MS-2000 controllers with firmware Version 8.0 and later may use the procedures outlined below to obtain optimal performance of the MS-2000 stage and/or Z-drive. Users with earlier firmware versions, or older hardware, should contact ASI for further information regarding upgrade options, or other service issues.

The MS-2000 controllers with Version 8.0+ firmware are highly configurable. Settings for the stage running speed, ramp up/down time, error thresholds, servo parameters, driver alignment settings, etc., may all be changed and saved in non-volatile memory. At ASI, we choose an optimal set of parameters suitable for most users before the stage leaves the factory, but our configuration may not necessarily the best for your application, or it is possible that conditions have changed and realignment is in order. Complete optimization consists of four parts. 1) Aligning the drive card to the stage and/or Z-drive motors, 2) Selecting speed and configuration parameters that you wish to use for the controller, 3) Saving the new settings to non-volatile memory so they are available upon power-up, and 4) Testing the new settings for suitability.

Proper calibration of the analog driver is required to get smooth, very slow speed operation of the stage with the digital closed-loop control. Proper drive card alignment also makes the joystick operation as smooth as possible under high magnification.

The purpose of aligning the analog feed back for the drivers is to reduce the dead zone near zero speed when low voltage is applied to the motor. The chart below shows typical a typical transfer function for various feed back adjustment values.

Click to Enlarge

As you can see, without any feedback, (blue diamonds, AA=1 case) there is a substantial region where supplying a commanded speed to the motor results in no motion whatsoever. The digital loop can somewhat overcome this nonlinearity, and still provide accurate positioning, but the dead zone will give rise to “wind-up” and overshoot in a digital loop that is well-tuned for the constant slope section of the transfer curve. Increasing the feedback (larger AA number) reduces the size of the dead region and significantly “linearizes” the transfer function near zero. Not surprisingly, problems associated with poor drive card alignment show up when the motors need to move slowly, i.e. when completing a move.

All of the steps below require a connection to a computer terminal program. Refer to RS-232 Communication

The following example will show alignment of the X axis. Alignment of the Y axis and Z drive channels is analogous.

Before we change anything we need to know the current feedback parameter setting. To find out, issue the command:

User’s typing shown in this typeface
MS-2000 responses are shown in red

AA X?     
X= 82    
:A

If you think the feedback is a little low (stage sluggish on landing) you can just increase the present value by a few counts using:

AA X=85
:A

Always issue the auto-zero command after changing the AA value to re-balance the output amplifiers.

AZ X
Zero C:0                  
C C:0 H:0 L:101
D C:1 H:0 L:109
Bracket H:125 L:109
E C:1 H:125 L:109         
E C:1 H:117 L:109         
Zerod at: 110

The procedure re-balances the amplifier now with the new Feedback value.

If you don’t get good results the first time, you can try higher or lower feedback values and re-test the motion.

Too much feedback can result in unstable jerky motion, the motor may tend to buzz, and the AZ command will often produce inconsistent results when repeated several times. Reduce the AA value is you see these symptoms. Too little feedback can result in no motion. With too little feedback, the motors will be trying to get to target but will be unable to do so, resulting in the status letter M (Motor) or B (Busy) on the LCD screen staying on for longer periods of time after a move.

The alignment routine contains a built-in self-test that generates the data used in the chart above. To start the test use the command:

AA X=500    
:A

The number 500 triggers this test.

The data from this test are saved in the controller and must be dumped to the screen for you to evaluate. To do this, type the dump command:

DU 
idmp = 52
pos  err   spd
   -25        0        0
   -31        0        0
   -38        0        0
   -47        0        0
   -58        0        0
   -72        0        0
   -90        0        0
  -112        0        0
  -140        0        0
  -175        0        0
  -218       -3        0
  -272      -11        0
  -340      -32        0
  -425      -46        0
  -531     -290        0
  -663     -488        0
  -828     -686        0
 -1035     -990        0
 -1293    -1326        0
 -1616    -1689        0
 -2020    -2172        0
 -2525    -2792        0
 -3156    -3520        0
 -3945    -4430        0
 -4931    -5673        0
 -6163    -7114        0
    25        0        0
    31        0        0
    38        0        0
    47        0        0
    58        0        0
    72        0        0
    90        0        0
   112        0        0
   140       11        0
   175        1        0
   218       21        0
   272       46        0
   340       77        0
   425      156        0
   531      284        0
   663      431        0
   828      578        0
  1035      932        0
  1293     1254        0
  1616     1636        0
  2020     2063        0
  2525     2723        0
  3156     3438        0
  3945     4353        0
  4931     5560        0
  6163     6983        0

The numbers show the results for moves in both directions. The first number in each triplet it the speed ‘drive’ value, the independent variable. The second number, the relative speed the stage is moving. You want to be sure that there is motion at the low drive levels (below +/-500), and that the speed is at least quasi-monotonic. The example shows a typical, well-adjusted stage axis.

If you see very slow, usually zero, movement for drive values < 500, you could increase the AA value.

