Measurement converter for sensor signals in modular systems

Abstract

We consider a facility for measurement of signals from different types of sensors; the system complies with the CAMAC and VME standards, and it is designed to increase measurement accuracy. We present the basic characteristics of the system.The measurement information system of wind tunnels makes use of various types of sensors: tensiometric, thermometric, potentiometric, sensors generating FM output signals, etc. As a rule, modern systems are modular, and based on standard interfaces (CAMAC, VME, VXI). The link between the signal convertors and the system bus efficiently uses structural and algorithmic methods for increasing accuracy.The greatest measurement accuracy, highest resolution, and broadest dynamic range is provided by measurement convertors that are constructed with the method of balanced charges. Devices designed with this method include, in particular, high-accuracy pulse-compensation voltage-to-frequency convertors [1]. There are frequency measuring instruments that can be programmed to meet accuracy and response requirements [2]. These devices have been used to develop an integrating instrument for conversion of signals from various types of sensors for modular systems.The convertor contains the following components ( Fig. 1): a differential instrumentation amplifier DIA with programmable gain; a voltage-to-frequency converter VFC and a frequency-to-code converter FCC; voltage and current sources VS and CS; a reference signal generator RS; a scaling circuit SU; a communication controller CU with a strain sensor SS; a circuit IU for separation of the analog and digital pairs of signals; and a decoder D for decoding signals from the bus master.For the DIA we used an M2USI~0084 hybrid chip that is based on a classical scheme using three operational amplifiers; this circuit permits selection of any of six gains (1, 10, 100, 200, 500, and 1000). The basic characteristics of the DIA (noise level, thermal drift in bias voltage, gain, attenuation of sin-phase noise) correspond to the requirements of measurements for aerodynamic experiments. There is a symmetric RC-filter at the input of the amplifier to suppress high-frequency noise.The voltage-to-frequency converter VFC is based on an M2PNM1781 hybrid chip. The circuit contains an integrator with a time constant of r = RIC a source for bias voltage U b with switchable polarity, a source of balancing current I o with a polarity switch, I 0 = +_ Ub/R 2, and a logic switch. Depending on the sign of U b, the input voltage for the VFC is converted to the 0-10 V range. To deal with a sign-varying signal from the strain gauge, there is a scaling unit that supports an inputsignal range of 0 to +_ 5 V. The scaling unit can also compensate for the signal from the initial equilibrium state in the strain gauge.The supply voltage U s for the strain gauge is drawn from the bias-voltage generator, which permits polarity switching. Changes in the polarity of Usg during measurement make it possible to reduce error due to drift in the amplifier, thermal effects, and other...