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[magnetic field effect transistor (MAGFET) arrays] set in a square arrangement. The sensor array is integrated onto a CMOS chip along with angle-detection ...
IEEE SENSORS JOURNAL, VOL. 5, NO. 5, OCTOBER 2005

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A CMOS Rotary Encoder Using Magnetic Sensor Arrays Kazuhiro Nakano, Student Member, IEEE, Toru Takahashi, and Shoji Kawahito, Senior Member, IEEE

Abstract—A new type of small magnetic rotary encoder is presented. The device detects the magnetic field of a permanent magnet attached to the end of the rotating shaft using complementary metal–oxide semiconductor (CMOS) magnetic sensors [magnetic field effect transistor (MAGFET) arrays] set in a square arrangement. The sensor array is integrated onto a CMOS chip along with angle-detection circuits, leading to the realization of a compact, cost-effective rotary encoder. A prototype sensor chip 4 3 mm2 is shown to provide error as with dimensions of 4 3 low as 3.5 without offset calibration and 0.36 with offset calibration, based on an angle calculation method with mean square estimation. This result shows that the CMOS rotary encoder can achieve resolution of 10 bits/rotation at the cost of calibration. Index Terms—Complementary metal–oxide semiconductor (CMOS), integrated sensor, magnetic field effect transistor (MAGFET), rotary encoder.

I. INTRODUCTION

A

ROTARY encoder is a fundamental device in many mechanical and electronic systems. For compact equipment, small and low-cost encoders capable of high resolution and absolute angle detection are required. However, commonly used contactless encoders, using magneto-resistive or Hall devices combined with a multipole magnet, have difficulty in meeting these requirements, requiring fine-pitched scales and some other means of detecting the absolute angle in a small space. As an example of such compact rotary encoders, a vector magnetic sensor using two elements for the and directions based on complementary metal–oxide semiconductor (CMOS) technology has been reported [1]. The integration of such a rotary encoder system onto a silicon chip is not only an ideal solution for miniature applications, but it also simplifies the manufacturing process and reduces cost. However, it has been found to be difficult to achieve the required resolution of 10 bits/rotation using this CMOS rotary encoder design. We have developed a magnetic rotary encoder using magnetic sensor arrays based on CMOS technology [2]. To achieve high resolution, the proposed system uses a large number of sensor elements set in a square arrangement to sense the magnetic field Manuscript received February 15, 2004; revised February 14, 2005. The associate editor coordinating the review of this paper and approving it for publication was Dr. David Lambeth. K. Nakano is with the Graduate School of Electronic Science and Technology, Shizuoka University, Hamamatsu, Shizuoka 432-8011, Japan (e-mail: [email protected]). T. Takahashi is with the R&D Center, NTN Corporation, Iwata, Shizuoka 438-8510, Japan (e-mail: [email protected]). S. Kawahito is with the Research Institute of Electronics, Shizuoka University, Hamamatsu, Shizuoka 432-8011, Japan (e-mail: [email protected]). Digital Object Identifier 10.1109/JSEN.2005.853597

Fig. 1. Arrangement of sensor chip and permanent magnet.

profile. The absolute angle is calculated by estimating the two zero-crossing points of the magnetic field profile using a meansquare estimation. The logic circuits for this calculation can be integrated onto a silicon chip, allowing for high-speed response and a compact unit. This paper describes the proposed system architecture and the estimated accuracy of angle detection and presents measurement results obtained for a prototype chip. II. ROTARY ENCODER SYSTEM The rotary encoder setup and block diagram of the rotary encoder chip are shown in Figs. 1 and 2. A permanent magnet with a magnetic yoke is attached to the end of the rotating shaft, and the rotary encoder chip is attached to a metal backing plate facing the magnet perpendicular to the end of the shaft. The magnetic field is detected by split-drain magnetic field effect transistor (MAGFET) [3]–[7] arrays set in a square arrangement. The output current is amplified and converted to a voltage signal by sample-and-hold amplifiers, and then converted to a digital signal by 8-bit analog-to-digital converters (ADCs). The rotation angle is then calculated in real time by logic circuits integrated with the sensor arrays. Using a fine-pitched sensor array and a digital domain signal processing technique, a compact encoder with good resolution can be realized. III. ANGLE DETECTION METHOD The geometric magnet angle is calculated by detecting two zero-crossing points of the magnetic field, as illustrated in Fig. 3. Another possible arrangement of MAGFET array is a circular pattern. Compared with the circular pattern, the square arrangement is suitable for this system because the square arrangement of the four linear sensor arrays ensures that angle detec-

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IEEE SENSORS JOURNAL, VOL. 5, NO. 5, OCTOBER 2005

Fig. 2. Block diagram of proposed sensor chip.