The discrete nature of encoders and timing cycles place a few limits on acceptable speed and up/down ramp settings. We do not try to go slower than one encoder count per servo-cycle, so this imposes certain discrete values for minimum slow speeds. It also makes no sense for a ramp to have a change in speed of less than one encoder count per servo-cycle. These constraints are built into the range-checking that is performed whenever either the speed (S) or the ramp time (AC) setting is changed. For instance, if we want to move at about 100 microns/second, we issue the command:

S X=.1 Y=.1

Then, ask for those values to be read back:

S X? Y?
: X=0.088110 Y=0.088110 A

We see that the controller has adjusted our choices somewhat. You can quickly see many of the controller parameter settings by issuing the info command:

I X
Axis Name      : X                  Error Status   :
Input Device   : JS_X     [J]       Motor Signal   :      128
Max Lim        :  110.947 [SU]      Min Lim        : -109.053 [SL]
Ramp Time      :       36 (ms)[AC]  Ramp Steps     :        6
Run Speed      :  0.08811 (mm/s)[S] vmax_enc       :        6
dv_enc         :        1           enc_bl_crossovr:       55
Drift Error    : 0.000500 (mm)[E]   enc_drift_err  :        5
Finish Error   : 0.000097 (mm)[PC]  enc_finish_err :        1
Backlash       : 0.040000 (mm)[B]   enc_backlash   :      453
Kp             :       20 [KP]      Ki             :        1 [KI]
Kv             :       25 [KV]
Axis Enable    :        1 [MC]      Motor Enable   :        0
CMD_stat       :  NO_MOVE           Move_stat      :   FINISH
Current pos    :  0.00000           enc position   :  8388608
Target pos     :  0.00000           enc target     :  8388608
enc pos error  :       -1           EEsum          :       -5
Lst Settle Time:       54 (ms)      Ave Settle Time:       84 (ms)

The parameters that are user-changeable are indicated by the presence of their command shortcut in the listing above. For instance, if you wished to reduce the Drift Error parameter you could issue the command:

E X=.0002 Y=.0002

Subsequent info requests would show the change to the Drift Error as well as to its corresponding parameter enc_drift_err, which is the Drift Error expressed in units of encoder counts.

Setting the appropriate error tolerances can have a dramatic effect on stage performance. Landing “on the count” is often possible, but will occasionally cause the controller to hang because mechanical “stiction” just doesn’t allow the motor to move such small amounts. Hence, we usually ship controllers with the Finish Error set to a distance equal to one encoder count. Once the stage moves to within the Finish Error distance of the target, it declares the move finished, clears the busy flag (the B on the LCD display), and enters the NO_MOVE state. Should the stage subsequently drift away from the target more than an amount specified by the Drift Error, the controller will re-energize the motors and bring the stage back to within the Finish Error distance once again.

In some instances, final positioning is very important so you may wish to reduce the Drift Error to one or two encoder counts. Doing so will cause the controller to attempt to reposition the stage more frequently. If you don’t care so much about accuracy, but are very concerned about the time it takes for the stage to settle to the target, you could increase the Finish Error parameter. You should notice a substantial decrease in the settling time.

On stages without linear encoders, we recommend using the built-in anti-backlash routine to improve the repeatability of the stage movement. The Backlash parameter specifies an offset distance from the final target position. A commanded move first moves to a target position offset by the Backlash parameter, and then subsequently moves on to the target. Although this helps to improve the repeatability of commanded moves, for some applications, it will cause more problems than it cures. To turn the Backlash off, just set its value to zero. (B X=0)

Changing the servo parameters should be done only if drive card alignment still doesn't get the performance that is required from the system. The servo parameters that you may want to adjust are the KP and KI settings that determine the response of the servo loop to trajectory errors during a move. The KP term sets the motor drive proportional to the error, and the KI term sets the motor drive proportional to the time integral of the error. Before changing anything always query the controller to find out the present values of the parameters.

KP X?
A: X=50
KI X?
A: X=5

In general, increasing the KP parameter increases the stiffness of the motion; the stage more closely follows the desired command trajectory. Increasing the KI parameter fixes persistent errors more quickly, especially finding target at the end of travel. Too much of either parameter can cause instability and overshoots. Here are a few rules of thumb for setting these parameters.

• Start with KI X=0. Increase KP value until unstable motion is detected on long moves. Back off KP value until motion is smooth.
• Start with KI approximately 10% of the KP value obtained above. Make several short commanded moves and be sure the Busy clears quickly. If not, increase KI.

Once you have made changes, be sure to save the settings as discussed below.

