I. Field of the Invention
This invention relates generally to apparatus for position measurement. In particular, this invention relates to interface circuits used with electromagnetic position transducers.
II. Description of the Prior Art
Position transducers which are of interest herein include resolvers and slider and scale systems producing AC output signals in response to AC excitation signals wherein a phase shift between the excitation signals and the output signals is introduced by the relative position of a transducer armature and stator. The position of the armature relative to the stator is measured by detecting this phase difference. Two alternative methods are known for detecting the phase difference: a phase discrimination technique wherein the excitation signals are applied to pairs of windings arranged in quadrature, and the position induced phase shift is detected by phase comparison of the output signal with a reference from which the excitation signals are derived; and, an amplitude technique wherein the output signals are produced by the quadrature windings and the position induced phase shift is detected from the ratio of the instantaneous magnitudes of the output signals.
FIG. 1a illustrates an arrangement used with the amplitude technique employing a resolver to measure position of a moveable member of, for example, machine tools, robots or other position controlled equipment. The resolver 10 includes a rotor 12 having an armature coil 14, and a stator having stator coils 16 and 18. The rotor 12 is rotated relative to the stator by, for example, a motor 28. The transducer 10 is located remotely from a control device 20 wherein a drive amplifier 22 produces an AC excitation signal applied to the armature coil 14. Output signals appearing at the stator coils 16 and 18 are returned to differential amplifiers 24 and 26 located in control 20. As shown, the return side of the drive amplifier output is grounded and the receiving amplifiers 24 and 26 present unmatched impedances to the signal and return paths because of the input resistor networks. Conducting cables 30, 32, and 34, typically twisted pairs, provide connection of excitation and output signals between the interface circuits of control 20 and the resolver 10.
FIG. 1b illustrates an arrangement used with the phase discrimination technique employing a resolver to measure position of a moveable member. In this arrangement excitation signals are produced by drive amplifiers 23 and 25 and applied to the resolver stator coils 17 and 19. An output signal appears at resolver armature coil 13 and is returned to differential amplifier 21 in control 19. The excitation signals are derived from a single reference signal and are phased displaced one from the other by .pi./2 radians.
FIG. 2 illustrates capacitive coupling between an excitation signal cable and an output signal cable which will exist as a result of proximity of the conducting cables 30, 32 and 34 of FIGS. 1a and 1b. In FIG. 2 capacitors C1, C2, C3, and C4 represent lumped values of the coupling capacitances distributed over the lengths of the conducting cables; source SD represents the source of excitation signals; and, load LD represents the load impedance presented to an output signal. Inductive coupling of the rotor and stator windings is intentionally omitted to simplify the analysis of the capacitive coupling in the conducting cables. It will be appreciated from FIG. 2 that by virtue of the grounded return paths only capacitance C1 contributes an error component to the output signal appearing across the load.
The voltage error component in the output signals arising from capacitive coupling as shown in FIG. 2 has a magnitude equal to the excitation signal magnitude and is phase displaced .pi./2 radians therefrom. The current due to this error component is added algebraically to the output signal current magnitude, resulting in a position error repeated over the range of position measured by the resolver. Such errors are referred to as "cyclic errors." It is common practice to provide individual shields for each conducting cable, such as shields 31, 33, and 35 to reduce or eliminate capacitive coupling between the excitation and output signal cables. The cost of such shielding significantly increases the material and labor costs associated with the installation of such cables.