Raster and serpentine scanning has been made simple and flexible with some special firmware and hardware options added to ASI's standard controller. Raster scanning is often used in conjunction with other equipment which collects data during the scan, the goal being data generation that can be mapped accurately in the two spatial scan dimensions. To facilitate this, the ASI controller with SCAN_MODULE firmware provides one or two TTL signals that are clocked to positional references. These signals are wired from the axis chosen by the customer when the controller is manufactured.

The first signal, SYNC, provides a “start-of-line” reference signal that occurs as the stage scans across a predetermined positioned for every raster-scanned line. Users can use the SYNC line and constant stage speed along with uniform data sampling to achieve imaging synchronization. The controller can keep the stage speed very constant with no accumulated error (on target to within a few encoder counts throughout the scan), so accompanied with a uniform sampling interval, accurate pixel rendition is possible without directly triggering the acquisition from the controller.

Two firmware options are available that involve how the TTL_IN interrupt is handled. The basic SCAN_MODULE operation offers only the SYNC output the controller. It is supplied with the IN0_INT firmware module which allows all of the normal TTL input functions to be available (described by the X-parameter options for the TTL command). For TG-1000 “Tiger” controllers the SYNC signal is available on the backplane and can be exposed easily using the Programmable Logic card or TTL card.

For users that want the controller to supply a pixel clock we supply the ENC_INT firmware module, and normal TTL input functions are not available. Significant parts of this document describe functionality of the ENC_INT module and do not apply to the SYNC modality. ENC_INT firmware is only available for MS-2000/RM-2000 controllers. Controllers running firmware with the ENC_INT interrupt option apply one channel of the stage encoder to the microprocessor's interrupt input. The controller “counts down” the encoder pulses and sends a output pulse on the TTL_OUT BNC connector after the programmable number of N encoder counts have gone by. In this way, the TTL_OUT signal can effectively be the pixel clock of the data acquisition system, where the user can program the distance between pixels.

To check if your controller has the required firmware modules you can use the command Command:BUILD (BU) to query the controller with BU X?. There are two modules required to enable scan functionality:

1. SCAN_MODULE
2. IN0_INT or ENC_INT

Use the commands Command:SCAN (SN), Command:SCANR (NR), and Command:SCANV (NV) to configure the scan parameters such as start and stop positions in both the scan and other axis, which axis has which role, the number of lines, etc. See the programming section for a full description of these commands, but this is a “quick start” guide.

NR X and NR Y sets up the raster scan start and stop positions in units of millimeters. The retrace speed can be specified using NR R in some firmware builds only. NR Z and NR F set up the divider and number of pixels and only apply to the ENC_INT module.

NV X and NV Y set up the start and stop positions in the “vertical” (non-scan) direction in units of millimeters. NV Z specifies many scans to perform (they will be evenly spaced in the “vertical” axis). NV F configures how much overshoot is applied along the scan axis to ensure that the stage is up to speed before it passes the scan start position where a SYNC pulse is generated.

SN without arguments is used to initiate the scan. SN F selects between raster and serpentine patterns, and which axis does which motion can be specified using SN Y and SN Z commands.

The user can select either Raster or Serpentine scanning modes using the SCAN F=mode command. During a Raster scan, the active scan begins at the same edge for each line, and is followed by a high speed retrace movement at the end of the scan line. For a Serpentine scan, the scan proceeds first in one direction, then in the opposite direction, advancing vertically one line each time the direction changes (there is no retrace movement). The “start-of-line” reference position is moved from one side of the scan area to the other depending upon the direction of the scan.

Raster scanning has the advantage that system backlash will be negligible, since the motion is always in the same direction. Serpentine scanning has the advantage of faster coverage (since there is no retrace operation) when very high accuracy is not required.

The encoder sync pulse is obtained directly from the encoder counting chip inside the controller. This encoder chip can be loaded with a “compare” position, so that whenever the current encoder count is equal to the “compare” count, a flag pin on the chip becomes active. We add additional circuitry to latch the sync pulse during the active part of the scan.

Pulses for the pixel clock are derived from a processed signal coming from the controller’s microprocessor (DIVIDE-by-N) on the normal TTL_OUT BNC connector. The pulses from the encoder are sent to an interrupt line on the controller’s microprocessor. The interrupt causes a programmable counter to count down. When the counter reaches zero, the microprocessor sends a pulse to the TTL_OUT connector. These pulses are only enabled during the active scan line.

Figure 1: Timing Example

Either the X or Y-axis can be used as the fast raster axis for scanning. Jumpers on the main controller card are used to select the appropriate wiring. The firmware defaults with the X-axis as the fast axis. When configured at the factory, the jumpers are set for a fast X-axis.

The output signals are found on header SV1. SV1 Pin 2 is the Encoder Divide by N output and SV1 Pin 7 is the Line Sync output. These pins should be connected to BNC connectors on the rear of the controller. (Controllers not specifically set up for the SCAN firmware will usually have SV1 Pin 2 connected to the TTL OUT BNC and SV1 Pin1 connected to the TTL IN BNC. In that case, switch the wire from Pin1 of the connector over to Pin 7 so that the Line Sync will appear on the TTL IN BNC.)