N = 175, n

Fig. 4. Sensor signal model (

= 12).

in the example shown in the figure. The zeroes with of the sensor outputs are determined by calculating the average of the sensor outputs, as follows:

(1) where is the output signal of the th sensor. The next step is to extract two straight lines that cross the zero sensor output line. To do this, sensors with output within a given range near zero, called the zero-cross window (ZCW), are selected. The ZCW should be chosen so as to be as large as possible in order to increase the data number for MSE while avoiding the noisy and nonlinear region. The ZCW is determined from the maximum and in the sensor output proand minimum outputs, is then given by files. The ZCW (2) Fig. 3.

Zero-crossing point detection.

tion is not sensitive to the rotation axis misalignment as described in Section III-B. If the sensor sensitivity is sufficiently large compared to the magnetic field of the permanent magnet, and if the resulting output signal is much larger than disturbances such as device mismatch, the zero-crossing points can be simply detected from the positions of the MAGFETs with zero output. However, due to the relatively low sensitivity of the silicon MAGFETs, the device is susceptible to large mismatch error associated with deviation of the threshold voltage of the MAGFETs. Therefore, to achieve high angle accuracy, detection algorithms with error reduction effects are necessary. The zero-crossing points are determined by mean-square estimation (MSE), as described in the next section. A. Zero-Crossing Point Detection Algorithm The typical sensor output, with mismatch error, is shown in Fig. 4. Each sensor line is assumed to consist of MAGFETs,

where is a constant that sets the ratio of the ZCW to the full signal range. In Fig. 4, is set at 0.3. To detect the zero-crossing points, the data profile in the ZCW is fitted to a straight line (3) where is the slope of the line, and is the intercept. The positions of MAGFETs is given by , where is the sensor pitch and is an integer. The detection of the zero-crossing point in region R1 in Fig. 4 is then performed by searching for the minimum of the following function: (4) The zero-crossing point in R1 can then be estimated by

(5)

NAKANO et al.: CMOS ROTARY ENCODER USING MAGNETIC SENSOR ARRAYS

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Fig. 6. Relationship between the standard deviation of calculated angle error and offset error (N = 175,  = 0 , worst case).

is the horizontal difference between the where zero-crossing points of the two lines. The magnet angle can also be calculated when the two zero-crossing points are on Lines 1 and 2, Lines 1 and 3, Lines 1 and 4, or Lines 4 and 3.

IV. SIMULATION A. Angle Detection Error

Fig. 5.

are esThe standard deviations of angle detection error timated by simulating 1000 different patterns of random offset. The angle resolution is a function of the deviation of the sensor’s and the signal amplitude. Fig. 6 shows the offset current as a function of the ratio of simulation result of to , where represents the standard deviation of the is the signal amplitude. For sensor’s offset current and an accuracy of 10 bits/rotation (1024 positions in 360 ), the LSB). Thereabsolute angle error must be within fore, must be smaller than 0.12 , if the maximum error is three times the standard deviation of the error. In order to of meet this condition, the ratio of the offset deviation must be smaller than the MAGFET to the signal amplitude 4%.

Angle calculation methods.

where

B. Influence of Rotation Axis Misalignment and is the number of data points. The other zero-crossing point is also calculated in R2. If the two zero-crossing points are on opposite array lines, as shown in Fig. 5(a), the magnet angle can be calculated by (6) where is the distance between the two parallel lines and is the vertical difference between the zero-crossing points of the two lines. If the rotation axis is misaligned, the two zerocrossing points may be on adjacent lines, as shown in Fig. 5(b), in which case is calculated by (7)

Figs. 7 and 8 show the calculated maximum angle error when the magnet rotation axis is misaligned in the and direc, ). In this simulation, the sensors are assumed tions ( to have no offset in order to clarify the effect of misalignment. Figs. 7 and 8 show the case for linear arrays extended 12 and 37 . In sensors beyond the intersection with the adjacent array Fig. 7, the maximum angle error is 0.62 for misalignment of m. The worst case occurs when the zero-crossing point occurs in the gap between two sensor lines. If the extension of is sufficiently large, the angle calculation is the sensor line robust to misalignment, as shown in Fig. 8. This result demonstrates that the device can be configured so as to be insensitive m, the to misalignment. In Fig. 8, for misalignment of maximum angle error is less than 0.23 .

