UBC Theses and Dissertations
A high precision angular encoder based on capacitive coupling Sankey, Todd
A method for measuring angular position based on sensing electrostatic fields is described. A device is presented which uses this method to achieve a resolution of 10 radians and is fabricated entirely from common electronic components mounted on printed circuit board. The angular sensor consists of two coplanar, coaxial disks, one static and the other freely rotating. An electrostatic field is generated by Nr active radiator electrodes on the static disk. Nm passive modifier electrodes on the rotating disk determine the magnitude of the signals measured by a detector circuit for each of the radiators. Nr The relationship between Nr and Nm is chosen to be Nm = Nr/2 +1 which produces a vernier effect between the two disks. This effect determines the relationship between successive signals measured at a given rotor position. This relationship is shown to be most easily analyzed using Fourier analysis because the angle of the rotor can be entirely determined from phase of the single frequency component at ώ = π - 2π/Nr. Also presented are the effects of two different kinds of misalignment of the sensor: an offset in the centres of rotation of the disks and a tilt in the axes of rotation. The errors are characterized and a method for measuring and reducing their impact is shown. Several tests of the sensor are presented and the results analyzed. A device simulator is used to examine the effect of changing the physical parameters of the sensor. A laser and spot detector are employed to test the accuracy of the sensor in measuring very small rotations. The repeatability of the sensor is examined by attaching the rotor to a stepper motor. Finally the current design is extrapolated to suggest possibilities for a more precise sensor or a sensor better suited to different environments.
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