SCAN firmware versions that have the IN0_INT module rather than the ENC_INT module should leave JP2 unconnected since encoder pulse counting is not enabled in these versions. The ENC_INT module provides the interrupt service handling so the encoder pulses can be counted. The internal interrupt service must be enabled using the TTL command, TTL X=1.

JP4: top two are shorted for X-axis as fast axis, bottom two are shorted for Y-axis as fast axis. The SYNC signal is available on the backplane and can be exposed easily using the Programmable Logic card or TTL card.

To perform a scan, you must first define the scan region, the pixel clock resolution, and the number of scan lines. The serial commands SCANR, SCANV, and SCAN let the user define the scan region, the scan mode, and initiate the scan process. See an earlier section of this document for a brief description of these commands.

Let’s go through an example where we wish to scan a 1 mm × 1 mm region at a resolution of about 0.5 μm on an XY stage. We will consider the X axis to be the ‘horizontal’ direction, and Y to be the ‘vertical’ direction. (Although any axis is eligible to be ‘horizontal’, only one will have been wired at the factory for encoder output signals – see the SCAN command’s parameters for alternative configurations.) For a standard 6.35 mm pitch lead-screw stage, the (rotary) encoder resolution is 45396 counts/mm or 0.022 μm/count. However, this includes all of the quadrature edges of the two encoder channels and the counting circuit is only looking at the positive edges of one encoder signal, so the input clock pulse to the controller would be at 0.0881 μm intervals. Hence the output clock pulse can only be set to be an integer multiple of 0.0881 μm which is 4 encoder counts (for 4 TPI leadscrew with rotary encoders; if you are not sure of the encoder resolution of the scanned axis, issue the command INFO X, where X is the axis in question, to query the controller about the parameters that are used.). This 0.0881 μm distance (or equivalent [4000/C]) is multiplied by the SCANR Z setting to get the distance between output clock pulses. The maximum output clock would is with SCANR Z set to 8, or just over 176 nm.

If we generate a clock pulse every 24 encoder counts (6 full periods of a single encoder signal), then our pixel clock resolution will be 0.022 × 24 = 0.528 μm. To set up the ‘horizontal’ raster, we issue the command:

SCANR X=0.0 Y=1.0 Z=24 (X= start, Y= stop, Z=encoder_divide)

The command above will generate a scan line with 1891 pixels, determined by multiplying the encoder resolution by the length of the scan and dividing by the encoder_divide parameter. If you wish to specify exactly the number of pixels you want to have in a scan row, you may issue the command using the F = # of pixels, instead of Y = stop_positon.

For the ‘vertical’ direction, if we want square pixels, then we will need 1891 scan lines as well. To set up the ‘vertical’ scan, we issue the command:

SCANV X=0.0 Y=1.0 Z=1891 (X=start, Y=stop, Z=number_of_lines)

In both commands, parameters “start” and “stop” are measured in millimeters from the current origin (“Home” position), and negative values are allowed.

Next, we need to determine the correct scan speed based upon the processing time for each pixel. If we assume a 1ms time for each pixel, then we would want to scan at a rate of 0.528 μm/ms or 0.528 mm/s. To set this up for the X-axis we issue the command:

SPEED X=0.528

We can specify that the scan mode will be a raster scan using the command:

SCAN F=0

Finally, we need to turn on the encoder interrupt with:

TTL X=1

Once all of the scan parameters are set up the way we want them, it is possible to save them to non-volatile memory so that we do not need to re-enter them each time we power up.

The command to save settings is: SS Z
When ready to initiate the scan, issue the command: SCAN

to start the process. Alternatively, you can use the @ button on the controller to start and stop the scan. To start a scan, hold the @ button down for more than 1 second. When you release the button the scan will start. To abort a scan in process, momentarily depress the @ button.

During set-up for a scan, the number of horizontal pulses (pixels) is calculated. During the scan, pulses are counted until the requisite number has gone by. Further interrupts are disabled and the current encoder position is read in. If the current position is not consistent with the expected position based upon the number of pulses and the encoder counts between pulses, an error code is placed in the error log.

The internal data size for certain variables limits the acceptable range some parameters. The encoder-divide parameter and the total number of pixels per scan line must be less than 32768.

With the MS2000WK controller, internal jumpers JP1 and JP2 are used to select either the X-axis (jumper positions 1-2) or the Y-axis (jumper positions 2-3) as the fast scanned axis. Controllers are shipped with the X-axis as the default fast raster axis. Please see the SCAN command to change the default axes for the scanning algorithms. Only the X (default) or Y axis may be used when generating the hardware encoder synchronization TTL pulses. The firmware controlled scan patterns can be used with any pair of axes, however internal errors (code 85 & 86) will be generated if the proper internal signals are not present or jumped correctly.