Investigation of errors in an electronic automatic potentiometer whose input is fed with a signal of the form U_m¦ sin (Ωt+ϕ)¦

Conclusions

• 1 When an electronic automatic potentiometer is fed with a sinusoidal signal transmitted through a full-wave rectifier, it is necessary to take into consideration the effect of the phase difference between the instrument supply voltage and the input signal, since this difference affects the calibration coefficient (Fig. 2, curve 2) and the undesirable effect of the out of phase component of the input-voltage fundamental (Fig. 2, curve 3).The effect of the out of phase component can be eliminated if the phase difference between the supply voltage and the transducer signal is set at 0 or 90‡ (Fig. 2, curve 3).

• 2 When an electronic automatic potentiometer is fed with a tachogenerator sinusoidal signal transmitted through a full-wave rectifier, regular instrument pointer oscillations at a frequency of Ω_p=2(Ω_g-Υ_pΩ_e arise close to certain fixed values of the signal frequency. These oscillations have a considerable amplitude (Fig. 3), which makes the instrument unsuitable for practical use.

• 3 In order to eliminate the above undesirable effects, it is necessary, whenever possible, to eliminate the input-voltage ac component by means of filters.

• 4 In measuring the angular velocity by means of a tachogenerator, the input signal has a low frequency at the beginning of the instrument scale, which impedes the production of an adequate filter. In this case it would appear advisable first to multiply the input signal frequency.

• 5 In recording signals with a constant frequency it is not necessary to use filters, if this is undesirable for any reason. It is then necessary to eliminate the effect of the out of phase input-voltage component by providing a zero phase difference between the instrument supply voltage and the signal and to take into consideration a corresponding correction in calibrating the instrument.