Engineers now can use VXI to characterize and evaluate system performance in critical areas such as avionics, radar positioning and fire-control systems on tanks. The ability to measure output signals of synchros and resolvers, which are used for position feedback in such applications, is essential for accurate test and evaluation.
To support the growing demand for VXI-based test implementations, synchro/resolver measurement and simulation instrumentation now is available in VXI. The register-based VXI cards offer accuracies extending to 18 arc seconds. They also provide dynamic capability by simulating a rotating synchro/resolver.
A synchro or resolver functions as an electro-mechanical transducer which is essentially a rotary transformer (Figure 1). A synchro consists of a rotor input (R1 and R2) and three stator windings (S1, S2 and S3) which are wound physically 120(degree) apart. Synchros are commonly used on shipboard applications to transmit position information from one area of the ship to another. When synchros are used on aircraft, the S2 output is usually grounded, eliminating the need to run the third output wire.
Resolvers consist of a rotor input (R1 and R2) and two stator windings, wound physically 90(degree) apart as shown in Figure 1. A reference oscillator is connected to the rotor to provide excitation. The outputs of each stator winding are sinusoidal waveforms whose magnitudes are proportional to the sine and cosine of the angle of rotation of the shaft.
Synchros and resolvers can operate with input voltages from 0.5 to 115 Vrms over frequencies ranging from 60 Hz to 100 kHz. Some standard rotor voltages are 26 Vrms, producing 11.8 V line-to-line, or 115 Vrms, producing 90 V line-to-line.
The synchro/resolver output signals may be converted to digital. The converter input circuitry used for this purpose differ for synchro, resolver and direct (sine/cosine) signals. An electronic Scott or T transformation, which transforms 30 to 20 signals, is used for synchro inputs; a resolver conditioner for resolver inputs; and sine and cosine followers for direct inputs.
In the main converter section, input and output signals are compared and a resultant error signal is developed. This DC error is integrated, yielding a velocity voltage, which in turn drives a voltage-controlled oscillator (VCO). The VCO is an incremental integrator (constant voltage input to position rate output), which together with the velocity integrator forms a type II servo feedback loop. A lead frequency response is introduced to stabilize the loop, and another lag at higher frequency is introduced to reduce the gain and ripple at the carrier frequency and above.
A single C-size VXI card can both simulate and measure synchro/resolver output signals. The functions and operations of the VXI card, which may be register-based, can be controlled by writing commands to the control word A register bit map.
By writing a 16-bit control word, you will set up each section of the VXI card for the desired line-to-line voltage, synchro or resolver format and reference level. Although sharing some common circuitry, each section of the VXI card is independent of the other and can be programmed to operate in either the synchro or resolver mode at 11.8, 26 or 90 V line-to-line over the frequency range of 60 Hz to 5,000 Hz.
The measurement section allows selection of 20- or 16-bit binary resolution. In 16-bit mode, the unit can track signals at 1,040(degree)/sec with an accuracy of 36 arc seconds. In 20-bit mode the maximum tracking rate is 65(degree)/sec with an accuracy of 18 arc seconds.
The generator section has a resolution of 16 bits with an accuracy of ±25 arc sec at no load and ±36 arc sec with a 1.5 VA synchro or resolver load. The generator section also contains a dynamic rotation capability. This feature provides for a programmed constant angular velocity, either clockwise or counterclockwise, in two ranges (low and high).
Using the internal clock, the low range steps the converter in 0.0055(degree) increments from 0.1676(degree)/sec with a resolution of 0.1676(degree)/sec. In the high range, which is effectively the low range times 16, the converter steps in 0.88(degree) increments and covers the rotational rate of 2.682(degree)/sec to 10,983.64(degree)/sec (30.5 rps). The card can also be programmed to accept external rate and direction signals for user-programmable dynamic applications.
You can execute a self-test by programming the instrument to connect the measurement section input to the generator section output, programming both sections for the same synchro/resolver mode and line-to-line level, and selecting the internal 400-Hz test reference. You don’t need external signals are required to perform this wraparound self-test, and it can be used to verify that both sections are functioning properly.
The status register bit map, shown in Figure 2, facilitates monitoring both the signal measurement section and the signal generation section. SG BIT (Signal Generator Built-in Test) will detect any potential overload condition. SM BIT (Signal Measurement Built-in Test) will detect if the input is rotating too fast. SG LOR and SM LOR will indicate a logic zero if the reference is missing.
The block diagram (Figure 3) shows that the R/D converter module and the digital-to-resolver (D/R) converter module are the foundation of the VXI card, with transformer isolation on the reference and signals. The VXI backplane is interfaced through the P1 connector, and the signals are brought off the circuit card assembly via a 25-pin D-type connector mounted on the front of the card.
With VXI systems becoming more popular, many functions can be integrated into a test system, providing a cost-effective solution in today’s rigorous testing environment. A typical application for such a VXI system is testing LRUs (line replaceable units).
LRUs often perform specific functions aboard aircraft. They are replaced when a problem is suspected, then tested to verify functionality. Many LRUs contain synchros, resolvers or gyros, in addition to other circuitry that requires VXI-based source/measurement instrumentation. These components usually send information to the cockpit display. Their performance and accuracy may be verified with the signal measurement section of the VXI card.
Conversely, some LRUs, such as a radar subassembly aboard the E-2C, contain synchro-to-digital converters for monitoring position information. The signal generator section of the VXI card can be used to exercise the synchro-to-digital converter of the LRU, both statically and dynamically.
LRU test systems traditionally have been rack-and-stack equipment. But now, VXI is capable of providing a modular architecture for multivendor instrument modules to work seamlessly together in a test system. This gives instrumentation users the ability to select the best instrument functionality based on their needs, providing a flexible, lower-cost means for testing a multitude of functions.
About the Author
David Dayton is a Synchro Applications Engineer at ILC Data Device Corp. He joined the Synchro Applications Department in 1991. He has more than 10 years experience testing and evaluating synchro products. ILC Data Device Corp., 105 Wilbur Place, Bohemia, NY 11716-2482, (516) 567-5600.
Copyright 1995 Nelson Publishing Inc.
April 1995