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Friday, 30 September 2016

Innovative Design and Signal Evaluation Improve Capabilities of Absolute Rotary Encoders

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A new encoder design produces the high resolution and accuracy of optical absolute encoders with the ruggedness and compact size of magnetic encoders

Dr. Markus Erlich, VP Marketing, Servotronix Motion Control Ltd.

sensAR, the new magnetic absolute rotary encoder developed by Servotronix, achieves the high resolution and accuracy of optical absolute encoders in a more compact, robust and durable product. The encoder’s strength stems from a non-periodic pattern and a patented method of signal evaluation to generate both Gray code and high-resolution absolute positions from a single circular track.

Optical Rotary Encoders

Conventional incremental encoders utilize a disk with either one or two concentric tracks. Each track is a pattern of equally spaced increments that are either transparent or opaque. A beam of light passing through the disk is detected by an optical sensor, which outputs equally spaced pulses as the disk rotates. In a dual track arrangement, shown in Figure 1, each track is fitted with a light emitter-detector pair; the two sensors generate two sinusoidal analog signals with a phase difference. In a single track arrangement, two signals are similarly produced by two sensors positioned at a quarter-period offset. These analog signals are then converted into digital signals (square waves) in which one channel leads the other by 90 electrical degrees. By monitoring the phase difference of the two output channels, the direction of rotation can be determined.

Figure 1: Disk with two incremental tracks

Figure 1: Disk with two incremental tracks

Optical encoders typically have an incremental pattern containing a number (n) of periodic sections, ranging from 250 to several thousand, spaced equally around the disk. The signal digitizing process thus produces 4xn counts for every turn. The analog values of the two signals are used to calculate the exact position within one period of the incremental pattern, thus achieving an incremental position with a high resolution. However, to obtain an absolute position within one turn, the starting position must be known.

To determine absolute positions, the disk of a rotary encoder includes an additional absolute track with a binary code pattern; an example is shown in Figure 2. The absolute track is composed of segments of different length, according to the pattern design. Each segment is a multiple of the divisional step, which is defined by the resolution.

The resolution of the absolute track must be as high as the resolution of the incremental track. The length of each divisional step of the absolute pattern is therefore equal to the length of the periodic section (graduation period) of the incremental pattern.

Figure 2: Disk with one absolute and one incremental track

Figure 2: Disk with one absolute and one incremental track

To read the absolute track, several sensors read consecutive steps of the pattern. The distance between adjacent sensors must be equal to the divisional step of the absolute track or the graduation period of the incremental track. Due to the consequent small size of such sensors, custom-designed miniature optical sensors are typically encapsulated as a sensor array on an application-specific semiconductor chip. Figure 3 shows an example of a sensor arrangement of an absolute encoder.

Figure 3: Sensor arrangement of an absolute encoder

Figure 3: Sensor arrangement of an absolute encoder

The digitized output signals from the sensor array provide a Gray code, a binary code in which two successive values differ in only one bit. Because successive incremental position codes differ by just one binary digit, the Gray code prevents the introduction of erroneous code at transitions between positions.

Magnetic Rotary Encoders

Magnetic encoders, which are based on the same principle as incremental encoders, have the advantage of being more robust than optical encoders, since they are less sensitive to shock, vibration and contamination. They are also more durable, since there is no degradation of light emitting diodes.

However, since the magnetic field rapidly decreases as the distance from the magnet surface increases, the number of periodic sections of the pattern is no more than a few ten. Moreover, if the graduation period is very small, the magnetic sensor must be very close to the magnet surface in order to sense a distinct transition. Magnetic encoders commonly compensate for their small number of periods by performing analog processing at a higher resolution. This results in a higher sensitivity to electrical noise. In addition, the signal in one period is less precise, rendering the overall accuracy of magnetic encoders inferior to that of comparable optical encoders.

