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Encoders maxon offers a range of different encoders. Their main characteristics are: Digital incremental encoders – Relative position signal, suitable for positioning tasks – Rotation direction detection – Speed information from number of pulses (counts) per time unit – The standard solution for many applications Digital absolute encoders – Absolute position signal, suitable for positioning without homing – Option to generate commutation signals Program – MILE (inductive encoders) – EASY, MAG, IMR, GAMA, TSX (magnetic encoders) – HEDL, RIO (optical encoders) – EMT (magnetic absolute encoder)
Encoder signals Digital incremental encoders
The line driver is a driver built into the encoder to improve the signal quality through steeper slopes. In addition, it gene- rates the complementary signals A, B, I. Differential signaling makes it possible to eliminate interference during transmission. Absolute encoders Absolute encoders return the absolute position as a bit sequence which is trans- mitted at the clock rate of the controller with a suitable protocol (SSI, BiSS-C). The resolution is given as a bit length; e.g., 12-bit equals 4096 positions. Single-turn absolute encoders output the position only within one motor revolution. Multi-turn absolute encoders determine the position unambiguously over several revo- lutions. They have the capability to detect motor revolutions even without a voltage supply. At startup, the motor position over multiple revolutions is thus known.
Position changes (relative position) are transmitted to the controller as square pulses. The controller evaluates these pulses for precise positioning or speed measurement. The signal transmission is implemented via two phase-shifted chan- nels (A and B) that are compared to deter- mine the direction of rotation. Usually the phasing of channels A and B applies for operation in a clockwise direction (CW), relative to the motor shaft seen from the outside. The maxon controllers evaluate the signal edges. This results in a four times higher positioning resolution relative to the counts per turn of the encoder. The technical term for this is quad counts or states. The abso- lute position can be determined by homing to a fixed position. The signal slopes of index channel I can be used for a precise reference position.
Recommendations for encoder selection
View of the output signals of a digital encoder
( ) Partially applies
90° e Phase shift A,B 360° e Cycle
✓ ✓ ( ✓ )
1 Very high speed
✓
Channel A
( ✓ ) ✓ ✓ ✓ ( ✓ ) ( ✓ ) ✓ ✓
2 Very low speed 3 Precise position 4 Line driver possible
Channel B
✓ ✓ ✓ ✓
5 Index channel possible
✓ ✓ ✓
Channel I
( ✓ ) ✓ ✓ ✓
6 Compact design
Index puls width Phase shift of index pulse
CW
( ✓ )
7 Dust, dirt, oil
✓ ✓ ✓
( ✓ ) ( ✓ ) ✓ ✓ ✓
8 External magnetic fields
9 Ionizing radiation
✓
Key points for encoder selection
Encoders and maxon controllers – The input frequency of the control electronics may limit the maximum possible counts per turn of the encoder. – The higher the counts per turn and the accuracy, the easier it is to achieve smooth, jolt-free operation even at low speeds. – The maxon controllers can be configured for encoders with low (at least 16 cpt) or high counts per turn.
The following applies for positioning systems: – The higher the counts per turn, the more accurate the positioning. At 500 counts (2000 statuses), a mechanical angle resolution of 0.18° is achieved. This is usually much higher than the precision of the mechanical drive components. – In positioning controllers, only encoders with an integrated line driver should be used (e.g., RS422). This prevents signal loss and accumulated positioning errors due to electromagnetic interference. – Positioning applications often require the encoder’s index channel for precise determination of the reference point.
Main characteristics of maxon incremental encoders: – Counts per turn (increments) – Accuracy – Use of an index channel – Use of a line driver – Maximum supported speed – Suitability for special ambient conditions (dust, oil, magnetic fields, ionizing radiation)
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