FAQ: How are pLimit_PositioningError and sPosition_CompletionWindow used to control positioning accuracy?
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The goal in a positioning system is to get from Point A to Point B (see attached SmplMove.pdf). But how close to Point B must the system be? Also, how much deviation is allowed on the way to point B? MotionWorks+ has a couple of system parameters that can help the programmer manage this. As with all of the system parameters, these are described in the "Programming Concept" section in your MotionWorks+ software manual.
pLimit_PositioningError: This parameter is the maximum allowable following error that the servo will be allowed to accumulate before an alarm is generated. This can occur during any part of the move. Usually, the maximum following error occurs during acceleration. If the following error exceeds this value, an A.9F alarm will be generated, and the system bit mPosition_Error_Exceeded will be set to "1".
Even if a system needs to be very precise at the end of the move, the maximum error during the move will be much higher in most instances, depending on the acceleration. Remember to set the value of pLimit_PositioningError sufficiently high. This value will have NO EFFECT on the final position precision.
sPosition_CompletionWindow: This parameter defines the maximum distance allowed between the final target and the servo for a move to be considered complete. If an index move with the "wait for completion" property set to "true" is used, program execution will be stalled until the position of the motor is at the commanded position, plus or minus this window. Setting this value larger just means that the final target need not be as close to the target before execution continues.
The default value of sPosition_CompletionWindow is 0.0 user units, meaning that the motor has to be EXACTLY at position before execution can continue. With the default tuning gains in the amplifier being so low, it is often difficult to achieve this level of precision, and the program will appear to "freeze" on the index block. In reality, the program is waiting for the position to get within the window, something that the tuning gains are not high enough to accomplish. You may even find that a small push into position will cause the move to complete and the program to continue.
Neither of these parameters will determine how closely a system will follow the command, only how the system will react to the error. The following error during the move and the precision at the end of the move are always determined by the tuning of the system. If the system is generating A.9F errors due to excessive position error, or if the program is hanging while waiting for a move to complete, then these are the parameters to check. If these parameters are set appropriately, then the tuning needs to be improved.
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Please read and understand the product instruction manual before installing, servicing or operating Yaskawa products. FAQ content and illustrations are provided as technical advice to augment the information in manual, not supersede it. It is not possible to give detailed instructions for all types of installation or support activities. The information described in the FAQs are subject to change without notice to improve the product or FAQ. Yaskawa assumes no responsibility for errors or omissions or damages resulting from the use of the information contained in any FAQ. All warnings, cautions and product instruction for product use must be followed. Installation, operation and maintenance should be carried out by qualified personnel. Failure to observe these and other precautions highlighted in the product manuals will expose the user to high voltages resulting in, serious injury or death. Qualified personnel are defined as individuals who are familiar with the installation, starting, operation and maintenance of Yaskawa products of the type described and have proper qualifications to perform the work.
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