GMR Multistack sensor for high resolution encoder application used for incremental and / or absolute applications.
Original Publication Date: 2004-Aug-12
Included in the Prior Art Database: 2004-Aug-12
Disclosed is a device that uses a new concept to decode the position of a magnetic encoder disc using one single track. The concept is based on a digital signal and signal treating and supports absolute encoding without signal interpolation. Because the resolution is dependent to the sensor stack geometric large magnetic pattern can be used on the encoder side with high magnetic flux. This has the advantage that a relative large distance between encoder plane and sensor stack can be realized. Starting from the vernier concept, what uses an array of sensors to improve the resolution this new concept uses a multistack sensor device. The stack height must be larger than the longest bit on the encoder plane. Then the output of each single sensor can be analyzed and the incremental resolution is improved, the index for the incremental decoding can be embedded into the signal track, absolute decoding can be done in parallel to the incremental decoding and the rotation speed and moving direction can be detected.
GMR Multistack sensor for high resolution encoder application used for
/// or absolute applications .
Summary: This new concept is based on a magnetic multi - stack sensor like in figure 1. This sensor head supports four different options to analyze the encoded information as illustrated in figure 2 to figure 5.
Usually an incremental position encoder uses one index track and one signal track. The resolution is equal the bit length if no signal interpolation is done . As shown in figure 2, with the sensor stack the resolution is set by the distance between two consecutive sensors.
Because the sensor stack can detect the length of each bit, as shown in figure 3, the index can be integrated into the signal track. No index track is needed anymore if the multi - stack sensor has at least two senor elements. By integrating the index track into the signal track more than one index can be used . By using bits of different length, each length can be associated to one unique index . Now this indexes can be used for absolute position decoding . For example a sensor stack with 11 sensor elements increase the incremental accuracy by the factor of 10 and supports 10 different indexes. That mean, for an encoder plane with 3600 bits the incremental resolution is increased from 0.1° to 0.01° and the absolute position can be detected with an accuracy of 36°.
To go for high end absolute position decoding a group of bits must be decoded together as shown in figure 4. With a number N = 1+(B+1)B/2 of sensor layers in the stack a number with a length of B bits can be detected . For a sensor - stack with 11 sensor elements the absolute position decoding is just 22.5° what is not much better than the 36° what can be reached by using multiple indexes. However a sensor - stack with 67 sensor elements supports a resolution of 0.18° by detecting a 11 bit consecutive bits on the encoder disc.
Alternatively the same sensor stack can be used to detect the bits in parallel tracks. To do this the sensor head must be assembled with a 90° rotation to the one track solution. Also a sensor can be created what means to each parallel data track a sensor stack is assigned. For example the 67 sensor element can be realized by 4 parallel data tracks. To each data track a sensor stack made of 19 sensors is assigned.
Additionally to the improved incremental resolution, embedded indexing and absolute position decoding, the sensor - stack can measure the velocity and direction of the movement. The principle of the speed and direction detection is demonstrated in figure 5. The detection of speed and movement direction is done at the transition between two bits. Because the sensor stack is higher than the length of the longest bit the sensor stack can detect two or more transitions between two bits at the same time. That means the algorithm described below can be applied to two or more different sensors at the same time . If this is done statistical error based...