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Fig. 9. Photograph of prototype sensor chip.

Fig. 7.

Maximum angle error versus rotation axis misalignment (N = 175,

n = 12).

Fig. 10. (a) MAGFET channel geometry and dimensions (in (b) equivalent circuit.

m)

and

V. EXPERIMENT

Fig. 8.

Maximum angle error versus rotation axis misalignment (N = 200,

n = 37).

Fig. 9 is a photograph of a prototype sensor chip fabricated using an 0.8- m 2P/2M CMOS process. Each sensor line consists of 175 MAGFETs set at a pitch of 15 m. As this prototype chip was fabricated to test the basic characteristics of the sensor, ADC and logic circuits for angle calculation were not integrated into the device. The circuits arranged inside the sensor arrays in Fig. 9 are test circuits for MAGFET characterization. The equivalent circuits and dimensions of each MAGFET are shown in Fig. 10. The test circuits were linear arrays of MAGFETs in three “pixel” configurations. The type-1 test circuit consisted of 255 pixels of one MAGFET at a pitch of 10 m, the type-2 circuit consisted of 170 pixels of one MAGFET at a pitch of 15 m, and the type-3 array consisted of 100 pixels of four MAGFETS at a pitch of 15 m. The MAGFETs of each pixel in the type-3 circuit were connected in parallel as shown in Fig. 11 in order to cancel out the inclination components introduced by the CMOS processing.

NAKANO et al.: CMOS ROTARY ENCODER USING MAGNETIC SENSOR ARRAYS

Fig. 11.

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Type-3 MAGFET connection.

Fig. 13.

Fig. 12. Relationship between signal (I = 20 A). (I

=I



=I

versus I

.

) and magnetic flux density

Fig. 14.

Block diagram of measurement setup.

A. Measurement of MAGFET Characteristics MAGFETs were characterized by applying a drain voltage of 5 V and varying the bias current via applied to the , generated by the applied gate. The output current magnetic field, was then measured. The measurement results for sensor sensitivity are shown in Fig. 12, where and . The sensor output is proportional to the magnetic flux density, and the magnetic sensitivities array types 1, 2, and 3 are 2.4%/T, 2.8%/T, and 2.9%/T, respectively. The standard deviation of the magnetic sensitivity for all the types is about 3%. Fig. 13 shows the deon the offset deviation , where pendence of represents the bias current in a single MAGFET. For A, is 0.9%, 0.6%, and 0.4%, respectively. B. Angle Detection Using the Prototype Chip The block diagram of the measurement setup is shown in Fig. 14. The sensor chip and ADC chip are controlled by a field-programmable gate array (FPGA), and the raw output digital data is stored on a personal computer. Fig. 16 shows the error of the detected angle calculated from the raw sensor output signal of Fig. 15. Despite the large offset deviation compared to the signal amplitude, as shown in Fig. 15, angle resolution of is achieved using the four linear magnetic sensor arrays and the MSE method. Higher angle resolution may be achieved by improving sensor sensitivity, increasing the magnetic coupling, and

Fig. 15.

Measured sensor output.

achieving better control of the offset deviation. These improvements would provide a calibration-free encoder chip, which leads to low costs. Another approach to provide better accuracy is the the use of an on-chip read-only memory (ROM) for offset deviation correction. This may be achieved by measuring the offset deviation in the absence of a magnetic field and storing the result in an on-chip 8-bit ROM. This value could then by subtracted from the sensor signal before angle estimation. The angle error as a function of magnet angle in this corrected case is shown in Fig. 17. The error of the detected angle is reduced to 0.31 to 0.29 . With this method, the required resolution can be

