Testing military laser tag assemblies

By Paul Hiller

While laser tag is a game to most of us, realistic combat training is vital to our military. Individual survival and combat effectiveness depend to a large degree on the level of training our soldiers receive. To be useful, this training must be realistic and accurately simulate actual combat situations. The weapons used, while nonlethal, are required to have the same look, feel, range of fire, and accuracy as the real weapon. Of necessity it must be small, lightweight, and nonintrusive. Otherwise the simulation is unrealistic and the training is flawed. This article explains how this is accomplished with the help of a PXI based test system.

Vest assembly
The latest military training systems do in fact provide an excellent simulation of combat. They utilize a vest and associated headband (attached to the helmet) with an array of optical detectors, and a corresponding rifle-mounted laser transmitter. These are then integrated together using a wireless RF link. When the soldier fires the weapon (using blanks), the system accurately simulates a kill or near-miss using optical pulses, calibrated for the actual weapon range.

Although manufactured in high volume, each assembly must be carefully tested and aligned to assure consistent performance in the field. Vest measurements include:

• Optical signal decoding and sensitivity
• Wireless RF link communication
• Battery current draw
• User control and display functionality
• Speaker output
• Shield/connector contact integrity

Of these, the RF and optical parameters are most critical for system performance.

PXI based test system
The test system used for alignment and final system test of this assembly is PXI based, providing a combination of low recurring cost and high throughput. An eight-slot chassis (NI PXI-1042) with MXI interface is utilized. Measurements are made primarily with the NI PXI-4070, which is a combination DMM and high speed digitizer (10-bit at 1.8 MSps). The latter allows for measurement of transient signals from the various station sensors. An IEEE 1394 interface in the PXI chassis is used to control the camera optics for display verification and character recognition. The full test system is shown in Figure 1.

Figure 1: IEEE 1394 interface in the PXI chassis is used to control the camera optics for display verification and character recognition

The PXI instrumentation is augmented by GPIB controlled instruments for optical signal generation, switching, and power measurement. A pulse generator with integrated laser optical output generates the initial optical signal. The output wavelength of the laser is fixed, but the amplitude is modulated to supply the coding required by the vest detectors. A programmable optical switch and attenuator adjusts the optical power and routes the output signal to each UUT detector in turn. A precision optical detector from Newport provides closed loop verification of the optical signal.

Symtx designed a special interface PWB to provide special conditioning circuitry not easily available with Commercial Off-The-Shelf (COTS) PXI modules. For example, generation of the optical pulse modulation and coding is handled using an FPGA on this PWB, with control managed via DIO from the PXI chassis. The interface PWB is shown in Figure 2.

Figure 2: PWB, with control managed via DIO from the PXI chassis.

Equally important, the interface PWB allows for the use of COTS cables within the test station. The PXI modules are each connected to this interface PWB using the cables normally supplied. Signal routing to interconnect these modules is then handled on this PWB, including signal routing to the UUT in each fixture. The result is lower documentation costs, as well as simplified spares and maintenance.

RF measurements
The UUT assemblies are connected and controlled via a local wireless network. This RF link allows the separate devices to operate via an integrated unit, while rejecting similar data from other transmitters in the immediate vicinity (the training devices used by other nearby soldiers). It utilizes an omni-directional antenna with receiver discrimination to filter out this extraneous data and noise.

To insure error-free operation, it is important to verify not only functional operation of this link but also parameters such as receiver sensitivity and transmitter power. A custom RF SIM PWB mounted in each fixture generates and measures these RF signals. This board (shown in Figure 3) is responsible for:

• RF signal generation and modulation
• RF signal reception and decoding
• Input sensitivity and output power level control
• RF power level measurement (calibration)

Figure 3: Custom RF SIM PWB mounted in each fixture generates and measures these RF signals

Data transmission occurs across one of the RS-232 interfaces in the eight-channel PXI module. Commands received over this interface are dynamically decoded on the RF SIM, with the appropriate signal then transmitted across the RF link. In parallel with this operation, input and/or output path attenuation is controlled via DIO lines from a separate PXI module. This attenuation is used to set the desired output power and receiver sensitivity.

For calibration purposes, a wide dynamic range RF detector is included on the RF SIM. This detector samples the output power level and supplies a proportional reference signal that is then measured by the DMM/digitizer. The detector is highly linear over a range of more than 50 dB.