Once you are happy with your settings, you can save them to flash EPROM so they will be used on subsequent start-ups. To save all of the current settings to Flash memory, use the SAVESET command

The MS-2000 controller has several built-in diagnostic capabilities that are useful for troubleshooting difficulties and for tuning the servo motion parameters. It is often useful to see how well the servo motion tracks the theoretical trajectory programmed by the controller for the move. The controller has a built-in buffer that can hold 200 move steps. For best results, restrict testing to a single axis at a time; otherwise, information from multiple axes will be interleaved in the dump buffer. Any motion from any axis will write information into the dump buffer until it is full. To begin a test, first reset the buffer with the command:

DU X [reset the dump buffer]

Then make a short move, e.g.:

M X=12345 [Moves about 1.2 mm] After the move is complete, you can dump the buffer to the screen:

DU
idmp = 121
pos  err   spd
 0 , 45 , 2250
 45 , 135 , 6310
 114 , 270 , 11348
 169 , 450 , 15840
 196 , 675 , 19218
 184 , 945 , 21034
 169 , 1215 , 20476
 118 , 1485 , 18464
 59 , 1755 , 16118
 18 , 2025 , 14482
 -1 , 2295 , 13722
 2 , 2565 , 13842
 22 , 2835 , 14646
 27 , 3105 , 14852
 50 , 3375 , 15784
 70 , 3645 , 16600
 74 , 3915 , 16778
 53 , 4185 , 15950
 45 , 4455 , 15640
 41 , 4725 , 15490
 39 , 4995 , 15418
 42 , 5265 , 15548
 42 , 5535 , 15558
 -5 , 5805 , 13678
 44 , 6075 , 15648
 61 , 6345 , 16342
 66 , 6615 , 16558
 59 , 6885 , 16292
 28 , 7155 , 15058
 27 , 7425 , 15024
 35 , 7695 , 15352
 47 , 7965 , 15842
 53 , 8235 , 16094
 41 , 8505 , 15624
 42 , 8775 , 15674
 44 , 9045 , 15764
 45 , 9315 , 15814
 40 , 9585 , 15624
 12 , 9855 , 14506
 22 , 10125 , 14910
 37 , 10395 , 15518
 52 , 10665 , 16130
 61 , 10935 , 16504
 57 , 11205 , 16358
 34 , 11475 , 15446
 29 , 11745 , 15252
 32 , 12015 , 15380
 38 , 12285 , 15628
 40 , 12555 , 15718
 38 , 12825 , 15646
 16 , 13095 , 14770
 23 , 13365 , 15054
 47 , 13635 , 16024
 -83 , 13790 , 10160
 -57 , 14015 , 8936
 -31 , 14195 , 7720
 -19 , 14330 , 5946
 -27 , 14420 , 3370
 -54 , 14433 , -4422
 -107 , 14343 , -8818
 -142 , 14208 , -12502
 -87 , 14190 , -8072
 -24 , 14145 , -3308
 -77 , 14010 , -3098
 11 , 14010 , 424
 78 , 14010 , 3122
 91 , 14010 , 3664
 71 , 14010 , 2880
 36 , 14010 , 1488
 7 , 14010 , 328
 -2 , 14010 , -32
 -9 , 14010 , -314
 -8 , 14010 , -276
 -5 , 14010 , -156
 -2 , 14010 , -36
 -2 , 14010 , -36
 0 , 14010 , 44    
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 3 , 14010 , 120
 0 , 14010 , 0
 -2 , 14010 , -80
 -2 , 14010 , -80
 -2 , 14010 , -80
 -2 , 14010 , -80
 -2 , 14010 , -80
 -2 , 14010 , -80
 -2 , 14010 , -80
 0 , 14010 , 0
 4 , 14010 , 160
 4 , 14010 , 160
 4 , 14010 , 160
 4 , 14010 , 160
 4 , 14010 , 160
 4 , 14010 , 160
 4 , 14010 , 160
 4 , 14010 , 160
 4 , 14010 , 160
 4 , 14010 , 160
 4 , 14010 , 160
 1 , 14010 , 40

All those numbers may not look particularly useful, but they have a lot of information about the system dynamics in them. Using HyperTerminal’s file capture capability, we can save the dumped information to a file and plot the results in a charting program like Microsoft’s Excel.

Click to Enlarge

The data shows the move trajectory, the error from desired path, as well as the drive signal given to the motor mover. For this move, there were six ramp-up and down points on the way to full velocity. Backlash of ~450 counts was enabled, the finish error was 1 encoder count, and the drift error was 2 encoder counts. During the time of maximum acceleration, there is also the most error. The short ramp (six cycles) contributes to the error (about 17 microns peak error from the theoretical path). Once the move is under way, the average error settles down to about 50 counts (4 microns). You can see the retrograde motion caused by the anti-backlash routine starting at about the 59th time step. There is a fair amount of overshoot and then the move settles in to find its final position. The move would clear the busy status at about step 77, but then it drifts outside the drift error range and cannot settle for another 45 time steps.

The user is advised that servo tuning can be a painstaking process, so only try new settings if you are unhappy with the default ones shipped from the factory.

The controller is designed to work with a wide variety of parameter settings, but there certainly could be conditions where the choice of parameters causes motion errors of one sort or another. To aid in diagnosing these problems, the MS-2000 controller has an error buffer of the last 256 error conditions it has encountered since last power up. The error codes can be accessed by the command:

DU Y [dump error codes] The buffer is cleared with:

DU X [clear buffers] Error codes are dumped to the screen with the last error code shown first. The table below lists the meanings of the error codes, as of the date of this publication.