TACHOMETER_DERIVED_FROM_BRUSHLESS_SHAFT_ANGLE_RESOLVER

Published:2009/6/22 23:26:00 Author:May | From:SeekIC

The tachometer circuit operates in conjunction with a brushless shaft-angle resolver. By performing a sequence of straight-forward mathematical operations on the resolver signals and utilizing a simple trigonometric identity, it generates a voltage pro-portional to the rate of rotation of the shaft.The figure illustrates an analog tachometer circuit that processes the input and output signals of a two-phase, brushless, transformer-type shaft-angle resolver into a signal with instantaneous amplitude proportional to the instantaneous rate of rota-tion of the shaft. The processing in this circuit effects a straightforward combination of mathematical operations leading to a fi-nal operation based on the well-known trigonometric identity [sin(x)]2 + [cos(x)]2 = 1 for any value ofx.The resolver is excited with a periodic waveform; a sinusoid is indicated in the figure, but a square, triangular, or other peri-odic waveform could be used instead. Thus, the two outputs of the resolver are k1 sin(ωt)sin(θ) and k1 sin(ωt)cos(θ), where k1 is a constant proportional to the amplitude of excitation, cot is 2nm the frequency of excitation, t is time, andθis the instantaneous shaft angle.The two outputs of the resolver are then processed, along with a replica of the sinusoidal excitation, by demodulators. These signals are then differentiated with respect to time in two differentiator circuits. Notice that dB/dt is the rate of change of the shaft angle and is the quantity that one seeks to measure.Next, a multiplier circuit forms a product of the demodulator and differentiator outputs proportional to sin(θ), and the prod-uct of the demodulator and differentiator outputs proportional to cos(θ), The output of the cosine multiplier is fed to a unit-gain inverting amplifier.



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