An Effective Method to Improve the Accuracy of a Vernier-Type Absolute Magnetic Encoder

This article proposes a method to improve the accuracy of a vernier absolute magnetic encoder. The encoder consists of a master and a nonius multipolar magnetic track. Sinusoidal signals from the master and nonius tracks are used to infer the absolute information. Unfortunately, these signals are contaminated by nonideal factors such as different amplitudes, dc-offsets, phase shifts, and random noise. Moreover, harmonics existing in the encoder signals distort the vernier principle and significantly affect the accuracy of the encoder. To address these problems, the present article proposes an efficient method with three main parts. The first is an observer phase-locked loop (OPLL), which is used to estimate the phase and eliminate the nonideal factors. The second is nonlinear phase compensation, which is used to correct the vernier principle that deviated due to the existing harmonics. Finally, a pole pitch compensation method is introduced to modulate the master phase angle from the OPLL to eliminate the harmonic distortion. The proposed method can eliminate the nonideal factors, harmonic distortion and improve the accuracy of the encoder. All the proposed methods were implemented on an ARM STM32F407ZG. The experimental results confirm the validity of the proposed method for practical applications.

I. Introduction

An absolute encoder is a device for measuring the rotation angle of a rotational motor in industrial applications. The two typical types of absolute encoders are optical and magnetic. Among these, the absolute magnetic encoder (AME) is widely used in applications because of its benefits, which include low cost and superior performance in industrial environments. It can achieve the same resolution and accuracy as an optical encoder. In recent years, extensive studies have been conducted on vernier-type absolute encoders [1]–[3]. In this article, we investigate a vernier absolute magnetic encoder (VAME), which is a multipolar dual track magnetic encoder. It has master and nonius tracks. The master track was located outside and split to pole pairs, while the nonius was located inside and split to pole pairs. Each pole pair of each track will generate a pair of sine and cosine signals and each period of the sine/cosine signal corresponds to a phase of a saw tooth wave in the range of  rad. Two phase values are extracted from the master and nonius tracks. The absolute information of any point on the two tracks can be obtained by the vernier principle [2], [3]. In this structure, the accuracy of the phase obtained from the master and nonius in each pole pair is important for improving the accuracy of the VAME. In practice, the sine/cosine signals from the master and nonius tracks are contaminated by nonideal factors, such as different amplitudes, dc-offsets, phase shifts, and random noise [4], [5]. Moreover, the harmonics existing in the encoder signals significantly affect the accuracy of the multipolar magnetic encoder [6], [7]. Therefore, a method to overcome the error factors is needed to obtain the absolute information.