Both optical and magnetic encoders have several disadvantages. They require at least two tracks and an array of sensors to determine the absolute position of the rotating disk. In particular for magnetic encoders, arranging two concentric magnetic patterns on an encoder disk is difficult. In both optical and magnetic absolute encoders the reliability of position detection depends largely on the accuracy of the code projection onto the sensor array.

Because of the low dimensional tolerances of the code tracks, absolute encoder disks must be manufactured with extreme precision and their size can only be as small as is feasible. This explains why absolute encoders commonly have 256 divisions, while incremental encoders of the same size usually have 1024 increments.

A New Design for Magnetic Absolute Rotary Encoders

The new magnetic absolute rotary encoder developed by Servotronix overcomes many of the disadvantages of conventional absolute encoders.

The Servotronix encoder design is shown in Figure 4. A number of permanent magnets of different sizes are positioned along the outer edge of the encoder disk in a single circular track, forming a magnetic code track whose pattern is non-periodic.

Magnetic (Hall) sensors are fixed to a static part of the encoder, spaced equidistantly from each other, arranged concentrically and in close proximity to the magnetic code track.

Figure 4: Single code track and equidistant sensors

Figure 4: Single code track and equidistant sensors

A patented algorithm utilized in the Servotronix design generates a Gray code with a maximum number of positions for a given number of sensors from a non-periodic pattern of a single magnetic code track.

In addition, the sensors’ analog output signals directly provide a high resolution absolute position, without needing additional incremental readings. The sensors produce electrical signals proportional to the strength of the magnetic field generated by the facing magnet. These analog signals are first digitized by comparing them to a threshold value, thus generating a Gray code which describes an absolute position at a low resolution. A configuration of seven sensors and seven magnets, for example, creates a Gray code that identifies 98 positions.

To achieve a higher absolute resolution, an additional patented method for signal evaluation is applied. Two analog signals are associated with each Gray code according to a predefined signal table. The absolute position of the disk corresponds to the associated position value in a prerecorded position table for the analog signal whose value is closest to the threshold.
When put into practice, the Servotronix technology achieves a resolution of 20 bits using a 12-bit analog to digital converter and a configuration for seven sensors.

sensAR™ Rotary Encoder Series

Servotronix recently introduced this new encoder design in their sensAR rotary encoder series. The first magnetic absolute encoder in this series offers a 20 bit resolution and an accuracy of ±0.02° (±72”). The encoder is initially available with a 36 mm diameter and a height of 28 mm.

Simplicity is the main advantage of this new encoder, which generates a Gray code from a single track, unlike other absolute encoders which need at least two tracks and an array of custom-designed sensors. High resolution is achieved by means of a patented signal evaluation, instead of the high-resolution incremental readings typically used in absolute encoders that make such devices larger and more complex. Moreover, the mechanical design, which utilizes off-the-shelf Hall sensors and does not require customized miniature sensor arrays, results in a cost-effective encoder.

Figure 5: Mechanically simple design

Figure 5: Mechanically simple design

Magnetic technology combined with a mechanically simple design award the sensAR encoder with compactness, robustness and durability.

Featuring only few mechanical parts and no optical components, sensAR encoders are less sensitive to contamination and can operate reliably in dirty, dusty, or humid environments, and at temperatures ranging from -20°C to 115°C. These encoders can also accommodate mechanical tolerance deviations with a permissible axial and radial motion of the motor shaft of ±0.3 mm and ±0.025 mm, respectively.

Furthermore, due to the elimination of both optical components and bearings the durability of the encoder is high and there is no need for maintenance. Even at rotational speeds up to 10,000 rpm and angular acceleration of up to 100,000 rad/s2, sensAR has a long service life (MTBF @ 80°C: 788400 hours).

The ruggedness of the sensAR encoders make them particularly reliable in motor-feedback applications that are exposed to severe shock, for example due to emergency braking, or high vibration, as in the mining, steel, cement and paper industries.

Servotronix