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stage_accuracy_settling_time_for_asi_stage [2016/05/24 19:34]
vik
stage_accuracy_settling_time_for_asi_stage [2016/10/04 22:13] (current)
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 What does this mean in terms of stage speed and settling time?  We tried to answer that question by running a test where we recorded the time to make many short moves using different accuracy and acceleration parameters. ​ We used Image Pro software, which queried the stage for its busy status about every 12 milliseconds. ​ As soon as the busy would clear, the program would issue the next move command. ​ We measured the time to make a whole series of moves, so besides the actual move and settling time there is also the serial communications overhead and the average polling time included in the measurement. What does this mean in terms of stage speed and settling time?  We tried to answer that question by running a test where we recorded the time to make many short moves using different accuracy and acceleration parameters. ​ We used Image Pro software, which queried the stage for its busy status about every 12 milliseconds. ​ As soon as the busy would clear, the program would issue the next move command. ​ We measured the time to make a whole series of moves, so besides the actual move and settling time there is also the serial communications overhead and the average polling time included in the measurement.
  
-[{{ :​documentation:​stage_acc_settling_time_1.jpg?​direct&​400 |Click to Enlarge}}]+[{{ stage_acc_settling_time_1.jpg?​direct&​400 |Click to Enlarge}}]
  
 The chart above shows the results of this test on a typical ASI stage equipped with linear encoders. ​ The encoder resolution was 20nm, so the best accuracy tested required that the stage settle to within one encoder count. ​ Top speed for this stage is about 6.5mm/s but for short moves the stage never gets up to full speed. ​ However, since acceleration is an adjustable parameter, we show move times for three different rates of acceleration. ​ Note that lower values of acceleration reduce the time per move when high accuracy moves are desired. ​ The jerky motion takes longer to settle, but if you don’t care as much about accuracy, then the higher acceleration will get the stage to target somewhat sooner. The chart above shows the results of this test on a typical ASI stage equipped with linear encoders. ​ The encoder resolution was 20nm, so the best accuracy tested required that the stage settle to within one encoder count. ​ Top speed for this stage is about 6.5mm/s but for short moves the stage never gets up to full speed. ​ However, since acceleration is an adjustable parameter, we show move times for three different rates of acceleration. ​ Note that lower values of acceleration reduce the time per move when high accuracy moves are desired. ​ The jerky motion takes longer to settle, but if you don’t care as much about accuracy, then the higher acceleration will get the stage to target somewhat sooner.
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 A related issue to settling time is how to best choose the lead screw pitch for quick moves. ​ Again, accuracy becomes an issue, since doubling the speed of the stage by using a fast pitch lead screw will in general double the inaccuracy. ​ The rate of acceleration is also a parameter that needs consideration. ​ The motors used in ASI stages are brushed DC motors that will give years of trouble free use if not abused. ​ Multiple rapid acceleration and de-acceleration are not good for the motor and can lead to failures. ​ A ramp time of 50ms or more is recommended to ensure long motor life.  The chart below shows the theoretical time it should take to make sub-millimeter moves for various lead screw and ramp time combinations. ​ The top speed of our standard 6.35mm pitch lead screw stage is about 6.5mm/​s. ​ ASI can also supply stages with 12.7mm pitch lead screws with 13mm/s top speed for applications where the added speed is desirable. A related issue to settling time is how to best choose the lead screw pitch for quick moves. ​ Again, accuracy becomes an issue, since doubling the speed of the stage by using a fast pitch lead screw will in general double the inaccuracy. ​ The rate of acceleration is also a parameter that needs consideration. ​ The motors used in ASI stages are brushed DC motors that will give years of trouble free use if not abused. ​ Multiple rapid acceleration and de-acceleration are not good for the motor and can lead to failures. ​ A ramp time of 50ms or more is recommended to ensure long motor life.  The chart below shows the theoretical time it should take to make sub-millimeter moves for various lead screw and ramp time combinations. ​ The top speed of our standard 6.35mm pitch lead screw stage is about 6.5mm/​s. ​ ASI can also supply stages with 12.7mm pitch lead screws with 13mm/s top speed for applications where the added speed is desirable.
  
-[{{ :​documentation:​stage_acc_settling_time_2.jpg?​direct&​400 |Click to Enlarge}}]+[{{ stage_acc_settling_time_2.jpg?​direct&​400 |Click to Enlarge}}]
  
-[{{ :​documentation:​stage_acc_settling_time_3.jpg?​direct&​400 |Click to Enlarge}}]+[{{ stage_acc_settling_time_3.jpg?​direct&​400 |Click to Enlarge}}]
   
 The chart above shows the measured average move time for different length moves assuming the controller doesn’t have to correct for the error on the move.  This usually means allowing errors of one to two microns. ​ Using various ramp times from 100ms to 25ms and top stage speed of 6.4mm/s, the time to complete moves is shown as a function of move length. ​ The calculated time for ramp up and down at a given acceleration is also shown. ​ The measured data closely matches the expected value if we include about 50ms of communications overhead that we observed previously. The chart above shows the measured average move time for different length moves assuming the controller doesn’t have to correct for the error on the move.  This usually means allowing errors of one to two microns. ​ Using various ramp times from 100ms to 25ms and top stage speed of 6.4mm/s, the time to complete moves is shown as a function of move length. ​ The calculated time for ramp up and down at a given acceleration is also shown. ​ The measured data closely matches the expected value if we include about 50ms of communications overhead that we observed previously.
stage_accuracy_settling_time_for_asi_stage.txt · Last modified: 2016/10/04 22:13 (external edit)