One of the technical challenges to the RF SIM board was the need for input sensitivity and output power level control over a range of more than 90 dB. This in turn requires high isolation for all aspects of the board layout and mounting. Otherwise, leakage signals will prevent the full dynamic range from being realized. The board uses 3 attenuation sections, each providing nominal attenuation of up to 31 dB in 1 dB steps. To prevent RF leakage, each section is separately shielded as shown. This is required to prevent high-level transmitter signals from coupling around the attenuation sections to the antenna.

In a similar manner, the RF SIM itself is mounted in an EMI enclosure (with an external antenna). For smaller dynamic ranges, this is not a requirement, given proper ground plane design. However, without this shielding, stray coupling from the transmitter and receiver limited the available range to about 60 - 70 dB, which was not sufficient to fully test these assemblies.

Finally, it should be noted that the wide dynamic range also required the use of EMI shielding for the overall fixture. This is needed due to the potential for interference between multiple fixtures in physical proximity on the factory floor. This is not an issue for normal field operation of these assemblies due to their operational mode and the use of unique addressing for each device. However, during manufacturing test, addresses have not yet been assigned. Further, the testing performed requires different operating modes that allow for potential interference between devices in nearby fixtures.

Optical measurements
During normal operation, the laser transmitter sends a series of coded optical pulses toward the intended target (vest). These are decoded and interpreted as a kill or near-miss based on a variety of factors, including the amplitude of the optical signal received. Thus, it is critical that both the transmitted power of the laser and optical sensitivity of the vest be carefully adjusted. This leads to the requirement for accurate calibration of the optical (radiated) power level at each of the vest sensors.

The test station utilizes a GPIB controlled reference laser to generate the optical signals. This laser is modulated using a custom FPGA, controlled via DIO lines from the PXI chassis, to generate the required optical codes. Laser amplitude is set using the optical attenuator, then switched to the selected vest sensor input. The same optical signal can also be switched to an auxiliary port, which contains a calibrated optical power sensor. This provides for automatic calibration of the optical power level.

During production, a vest is mounted such that each sensor is located and clamped in its own optical fixture. This fixture aligns the sensor with the test system optical source, assuring tight coupling between the transmitted and received optical power levels. Note that radiating the optical signal directly from an optical fiber or collimating lens does not suffice. Because the detector used in the vest is large and designed for far-field optical signals, accurate calibration of the vest requires that the test station inject similar far-field signals during optical alignment. A lens system is therefore provided that spreads the beam to the dimensions of the detector and collimates the beam to simulate a far-field signal.

Calibration of the resulting optical power at each sensor requires a corresponding lens arrangement to capture this wide output beam and focus on an optical detector. With all radiated power collected at the detector, it becomes the reference used to correct for:

• Laser drift
• Cable and connector losses
• Optical attenuator non-linearity
• Losses in the output fixture (lens system and dust accumulation)

Typical overall system optical accuracy is held to better than 0.05 dB. This is obtained by first measuring the loss between the reference optical detector and each sensor fixture using an optical calibrator moved manually to each fixture during the calibration process. The reference detector at the auxiliary port of the optical switch is then used to calibrate the laser power level and attenuator settings. During production testing and alignment of the vest, the power level at each sensor can be accurately set by adding the loss from the auxiliary port to the selected sensor input (from the calibration table) to the desired optical power level, then using this value to look up the required attenuator setting in the power level calibration table. This provides a very fast and accurate means to set the power level.

Calibration of the attenuator and laser settings is performed on a daily basis to assure that any drift with time and temperature is corrected. Calibration of the loss to each fixture is primarily a function of dust accumulation on the fixture lens and is therefore run periodically based on particle levels in the factory. An optical calibration system is shown in Figure 4.

Figure 4: Calibration of the loss to each fixture is primarily a function of dust accumulation on the fixture lens

Summary
Modern combat training systems pose numerous production test challenges, particularly in optical and wireless RF test. However, these challenges are not insurmountable. With careful system design, a PXI based test station provides a suitable solution.

Paul Hiller is Chief Technical Officer for Symtx, a leading supplier of custom functional test systems. He holds both a Bachelor’s of Business Administration and an MSEE with highest honors from the University of Wyoming. Hobbies include golf, weight lifting, spending time with his four sons and two grandsons, and teaching college classes at his church.

For more information, contact Symtx directly:

Symtx, Inc.
Tel: 512-328-7799
E-mail: info@symtx.com
Website: www.symtx.com