A circuit generates sinusoidal excitation signals for a shaft-angle resolver and, like the arctangent circuit described in the preceding article, generates an analog voltage proportional to the shaft angle. The disadvantages of the circuit described in the preceding article arise from the fact that it must be made from precise analog subcircuits, including a functional block capable of implementing some trigonometric identities; this circuitry tends to be expensive, sensitive to noise, and susceptible to errors caused by temperature-induced drifts and imprecise matching of gains and phases. These disadvantages are overcome by the design of the present circuit.
The present circuit (see figure) includes an excitation circuit, which generates signals K sin(ωt) and K cos(ωt) [where K is an amplitude, ω denotes 2π × a carrier frequency (the design value of which is 10 kHz), and t denotes time]. These signals are applied to the excitation terminals of a shaft-angle resolver, causing the resolver to put out signals C sin(ωt– θ) and C cos(ωt– θ). The cosine excitation signal and the cosine resolver output signal are processed through inverting comparator circuits, which are configured to function as inverting squarers, to obtain logic-level or square-wave signals –LL[cos(ωt)] and –LL[cos(ωt– θ)], respectively. These signals are fed as inputs to a block containing digital logic circuits that effectively measure the phase difference (which equals θ between the two logic-level signals). The output of this block is a pulse-width-modulated signal, PWM(θ), the time-averaged value of which ranges from 0 to 5 VDC as θ ranges from –180 to +180°.
PWM(θ) is fed to a block of amplifying and level-shifting circuitry, which converts the input PWM waveform to an output waveform that switches between precise reference voltage levels of +10 and –10 V. This waveform is processed by a two-pole, low-pass filter, which removes the carrier-frequency component. The final output signal is a DC potential, proportional to θ that ranges continuously from –10 V at θ = –180° to +10 V at θ = +180°.