ENCODERS
1.Intro
Encoders can be either absolute or incremental,
1.1 Absolute encoder
Absolute encoders have a unique code for each shaft position. The absolute encoder interprets a system of coded tracks to create position information where no two positions are identical. Another feature is that absolute encoders do not lose position whenever power is switched off. Single-turn encoders are used for position verification with a single revolution of the encoder axis
Absolute encoders have a number of advantages. First is the non-volatility of memory. An absolute encoder works as a non-volatile position verification device. True position is not lost if power is lost or the system moves while power is switched off. A continuous reading of position is not needed. This is specifically useful in those applications, such as satellite-tracking antennas, where position verification is key.
1.2 Incremental encoders
An incremental encoder provides a specified amount of pulses in one rotation of the encoder. The output can be two lines of pulses (an “A” and “B” channel) that are offset in order to determine rotation direction. This phasing between the two signals is called quadrature. An disadvantage is that incremental encoders lose position whenever power is switched off.
2. Principle
2.1 The principle of an incremental encoder
An incremental encoder generates a pulse for each incremental step in it's rotation.
| Incremental encoder, with corresponding A/B signal states shown on the right |  |
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The phasing between the two signals is called quadrature, it is giving the rotation direction Chanel B leading A means CW. Chanel A leading B means CCW. |
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2.2 The principle of an absolute encoder
An absolute encoder generates a code for each step in it's rotation, it has a disk with a gray code, that means with every code change only one codetrack change.
| An 3 bits absolute encoder with the Gray-code, every position has his own code. |  |
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An comparison of an incremental disk and an absolute disk (10 bits). One rotation of the disk means 360 degrees, also one step is 360/1024 = 0,3515625 degree. |
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3 How are both encoders used by FreeGo2
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The incremental encoder is directly mounted on the motor shaft and the absolute encoder is mounted on the declination axis. Example Absolute encoder The 17 bits Absolute encoder makes 217 =131.072 steps for 360 degrees rotation. 360 Degrees = 360 x 3600 = 1.296.000 bgsec Also 1 codestep of the absolute encoder means 1.296.000 / 131.072 = 9,887695 bgsec |
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Incremental encoder The 11bits incremental encoder on the motor shaft makes 211 =2048 steps for one rotation of the motor shaft. What means one incremental codestep on the declination axis ? With the reductions from the motor shaft to the declination axis (Reduction 1 = 510 and Reduction 2 = 47) the motor has For 360 degrees rotation of the declination axis the motor has to rotate 1 x 510 x 47 = 23970 times, that are 23970 x 2048 = 49.090.560 code steps. Also 1 code step of the incremental encoder means 1.296.000 / 49.090.560 = 0,026400 bsec Theoretical the smallest step of the absolute encoder (9,887695 bgsec) can filled up with about 374 steps of the incremental encoder, but there are also mechanical and optical limitations. |
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