What is the position (time) delay of the encoder signals?
Position or time delay in signals is common to all encoders. It can be corrected if needed in the control system by putting in a factor derived from the calculated delay.
At high rotation speed a position delay between magnet position and the electrical output appears because of filtering. Filtering is carried out with an RC filter. Position delay can be calculated with formula:
Δφ = -Arctan{ƒ/ƒ0} (ƒ = frequency, ƒ0 = (2πRC)-1)
For the AM256 and AM512B (8, 9 bit encoder ICs) and up to 9 bit RM/RE, the value of the resistor is 10 kΩ and the recommended value of capacitor is 10 nF.
|
Parameter |
Symbol |
Typ. |
|
Position delay @ 10 Hz (600 rpm), |
Δφpos |
0.36° |
|
Position delay @ 100 Hz (6000 rpm), |
Δφpos |
3.6° |
For 10+ bit encoders the resistor value is 10 kΩ and the value of capacitor is 22 nF.
|
Parameter |
Symbol |
Typ. |
|
Position delay @ 10 Hz (600 rpm), |
Δφpos |
0.8° |
|
Position delay @ 100 Hz (6000 rpm), |
Δφpos |
7.87° |
Using the above equation, we can therefore calculate the position delay in both degrees and time. In the following example we have a 10 bit encoder rotating at 1000 rpm.
|
1000 rpm |
= 16.7 Hz |
|
|
ƒ0 |
= (2πRC)-1 |
|
|
|
= (2 x π x 10 x 103 x 22 x 10-9)-1 |
|
|
|
= (1.38 x 10-3)-1 = 723.43 |
|
|
Δφ |
= tan-1{ƒ⁄ƒ0} |
|
|
|
= tan-1{16.7⁄723.48} = tan-1{0.02308} = 1.3° |
|
|
@ 1000 rpm - 1 rotation = 0.06 s |
||
|
|
(0.06⁄360) x 1.3 = 2.16 x 10-4 = 0.216 ms |
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