stage. The command motion measurement loop is executed repeatedly until it can be determined whether positioning errors occur and, if so, whether they are always equal. Positioning errors can vary over the course of a stroke, so repeatability testing over a series of points in a linear motion system is necessary.

When repeatability errors occur, they occur predictably, 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

If you want to consider whether harmonic compensation should be performed on the servo control loop, the disturbances within the motor cycle need to have a fixed pattern. This indicates the presence of harmonic errors in the system. For example, motor cogging torque is caused by the mechanical structure of the motor. Cogging torque is commonly found in iron core linear motors and, therefore, can be corrected through harmonic compensation.

CDHD2 The servo drive contains a harmonic compensation algorithm that corrects for torque and feedback disturbances that may be caused by mechanical defects in the motor and/or defects in the feedback. Harmonic correction algorithms can handle disturbances with repeatable patterns in one motor pitch in linear motors or in one mechanical speed in rotary motors.

It is also important to correctly identify interference sources and use the correct type of harmonic compensation before applying the algorithm. If a system uses resolver feedback and a pattern of two disturbances is detected per cycle, then feedback-based harmonic compensation is most likely required.

Harmonic Compensation Control Loop – Torque Based

Error mapping correction

Some repeatable positioning errors cannot be corrected by analyzing expressions. The motion system can lose accuracy and only have to compensate at a few points along the stroke. For errors such as these, an external measurement device can be used to generate an error map, which can then be used by the drive to compensate for errors at specific points.

For example, the position of a load on a linear axis can be measured with a laser interferometer. For simplicity, let's assume the axis has a travel distance of one meter. The driver software sends an instruction to move the motor at an interval of 100mm, so that the motor moves within a range of 10 positions. As the motor moves a load, the interferometer measures the distance traveled by the load, comparing that distance value to the motor encoder position at each point. The difference between the two values ​​is the positioning error.

Once an error map is generated, it is stored in the drive's non-volatile memory and error compensation can be activated in the drive.

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