The NSI optical subsystem provides measurements of
both the probe X and Y positionS, and the X, Y and Z position
errors, with a novel patent-pending design approach which minimizes
hardware costs and alignment concerns. The system uses an XY beam
monitor assembly (BMA) inserted into the laser beam path to derive
the scanner XY straightness errors. This configuration does not
require the addition of another laser head for the straightness
measurements, as is typical of most other systems. For customers
with existing near-field ranges using HP laser interferometer
equipment, only a re-arrangement of the optical components is
required to support the BMA. The independent Z-plane subsystem
uses a spinning plane laser to measure the probe's out-of-plane
Optical subsystem description
The Z-plane laser is also mounted at the -X,-Y corner
of the scanner, and sweeps out a vertical or horizontal plane,
depending on the scanner configuration. The Z-plane sensor is
mounted near the probe.
NSI optics assembly
Standard HP doppler receivers were used to derive the path length differences between the reference beams from the interferometers and the test beams to the externally mounted retroreflectors. The output of the receivers were converted to quadrature signals for X and Y and sent differentially back to the computer for processing in the DSP card.
The optics assembly is precision aligned at NSI on a granite bench, leaving only the laser and plate relative alignment to the scanner to be performed in the field. Figures 2 and 3 show the optics assembly and components.
The Z-plane measurement system is based on the use of a spinning plane laser mounted at one corner of the scanner, and a lateral sensor mounted near the probe. The plane laser sweeps out a plane against which the probe position errors are tracked. The laser is calibrated in software to correct for coning and runout errors. The Z-sensor is a custom NSI designed package which includes the sensor, gain ranging and detection electronics. The sensing package is 6" by 3" by 1" and weighs only 0.75 lbs, making it quite easy to mount at the probe. The sensor's linear operating range is about 0.2 inches. Figure 4 shows the sensor package.
The deformation table is used because the optical sensor measurements are subject to atmospheric noise, and require averaging over time to enhance the signal to noise ratio. The table size is selectable and is dependant on the straightness of the scanner. Low frequency warpage due to manufacturing tolerances or rail bowing can be handled quite well with small grid sizes -- a 7 x 7 size is typical.
The real-time sensor displays, and ability to plot the sensor data versus X or Y motion are invaluable tools for scanner rail alignment, as well as aiding the system optical alignment process. During RF data acquisition, the NSI software will scan the Y-axis, while performing cross-axis corrections of the probe position by stepping the X-axis and Z-axis stepper motors, according to the interpolated values derived from the deformation table.
In one large near-field design using a 19m by 8m near-field scanner, the scanner errors caused sufficient misalignment in the return laser beam from the probe Y-axis retroreflector that it exceeded the capture range of the HP laser receivers. This problem was solved by adding a small dual-axis stepper control system to the Y-axis retroreflector and dynamically adjusting its position while scanning in X and Y, to keep the return beam from the retro-reflector lined up on the receiver aperture.
Figure 5 shows a series of measurements of the probe retroreflector Z motion, as Z vs. Y cuts for 20 different X positions spaced 1 meter apart on this 19 meter scanner. The error was caused by a combination of scanner tower tilt and scanner-induced twist in the laser optics plate. These errors were significantly beyond the ability of the standard laser receiver to operate. With use of the real-time software tracking and the small retro adjuster, the Y-axis retroreflector was tracked to eliminate the errors and keep the laser system aligned. Figure 6 shows the results with the NSI optics system enabled.
Other error components, such as the scanner orthogonality and X rail leveling errors are handled in a similar manner. The residual error of the system after the optical correction is on the order of 2 mils (0.05 mm) rms.
This paper has described a unique system for measuring and correcting for scanner and probe positioning errors in large scanners. NSI has implemented 4 of these systems on large near-field scanners. Significant improvements in scanner accuracy can be achieved through optical means. Real-time correction of scanner errors are accomplished through software control, allowing the nearfield probe to travel in an essentially perfect plane. The combination of probe position correction and NSI's Motion Tracking Interferometer2 (MTI) which corrects for AUT motion and RF cable phase change during a scan due to thermal variations can greatly enhance the accuracy of large near-field test facilities.
1. Implementation of a 22' x 22' Planar Near-field Measurement System for Satellite Antenna Measurements, by Greg Hindman, Greg Masters, Antenna Measurement Techniques Association (AMTA) symposium, October 1993.
2. A 550 Ghz Near-field Antenna Measurement System for the NASA Submillimeter Wave Astronomy Satellite by Dan Slater, Antenna Measurement Techniques Association (AMTA) symposium, October 1994.
Figure 5 - Scanner Z erroe before correction. This shows the large errors detected by the optical system which are a combination of scaner errors, and scanner-induced twist in the optics assembly. The magnitude exceeds the alignment tolerance of the laser receiver.
Figure 6 - Residual error after crrection. The BMA sensor readings have been used to correct the retroreflector position for each X,Y probe position, keeping the return beam from the retroreflector well aligned.