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[2] K. Nakano, T. Takahashi, and S. Kawahito, “A CMOS smart rotary encoder using magnetic sensor arrays,” in Proc. IEEE Sensors, 2003, pp. 206–209. [3] J. Lau, P. K. Ko, and P. C. H. Chan, “Modeling of split-drain magnetic field-effect transistor (MAGFET),” Sens. Actuators A, vol. 49, pp. 155–162, 1995. [4] S. Kawahito et al., “An integrated MOS magnetic sensor with chopperstabilized amplifier,” Sens. Mater., vol. 8, pp. 001–012, 1996. [5] A. Nathan, A. M. J. Huiser, and H. P. Baltes, “Two-dimensional numerical modeling of magnetic-field sensors in CMOS technology,” IEEE Trans. Electron Devices, vol. ED-32, no. 7, pp. 1212–1219, Jul. 1985. [6] H. P. Baltes and R. S. Popovic´ , “Integrated semiconductor magnetic field sensors,” Proc. IEEE, vol. 74, no. 8, pp. 1107–1132, Aug. 1986. [7] J. J. Clark, “Split-drain MOSFET magnetic sensor arrays,” Sens. Actuators A, vol. 24, pp. 107–116, 1990.

Fig. 16.

Fig. 17.

Measured angle error versus magnet angle without offset correction.

Measured angle error versus magnet angle with offset correction.

achieved with the present performance of MAGFET arrays at the cost of calibration.

Kazuhiro Nakano (S’03) was born in Shizuoka, Japan, in 1979. He received the B.E. and M.E. degrees in electrical and electronic engineering from Shizuoka University, Hamamatsu, Japan, in 2002 and 2004, respectively. He is currently pursuing the D.E. degree at Shizuoka University. His research interests include mixed analog/digital LSI circuits and smart silicon sensors. Mr. Nakano is a member of the Institute of Electronics, Information, and Communication Engineers of Japan. He received the Beatrice Winner Award at the 2005 IEEE International Solid-State Circuits Conference.

Toru Takahashi was born in Shizuoka, Japan, in 1968. He received the B.E. and M.E. degrees in electrical engineering from Shizuoka University, Hamamatsu, Japan, in 1990 and 1992, respectively. He joined NTN Corporation, Iwata, Shizuoka, in 1992, as an Engineer of the R&D center. His activities include circuit and system design and sensor applications. From 2002 to 2004, he was a Researcher at the Research Institute of Electronics, Shizuoka University. Mr. Takahashi received the Beatrice Winner Award at the 2005 IEEE International Solid-State Circuits Conference.

VI. CONCLUSION A new CMOS-based rotary encoder system and angle detection algorithm were presented. Simulation results showed that the proposed system can realize high-resolution angle detection and is not sensitive to misalignment of the sensor or rotating magnet. Measurement of a prototype chip revealed that the MAGFET arrays exhibit significant offset error due to deviations of device characteristics. Despite this large offset error, using however, the error of angle detection was within MSE-based angle calculation. Using an 8-bit ROM to store and correct for offset deviation, the error of angle detection was successfully reduced to less than 0.36 , using MSE-based angle calculation and at the cost of calibration, corresponding to a resolution of more than 10 bits/rotation. Temperature stability is very important for practical applications. The characterization of the temperature stability is left as a future work. REFERENCES [1] A. Häberli et al., “Two-dimensional magnetic microsensor with on-chip signal processing for contactless angle measurement,” IEEE J. SolidState Circuits, vol. 31, no. 12, pp. 1902–1907, Dec. 1996.

Shoji Kawahito (S’86–M’88–SM’00) was born in Tokushima, Japan, in 1961. He received the B.E. and M.E. degrees in electrical and electronic engineering from the Toyohashi University of Technology, Toyohashi, Japan, in 1983 and 1985, respectively, and the D.E. degree from Tohoku University, Sendai, Japan, in 1988. In 1988, he joined Tohoku University as a Research Associate. From 1989 to 1999, he was with the Toyohashi University of Technology. From 1996 to 1997, he was a Visiting Professor at ETH, Zurich, Switzerland. Since 1999, he has been a Professor with the Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan. His research interests are in mixed analog/digital circuit design for imaging and sensing devices and systems. Dr. Kawahito is a member of the Institute of Electronics, Information, and Communication Engineers of Japan, the Institute of Image Information and Television Engineers of Japan, the International Society for Optical Engineering, and a Senior Member of the Institute of Electrical and Electronic Engineers. He received the Outstanding Paper Award at the 1987 IEEE International Symposium on Multiple-Valued Logic, the Special Feature Award in LSI Design Contest at the 1998 Asia and South Pacific Design Automation Conference, and the Beatrice Winner Award at the 2005 IEEE International Solid-State Circuits Conference.