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A Capacitive Rotary Encoder Based on Quadrature Modulation and Demodulation Dezhi Zheng, Member, IEEE, Shaobo Zhang, Shuai Wang, Chun Hu, and Xiaomeng Zhao
Abstract— This paper presents a capacitive rotary encoder for both angular position and angular speed measurements. The encoder is mainly composed of three parts: 1) the transmitting segments; 2) a pair of reflecting electrodes; and 3) a pair of receiving electrodes. The transmitting segments together with four mutual quadrature carrier voltages provide a modulated electric field. The reflecting electrodes, which are patterned sinusoidally can encode the angular position to a phase/frequency modulated signal based on quadrature modulation. The modulated signal is then digitally decoded to the angular position in a field programmable gate array processor based on the quadrature demodulation and the coordinate rotational digital computer algorithm. Through a universal serial bus, the digital angular position is transmitted to a computer for further analysis in National Instruments’ LabVIEW software. A prototype of the capacitive encoder shows that its precision is better than 0.006° and the resolution is 0.002°. The dynamic nonlinearity is evaluated at ±0.4° when the rotor is rotating at 1000 r/min. Index Terms— Angular velocity, capacitance measurement, encoding, phase modulation, quadrature amplitude modulation, rotating machine measurements.
I. I NTRODUCTION
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OTARY encoders, which can measure the angular position and the angular speed together, are widely used in industrial automation systems. In many cases, such as the speed and position feedback control system, rotary encoders need to provide noncontact sensing, full-circle measurement range, high resolution, high accuracy, and high reliability in harsh environments. Currently, commercially available, noncontact, and widely used rotary encoders are almost exclusively either optical encoders or electromagnetic resolvers [1], [2]. Optical encoders can measure angular position with a very fine resolution, which is up to nanometer accuracy after interpolation. However, they cannot work in harsh environments like those with dust, moisture, frequent mechanical vibrations, and high temperature, because the optical disc inside the encoder is
Manuscript received January 14, 2014; revised May 7, 2014; accepted May 8, 2014. This work was supported in part by the Foundation Item of Chinese National High-Tech Research and Development Program under Grant 2014BAF08B01 and in part by the TransCentury Training Programme Foundation for the Talents of Humanities and Social Science through the State Education Commission. The Associate Editor coordinating the review process was Dr. Subhas Mukhopadhyay. The authors are with the Science and Technology on Inertial Laboratory, Department of Instrumentation Science and Opto-Electronics Engineering, Beihang University, Beijing 100191, China (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2014.2328456
easily affected and damaged by ambient factors. Besides, it is difficult to manufacture small size items with high resolution due to the limitation of grating pitch on glass scale. Resolvers are classified into magnetic-field-based sensors, which are immune to dust, moisture, vibrations, and high temperature, but can be affected by electromagnetic interference. Hence, resolvers have better mechanical robustness in diverse environments, but cannot provide a sufficiently high resolution and high accuracy. To reject electromagnetic interference, a resolver usually requires magnetic shields, which make it bulky. On the other hand, both optical encoders and electromagnetic resolvers are relatively expensive and are usually used in high-end application scenarios only. Capacitive encoders would be expected to provide some significant advantages over optical encoders and resolvers [3]–[5]. Unlike optical encoders in which only a light beam is focused at a certain point on optical disc, the whole area of a capacitive encoder board contributes to the output signal. As the measuring mechanism, the susceptibility of the capacitive encoder to mechanical vibrations and mechanical misalignment (such as tilt and eccentricity of the rotor) is far lower than that of the optical encoder. Compared with electromagnetic resolvers, the capacitive encoder has the unique qualities of low cost and simple structure, since the main components of the capacitive encoder are electrode plates, which are made from standard printed circuit board (PCB) technology. In spite of its sensitivity to industrial pollution, such as moisture and oils, which would be prevented by a good shielding, the capacitive encoder still tend to be critical device as it not only combines good robustness with high accuracy, but it also realizes low cost, simple structure, and low-power consumption. II. S TATE - OF - THE -A RT C APACITIVE E NCODER In recent years, various capacitive sensors have been becoming more and more popular. However, capacitive encoders have been almost absent from the market, except for niche markets, due to various technical difficulties, which were discussed in [1] and [6]. Brasseur [7]–[11] presented a variety of capacitive encoders to meet different special application demands. All of these capacitive encoders are based on ratio metric measurement principle, which need to apply four different excitation signals to four transmitting segments and detect the capacitances between the transmitting segments and the receiving electrode in sequence. The four excitation and detecting periods make
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the measurement time too long so that these encoders can only be used to detect angular position when shaft is stopped or rotating at very low speed (
